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Chapter 3 Data Link Layer.docx

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Information Technology Management
ITM 301
Franklyn Prescod

Chapter 3 The Data Link Layer31 Data Link Layer Design IssuesThe data link layer uses the services of the physical layer to send and receive bits over communication channels It has a number of functions such as providing a well defined service interface to the network layer dealing with transmission errors and regulating the flow of data so that slow receivers are not swamped by fast senders Frames the data link layer takes the packets it gets from the network layer and encapsulates them into frames for transmission Each frame contains a frame header a payload field for holding the packet and a frame trailer Frame management forms the heart of what the data link does311 Services Provided to the Network LayerUnacknowledged Connectionless Service consists of having the source machine send independent frames to the destination machine without have the destination machine acknowledge them Ethernet is an example No logical connection is established beforehand or afterward If a frame is lose due to noise on the line no attempt is made to detect the loss or recover from it in the data link layer This is appropriate when the error rate is very low so recovery is left to higher layers Acknowledged Connectionless Service there are no logical connections used each frame sent is individually acknowledged The sender knows whether a frame has arrived correctly or has been lost It if has not arrived within a specified time interval it can be sent again Good for using in unreliable networks such as WiFi Connection Oriented Service the source and destination machines establish a connection before any data are transferred Each frame sent over the connection is numbered and it guarantees that each frame is received exactly once and that all frames are received in the right order It this provides the network layer processes with the equivalent of a reliable bit stream PhasesFirst Phase connection is established by having both sides initialize variables and counters needed to keep track of which frames have been received and which ones have not Second Phase one or more frames are actually transmittedThird Phase the connection is released freeing up the variables buffers and other resources used to maintain the connection312 Framing What the physical layer does is accept a raw bit stream and attempt to deliver it to the destination If the channel is noisy the physical layer will add some redundancy to its signals to reduce the bit error rate to a tolerable level However the bit stream received by the data link later is not guaranteed to be error free Some bits have different values and the numbers of bits received may be out of orderThe usual approach is for the data link layer to break up the bit stream into discrete frames compute a short token called a checksum for each frame and include the checksum in the frame when it is transmitted When a frame arrives at the destination the checksum is recomputed If the newly computed checksum is different from the one contained in the frame the data link layers knows that an error has occurredFour Methods of Breaking up Bit Streams 1 Byte Countuse a field in the header to specify the number of bytes in the frame When the data link layer at the destination sees the byte count it knows how many byes follow and hence where the frame ends The disadvantages are that the count can be garbled by transmission errors2 Flag Bytes with Byte Stuffing
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