Lecture 6 Data Link Layer Rachit Agarwal, Summaries of Communication

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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