1. Link Layer

2. 2 Content Error detection and correction MAC sub-layer Ethernet Token Ring

3. 3 Access Protocols Who gets to use the channel next? Fixed/Static assignment Demand assignment Contention Turn-Based

4. 4 Contention Access Protocols No coordination between hosts Control is completely distributed Outcome is probabilistic Examples: ALOHA, CSMA, CSMA/CD

5. 5 Contention Access (cont’d) Advantages: Short delay for bursty traffic Simple (due to distributed control) Flexible to fluctuations in the number of hosts Fairness

6. 6 Contention Access (cont’d) Disadvantages: Can not be certain who will acquire the media/channel Low channel efficiency with a large number of hosts Not good for continuous traffic (e.g., voice) Cannot support priority traffic High variance in transmission delays

7. 7 Contention Access Methods Pure ALOHA Slotted ALOHA CSMA 1-Persistent CSMA Non-Persistent CSMA P-Persistent CSMA CSMA/CD

8. 8 Slotted ALOHA Assumptions all frames same size time is divided into equal size slots, time to transmit 1 frame nodes start to transmit frames only at beginning of slots nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Operation when node obtains fresh frame, it transmits in next slot no collision, node can send new frame in next slot if collision, node retransmits frame in each subsequent slot with prob. p until success

9. 9 Slotted ALOHA Pros single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple Cons collisions, wasting slots idle slots nodes may be able to detect collision in less than time to transmit packet clock synchronization

10. 10 Slotted Aloha efficiency Suppose N nodes with many frames to send, each transmits in slot with probability p prob that node 1 has success in a slot = p(1-p) N-1 prob that any node has a success = Np(1-p) N-1 For max efficiency with N nodes, find p* that maximizes Np(1-p) N-1 For many nodes, take limit of Np*(1-p*) N-1 as N goes to infinity, gives 1/e =.37 Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send At best: channel used for useful transmissions 37% of time!

11. 11 Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame first arrives transmit immediately collision probability increases: frame sent at t 0 collides with other frames sent in [t 0 -1,t 0 +1]

12. 12 Pure Aloha efficiency P(success by given node) = P(node transmits). P(no other node transmits in [p 0 -1,p 0 ]. P(no other node transmits in [p 0 -1,p 0 ] = p. (1-p) N-1. (1-p) N-1 = p. (1-p) 2(N-1) … choosing optimum p and then letting n -> infty... = 1/(2e) =.18 Even worse !

13. 13 Carrier Sense Multiple Access (CSMA) We could achieve better throughput if we could listen to the channel before transmitting a packet This way, we would stop avoidable collisions. To do this, we need “Carrier Sense Multiple Access,” or CSMA, protocols

14. 14 Assumptions with CSMA Networks 1. Constant length packets 2. No errors, except those caused by collisions 3. No capture effect 4. Each host can sense the transmissions of all other hosts 5. The propagation delay is small compared to the transmission time

15. 15 CSMA collisions collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted spatial layout of nodes note: role of distance & propagation delay in determining collision probability

16. 16 CSMA (cont’d) There are several types of CSMA protocols: 1-Persistent CSMA Non-Persistent CSMA P-Persistent CSMA

17. 17 1-Persistent CSMA Sense the channel. If busy, keep listening to the channel and transmit immediately when the channel becomes idle. If idle, transmit a packet immediately. If collision occurs, Wait a random amount of time and start over again.

18. 18 1-Persistent CSMA (cont’d) The protocol is called 1-persistent because the host transmits with a probability of 1 whenever it finds the channel idle.

19. 19 The Effect of Propagation Delay on CSMA AB carrier sense = idle Transmit a packet Collision packet

20. 20 Propagation Delay and CSMA Contention (vulnerable) period in Pure ALOHA two packet transmission times Contention period in Slotted ALOHA one packet transmission time Contention period in CSMA up to 2 x end-to-end propagation delay Performance of CSMA > Performance of Slotted ALOHA > Performance of Pure ALOHA

21. 21 1-Persistent CSMA (cont’d) Even if prop. delay is zero, there will be collisions Example: If stations B and C become ready in the middle of A’s transmission, B and C will wait until the end of A’s transmission and then both will begin transmitted simultaneously, resulting in a collision. If B and C were not so greedy, there would be fewer collisions

22. 22 Non-Persistent CSMA Sense the channel. If busy, wait a random amount of time and sense the channel again If idle, transmit a packet immediately If collision occurs wait a random amount of time and start all over again

23. 23 Tradeoff between 1- and Non- Persistent CSMA If B and C become ready in the middle of A’s transmission, 1-Persistent: B and C collide Non-Persistent: B and C probably do not collide If only B becomes ready in the middle of A’s transmission, 1-Persistent: B succeeds as soon as A ends Non-Persistent: B may have to wait

24. 24 P-Persistent CSMA Optimal strategy: use P-Persistent CSMA Assume channels are slotted One slot = contention period (i.e., one round trip propagation delay)

25. 25 P-Persistent CSMA (cont’d) 1. Sense the channel If channel is idle, transmit a packet with probability p if a packet was transmitted, go to step 2 if a packet was not transmitted, wait one slot and go to step 1 If channel is busy, wait one slot and go to step 1. 2. Detect collisions If a collision occurs, wait a random amount of time and go to step 1

26. 26 P-Persistent CSMA (cont’d) Consider p-persistent CSMA with p=0.5 When a host senses an idle channel, it will only send a packet with 50% probability If it does not send, it tries again in the next slot.

27. 27 Comparison of CSMA and ALOHA Protocols (Number of Channel Contenders)

28. 28 CSMA/CD In CSMA protocols If two stations begin transmitting at the same time, each will transmit its complete packet, thus wasting the channel for an entire packet time In CSMA/CD protocols The transmission is terminated immediately upon the detection of a collision CD = Collision Detect

29. 29 CSMA/CD (Collision Detection) collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals difficult in wireless LANs: receiver shut off while transmitting human analogy: the polite conversationalist

30. 30 CSMA/CD collision detection

31. 31 CSMA/CD Sense the channel If idle, transmit immediately If busy, wait until the channel becomes idle Collision detection Abort a transmission immediately if a collision is detected Try again later after waiting a random amount of time

32. 32 CSMA/CD (cont’d) Carrier sense reduces the number of collisions Collision detection reduces the effect of collisions, making the channel ready to use sooner

33. 33 Collision detection time How long does it take to realize there has been a collision? Worst case: 2 x end-to-end prop. delay AB packet

34. 34 Turn-Based Access Protocols A D C B

35. 35 IEEE 802 LANs LAN: Local Area Network What is a local area network? A LAN is a network that resides in a geographically restricted area LANs usually span a building or a campus

36. 36 Characteristics of LANs Short propagation delays Small number of users Single shared medium (usually) Inexpensive

37. 37 Common LANs Bus-based LANs Ethernet (*) Token Bus (*) Ring-based LANs Token Ring (*) Switch-based LANs Switched Ethernet ATM LANs (*) IEEE 802 LANs

38. 38 IEEE 802 Standards 802.1: Introduction 802.2: Logical Link Control (LLC) 802.3: CSMA/CD (Ethernet) 802.4: Token Bus 802.5: Token Ring 802.6: DQDB 802.11: CSMA/CA (Wireless LAN)

39. 39 IEEE 802 Standards (cont’d) 802 standards define: Physical layer protocol Data link layer protocol Medium Access (MAC) Sublayer Logical Link Control (LLC) Sublayer

40. 40 OSI Layers and IEEE 802 802.2 Logical Link Control 802.3802.4802.5 Medium Access Control Data Link Layer Physical Layer Higher Layers OSI layers IEEE 802 LAN standards Higher Layers CSMA/CD Token-passing Token-passing bus bus ring

41. 41 IEEE 802 LANs (cont’d) Ethernet Token Ring

42. 42 Ethernet (CSMA/CD) IEEE 802.3 defines Ethernet Layers specified by 802.3: Ethernet Physical Layer Ethernet Medium Access (MAC) Sublayer

43. 43 Ethernet (cont’d) Possible Topologies: 1. Bus 2. Branching non-rooted tree for large Ethernets

44. 44 Minimal Bus Configuration Host Transceiver Cable Coaxial Cable Terminator

45. 45 Typical Large-Scale Configuration Host Repeater Ethernet segment

46. 46 Ethernet Physical Layer Transceiver Transceiver Cable 4 Twisted Pairs 15 Pin Connectors Channel Logic Manchester Phase Encoding 64-bit preamble for synchronization

47. 47 Ethernet Cabling Options 10Base5: Thick Coax 10Base2: Thin Coax (“cheapernet”) 10Base-T: Twisted Pair 10Base-F: Fiber optic Each cabling option carries with it a different set of physical layer constraints (e.g., max. segment size, nodes/segment, etc.)

48. 48 Ethernet Physical Configuration For thick coaxial cable Segments of 500 meters maximum Maximum total cable length of 1500 meters between any two transceivers Maximum of 2 repeaters in any path Maximum of 100 transceivers per segment Transceivers placed only at 2.5 meter marks on cable

49. 49 Manchester Encoding 1 bit = high/low voltage signal 0 bit = low/high voltage signal 1 0 1 1 0 0Data stream Encoded bit pattern

50. 50 Ethernet Synchronization 64-bit frame preamble used to synchronize reception 7 bytes of 10101010 followed by a byte containing 10101011 Manchester encoded, the preamble appears like a sine wave

51. 51 Ethernet: MAC Layer Data encapsulation Frame Format Addressing Error Detection Link Management CSMA/CD Backoff Algorithm

52. 52 Frame Check Seq. (4 bytes) MAC Layer Ethernet Frame Format Destination (6 bytes) Length (2 bytes) Data (46-1500 bytes) Pad Source (6 bytes) Multicast bit

53. 53 Ethernet MAC Frame Address Field Destination and Source Addresses: 6 bytes each Two types of destination addresses Physical address: Unique for each user Multicast address: Group of users First bit of address determines which type of address is being used 0 = physical address 1 = multicast address

54. 54 Ethernet MAC Frame Other Fields Length Field 2 bytes in length determines length of data payload Data Field: between 0 and 1500 bytes Pad: Filled when Length < 46 Frame Check Sequence Field 4 bytes Cyclic Redundancy Check (CRC-32)

55. 55 CSMA/CD Recall: CSMA/CD is a “carrier sense” protocol. If channel is idle, transmit immediately If busy, wait until the channel becomes idle CSMA/CD can detect collections. Abort transmission immediately if there is a collision Try again later according to a backoff algorithm

56. 56 Ethernet Backoff Algorithm: Binary Exponential Backoff If collision, Choose one slot randomly from 2 k slots, where k is the number of collisions the frame has suffered. One contention slot length = 2 x end-to-end propagation delay This algorithm can adapt to changes in network load.

57. 57 Binary Exponential Backoff (cont’d) slot length = 2 x end-to-end delay = 50  s AB t=0  s:Assume A and B collide ( k A = k B = 1 ) A, B choose randomly from 2 1 slots: [0,1] Assume A chooses 1, B chooses 1 t=100  s:A and B collide ( k A = k B = 2 ) A, B choose randomly from 2 2 slots: [0,3] Assume A chooses 2, B chooses 0 t=150  s:B transmits successfully t=250  s:A transmits successfully

58. 58 Binary Exponential Backoff (cont’d) In Ethernet, Binary exponential backoff will allow a maximum of 15 retransmission attempts If 16 backoffs occur, the transmission of the frame is considered a failure.

59. 59 Ethernet Performance

60. 60 Ethernet Features and Advantages 1. Passive interface: No active element 2. Broadcast: All users can listen 3. Distributed control: Each user makes own decision Simple Reliable Easy to reconfigure

61. 61 Ethernet Disadvantages Lack of priority levels Cannot perform real-time communication Security issues

62. 62 Hubs, Switches, Routers Hub: Behaves like Ethernet Switch: Supports multiple collision domains A collision domain is a segment Router: operates on level-3 packets

63. 63 Why Ethernet Switching? LANs may grow very large The switch has a very fast backplane It can forward frames very quickly to the appropriate subnet Cheaper than upgrading all host interfaces to use a faster network

64. 64 Ethernet Switching Connect many Ethernet through an “Ethernet switch” Each Ethernet is a “segment” Make one large, logical segment to segment 1 to segment 2to segment 3 to segment 4

65. 65 Collision Domains Host switch Ethernet Hub A B C D E F A,B,CD,E,F G H Z Each segment runs a standard CMSA protocol

66. 66 Layer-2 routing tables Host switch Ethernet Hub A B C D E F A,B,CD,E,F G H Z Switch must forward packets from A,B,C to the other segment Switch builds a large table For each packet, look up in table and maybe forward the packet

67. 67 Learning MAC addresses Host switch Ethernet segment A B C D E F A,B,CD,E,F Per-port routing table G H Z Switch adds hosts to routing table when it sees a packet with a given source address

68. 68 Spanning Trees Want to allow multiple switches to connect together What If there is a cycle in the graph of switches connected together? Can’t have packets circulate forever! Must break the cycle by restricting routes

69. 69 Spanning Trees Host switches A B C D E F G H Z J k 12 3

70. 70 Spanning Trees Host switches A B C D E F G H Z J k 12 3 no cycles in the graph of switches

71. 71 Spanning Tree Protocol 1. Each switch periodically sends a configuration message out of every port. A message contains: (ID of sender, ID of root, distance from sender to root). 2. Initially, every switch claims to be root and sends a distance field of 0. 3. A switch keeps sending the same message (periodically) until it hears a “better” message. 4. “Better” means: A root with a smaller ID A root with equal ID, but with shorter distance The root ID and distance are the same as we already have, but the sending bridge has a smaller ID. 5. When a switch hears a better configuration message, it stops generating its own messages, and just forwards ones that it receives (adding 1 to the distance). 6. If the switch realizes that it is not the designated bridge for a segment, it stops sending configuration messages to that segment. Eventually: Only the root switch generates configuration messages, Other switches send configuration messages to segments for which they are the designated switch

72. 72 Token Ring IEEE 802.5 Standard Layers specified by 802.5: Token Ring Physical Layer Token Ring MAC Sublayer

73. 73 Token Ring (cont’d) Token Ring, unlike Ethernet, requires an active interface Host Ring interface

74. 74 Token Ring Physical Layer Ring Interfaces Listen and Transmit Modes Channel Logic Differential Manchester Encoding

75. 75 Token Ring Interface Modes To station From station To station From station Listen Mode Transmit Mode one-bit delay

76. 76 Differential Manchester Encoding 1 0 0 1 1 Transitions take place at midpoint of interval 1 bit: the initial half of the bit interval carries the same polarity as the second half of the previous interval 0 bit: a transition takes place at both the beginning and the middle of the bit interval Differential Manchester is more efficient than standard Manchester encoding

77. 77 Token Ring MAC Sublayer Token passing protocol Frame format Token format

78. 78 Token Passing Protocol A token (8 bit pattern) circulates around the ring Token state: Busy: 11111111 Idle: 11111110

79. 79 Token Passing Protocol (cont’d) General Procedure: Sending host waits for and captures an idle token Sending host changes the token to a frame and circulates it Receiving host accepts the frame and continues to circulate it Sending host receives its frame, removes it from the ring, and generates an idle token which it then circulates on the ring

80. 80 Token Ring Frame and Token Formats SDACED SCACFC Destination Address Source Address Data Checksum EDFS Token Format Frame Format 1 1 1 1 1 1 2/6 2/6 unlimited 4 1 1 Bytes

81. 81 Token Ring Delimiters SD = Starting Delimiter ED = Ending Delimiter They contains invalid differential Manchester codes SDACED SCACFC Destination Address Source Address Data Checksum EDFS

82. 82 Token Ring Access Control Field P = Priority bits provides up to 8 levels of priority when accessing the ring T = Token bit T=0: Token T=1: Frame SDACED P P P T M R R R (Note: The AC field is also used in frames)

83. 83 Token Ring Access Control Field (cont’d) SDACED P P P T M R R R M = Monitor Bit Prevents tokens and frames from circulating indefinitely All frames and tokens are issued with M=0 On passing through the “monitor station,” M is set to 1 All other stations repeat this bit as set A token or frame that reaches the monitor station with M=1 is considered invalid and is purged

84. 84 The Token Ring Monitor Station One station on the ring is designated as the “monitor station” The monitor station: marks the M bit in frames and tokens removes marked frames and tokens from the ring watches for missing tokens and generates new ones after a timeout period

85. 85 Token Ring Access Control Fields (cont’d) SDACED P P P T M R R R R = Reservation Bits Allows stations with high priority data to request (in frames and tokens as they are repeated) that the next token be issued at the requested priority

86. 86 Token Ring Frame Control Field FC = Frame Control Field Defines the type of frame being sent Frames may be either data frames or some type of control frame. Example control frames: Beacon: Used to locate breaks in the ring Duplicate address test: Used to test if two stations have the same address SCACFC Destination Address Source Address Data Checksum EDFS

87. 87 Token Ring Address & Data Fields Address Fields: Indicate the source and destination hosts Broadcast: Set all destination address bits to 1s. Data No fixed limit on length Caveat: Hosts may only hold the token for a limited amount of time (10 msec) SCACFC Destination Address Source Address Data Checksum EDFS

88. 88 Token Ring Checksum and Frame Status Checksum: 32-bit CRC FS = Frame Status Contains two bits, A and C When the message arrives at the destination, it sets A=1 When the destination copies the data in the message, it sets C=1 SCACFC Destination Address Source Address Data Checksum EDFS

89. 89 Using Priority in Token Ring If a host wants to send data of priority n, it may only grab a token with priority value n or lower. A host may reserve a token of priority n by marking the reservation bits in the AC field of a passing token or frame Caveat: The host may not make the reservation if the token or frame’s AC field already indicates a higher priority reservation The next token generated will have a priority equal to the highest reserved priority

90. 90 Priority Transmission: Example A D C B Host B has 1 frame of priority 3 to send to A Host C has 1 frame of priority 2 to send to A Host D has 1 frame of priority 4 to send to A Token starts at host A with priority 0 and circulates clockwise Host C is the monitor station

91. 91 Example (cont’d) Event Token/Frame AC Field A generates a token P=0, M=0, T=0, R=0 B grabs the token and sets the message destination to A P=3, M=0, T=1, R=0 Frame arrives at C, and C reserves priority level 2. Monitor bit set.P=3, M=1, T=1, R=2 Frame arrives at D, and D attempts to reserve priority level 4:P=3, M=1, T=1, R=4 Frame arrives at A, and A copies itP=3, M=1, T=1, R=4 Frame returns to B, so B removes it, and generates a new tokenP=4, M=0, T=0, R=0 Token arrives at C, but its priority is too high. C reserves priority 2. M bit.P=4, M=1, T=0, R=2

92. 92 Example (cont’d) Event Token/Frame AC Field Token arrives at D, and D grabs it, sending a message to A P=4, M=0, T=1, R=2 Frame arrives at A, and A copies it P=4, M=0, T=1, R=2 Frame arrives at B, which does nothing to itP=4, M=0, T=1, R=2 Frame arrives at C, which sets the monitor bitP=4, M=1, T=1, R=2 Frame returns to D, so D removes it and generates a new token with P=2P=2, M=0, T=0, R=0 etc… Attempt to complete this scenario on your own.