1. TCP/IP Protocol Suite 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 3 Underlying Technology

2. TCP/IP Protocol Suite 2OBJECTIVES:  To briefly discuss the technology of dominant wired LANs, Ethernet, including traditional, fast, gigabit, and ten-gigabit Ethernet.  To briefly discuss the technology of wireless WANs, including IEEE 802.11 LANs, and Bluetooth.  To briefly discuss the technology of point-to-point WANs including 56K modems, DSL, cable modem, T-lines, and SONET.  To briefly discuss the technology of switched WANs including X.25, Frame Relay, and ATM.  To discuss the need and use of connecting devices such as repeaters (hubs), bridges (two-layer switches), and routers (three-layer switches).

3. TCP/IP Protocol Suite 3 Chapter Outline 3.1 Wired Local Area Network 3.2 Wireless LANs 3.3 Point-to-Point WANs 3.4 Switched WANs 3.5 Connecting Devices

4. TCP/IP Protocol Suite 4 3-1 WIRED LOCAL AREA NETWORKS A local area network (LAN) is a computer network that is designed for a limited geographic area such as a building or a campus. Although a LAN can be used as an isolated network to connect computers in an organization for the sole purpose of sharing resources, most LANs today are also linked to a wide area network (WAN) or the Internet. The LAN market has seen several technologies such as Ethernet, token ring, token bus, FDDI, and ATM LAN, but Ethernet is by far the dominant technology.

5. TCP/IP Protocol Suite 5 Topics Discussed in the Section IEEE Standards Frame Format Addressing Ethernet Evolution Standard Ethernet Fast Ethernet Gigabit Ethernet Ten-Gigabit Ethernet

6. TCP/IP Protocol Suite 6 Figure 3.1 IEEE standard for LANs

7. TCP/IP Protocol Suite 7 Figure 3.2 Ethernet Frame

8. TCP/IP Protocol Suite 8 Figure 3.3 Maximum and minimum lengths

9. TCP/IP Protocol Suite 9 Minimum length: 64 bytes (512 bits) Maximum length: 1518 bytes (12,144 bits) Note

10. TCP/IP Protocol Suite 10 Figure 3.4 Ethernet address in hexadecimal notation

11. TCP/IP Protocol Suite 11 Figure 3.5 Unicast and multicast addresses

12. TCP/IP Protocol Suite 12 The broadcast destination address is a special case of the multicast address in which all bits are 1s. Note

13. TCP/IP Protocol Suite 13 The least significant bit of the first byte defines the type of address. If the bit is 0, the address is unicast; otherwise, it is multicast. Note

14. TCP/IP Protocol Suite 14 Define the type of the following destination addresses: a. 4A:30:10:21:10:1A b. 47:20:1B:2E:08:EE c. FF:FF:FF:FF:FF:FF Solution To find the type of the address, we need to look at the second hexadecimal digit from the left. If it is even, the address is unicast. If it is odd, the address is multicast. If all digits are F ’ s, the address is broadcast. Therefore, we have the following: a. This is a unicast address because A in binary is 1010 (even). b. This is a multicast address because 7 in binary is 0111 (odd). c. This is a broadcast address because all digits are F ’ s. Example Example 3.1

15. TCP/IP Protocol Suite 15 Show how the address 47:20:1B:2E:08:EE is sent out on line. Solution The address is sent left-to-right, byte by byte; for each byte, it is sent right-to-left, bit by bit, as shown below: Example Example 3.2 ← 11100010 00000100 11011000 01110100 00010000 01110111

16. TCP/IP Protocol Suite 16 Figure 3.6 Ethernet evolution through four generations

17. TCP/IP Protocol Suite 17 Figure 3.7 Space/time model of a collision in CSMA TimeTime BACD

18. TCP/IP Protocol Suite 18 Figure 3.8 Collision of the first bit in CSMA/CD

19. TCP/IP Protocol Suite 19 In the standard Ethernet, if the maximum propagation time is 25.6 μs, what is the minimum size of the frame? Solution The frame transmission time is T fr = 2 × T p = 51.2 μs. This means, in the worst case, a station needs to transmit for a period of 51.2 μs to detect the collision. The minimum size of the frame is 10 Mbps × 51.2 μs = 512 bits or 64 bytes. This is actually the minimum size of the frame for Standard Ethernet, as we discussed before. Example Example 3.3

20. TCP/IP Protocol Suite 20 Figure 3.9 CSMA/CD flow diagram

21. TCP/IP Protocol Suite 21

22. TCP/IP Protocol Suite 22 Figure 3.10 Standard Ethernet implementation

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24. TCP/IP Protocol Suite 24 Figure 3.11 Fast Ethernet implementation

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26. TCP/IP Protocol Suite 26 In the full-duplex mode of Gigabit Ethernet, there is no collision; the maximum length of the cable is determined by the signal attenuation in the cable. Note

27. TCP/IP Protocol Suite 27 Figure 3.12 Gigabit Ethernet implementation

28. TCP/IP Protocol Suite 28

29. TCP/IP Protocol Suite 29 3-2 WIRELESS LANS Wireless communication is one of the fastest growing technologies. The demand for connecting devices without the use of cables is increasing everywhere. Wireless LANs can be found on college campuses, in office buildings, and in many public areas. In this section, we concentrate on two wireless technologies for LANs: IEEE 802.11 wireless LANs, sometimes called wireless Ethernet, and Bluetooth, a technology for small wireless LANs.

30. TCP/IP Protocol Suite 30 Topics Discussed in the Section IEEE 802.1 MAC Sublayer Addressing Mechanism Bluetooth

31. TCP/IP Protocol Suite 31 Figure 3.13 Basic service sets (BSSs)

32. TCP/IP Protocol Suite 32 Figure 3.14 Extended service sets (ESSs)

33. TCP/IP Protocol Suite 33 Figure 3.15 CSMA/CA flow diagram

34. TCP/IP Protocol Suite 34 Figure 3.16 CSMA/CA and NAV

35. TCP/IP Protocol Suite 35 Figure 3.17 Frame format

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37. TCP/IP Protocol Suite 37 Figure 3.18 Control frames

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39. TCP/IP Protocol Suite 39

40. TCP/IP Protocol Suite 40 Figure 3.19 Hidden station problem

41. TCP/IP Protocol Suite 41 The CTS frame in CSMA/CA handshake can prevent collision from a hidden station. Note

42. TCP/IP Protocol Suite 42 Figure 3.20 Use of handshaking to prevent hidden station problem

43. TCP/IP Protocol Suite 43 Figure 3.21 Exposed station problem

44. TCP/IP Protocol Suite 44 Figure 3.22 Use of handshaking in exposed station problem

45. TCP/IP Protocol Suite 45 Figure 3.23 Piconet

46. TCP/IP Protocol Suite 46 Figure 3.24 Scatternet

47. TCP/IP Protocol Suite 47 Figure 3.25 Frame format types

48. TCP/IP Protocol Suite 48 3-3 POINT-TO-POINT WANS A second type of network we encounter in the Internet is the point-to-point wide area network. A point-to-point WAN connects two remote devices using a line available from a public network such as a telephone network. We discuss traditional modem technology, DSL line, cable modem, T-lines, and SONET.

49. TCP/IP Protocol Suite 49 Topics Discussed in the Section 65K Modems DSL Technology Cable Modem T Lines SONET PPP

50. TCP/IP Protocol Suite 50 Figure 3.26 56K modem

51. TCP/IP Protocol Suite 51 ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses. Note

52. TCP/IP Protocol Suite 52 Figure 3.27 Bandwidth division

53. TCP/IP Protocol Suite 53 Figure 3.28 ADSL and DSLAM

54. TCP/IP Protocol Suite 54 Figure 3.29 Cable bandwidth

55. TCP/IP Protocol Suite 55 Figure 3.30 Cable modem configuration

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58. TCP/IP Protocol Suite 58 Figure 3.31 PPP frame

59. TCP/IP Protocol Suite 59 3-4 SWITCHED WANS The backbone networks in the Internet can be switched WANs. A switched WAN is a wide area network that covers a large area (a state or a country) and provides access at several points to the users. Inside the network, there is a mesh of point- to-point networks that connects switches. The switches, multiple port connectors, allow the connection of several inputs and outputs. Switched WAN technology differs from LAN technology in many ways.

60. TCP/IP Protocol Suite 60 Topics Discussed in the Section X.25 Frame Relay ATM

61. TCP/IP Protocol Suite 61 A cell network uses the cell as the basic unit of data exchange. A cell is defined as a small, fixed-size block of information. Note

62. TCP/IP Protocol Suite 62 Figure 3.32 ATM multiplexing

63. TCP/IP Protocol Suite 63 Figure 3.33 Architecture of an ATM network

64. TCP/IP Protocol Suite 64 Figure 3.34 Virtual circuit

65. TCP/IP Protocol Suite 65 A virtual connection is defined by a pair of numbers: the VPI and the VCI. Note

66. TCP/IP Protocol Suite 66 Figure 3.35 ATM layers

67. TCP/IP Protocol Suite 67 Figure 3.36 Use of the layers

68. TCP/IP Protocol Suite 68 The IP protocol uses the AAL5 sublayer. Note

69. TCP/IP Protocol Suite 69 Figure 3.37 AAL5

70. TCP/IP Protocol Suite 70 Figure 3.38 ATM layer

71. TCP/IP Protocol Suite 71 Figure 3.39 An ATM cell

72. TCP/IP Protocol Suite 72 3-5 CONNECTING DEVICES LANs or WANs do not normally operate in isolation. They are connected to one another or to the Internet. To connect LANs and WANs together we use connecting devices. Connecting devices can operate in different layers of the Internet model. We discuss three kinds of connecting devices: repeaters (or hubs), bridges (or two-layer switches), and routers (or three-layer switches).

73. TCP/IP Protocol Suite 73 Topics Discussed in the Section Repeaters Bridges Routers

74. TCP/IP Protocol Suite 74 Figure 3.40 Connecting devices

75. TCP/IP Protocol Suite 75 Figure 3.41 Repeater or hub

76. TCP/IP Protocol Suite 76 A repeater forwards every bit; it has no filtering capability. Note

77. TCP/IP Protocol Suite 77 A bridge has a table used in filtering decisions. Note

78. TCP/IP Protocol Suite 78 A bridge does not change the physical (MAC) addresses in a frame. Note

79. TCP/IP Protocol Suite 79 Figure 3.42 Bridge

80. TCP/IP Protocol Suite 80 Figure 3.43 Learning bridge MM M M

81. TCP/IP Protocol Suite 81 A router is a three-layer (physical, data link, and network) device. Note

82. TCP/IP Protocol Suite 82 A repeater or a bridge connects segments of a LAN. A router connects independent LANs or WANs to create an internetwork (internet). Note

83. TCP/IP Protocol Suite 83 Figure 3.44 Routing example

84. TCP/IP Protocol Suite 84 A router changes the physical addresses in a packet. Note

85. TCP/IP Protocol Suite 85 Ethernet Supplement

86. TCP/IP Protocol Suite 86

87. 87 Ethernet First practical local area network, built at Xerox PARC in 70’s “Dominant” LAN technology: Cheap Kept up with speed race: 10, 100, 1000 Mbps Metcalfe’s Ethernet sketch

88. 88 Ethernet MAC – Carrier Sense Basic idea: Listen to wire before transmission Avoid collision with active transmission Why didn’t ALOHA have this? In wireless, relevant contention at the receiver, not sender Hidden terminal Exposed terminal NY CMU Chicago St.Louis Chicago CMU NY HiddenExposed

89. 89 Ethernet MAC – Collision Detection But: ALOHA has collision detection also? That was very slow and inefficient Basic idea: Listen while transmitting If you notice interference  assume collision Why didn’t ALOHA have this? Very difficult for radios to listen and transmit Signal strength is reduced by distance for radio Much easier to hear “local, powerful” radio station than one in NY You may not notice any “interference”

90. 90 Ethernet MAC (CSMA/CD) Packet? Sense Carrier Discard Packet Send Detect Collision Jam channel b=CalcBackoff() ; wait(b); attempts++; No Yes attempts < 16 attempts == 16 Carrier Sense Multiple Access/Collision Detection

91. 91 Ethernet CSMA/CD: Making it word Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Exponential Backoff: If deterministic delay after collision, collision will occur again in lockstep Why not random delay with fixed mean? Few senders  needless waiting Too many senders  too many collisions Goal: adapt retransmission attempts to estimated current load heavy load: random wait will be longer

92. 92 Ethernet Backoff Calculation Exponentially increasing random delay Infer senders from # of collisions More senders  increase wait time First collision: choose K from {0,1}; delay is K x 512 bit transmission times After second collision: choose K from {0,1,2,3}… After ten or more collisions, choose K from {0,1,2,3,4,…,1023}

93. 93 Outline Aloha Ethernet MAC Collisions Ethernet Frames

94. 94 Collisions Time ABC

95. 95 Minimum Packet Size What if two people sent really small packets How do you find collision?

96. 96 Ethernet Collision Detect Min packet length > 2x max prop delay If A, B are at opposite sides of link, and B starts one link prop delay after A Jam network for 32-48 bits after collision, then stop sending Ensures that everyone notices collision

97. 97 End to End Delay c in cable = 60% * c in vacuum = 1.8 x 10^8 m/s Modern 10Mb Ethernet 2.5km, 10Mbps ~= 12.5us delay +Introduced repeaters (max 5 segments) Worst case – 51.2us round trip time! Slot time = 51.2us = 512bits in flight After this amount, sender is guaranteed sole access to link 51.2us = slot time for backoff

98. 98 Packet Size What about scaling? 3Mbit, 100Mbit, 1Gbit... Original 3Mbit Ethernet did not have minimum packet size Max length = 1Km and No repeaters For higher speeds must make network smaller, minimum packet size larger or both What about a maximum packet size? Needed to prevent node from hogging the network 1500 bytes in Ethernet

99. 99 10BaseT and 100BaseT 10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair (wiring) Minimum packet size requirement Make network smaller  solution for 100BaseT

100. 100 Gbit Ethernet Minimum packet size requirement Make network smaller? 512bits @ 1Gbps = 512ns 512ns * 1.8 * 10^8 = 92meters = too small !! Make min pkt size larger! Gigabit Ethernet uses collision extension for small pkts and backward compatibility Maximum packet size requirement 1500 bytes is not really “hogging” the network Defines “jumbo frames” (9000 bytes) for higher efficiency

101. 101 Outline Aloha Ethernet MAC Collisions Ethernet Frames

102. 102 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

103. 103 Ethernet Frame Structure (cont.) Preamble: 8 bytes 101010…1011 Used to synchronize receiver, sender clock rates CRC: 4 bytes Checked at receiver, if error is detected, the frame is simply dropped

104. 104 Ethernet Frame Structure (cont.) Each protocol layer needs to provide some hooks to upper layer protocols Demultiplexing: identify which upper layer protocol packet belongs to E.g., port numbers allow TCP/UDP to identify target application Ethernet uses Type field Type: 2 bytes Indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk)

105. 105 Addressing Alternatives Broadcast  all nodes receive all packets Addressing determines which packets are kept and which are packets are thrown away Packets can be sent to: Unicast – one destination Multicast – group of nodes (e.g. “everyone playing Quake”) Broadcast – everybody on wire Dynamic addresses (e.g. Appletalk) Pick an address at random Broadcast “is anyone using address XX?” If yes, repeat Static address (e.g. Ethernet)

106. 106 Ethernet Frame Structure (cont.) Addresses: 6 bytes Each adapter is given a globally unique address at manufacturing time Address space is allocated to manufacturers 24 bits identify manufacturer E.g., 0:0:15:*  3com adapter Frame is received by all adapters on a LAN and dropped if address does not match Special addresses Broadcast – FF:FF:FF:FF:FF:FF is “everybody” Range of addresses allocated to multicast Adapter maintains list of multicast groups node is interested in

107. 107 Why Did Ethernet Win? Failure modes Token rings – network unusable Ethernet – node detached Good performance in common case Deals well with bursty traffic Usually used at low load Volume  lower cost  higher volume …. Adaptable To higher bandwidths (vs. FDDI) To switching (vs. ATM) Easy incremental deployment Cheap cabling, etc

108. 108 And.. It is Easy to Manage You plug in the host and it basically works No configuration at the datalink layer Today: may need to deal with security Protocol is fully distributed Broadcast-based. In part explains the easy management Some of the LAN protocols (e.g. ARP) rely on broadcast Networking would be harder without ARP Not having natural broadcast capabilities adds complexity to a LAN Example: ATM

109. Ethernet Problems: Unstable at High Load Peak throughput worst with More hosts – more collisions to identify single sender Smaller packet sizes – more frequent arbitration Longer links – collisions take longer to observe, more wasted bandwidth But works well in practice Can improve efficiency by avoiding above conditions S = throughput = “goodput” (success rate) G = offered load = N X p 0.51.0 1.5 2.0 0.1 0.2 0.3 0.4 Pure Aloha Slotted Aloha 1/e = 37%

110. 110 Virtual LANs Single physical LAN infrastructure that carries multiple “virtual” LANs simultaneously. Each virtual LAN has a LAN identifier in the packet. Switch keeps track of what nodes are on each segment and what their virtual LAN id is Can bridge and route appropriately. Broadcast packets stay within the virtual LAN. Limits the collision domain for the packet host Switch

111. 111 Summary CSMA/CD  carrier sense multiple access with collision detection Why do we need exponential backoff? Why does collision happen? Why do we need a minimum packet size? How does this scale with speed? Ethernet What is the purpose of different header fields? What do Ethernet addresses look like? What are some alternatives to Ethernet design?

112. McGraw-Hill©The McGraw-Hill Companies, Inc., 2000 EXTRA SLIDES

113. 113 Outline Random Access Analysis

114. 114 Slotted Aloha Efficiency Q: What is max fraction slots successful? A: Suppose N stations have packets to send Each transmits in slot with probability p Prob. successful transmission S is: by single node: S = p (1-p) (N-1) by any of N nodes S = Prob (only one transmits) = N p (1-p) (N-1) … choosing optimum p as N -> infty... … p = 1/N = 1/e =.37 as N -> infty At best: channel use for useful transmissions 37% of time!

115. 115 Pure Aloha (cont.) P(success by given node) = P(node transmits) X P(no other node transmits in [p 0 -1,p 0 ] X P(no other node transmits in [p 0 -1,p 0 ] = p X (1-p) (N-1) X (1-p) (N-1) P(success by any of N nodes) = N p X (1-p) (N-1) X (1-p) (N-1) = 1/(2e) =.18 … choosing optimum p as N  infty  p = 1/2N … S = throughput = “goodput” (success rate) G = offered load = N X p 0.51.0 1.5 2.0 0.1 0.2 0.3 0.4 Pure Aloha Slotted Aloha protocol constrains effective channel throughput!

116. 116 Simple Analysis of Efficiency Key assumptions All packets are same, small size Packet size = size of contention slot All nodes always have pkt to send p is chosen carefully to be related to N p is actually chosen by exponential backoff Takes full slot to detect collision (I.e. no “fast collision detection”)

117. 117 Ethernet Technologies: 10Base2 10: 10Mbps; 2: under 185 (~200) meters cable length Thin coaxial cable in a bus topology Repeaters used to connect up to multiple segments Repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!

118. 118 Gbit Ethernet Use standard Ethernet frame format Allows for point-to-point links and shared broadcast channels In shared mode, CSMA/CD is used; short distances between nodes to be efficient Uses hubs, called here “Buffered Distributors” Full-Duplex at 1 Gbps for point-to-point links