CS4450 Lecture 6: Data Link Layer, Lecture notes of Network Technologies and TCP/IP

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Computer Networks:

Architecture and Protocols

CS

Lecture 6

Data Link Layer

Rachit Agarwal

Announcements

• Exam 1 on 09/
  • Material: everything covered until Wed lecture
    • Slides, Problem set 1, Problem set 2 (up to Question 4)
  • Infinite time, should be doable in ~90 minutes
  • Open-notes, open-book, open-Internet, open-everything, except…
  • Talking to any human or alien
• Exam structure
  • Several conceptual questions
  • Several “problems” (e.g., Q1 and Q2 on pset 2 were in past exams)
• For all those who declared their conflicts
  • We have already sent an email; please respond by tomorrow
  • If we missed you, meet me after the lecture today

Context for Today’s Lecture

• You now understand
  • Network sharing (in depth)
  • Architectural principles (in depth)
  • Design goals for the Internet (& computer networks, in depth)
  • End-to-end working of the Internet (at a high-level)
• Now, time to dive deeper:
  • Link Layer (~1 week)
  • Network Layer (~4 weeks)
  • Transport Layer (~3 weeks)
• Today:^ Link layer

Goals for Today’s Lecture

• Link layer:
  • Broadcast medium
  • Sharing broadcast medium
  • Carrier Sense Multiple Access - Collision Detection (CSMA/CD)

• Two types of communication mediums
  • 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

Point-to-Point vs. Broadcast Medium

• Point-to-point:^ dedicated^ pairwise communication
  • E.g., long distance fiber link
  • E.g., Point-to-point link between two routers
• Broadcast:^ shared^ wire or medium
  • Traditional Link Layer (Ethernet)
  • 802.11 wireless LAN

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 descnacon

Where it all Started: AlohaNet

• Norm Abramson:
  • Left Stanford in 1970
    • So he could SURF
  • Set up first data communication system for Hawaiian islands
  • Central hub at University of Hawaii, Oahu

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!

• Frequency sharing
  • Divide the channel into^ frequencies
  • Every source is assigned a subset of frequencies
    • And transmits data only on its assigned frequency
• Goods: no collisions
• Not-so-good:
  • A source may have nothing to send (frequency wasted)
  • Interference may cause disruption
  • Hard to implement for wired networks
• Used in many wireless networks
  • E.g., radio

Frequency-Division Multiple Access (FDMA)

Random Access

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

Life lesson:

If you want to invent great things,

go to Hawaii :-)

LETS TRY!

Multiple source-destination pairs

Design a protocol that allows sharing the broadcast medium

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

CSMA (Carrier Sense Multiple Access)

• CSMA:^ listen^ before transmit
  • If channel sensed idle: transmit entire frame
  • If channel sensed busy: defer transmission
• Human analogy: don’t interrupt others!
• Does this eliminate all collisions?
  • No , because of nonzero propagation delay
• Solution:
  • Include a^ Collision Detection (CD)^ mechanism
  • If a collision detected
    • Retransmit