Lecture5_Link Layer1

Lecture5_Link Layer1 - I1070 / T2000 Intro to Telecom The...

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I1070 / T2000 Intro to Telecom The Data Link Layer Dr. Richard A. Thompson Telecommunications Program University of Pittsburgh
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Learning Objectives Describe the role and functions of the link layer Describe some common link layer protocols Explain the rationale for flow and error control Understand ARQ systems Describe how stop and wait and sliding window flow control work Explain and characterize the performance differences between these two mechanisms
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Lecture outline Link layer functions Flow Control Error Control Example of HDLC Inherited slides: provided, but skipped in class Media Access Control - principles
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OSI Reference Model Not a very good model Usually merge Layers 5-7 Implies layers are independent Functions move around
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Functions Confusing issues Link LAN Multiplexing in HW or SW? Functions evolve to different layers
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Link Between routers Which functions are needed? Internet R G/R G/R R G/R G/R uniform N N N Y Y N frag Y Y Y Y Y N mux Y N Y N N N
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LAN No internal IP Which functions are needed? Local Area Network H H G/R N Y Y Y Y Y H H H H Internet
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Software Multiplexing (Application Layer) Many app-to-app connections over the same host-to -host connection Which functions are needed? Y N N N N Y
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Internet Hardware Multiplexing (Physical Layer) - 2Rs Reconnect Physically Which functions are needed? M G/R G/R M G/R G/R Y N N N N Y a b y z 4 sub-channels: ay, az, by, bz R R
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Functions evolve to different layers Related Link quality Where? Poor Good Link wasted O/H Ends retransmit over entire network (many links)
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Systems Context What are the end to end protocols? What are the network layer protocols? How are network layer protocols different from link layer? End-to-end protocols Network layer protocol Link Layer Link Layer Link Layer
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Link Layer Functions Provide service interface to the network layer Addressing Flow control Error Control
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Services Provided to Network Layer Placement of the data link protocol. Remember, this might be a LAN; in which case, Layer 2 has additional work
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Link Layer Addresses Physical delivery address Need only be locally unique Typically known as the MAC address Often physically embedded in Network Interface Card (NIC)
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Finding a Host on a Subnet Hosts are known to a subnet (eg. LAN) by their local attachment address (eg. Ethernet or MAC address) An IP address does not help deliver a packet
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Solution: Make a table associating IP with network attachment addresses How? Broadcast unknown IP addresses to all
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Lecture5_Link Layer1 - I1070 / T2000 Intro to Telecom The...

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