William Stallings Data and Computer Communications 7 th Edition Chapter 1 Data Communications and Networks Overview

A Communications Model Source —generates data to be transmitted Transmitter —Converts data into transmittable signals Transmission System —Carries data Receiver —Converts received signal into data Destination —Takes incoming data

Communications Tasks Transmission system utilizationAddressing InterfacingRouting Signal generationRecovery Synchronization Must be some form of synchronization between transmitter and receiver Message formatting Exchange management A variety of requirement for communication between two parts Security Error detection and correctionNetwork management Flow control

Flow Control: Aims to control the rate at which segments (packets) are communicated. A segment acknowledgment mechanism is used. From time to time, the receiver informs the transmitter of the highest sequence number received together with a window size. The window size informs the sender with the number of segments the receiver can handle.

Simplified Communications Model - Diagram

Simplified Data Communications Model

Networking Point to point communication not usually practical —Devices are too far apart —Large set of devices would need impractical number of connections Solution is a communications network —Wide Area Network (WAN) —Local Area Network (LAN)

Wide Area Networks Large geographical area Crossing public rights of way Rely in part on common carrier circuits Alternative technologies —Circuit switching —Packet switching —Frame relay —Asynchronous Transfer Mode (ATM)

Wide Area Networks – Support the exchange of messages at lower speeds between computers in different organizations, usually separated by large distances. – The network is managed by a set of linked routers which are dedicated computers that route messages to their destinations. Routing at each router introduces a delay.

Circuit Switching Dedicated communications path established for the duration of the conversation e.g. telephone network Switches are used to build communication circuits between network nodes. A circuit is built between nodes at the start of communication session and the communication circuit is broken at the end of such session. Thus a dedicated communication channel is constructed during the session.

Packet Switching Data sent out of sequence Small chunks (packets) of data at a time Packets passed from node to node between source and destination Used for terminal to computer and computer to computer communications

Packet Switching Messages to be transmitted are divided into packets (of equal length in simplest form). Rather than making fixed circuits for a communication, the network just forwards packets individually from source to destination. Each switching node in the network is represented by a computer.

Packet Switching – A Store-and-Forward technique is used in which each switching node (computer): A packet arriving at a switching computer is stored in memory. The packet is processed to identify details such as packet destination, Such a packet is then forwarded through an outgoing channel to another - node moving the packet closer to its destination.

Frame Relay Packet switching systems have large overheads to compensate for errors Modern systems are more reliable Errors can be caught in end system Most overhead for error control is stripped out

Frame Relay – The store-and-forward technique does not support instantaneous transmission of packets. – In the Internet, a packet of small size could take around 200 ms to move from source to destination. – This speed of communication is not suitable for applications such as video streaming and telephony (requires a 50 ms delay to support a conversation). – To overcome such delay problems, a frame rely switching technique is used which combines feature of circuit and packet switching.

Frame Relay Frame rely is fast packet switching technique in which switching speed is increased by: Avoiding flow control and error checking at intermediate nodes. The small and fixed size of packets (cells) transmitted which reduces buffer size and queuing delays at switching nodes.

Frame Relay – In frame rely, a connection is established only if there are sufficient resources available to provide a guaranteed Quality of Service (QoS). – Asynchronous Transfer Mode (ATM) is based on the frame rely mechanism.

Asynchronous Transfer Mode ATM Evolution of frame relay Little overhead for error control Fixed packet (called cell) length Anything from 10Mbps to Gbps Constant data rate using packet switching technique

Local Area Networks Smaller scope —Building or small campus Usually owned by same organization as attached devices Data rates much higher Usually broadcast systems Now some switched systems and ATM are being introduced

Local Area Networks – LANs support the exchange of messages at relatively high speeds between computers connected by a single communication medium. – A segment is a section of a cable that can connect many computers in,e.g., a building. – No routing is required in a segment. – Larger LANs can be constructed by connecting more that one segment using a hub. – LANs can be implemented using the Ethernet, Token Rings.

LAN Configurations Switched —Switched Ethernet May be single or multiple switches —ATM LAN —Fibre Channel Wireless —Mobility —Ease of installation

Metropolitan Area Networks MAN Middle ground between LAN and WAN Private or public network High speed Large area – Based on the high bandwidth copper and fiber optic cables which are installed across towns and cities. – Can be implemented in many technologies ranging from the Ethernet to ATM. – IEEE is a standard specification of such networks.

Networking Configuration

Comparison of Different Network Types

Further Reading Stallings, W. [2003] Data and Computer Communications (7th edition), Prentice Hall, Upper Saddle River NJ, chapter 1 Web site for Stallings book —