Data Communication 1

Chapter 1

Data Communications and Networking Overview

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A Communications ModelKey elements of a simple model of communications:(Purpose: the exchange of data between two parties)• Source

—generates data to be transmitted (e.g. telephones & PCs)

• Transmitter—Converts data into transmittable electromagnetic signals

• Transmission System—Carries data (single transmission line or network)

• Receiver—Converts received signal into data

• Destination—Takes incoming data

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Simplified Communications Model - Diagram

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Communications Tasks

Transmission system utilization Make efficient use of transmission facilities

Interfacing A device must interface to cummunicate

Signal generation Capable of being propagated through the transmission system, interpretable as data at the receiver

Synchronization The receiver must be able to determine when a signal begins to arrive and when it ends

Exchange management The two parties must cooperate (transmit simultaneously or take turn, data format)

Error detection and correction In all communications systems, there is a potential for error

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Communications Tasks

Flow control Sending data faster than destination can be processed

Addressing Only intended destination system must receive the data

Routing Specific route through the network

Recovery Resume activity at the point of interruption or restore the state

Message formatting Form of the data to be exchanged

Security Receiver may wish the data have not been altered in transit, ..

Network management Configuring the system, monitoring its status

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Simplified Data Communications Model

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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)

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Wide Area Networks• Large geographical area• Crossing public rights of way• Consists of a number of interconnected

switching nodes• Alternative technologies

—Circuit switching—Packet switching—Frame relay—Asynchronous Transfer Mode (ATM)

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Circuit Switching• Dedicated communications path

established for the duration of the conversation

• Path: a connected sequence of physical links between nodes

• e.g. telephone network

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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

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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

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Asynchronous Transfer Mode• ATM• Evolution of frame relay• Little overhead for error control• Fixed packet (called cell) length• Constant data rate using packet switching

technique (by using small – fixed sized cells)

• By using a fixed packet length, the processing overhead is reduced

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Local Area Networks• Smaller scope

—Building or small campus

• Usually owned by same organization as attached devices—There may be a substantial capital invesment

for both purchase and maintenance—One user for network management

responsibility

• Data rates much higher

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LAN Configurations• Switched

—Switched Ethernet• May be single or multiple switches

—ATM LAN—Fibre Channel

• Wireless—Mobility—Ease of installation

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Metropolitan Area Networks• MAN• Middle ground between LAN and WAN• Private or public networks that provide

high capacity at low costs• Large area

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NetworkingConfiguration

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Further Reading• Stallings, W. Data and Computer

Communications (7th edition), Prentice Hall, Upper Saddle River NJ, chapter 1