1. Medium Access Control Sublayer Chapter 4 CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Channel Allocation Problem Multiple Access Protocols Ethernet Wireless LANs Broadband Wireless Bluetooth RFID Data Link Layer Switching Revised: August 2011

2. The MAC Sublayer CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Responsible for deciding who sends next on a multi-access link An important part of the link layer, especially for LANs Physical Link Network Transport Application MAC is in here!

3. Channel Allocation Problem (1) For fixed channel and traffic from N users Divide up bandwidth using FTM, TDM, CDMA, etc. This is a static allocation, e.g., FM radio This static allocation performs poorly for bursty traffic Allocation to a user will sometimes go unused CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

4. Channel Allocation Problem (2) Dynamic allocation gives the channel to a user when they need it. Potentially N times as efficient for N users. Schemes vary with assumptions: AssumptionImplication Independent traffic Often not a good model, but permits analysis Single channelNo external way to coordinate senders Observable collisions Needed for reliability; mechanisms vary Continuous or slotted time Slotting may improve performance Carrier senseCan improve performance if available CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

5. Multiple Access Protocols CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 ALOHA » CSMA (Carrier Sense Multiple Access) » Collision-free protocols » Limited-contention protocols » Wireless LAN protocols »

6. ALOHA (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 In pure ALOHA, users transmit frames whenever they have data; users retry after a random time for collisions Efficient and low-delay under low load ` Collision Time User A B C D E

7. ALOHA (2) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Collisions happen when other users transmit during a vulnerable period that is twice the frame time Synchronizing senders to slots can reduce collisions

8. ALOHA (3) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Slotted ALOHA is twice as efficient as pure ALOHA Low load wastes slots, high loads causes collisions Efficiency up to 1/e (37%) for random traffic models

9. CSMA (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 CSMA improves on ALOHA by sensing the channel! User doesn’t send if it senses someone else Variations on what to do if the channel is busy: 1-persistent (greedy) sends as soon as idle Nonpersistent waits a random time then tries again p-persistent sends with probability p when idle

10. CSMA (2) – Persistence CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 CSMA outperforms ALOHA, and being less persistent is better under high load

11. CSMA (3) – Collision Detection CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 CSMA/CD improvement is to detect/abort collisions Reduced contention times improve performance Collision time is much shorter than frame time

12. CSMA (4) If two stations sense the channel to be idle and begin transmitting simultaneously, their signals will still collide. An improvement is for the stations to quickly detect the collision and abruptly stop transmitting since they are irretrievably garbled anyway. This strategy saves time and bandwidth. This protocol, is known as CSMA/CD(CSMA with Collision Detection). In a hub, all stations are in the same Collision domain. They must use the CSMA/CD algorithm to schedule their transmissions.

13. Collision-Free (1) – Bitmap CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Collision-free protocols avoid collisions entirely Senders must know when it is their turn to send The basic bit-map protocol: Sender set a bit in contention slot if they have data Senders send in turn; everyone knows who has data

14. Collision-Free (2) – Token Ring CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Token sent round ring defines the sending order Station with token may send a frame before passing Idea can be used without ring too, e.g., token bus Station Direction of transmission Token

15. Collision-Free (3) – Countdown CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Binary countdown improves on the bitmap protocol Stations send their address in contention slot (log N bits instead of N bits) Medium ORs bits; stations give up when they send a “0” but see a “1” Station that sees its full address is next to send

16. Limited-Contention Protocols (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Idea is to divide stations into groups within which only a very small number are likely to want to send Avoids wastage due to idle periods and collisions Already too many contenders for a good chance of one winner

17. Limited Contention (2) –Adaptive Tree Walk CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Tree divides stations into groups (nodes) to poll Depth first search under nodes with poll collisions Start search at lower levels if >1 station expected Level 0 Level 1 Level 2

18. Characteristics of selected wireless link standards Indoor 10-30m Outdoor 50-200m Mid-range outdoor 200m – 4 Km Long-range outdoor 5Km – 20 Km.056.384 1 4 5-11 54 IS-95, CDMA, GSM 2G UMTS/WCDMA, CDMA2000 3G 802.15 802.11b 802.11a,g UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO 3G cellular enhanced 802.16 (WiMAX) 802.11a,g point-to-point 200 802.11n Data rate (Mbps) data

19. Wireless network taxonomy single hop multiple hops infrastructure (e.g., APs) no infrastructure host connects to base station (WiFi, WiMAX, cellular) which connects to larger Internet no base station, no connection to larger Internet (Bluetooth, ad hoc nets) host may have to relay through several wireless nodes to connect to larger Internet: mesh net no base station, no connection to larger Internet. May have to relay to reach other a given wireless node MANET, VANET

20. Wireless Link Characteristics Differences from wired link …. decreased signal strength: radio signal attenuates as it propagates through matter (path loss) interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult”

21. Wireless & MAC A station on a wireless LAN may not be able to transmit frames to or receive frames from all other stations because of the limited radio range of the stations. In wired LANs, when one station sends a frame, all other stations receive it. The absence of this property in wireless LANs causes a variety of complications. On a wireless network, the problem of a station not being able to detect a potential competitor for the medium because the competitor is too far away is called the hidden terminal problem. We look at that next.

22. Wireless LAN Protocols (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Wireless has complications compared to wired. Nodes may have different coverage regions Leads to hidden and exposed terminals Nodes can’t detect collisions, i.e., sense while sending Makes collisions expensive and to be avoided

23. Wireless LANs (2) – Hidden terminals CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Hidden terminals are senders that cannot sense each other but nonetheless collide at intended receiver Want to prevent; loss of efficiency A and C are hidden terminals when sending to B

24. Wireless LANs (3) – Exposed terminals CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Exposed terminals are senders who can sense each other but still transmit safely (to different receivers) Desirably concurrency; improves performance B  A and C  D are exposed terminals

25. Wireless LANs (4) – MACA CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 MACA protocol grants access for A to send to B: A sends RTS to B [left]; B replies with CTS [right] A can send with exposed but no hidden terminals A sends RTS to B; C and E hear and defer for CTS B replies with CTS; D and E hear and defer for data

26. Ethernet CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Classic Ethernet » Switched/Fast Ethernet » Gigabit/10 Gigabit Ethernet »

27. Classic Ethernet (1) – Physical Layer CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 One shared coaxial cable to which all hosts attached Up to 10 Mbps, with Manchester encoding Hosts ran the classic Ethernet protocol for access

28. Classic Ethernet (2) – MAC CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 MAC protocol is 1-persistent CSMA/CD (earlier) Random delay (backoff) after collision is computed with BEB (Binary Exponential Backoff) Frame format is still used with modern Ethernet. Ethernet (DIX) IEEE 802.3

29. Classic Ethernet (3) – MAC CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Collisions can occur and take as long as 2  to detect  is the time it takes to propagate over the Ethernet Leads to minimum packet size for reliable detection

30. Classic Ethernet (4) – Performance CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Efficient for large frames, even with many senders Degrades for small frames (and long LANs) 10 Mbps Ethernet, 64 byte min. frame

31. Switched/Fast Ethernet (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Hubs wire all lines into a single CSMA/CD domain Switches isolate each port to a separate domain −Much greater throughput for multiple ports −No need for CSMA/CD with full-duplex lines

32. Switched/Fast Ethernet (2) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Switches can be wired to computers, hubs and switches Hubs concentrate traffic from computers More on how to switch frames the in 4.8 Switch Twisted pair Switch ports Hub

33. Switched/Fast Ethernet (3) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Fast Ethernet extended Ethernet from 10 to 100 Mbps Twisted pair (with Cat 5) dominated the market

34. Gigabit / 10 Gigabit Ethernet (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Switched Gigabit Ethernet is now the garden variety With full-duplex lines between computers/switches

35. Gigabit / 10 Gigabit Ethernet (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Gigabit Ethernet is commonly run over twisted pair 10 Gigabit Ethernet is being deployed where needed 40/100 Gigabit Ethernet is under development

36. Wireless LANs CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 802.11 architecture/protocol stack » 802.11 physical layer » 802.11 MAC » 802.11 frames »

37. 802.11 Architecture/Protocol Stack (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Wireless clients associate to a wired AP (Access Point) Called infrastructure mode; there is also ad-hoc mode with no AP, but that is rare. Access Point Client To Network

38. 802.11 Architecture/Protocol Stack (2) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 MAC is used across different physical layers

39. 802.11 physical layer CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 NICs are compatible with multiple physical layers −E.g., 802.11 a/b/g NameTechniqueMax. Bit Rate 802.11bSpread spectrum, 2.4 GHz11 Mbps 802.11gOFDM, 2.4 GHz54 Mbps 802.11aOFDM, 5 GHz54 Mbps 802.11nOFDM with MIMO, 2.4/5 GHz600 Mbps Remember, most LAN interfaces have a promiscuous mode, in which all frames are given to each computer, not just those addressed to it.

40. 802.11 physical layer CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 All of the 802.11 transmission methods define multiple rates. The idea is that different rates can be used depending on the current conditions. If the wireless signal is weak, a low rate can be used. If the signal is clear, the highest rate can be used. This adjustment is rate adaptation.

41. 802.11 MAC (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 CSMA/CA inserts backoff slots to avoid collisions MAC uses ACKs/retransmissions for wireless errors

42. 802.11 MAC (2) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Virtual channel sensing with the NAV and optional RTS/CTS (often not used) avoids hidden terminals

43. 802.11 MAC (3) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Different backoff slot times add quality of service −Short intervals give preferred access, e.g., control, VoIP MAC has other mechanisms too, e.g., power save

44. 802.11 Frames CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Frames vary depending on their type (Frame control) Data frames have 3 addresses to pass via APs

45. Wireless, Mobile Networks 6-45 IEEE 802.11: multiple access avoid collisions: 2 + nodes transmitting at same time 802.11: CSMA - sense before transmitting don’t collide with ongoing transmission by other node 802.11: no collision detection! difficult to receive (sense collisions) when transmitting due to weak received signals (fading) can’t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/C(ollision)A(voidance) A B C A B C A’s signal strength space C’s signal strength

46. IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat 2 802.11 receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) sender receiver DIFS data SIFS ACK

47. Avoiding collisions (more) idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they’re short) BS broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes sender transmits data frame other stations defer transmissions avoid data frame collisions completely using small reservation packets!

48. Collision Avoidance: RTS-CTS exchange AP A B time RTS(A) RTS(B) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer

49. frame control duration address 1 address 2 address 4 address 3 payloadCRC 226662 6 0 - 2312 4 seq control 802.11 frame: addressing Address 2: MAC address of wireless host or AP transmitting this frame Address 1: MAC address of wireless host or AP to receive this frame Address 3: MAC address of router interface to which AP is attached Address 4: used only in ad hoc mode

50. Internet router AP H1 R1 AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.11 frame R1 MAC addr H1 MAC addr dest. address source address 802.3 frame 802.11 frame: addressing

51. frame control duration address 1 address 2 address 4 address 3 payloadCRC 226662 6 0 - 2312 4 seq control Type From AP Subtype To AP More frag WEP More data Power mgt RetryRsvd Protocol version 2 2411111111 802.11 frame: more duration of reserved transmission time (RTS/CTS) frame seq # (for RDT) frame type (RTS, CTS, ACK, data)

52. hub or switch AP 2 AP 1 H1 BBS 2 BBS 1 802.11: mobility within same subnet router H1 remains in same IP subnet: IP address can remain same switch: which AP is associated with H1? self-learning: switch will see frame from H1 and “remember” which switch port can be used to reach H1

53. 802.11: advanced capabilities Rate Adaptation base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) 1020 30 40 SNR(dB) BER 10 -1 10 -2 10 -3 10 -5 10 -6 10 -7 10 -4 operating point 1. SNR decreases, BER increase as node moves away from base station 2. When BER becomes too high, switch to lower transmission rate but with lower BER

54. Broadband Wireless CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 802.16 Architecture / Protocol Stack » 802.16 Physical Layer » 802.16 MAC » 802.16 Frames »

55. Google Project Loon Many of us think of the Internet as a global community. But two- thirds of the world’s population does not yet have Internet access. Project Loon is a network of balloons traveling on the edge of space, designed to connect people in rural and remote areas, help fill coverage gaps, and bring people back online after disasters. https://www.youtube.com/watch?v=mcw6j-QWGMo#t=60www.youtube.com/watch?v=mcw6j-QWGMo#t=60

56. 802.16 Architecture/Protocol Stack (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Wireless clients connect to a wired basestation (like 3G)

57. 802.16: WiMAX like 802.11 & cellular: base station model transmissions to/from base station by hosts with omnidirectional antenna base station-to-base station backhaul with point-to-point antenna unlike 802.11: range ~ 6 miles (“city rather than coffee shop”) ~14 Mbps point-to-multipoint point-to-point

58. 802.16 Architecture/Protocol Stack (2) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 MAC is connection-oriented; IP is connectionless Convergence sublayer maps between the two

59. 802.16 Physical Layer CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Based on OFDM; base station gives mobiles bursts (subcarrier/time frame slots) for uplink and downlink

60. 802.16 MAC CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Connection-oriented with base station in control Clients request the bandwidth they need Different kinds of service can be requested: Constant bit rate, e.g., uncompressed voice Real-time variable bit rate, e.g., video, Web Non-real-time variable bit rate, e.g., file download Best-effort for everything else

61. 802.16 Frames CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Frames vary depending on their type Connection ID instead of source/dest addresses (a) A generic frame. (b) A bandwidth request frame (b) (a)

62. 802.16: WiMAX: downlink, uplink scheduling transmission frame down-link subframe: base station to node uplink subframe: node to base station pream. DL- MAP UL- MAP DL burst 1 SS #1 DL burst 2 DL burst n Initial maint. request conn. downlink subframe SS #2 SS #k uplink subframe … … … … base station tells nodes who will get to receive (DL map) and who will get to send (UL map), and when  WiMAX standard provide mechanism for scheduling, but not scheduling algorithm

63. Bluetooth CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Bluetooth Architecture » Bluetooth Applications / Protocol » Bluetooth Radio / Link Layers » Bluetooth Frames »

64. M radius of coverage S S S P P P P M S Master device Slave device Parked device (inactive) P 802.15: personal area network less than 10 m diameter replacement for cables (mouse, keyboard, headphones) ad hoc: no infrastructure master/slaves: slaves request permission to send (to master) master grants requests 802.15: evolved from Bluetooth specification 2.4-2.5 GHz radio band up to 721 kbps

65. Bluetooth Architecture CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Piconet master is connected to slave wireless devices Slaves may be asleep (parked) to save power Two piconets can be bridged into a scatternet

66. Bluetooth Applications / Protocol Stack CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Profiles give the set of protocols for a given application 25 profiles, including headset, intercom, streaming audio, remote control, personal area network, …

67. Bluetooth Radio / Link Layers CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Radio layer Uses adaptive frequency hopping in 2.4 GHz band Link layer TDM with timeslots for master and slaves Synchronous CO for periodic slots in each direction Asynchronous CL for packet-switched data Links undergo pairing (user confirms passkey/PIN) to authorize them before use

68. Bluetooth Frames CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Time is slotted; enhanced data rates send faster but for the same time; addresses are only 3 bits for 8 devices (b) (a) (b)

69. RFID CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Gen 2 Architecture » Gen 2 Physical Layer » Gen 2 Tag Identification Layer » Gen 2 Frames »

70. Gen 2 Architecture CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Reader signal powers tags; tags reply with backscatter

71. Gen 2 Physical Layer CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Reader uses duration of on period to send 0/1 Tag backscatters reader signal in pulses to send 0/1

72. Gen 2 Tag Identification Layer CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Reader sends query and sets slot structure Tags reply (RN16) in a random slot; may collide Reader asks one tag for its identifier (ACK) Process continues until no tags are left

73. Gen 2 Frames CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Reader frames vary depending on type (Command) −Query shown below, has parameters and error detection Tag responses are simply data −Reader sets timing and knows the expected format Query message

74. Data Link Layer Switching CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Uses of Bridges » Learning Bridges » Spanning Tree » Repeaters, hubs, bridges,.., routers, gateways » Virtual LANs »

75. Uses of Bridges Common setup is a building with centralized wiring Bridges (switches) are placed in or near wiring closets CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

76. Learning Bridges (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 A bridge operates as a switched LAN (not a hub) Computers, bridges, and hubs connect to its ports

77. Learning Bridges (2) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Backward learning algorithm picks the output port: Associates source address on frame with input port Frame with destination address sent to learned port Unlearned destinations are sent to all other ports Needs no configuration Forget unused addresses to allow changes Bandwidth efficient for two-way traffic

78. Learning Bridges (3) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Bridges extend the Link layer: Use but don’t remove Ethernet header/addresses Do not inspect Network header

79. Spanning Tree (1) – Problem CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Bridge topologies with loops and only backward learning will cause frames to circulate for ever Need spanning tree support to solve problem

80. Spanning Tree (2) – Algorithm CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Subset of forwarding ports for data is use to avoid loops Selected with the spanning tree distributed algorithm by Perlman I think that I shall never see A graph more lovely than a tree. A tree whose crucial property Is loop-free connectivity. A tree which must be sure to span. So packets can reach every LAN. First the Root must be selected By ID it is elected. Least cost paths from Root are traced In the tree these paths are placed. A mesh is made by folks like me Then bridges find a spanning tree. – Radia Perlman, 1985.

81. Spanning Tree (3) – Example CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 After the algorithm runs: −B1 is the root, two dashed links are turned off −B4 uses link to B2 (lower than B3 also at distance 1) −B5 uses B3 (distance 1 versus B4 at distance 2)

82. Repeaters, Hubs, Bridges, Switches, Routers, & Gateways CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Devices are named according to the layer they process A bridge or LAN switch operates in the Link layer

83. Virtual LANs (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 VLANs (Virtual LANs) splits one physical LAN into multiple logical LANs to ease management tasks Ports are “colored” according to their VLAN

84. Virtual LANs (2) – IEEE 802.1Q CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Bridges need to be aware of VLANs to support them In 802.1Q, frames are tagged with their “color” Legacy switches with no tags are supported

85. Virtual LANs (3) – IEEE 802.1Q CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 802.1Q frames carry a color tag (VLAN identifier) Length/Type value is 0x8100 for VLAN protocol

86. End Chapter 4 CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011