BG Chapter 01

Communication Networks

Prepared by

Bore Gowda S B Associate Professor

MIT, Manipal

COMMUNICATIONS

Communication? o Sharing information - sharing can be local

or remote.

Telecommunication: o Used for distance communication.

o Includes telephony, telegraph, and

television.

DATA COMMUNICATIONS

Data communications ?

o The exchange of data between two devices via

some form of transmission media.

- media wired or wireless

Data ?

Data is represented as bit pattern

o Text Ex: ASCII o Numbers

o Images color image represented as RGB/YCM o Audio continuous, not discrete o Video continuous

Data Communication System

Communicating devices made up of combination of hardware (physical equipment) and software (programs)

Effectiveness of Data Communication System

Depends on following fundamental characteristics: o Delivery: data to the correct destination

o Accuracy: Errorless delivery

o Timeliness: Delivery of data in time

o Jitter: Variation in the packet arrival time

Data Flow Communication between two devices can be:

o Simplex Unidirectional

o Half-duplex -- Bidirectional, But not at the same time

o Full-duplex Bidirectional (simultaneously)

NETWORKS

o A network is the interconnection of a set of devices capable of

communication.

o A device can be a host such as:

a large computer Desktop Laptop Workstation cellular phone, or security system.

o A device can also be a connecting device such as:

router, switch, modem etc. o Devices in a network can be connected using wired or wireless

transmission media.

o How do we create a network?

NETWORKS

Network Criteria

o A network must be able to meet a certain

number of criteria.

o The most important of these are:

Performance: Metrics: Transit time and response time.

Throughput and delay

Also depends on number of users, type of transmission

media, capability of the connected device and efficiency

of software.

Reliability: Measured by frequency of failure

Security: Protecting data from unauthorized access

Types of Connection

Devices are connected by using links 1. Point to Point connection Dedicated link cable / microwave / satellite Simple Ex: Establishing connection between remote control and TV

2. Multipoint connection Capacity is shared Either spatially / temporarily

Physical Topology

The way in which a network is laid out Physically Two or more devices connect to link Two or more links form topology Topology of the network is the geometric

representation of the relationship of the links and Linking devices to one another

Basic Topologies: MESH STAR BUS RING

Mesh Topology

Figure : A fully-connected mesh topology

Advantages

Dedicated links guarantees that each connection can carry its own data load eliminates the traffic problems that can occur when links are shared by the multiple deices

Robust, failure of single link does not degrade the network operation.

Privacy or security. Physical boundaries prevent other users from gaining Access to message.

P2P links make fault identification and fault isolation Easy.

Disadvantages

The amount of cabling and the number of i/o ports.

Sheer bulk of the wiring can be greater than the available space can accommodate.

The hardware required to connect each link can be prohibitively expensive.

Example : Connection of telephone regional offices, in which each office need to connected to every other regional office.

Star Topology

Figure : A star topology

Star Topology

Dedicated P2P link. The devices are not directly linked to one

another. No direct traffic between the devices. Hub/controller acts as an exchange. A star topology is less expensive than mesh. Each device needs only one link & I/O port to

connect it to any number of others. Example : LANs

Advantages

Robustness. Easy fault identification and fault isolation.

Disadvantages

The dependence of the whole topology on single Point, the hub. Although It requires far less cable than mesh, each Node must be linked to be a central Hub. For this reason, It requires more cabling than other Topologies(ring & bus)

Bus Topology

Figure : A bus topology

Multipoint.

Backbone

Bus Topology

Advantages

Ease of Installation

Bus uses less cabling than mesh or star topologies

Disadvantages Difficult reconnection and fault isolation Signal reflections at the taps can cause

degradation In quality

Bus Topology

Fault or break in the bus cable stops all transmission, even between devices on the same side of the Problem

The damaged area reflects signal back in the direction of in the origin, creating noise in both directions

Ex: Ethernet LANs

Ring Topology

Figure 1.7: A ring topology

Ring Topology

Each device has a dedicated P2P connection with only the two devices on either side of it.

A signal is passed along the ring in one direction,

from device to device, until it reaches its destination.

Advantages Easy to install and reconfigure. Add or delete a device requires changing only two

connections

Fault isolation is simplified

In a ring a signal is circulating at all times. If one device does not receive a signal with in a Specified period, It can issue alarm .

The alarm alerts the network operator to the problem and its location.

Ring Topology

Ring Topology

Disadvantages

Unidirectional Traffic In a simple ring, a break in the ring can disable the Entire network

Hybrid Topology It is the combination of two or more topologies Draw the hybrid topology with star backbone and three bus networks Draw the hybrid topology with star backbone and four ring networks

Topology along with number of links

Topology Number of links

Mesh

Ring

Bus

Star

n(n-1)/2

n

(n drop lines + one line for bus)

n

Network Types

The criteria of distinguishing one type of network from another is difficult and sometimes confusing. Following criteria's are used to classify the network types such as: Size Geographical coverage, and Ownership

Classification of interconnected processors by scale.

Network Types

Local Area Network A local area network (LAN) is usually

privately owned and connects some hosts in a

single office, building, or campus.

Each host in a LAN has an identifier, an

address, that uniquely defines the host in the

LAN.

A packet sent by a host to another host carries

both the source hosts and the destination

hosts addresses.

Local Area Network

LAN connected with common cable Connected through a common cable. Packet sent from one host to another was received by all hosts. The intended recipient kept the packet, others dropped.

LAN connected with a switch LANs use a smart connecting switch Able to recognize the destination address of the packet Guide the packet to its destination without sending it to all other hosts

Local Area Network

The switch alleviates the traffic in the LAN and allow more than one pair to communicate with each other at the same time if there is no common source and destination among them Traditional LANs run at speed of 10Mbps to 100Mbps, low delay(micro or nanoseconds) Newer version LANs operates up to 10Gbps IEEE 802.3 ETHERNET(Bus topology) 10Mbps-10Gbps IEEE 802.5 (IBM Token ring) 4 & 16Mbps

Local Area Network

Metropolitan Area Networks

A metropolitan area network based on cable TV.

Cable TV High speed wireless Internet access Standardized as IEEE802.16

Wide Area Networks A wide area network (WAN) is also an connection of

devices capable of communication.

Differences between a LAN and a WAN.

A LAN is limited in size; a WAN has a wider geographical span, spanning a town, a state, a

country, or even the world.

A LAN interconnects hosts; a WAN interconnects connecting devices such as switches, routers, or

modems.

A LAN is owned by the organization that uses it; a WAN is created and run by communication

companies and leased by an organization that uses it.

Wide Area Networks Examples of WANs Point-to-Point WAN

Switched WAN

Internetworks

When two or more networks are connected

Figure : An internetwork made of two LANs and one WAN

Internetworks

Figure : A heterogeneous network

made of WANs and LANs

Uses of Computer Networks

Business Applications

Home Applications

Mobile Users

Social Issues

Network Software A protocol defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to communicate effectively.

A protocol architecture is the layered structure of hardware and software that supports the exchange of data between systems and supports distributed applications, such as electronic mail and file transfer

The peer layers communicate by means of formatted blocks of data that obey a set of rules or conventions known as a protocol

The key features of a protocol are as follows: Syntax : Concerns the format of the data blocks Semantics : Includes control information for

coordination and error handling Timing : Includes speed matching and sequencing

When communication is simple, we may need only one simple protocol When the communication is complex, we need a protocol at each layer, or protocol layering

Network Software

Protocol Hierarchies Let us develop two simple scenarios to better

understand the need for protocol layering.

In the first scenario, communication is so simple that it can occur in only one layer.

Figure : A single-layer protocol

Network Software

Figure : A three-layer protocol

In the second, the communication between Maria

and Ann takes place in three layers.

Network Software

First principles of protocol layering.

Dictates that if we want bidirectional communication, we need to make each layer so

that it is able to perform two opposite tasks, one

in each direction.

Second principles of protocol layering

In protocol layering the two objects under each layer at both sites should be identical.

A set of layers and protocols is called a network

architecture

Principles of Protocol Layering

Network Software A list of the protocols used by a certain system, one

protocol per layer, is called a protocol stack

At each layer of a protocol architecture, one or more

common protocols are implemented in

communicating systems.

Each protocol provides a set of rules for the exchange

of data between systems

Figure: Layers, protocols, and interfaces

Protocol Hierarchies

The entities comprising the corresponding layers on different machines are called peers. The peers that communicate by using the protocol to talk to each other In reality, no data are directly transferred from layer n on one machine to layer n on another machine Instead, each layer passes data and control information to the layer immediately below it, until the lowest layer is reached.


Below layer 1 is the physical medium through which actual communication occurs. In Fig. virtual communication is shown by dotted lines and physical communication by solid lines. Between each pair of adjacent layers is an interface. The interface defines which primitive operations and services the lower layer makes available to the upper one Each layer perform a specific collection of well-understood functions



Example information flow supporting virtual communication in layer 5.

A message, M, is produced by an application process running in layer 5 and given to layer 4 for transmission. Layer 4 puts a header in front of the message to identify the message and passes the result to layer 3. The header includes control information, such as addresses, to allow layer 4 on the destination machine to deliver the message.


Layer 3 must break up the incoming messages into smaller units, packets, prepending a layer 3 header to each packet. In this example, M is split into two parts, M1 and M2, that will be transmitted separately. Layer 3 decides which of the outgoing lines to use and passes the packets to layer 2.


Layer 2 adds to each piece not only a header but also a trailer, and gives the resulting unit to layer 1 for physical transmission. At the receiving machine the message moves upward, from layer to layer, with headers being stripped off as it progresses. None of the headers for layers below n are passed up to layer n.


Design Issues for the Layers Addressing There are many computers on the network, every

layer needs a mechanism for identifying the senders and receivers that are involved in a particular message.

Error Control It is most important because physical

communication circuits are not perfect Finding errors in received information uses codes

for error detection and error correction. Information that is incorrectly received can be

retransmitted until it is received correctly.

Flow Control How to keep a fast sender from swamping a slow

receiver with data. Feedback from the receiver to the sender is often

used. This subject is called flow control.

Sometimes the problem is that the network is

oversubscribed because too many computers want to send too much traffic, and the network cannot deliver it all.

This overloading of the network is called congestion.

Design Issues for the Layers

Multiplexing

Many designs share network bandwidth dynamically, according to the short term needs of hosts, rather than by giving each host a fixed fraction of the bandwidth that it may or may not use.

This design is called statistical multiplexing Sharing based on the statistics of demand. It can be applied at low layers for a single link,

or at high layers for a network or even applications that use the network.


Routing

Finding a working path through a network. There are multiple paths between a source

and destination, and in a large network, there may be some links or routers that are broken.

Protocol layering Networks grow larger and new designs

emerge that need to be connected to the existing network.

Structuring mechanism used to support change by dividing the overall problem and hiding implementation details


Scalable

Designs that continue to work well when the network gets large

Confidentiality

To secure the network by defending it against different kinds of threats

Authentication

Prevent someone from impersonating someone else

Used to tell fake banking Web sites from the real one


Advantages of layering in a network

It resolve complicated tasks by breaking it into smaller and manageable pieces Each layer can be analyzed and tested

independently Functionalities are carried out in logical

sequential manner Implementation of the layer can be changed

without disturbing other layer, as the details of all the layers are hidden from each other Layering allows reuse in a way that once a

common functionality is implemented in a lower layer, the upper layer can share it

OSI Reference model Established in 1947, the International Standards Organization (ISO) It is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s. An open system is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture. The purpose of the OSI model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software. The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable. The OSI model was intended to be the basis for the creation of the protocols in the OSI stack.

OSI Reference model

OSI Reference model The seven layers can be thought of as belonging to three subgroups. Layers 1, 2, and 3physical, data link, and networkare the network support layers; they deal with the physical aspects of moving data from one

device to another (such as electrical specifications, physical connections, physical addressing, and transport timing and reliability).

Layers 5, 6, and 7session, presentation, and applicationcan be thought of as the user support layers; they allow interoperability among unrelated software systems.

Layer 4, the transport layer links the two subgroups and ensures that what the lower

layers have transmitted is in a form that the upper layers can use.

OSI Reference model D7 data means the data unit at layer 7, D6 data means the data unit at layer 6, and so on. The process starts at layer 7 (the application layer), then moves from layer to layer in descending, sequential order. At each layer, a header can be added to the data unit. At layer 2, a trailer may also be added. When the formatted data unit passes through the physical layer (layer 1), it is changed into an electromagnetic signal and transported along a physical link.

OSI Reference model Upon reaching its destination, the signal passes into layer 1 and is transformed back into digital form. The data units then move back up through the OSI layers. As each block of data reaches the next higher layer, the headers and trailers attached to it at the corresponding sending layer are removed, and actions appropriate to that layer are taken. By the time it reaches layer 7, the message is again in a form appropriate to the application and is made available to the recipient.

OSI Reference model

Encapsulation Figure reveals another aspect of data communications in the OSI model: encapsulation. A packet at level 7 is encapsulated in the packet at level 6. The whole packet at level 6 is encapsulated in a packet at level 5, and so on. In other words, the data part of a packet at level N is carrying the whole packet (data and overhead) from level N+ 1. The concept is called encapsulation because level N is not aware what part of the encapsulated packet is data and what part is the header or trailer. For level N, the whole packet coming from level N + 1 is treated as one integral unit.

OSI Reference model Physical Layer

The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission media. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur. The physical layer is also concerned with the following:

Physical characteristics of interfaces and media. Representation of bits Data rate Synchronization of bits Line configuration Physical topology Transmission mode

OSI Reference model Data Link Layer

The data link layer transforms the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer (network layer). Other responsibilities of the data link layer include the following: Framing. The data link layer divides the stream of bits received from the

network layer into manageable data units called frames. Physical addressing. If frames are to be distributed to different systems on

the network, the data link layer adds a header to the frame to define the sender and/or receiver of the frame.

Flow control. If the rate at which the data is absorbed by the receiver is less than the rate produced at the sender, the data link layer imposes a flow control mechanism to prevent overwhelming the receiver.

Error control. The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. It also uses a mechanism to recognize duplicate frames.

Access control. When two or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time.

OSI Reference model Network Layer

The network layer is responsible for the source-to-destination delivery of a packet, possibly across multiple networks (links). The network layer ensures that each packet gets from its point of origin to its final destination. If two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached to different networks (links) with connecting devices between the networks (links), there is often a need for the network layer to accomplish source-to-destination delivery.

OSI Reference model Other responsibilities of the network layer include the following: Logical addressing.

o If a packet passes the network boundary, we need another addressing system to help distinguish the source and destination systems.

o The network layer adds a header to the packet coming from the upper layer that, among other things, includes the logical addresses of the sender and receiver.

Routing. o When independent networks or links are connected

together to create internetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination.

OSI Reference model Transport Layer

The transport layer is responsible for process-to-process delivery of the entire message. A process is an application program running on the host. Whereas the network layer oversees source-to-destination delivery of individual packets. It treats each one independently, as though each piece belonged to a separate message. The transport layer, on the other hand, ensures that the whole message arrives intact and in order, overseeing both error control and flow control at the source-to-destination level. Other responsibilities of the transport layer include the following: Service-point addressing: specific process (running program) on one

computer to a specific process (running program) on the other

Segmentation and reassembly Connection control Flow control Error control

OSI Reference model Session Layer

The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction between communicating systems. Specific responsibilities of the session layer include the following: Dialog control: The session layer allows two systems to enter

into a dialog. It allows the communication between two processes to take place in either half-duplex or full-duplex mode.

Synchronization: The session layer allows a process to add checkpoints (synchronization points) into a stream of data. o For example, if a system is sending a file of 2,000 pages, it

is advisable to insert checkpoints after every 100 pages to ensure that each 100-page unit is received and acknowledged independently.

OSI Reference model Presentation Layer

It is concerned with the syntax and semantics of the information exchanged between two systems. Specific responsibilities of the presentation layer include the following:

Translation o The presentation layer at the sender changes the information

from its sender-dependent format into a common format. o The presentation layer at the receiving machine changes the

common format into its receiver-dependent format.

Encryption o To carry sensitive information a system must be able to assure

privacy.

Compression o Data compression reduces the number of bits contained in the

information. Data compression becomes particularly important in the transmission of multimedia such as text, audio, and video.

OSI Reference model Application Layer

The application layer enables the user, whether human or software, to access the network. It provides user interfaces and support for services such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services. Specific services provided by the application layer include the following: Network virtual terminal File transfer, access, and management (FTAM) E-mail services Directory services

TCP/IP Reference Model

The TCP/IP protocol suite was developed prior to the OSI model. The layers in the TCP/IP protocol suite do not match exactly with those in the OSI model. The original TCP/IP protocol suite was defined as four software layers built upon the hardware. Today, however, TCP/IP is thought of as a five-layer model with the layers named similarly to the ones in the OSI model


HTTP (Hyper Text Transfer Protocol), SMTP (Simple Mail Transfer Protocol) RTP(Real-time Transport Protocol), DNS(Domain Name System) TCP (Transmission Control Protocol), UDP (User Datagram Protocol) IP (Internet Protocol), ICMP (Internet Control Message Protocol) DSL (Digital Subscriber Line), SONET (Synchronous Optical NETwork),

Similarity between OSI and TCP/IP

Both are based on the concept of a stack of independent protocols.

The functionality of the layers is roughly similar.

Differences between OSI and TCP/IP

1. OSI makes the distinction between services, interfaces, and protocol.

2. The OSI model was devised before the protocols were invented. It can be made to work in diverse heterogeneous networks.

1. TCP/IP does not originally clearly distinguish between services, interface, and protocol.

2. TCP/IP model was just a description of the existing protocols. The model and the protocol fit perfectly.

3. The OSI model supports both connectionless and connection-oriented communication in the network layer, but only connection-oriented communication in the transport layer.

4. OSI has seven layers

5. Host on OSI implementations do not handle network operations.

3. The TCP/IP model has only one mode in the network layer (connectionless) but supports both modes in the transport layer, giving the user choice.

4. TCP/IP has four layers

5. TCP/IP hosts participate in most network protocols.


6. OSI emphasis on providing a reliable data transfer service, Each layer of the OSI model detects and handles errors, all data transmitted includes checksums. The transport layer checks source-destination reliability.

6. TCP/IP treats reliability as an end to end Problem. The transport layer handles all error detection and recovery, it was checksums, acknowledgments, and timeouts to control transmissions and provides end-to-end verification.


Switching

An internet is a switched network in which a switch connects at least two links together. A switch needs to forward data from a network to another network when required. Common types of switching are: Circuit-Switching Packet-Switching Message-Switching

Circuit switching In circuit switching, a connection is established between two

network nodes before they begin transmitting data.

Bandwidth is dedicated to this connection and remains available

until the users terminate communication between the two nodes.

While the nodes remain connected, all data follows the same path

initially selected by the switch.

This works much like a normal telephone line works for voice

communication.

Circuit switching Circuit establishment In this phase, the circuit must be established, when signals are transmitted from one network node to other node. This can send information through signals before that the receiving end and sending end must establish their circuit. Data transfer In this data transfer phase, information can be transmitted from one network node to other network node through network. The data may be analog signals or digital signals that depend on the nature of the network protocol. Circuit disconnect After some time of data transfer, the connection is terminated, generally by the action of one of the two network nodes. Signals must be propagated to given nodes to deallocate the dedicated resources

Advantages of Circuit Switching Guaranteed bandwidth Predictable communication performance Not best-effort delivery with no real guarantees

Simple abstraction Reliable communication channel between hosts No worries about lost or out-of-order packets

Simple forwarding Forwarding based on time slot or frequency No need to inspect a packet header

Low per-packet overhead Forwarding based on time slot or frequency No IP (and TCP/UDP) header on each packet

Disadvantages of Circuit Switching

Wasted bandwidth Bursty traffic leads to idle connection during silent period Unable to achieve gains from statistical multiplexing

Blocked connections Connection refused when resources are not sufficient Unable to offer okay service to everybody

Connection set-up delay No communication until the connection is set up Unable to avoid extra latency for small data transfers

Network state Network nodes must store per-connection information Unable to avoid per-connection storage and state

Packet Switched Network

In packet-based networks, the message gets broken into small data packets.

These packets are sent out from the computer and they travel around the network seeking out the most efficient route to travel as circuits become available.

This does not necessarily mean that they seek out the shortest route.

Each packet may go a different route from the others.

Packet Switching

Each packet is sent with a header address which tells it where its final destination is, so it knows where to go.

The header address also describes the sequence for reassembly at the destination computer so that the packets are put back into the correct order.

One packet also contains details of how many packets should be arriving so that the recipient computer knows if one packet has failed to turn up.

If a packet fails to arrive, the recipient computer sends a message back to the computer which originally sent the data, asking for the missing packet to be resent.

Packet Switching

Advantages Security

Bandwidth used to full potential

Devices of different speeds can communicate

Not affected by line failure (redirects signal)

Availability no waiting for a direct connection to become available

During a crisis or disaster, when the public telephone network might stop working, e-mails and texts can still be sent via packet switching

Packet Switching

Disadvantages

Under heavy use there can be a delay

Data packets can get lost or become corrupted

Protocols are needed for a reliable transfer

Not so good for some types data streams (e.g. real-

time video streams can lose frames due to the way

packets arrive out of sequence)

Packet Switching

Two approaches are used in packet switching: datagram and virtual circuit

Datagram approach In the datagram approach, each packet is treated independently, with no reference to packets that have gone before. Each node chooses the next node on a packets path, taking into account information received from neighboring nodes on traffic, line failures, and so on. All the packets do not follow the same route, and they may arrive out of sequence at the exit point. The exit node/destination do the reordering. It is up to either the exit node or the destination to detect the loss of a packet and decide how to recover it.

Packet switching : Datagram approach

Packet Switching: Virtual circuit approach

Pre-planned route is established before any packets are sent. All the packets between a pair of communicating parties follow this same route through the network. Route is fixed for the duration of the logical connection and is referred to as a virtual circuit. Each packet contains a virtual circuit identifier as well as data. Each node on the pre-established route knows where to direct such packets; no routing decisions are required. At any time, each station can have more than one virtual circuit to any other station and can have virtual circuits to more than one station.

Packet switching : Virtual circuit approach

Message Switching

With message switching there is no need to establish a dedicated path between two stations.

When a station sends a message, the destination address is appended to the message.

The message is then transmitted through the network, in its entirety, from node to node.

Each node receives the entire message, stores it in its entirety on disk, and then transmits the message to the next node.

This type of network is called a store-and-forward network.

A message-switching node is typically a general-purpose

computer. The device needs sufficient secondary-storage

capacity to store the incoming messages, which could be

long.

A time delay is introduced using this type of scheme due to

store- and-forward time, plus the time required to find the

next node in the transmission path.

Message Switching

Advantages: Channel efficiency can be greater compared to circuit-

switched systems, because more devices are sharing the

channel.

Traffic congestion can be reduced, because messages may be

temporarily stored in route.

Message priorities can be established due to store-and-

forward technique.

Message broadcasting can be achieved with the use of

broadcast address appended in the message.

Disadvantages : Message switching is not compatible with interactive

applications.

Store-and-forward devices are expensive, because they

must have large disks to hold potentially long messages.

Message Switching

Connection-oriented Service It is modeled based on telephone system

Requires a session connection be established before any data

can be sent

This method is often called a "reliable" network service.

It can guarantee that data will arrive in the same order.

Connection-oriented services set up virtual links between end

systems through a network

Examples of a connection-oriented networks

X.25

Frame Relay

ATM

Connectionless service is modeled after the postal system.

Used in packet switching networks by which each data unit is

individually addressed and routed based on information carried in

each unit

Connectionless protocols are usually described as stateless

protocols because the end points have no protocol-defined way to

remember where they are in a "conversation" of message exchanges.

Message can be sent from one end point to another without prior

arrangement

The device at one end of the communication transmits data addressed

to the other, without first ensuring that the recipient is available and

ready to receive the data.

Example: Internet Protocol (IP) and User Datagram Protocol (UDP)

are connectionless protocols

Connectionless Service

Asynchronous Transfer Mode(ATM) By the mid 1980s, three types of communication networks had evolved. Telephone network - carries voice calls Television network - carries video

transmissions Computer network - carries data. Telephone companies realized that voice communication was becoming a commodity service and that the profit margin would decrease over time. They realized that data communication was increasing. The telecommunication industry decided to expand its business by developing networks to carry traffic other than voice

Asynchronous Transfer Mode(ATM)

Goals

Universal Service

Support for all users

Single, unified infrastructure

Service guarantees

Support for low-cost Devices

ATM Architecture

ATM is a cell-switched network.

The user access devices, called the endpoints, are connected

through a user-to-network interface (UNI) to the switches inside

the network.

The switches are connected through network-to-network

interfaces (NNIs).


Connection between two endpoints is accomplished through transmission paths, virtual paths, and virtual circuits. Transmission path (TP): It is the physical connection (wire, cable, satellite, and so on) between an endpoint and a switch or between two switches. Virtual path (VP): provides a connection or a set of connections between two switches. Cell networks are based on virtual circuits (VCs). All cells belonging to a single message follow the same virtual circuit and remain in their original order until they reach their destination.


ATM Cells The basic data unit in an ATM network is called a cell. A cell is only 53 bytes long with 5 bytes allocated to the header and 48 bytes carrying the payload Header contains connection identifier, so the sending, receiving hosts and all the intermediate routers can tell which cell belongs to which connections This allows each router to know how to route each incoming cell Cell routing is done in hardware at high speed ATM uses two types of connections: PVC and SVC. PVC: A permanent virtual-circuit connection is established

between two endpoints by the network provider. SVC: In a switched virtual-circuit connection, each time an

endpoint wants to make a connection with another endpoint, a new virtual circuit must be established (on demand).

ATM Reference model

Physical Layer:

The lowest layer in the ATM protocol. It describes the

physical transmission media.

Examples: shielded and unshielded twisted pair, coaxial

cable, and fiber-optic cable.

ATM Layer:

It performs all functions relating to the routing and

multiplexing of cells over VCs.

It generates a header to the segment streams generated by

the AAL.

Similarly, on receipt of a cell streams, it removes the header

from the cell and pass the cell contents to the AAL

protocol.

ATM Reference model

ATM Reference model ATM Adaptation Layer:

Top layer in the ATM protocol Model.

It converts the submitted information into streams of 48-bytes segments and transports these in the payload field of multiple ATM cells.

Similarly, on receipt of the stream of cells relating to the same call, it converts the 48-byte information field into required form for delivery to the particular higher protocol layer.

Currently five service types have been defined ( i.e. AAL1-5)

AAL1 and AAL2 are connection oriented. AAL1 provides a constant bit rate (CBR) service, where as AAL2 provides a variable bit rate (VBR) service.

Initially, AAL 3 was defined to provide connection oriented and VBR service.

AAL : AAL3 was intended to support connection-oriented data services and AAL4 to support connectionless services

AAL 5 provides connectionless VBR service.

ATM Reference model AAL layer has 2 sublayers: Segmentation and Reassembly Layer (SAR):

Responsible for segmentation of payload at the sender side and reassembling the segments to create the original payload at the receiver side

Convergence Sublayer (CS) Responsible for ensuring the integrity of data and preparing it for segmentation by the SAR

Physical layer has 2 sublayers: Physical Medium Dependent(PMD)

It interfaces to the actual cable It moves the bits on and off and handles the bit timing

Transmission convergence(TC) When cells are transmitted, it sends them as a string of bits to PMD layer At the receiver it converts bit stream into cell stream for ATM layers

ATM Reference model

User Plane: Deals with data transport, flow control, error correction

Control plane: Performs call control and connection control functions

Management plane

Layer Management: Manages layer specific functions such as detection of failures and protocol problems Plane Management: Manages and coordinates functions related to the complete system

X.25 Network First Public data network was developed by ITU-Tin 1976 Allows devices to communicate over private digital links instead of leased lines It specifies an interface for exchange data packets between the end system(DTE)and access node(DCE-Data circuit Terminating Equipment) DTE is operated by user while DCE is service provider It is used for packet switching network It is a virtual circuit switching network requires prior establishment of virtual connection Each connection is assigned unique connection number included in the packet

Data packets consisting of 3-byte header followed by data up to 128 bytes The header contains 12-bit connection number, a packet sequence number, acknowledgement number etc.. It provides flow and error control at both data link & network layer It offers end to end virtual connection between sender and receiver Virtual path can be two types Switched Virtual circuit Permanent Virtual circuit

X.25 Network

Frame Relay It is virtual circuit WAN came into existence in late 1980 It is a connection oriented network with no error control and flow control Provides higher transmission speed of 44.376Mbps Allows a frame size up to 9000 bytes Operates in physical and data link layer only Limitations Frame length is not fixed, user may transmit

frames of different size Due to variable frame length, it is not suitable for

real time applications

Ethernet Ethernet is by far the most widely used local area networking (LAN) technology in the world today. Market surveys indicate that hundreds of millions of Ethernet network interface cards (NICs), repeater ports, and switching hub ports have been sold to date, and the market continues to grow. In 1970s, Norman Abramson and his colleagues at the University of Hawaii who were trying to connect users on remote islands to the main computer in Honolulu Each user terminal was equipped with a small radio having two frequencies: upstream (to the central computer) and downstream (from the central computer).

Ethernet Upstream channel is used send data to the central computer by user. Downstream channel is used send acknowledgement to user by the central computer If no one else was transmitting at that instant, the packet probably got through and was acknowledged on the downstream channel. If there was contention for the upstream channel, the terminal noticed the lack of acknowledgement and tried again. Since there was only one sender on the downstream channel (the central computer), there were never collisions there. This system, called ALOHANET. Worked fairly well under low traffic but bogged down badly when the upstream traffic was heavy.

Ethernet Ethernet was developed in 1976 at Xeroxs Palo Alto Research Center(PARC) They called the system Ethernet after the luminiferous ether, through which electromagnetic radiation was once thought to propagate Category of Ethernet: Standard Ethernet Fast Ethernet Gigabit Ethernet 10-Gigabit Ethernet Features: Txn medium is co-axial cable upto 2.5 km long with

repeaters at every 500 meters Upto 256 machines can be connected System runs at 2.94 Mbps

Ethernet

Ethernet major improvement over ALOHANET: Before transmitting, a computer first listened to the cable to see if someone else was already transmitting. If so, the computer held back until the current transmission finished. Doing so avoided interfering with existing transmissions, giving a much higher efficiency. ALOHANET did not work like this because it was impossible for a terminal on one island to sense the transmission of a terminal on a distant island.

Ethernet Xerox Ethernet was so successful that DEC, Intel, and Xerox drew up a standard in 1978 for a 10-Mbps Ethernet, called the DIX standard With minor changes, the DIX standard became the IEEE 802.3 standard in 1983. The IEEE 802 standards are further divided into many parts. They are, IEEE 802.1 -Bridging (networking) and Network Management IEEE 802.2 -Logical link control (upper part of data link layer) IEEE 802.3 -Ethernet (CSMA/CD) IEEE 802.4 -Token bus (disbanded) IEEE 802.5 -Defines the MAC layer for a Token Ring (inactive) IEEE 802.6 -Metropolitan Area Networks (disbanded) IEEE 802.7 -Broadband LAN using Coaxial Cable (disbanded) IEEE 802.8 -Fiber Optic TAG (disbanded) IEEE 802.9 -Integrated Services LAN (disbanded) IEEE 802.10 -Interoperable LAN Security (disbanded) IEEE 802.11 -Wireless LAN & Mesh (Wi-Fi certification)

Wireless LAN Wireless communication is one of the fastest-growing technologies. The demand for connecting devices without the use of cables is increasing everywhere Wireless LANs can be found on college campuses, in office buildings, and in many public areas. IEEE 802.11 is a standard for wireless LANs, sometimes called wireless Ethernet It is commonly called as WiFi The proposed standard had to work in two modes: Presence of a base station(Access point) Absence of a base station (Ad-Hoc)

Wireless LAN

Wireless LAN Some of the challenges are: Suitable frequency band Transmission range of radio signals Privacy Limited battery Human safety Nodes mobility Bandwidth Multi path fading Hand off

Problems in wireless LAN: Hidden terminal problem Exposed terminal problem

Wireless LAN Hidden terminal problem

Assume that station A is sending data to station B. In the middle of this transmission, station C also has data to send to station B. Station C is out of A's range and transmissions from A cannot reach C.

C thinks the medium is free. Station C sends its data to B, which results in a collision at B because

this station is receiving data from both A and C. Stations A and C are hidden from each other with respect to B. Hidden stations can reduce the capacity of the network because of the

possibility of collision.

Exposed terminal problem

This problem arises when a station restricts itself from using another station that is in fact available for use

Assume that station B is sending data to station A. In the middle of this transmission, station C also has data to send to station D.

Station C is out of A's range and transmissions from A cannot reach C. The transmission from C to D can be done without disturbing the

transmission from B to A Station C is exposed to the transmission range of B, it stops itself from

transmitting to D after realizing that B is transmitting some data Such a situation is known as exposed terminal problem

Wireless LAN

A multi-cell 802.11 network

802.11 2 Mbps (rare) 2.4 GHz band (limited in bandwidth)

802.11b 11 Mbps, 2.4 GHz 3 channels/access point

802.11a 54 Mbps, 5 GHz (> bandwidth than 2.4 GHz) 11 channels/access point

802.11g 54 Mbps, 2.4 GHz limited bandwidth

802.11 Wireless LAN Versions

Network Interface Cards (NICs)

Connects the clients, servers and peripherals to the network through a port

It comes as a small circuit board that can be inserted onto one of the mother board s slots

Each NICs is associated with unique MAC address which helps in sending information to the intended destination

Puts the data into packets and transmits

packet onto the network.

May be wired or wireless.

Network Hardware

Gateway Connects networks with different protocols like TCP/IP

network and IPX/SPX networks.

It is known as protocol converter

It accepts the packet formatted for one protocol and converts into another protocol

It can be implemented completely in software, hardware or as a combination of both

Routers and Gateways often refer to the same device.

Proxy server Isolates internal network computers from the internet.

The user first access the proxy server and the proxy server accesses the internet and retrieves the requested web page or document. The user then gets a copy of that page from the proxy server.

Network Hardware

Network Hardware Hub A device that provides a central connection

point for cables from workstations, servers

and peripherals

Hubs are multi-slot concentrators

An unintelligent network device that takes the

data that comes into a port and sends it out

all the other ports in the hub.

Hubs are used for star topology networks, but

they are often used with other configurations

to make it easy to add and remove computers

without bringing down the network.

Typically hubs have anywhere from 4 to over

400 ports

Switch Split large networks into small

segments, decreasing the number of users sharing the same network resources and bandwidth.

When two devices want to talk to each other, and gives them a switched connection

Helps prevent data collisions and reduces network congestion, increasing network performance.

Network Hardware

Bridge

It operates both in physical and data link layer

As a physical layer it regenerates the signal and as

data link layer operates like a bridge

Connects two LANs and forwards or filters data

packets between them

A bridge is a device that allows you to segment a

large network into two smaller, more efficient

networks

Creates an extended network in which any two

workstations on the linked LANs can share data

Transparent to protocols and to higher level devices

like routers.

Forward data depending on the Hardware (MAC)

address, not the Network address (IP).

Network Hardware

Repeater Used to boost the signal between two cable

segments or wireless access points.

Can not connect different network

architecture

It connects the segments of a LAN

It has no filtering capability, it forwards every

frames

Does not simply amplify the signal, it

regenerates the packets and retimes them

Resides on Layer 1 of the OSI model.

Network Hardware

Router A device that connects any number of LANs.

Uses standardized protocols to move packets efficiently to their destination.

More sophisticated than bridges, connecting networks of different types (for example, star and token ring)

Forwards data depending on the Network address (IP), not the Hardware (MAC) address.

A router translates information from one network to another

The router directs traffic to prevent head-on collisions

Network Hardware

Router

An example of a network with a bridge

Internet

Segment Node

Hub

Hub

Bridge

BG Chapter 01

Documents

Transcript of BG Chapter 01