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COE Chapter 11: Data Link Control (DLC), Automatic repeated request (ARQ) …
COE Chapter 11: Data Link Control (DLC)
Services provided by data link layer
Flow control
Go-Back-N ARQ
Allows sender to send more than one frame without worrying about ACK
A copy of each frame is kept in the sender until ACK arrives. Each frame is numbered. (sequence number in header)
If m bits are used, Sequence numbers range from 0 to 2^m - 1
e.g if m = 3 bit --> seq. no: 0,1,2,3,4,5,6,7,0,1,2,...
To hold until ACK is received, we used the concept of window
We imagine all frames are stored in buffer. The outstanding frames (waiting for ACK) are enclosed in a window.
Frames to the left of the window are already ACKed & can be purged; those in right can't be sent until the window slides over them.
Size of the window is at most 2^m - 1
1 more item...
The receiver also needs to use sliding window to track frames.
2 more items...
the sequence no. are modulo 2^m, m os the size of seq no. filed in bits
Go-Back-N ARQ: Timers
Sender sets a timer for each frame. Receiver has no timer
Problems of this method is: very inefficient for noisy link.
Error control
Collectively known as data link control procedures
Framing
Framing
Data-link layer at each node, needs to encapsulate the datagram in a frame before sending. ---> The recieving node needs to decapsulate the datagram from the frame.
May have both a
header
&
trailer
The block of data at the data-link layer is called a frame.
Flow Control
Is a set of procedure used to ensure that the sender doesn't overwhelm the receiver with data.
Restricts the amount of data's, the sender can send before waiting for ACK
Any reciever has limited speed and memory
It must inform the sender to slow down and not exceed its limits.
Has a block of memory (a buffer) reserved for storing incoming data until processed.
Error control
Mechanism for detection & correction of error.
Data are sent as sequence of frames (order maintained)
Types of error
Damaged frame
Frame arrives with some bits in error
Lost ACK
Lost frames
receiver is not aware that a frame has been transmitted
When error is detected --> specified frames are retransmitted.
Stop & wait ARQ
A form of stop and wait flow control with retransmission
sender keeps a copy of the last frame transmitted until receiving a ACK. Allows senders to retransmit lost or corrupted frames until they are received correctly.
Step 1.) Source transmits a single frame and waits for ACK
Step2.) A timer is started when a frame is transmitted
Step 3.) If received frame is correct, receiver accepts the frame
Step 4.) and sends a positive ACK
No ack for a frame with error; damage frame is discarded
The sender assumes the frame was lost, or damaged & resends it.
Pros & Cons:
Simple
Inefficient (slow)
Special case of Go-back-N ARQ, where sender window is 1
Automatic repeated request (ARQ)
