1 / 13

Link-layer

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-F9-74-75 Use ipconfig (Windows) or ifconfig (Linux). Address Resolution Protocol.

imelda
Download Presentation

Link-layer

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Link-layer EE122 Discussion 10/14/2011

  2. MAC Addresses • Link-layer deals with MAC addresses • 48-bit addresses, flat name space • E.g., 88-9F-FA-F9-74-75 • Use ipconfig(Windows)orifconfig (Linux)

  3. 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

  4. Question 1 • Use subnet mask; not in same network • ARP: <MACA, Bcast> - “Who has 10.1.1.7?” • ARP Response: <MACR, MACA> - “I do” • A <MACA, MACR, 10.1.1.4, 198.56.7.99> • ARP: <MACR, Bcast> - “Who is 198.56.7.99?” • ARP Response: <MACB, MACR> - “I do” • R <MACR, MACB, 10.1.1.4, 198.56.7.99>

  5. 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

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

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

  8. 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)

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

  10. Question 3 • p is low  lower probability of collisions  CSMA/CD is good • p is high  high probability of collisions  channel partitioning is better

  11. Question 4 • Host must send data at least for twice propagation delay • Propagation Delay = (d/2*108) • Min size of frame ≥ 2 * (d/2*108) * 1Gbps • …d ≤ 100m

  12. Question 5 • Transmission delay for one bit in 10Mbps link is 0.1us • Ethernet’s back off is K * 512 bit times where K Ɛ {0, 23-1} • Expected back-off = (1/8) = 179.2µs

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

More Related