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BEGINNER LEVEL

History of Computer Networks

The Beginning

Computer networking is a very recent phenomenon. Although computer networks have been in use for about 30 years, their impact on our daily life and work increased dramatically in the late 1980s and early 1990s with the integration of desktop systems.

In the 1960s the development of mainframes and minicomputers created a technological explosion of ideas concerning the way computers were to be used. Engineers and researchers started developing new applications and techniques for using computers, including the possibility of having computers communicate directly with each other directly over telephone lines.

In the early 1970s new technology to connect computer systems was developed which proved that computer systems could be linked together effectively, using long distance communications media. As technology progressed, several different approaches emerged, each driven by different assumptions and goals. Some approaches primarily addressed the need to connect terminals to central concentrations of computers or mainframes, while others focused on a flexible interconnection of computers.

Due to the increasing complexity and importance of computer networks, computer manufacturers began developing comprehensive network architectures. Examples of such network architectures are IBMs SNA (Systems Network Architecture), DECs DNA (Digital Network Architecture) and TCP/IP.

1970 - 1990

In the late 1970s and early 1980s, the emergence of several new technologies made the concept of networking both an opportunity and a requirement. The introduction of new local area networking (LAN) and wide area networking (WAN)

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technologies made the communication between computer systems simpler, faster and more cost-effective. Typical examples for such WAN and LAN technologies are X.25, Ethernet and Token Ring.

During the early 1980s another new concept was introduced: desktop systems and personal computing. With desktop hardware and software becoming cheaper and at the same time more powerful, it became necessary to develop possibilities to integrate these systems into the traditional computing environment.

In the mid 1980s the first "real" networking solutions for desktop systems appeared on the market. At that time these networks were targeted strictly to homogeneous (single vendor) environments. Typical examples of such networks are the early versions of AppleTalk (just for Apple computer systems) and Novell NetWare (just for IBM PCs and compatibles). These early networks for desktop systems have several principles in common. First of all they are mainly based on LAN technology, and second they are typically based on the client-server type of computing. The client-server type of computing permits the desktop system to act as a client in requesting services from other computer systems (servers) across the network.

1990 - Today

During the 1980s and the early 1990s the trend was clearly to incorporate other systems and architectures into what were formerly homogeneous networks.

Novell for example, incorporated networking technology into NetWare that allows Apple Macintosh computers to take part in a Novell NetWare network. Many vendors of computer systems also started to offer protocols like TCP/IP to allow connectivity between systems.

The last 10 years brought a lot more consolidation and standardization in regards of networking hardware and protocols with the Internet becoming the by far largest open WAN infrastructure and TCP/IP as the networking protocol of choice to interconnect computer systems. Also in regards of LAN technology there has been a visible trend towards the simpler and cost effective Ethernet infrastructures, where the demand for more bandwidth led to the development to Fast Ethernet and Gigabit Ethernet technology.

Terminology and Concepts

Terms and Definitions

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There are a number of basic networking terms and concepts that form the basis of the theory of computer networks:

LAN (Local Area Network), WAN (Wide Area Network)

Nodes and node addresses

Packets (or frames)

Different communication media and technologies

Internetworking

A general problem when dealing with networking" in general is the existence of many different networking solutions, technologies, products and concepts. In order to be able to position and compare these different alternatives, it is important to have a basis with which to relate.

One way to achieve this is to analyse the overall structure, or in networking terms, the network topology.

An other way of understanding and describing different networking solutions is to introduce the very abstract concept of network architectures". The so called OSI-Model (or OSI architecture) is typically used as the reference model to describe and compare different networking architectures.

Other attempts to describe and explain networking concepts focus on implementation and functional principles and divide networking solutions into client-server or peer-to-peer solutions.

Packet or Frames

Data being sent between computer systems is broken down into smaller pieces called packets or frames. The specific structure and content of these packets very much depends on the communication technology. In general there is information needed in addition to the to be communicated data to ensure proper delivery of that data. This additional information typically consists of the sender and the receiver address, protocol specific parameters and data that ensures data integrity.

Ethernet packets or frames for example are composed of three sections:

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Packet header - contains information like sending and receiving station and protocol type

Payload or data section - holds the to be transmitted data

Trailer - contains status information about the packet and error checking information

Network. Architecture / OSI Model

Fundamentals

Already in the early days of networking it became clear that due to the complexity of inter-connecting computers and the steadily changing and improving technology, computer networks would have to be designed in a highly structured way. This overall structure is what we call a network architecture.

Definition: A network architecture specifies common communication mechanisms and interfaces that computer systems of different types must adhere to, when passing data between systems.

Network architectures are neccessary for several reasons:

Communication technolgy is continuously changing

A broad variety of operating systems, communication devices and computer hardware exists

Network management, error recording and maintenance become simpler tasks when standardized

Networks need to be adaptable to different communication situations and requirements

To reduce the complexity of their design, most network architectures are built as a set of layers with each layer performing a different function - this means each layer has a specific set of tasks that it has to accomplish.The function of the different layers, their names, and the actual number of layers differ among the network architectures. One reason is that computer networks from different vendors have been designed to solve their specific communication needs.

Other attempts to describe and explain networking concepts focus on implementation and functional principles and divide networking solutions into client-server or peer-to-peer solutions.

The OSI Reference Model

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The Open Systems Interconnection (OSI) model is the basis for a set of international communication standards established by the International Standards Organization (ISO). The OSI Model is an internationally accepted framework of standards for communication between computer systems.

The OSI model will help us to get a basic understanding of how network architectures are structured, and will be used as a reference for comparing different architectures.

There are two important aspects to be understood about the layers of the OSI model: Each layer communicates with its peer on another node using a specific protocol - and - each layer represents a defined set of services to the layer above it.

Within the OSI model there are seven defined layers:

Layer 7 - Application Layer

Layer 6 - Presentation Layer

Layer 5 - Session Layer

Layer 4 - Transport Layer

Layer 3 - Network Layer

Layer 2 - Data Link Layer

Layer 1 - Physical Layer

Networking Protocols

A protocol is simply a set of rules for communicating. This set of rules determines how data is transmitted in a network, such as:

How the data will be transmitted

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Size of the packet

Error control

Recovery procedures

Within a layered network architecture, every layer uses its own specific set of protocols. This is done by adding some control information (header, trailer) to the actual user data.

To understand how different protocols at different layers in our network architecture work it is best to look at similar principles when people communicate with each other. A good example is the communication between two people by phone. Certain layers or levels" of the communication must use certain protocols to work properly:

Telephone line (communication media)

Language

Syntax

Context

Conventions of telephone conversations, i.e. only one person talks at a time

The Seven OSI Model Layers

The Physical Layer is concerned with the mechanical and electrical (optical, ...) transmissions of signals between computer systems. Physical layer standards control such matters as connector specification, modulation, encoding techniques.

The Data Link Layer establishes a communication path over the physical channel, manages access to the communications channel, and ensures the proper transmission of data at this level.

The Network Layer has as its most critical function with the allocation and interpretation of network addresses. The Network Layer sets up the path between communicating nodes, routes messages through intervening nodes to their destination, and controls the flow of messages between nodes.

The Transport Layer provides end-to-end control of a communication session once the path has been established. This layer allows processes to exchange data reliably and sequentially independent of the systems which are communicating, or their location in the network.

Session Layer is concerned with dialogue management. It establishes and controls system dependant aspects of communications sessions.

The Presentation Layer masks out the variation in data formats between systems from different vendors. This layer works by transferring data in a system-independent way, performing appr. conversions within each system.

The Application Layer provides services that directly support user and application tasks such as file transfer, remote file access and mail.

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Network Topologies

Introduction

Mesh (point-to-point)

Multipoint

Star

Ring (Loop)

Bus

A computer network can be configured in an almost endless variety of ways. The particular user requirements and chosen media are the most important factors in determining the shape" of a network.

Despite the variety among networks, there are general categories of network shapes, called topologies. The Topologies are helpful when discussing or comparing various networks and their design goals.

Mesh Topology

The simplest network structure is based on point-to-point connections. A point-to-point link connects two (and only two) nodes without passing through an intermediate node. A mesh topology is built of just point-to-point links.

Multipoint Topology

In a multipoint configuration, several remote nodes share the same physical link. One node is designated as the control node which asks the other nodes in turn (polling) to send data.

Star Topology

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In a star, or centralized network, all nodes communicate via a central node that controls the network. All data flows toward or outward from this central device, node or computer

Ring Topology

In a ring topology the nodes are arranged to form an unbroken circular configuration. Transmitted messages travel from node to node around the ring.

Bus Topology

The bus topology works to some extent in the same way as a multipoint network - a single communication media which is shared by a number of nodes. However, in the event of node failures, network operation will continue due to the passive role nodes play in transmissions on the bus. There is no single device or node controlling or prioritising the transmissions.

Peer-to-Peer vs. Clients-Server

Peer-to-Peer Networking

In peer-to-peer networks users share information between each other in a de-centralized way. Individuasl systems have all necessary capabilities.

Typical advantages of peer-to-peer networks are:

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Easy to install

Inexpensive

No dedicated server - no single point of failure

Typical disadvantages are:

Difficult to manage

Limited security

Reduced performance as the number of users increases

Client-Server Networking

In client-server networks users access and share information in a centralized way. Servers and clients have very different capabilities.The asymmetric implementation of functions allow simple less overhead end user applications while concentrating advanced functions in the servers.

Typical advantages of client-server networks are:

Easy to manage

Easy to maintain, backup

Good performance

Security measures are easily implemented

Typical disadvantages are:

Dedicated server - single point of failure

Management necessary

Introduction

The first two OSI Layers

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The lowest two layers of the OSI model are not always easy to separate when it comes to group them into LAN and WAN standards.

Most importantly the first two layers build the basis for all protocols of the higher layers by specifying what kind of language" is used on what type of media.

Many different types of transmission media and access protocols have been and are used in computer networks, ranging from two wire cables to satellite links. Each transmission media uses specially designed protocols that specify how it is to be accessed

WAN Protocols

In the early days of data communication computer networks were built upon point-to-point connections (serial lines). Because of the bad quality of the media that were available at that time (i.e. telephone lines) WAN protocols like HDLC and DDCMP were designed to ensure sequencing and integrity of data in the event of transmission errors.

The need for more reliable WAN connections then lead to the development and implementation of packet switching networks such as X.25, Frame Relay and ISDN.

Many of the WAN standards and specifications are older than the OSI reference model wich makes it often difficult to precisely assign them to a specific OSI model layer. The grouping is relatively easy when it comes to standards and specifications like V.35, RS232, X.21 that describe interfaces and signalling which clearly is OSI Physical Layer related. The same is true for Data Link protocols like PPP, PPPoE or DDCMP these standards and specifications fulfill exactly the functions described in the OSI Data Link Layer. A clear decision of whether protocols like ISDN, X.25 or Frame Relay should be seen as pure Data Link protocols is somehow difficult. These standards and specifications include many functions, interface descriptions, etc. that could be seen as OSI model layer 1 (for example ISDN signalling) or even OSI model layer 3 (for example addressing and forwarding of packets in X.25, ISDN) related.

LAN Protocols - IEEE 802.x

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In the last 25 years the work place has been filled more and more with increasingly intelligent machines such as personal computers, workstations, scanners, plotters, printers, and so on. These machines assist in carrying out the day to day tasks and communications; therefore there has been an increasing need to interconnect these separate machines within a limited area. This has led to the development of what we call Local Area Networks (LAN).

Typical characteristics of LANs are:

Limited to a small area (i.e. building, factory, campus)

High bandwidth compared to WANs

Relatively low cost for high bandwidth

Usually owned by the user (company)

Due to the fact that lots of different devices and applications from different vendors should be able to access the same media, standardization within the LAN environment was, and is, a crucial issue.

Most of the widely used LAN technologies are either part of the ANSI / IEEE 802.x standards or are tightly related to ensure compatibility and easy integration of different LANs.

Examples for such protocols are:

IEEE 802.3 (Ethernet)

IEEE 802.5 (Token Ring)

IEEE 802.11 (Wireless LAN)

FDDI

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Transmission Media Types

Transmission media are the physical paths over which information flows from sender to receiver. Transmission refers to the method of carrying data from one place to another.

In computer networks a broad range of different media is used, from simple two wire cables to radio or microwaves.

There are three main media types used in LANs:

Coaxial cable (ARCNet, Ethernet)

Shielded / unshielded twisted pair cable (ARCNet, Ethernet, Fast Ethernet, Token Ring, FDDI/CDDI)

Fibre optic cable (Ethernet, Fast Ethernet, Gigabit Ethernet, FDDI)

Ethernet / IEEE 802.3

The ALOHA Network

The idea of Ethernet grew out of the packet radio broadcast network, known as the ALOHA network. This system, designed in the early 1970s for the University of Hawaii, used a distributed radio transmission network.

The special thing about ALOHA is the fact that it did not use FDM (Frequency Division Multiplexing) which is used by conventional broadcast systems to give each site its own share of the communication bandwidth. Instead it uses a special contention scheme in which a node simply transmits on the single channel when it needs to. If one node is already transmitting, or starts to transmit, while an other node is also beginning to transmit, a collision" occurs.These collisions can be detected and retransmits can be initiated.

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In approximately 1976 the Xerox PARC experimental Ethernet was developed, in which the techniques used for ALOHA were improved and applied to a coaxial cable medium.

In 1980, a new version was introduced in a specification document published jointly by Digital Equipment, Intel and Xerox. This specification, called DIX-Ethernet became then quickly a de facto industry standard.

The DIX-Ethernet was later adopted with minor changes and enhancements in the IEEE 802 local area networks standards committee and became the IEEE 802.3 (CSMA/CD) standard.

Ethernet / IEEE 802.3

History

1973 Xerox develops Ethernet, named after the "luminiferous ether", a medium once thought to fill all space and control the transmission of electromagnetic waves (operated at 2 Mbps)

1980 - First formal specifications created in a joint effort by Digital, Intel and Xerox named DIX Ethernet (operated at 10 Mbps)

1985 IEEE modifies DIX and creates 802.3 standard

1995 IEEE creates 802.3u standard for Fast Ethernet

1998 IEEE creates 802.3z standard for Gigabit Ethernet

2002 - IEEE releases the 802.3ae standard for 10 Gigabit Ethernet

Topologies / Transm. Medias

Ethernet not only has evolved over time to deliver more and more bandwidth but also to support a broad variety of transmission media. Some of the implementations are listed below:

10Base5, or (DIX) Ethernet, Thickwire

10Base2, or Thinwire, Cheapernet

10BaseT, or Twisted-pair Ethernet

10Broad36, or Broadband Ethernet

100BaseT, or Fast Ethernet (twisted-pair cables)

1000BaseT, or Gigabit Ethernet (twisted-pair cables)

Connector Types

Connectors typically found in Ethernet installations:

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AUI (Attachment Unit Interface) used to connect to external transceivers

BNC

RJ45

SC, ST, VF45, MTRJ

An Ethernet transceiver is a device that transmits and receives information to and from the wire. When you plug into an RJ45, BNC or Fiber port on a NIC, you are connecting to a transceiver. Transceivers are also available as an external device that attach to the AUI port of the network computer.

CSMA/CD

Although there are minor differences between (DIX) Ethernet and IEEE 802.3, both use the same basic scheme, called CSMA/CD to control the access to the communication channel.

CSMA/CD is an abbreviation for:

CS - Carrier Sense. The node constantly monitors the cable to see if there is any activity on the Ethernet. If some other node is already transmitting, then the node waits (defers its transmission) until the other node has finished transmitting.

MA - Multiple Access. Any station connected to the Ethernet can transmit as soon as it is free. This means all nodes have equal access to the communication channel

CD - Collision Detection. Most of the time the carrier sense works well, however on occasion, two nodes might start transmitting at the same time. In this case, they both interfere with each others signals and generate a collision. When a collision is detected, the transmission is aborted and started again at a random time.

Configuration Rules

In order to ensure that all nodes, including the one that is transmitting, are able to detect a collision on the channel, packets must be of a certain minimum length.

The minimum time a node has to send (minimum packet length) is called slot time, which is slightly greater than the round trip propagation delay between two furthest points in the network.

This is the reason why it is this important to follow the configuration rules to be found in the IEEE 802.3 specifications. The configuration rules are based on a worst case" calculation which takes all components causing propagation delays into

account.

Violation of configuration rules can have severe effects on performance and stability of the Ethernet network.

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Round Trip Prop. Delay

The Ethernet configuration rules are based on a worst case" calculation which takes all components causing propagation delays into account.

The above calculation is from the early Ethernet V2.0 spec and is only meant to visualize the calculation.

Ethernet Collision Domain

Ethernet bandwidth is 10Mbps. In an environment using only hubs, the entire network is on a single collision domain.

As a consequence of this all users are sharing the 10Mbps bandwidth. As more users connect to the LAN, the number of collisions in the domain rises, and the bandwidth available per user is reduced. This mechanism is called contention.

It is also important to remember that an Ethernet collision domain is limited in "size" because of CSMA/CD and therefore Ethernet configuration rules have to be followed precisely.

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Typical Ethernet Activity

Ethernet uses CSMA/CD a contention protocol that resolves a collision after it occurs. It executes the collision resolution protocol after each collision.

A sending node on the Ethernet attempts to avoid contention with other traffic on the channel by monitoring the carrier sense signal and deferring to passing traffic. When the traffic is clear, the frame transmission is started(after a brief interframe delay).

At the receiving station, the arriving frame is detected, synchronizes with the incoming preamble. The frame's destination address field is checked to decide whether the frame should be received by the station. If yes the contents of the frame is passed to the next higher layer.

If multiple stations attempt to transmit at the same time, it is possible that they interfere with each other's transmissions, in spite of their attempts to avoid this by deferring. When two station's transmissions overlap, the resulting contention is called a collisison. As soon as a collision is detected the transmission is stopped and attempted after a short delay again.

Minimum time a host must transmit for before it can be sure that no other host's packet has collided with its transmission is called contention slot.

Typical Ethernet activity therefore shows between the ransmission periods, idle periods (where no transmission is attempted) and contention periods.

Ethernet Addressing

Node addressing provides a means of uniquely identifying each node connected to the local area network.

An Ethernet address is 48 bits in length. It is represented by six pairs of hexadecimal digits. For example: F0-2E-25-6C-77-3B

These digit pairs are typically separated by single hyphens. The order of transmission on the Ethernet is from the leftmost octet to the rightmost octet. The order of bits within the octets is from the least significant bit of the rightmost digit to the most significant bit of the leftmost digit.

Normally, one address is permanently associated with each interface. This means that each Ethernet device is manufactured with an unique address stored in ROM. This individual address is called the Hardware Address.

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These (globally administered) unique addresses are allocated in address blocks to organizations in a centralized manner. SMC for example has the following ranges of addresses (and others) assigned to it:

00-80-0F-xx-xx-xx

00-E0-29-xx-xx-xx

There are specific types of addresses that are essential for some of the higher layer protocols:

Multicast Address - a multi-destination address ( for one or more nodes)

Broadcast Address - a single Multicast address intended for all nodes

MAC - Media Access Control

Due to the fact that IEEE did introduce some changes to the original Ethernet V2.0 specification, we know today two slightly different Ethernet MAC frames.

The MAC Protocol adds address information to the packet, and checks to see that the packet arrives intact. For this purpose the vendor's hardware address (3 bytes) from the NIC is read. This information is used to create a 6 byte MAC Address.

To transmit a packet, the source MAC Address and the destination MAC Address are added to the packet, creating a new packet. This process is called encapsulation.

In addition to adding address information, the MAC Protocol also adds:

A 2 byte field. In Ethernet V2.0 it contains frame type information that tells the OS what protocol is being used (IP, IPX, etc.) In IEEE 802.3 these 2 bytes specify the length of the data field in bytes.

A 4 byte CRC (Cyclic Redundancy Check) field that is used to check for errors in the received data

Once the packet is encapsulated, it is send out on the "wire".

As the packet passes by a computer attached to the LAN, the NIC in that computer checks the packet's destination address.

If the packet is addressed to that NIC, the driver copies the packet and the OS decodes the packet and delivers the data to the appropriate application

Fast Ethernet

Introduction

Fast Ethernet transmits at 10 times the speed of Ethernet and as with Ethernet, signal loses strength and coherence as it travels the wire.

The developers of Fast Ethernet had to ensure compatibility at the frame level with Ethernet. CSMA/CD relies on a minimum time that every station on the network is sending a frame. This is guaranteed by the minimum packet length. The speed of transmission can therefore be increased by decreasing the signals worst

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case round trip delay. This means reducing also the maximum allowed distance between any two stations in the network.

As a result the maximum allowed distance between any two stations with Fast Ethernet (copper) is only 205m compared to 2.5km with Ethernet.

Physical Layer

The Physical Layer of Fast Ethernet uses a mixture of proven technologies from the original Ethernet and the ANSI FDDI Specification. The physical media types are defined in 802.3u.

Fast Ethernet works with category 3,4 and 5 unshielded twisted pair (UTP), type-1 shielded twisted pair (STP), and fiberoptic cables.

The MII provides a media-independent interface and performs the same function in Fast Ethernet as the AUI in 10 Mbps Ethernet.

The Fast Ethernet standard also offers a media-independent interface. This interface is called MII (Media Independent Interface) and performs the same function in Fast Ethernet as the AUI in 10Mbps Ethernet.

Gigabit Ethernet

Introduction

Gigabit Ethernet transmits at 100 times the speed of Ethernet.

The developers of Gigabit Ethernet had to ensure compatibility at the frame level with Ethernet and Fast Ethernet. This and the requirement to support still transmission distances that are acceptable results not only in the use of switching technology but also in changed layer one operation.

The use of Gigabit Ethernet switches instead of repeaters also means that there are hardly any configuration rules besides max cable length to be followed.

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Physical Layer

The Physical Layer of Gigabit Ethernet uses a mixture of proven technologies from the original Ethernet and the ANSI X3T11 Fibre Channel Specification. The physical media types are defined in 802.3z (1000Base-X) and 802.3ab (1000Base-T).

The 1000Base-X standard is based on the Fibre Channel Physical Layer. Fibre Channel is an interconnection technology for connecting workstations, supercomputers, storage devices and peripherals. Three types of media are include in the 1000Base-X standard:

1000Base-SX 850 nm laser on multi mode fiber.

1000Base-LX 1300 nm laser on single mode and multi mode fiber.

1000Base-CX Short haul copper "twinax" STP cable

1000Base-T is a standard for Gigabit Ethernet that utilizes long haul copper UTP. Up to 100m over 4 pairs of Category 5 UTP are possible.

Some Gigabit Ethernet switching devices offer a modular standardized media interface called GBIC. The Gigabit interface converter (GBIC) allows the network administrator to configure each gigabit port on a port-by-port basis for short-wave (SX), long-wave (LX), long-haul (LH), and copper physical interfaces (CX). Media Access Control / Frame Format

The developers of Gigabit Ethernet had to ensure compatibility at the frame level with Ethernet. The general structure of a Gigabit Ethernet frame and a 10Mbps or 100Mbps Ethernet frame are identical. Todays Gigabit Ethernet networks are nearly entirely implemented using switched full-duplex connections.

But due to the fact that also Gigabit Ethernet was designed to work in (half duplex) shared media implementations the developers of the standard had to ensure that the sending station still can sense collisions.CSMA/CD relies on a minimum time that every station on the network is sending a frame. In Ethernet and Fast Ethernet this is guaranteed by the minimum packet length. Because of the speed of transmission with Gigabit Ethernet this would result in a maximum allowed distance between any two stations of only about 10m.

In order to overcome this severe length limitation in Gigabit Ethernet the frame length has to be artificially increased by appending an extension field at the end of the frame, right after the frame check sequence (FCS). To minimize the waste of

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bandwidth introduced with the extension field, the Gigabit Ethernet standard allows the sending station to send a sequence of frames (frame bursting) for a pre defined period of time.

10 Gigabit Ethernet

Introduction

10 Gigabit Ethernet (10GBASE-T) as standardized in IEEE 802.3an, is a telecommunication technology that offers data speeds up to 10 billion bits per second - or - 1000 times the speed of Ethernet. Built on the Ethernet technology used in most of today's LANs, 10-Gigabit Ethernet offers a more efficient and less expensive alternative for backbone connections while also providing a consistent technology end-to-end.

10 Gigabit Ethernet uses the familiar IEEE 802.3 Ethernet media access control (MAC) protocol and its frame format and size. Additionally, this standard is moving away from half-duplex design, with broadcasting to all nodes, towards only supporting switched full-duplex networks.

Unlike earlier Ethernet systems, 10-Gigabit Ethernet is mainly based on the use of optical fiber connections. However, the IEEE is working on a standard for 10-Gigabit Ethernet over Cat-6 or Cat-7 twisted pair cable.

IEEE 802.3ae Physical Layer

The IEEE 802.3ae* standard describes a physical layer that supports specific link distances for fiber-optic media. To meet the distance objectives, four PMDs (physical-media-dependent devices) were selected:

1310nm serial

1550nm serial

850nm serial

1310nm WWDM (wide-wave division multiplexing)

There are two types of optical fiber, multimode and singlemode fiber, that are currently used in data networking and telecommunications applications. The IEEE 802.3ae* standard, supports both optical fiber types. However, the distances supported vary based on the type of fiber and wavelength (nm) is implemented in the application.

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IEEE 802.3* has formed two study groups to investigate 10 Gigabit Ethernet over copper cabling. The 10GBASE-CX4 group is working on a standard for transmitting and receiving via a 4-pair twinax- cable. The 10GBASE-T group is working on a standard for the transmission and reception of 10 Gigabit Ethernet via a Category 5 unshielded twisted pair (UTP) copper cable up to 100 m.

Media Types

The 10 Gigabit Ethernet standard includes several different media types, that are currently specified by a supplementary standard, IEEE 802.3ae:

10GBASE-SR (short range)

10GBASE-CX4 (Copper interface)

10GBASE-LX4

10GBASE-LR (long range)

10GBASE-ER (extended range)

10GBASE-LRM

10GBASE-SW

10GBASE-LW

10GBASE-EW

10GBASE-LR

10GBASE-ER

10GBASE-SR ("short range") is designed to cover short distances using existing multi-mode fiber cabling. It has a range of between 26m and 8 m depending on the used cable type. With a new developed multi-mode fiber distances with up to 300m are possible.

10GBASE-CX4 describes a copper interface using twinax-cable ( InfiniBand) for short-reach (15 m maximum) applications.

10GBASE-LX4 uses wavelength division multiplexing to support ranges of between 240m and 300m over multi-mode cabling and also supports distances of up to 10km over single-mode fiber.

10GBASE-LR (long range) and 10GBASE-ER (extended range) are standards that allow distances of up to 10 km and 40km respectively over single-mode fiber.

10GBASE-LRM describes 10 Gbps on FDDI-grade 62.5 m multi-mode cable.

10GBASE-SW, 10GBASE-LW and 10GBASE-EW use the WAN PHY, designed to interoperate with OC-192/STM-64 SONET/SDH equipment. They relate at the physical layer to 10GBASE-SR.

10GBASE-LR and 10GBASE-ER respectively, and therefore use the same types of fiber and support the same distances.

Wireless Networks

Applications

Alternative and/or extension of wired infrastructures

Simple integration into existing networking infrastructures

Solutions for environments and applications where conventional wired infrastructures are not feasible:

Temporary networks

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Architectural reasons (building codes, protection of-histhistoric buildings, )

Mobile applications

Flexible networking solutions

Interconnecting LANs

Quite often communication infrastructures based on standard wiring schemes are not feasible because of cost or technical reasons. In this case wireless products offer flexible alternatives to wired network solutions.

Wireless technology also provides excellent solutions where there is a need for temporary networking installations.

In many cases where more traditional communication solution cannot be envisioned with conventional wired technologies, wireless technology makes the seemingly impossible quite feasible, easy to implement, and cost effective.

Implementing wired infrastructures into existing building structures can present complex problems. Building codes or city ordinances that seek to protect historic buildings from any structural damage can create severe costs and technical problems for the network designer implementing wired technologies.

Overview Technologies

In general we can separate the different wireless technologies into the following categories:

WPAN (Wireless Personal Area Networking)

Bluetooth / IEEE 802.15.1

IEEE 802.15.3

WLAN (Wireless Local Area Networking)

IEEE 802.11a/b/g

WMAN (Wireless Metropolitan Area Networking)

WiMAX / IEEE 802.16

WWAN (Wireless Wide Area Networking)

GPRS

UMTS

GSM

Today different wireless(mostly RF) technologies have been developed or are under development to address a broad range of wireless communication applications and scenarios.

The requirements for these applications are mainly based on a varitey of variables including the needed bandwith, the distances that have to be covered, the geographic reach, power consumption and the kind of services offered.

In general we can separate the different wireless technologies into the following categories:

Each categoriy shows one (or more) corresponding wireless technologies that solve the specific communication issues of that category or application. Although overlaps (WPAN/WLAN, WLAN/WMAN) exist, the deployed technologies are extremely different and supplement each other to a very high degree.

Overview Scenarios

WPAN (Wireless Personal Area Netwrking) technologies like Bluetooth / / IEEE 802.15 solve connectivity problems between devices and systems in a very limited geographical area. Typical network coverage in the WPAN is up to 10m, the data transfer rates depend on the standards emplyed. Applications are for example the synchronisation of data and file transfers between PDAs, laptops and mobile phones but also the wireless

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connection of peripherals and devices like head sets, printers, etc.

WLAN (Wireless Local Area Networking) applications typically solve wireless data communication problems in an building, enterprize or campus environment. The dominant technology used is absed on the IEEE 802.11 standards.

WMAN (Wireless Metropolitan Area Networking) infrastructures are designed to overcome "last mile" access issues by providing wireless connectivity in an metropolitan environment. Example for an emerging WMAN standard is WiMAX (IEEE 802.16)

WWAN (Wireless Wide Area Networking) technologies offer wireless mobility solutions, typically offering lower bandwitdth but covering large geographical areas. Typical examples for such technologies are GPRS, UMTS and GSM.

Standards - WPAN/WLAN/WMAN

The Bluetooth technology standard was originally deveolperd as an industry standard driven by a group of manufacturers but the standardisation process is now also taken care of by an IEEE working group (802.15). The first version IEEE 802.15.1 was derived from the original Bluetooth specification and is compatible to Bluetooth V.1.1. This standard supports data rates up to 1Mbps and is primarily used for wireless connectivity with computer peripherals and other devices like printers, headsets, mobile phones and PDAs. IEEE 802.15.3 (also called ultra-wide band or UWB) is designed for much higher speeds and multimedia services. This standard supports

speeds up to 400Mbps, allowing the transmission of video (of DVD quality) and audio signals throughout the home.

Within the IEEE the 802.11 working group is responsible for developing standards for WirelessLocal Area Networks (WLANs). WLANs typically serve a lot more users than WPANs and cover a larger area. The IEEE 802.11 standard is based on the same framework and principles that also form the basis for Ethernet (IEEE 802.3). This ensures a high level of compatibility and interopearbility between 802.11 and 802.3 devices and infrastructures. Until now three major revisions or versions of the physical layer have been released supporting speeds up to 54Mbps.

The Wireless Metropolitan Area Network (WWAN) typically covers areas up to 50km and competes directly with other access technologies like xDSL or (DOCSIS) cable. WiMAX is an example for a new generation of standardized wireless broadband internet access technologyies. WiMAX is aworldwide certification adddressing interoperability issues across IEEE 802.16 products.

PSTN / Modem

Introduction

The word modem is a contraction of the words modulator-demodulator. A modem is typically used to send digital data over a phone line. The sending modem modulates the data into a signal that is compatible with the phone line, and the receiving modem demodulates the signal back into digital data.

Modems came into existence in the 1960s as a way to allow terminals to connect to computers over the phone lines. Once people started transferring large programs and images 300 BPS became intolerable.

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Modem speeds increased in a series of steps at two year or so intervals:

300 bit per second - 1960s through 1983 or so

1200 bit per second - gained more popularity in 1984 and 1985

2400 bit per second

9600 bit per second - ( late 1990 and early 1991)

19.2K bit per second

28.8K bit per second

33.6K bit per second

56K bit per second - became the standard in 1998

Modems use a hand-shaking sequence to negotiate the best modulation technique supported on both ends of the communication path.

ISDN

Introduction

Telephone networks around the world have been evolving toward the use of digital transmission facilities and switches for many years.

The CCITT which is largely responsible for todays international ISDN standards, defines an Integrated Services Digital Network (ISDN) as: "A network evolved from the telephony Integrated Digital Network (IDN), that provides end-to-end digital connectivity to support a wide variety of services, to which users have access by a limited set of standard multi-purpose user-network interfaces."

In other words, an ISDN is a network designed to carry many different types of data over medium-to-large distances, and between a wide

variety of equipment types, such as computers, telephones, facsimile and telex machines.

Features and functions associated with ISDN include:

End-to-end digital service

Standardized access interface

Well defined basic services and supplementary services like telephone (voice)

2B+D for small users (B=64 KB/sec, D=16 KB/sec)

23B+D (30B+D) for large users (B=64 KB/sec, D=64 KB/sec)

More than a "Digital Network"

ISDN has some very important advantages as a technology to be used for data communication:

Standardised

Flexible, 2 available simultaneous channels

Bandwidth (2 x 64 Kbps)

High transmission quality (digital)

Attractive pricing (in many countries)

Availability, good geographical coverage

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Fast call establishment

Integral security functions

Standards are the basis for the development of attractively priced communication solutions for large markets.

ISDN is more than Digital Network". Integrated Services" stands for the seamless integration of voice and data. A variety of advanced communication services, tele-services and fast and reliable connections into the Internet or to other remote networks today rely on ISDN. All this and the ability to use two individual communication channels with a single S0 connection explain the flexibility of ISDN. For small and medium-sized enterprises ISDN is very attractive. ISDN delivers to customers attractive tariffs for lines and high quality digital transmission in combination with relatively large bandwidth. ISDN is also available in most European countries.

Functions like dial-on-demand" or bandwidth-on-demand" are only possible because of the short times needed for call establishment when using ISDN.

ISDN is also popular because of its built-in security features. Typical examples for such functions are calling line identification or closed user group.

Broadband WAN Services

Introduction

The demand for high performance WAN services - also for small and medium enterprises - grows steadily. Both old and new "bandwidth hungry" applications require more WAN bandwidth.

Groupware and other Client Server solutions

Multi media solutions

Video streaming and video conferencing

Internet Access for individual systems and complete LANs

The Internet used as company WAN backbone

The number of subscribers for broadband services is growing rapidly. Many different tariff models, technological alternatives and attractive pricing attract a large number of users to change their existing Internet access technology

to one of the new broadband alternatives.

There are a number of important factors causing this fast development: Cost efficient use and upgrade of existing communication infrastructures Standardized products and technologies Competing service providers in most markets A large number of manufacturers of broadband products

Overview

There are currently several alternative and competing broadband technologies available or under development:

xDSL (Digital Subscriber Line)

Cable network (cable modem)

Satellite Transmission

Wireless (RF) Networks

Communication solutions utilising the electrical power infrastructure

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xDSL (Digital Subscriber Line) today clearly has the largest market share of all broadband internet access technologies.

Other technologies and solutions have nevertheless also a large growth potential because of the specific features and advantages some of theses technologies offer. Still there are criteria to be met to gain broad market acceptance which are not all met by current xDSL alternatives:

Complete geographical coverage

Different services and tariff models to optimally solve specific customer requirements

Low cost for both subscriber and service provider (equipment, installation, service, tariffs, operational costs)

Standards, compatible products and solutions

xDSL - Digital Subscriber Line

xDSL services clearly have today the largest market share of all broaband internet access services offered. xDSL is a term used to describe a whole range of different DSL (Digital Subscriber Line) technologies. xDSL - with very few exceptions - utilizes existing telephone infrastructures (last mile). xDSL is based on new advanced modem-technologies that allow very high transmission rates.

Advantages:

Complete geographical coverage

Different services and tariff models offered

Low cost, good price performance ratio

Disadvantages:

many different standards and therefore only partial compatibility of products and solutions

Cable Modem Solutions

Cable modem solutions utilize existing Cable TV infrastructures. Early standardisation efforts led to commonly accepted specifications and modem products. (DOCSIS - Data Over Cable Service Interface Specifications). Cable modem solutions allow only asymmetrical data streams. They offer higher bandwidth downstream (from the Internet) connections and are therefore suitable for Internet access applications for SME networks or individual systems (home).

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Advantages:

Theoretically complete geographical coverage. (Not all cable networks support bi-directional transmissions)

Low costs

Standards and compatible products

Disadvantages:

Very restricted service offerings (only usable for asymmetrical traffic)

Shared medium

Satellite Communication

Affordable satellite solutions typically allow only asymmetrical data streams. They offer higher bandwidth downstream (from the Internet) connections and are therefore suitable for Internet access applications for SME networks or individual systems (home).

Advantages:

Theoretically complete geographical coverage.

Appropriate solution where huge down link capacities are required

Disadvantages:

Relatively high costs (dial-up back channel)

Hardly any standards or compatible products

Very restricted service offerings (only usable for asymmetrical traffic)

Shared medium

Wireless (RF) Solutions

Affordable wireless solutions are not based on a single standard. Some solutions only allow asymmetrical data streams while other solutions have severe transmitting distance limitations. Whether wireless technology is an acceptable alternative to other broadband technologies largely depends on the local service offerings.

Advantages:

Several different services and tariff models are possible

Easy to deploy - no existing infrastructure is necessary

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Disadvantages:

No broad geographical coverage

Hardly any standards or compatible products

Very restricted service offerings

Shared medium

Utilising the Electrical Power Infrastructure

Internet access solutions utilizing the electrical power infrastructure are in very early development phases. These kinds of services are today only available in field test environments.

Advantages:

Theoretically complete geographical coverage.

Relatively low cost because it is using the power lines to cover the "last mile".

Disadvantages:

Currently only available in test installations

Hardly any standards or compatible products

Very restricted service offerings

Internetworking

Overview

The major components that provide extended connectivity capabilities between LANs or LANs and WANs are:

Repeaters

Bridges / Switches

Routers

Gateways

These devices have very different functions and capabilities. The easiest way to define these terms is to use the OSI model for reference.

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Repeater / Hub

Definition

Repeaters operate within the physical layer of the OSI model and provide connectivity normally between similar media.

Technical features of repeaters are:

They repeat and amplify electrical signals (also noise)

All LANs connected by repeaters sense the same traffic

LAN segments that are connected by a repeater are still on the same network

With Ethernet, hubs are multi-port repeaters.

Bridge / Switch

Definition

Bridges connect networks of similar technology. They work at the Data Link layer of the OSI model.

Typical features / functions of bridges are:

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They typically connect similar hardware networks like an Ethernet network to an Ethernet network

As repeaters connect (cable) segments together within a LAN, a bridge can connect LANs together to form an extended LAN

Bridges are able to connect networks regardless of the high level protocols (TCP/IP, AppleTalk, IPX, ) being used.

Bridges can filter traffic so that only the intended traffic passes through. They also do not forward faulty packets and noise on the lines

Some special bridges can connect LANs based on different technologies. Examples for such bridges are Ethernet / FDDI or Ethernet / Token Ring bridges.

Two fundamentally different kind of bridging technologies have been used to interconnect / extend local area networks:

Source Routing

Transparent Bridging

Of these two techniques only transparent bridging has significant relevance as it is used in todays Ethernet networks a lot.

Bridge / Switch

Basic Transparent Bridge Operation

The bridge learns with each received frame the source address (MAC address) of the frame and the interface (port) via which the frame has been received. This information is stored in the bridges station cache. ( MAC Address Table)

Each received frame also contains a destination address (MAC address). This address is compared to the entries in the bridges station cache. Afterwards the following forwarding rules are applied:

If the address is not found in the station cache then the frame is forwarded on all bridge interfaces, except the interface where the frame was received.

If the address is found in the station cache then the frame is forwarded to the interface associated wit the address.

If the specified interface is the one from which the packet was received, the bridge drops the frame.

In order to accomodate dynamic changes in the network and to keep the tables at an appropriate size, each entry in the station cache is aged". This means entries in the station cache are deleted after a specified period of time (aging timer) if no frame with this address (source address) is received.

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Bridge / Switch

Bridges - Multiple Path

For this example we assume that all station caches are empty and that station F sends the first fame.

Initially each of the three bridges (A, B, C) receives the frame that station F sends to station H. Each of the bridges then notes that station F resides on LAN 1 and queue the frame for forwarding to LAN 2.

One of the bridges (in our example bridge A) will be the first to successfully forward the frame to LAN 2. Because bridge operation is transparent (also to other bridges) the frame appears on LAN 2 exactly as if the originating station is on LAN 2. Therefore the bridges B and

C will receive the packet, note in their tables that station F now resides on LAN 2 and queue the packet for forwarding to LAN 1.

This looping of frames will occur forever with an exponentially increasing number of frames.

To ensure proper operation of learning bridges in a topology with loops an algorithm has been introduced that automatically changes the topology into a loop free structure called a "spanning tree".

Bridge / Switch

The Spanning Tree Protocol

The Spanning Tree protocol / algorithm takes care of link management and loop prevention in extended LANs.

The Spanning Tree Algorithm is used by bridges in redundantly configured networks to dynamically block ports to avoid network loops and open them again if a changed network situation makes this necessary.

In order to implement the process, the bridges exchange special messages with each other that allow them to calculate a spanning tree. The bridges perform the following steps:

Among all briges on the extended LAN one bridge is elected to be the Root Bridge.

All other bridges then calculate the shortest path from themselves to the Root Bridge.

On each LAN the one bridge that is closest to the Root Bridge is elected to be the Designated Bridge for this LAN. The Designated Bridge will forward frames from that LAN towards the Root Bridge.

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Each bridge identifies the port that gives the best path from themselves to the Root Bridge.

Eventually the ports that are neccessary to build the spanning tree are selected.

Data frames are forwarded to and from ports that are included in the spanning tree. On ports that have not been selected for the spanning tree data frames are not forwarded and discarded.

Router

Definition

Routers work at the Network layer of the OSI model and are independent of the network media and LAN technology. Instead of forwarding Data Link level packets like bridges, routers forward the data based on the higher layer information in those packets. This means it uses the routing information of the higher level protocol like TCP/IP or IPX/SPX.

Typical features / functions of routers are:

They work protocol oriented

They can be used to link different LAN/WAN technologies

Some vendors offer devices called brouters or bridge-routers that have both bridging and routing capabilities implemented.

Routing Protocols

Routers collect and store information about the network in routing tables. These tables are used to determine the optimum path for a packet to be transmitted. Routing protocols are used to maintain and exchange information necessary to calculate these tables.

Routing protocols typically fall into two main categories, Distance Vector routing or Link State routing.

Distance Vector routing protocols determine the best path on how far the destination is based on

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basic information like the number of intermediate routing systems (hops).

Link State protocols are capable of using more sophisticated methods to determine the best path for a to be transmitted packet. These methods may take into consideration link variables like bandwidth, delay, reliability and load.

Routing metrics and cost values are used by routers to determine the best path to the destination network or node.

Hop

Bandwidth

Delay

Reliability

Load

Cost

Routing Protocols - Examples

For several decades network architectures and therefore also routing protocols have been developed an deployed.

With increased numbers of networks and nodes to be connected also the routing protocols had to evolve. New levels of flexibility, performance and control were introduced with more powerful routing algorithms and techniques.

Examples for routing protpocols:

RIP v1 and RIP v2 (Routing Information Protocol)

OSPF (Open Shortest Path First)

BGP (Border Gateway Protocol)

IGRP (Interior Gateway Protocol) Cisco

DECnet Phase IV DRP (DECnet Routing Protocol)

RTMP (Routing Table Maintenance Protocol) a and ZIP (Zone Information Protocol) AppleTalk

Novell NetWare RIP (Routing Information Protocol)

Gateway

Definition

Gateways are typically used to connect two different network architectures and therefore work at the level of the Application Layer of the OSI model. This means it can "understand" and convert between different high level protocols. Examples for such gateways are DECnet / SNA or AppleTalk / TCP/IP gateways.

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Typical features / functions of gateways are:

They provide protocol conversion

They can support different network technologies (like routers)

A gateway typically has two complete architectures implemented

Architectures and Prot. Suites

Examples

Over the years a broad range of network architectures and protocol suites have been developed and deployed. Typical examples for such widely used architectures are:

TCP/IP

Novell Netware (IPX/SPX)

AppleTalk

DNA and DECnet (Digital Equipment)

LAT (Digital Equipment)

SNA (IBM)

OSI

NetBIOS/ NetBEUI (Microsoft, IBM)

Banyan Vines

In the past many manufacturers offered their own proprietary protocols and networking solutions to support their specific hardware and software in an optimum way.

Over the last years there was a clear trend towards "standardised" networking solutions based on the TCP/IP protocol suite.

Many network architectures now have TCP/IP protocols integrated to ensure a high level compatibility and interoperability.

Architectures and Prot. Suites

TCP/IP

TCP/IP is a widely accepted protocol suite and is the basis for the worldwide Internet. It supports a broad variety of Data Link protocols and transmission media and is implemented on a broad range of different operating systems and hardware platforms.

The TCP/IP protocol suite is organized into four conceptual layers:

The Network Access or Local Network Layer is the equivalent to the combined Physical and Data Link Layers of the OSI model. The architecture does not specify a particular Data Link protocol to be used, but there are existing standards to support for example Ethernet, Token Ring, X.25 and PPP.

The principal protocol of the Internet Layer is IP (Internet Protocol). It is used to connect one or more networks into an internet. It offers it services to various higher layer protocols by assisting the delivery of data (packets) in one or more IP datagrams.

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The Host-to-Host or Transport Layer has the task of providing end-to-end communication between processes rather than systems. TCP/IP provides at that level two principal protocols: TCP (Transmission Control Protocol) that provides reliability with a high overhead and UDP (User Datagram Protocol) which provides unreliable services with less overhead.

The Application Layer is the equivalent to the three highest layers of the OSI Model.

Architectures and Prot. Suites

Win NT Network Architecture

The network architecture which is part of the overall Windows NT system architecture provides a good example for Microsoft networking solutions.

A broad range of server and workstation applications and services can use (alternatively) different widely available networking protocol suites.

Besides IPX/SPX (NWLINK), TCP/IP and DLC (Data Link Control), Microsoft networking solutions often rely on the NetBEUI protocol. NetBEUI was developed to work effectively with LAN technologies and provides therefore no routing functionality.

The Internet and TCP/IP

History

In the US, government agencies already recognized in the late 1960s the need for a technology that would interconnect many different networks in order to make them all function as one unit with a high level of redundancy.

The internet technology that resulted from research funded by the Defense Advanced Research Projects Agency (DARPA) was a set of layered protocols called TCP/IP named after two of its main protocols. (Transmission Control Protocol and Internet Protocol).

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In about 1983, TCP/IP became the standard protocol suite used on the DoD Internet (Department of Defence Internet) including the ARPANET which was the first available packet switching network.

The ARPANET research resulted in the establishment of additional networks that are referred to as the DARPA Internet or simply the Internet. (The term Internet written with a capital "I" is used when referring to the DARPA Internet. If it is written with a small "I" then the term is used in a generic way.)

The Internet is today a worldwide grouping of networks, all of which use TCP/IP. These networks include large and small private networks, science and research networks and military networks like the DDN (Defence Data Network).

Since years the Internet grows at an incredible speed. In January 1993 only about 1.3 million hosts were connected to the Internet. January 2008 already close to 550 million hosts have been counted by the ISC (Inernet Software Consortium) an organization that regulary determines the approximate number of computer systems connected to the Internet.

Reference Model

The TCP/IP protocol suite contains a large number protocols at all layers within the architecture. Some of the more common protocols are:

IP Internet Protocol

ICMP Internet Control Message Protocol

ARP Address Resolution Protocol

RARP Reverse Address Resolution Protocol

RIP Routing Information Protocol

TCP Transmission Control Protocol

UDP User Datagram Protocol

FTP File Transfer Protocol

RPC Remote Procedure Call

NFS Network File Server

SMTP Simple Mail Transfer Protocol

Ping Packet Internet Groper

HTTP Hypertext Transfer Protocol

IP Characteristics

IP datagrams are sent from one host to another, possibly through interconnecting routers. These routers (in IP terminology also called gateways) forward IP packets from one network to another.

IP service is unreliable, connectionless, best-effort packet delivery system

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The IP service does not guarantee the delivery of packets. The packets may be duplicated, lost or delivered in wrong order. Error detection is only provided for the IP header not for the payload portion of the packet.

The IP service is called connectionless because each packet is processed independently from all others. IP datagrams contain all the information necessary for intermediate routers to process the packets and forward them accordingly.

The IP service which is used by all other protocols of the TCP/IP protocol suite provides network level services like:

Host addressing

Routing

Packet fragmentation and reassembly (if necessary)

All other protocols use IP services

TCP/IP Protocol Suite

IP Datagrams

To send an IP datagram , the sending machine encapsulates the datagram inside a network frame for transmission across a directly connected network. If for example the network technology used is Ethernet, then the IP datagram is placed in the data portion of the Ethernet frame, and the frames type field is set to IP.

After the network delivers the frame to the destination, the receiver uses the type field to identify data portion of the frame as an IP datagram and forwards the datagram to the software that processes them.

The IP service is called connectionless because each packet is processed independently from all

others. IP datagrams contain all the information necessary for intermediate routers to process the packets and forward them accordingly.

The IP service which is used by all other protocols of the TCP/IP protocol suite provides network level services like:

Host addressing

Routing

Packet fragmentation and reassembly (if necessary)

All other protocols use IP services

TCP and UDP Characteristics

The Transport Layer identifies which processes (programs) are active on each host and provides either connection-oriented or connection-less services to these processes.

Connection-oriented services ensure a reliable transmission of data. TCP (Transmission Control Protocol) provides such reliable services to upper layer protocols like FTP or HTML.

Connection-less services provide faster, less overhead transmissions but offer no reliability. UDP (User Datagram Protocol) is used to provide connection-less services to upper layer protocols like NFS or TIME.

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The Transport layer uses ports to identify upper-layer processes or programs. Port addresses are used to distinguish between the different programs running within a system.

The combination of an end systems IP address and transport layer port is called socket and uniquely identifies a process running on a specific host. A socket pair includes each end systems IP address and port address and identifies a logical communication channel between the systems (processes).

Client- and server-based addresses are used (with TCP and UDP) to identify processes running on a host. Server ports have a range of 1 to 1023. Industry wide recognized port addresses are within the range of 1 through 255. Client port addresses can be anywhere between 1024 to 65536.

Addressing in IP

Binary-to-Decimal Conversion

To understand the derivation of network addresses it is important to get a basic understanding of decimal and binary numbering. The example below shows the "translation" of the binary number 10101101 (1octet) into its decimal representation

The decimal number system consists of the 10 unique digits of 0 to 9. Decimal numbering uses therefore powers of 10. This number system is also referred to as the base-10 system.

The binary number system consists only of two unique numbers 0 and 1. Unlike decimal numbering, the binary numbering systems uses power of 2 rather than power of 10. This number system is also referred to as base-2 system.

A byte or octet is composed of 8-bit positions with possible values ranging from 0 (all bits are 0) to 255 (all bits are 1).

Internet Addresses (1)

An Internet host address is a 32 bit number that identifies both the network on which a host is located and the host on that network. Network addresses (Internet addresses) are assigned by a central agency, while host numbers are assigned individually by the local network administrator.

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The most significant bits of the network portion of the Internet address determine the class of an address. There are three classes defined:

A, with high-order bit "0", 8 bits network portion

B, with high-order bits "10", 16 bits network portion

C, with high-order bits "110", 24 bits network portion

Each class has fewer bits for the host part of each Internet address and therefore supports fewer hosts than the higher classes.

Addressing in IP

Internet Addresses (2)

The example below shows the Internet Address 130.44.79.34 converted into binary format

The numeric representation of an Internet address is as follows: Each 8 bit field of the address is denoted by a decimal number, separated from the other fields with a period.

Reserved Internet Addresses

Class D addresses have the first four bits set to "1110" and are reserved for use as multicast addresses and are not for use by individual hosts.

Class E addresses have the first five bits set to "11110" and have been reserved for future use.

255.255.255.255 is the decimal representation of an IP address with all binary digits set to 1. It identifies a message sent to all nodes on all networks and is therefore used for broadcast purposes.

The address 0.0.0.0 is the decimal representation of an IP address with all binary digits set to 0. This number typically represents an unknown network/host.

The address 127.0.0.1 is a special address (Class A) used for internal loop-back testing. It designates the the local node and does not generate any traffic on the network.

Private addresses defined in RFC 1918 may be used internally by private networks.

The reserved address ranges are:

10.0.0.0 - 10.255.255.255 (Class A)

172.16.0.0 - 172.31.255.255 (Class B)

192.168.0.0 - 192.168.255.255 (Class C)

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These Addresses are not routable through the Internet. These addresses are used to overcome addressing issues in the current Internet (IP V4 ) address space and give companies more flexibility by providing larger usable address ranges. To communicate with the Internet subnets using RFC 1918 addresses need to be connected using some form of address translation with registered Internet addresses like NAT (Network Address Translation) or PAT (Port Address Translation).

Subnetting

In 1985, RFC 950 defined a standard procedure to support the subnetting, or division, of a single Class A, B, or C network number into smaller pieces. Subnetting was introduced to overcome some of the problems that parts of the Internet were beginning to experience with the classful two-level addressing hierarchy:

growing internet routing tables

local administrators had to request another network from the Internet before a new network could be installed at their site.

Subnetting divides the addressing hierarchy into three levels. Adding another level makes it unnecessary to have a knowledge of the internal subnet structure outside of the organization. Since the subnets for a given network number all use the same network prefix, the route in from outside to any subnet is the same. This means that for one entry in the global routing tables, there can exist many individual sub-networks.

The network prefix is effectively extended - the most significant bits after the network number and the next most significant bits to the subnet.

Subnet Mask (Examples)

The subnet mask is used to define the host part of the IP address. The bits in the mask are set to 1 for the digits that are to be a part of the extended network prefix and are set to 0 for the digits that are part of the host number.

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IP Routing

RIP

RIP v.1

Distance Vector

15 Hops or less

Best for Star Topologies

Cannot do load balancing

RIP v.2

Extensions to carry subnet mask and next hop information

Routing Information Protocol (RIP) is described fully in RFC 1058. Extensions for RIP version 2 are described in RFC 1723. Extensions for RIP on demand is described in RFC 1582. RIP is a fairly simple distance vector protocol which defines networks based on how many hops they are from the router. Once a network is more than 15 hops away (one hop is one link) it is not included in the routing table. The possible routes (there may be more than one) to a particular host are selected on the basis of the shortest one. If two routes have the same metric (hop count) or cost, the first one found will be chosen. RIP does not cope very well with a meshed (multiply connected) network. It suits star topologies very well.

Each router configured for RIP maintains a relatively simple route table as described earlier. The router will periodically broadcast its routing information to other routers. Similarly it will need to obtain this information from neighbouring routers to improve its own picture of the network. Routes are removed from the table if they are not kept up to date (refreshed) by the neighbouring routers. The RIP version 2 extensions allow the RIP updates to contain subnet masks and next hop information. The ability to carry subnet masks allows the use of different sized subnet masks on different subnets within the same network.

OSPF (1)

The Open Shortest Path First (OSPF) protocol is a relatively recent standard which is documented in RFC 1247. It has a number of significant benefits over older distance vector based protocols like RIP, including:

OSPF is an open, published specification. It is not proprietary to any manufacturer. OSPF supports the concept of areas to allow networks to be administratively partitioned as they grow in size. Load balancing, in which multiple routes exist to a destination is also supported. OSPF distributes traffic over these links.

OSPF routes IP packets based solely on the destination IP address and IP Type of Service found in the IP packet header. OSPF is a dynamic routing protocol. It quickly detects topological changes in the network and calculates new loop-free routes after a period of convergence. This period of convergence is short and involves a minimum of routing traffic.

OSPF supports the concept of areas to allow networks to be administratively partitioned as they grow in size.

OSPF (2)

In an OSPF-based routing protocol, each router maintains a database describing the Autonomous System's topology. Each participating router has an identical database. Each individual piece of this database is a particular router's local state (e.g., the router's usable interfaces and reachable neighbours). The router distributes its local state throughout the Autonomous System by flooding. All routers run the exact same algorithm, in parallel. From the topological database, each router constructs a tree of

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shortest paths with itself as root. This shortest-path tree gives the route to each destination in the Autonomous System. Externally derived routing information appears on the tree as leaves.

OSPF calculates separate routes for each Type of Service (TOS). When several equal-cost routes to a destination exist, traffic is distributed equally among them. The cost of a route is described by a single dimensionless metric. OSPF allows sets of networks to be grouped together. Such a grouping is called an area. The topology of an area is hidden from the rest of the Autonomous System. This information hiding enables a significant reduction in routing traffic. Also, routing within the area is determined only by the area's own topology, lending the area protection from bad routing data. An area is a generalization of an IP subnetted network.

Address Resolution

ARP (Address Res. Protocol)

Whenever the IP process running on a source node is attempting to send an IP datagram, it examines whether the destination internet address is on its own physical network. If the IP datagram is destined for a host on its own local network the IP process delivers the IP datagram directly. If the IP datagram is destined for a host on some other network it sends it to a router on the local network.

To make this direct delivery possible, each node maintains an ARP (Address Resolution Protocol) cache (or table) containing the mappings of internet addresses to physical (hardware) addresses.

To add an entry to the ARP table for a destination host that has not been contacted for some time, ARP multicasts an ARP Request packet containing the destination nodes internet address. The destination node (or router) replies with an ARP Response packet containing its physical (hardware) address.

RARP (Reverse ARP) allows a host that only knows its physical (hardware) address to obtain the internet address that it should use in communicating with other systems.

Dynamic Host Configuration

DHCP

IP networks require each node in the network to be provided with:

IP address

Subnet mask

DNS address

Domain name

Gateway

DHCP (Dynamic Host Configuration Protocol) enables network servers to assign a range of IP addresses automatically to client stations logging into a TCP/IP network eliminating the need to manually assign permanent IP addresses to each node. It is also a

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means to provide other necessary IP setup information automatically.

Whenever a computer supporting DHCP is switched on, it sends out a DHCP request to obtain TCP/IP setup information.

Domain Name System

Domain Names / Host Names

Examples for top-level domains:

.com Commercial organizations

.edu Educational organizations

.gov US Government and government agencies

.net Network providers (like ISPs, etc.)

.org Misc. organizations

.mil US military organizations

.int International organisations such as UNO, NATO, etc.

Countries are assigned domains that start with their ISO country code: .de Germany .ch Switzerland .

Before 1984 when there were only a few hundred machines connected to the ARPANET. A simple file called "hosts.txt" was maintained to provide name to address information. This file was then copied to the individual hosts.

In the mid 1980s it became clear that this method would soon be unworkable. The Internet was growing at a very fast rate and new system were connected every day.

The names used with the Domain Name System (DNS) are constructed hierarchically, so that responsibility for portions of the namespace can be assigned to different organisations. These parts of the namespace are called "Domains". The domain names can be read from right to left, with each portion of the domain being more specific.

The top-level domains (.com, .edu, .net, .int, etc.) are administered by the Internic (Internet Network Information Centre). National organizations in each country manages name assignment for the respective domains (.fr, .de, .at, etc.).

Domain Name System

DNS (Domain Name System)

The Domain Name System (DNS) is the distributed Internet service that provides translation from hostnames to the numeric addresses used to uniquely identify a host in the Internet.

To perform a name to address translation two elements/functions are involved. One element is part of the operating system requesting the translation and is called the "resolver". In order to perform the translation the resolver has to interact with name servers. Name servers store and distribute the information about what address corresponds with which name.

When the resolver needs an IP address, it sends a query to the name server. The name server may have the answer, and if so, it returns the information to the

resolver. If the server does not know the answer, it asks a neighbouring name server.

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TCP/IP Tools and Applications

Overview

The Transport layer uses ports to identify upper-layer processes or programs. Port addresses are used to distinguish between the different programs / applications running within a system.

Well known ports reserved for use with specific applications / protocolls are for example:

TCP Port 80, HTTP (WWW Server)

TCP Port 23, TELNET

TCP Port 25, SMTP

TCP Port 110, POP3

TCP Port 20, FTP Data

TCP Port 21, FTP Command

UDP Port 69, TFTP (Trivial File Transfer Protocol)

UDP Port 123, NTP (Network Time Protocol)

UDP Port 53, DNS Request

TCP Port 53, DNS Table Exchanges

The combination of an end systems IP address and transport layer port is called socket and uniquely identifies a process (application) running on a specific host. A socket pair includes each end systems IP address and port address and identifies a logical communication channel between the systems processes (applications).

Client- and server-based addresses (16 bit code, from 0 to 65535) are used (with TCP and UDP) to identify processes (applications) running on a host. The server ports have a range of 1 to 1023 and are assigned by the IANA (Internet Assigned Numbers Authority) and reserved for the specific server application. Industry wide recognized (well-known) port addresses are within the range of 1 through 255. Client port addresses can be anywhere between 1024 to 65536.

HTTP

HTTP (Hypertext Transfer Protocol) is the basis for a very popular Internet application - the World Wide Web (WWW). It contains the set of rules for transferring files (text, graphic images, sound, and other multimedia data) fromn a Web server. As soon as a Web user opens their Web browser, the user is indirectly making use of HTTP.

HTTP concepts include (as the Hypertext part of the name implies) the idea, that files contain references to other files whose selection will lead to automatically access those files. Any Web server contains, in addition to the Web page files it can serve, an HTTP server program, that is designed to listen for HTTP requests and respond to them as soon as they arrive. A Web browser is basically a HTTP client, sending requests to server machines.

As soon as the browser user enters file requests by either typing in a Uniform Resource Locator (URL) or by clicking on a hypertext link, the browser sends an HTTP request to the IP address indicated by the URL. The HTTP server process receives the request and sends back the requested files associated with the request.

The HTML session uses the TCP transport layer protocol for connecting the client and server processes. The standard well-known port that clients connect to at the WWW server side is port 80.

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TELNET

TELNET is a simple text-based remote terminal protocol that allows an user to log in on a remote host. Using a telnet session to another computer is like using a lokal terminal of that system. TELNET is typically used with Unix-oriented systems and to access many networking devices for management and configuration purposes.

TELNET is based on a client/server principle in which one host (the telnet client) negotiates opening a session on another computer (the remote host, running the TELNET server ). During the negotiation process, the two computers agree on the parameters relating the session including the terminal type (virtual terminal) to be used. In this context virtual terminal refers to a set of terminal characteristics and functionalities that both sides of a

TELNET connection agree to use to transmit data across the network.

The TELNET session uses the TCP transport layer protocol for connecting the client and server processes on the system. The standard well-known port for TELNET terminal access is port 23 on the server side.

FTP

The File Transfer Protocol (FTP) allows the user to transfer data in both directions between the local host (FTP client) and a remote host (FTP sever). FTP is a TCP based service. FTP is an unusual service in that it utilizes two ports, a 'data' port and a 'command' port (also known as the control port). Traditionally these are port 21 for the command port and port 20 for the data port although depending on the FTP mode, the data port may be on an other port than 20.

In active mode FTP the client connects from a random unprivileged port (N > 1024) to the FTP server's command port, port 21. Then, the client starts listening to the next higher port (N+1) and sends the FTP command PORT N+1 to the server. The server will then connect back to the client's specified data port from its

local data port, which is port 20.

In order to resolve the issue of the server initiating the connection to the client an other method for FTP connections - called "passsive mode" was introduced. In passive mode FTP the client initiates both connections to the server. This solvies the problem of some firewalls filtering the incoming data port connection from the server. When opening an FTP connection, the client opens two random unprivileged ports locally. As in the example with active mode before the first port contacts the server on port 21, but instead of then allowing the server to connect back to its data port, the client will send the passive mode instruction. Because of this is, the server then opens a random unprivileged port (P) and sends the PORT P command back to the client. The client then initiates the connection from port N+1 to port P on the server to transfer data.

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Email

Electronic mail is one of the most commonly used networking applications resulting in a number of different protocols that have beed developed over time to transfer emails across TCP/IP networks (and the Internet).

The Simple Mail Transfer Protocol (SMTP) is the classic Internet standard for transfering emails between computers. SMTP deals with the exchanges that occur between a process with mail to be sent (SMTP client) and a SNMP process that receives mail (SMTP server).

Other standards define extensions to SNMP that enable it to transport any type of information. Multipart messages are described in Multipurpose Internet Mail Extensions (MIME) standards that allow the transfer of word processor documents, binary files or multimedia data.

The Post Office Protocol (POP) enables a desktop mail client to retrieve mail from a mail server. An alternative technology is the Internet Message Access Protocol (IMAP), that enables a user to work with his emails actually stored at a server.

PING

Packet Internet Groper (PING) is a protocol that uses ICMP as a transport mechanism. It is used to send a message to a host and wait for that