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LECTURE
Communication Tasks
There are some key tasks that must be performed in a data communication system Elements can be added, deleted, or merged together
Transmission System utilization Interfacing
Signal Generation Synchronization
Exchange Management Error Detection and Correction
Flow Control Addressing
Routing Recovery
Security Network Management
Transmission System Utilization
Need to make efficient use of Transmission facilities that are shared among a no. of communicating devices
For Example: Techniques like Multiplexing to allow multiple users to share total capacity of a Transmission Medium Congestion Control : TX. System should not be overwhelmed by traffic
Interfacing A device must have an Interface with the Transmission System/Transmission Medium
Signal Generation
Electromagnetic Signals travel over Transmission Medium. Once an interface is established, Signal generation is required
Properties of Signals Capable of being propagated over TX. Medium Interpretable as data at the Receiver
Synchronization The transmission and the reception should be properly synchronized. Synchronization means that the receiver must be able to determine, when to
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expect a new transmission and when to send acknowledgements. In other words transmitter and receiver should have an agreement on the nature as well as timing of the signals
Exchange Management If the data needs to be exchanged in both directions over a period of time, both parties must cooperate as follows
Whether both devices must transmit simultaneously or take turns Amount of Data to be sent at one time Format f the Data What to do when an Error Arises
Error Detection and Correction In all comm. Systems, there is a potential risk for errors and impairments. Tx. Signals are distorted to some extent before reaching their destination. Error Detection & Correction needs to be employed in Data Processing Systems where a change in say the contents of a file cannot be tolerated
Flow Control To make sure that source does not overwhelm destination by sending data faster than it can be handled and processed
Addressing & Routing If TX facility is shared by two or more devices, source must specify the identity or the address of the destination system and if Tx. System is itself a system, a proper route must be allocated that the data will take in order to reach the desired destination
Recovery If a data transmission is interrupted due to a fault somewhere in the system, recovery techniques are needed. The objective is either to resume activity at the point of interruption and to restore the state of the system to what it was prior to the interruption
Security
Security is very important issue in a Data Communication System. The sender needs to be assured that
Only the Intended receiver receives the data Data is delivered unaltered
Introduction to Protocol
In computer Networks, communication occurs between two entities in different systems.
o Entity is anything sending and receiving information o SYSTEM is a physical object containing more than one entities
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Protocol Architecture
Instead of having a single Module for performing communication, there is a structured set of modules that implement communications function” This structure is called Protocol Architecture Let’s explain it by an example of File transfer system.
Simplified File Transfer Architecture
In the above example File transfer could use three modules
File transfer application Communication service module Network access module
File transfer application
File Transfer contains all of the logic is unique to the file transfer application such as: Transmitting passwords File Commands
Checking File System on other machine if it is ready Check File System Compatibility File records
Communication service module Instead of allowing File Transfer Module to deal with actual transfer of data and commands, we can have a separate module for this transfer. This module must make sure that the receiver system is ready to receive and look into the reliable exchange of data
Network access module
Nature of the exchange between systems is independent of the network that connects them. That allows us to have a 3rd module that handles the details of the Network interface and interacts with the network. If Network to be used changes, only Network access Module has to change
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Characteristics of a Protocol
Direct or indirect Monolithic or structured Symmetric or asymmetric Standard or nonstandard
Direct
Systems share a point to point link or Data can pass without intervening active agent Simple Protocol
Indirect
Switched networks or Interne works or internets Data transfer depend on other entities Complex Protocol
Monolithic or Structured
Communications is a complex task To complex for single unit Structured design breaks down problem into smaller units Layered structure
Symmetric or Asymmetric
Symmetric Communication between peer entities
Asymmetric Client/server
Standard or Nonstandard Nonstandard protocols built for specific computers and tasks
Summary
Communication Tasks Protocols Protocol Architecture Characteristics of a Protocol
Reading Sections
Section 1.4 “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan Sections 1.1,2.1 “Data and Computer Communication” 6th Edition by William Stallings
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