dlc_general - Data Link Control Layer General Concepts...

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Data Link Control Layer Data Link Control Layer General Concepts General Concepts Rudra Dutta ECE/CSC 570 - Fall 2007, Section 001
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Copyright Spring 2007, Rudra Dutta, NCSU 2 Data Link Layer Perspective Data Link Layer Perspective Second of the OSI layers Utilizes (unreliable) bitpipe Provides service to Networking layer Equipments: switch, bridge, … PDUs: “Frames”
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Copyright Spring 2007, Rudra Dutta, NCSU 3 Data Link Layer Services Data Link Layer Services Services Provided to the Network Layer Framing Logical bit groupings – more use at higher layers Error Control Error control overlaps with physical layer DLC – retransmission strategies Flow Control Matches dissimilar endpoint processing speeds Slow receiver should not be swamped by fast sender Also a backward error correction mechanism Optionally, mediate contention for shared medium Such as Ethernet
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Copyright Spring 2007, Rudra Dutta, NCSU 4 Services Provided to Network Layer Services Provided to Network Layer DLC fronts the transmission link DLC PHY NET DLC PHY DLC PHY NET A R D DLC PHY NET
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Copyright Spring 2007, Rudra Dutta, NCSU 5 Utilizing Unreliable Bitpipe Utilizing Unreliable Bitpipe Sometimes sender sends bits, sometime not Bitpipe can sometime lose bits, or “manufacture” bits How does the receiver (receiving DLC) know when bits are being received, and when not? Must create own (DLC layer) convention (protocol) about transmission of bits Start with preamble, end with conclusion Creates logical groups at DLC layer - “frames” Also serves as logical groups to encapsulate higher layer PDUs Higher layers want service in multiple-byte chunks Logical units determined by the logic of the higher layers Frame delineation - must start with (and end with) an easily recognizable set of bits that are unlikely to arise randomly due to noise
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Copyright Spring 2007, Rudra Dutta, NCSU 6 Framing with FLAGs Framing with FLAGs FLAG is a distinctive bit pattern Such as 01111110 (01 6 0) Must not appear inside data
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Copyright Spring 2007, Rudra Dutta, NCSU 7 Bit Stuffing Bit Stuffing Bit stuffing method Delineate by pattern of many bits Prevent pattern from occurring in data by few bits Must be completely reversible, i.e. destuffable
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Copyright Spring 2007, Rudra Dutta, NCSU 8 Error Control Error Control Errors happen Dual concerns from a communication POV Detection Can at least catch errors ARQ strategies may come in Correction (FEC) Better if we can Sometimes essential due to link characteristics Involves error codes Coding, not encryption
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Copyright Spring 2007, Rudra Dutta, NCSU 9 Flow Control Flow Control Potential data generation and transmission
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dlc_general - Data Link Control Layer General Concepts...

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