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Link Layer Review (PowerPoint)

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					Link Layer Review

     CS176A
 Ramya Raghavendra
• Suppose two nodes start to transmit at the
  same time a packet of length L over a
  broadcast channel of Rate R. Denote the
  propagation delay between the two nodes
  as tprop. Will there be a collision if tprop <
  L/R? Why or why not?
• Transmission delay = L/R
• Propagation delay = d/s
        Link Layer Protocols
• Desirable Properties of a broadcast
  medium of rate R bps
  – When only one node has data to send, that
    node has a throughput of R bps.
  – When M nodes have data to send, each node
    should have R/M average rate
  – Decentralized
  – Simple and inexpensive to implement
               Slotted ALOHA
• How many of these desirable properties does
  ALOHA have?
  – All frames consist of L bits
  – Time is divided into slots of size L/R secs
  – Nodes start to transmit frames at the beginning of a
    time slot
  – Nodes are synchronized (Why? )
  – Nodes detect collision before the time slot ends

  – When node has a frame to send, transmits at the
    beginning of the next slot
  – If there is a collision, retransmits with probability
                Ethernet
     A            B               C




• A sends IP datagrams to B, encapsulating
  MAC address to B, does C receive it?
• Do the IP datagrams get passed to C?
• What if A sent broadcast packets?
                     ARP
•   IP addresses at the network layer
•   MAC addresses at the link layer
•   Need to translate
•   Address Resolution Protocol
    – same subnet
• Broadcast ARP Packet
• Unicast Reply
• Why?
                      ARP (Cont)


                                    R




R has 2 ARP modules
Can a MAC address appear on both?
                        CSMA
1. Carrier Sensing
2. Collision Detection
• CSMA/CD
    –   Ethernet
    –   Senders detect collision
    –   Can this be possible in all media? Wireless?
•   CSMA/CA
    –   Intent to transmit
    –   Clear notification
    –   Acknowledgement
                CSMA/CA
• Adapter waits K * 512 bit-times, K being
  random, when it detects collision
• K=100, how long does it wait for
  – 10 Mbps Ethernet
  51200 bit-times =>
  [51.2 * 10^3] / [10 * 10 ^6] = 5.12 ms
  – 100 Mbps = 512 microsec

				
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posted:8/14/2011
language:English
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