Link-layer EE122 Discussion 10/14/2011

MAC Addresses Link-layer deals with MAC addresses – 48-bit addresses, flat name space – E.g., 88-9F-FA-F – Use ipconfig (Windows) or ifconfig (Linux)

Address Resolution Protocol “I want to reach IP address a.b.c.d. What is the MAC address of the corresponding machine?” – Broadcast Response from appropriate machine – Cache mappings …default, use the gateway router

Question 1 1.Use subnet mask; not in same network 2.ARP: - “Who has ?” 3.ARP Response: - “I do” 4.A  5.ARP: - “Who is ?” 6.ARP Response: - “I do” 7.R 

Multiple Access “How can multiple hosts share a medium?” Channel Partitioning – Access to channel in fixed slots (TDMA/FDMA) – Easy to implement and reason about

Random Access Carrier Sense Multiple Access (CSMA/CD) 1.Listen and transmit only if free 2.If collisions detected, abort transmission – Why would there be collisions?? 3.Resend after a probabilistic wait

Limitations of length Collision is detected after twice the propagation delay – Keep transmitting in that interval Frame needs to be big enough!

Efficiency of CSMA/CD Time wasted in collisions – Proportional to distance d Time spend transmitting a packet – Packet length p divided by bandwidth b Rough estimate for efficiency (k some constant) = (p/b) / ((p/b) + kd)

Binary Exponential Back-off Hold on for a while after you detect collision – Introduce some randomness After each collision, choose K randomly from {0, …, 2 m-1 } – m is the #collisions since last successful transmission – Wait for K * 512 bit times before trying again

Question 3 a)p is low  lower probability of collisions  CSMA/CD is good a)p is high  high probability of collisions  channel partitioning is better

Question 4 Host must send data at least for twice propagation delay Propagation Delay = (d/2*10 8 ) Min size of frame ≥ 2 * (d/2*10 8 ) * 1Gbps …d ≤ 100m

Question 5

Question 6 Efficiency of CSMA/CD is (p/b) / ((p/b) + kd) As link speed increases, the efficiency goes down – (c)