1. CMPE 150 – Winter 2009 Lecture 8 January 29, 2009 P.E. Mantey

2. CMPE 150 -- Introduction to Computer Networks Instructor: Patrick Mantey mantey@soe.ucsc.edu http://www.soe.ucsc.edu/~mantey/ mantey@soe.ucsc.edu Office: Engr. 2 Room 595J Office hours: Tues 3-5 PM, Mon 5-6 PM* TA: Anselm Kia akia@soe.ucsc.edu Web site: http://www.soe.ucsc.edu/classes/cmpe150/Winter09/ Text: Tannenbaum: Computer Networks (4 th edition – available in bookstore, etc. )

3. Syllabus

4. Assignment #4 Available on the web site: http://www.soe.ucsc.edu/classes/cmpe150/ Winter09/ Due Tuesday, February 3, 2009

5. Today’s Agenda LAN topologies Wireless 802.11 802.16 Bluetooth Wireless USB Repeaters, Hubs, Switches, Routers, … VLANS

6. http://www.javvin.com/protocolDQDB.html

7. LAN Topologies Star Central node Ring Bus Tree

8. Bus Topology Use of multipoint medium. Stations attach to bus through tap. –Full-duplex communication allows data to be sent to/received from bus. Transmission from any station propagates in both directions and is received by all. –At each end, terminator absorbs and removes signal from bus.

9. Tree Topology Tree is generalization of bus. Headend: start of 1 or more cables (branches). Transmission from one station propagates to all others.

10. Issues Inherently broadcast. –Frames to transmit data. –Need for specifying the destination. –Addresses. Multi-access. –Need for controlling access to medium. Avoid collisions. MAC protocol.

11. Ring Topology Stations attach to repeaters. Repeaters are linked to each other by point-to-point links forming a closed loop. Links are unidirectional. Repeaters: receive data from one link and repeat it on the other with no buffering.

12. Ring Stations transmit/receive via repeater. Frames circulate past all stations; destination copies frame as it goes by; source removes frame. Ring shared by multiple stations. –Need MAC protocol. Determine when each station may insert frame.

13. Star Topology Each station directly connected to central node via point-to-point link. Central node’s modes of operation: –Broadcast mode: node broadcasts received frame on all other links; logically works like bus. –Switching mode: node sends frame out only on the link to the destination. Central node as single-point of failure.

14. Medium Access Control Control access to shared medium. Where and how? –Where: centralized versus decentralized. –How: synchronous versus asynchronous.

15. Centralized versus Distributed MAC Centralized approaches: –Controller grants access to medium. –Simple, greater control: priorities, qos. –But, single point of failure and performance bottleneck. Decentralized schemes: –All stations collectively run MAC to decide when to transmit.

16. Synchronous versus Asynchronous Synchronous approaches: –Static channel allocation. –Examples: FDM, TDM. –Simple but inefficient. Asynchronous or dynamic: –Example: STDM. –3 categories: round-robin, reservation, and contention.

17. Round-Robin MAC Each station is allowed to transmit; station may decline or transmit (bounded by some maximum transmit time). Centralized (e.g., polling) or distributed control of who is next to transmit. When done, station relinquishes and right to transmit goes to next station. Efficient when many stations have data to transmit over extended period (stream).

18. Reservation Time divided into slots. Station reserves slots in the future. Multiple slots for extended transmissions. Suited to stream traffic.

19. Contention No control. Stations try to acquire the medium. Distributed in nature. Perform well for bursty traffic. Can get very inefficient under heavy load. NOTE: round-robin and contention are the most common.

20. Standardized MACs Topologies Bus Ring Round robin Reservation Contention Token bus (802.4) Polling (802.11) DQDB (802.6) CSMA/CD (802.3) CSMA(802.11) Token ring (802.5; FDDI) Techniques

21. LLC for LANs Similar functions as general LLCs. But it has to interface with MAC sublayer. LLC functions: –Addressing: source and destination. LLC address versus MAC address. –Control data exchange between 2 users. User as higher-layer protocol in the station.

22. Encapsulation Application data header TCP IP LLC MAC trailer TCP segment IP datagram LLC PDU MAC frame

23. LLC Services 3 different services: 1.Unacknowledged connectionless (type 1). No error or flow control. No delivery guarantees. 2.Connection-mode (type 2). Logical connection established. Flow and congestion control provided. 3.Acknowledged connectionless (type 3). No logical connection. Flow and error control.

24. IEEE 802 Standards The 802 working groups. The important ones are marked with *. The ones marked with  are hibernating. The one marked with † gave up.

25. LLC (802.2) Protocol –Similar to HDLC (ISO standard). –LLC PDU (protocol data unit): DSAPSSAPLLC control Information 1 byte 1 or 2 bytes variable

26. Wireless LANs Use wireless transmission media. –Infrared (IR): limited to indoors and single room (IR is light -- doesn’t penetrate walls). –Radio Narrowband microwave. Spread Spectrum LANs.

27. Wireless LAN Applications –Nomadic access (e.g., users roaming around campus). –LAN interconnection (e.g., across buildings). –Ad Hoc Networks (e.g., disaster relief crew). –Sensor Networks

28. MAC Protocols Contention-based –ALOHA and Slotted ALOHA. –CSMA. –CSMA/CD. Round-robin : token-based protocols. –Token bus. –Token ring.

29. CSMA Maximum utilization is function of frame size and propagation time. –Longer frames or shorter propagation time, higher utilization.

30. 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. P-persistent: when channel idle detected, transmits packet in the first slot with probability = p.

31. 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. Why not listen while transmitting?

32. 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.

33. CSMA with Collision Detection If collision detected, immediately abort transmission CSMA/CD can be in one of three states: contention, transmission, or idle.

34. CSMA/CD Performance Wasted capacity restricted to time to detect collision. Time to detect collision < 2*maximum propagation delay. (page 258 of Tannenbaum Text) Rule in CSMA/CD protocols: frames long enough to allow collision detection prior to end of transmission. (need to be still listening) CSMA/CD is thus half-duplex

35. Persistent and Nonpersistent CSMA Comparison of the channel utilization versus load for various random access protocols.

36. IEEE 802 Standards The 802 working groups. The important ones are marked with *. The ones marked with  are hibernating. The one marked with † gave up.

37. IEEE 802.2: Logical Link Control (a) Position of LLC. (b) Protocol formats.

38. Wireless LANs The 802.11 Protocol Stack The 802.11 Physical Layer The 802.11 MAC Sublayer Protocol The 802.11 Frame Structure Services

39. Wireless LANs (a) Wireless networking with a base station. (b) Ad hoc networking.

40. Wireless LANs (2) The range of a single radio may not cover the entire system.

41. Wireless LANs (3) A multicell 802.11 network.

42. Wireless LANs IEEE 802.11 Distributed access control mechanism (DCF) based on CSMA with optional centralized control (PCF). Physical Layer DCF PCF MAC layer Contention-free Service (polling) Contention Service (CSMA)

43. MAC in Wireless LANs Distributed coordination function (DCF) uses CSMA-based protocol (e.g., ad hoc networks). CD does not make sense in wireless. –Hard for transmitter to distinguish its own transmission from incoming weak signals and noise. Point coordination function (PCF) uses polling to grant stations their turn to transmit (e.g., cellular networks).

44. The 802.11 Protocol Stack Part of the 802.11 protocol stack.

45. The 802.11 MAC Sublayer Protocol (a) The hidden station problem. (b) The exposed station problem.

46. The 802.11 MAC Sublayer Protocol (2) The use of virtual channel sensing using CSMA/CA.

47. The 802.11 MAC Sublayer Protocol (3) A fragment burst.

48. The 802.11 MAC Sublayer Protocol (4) Interframe spacing in 802.11.

49. The 802.11 Frame Structure The 802.11 data frame.

50. 802.11 Services Association Disassociation Reassociation Distribution Integration Distribution Services

51. 802.11 Services Authentication Deauthentication Privacy Data Delivery Intracell Services

52. Broadband Wireless Comparison of 802.11 and 802.16 The 802.16 Protocol Stack The 802.16 Physical Layer The 802.16 MAC Sublayer Protocol The 802.16 Frame Structure

53. The 802.16 Protocol Stack The 802.16 Protocol Stack.

54. The 802.16 Physical Layer The 802.16 transmission environment.

55. The 802.16 Physical Layer (2) Frames and time slots for time division duplexing.

56. The 802.16 MAC Sublayer Protocol Service Classes Constant bit rate service Real-time variable bit rate service Non-real-time variable bit rate service Best efforts service

57. The 802.16 Frame Structure (a) A generic frame. (b) A bandwidth request frame.

58. Bluetooth Bluetooth Architecture Bluetooth Applications The Bluetooth Protocol Stack The Bluetooth Radio Layer The Bluetooth Baseband Layer The Bluetooth L2CAP Layer The Bluetooth Frame Structure

59. Bluetooth Architecture Two piconets can be connected to form a scatternet.

60. Bluetooth Applications The Bluetooth profiles.

61. The Bluetooth Protocol Stack The 802.15 version of the Bluetooth protocol architecture.

62. The Bluetooth Frame Structure A typical Bluetooth data frame.

63. See more recent info on this at: http://www.usb.org/developers/wusb/

65. Summary Channel allocation methods and systems for a common channel.

66. Data Link Layer Switching Bridges from 802.x to 802.y Local Internetworking Spanning Tree Bridges Remote Bridges Repeaters, Hubs, Bridges, Switches, Routers, Gateways Virtual LANs

67. Bridges Bridges used to connect multiple LANS Link Layer –Do not look at anything in packets Work for any payload within the frame –IPv4, IPv6, Apple Talk, etc. –(vs. routers which work at packet (IP) level) Individual LANS are shared media (cable or hub) (Few hubs still in use – now switched Ethernet) LANS have promiscuous mode (i.e. “party line”) –Bridges provide isolation between LANS

68. Data Link Layer Switching Multiple LANs connected by a backbone to handle a total load higher than the capacity of a single LAN.