Lecture 6 Data Link Layer Rachit Agarwal, Summaries of Communication

Link layer: • Broadcast medium. • Sharing broadcast medium. • Carrier Sense Multiple Access - Collision Detection (CSMA/CD). 7. Page 8. • Communication Medium.

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2022/2023

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Computer)Networks:)
Architecture)and)Protocols
CS4450
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Rachit'Agarwal
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Download Lecture 6 Data Link Layer Rachit Agarwal and more Summaries Communication in PDF only on Docsity!

Computer Networks:

Architecture and Protocols

CS

Lecture 6

Data Link Layer

Rachit Agarwal

Announcements

  • We will have a “live” coding class on 02/
    • Please bring your laptops
    • Its gonna be a lot of fun!
    • We will learn how to implement sockets, etc.
  • Please read Chapter 1 of the textbook!
    • And try to solve problems at the end of Chapter 1 of the textbook
      • For extra practice
      • Ask us questions on Piazza
  • In-class Quiz policy
    • We understand that sometimes its impossible to attend a lecture
    • Email AT LEAST AN HOUR BEFORE the lecture if you can’t attend
    • If legitimate reasons, we will ignore that quiz for you
  • How to break system into modules
    • Layering
  • Where are modules implemented
    • End-to-End Principle
  • Where is state stored?
    • Fate-Sharing

Recap: Three design principles

  • Build something that works
  • Connect existing networks
  • Robust in face of failures
  • Support multiple types of delivery service
  • Accommodate a variety of networks
  • Allow distributed management
  • Easy host attachment
  • Cost effective
  • Allow resource accountability

Recap: Internet Design Goals

Goals for Today’s Lecture

  • Link layer:
    • Broadcast medium
    • Sharing broadcast medium
    • Carrier Sense Multiple Access - Collision Detection (CSMA/CD)
  • Communication Medium
    • Point-to-point
      • The high-level ideas discussed so far were for point-to-point
    • Broadcast
      • Original design of Link layer protocols
      • More recent versions have moved to point-to-point
        • We will discuss why so!
  • Network Adapters (e.g., NIC — network interface card)
    • The hardware that connects a machine to the network
    • Has a “name” — MAC (Medium access control) address

Data Link Layer

  • Ever been to a party?
    • Tried to have an interesting discussion?
  • Fundamental challenge?
    • Collisions

Data Link Layer: Broadcast (until ~2000s)

Broadcast Medium: Desirable properties

  • One and only one: data delivery
  • How do we design a broadcast medium protocol for data delivery? link-layer “protocol” source NIC NIC (^) NIC desanaaon

Aloha Signaling

  • Two channels: random access, broadcast
  • Sites send packets to hub
    • Random access channel
    • Each site transmits packets at “random” times
    • If a packet not received (due to collision), site resends
  • Hub sends packets to all sites
    • Broadcast channel
    • Sites can receive even if they are also sending
  • Challenge: Requires a centralized hub
    • If the hub fails, the entire network fails
    • Not always a good design

Sharing a broadcast channel

  • Context: a shared broadcast channel
    • Must avoid/handle having multiple sources speaking at once
    • Otherwise collisions lead to garbled data
    • Need distributed algorithm for sharing channel
    • Algorithm determines when and which source can transmit
  • Three classes of techniques
    • Frequency-division multiple access : divide channel into pieces
    • Time-division multiple access : divide channel into time slots
    • Random access : allow uncoordinated access
      • Detect collisions, and if needed, recover from collisions
      • More in the Internet style!
  • Time sharing
    • Divide time into^ slots
    • Divide data into^ frames
      • Such that a frame can be transmitted in one slot
    • Every source is assigned a subset of slots
      • And transmits a frame only in its assigned slot
  • Goods: no collisions
  • Not-so-good: Underutilization of resources
    • During a slot, a source may have nothing to send
    • When the source has something to send, wait for its slot

Time-Division Multiple Access (TDMA)

Random Access

  • Bob Metcalfe:
    • Xerox PARC
    • Visits Hawaii, and gets the idea
    • Shared wired medium

Link Layer (MAC) Protocol

  • When source has a frame to send
    • Transmit at full bandwidth
    • No a priori coordination among nodes
  • Two or more transmitting sources => collision
    • Frame lost
  • Link-layer protocol specifies:
    • How to detect collision
    • How to recover from collisions

LETS TRY!

Group Exercise:

Multiple source-destination pairs

Design a protocol that allows sharing the broadcast medium

link-layer “protocol” source Adapter source Adapter (^) Adapter desanaaon