Introduction to Data Communications

NETE0510 Transmission media and data

communications

Dr.Apichan Kanjanavapastit

How to contact your lecturer?

• Office: F405 (Telecommunication System Laboratory)

• Telephone: 029883655 Ext. 113 (office)

• Email: apichan@mut.ac.th

• Consultation:– 9.30AM-11.30AM Saturday– prefer contact via email first in another day

What is Data Communication?

• Data communication is the exchange of data (in the form of 0s and 1s) between two devices via some form of transmission medium (such as a wire cable)

• Data communication is considered local if the communicating devices are in the same building or a similarly restricted geographical area

• It is considered remote if the devices are farther apart

Fundamental characteristics of effectiveness data communication

• The effectiveness of a data communication system depends on 3 fundamental characteristics:

1. Delivery. The system must deliver data to the correct destination

2. Accuracy. The system must deliver data accurately

3. Timeliness. The system must deliver data in a timely manner

Components of a data communication system

ReceiverSender

Transmission medium

Message

Step 1:Step 2:Step 3:

………...

Step 1:Step 2:Step 3:

………...

Protocol Protocol

Components of a data communication system (cont.)

• Message. The message is the information to be communicated. It can consist of text, number, pictures, sound, or video-or any combination of these (multimedia).

• Sender. The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.

• Receiver. The receiver is the device that receive the message

• Medium. The transmission medium is the physical path by which a message travels from sender to receiver. It can consist of twisted par wire, coaxial cable, laser, or radio wares (terrestrial or satellite microwave).

• Protocol. A protocol is a set of rules that govern data communication. It represents an agreement between the communicating devices.

What are Networks?

• A network is a set of devices (often referred to as node) connected by media links

• A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network

• The links connecting the devices are often called communication channels

Network Criteria

• To be considered effective and efficient, a network must meet a number of criteria. The most important of these are performance, reliability, and security

Performance Reliability Security

Data communicationnetwork criteria

Performance

• The performance of a network depends on a number of factors:– Number of users. Having a large number of

concurrent users can slow response time in a network not designed to coordinate heavy traffic loads

– Type of transmission medium. The medium defines the speed at which data can travel through a connection (the data rate)

– Hardware. A higher-speed computer with greater storage capacity provides better performance

– Software. Well-designed software can speed the process and make transmission more effective and efficient

Reliability

• In addition to accuracy of delivery, network reliability is measured by:– Frequency of failure. A network that fails

often, however, is of little value of a user– Recovery time of a network after a failure. A

network that recovers quickly is more useful than one that does not

– Catastrophe. Networks must be protected from catastrophe events such as fire, earthquake.

Security

• Network security issues include protecting data from unauthorized access and viruses– Unauthorized access. Protection can be

accomplished at a number of levels. At the lowest level are user identification codes and passwords. At a higher level are encryption techniques

– Viruses. A virus is an illicitly introduced code that damages the system. A good network is protected from viruses by hardware and software designed specifically fro that purpose

Basic concepts of relationship between communicating devices

• There are five general concept provide the basis for this relationship:– Line configuration– Topology– Transmission mode– Categories of networks– Internetworks

Line Configuration

• Line configuration refers to the way two or more communication devices attach to a link

• A link is the physical communication pathway that transfers data from one device to another

• There are two possible line configuration: point-to-point and multipoint

Point-to-Point

• A point-to-point line configuration provides a dedicated link between two device

• The entire capacity of the channel is received for transmission between those two devices

Modem

Personal Computer

Serial communication link

Workstation

Satellite dish

Workstation

Satellite dish

Microwave link

Television

Remote Controller

Infrared link

Multipoint

• A multipoint (also called multidrop) line configuration is one which more than two specific devices share a single link

Printer

File ServerPersonal Computer

Topology

• The term topology refers to the way a network is laid out, either physically or logically

• The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to each other

• There are five basic topologies possible: mesh, star, tree, bus, and ring

Mesh

• In a mesh topology, every device has a dedicated point-to-point link to every other node

• A fully connected mesh network therefore has n(n-1)/2 physical channels to link n devices

• To accommodate that many link, every device on the network must have n-1 input/output ports

Star

• In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub

Tree

• A tree topology is a variation of a star. As in a star, nodes in a tree are linked to a central hub that controls the traffic to the network

• However, not every device plugs directly into the central hub. The majority of devices connect to a secondary hub that in turn is connected to the central hub

Bus

• A bus topology is multipoint. One long cable acts as a backbone to link all the devices in the network

Ring

• In a ring topology, each device has a dedicated point-to-point line configuration only with 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

Hybrid Topologies

• Often a network combines several topologies as subnetworks linked together in a larger topology

Transmission Mode

• The term transmission mode is used to define the direction of signal flow between two linked devices

• There are three types of transmission mode: simplex, half-duplex, and full-duplex

Simplex Half-duplex Full-duplex

Transmission mode

Simplex

• In simplex mode, the communication is unidirectional, as on a one-way street

• Only one of the two stations on a link can transmit; the other can only receive

• Keyboards and traditional monitors are both example of simplex devices

Half-Duplex

• In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa

• Walkie-talkie and CB radios are both half-duplex systems

Full-Duplex

• In full-duplex mode (also called duplex), both stations can transmit and receive simultaneously

• One common example of full-duplex communication is the telephone network

Categories of Networks

• When we speak of networks, we are generally referring to three primary categories: local area network (LAN), metropolitan area network (MAN), and wide area network (WAN)

Local Area Network(LAN)

Metropolitan Area Network (MAN)

Wide Area Network(WAN)

Network

Local Area Network (LAN)• A local area network is usually privately owned and links the devices

in a single office, building, or campus• LANs are designed to allow resources to be shared (i.e., printers or

applications programs )between PCs or workstations• LANs are distinguished from other types of networks by their

transmission media and topology. In general, a given LAN will use only one type of transmission medium and the most common topologies are bus, ring, and star

Metropolitan Area Network (MAN)

• A metropolitan area network is designed to extend over an entire city. It may be a single network such as a CATV network

• In addition, it may be a means of connecting a number of LANs into a larger network

• A MAN may be wholly owned and operated by a private company, or it may be a service provided by a public company (i.e., a local telephone company)

Wide Area Network (WAN)• A wide area network (WAN) provides long-distance transmission of

data, voice, image, and video information over large geographical areas that may comprise a country, a continent, or even the whole world

• WANs may utilize public, leased, or private communication devices, usually in combinations, and can therefore span an unlimited number of miles

• A WAN that is wholly owned and used by a single company is often referred to as an enterprise network

Internetworks

• When two or more networks are connected, they become an internetwork, or internet

• Individual networks are joined into internetworks by the use of internetworking devices (i.e., routers and gateways)

• The term internet (lowercase i) should not be confused with the Internet (uppercase I)

The OSI Model

• An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model

• An open system is a model that allows any two different systems to communicate regardless of their underlying architecture

• The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable

Fundamental of the OSI Model

• Understanding the following fundamentals of the OSI model provides a solid basis for exploration of data communication– Layered Architecture– Peer-to-Peer Processes– Interfaces between Layers– Organization of the Layers

Layered Architecture

• The OSI model is built of 7 ordered layers: physical, datalink, network, transport, session, presentation, and application

• In developing the model, the designers distilled the process of transmitting data down to its most fundamental elements

• They identified which networking functions had related uses and collected those functions into discrete groups that became the layers

Layered Architecture (cont.)

• By defining and localizing functionality in this fashion, the designers created an architecture that is both comprehensive and flexible

• Most important, the OSI model allows complete transparency between otherwise incompatible systems

Peer-to-Peer Processes

• Within a single machine, each layer calls upon the services of the layer just below it. For example, layer 3 uses the services provided by layer 2

• Between machines, a process of layer x on one machine communicate with the same process of layer x on another machine; this is called peer-to-peer processes

• This communication is governed by an agreed-upon series of rules and conventions called protocols

Peer-to-Peer Processes (cont.)

• At the higher layers, communication must move down through the layers on the sending machine over to the destining machine, and then back up through the layers

• Each layer in the sending machine adds its own information (called headers or trailers) to the message it receives from the layer just above it and passes the whole package to the layer just below it

• Headers are added to the message at layers 6, 5, 4, 3, and 2. A trailers is added at layer 2

Peer-to-Peer Processes (cont.)

• At the receiving machine, the message is unwrapped layer by layer, with each process receiving and removing the data meant for it and then passes the rest to the process in the upper layer

Interfaces between Layers

• The passing of the data and network information down through the layers of the sending machine and back up through the layers of the receiving machine is made by an interface between each pair of adjacent layers

• Each interface defines what information and services a layer must provide for the layer above it

• As long as a layer still provides the expected services to the layer above it, the specific implementation of its functions can be modified or replaced without requiring changes to the surrounding layers

Organization of the Layers

• The 7 layers can be thought of as belonging to 3 subgroup:– Layers 1-3 are the network support layers, they deal with the

physical aspects of moving data from one device to another– Layers 5-7 can be thought of as the user support layers; they

allow interoperability among unrelated software system– Layer 4 ensures end-to-end reliable data transmission while

layer 2 ensures reliable transmission on a single link

• The upper OSI layers are almost always implemented in software; lower layers are a combination of hardware and software, except for the physical layer, which is mostly hardware

Functions of the Physical Layers

• The physical layer coordinates the functions required to transmit a bit stream over a physical medium

• It deals with the mechanical and electrical specifications of the interface and transmission medium

• It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur

Functions of the Physical Layers (cont.)

• The physical layer is concerned with the following:– Physical characteristics of interface and media– Representation of bits– Data rate (transmission rate)– Synchronization of bits– Line configuration– Physical topology– Transmission mode

Functions of the Data Link Layer

• The data link layer transforms the physical layer to a reliable link and is responsible for node-to-node delivery

• It makes the physical layer appear error free to the upper layer (network layer)

Functions of the Data Link Layer (cont.)

• Specific responsibilities of the data link layer include the following:– Framing– Physical addressing– Flow control– Error control– Access control

Example

• a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link. At the data link level this frame contains physical addresses in the header. These are the only addresses needed. The rest of the header contains other information needed at this level. The trailer usually contains extra bits needed for error detection

Functions of the Network Layer • The network layer is responsible for the source-to-destination delivery of

a packet possibly across multiple networks• Whereas the data link layer oversees the delivery of the packet between

two systems on the same network, the network layer ensures that each packet gets from its point of origin to its final destination

• Specific responsibilities of the network layer include the following:– Logical addressing– Routing

Functions of the Transport Layer

• The transport layer is responsible for source-to-destination (end-to-end) delivery of the entire message

• Whereas the network layer threats each one independently, the transport layer ensures that the whole message arrives intact and in order

• For added security, the transport layer may create a connection between the two end ports

• A connection is a single logical path between the source and destination that is associated with all packet in a message

• Creating a connection involves 3 steps: connection establishment, data transfer, and connection release

Functions of the Transport Layer (cont.)

Functions of the Transport Layer (cont.)

• Specific responsibilities of the transport layer include the following:– Service-point addressing (or port address)– Segmentation and reassembly– Connection control. There are 2 types of

connection control: connectionless or connection oriented.

– Flow control– Error control (damage, loss, or duplication)

• The services provided by the lowest four layers of the OSI model may be not sufficient

• For example, a remote terminal access application might require a half-duplex dialogue. A transaction-processing application might require checkpoints in the data transfer stream to permit backup and recovery

• Since these types of dialogue-structuring tools have widespread applicability, it make sense to organize them into a separate layer: the session layer which establishes, maintains, and synchronizes the interaction between communicating systems

Functions of the Session Layer

Functions of the Session Layer (cont.)

• Specific responsibilities of the session สฟำ��พ include the following:– Dialog control– half duplex or full duplex– Synchronization. The session layer allows a process

to add checkpoints (synchronization points)

Data

From presentation layer

H

Synchronizing point

Data

To transport layer

Session layer

• The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems

• Specific responsibilities of the presentation layer include the following:– Translation. Since different computers use different

encoding systems, the presentation layer responsible for interoperability between these different encoding methods

– Encryption. To carry sensitive information, a system must be able to assure privacy

– Compression. Data compression reduces the number of bits to be transmitted

Functions of the Presentation Layer

Functions of the Application Layer

• The application layer enables application programs to access the network

• It provides interfaces and support for services such as electronic mail, remote file access and transfer

Summary of Layer Functions

Homework

1. Give your own example of how the type of transmission medium can affect the performance of a network

2. List the advantages and disadvantages of each topology type

3. Draw a diagram to illustrate the correlation between layers of the TCP/IP protocol suite and layers of the OSI model