DEPARTMENT OF

ELECTRICAL

ENGINEERING

EP601 DATA

COMMUNICATION

At the end of this learning session, student must be able to;

Define :

a. Data Network

b. Value Added Network (VAN)

c. Packet Switching Network

Identify types of common switching network:

a. Circuit switching

b. Message switching

c. Packet switching

Compare among circuit switching, message switching, packet switching network

Categorize packet switching methods:

a. Datagram

b. Virtual circuit

Explain protocol used in packet switched services:

a. X.25

b. Frame relay

C. Asynchronous transfer mode (ATM)

Definition:

A data network is an electronic communications process

that allows for the orderly transmission and receptive of

data, such as letters, spreadsheets, and other types of

documents.

What sets the data network apart from other forms of

communication, such as an audio network, is that the data

network is configured to transmit data only.

This is in contrast to the audio or voice network, which is

often employed for both voice communications and the

transmission of data such as a facsimile transmission.

Definition:

A value-added network adds value to the services or facilities provided by a common carrier to provide new types of

communication services.

Examples of added values are error control, enhanced connection

reliability, dynamic routing, failure protection, logical multiplexing

and data format conversion.

Examples of value-added networks are GTE Telnet, DATAPAC,

TRANSPAC and Tymnet Inc.

Definition :

A digital communication network which operates by dividing

each piece of information to be sent into discrete packets.

These packets are then sent individually across the network

and reassembled, in order, at the information's destination.

Since 1970, packet switching has evolved substantially for

digital data communications.

It was designed to provide a more efficient facility than

circuit switching for bursty data traffic.

Data is transmitted as stream of bits, no packetizing is needed.

Circuit switching:

o There is a dedicated communication path between two stations (end-to-end)

o The path is a connected sequence of links between network nodes. On each physical link, a logical channel is dedicated to the connection.

Communication via circuit switching has three phases:

Circuit establishment (link by link)

Routing & resource allocation (FDM or TDM) Data transfer

Circuit disconnect

Deallocate the dedicated resources

The switches must know how to find the route to the destination and how to allocate bandwidth (channel) to establish a connection.

Example of circuit switching network to connect

eight telephones in a area.

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Subscribers: the devices that attach to the network.

Subscriber loop: the link between the subscriber and the network.

Exchanges: the switching centers in the network.

End office: the switching center that directly supports subscribers.

Trunks: the branches between exchanges. They carry multiple voice-

frequency circuits using either FDM or synchronous TDM.

Normal telephone service is based on a circuit-switching technology, in which a dedicated line is allocated for transmission between two parties.

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.

Application : Mail Delivery

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.

Packet switching can be seen as a solution that tries to combine the advantages of message and circuit switching and to minimize the disadvantages of both.

A station breaks long message into packets. Packets are sent out to the network sequentially, one at a time (individually) and can even follow different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message.

Data are transmitted in short packets Typically at the order of 1000 bytes Longer messages are split into series of packets Each packet contains a portion of user data plus some control info

store and forward On each switching node, packets are received, stored briefly (buffered) and

passed on to the next node.

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Most modern Wide Area Network (WAN) protocols, including TCP/IP, X.25, and Frame Relay, are based on packet-switching technologies.

No Circuit switching Message switching Packet switching

1 dedicated transmission

path

no dedicated transmission

path

no dedicated transmission

path

2 continuous transmission

of data

transmission of messages transmission of packets

3 operates in real time not real time near real time

4 messages not stored messages stored messages held for short

time

5 path established for entire

message

route established for each

message

route established for each

packet

6 call setup delay message transmission

delay

packet transmission delay

7 blocking may occur blocking cannot occur blocking cannot occur

8 no speed or code

conversion

speed or code conversion speed or code conversion

There are two approaches:

Datagram Virtual circuit

Datagram - Connectionless service :No handshaking, each packet is sent and routed independently and can follow different paths to reach to the destination. The full address of the source and destination must be attached to each packet.

o No setup delay

o Packets are not guaranteed to arrive in the order they were sent

o Robust: If a router crashes only packets inside the router will be lost, other packets can follow other path

It is up to the receiver to re-order packets and recover from missing packets.

Example: Internet

Virtual circuit - Connection-oriented : A connection

(handshaking) between the sender and the receiver is

established and the complete path for the packets to reach

to the destination is determined before transmission of

any packets. This path is called virtual circuit or a

connection and the address given for each packet is the

sequence number of the virtual circuit called Virtual

Circuit Identifier (VCI)

o No dedicated path: the path can be used by other virtual

circuits

o Data is packetizied before transmission

o Packets are guaranteed to arrive in the order they were

sent

o Packets are logically connected to each other, packets travel

one after the other

o The virtual circuit has to be terminated after all packets of

a message have been arrived

o If the virtual circuit router crashes all virtual circuits that

go through the router are terminated and paths are lost

o Used in WAN (Frame relay, ATM)

Virtual circuits

o Network can provide sequencing (packets arrive at the same order) and error control (retransmission between two nodes).

o Packets are forwarded more quickly

Based on the virtual circuit identifier

No routing decisions to make

o Less reliable

If a node fails, all virtual circuits that pass through that node fail.

Datagram

o No call setup phase

Good for bursty data, such as Web applications

o More flexible

If a node fails, packets may find an alternate route

Routing can be used to avoid congested parts of the network

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There are divided in three services:

X.25 Frame Relay Asynchronous Transfer Mode (ATM)

In case of packet switching networks, the attached stations must organize their data into packets for transmission.

This requires a certain level of cooperation between the network and the attached stations.

X.25 is an ITU-T standard that specifies on interface between a host system and packet switching network.

Implemented at the network layer.

Implements extensive error correction and flow control due to early unreliable links.

X.25 Layers in Relation to the OSI Layers

Note:

Link Level (LAPB- Link Access Protocol Balanced) Packet level (PLP-Packet Layer Protocol)

Layer 1 - Physical level:

Physical level deals with the physical interface between an

attached station and the link that attaches that station to the

packet switching node.

Layer 2 - Link level:

The link level provides for the reliable transfer of data across

the physical link, by transmitting the data as a sequence of frames.

The link level standard is referred to as LAPB (link access protocol

balanced), LAPB is subset of HDLC (High-level Data Link Control).

Layer 3 - Packet level:

The packet level provides a virtual circuit service.

This service enables any subscriber to the network to setup

logical connections called virtual circuits, to other subscribers.

X.25 is a standard for interface between the host system

with the packet switching network in which it defines

how DTE is connected and communicates with packet

switching network.

Note:

S-Frames flow and error control in the frame layer

U-Frames- used to set up and disconnect the links between a DTE

and a DCE. In the frame layer, communication between a DTE -

DCE involves three phases:

1: Link Setup ; 2: Packet Transfer ; 3: Link Disconnect

Internationally, Frame Relay (FR) was standardized by the

International Telecommunication UnionTelecommunications Standards Section (ITU-T).

FR originally was designed for use across Integrated Service Digital Network (ISDN) interfaces, allows for digital voice communication (VOFR).Today, it is used over a variety of other network interfaces as well.

Improvement of previous technology X.25

Operate only at the Physica and Data link layer. Error

detection at the data link layer. No flow control or error correction

control (less overhead).

FR Layers in Relation to the OSI Layers

Layer 1 : Physical layer

o No specific protocol, it is left to the implementer to use

whatever is available

o Supports any of the protocols recognized by ANSI

Layer 2 : Data link layer

o Employs a simplified version of HDLC called core LAPF

(Link Access Procedure for Frame Mode Bearer Services) with

no extensive error and flow control fields.

LAPF core: minimal data link control

Preservation of order for frames

Small probability of frame loss

LAPF control: additional data link or network layer end-to-end

functions

Uses asynchronous time division multiplexing.

Designed to take advantage of the bandwidth of

optical fiber transmission media.

Many of the protocol functions are implemented in

hardware (not software) to insure the best possible

performance.

ATM uses fixed packet lengths of 53 bytes (5 bytes of overhead and 48 bytes of user data), which is more suitable

for voice transmissions.

ATM provides extensive quality of service information that enables the setting of very precise priorities among

different types of transmissions (i.e. voice & video, internet,

etc).

ATM provides connection-oriented services only.

Note :

All cells are 53 bytes

5 byte header

48 byte data payload

An ATM Cell

ATM is implemented in the physical and data link

layers.

Any physical layer carrier can carry ATM cells (wired, optical,

wireless).

The Data Link layer for ATM provides :

Routing/Switching

Multiplexing

Flow control (quality of service)

Error detection and correction

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ATM Layers

ATM layers in endpoint devices and switches

Architecture of an ATM network

Note :

UNI user to network interfaces NNI network to network interfaces

Cell switched architecture