Lecture04 - Data Link Layer

Lecture04 - Data Link Layer - 10/5/2009 1 CNIT 24000 CNIT...

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Unformatted text preview: 10/5/2009 1 CNIT 24000 CNIT 24000 Data Data Communications & Communications & Networking Networking The Data Link Layer Administrivia Exam I Sept 30 2 10/5/2009 2 Objectives Build on the knowledge of the Physical Layer Continue to develop our understanding of the OSI model Understand how the major pieces of the Data Link layer interoperate Differentiate between major deployment types Understand the different types of LAN hardware and their interactions 3 The Data Link Layer Provides Point-to-Point connectivity between devices over the physical connections provided by the underlying physical layer Data is formatted into frames Frames vs Cells Frames vs Packets Provides reliable communications Error Detection Error Correction Flow Control 4 10/5/2009 3 Error Handling Error Prevention Signal-based approach Protocol-based approach Error Detection The process of guaranteeing that the received signals are the exact same as the transmitted signals Sender & receiver must agree on detection method Sender calculates error check and transmits with data Receiver recalculates error check upon reception of data Error check values are compared Next action is based on that comparison 5 Error Detection Parity (Vertical Redundancy Check) Simplest form of error detection Even vs Odd Parity Longitudinal Redundancy Check Two-dimensional detection that overcomes inherent weakness of VRC 6 10/5/2009 4 Error Detection cont Checksum Less overhead than LRC (fewer calculations) Block-oriented summation of character values modulo 255 Cyclic Redundancy Check Entire block is treated as a single binary value Divide by predetermined prime number of specified length The most accurate of detection methods 7 Error Correction Once an error is detected, there must be a method to provide the correct data to the receiver This method must be exactso no guessing as to the intent of the received data Receiver must detect error & request retransmission Sender must retransmit the erred transmission We want to minimize: The process for requesting retransmission The amount of data retransmitted The amount of time elapsed from original transmission to reception of accurate retransmission 8 10/5/2009 5 Error Correction cont ARQ Automatic Retransmission Request Discrete ARQ ACK/NAK Stop and Wait Continuous ARQ ACK/NAK Sliding Window & Sequence Numbers...
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Lecture04 - Data Link Layer - 10/5/2009 1 CNIT 24000 CNIT...

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