Interlayer Communication-Data Communication-Lecture Slides, Slides of Data Communication Systems and Computer Networks

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Data Communication

Previous Lecture

• What is data communication
  • Transfer of encoded information between functional units according to some protocol..
• A simple communication model
  • A simple communication has a source, transmitter, communication network, receiver, destination
• Essential functions of a communication model
  • Communication model implements some essential functions e.g. addressing, error correction and detection etc..
• Communication Networks
  • Set of inter-connected nodes, broadly divided into WANs, and LANs (geographical classification)

Protocol Architecture

• Protocol
  • Set of rules agreed upon by the devices taking part in communication
• Functions performed for communication are grouped into layers according to their nature
• Each layer performs its specific functions
  • e.g. Network layer perform network related functions where as application layer performs user related functions
• Each layer has a well defined interface for interaction with other layers/programs
• Interlayer communication
  • Each layer accepts data from higher layer
  • Performs necessary actions

Interlayer Communication

• Interlayer communication
  • Each layer accepts data from higher layer
  • Performs necessary actions
  • Adds layer specific information
  • Pass it to the lower layer
• Example
  • A file transfer program

A Three Layer Model-(generic model)

• Communication tasks can be divided into three

independent layers

• Network access layer
• Transport layer
• Application layer

Network Access Layer

• Concerned with exchange of data between a computer and the

network to which it is attached

• Performs network relating functions
  • Selection of communication method i.e. circuit switching or packet switching
  • Network addressing
  • Network paths/routes selection
  • Invoke network services, quality, throughput etc..
  • Separates the network logic from higher layer.

All network related functions are performed by the network layer layers

using this layer need not worry about network matters

Application Layer

• Provide services to the user
• A user could be a software or a human being
• Specific modules require to deal with variety of application

Example

• File Transfer Service
• E-mail service

Service Access Point

• Two levels of addressing is used for communication
  • Network Address
    • Each computer needs unique network address
  • Application Address
    • Each application on a computer needs a unique address within the computer termed as SAP or Port Address
  • Communication between two systems requires
    • Source and destination system’s network address
    • Source and destination application’s SAP

Protocol Data Units (PDU)

• Data from next layer added with layer specific information

forms a protocol data unit.

• Transport layer may fragment user data
  • Each fragment has a transport header that includes
    • Destination SAP
    • Sequence number
    • Error detection code
• Network Layer
  • Adds network header
    • Network address for destination computer
    • Facilities requests

Protocol Architecture-Operational View

De-facto Standards

• Adopted by convention or some other reason but not according to

the defined laws e.g. MS-windows as operating system

• The de-facto standards can be
  • Proprietary or closed
  • Nonproprietary or open

De-Jure Standards

• De-Jure
  • Legislated by an official authority
  • e.g. http ( hyper text transfer protocol ) legislated by w3c or

W.W.W.C ( world wide web consortium )

Organization for Standards

• Standards are developed by co-operation among standards

creation committees, forums, and government regulatory

authorities

• Standards Creation Committees
• Procedural organization, processes are slow
  • ISO
  • ITU-T
  • ANSI
  • IEEE
  • EIA
  • Telcordia

Annexure

Protocol When data is being transmitted between two or more devices something needs to govern the controls that keep this data intact. A formal description of message formats and the rules two computers must follow to exchange those messages. Protocols can describe low-level details of machine-to- machine interfaces (e.g., the order in which hits and bytes are sent across wire) or high-level exchanges between application programs (e.g., the way in which two programs transfer a file across the Internet). [San Diego State University] “

Port Number

1-A physical port is an interface where other devices can be attached such as printer port, network port etc..

2-A logical port is a number assigned to a program by an operating system