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Lecture2 - DLC - Lecture 2 6.263/16.37 The Data Link Layer...

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Lecture 2 6.263/16.37 The Data Link Layer: Framing and Error Detection Eytan Modiano MIT, LIDS Eytan Modiano Slide 1
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Data Link Layer (DLC) Responsible for reliable transmission of packets over a link Framing: Determine the start and end of packets (sec 2.5) Error Detection: Determine when a packet contains errors (sec 2.3) Error recovery: Retransmission of packets containing errors (sec 2..4) DLC layer recovery May be done at higher layer Eytan Modiano Slide 2
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Framing _____________________________________ 010100111010100100101010100111000100 Where is the DATA?? Three approaches to find frame and idle fill boundaries: 1) Character oriented framing 2) Length counts - fixed length 3) Bit oriented protocols (flags) Eytan Modiano Slide 3
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Character Based Framing Frame SYN Packet SYN Header STX SYN SYN CRC ETX SYN is synchronous idle STX is start text ETX is end text Standard character codes such as ASCII and EBCDIC contain special communication characters that cannot appear in data Entire transmission is based on a character code Eytan Modiano Slide 4
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Issues With Character Based Framing Character code dependent How do you send binary data? Frames must be integer number of characters Errors in control characters are messy NOTE: Primary Framing method from 1960 to ~1975 Eytan Modiano Slide 5
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Length field approach (DECNET) Use a header field to give the length of the frame (in bits or bytes) Receiver can count until the end of the frame to find the start of the next frame Receiver looks at the respective length field in the next packet header to find that packet’s length Length field must be log 2 (Max_Size_Packet) + 1 bits long This restricts the packet size to be used Issues with length counts Difficult to recover from errors Resynchronization is needed after an error in the length count Eytan Modiano Slide 6
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Fixed Length Packets (e.g., ATM) All packets are of the same size In ATM networks all packets are 53 Bytes Requires synchronization upon initialization Issues: Message lengths are not multiples of packet size Last packet of a message must contain idle fill (efficiency) Synchronization issues Fragmentation and re-assembly is complicated at high rates Eytan Modiano Slide 7
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Bit Oriented Framing (Flags)
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