CN UNIT-3 PPT R20 computer networks .pptx

Computer Networks
(Professional Elective )
Class: III/II Branch: ECE-B
Prepared by
Mr.D.Durga Prasad
Assistant Professor
ECE Dept. VIT, Bhimavaram

UNIT-3 Syllabus
 The Data Link Layer
Services Provided to the Network Layer
 Framing
Error Control
Flow Control
 Error Detection and Correction
 Error-Correcting Codes Error Detecting Codes
 Elementary Data Link Protocols
A Simplex Stop and Wait Protocol for an Error free channel
A Simplex Stop and Wait Protocol for a Noisy Channel
 Sliding Window Protocols
A One Bit Sliding Window Protocol
A Protocol Using Go-Back-N
A Protocol Using Selective Repeat.

Contd..
 Above Figure provides an outline of the three basic steps in CRC.
 1. A string of n 0s is appended to the data unit. The number n is 1 less
than the number of bits in the predetermined divisor, which is n + 1 bit.
 2. The newly elongated data unit is divided by the divisor, using a
process called binary division. The remainder resulting from this
division is the CRC.
 3. The CRC of n bits derived in step 2 replaces the appended 0s at the
end of the data unit. Note that the CRC may consist of all 0s.

Data Link Layer protocols
 Data Link Layer protocols can be divided into two types :
 1.Elementary protocols
 2.Sliding window protocols
 Elementary DLL protocols are further divided into three types:
 1.An Unrestricted Simplex Protocol(Utopian simplex protocol)
 2.A Simplex Stop-and-Wait Protocol
 3.A Simplex Protocol for a Noisy Channel

A Simplex Stop and Wait Protocol for an
Error free channel
 In this protocol the sender sends a single frame to receiver & waits for an
acknowledgment before sending another frame.
 The next frame is sent by sender only when acknowledgment of previous
frame is received.
 This process continues as long as the sender has data to send.
 The advantage of this protocol is that it is able to prevent the sender from
flooding the receiver.
 The disadvantage is that the sender should wait till it gets an ACK of
previous frame before it sends next frame.
 If the acknowledgment frame gets corrupted, the sender cannot perform
anymore.

Contd..
 In this protocol we assume that:
 Data are transmitted in one direction only
 No errors occur (noiseless channel)
 The receiver can only process the received information at a finite rate.

A Simplex Stop and Wait Protocol
for a Noisy Channel
 In this protocol the error free assumption in protocol 2 is dropped i.e. frames may
be either damaged or lost completely.
 The sender will send a frame, the receiver will send an ACK frame only if the
frame is received correctly. If the frame is in error the receiver simply ignores it; the
transmitter would time out and would retransmit it.
 Protocols in which the sender waits for a positive acknowledgement before
advancing to the next frame are often called stop and wait Automatic Repeat
Request (ARQ).
 One problem with the above scheme is that if the ACK frame is lost or damaged,
duplicate frames are accepted at the receiver without the receiver knowing it.

Contd..
 To overcome this problem sender put a sequence number in the header of each
frame it sends.
 The receiver then can check the sequence number of each arriving frame to see
if it is a new frame or a duplicate to be discarded.
 This protocol can transmit one frame at a time.
 sender always waits for acknowledgement even if it has next frame ready to
send(poor utilization of band width).

CN UNIT-3 PPT R20 computer networks .pptx

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