Data Link Control: Flow Control and Error Control Mechanisms, Slides of Data Communication Systems and Computer Networks

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Lecture- BS(CIS) Semester-IV

Data Communication

Previous Lectures

• Communication Model
• Protocols  Protocol Architecture
• Transmission Media
• Data Encoding Technique

Major Emphasis: Transmission of data over a transmission

medium

Transmission over a link..

 Addressing

• How a receiver will recognize the frames sent for it?  Link Management
• How a sender will inform a receiver that it is about to send the data
• How a receiver will inform a sender about its readiness to accept the data?

Data Link Control

• We need to add a logic layer above the physical to answer

the afore mentioned needs of a sender and a receiver

 Such a layer is called data link control protocol

• Key Mechanisms of Data Link Control Protocol  Flow Control  Error Control  (Other parts of data link control protocol will be discussed consequently)

Flow Control

 Propagation Time

• Time taken by a bit to traverse the link between source and destination  Transmission time is directly proportional to the length of a frame  Propagation Time depends upon nature of the medium and signals used to for bits

Flow Control Model

 Successful reception  No sequencing Error  No Frame errors ( damage frames )  Each frame suffers form a transmission delay

Fragmentation

• Large block of data may be split into small frames  Limited buffer size  Errors detected sooner (when whole frame received)  On error, retransmission of smaller frames is needed  Prevents one station occupying medium for long periods
• Stop and wait becomes inadequate

Stop and Wait Link Utilization

Sliding Window Diagram

Example Sliding Window