2. 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.
37. 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.
40. 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
41. 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.
42. 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.
44. 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.
45. 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).
