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ECE155ALecture2

ECE155ALecture2 - Computer Networks Lecture 2 Professor...

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1 Computer Networks Lecture 2 Professor Louise E. Moser Winter 2010
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2 Service vs. Protocol Service - A set of primitive operations that a layer provides to the layer above it Protocol - A set of rules that govern the format and meaning of frames, packets, and messages that are exchanged by peers within a layer
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3 Primitive Service Operations Request - Requester wants to establish connection or send data Indication - Requester announces that it wants to set up a connection Response - Responder accepts or rejects Confirm - Requester is informed about its request Parameters of Primitives Machine to connect to, type of service desired, max message size to be used, etc. Negotiated as part of protocol EX: Always choose smaller of two message sizes
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4 Quality of Service (QoS) Reliable Service “Never” loses data Usually uses acks which incur substantial overhead Unreliable or Datagram Service Data received with high probability EX: Voice and video Request-Reply Service Sender sends requests and waits for reply from receiver EX: Request a file from a file server
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5 Reference Models ISO/OSI Reference Model ISO – International Standards Organization OSI – Open Systems Interconnection Open for communication with other systems TCP/IP Reference Model TCP – Transmission Control Protocol IP – Internet Protocol A Comparison of ISO/OSI and TCP/IP Reference Models
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6 ISO/OSI Reference Model Distinguishes Interface - Tells upper layer how to access services of lower layer Layer - Implements protocols and mechanisms Distinguishes Specifications - Requirements Implementation - Code to carry out specs Distinguishes Services - What layer does Protocols - How layer does it
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7 The ISO/OSI Reference Model
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8 1 - The Physical Layer Transmits raw bits over the communication medium No regard for meaning or structure Ensures that, if a 0/1 bit is sent, then a 0/1 bit is received Design issues concern Electrical, mechanical, and procedural interfaces Physical communication medium
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9 2 - The Data Link Layer Takes raw bits and ensures that they’re free of transmission errors Breaks data into frames (a few hundred bytes) Transmits frames sequentially Processes ack frames sent back by receiver Creates and recognizes frame boundaries by attaching special bit patterns to beginning and end of frame Retransmits lost, corrupted, and duplicated frames
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