THE DATA LINK LAYER. Out of order with the book Dr. John P. Abraham University of Texas, Panam. The data link layer. Provides data transport across a physical link
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THE DATA LINK LAYER Out of order with the book Dr. John P. Abraham University of Texas, Panam
The data link layer • Provides data transport across a physical link • Handles physical addressing, network topology, line discipline,Frame synchronization, error notification, orderly delivery of frames and flow control.
Services provided by data link layer • Unacknowledged connectionless service • Acknowledged connectionless service • Acknowledged connection-oriented service
Unacknowledged connectionless service • No connection established. • Send frames. No acknowledgment by receiving machine. • If lost, no attempt to recover by this layer. Recovery is left up to the higher layers • Good for reliable lines such as fiber, use for speech, video, etc. where late is worse than lost frames.
Acknowledged connectionless service • No connection established • Frames are sent and each is individually acknowledged • Frames are resent if acknowledgement received within timeout period. • Useful for unreliable channels such as wireless.
Acknowledged connection-oriented service • Reliable service • Source and destination establish connection • All frames are received in the right order • After transmission the connection is releasedaming
Framing • In order to provide service to the network layer, the data link layer must use the service provided to it by the Physical layer. The physical layer only accept a raw bit stream. For a long bit stream, if error occurred during transmission, the entire stream has to be sent again. Therefore, it is broken to smaller frames.
Each frame will have to be distinguished somehow. Three methods are used Character count Starting and ending characters, with character stuffing Starting and ending flags, with bit stuffing
Character count The first byte indicates how many characters are in the frame including the first byte. Trouble with this scheme is that if the first byte of any frame gets corrupted, the destination will get out of sequence, having no way to find where the beginning of the next frame is. This method is rarely used any more.
Start and end with control chars with character stuffing Resynchronize at the beginning of each frame Start with ASCII chars DLE & STX Data Link Escape, Start of Text End with the same sequence If these ASCII char is part of the text then double it like Jim’’s book - in pascal Problem - must use 8 bit ASCII which is not suitable for all communications
Start and end with flags with stuffing Each frame begins with bit pattern 01111110 If senders data has five consecutive ones then stuff it with a zero which is deleted at the receiving end This bit pattern called flag byte is added at the end of each frame as well With this method boundary between two frames are easily recognized
Error control How to make sure that all data arrived at the destination correctly and in the proper order. The receiver sends special control characters indicating positive or negative acknowledgement about incoming frames. What to do if the acknowledgment gets lost? -the computer will hang. Introduce timers for each frame. The time has to be set long enough to send/receive in both directions The time will vary depending upon distance, conditions, etc. We will examine this in more detail later.
Receives packets from network layer The packet is treated as pure data, even though it may contain data and control information. It is the data link’s job to transmit it to the receiver’s data link layer, which in turn gives it to the network layer. Data link encapsulates it in a frame by adding data link header and trailer to it. Appropriate checksums are computed and included in the frame. Elementary data link protocols
computes the checksum if checksum does not match send error message If checksum is ok notify sending data link layer Check all header trailer information If all OK strip off the header and trailer and pass the packet to the data link layer Receiving data link layer
Wait for an event to happen Fetch packet from the network layer for transmission on the channel Deliver information from inbound frame to network layer Go get an inbound frame from the physical layer Pass the frame to physical layer for transmission Start clocks and stop clocks as necessary etc, etc Pseudocode for data link layer routine
Sliding Window Protocols For full duplex two transmission lines are required One to send data and the other to receive data and/or ACKs Sending ACKs in a packet of its own is waste of resources May use piggybacking The acknowledgement is attached to an outgoing data frame(ack field in the frame header) Data link layer must wait until it has some thing to send before it can ack. This may cause time out and the frame may be re-sent.
The essence of sliding window protocols is that at any instant of time, the sender maintains a set of sequence numbers corresponding to frames it is permitted to send. These frames are said to fall within the sending window. Similarly, the receiver also maintains a receiving window corresponding to the set of frames it is permitted to accept. It is NOT necessary for the sender and receiver to have same number limits. Sequence numbers of frames sent but not acknowledged are kept in the senders window. Th receivers window corresponds to the frames it may accept.
One bit sliding window protocol uses stop and wait scheme, since the sender transmits a frame and waits for its acknowledgement before sending the next one. The acknowledgement field of the frame contains the number of the last frame received without error. If this number agrees with the sequence number of the frame the sender is trying to send, the sender knows that it is done and can proceed with the next frame.
A protocol using Go-Back-N Takes into account of round trip transmission time. Enough frames should be send to stuff the band with to avoid channel idling Consider a 50-kbps satellite channel with 500 m-sec round-trip propagation delay Let us send 1000 bit frames At t=0 starts sending the first frame, at t=20 msec the frames has been send Receiver fully receives only at t=270msc, and ack will be received at t=520 msec. Sender would be blocked 96% of time- only 4% of available band width was used.
Go-Back-N continued. Instead of sending 4 frames, the sender should have sent 26 frames. This will take t=520 msec. at t=520 the acknowledgement for the first frame will be received. The senders maximum window size is 26. This technique is called pipelining. If a frame in the middle of a stream gets damaged, what should the receiver do? Should all frames starting at that point be re-sent? Go-Back-N simply discards all frames including the damaged one, not sending acks. The time out at the sender will resend all subsequent frames. Can waste a lot of bandwidth in an unreliable channel.
Selective Repeat Receiver stores all good frames after the damaged frame. Notifies the sender of the bad frame, and sender re-sends that one. This technique requires the data link layer to have large amount of memory for the cache.
Examples of Data Link Protocols HDLC - HIGH LEVEL DATA LINK CONTROL DERIVED FROM IBM’S SNA - LAYER CALLED SDLC (SYNCHRONOUS DATA LINK PROTOCOL). ANSI MODIFIED IT TWICE FINALLY IT IS NOW HDLC CCITT MODIFIED HDLC AS A PART OF X.25 CALLED LAPB (LINK ACCESS PROCEDURE B) ALL ARE BIT ORIENTED PROTOCOLS FRAME EXAMPLE 01111110/ADDRESS/CONTROL/DATA/CHECKSUM/01111110 CONTROL FIELD IS FOR FRAME SEQUENCE NOS
DATA LINK LAYER IN THE INTERNET HOME USER DIALS UP THE INTERNET PROVIDER THE PC AT HOME FUNCTIONS AS A CHARACTER ORIENTED TERMINAL LOGGED INTO THE INTERNET SERVICE PROVIDER’S TIME SHARING SYSTEM. -This is called a SHELL ACCOUNT. CAN’T USE WWW. THE HOME PC CAN CALL THE INTERNET PROVIDER’S ROUTER AND THEN ACT LIKE A FULL-BLOWN INTERNET HOST. CAN USE WWW. TO DO THIS SOME POINT-TO-POINT DATA LINK PROTOCOL IS REQUIRED.
FRAMING, ERROR CONTROL AND OTHER DATA LINK LAYER FUNCTIONS TWO POINT-TO-POINT PROTOCOLS SLIP - SERIAL LINE IP PPP - POINT-TO-POINT PROTOCOL POINT-TO-POINT DATA LINK PROTOCOL
DEVISED IN 1984 TO CONNECT SUNS TO INTERNET USING MODEMS SLIP DOES NOT DO ERROR CORRECTION ONLY SUPPORTS IP -NOT IPX EACH SIDE MUST KNOW EACH OTHER’S IP ADDRESS IN ADVANCE SLIP DOES NOT PROVIDE USER AUTHENTICATION SLIP IS NOT AN INTERNET APPROVED STANDARD THEREFORE MANY VERSIONS EXIST SLIP (SERIAL LINE IP)
APPROVED INTERENT STANDARD HANDLES ERROR CORRECTION SUPPORTS MULTIPLE PROTOCOLS IP ADDRESS CAN BE NEGOTIATED AT CONNECT TIME PERMITS AUTHENTICATION A LINK PROTOCOL CALLED LCP (LINK CONTROL PROTOCOL) CONNECTS, NEGOTIATES, AND TERMINATES CONNECTIONS PPP - POINT TO POINT PROTOCOL
PC CALLS PROVIDER’S ROUTER VIA MODEM, ROUTER ESTABLISHES A PHYSICAL CONNECTION, THE PC THEN SENDS THE ROUTER A SERIES OF LCP (LINK CONTROL PROTOCOLS) TO NEGOTIATE PPP PERAMETERS TO BE USED. THEN A SERIES OF NCP(NETWORK CONTROL PROTOCOLS) ARE SENT TO CONFIGURE THE NETWORK LAYER. A NETWORK ADDRESS IS ISSUED - THE PROVIDER HAS A LIMITED NUMBER. IF TOTAL CUSTOMER CONNECTION EXCEEDS THE AVAILABLE NO OF IP ADDRESSES CAN’T CONTINUE. EXAMPLE OF PC CONNECTION