1. CMPE 150- Introduction to Computer Networks 1 CMPE 150 Fall 2005 Lecture 17 Introduction to Computer Networks

2. CMPE 150- Introduction to Computer Networks 2 Announcements Midterm on 11.04. –In class, closed books/notes. Homework 3 is up. –Due on 11.07.05. Lab this week: discussion/review sessions for midterm.

3. CMPE 150- Introduction to Computer Networks 3 Today MAC (cont’d).

4. CMPE 150- Introduction to Computer Networks 4 Types of MAC Control: –Distributed. –Centralized. How they coordinate medium access: –Round-robin. –Scheduled-access. –Contention-based.

5. CMPE 150- Introduction to Computer Networks 5 Round-Robin MAC Centralized: polling. Distributed: token passing.

6. CMPE 150- Introduction to Computer Networks 6 Scheduled Access MAC Time divided into slots. Station reserves slots in the future. Multiple slots for extended transmissions. Suited to stream traffic.

7. CMPE 150- Introduction to Computer Networks 7 Scheduled-Access MAC: Example The basic bit-map protocol.

8. CMPE 150- Introduction to Computer Networks 8 Contention-Based MAC No control. Stations try to acquire the medium. Distributed in nature.

9. CMPE 150- Introduction to Computer Networks 9 MAC Protocols Contention-based –ALOHA and Slotted ALOHA. –CSMA. –CSMA/CD. Round-robin : token-based protocols. –Token bus. –Token ring.

10. CMPE 150- Introduction to Computer Networks 10 Contention-Based MACs ALOHA family. CSMA family.

11. CMPE 150- Introduction to Computer Networks 11 ALOHA Protocols: Performance Throughput versus offered traffic for ALOHA systems.

12. CMPE 150- Introduction to Computer Networks 12 ALOHA Protocols: Summary Simple. But, poor utilization… –When?

13. CMPE 150- Introduction to Computer Networks 13 CSMA Protocols

14. CMPE 150- Introduction to Computer Networks 14 Carrier Sense Multiple Access The capacity of ALOHA or slotted ALOHA is limited by the large vulnerability period of a packet. By listening before transmitting, stations try to reduce the vulnerability period to one propagation delay. This is the basis of CSMA (Kleinrock and Tobagi, UCLA, 1975).

15. CMPE 150- Introduction to Computer Networks 15 CSMA Station that wants to transmit first listens to check if another transmission is in progress (carrier sense). If medium is in use, station waits; else, it transmits. Collisions can still occur. Transmitter waits for ACK; if no ACKs, retransmits.

16. CMPE 150- Introduction to Computer Networks 16 Collisions Can collisions still occur?

17. CMPE 150- Introduction to Computer Networks 17 CSMA Flavors 1-persistent CSMA (IEEE 802.3) –If medium idle, transmit; if medium busy, wait until idle; then transmit with p=1. –If collision, waits random period and starts again. Non-persistent CSMA: if medium idle, transmit; otherwise wait a random time before re-trying. –Thus, station does not continuously sense channel when it is in use. P-persistent: when channel idle detected, transmits packet in the first slot with p. –Slotted channel, i.e., with probability q = p-1, defers to next slot.

18. CMPE 150- Introduction to Computer Networks 18 CSMA vesrsus Aloha Comparison of the channel utilization versus load for various random access protocols.

19. CMPE 150- Introduction to Computer Networks 19 CSMA/CD CSMA with collision detection. Problem: when frames collide, medium is unusable for duration of both (damaged) frames. For long frames (when compared to propagation time), considerable waste. What if station listens while transmitting?

20. CMPE 150- Introduction to Computer Networks 20 CSMA/CD Protocol 1. If medium idle, transmit; otherwise 2. 2. If medium busy, wait until idle, then transmit with p=1. 3. If collision detected, transmit brief jamming signal and abort transmission. 4. After aborting, wait random time, try again.

21. CMPE 150- Introduction to Computer Networks 21 CSMA/CD Performance Wasted capacity restricted to time to detect collision. Time to detect collision < 2*maximum propagation delay. Rule in CSMA/CD protocols: frames long enough to allow collision detection prior to end of transmission. Thus frames need to be at least “2*RTT” long.

22. CMPE 150- Introduction to Computer Networks 22 CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle.

23. CMPE 150- Introduction to Computer Networks 23 Ethernet

24. CMPE 150- Introduction to Computer Networks 24 Ethernet IEEE 802. family. Standards for LANs and MANs. Ethernet defined in the IEEE 802.3 standard. PHY, MAC, and LLC.

25. CMPE 150- Introduction to Computer Networks 25 Where in the Stack? (a) Position of LLC. (b) Protocol formats.

26. CMPE 150- Introduction to Computer Networks 26 Ethernet MAC CSMA/CD. Binary exponential back-off.

27. CMPE 150- Introduction to Computer Networks 27 Ethernet MAC (Cont’d)

28. CMPE 150- Introduction to Computer Networks 28 Ethernet Frame Length At 10Mbps with 2,500 m maximum distance: –RTT ~ 50 microsec. –Thus, at least 500-bit frames. –It is actually 512 bits. –If fewer bits than that, add “padding”.

29. CMPE 150- Introduction to Computer Networks 29 Ethernet Frame Frame formats. (a) DIX Ethernet, (b) IEEE 802.3. Destination address: “1” for “group” addresses. Type: mux/demux of network layer protocols. Data:max. of 1500 bytes.

30. CMPE 150- Introduction to Computer Networks 30 Binary Exponential Backoff Randomization after collision.

31. CMPE 150- Introduction to Computer Networks 31 BEB (Cont’d) After first collision, each station waits for 0 or 1 slot before trying again. After second collision, they pick either 0, 1, 2, or 3 slots at random to wait. After 3 rd. collision, number of slots to wait is between 0 and 2 3 -1. In general, after I collisions, wait is between 0 and 2 i – 1. After 10 collisions, randomization interval frozen at 1023 slots. After 16 collisions, error!

32. CMPE 150- Introduction to Computer Networks 32 Ethernet Performance

33. CMPE 150- Introduction to Computer Networks 33 Ethernet Cabling The most common kinds of Ethernet cabling.

34. CMPE 150- Introduction to Computer Networks 34 Ethernet Cabling (Cont’d) Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.

35. CMPE 150- Introduction to Computer Networks 35 Ethernet Topologies Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

36. CMPE 150- Introduction to Computer Networks 36 Switched Ethernet A simple example of switched Ethernet.

37. CMPE 150- Introduction to Computer Networks 37 Fast Ethernet The original fast Ethernet cabling.

38. CMPE 150- Introduction to Computer Networks 38 Gigabit Ethernet (a) A two-station Ethernet. (b) A multistation Ethernet.

39. CMPE 150- Introduction to Computer Networks 39 Gigabit Ethernet (Cont’d) Gigabit Ethernet cabling.