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