Data Link Layer.pdf - CS4344 – Computer Networks and Distributed Systems Data Link Layer Dr Padmaraj Nair Data Link Control Protocols • Requirements

Data Link Layer.pdf - CS4344 – Computer Networks and...

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CS4344 – Computer Networks and Distributed Systems Data Link Layer Dr. Padmaraj Nair
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Data Link Control Protocols Requirements and objectives for effective data communication between two directly connected transmitting-receiving stations: Frame synchronization Flow control Error control Addressing Control and data Link management
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Flow Control Technique for assuring that a transmitting entity does not over-whelm a receiving entity with data The receiving entity typically allocates a data buffer of some maximum length for a transfer When data are received, the receiver must do a certain amount of processing before passing the data to the higher-level software In the absence of flow control, the receiver’s buffer may fill up and overflow while it is processing old data
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Frame 1 Source Destination Time Frame 1 Frame 2 Frame 2 Frame 3 Frame 3 Frame 4 Frame 4 Frame 5 (a) Error-free transmission Frame 5 Frame 1 Source Destination Frame 1 Frame 2 Frame 3 Frame 3 Frame 4 Garbled frame Frame 5 (b) Transmission with losses and errors Frame 5 Model of Frame Transmission
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Stop-and-Wait Flow Control Simplest form of flow control It is often the case that a source will break up a large block of data into smaller blocks and transmit the data in many frames The buffer size of the receiver may be limited The longer the transmission, the more likely that there will be an error, necessitating retransmission of the entire frame On a shared medium it is usually desirable not to permit one station to the medium for an extended period, thus causing long delays at the other sending station
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t 0 T R T R t 0 + 1 T R T R t 0 + a T R T R t 0 + 1 + a T R T R t 0 + 1 + 2 a ACK Frame t 0 t 0 + a t 0 + 1 t 0 + 1 + a t 0 + 1 + 2 a T R T R (b) a > 1 (a) a < 1
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Sliding Windows Flow Control Allows multiple numbered frames to be in transit Receiver has buffer W long Transmitter sends up to W frames without ACK ACK includes number of next frame expected Sequence number is bounded by size of field (k) Frames are numbered modulo 2 k Giving max window size of up to 2 k – 1 Receiver can ACK frames without permitting further transmission (Receive Not Ready) Must send a normal acknowledge to resume If have full-duplex link, can piggyback ACKs
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0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Window of frames that may be transmitted Frames already transmitted Frames buffered until acknowledged Last frame transmitted Last frame acknowledged Frame sequence number Window shrinks from trailing edge as frames are sent Window expands from leading edge as ACKs are received (a) Sender's perspective 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 Window of frames that may be accepted Frames already received Last frame received Last frame acknowledged Window shrinks from trailing edge as frames are received Window expands from leading edge as ACKs are sent (b) Receiver's perspective
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F0 F1 F2 RR 3 F3 F4 F5 F6 RR 4 Source System A Destination System B 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0
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