ECE155ALecture5

ECE155ALecture5 - Computer Networks Lecture 5 Professor...

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1 Computer Networks Lecture 5 Professor Louise E. Moser Winter 2010
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2 Network Architecture The network architecture used in the textbook and this course
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3 Data Link Layer (DLL) Services Provides Service to the Network Layer (NL) above it Point-to-point, source-to-destination Types of Service Unacked connectionless OK if low error rate, real-time Acked connectionless Connection-oriented Aim: Each frame sent is received exactly once and in the right order Three Phases of Connection-Oriented Connection established Variables initialized Frames transmitted Connection released Buffers, variables, etc freed
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4 Data Link Layer Functions Framing Physical bit stream divided up into frames Error Detection and Correction Nacks and/or Acks Nacks – If sender gets Nack from receiver, it transmits frame again Acks – Sender has a timer – If sender does not get Ack from receiver within a timeout, it transmits frame again If frame transmitted multiple times, receiver might get several copies Use sequence numbers to detect and suppress duplicates Flow Control (Sliding Window Protocols) Sender might transmit frames faster than receiver can receive them Throttle sender, so it transmits frames no faster than receiver can receive them
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5 Communication between NL and DLL (a) Virtual (logical) communication (b) Actual (physical) communication
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6 Communication between NL and DLL Four Primitives Request Indication Response Confirm Network Layer Data link Layer Physical Layer Request Confrm Response Indication Service user (Network Layer) Service user (Network Layer) Service provider (Data link Layer) A B
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7 Relationship between Packets and Frames Packet – NL Protocol Data Unit Frame – DLL Protocol Data Unit
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8 DLL Process vs. NL Process When frame arrives at router Hardware verifies checksum to detect errors Hardware passes frame to DLL software DLL software checks if frame is one expected and, if so, gives packet in payload of frame to NL routing software NL software chooses appropriate outgoing line NL software passes packet back down to DLL software DLL software transmits it
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9 Framing DLL divides a physical bit stream into frames Source computes a checksum and includes it in the header of the frame Destination re-computes the checksum and checks against the checksum in the header Discards or recovers bad frames, notifies source
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10 Framing How does DLL form frames? Insert time gaps between frames Too risky, no timing guarantees, not used Character count Flag bytes with byte stuffing Flag bytes with bit stuffing Physical layer coding
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