1. Small Ethernet LANs Chapter 7 Copyright 2001 Prentice Hall Revision 2: July 2001

2. 2 Ethernet n The Most Popular LAN Technology – Carries perhaps 80% of all LAN traffic n Created at the Xerox Palo Alto Research Center (PARC) n Initially standardized by Digital Equipment Corporation, Intel, and Xerox – Ethernet Version 2 (Ethernet II) was the final standard of this partnership – Still used on some LANs

3. 3 LAN Standards n Now, most LAN Standards are Developed by the IEEE – Institute for Electrical and Electronics Engineers – Not just Ethernet LAN standards

4. 4 LAN Standards n Now, most LAN Standards are Developed by the IEEE – Developed through the IEEE’s 802 LAN MAN Standards Committee n MAN is a metropolitan area network (for a city and its suburbs) – IEEE LAN standards are submitted to ISO for ratification as OSI standards

5. 5 LAN Standards n 802 Committee has Working Groups – Working groups develop individual standards – Submit to whole 802 committee – 802.1 develops priority standards and other general standards – 802.3 has taken over the development of new Ethernet standards – 802.5 develops Token-Ring Network standards – 802.11 develops wireless LAN standards

6. 6 LANs are Subnet Standards n Only Physical and Data Link Layer standards n Of course, clients and servers must be compatible at other layers as well Application Transport Internet LAN Subnet (NIC) Application Transport Internet LAN Subnet (NIC)

7. 7 LANs are Subnet Standards n Implemented by the NICs – NICs on the two machines must talk to one another n Hubs and Switches Merely Relay Transmissions – Hubs implement Physical layer only (no Data Link layer needed) – Switches implement the Physical and Data Link layers n Wiring Implements Physical Layer

8. 8 802 Layering n 802 Committee Subdivided the Data Link Layer – Media access control (MAC) layer – Logical link control (LLC) layer OSI802 Data Link LLC MAC PHY

9. 9 802 Layering n Media Access Control (MAC) Layer – Only one station may transmit at a time or signals will be scrambled – MAC layer standards ensure that only one can transmit (access the medium) at a time – Also defines the lowest-layer frame format

10. 10 802 Layering n Logical Link Control (LLC) Layer – Adds optional error correction (rarely used) – Connects to next-higher-layer (internet) – Single LLC standard for all LANs: 802.2 802.2 Logical Link Control Layer Standard IPIPXEtc. 802.3802.5802.11

11. 11 Higher Layers n With OSI LAN standards, six-layer model – Hybrid TCP/IP-IEEE framework n Application n Transport n Internet n Logical Link Control n Media Access Control n Physical – Client and server must use same standard for each layer

12. 12 Ethernet 802.3 Physical Layer n Topology: Order in which stations receive bits n Ethernet hubs use a bus topology – Signal is broadcast – All stations receive almost simultaneously

13. 13 Ethernet 802.3 Physical Layer n Topology: Order in which stations receive bits n Early Ethernet standards arranged stations in a daisy chain – Stations broadcast on the chain in both directions – All stations receive almost simultaneously – Original idea of bus Mod C

14. 14 Ethernet 802.3 Physical Layer n Topology: Order in which stations receive bits n Ethernet switches use a switched topology – Signal only goes to one station

15. 15 Ethernet 802.3 Physical Layer n Ethernet began as a bus network n Some question whether Ethernet switching is really Ethernet n However, hubs will be disappearing in the next few years, and almost all Ethernet will be switched

16. 16 Ethernet 802.3 Physical Layer n Recent Ethernet 802.3 Standards use Unshielded Twisted Pair (UTP) Wiring or Optical Fiber n For Small LANs with a Single Hub or Switch, use UTP Exclusively

17. 17 Physical Layer 802.3 UTP Standards n Ethernet 802.3 10Base-T – Physical layer standard – Created by the 802.3 Working Group – 10 Mbps – Baseband transmission n Insert signal directly into wire n No channels – T means uses UTP telephone wire 10 Mbps 802.3

18. 18 Physical Layer 802.3 UTP Standards n Ethernet 802.3 100Base-TX – 100 Mbps – 100Base-TX: Not just 100Base-T because other 100Mbps UTP standards were created but were not used significantly n Ethernet 802.3 1000Base-T – Gigabit Ethernet – Overkill for small LANs

19. 19 Physical Layer 802.3 UTP Standards n Wiring – Unshielded Twisted Pair – Bundle of 4 pairs (only uses 2 pairs) n One pair to send n One pair to receive – Terminates in RJ-45 connector n Slightly larger than RJ-11 home phone connector

20. 20 Physical Layer 802.3 UTP Standards n Categories of UTP Wiring n For 10Base-T – Categories 3, 4, or 5 are OK – However, most installed wiring is Cat 5 n For 100Base-TX, Cat 5 is required n For Gigabit Ethernet, better to use Enhanced Category 5 (Cat5e) n Cat5e is now recommended for all new LANs in the TIA/EIA-568 standard New

21. 21 Physical Layer: 802.3 UTP Standards n Wiring – 100 meters maximum UTP distance hub-to- station or hub-switch – 200 meters maximum distance between stations 100 m 200 m

22. 22 Physical Layer 802.3 Standards n NIC-Hub Communication – NIC transmits on one pair (Pins 1&2) – Hub or switch transmits on another pair (Pins 3 & 6) – Other 4 wires are not used To Hub or Switch (Pins 1&2) From Hub or Switch (Pins 3&6)

23. 23 Physical Layer 802.3 Standards n Upgrading from 10Base-T to 100Base-TX – Need new hub or switch n May have autosensing 10/100 ports that handle either 10 Mbps or 100 Mbps NICs – Need new NICs n Only for stations that need more speed – No need to rewire n This would be expensive

24. 24 Electrical Signaling: Serial Ports n EIA/TIA-232 Serial Ports (Chapter 4) – One is a low voltage (-3 to -15 volts) – Zero is a high voltage (+3 to +15 volts) – 300 bps to 115.2 kbps – Length of clock cycle is 1/bit rate 01001

25. 25 Electrical Signaling: Loss of Synch n Problem of Long String of Ones or Zeros – No transition to resynchronize receiver’s clock – Receiver may interpret bit N as N-1 or N+1 – At 10 Mbps or 100 Mbps, bit periods are so brief that synchronization must be very exact 12345 123456 Sender Receiver

26. 26 Electrical Signaling: 10Base-T n Manchester Encoding – Used in 10Base-T only – Two voltage levels n High: TD+ (Pin 1) is 2.2 to 2.8 volts higher than TD- (Pin 2) n Low: TD+ is 2.2 to 2.8 volts lower than TD- 1101 10Base-T High Low

27. 27 Electrical Signaling: 10Base-T n Manchester Encoding – Used in 10Base-T – Transition in middle of each bit period – One ends high; zero ends low – Resynchronizes receiver’s clock every bit 1101 Transition in mid-bit 1 ends high 10Base-T

28. 28 Electrical Signaling: 10Base-T n Manchester Encoding is Inefficient – Baud rate is number of possible transitions per second – Baud rate is the limiting factor technically – 20 Mbaud to deliver only 10 Mbps 1101 8 possible transitions 4 bits 10Base-T

29. 29 Older Ethernet Standards n Do Not Use Hubs or Switches n Daisy-Chain Layouts n 10Base5 – Uses attachment unit interface (AUI) ports – D connector with 8 holes in top row, 7 holes in bottom row (15 total) – AUI is the normal Ethernet connector in Cisco routers – Must have an AUI-to-RJ 45 converter to connect UTP to an AUI connector Mod C New

30. 30 Ethernet 10Base2 (802.3a) n Cheaper Physical Layer Standard – NICs have BNC plug (barrel-shaped) – Twist-on T-connector attaches to NIC – T-connector has BNC plugs for cable runs attaching it to adjacent stations NIC BNC T-connector To next NIC To next NIC Mod C

31. 31 Ethernet 10Base2 (802.3a) n Segments are thin coaxial cable – Run only between NICs – Daisy chain of NICs is a segment – Terminator at end of each segment – Up to 30 stations per segment – 5 segments (4 repeaters) maximum – 10Base2: 185 meters/segment NIC Terminator Mod C

32. 32 802.3 MAC Layer: Access Control n Media Access Control (MAC) Layer – Control over when a station may transmit – Only one station may transmit at a time with a hub – Otherwise, their signals would be scrambled Hub

33. 33 802.3 MAC Layer: Access Control n Access Control in Ethernet: CSMA/CD n Carrier Sense Multiple Access (CSMA) – Carrier sense = listen to traffic – Multiple access = control multiple stations

34. 34 802.3 MAC Layer: Access Control n Access Control in Ethernet: CSMA/CD n CSMA Operation – If no one else is transmitting, NIC may transmit – If anyone else is transmitting, NIC must wait until nobody is transmitting If Incoming Traffic, wait If No Incoming Traffic, send

35. 35 802.3 MAC Layer: Access Control n CSMA/CD n Collision Detection (CD) – If two stations transmit at the same time, each hears the other – Both stop, wait random amounts of time – Transmit after wait, but only if the line is free

36. 36 802.3 MAC Layer: Access Control n CSMA/CD n Collision Detection – If there is another collision – Stations back off a longer random time period – After 16 collisions, discard the frame

37. 37 802.3 MAC Layer: Access Control n How to Describe CSMA/CD n 1.First describe CSMA n 2.Second, describe collision detection n 3.Third, describe what happens if there are multiple collisions

38. 38 802.3 MAC Layer: Access Control n Switches Do Not Need CSMA/CD – No danger of collision – Can even work in full duplex (802.3x), with NICs sending and receiving at the same time n However, Ordinary NICs Can Work With Switches – Only hear other traffic if the traffic is directed at them, so waits to transmit are rare and brief

39. 39 802.3 Ethernet MAC Layer Frame n MAC Standard Also Defines 802.3 Ethernet MAC Frame – Header – Data Field – Trailer n Header Has Multiple Fields – Measure size in octets (bytes) TrailerData Field Header Fields Ethernet Frame

40. 40 802.3 Ethernet MAC Layer Frame n Preamble and Start of Frame Delimiter – To synchronize receiver’s clock – Preamble is 56-bit alternating 101010… pattern – SFD is 10101011 to end the synchronization – Together, 64-bit synchronizing pattern PreSFDDASALenDataPADFCS Ethernet 802.3 MAC Layer Frame

41. 41 802.3 Ethernet MAC Layer Frame n Destination Address Field – Address of destination device (receiver) n Source Address Field – Address of source device (sender) n 48-bit MAC Addresses – Must be unique – All NICs are sold with unique MAC addresses PreSFDDASALenDataPADFCS

42. 42 802.3 Ethernet MAC Layer Frame n Source and Destination Addresses are Expressed in Hexadecimal Notation (hex) – Base 16 – 48 bits are divided into twelve 4-bit units – Each unit is represented by a hex symbol (0-9, A-F) – Grouped in pairs of symbols, followed by a lower-case h for Hex PreSFDDASALenDataPADFCS A1-BD-23-0C-09-C3 h

43. 43 802.3 Ethernet MAC Layer Frame n Hex Symbols BitsHex Symbol BitsHex Symbol 0000010008 0001110019 001021010A 001131011B 010041100C 010151101D 011061110E 011171111F

44. 44 802.3 Ethernet MAC Layer Frame n Length Field (2 Octets) – Length of the Data Field, not of the entire frame – Maximum data field size is 1500 octets LenDataPAD

45. 45 802.3 Ethernet MAC Layer Frame n Data Field – Frame of next higher layer, LLC n PAD Field – 46-octet minimum size for MAC data field plus PAD – If Data Field is smaller, add PAD field to bring data field plus PAD to 46 octets LenDataPAD

46. 46 802.3 Ethernet MAC Layer Frame n Frame Check Sequence Field (2 Octets) – Error checking information – Sending computer computes FCS number and places it in FCS field – Uses cyclical redundancy check (CRC) method PreSFDDASALenDataPADFCS

47. 47 802.3 Ethernet MAC Layer Frame n Frame Check Sequence (2 Octets) – Receiving NIC recomputes FCS number – If disagrees with transmitted FCS field, discards the frame! – Does not ask for a retransmission – A higher layer must do this PreSFDDASALenDataPADFCS

48. 48 802.3 Ethernet MAC Layer Frame n Tag Fields Being Added – Added after address fields – To designate priority (frames with higher priority go first if there is congestion) – To designate VLANs (Ch. 8) – 802.1Q standardizes overall structure – 802.1p standardizes priority levels PreSFDDASALenDataPADFCSTPIDTCI

49. 49 802.3 Ethernet MAC Layer Frame n Tag Protocol ID (TPID) (2 Octets) – Located where length field normally goes – Identifies frame as tagged – If a length field, must be less than 1500, because the maximum length of the data field is 1500 octets – TPID field is given the value 81-00 hex (33,024 decimal) PreSFDDASALenDataPADFCSTPIDTCI

50. 50 802.3 Ethernet MAC Layer Frame n Tag Control Information (TCI) (2 Octets) – Gives specific tagging information – Three priority bits (000 to 111) – Eight priority levels, with 111 being high – 12-bit VLAN ID (see Chapter 8) – One bit canonical form indicator (rarely used) PreSFDDASALenDataPADFCSTPIDTCI

51. 51 Processing an Incoming MAC Frame n Receiving NIC reads Preamble and SFD – Synchronizes itself to the incoming bit stream n Receiving NIC reads Source and Destination Address – Discards frame if destination address is not its own – If destination address is its own, continues

52. 52 Processing an Incoming MAC Frame n Reads Next two Octets – If Length field (values <= 1500), sets aside room in RAM for data field – If TPID, handles TCI information, then goes on and reads Length Field – Note: reads next two octets; Not “the length field” n Places Data Field in RAM n Discards PAD if Present – Note: sender adds the PAD, not the receiver

53. 53 Processing an Incoming MAC Frame n Examines Frame Check Sequence – Recomputes the Value based on bits in other fields n If same value as transmitted, the frame is good – Passes deencapsulated data field to LLC layer n If different value than transmitted, frame is bad – Discards the frame – There is no error correction (retransmission)

54. 54 Other LAN Standards n There are Other Physical and MAC Layer Standards – 802.11 Wireless LAN standards – 802.5 Token-Ring Network standards – Etc. Box

55. 55 802.11 Wireless LANs n Wireless Technologies for LANs – Radio – Infrared light (as in TV remote control) – Ideal for mobile devices – Useful when wiring would be costly Box

56. 56 802.11 Wireless LAN Standards n Standards come from the 802.11 Working Group – Initially, 1 Mbps and 2 Mbps. Ignored by the market – Now, 11 Mbps (802.11b) n Becoming popular n Can only serve a few wireless stations in each area – 802.11a is being finalized n 54 Mbps, so can serve many more stations n Should create an explosion in wireless LAN use n Vendors building products before standard is finalized Box New

57. 57 802.11 Wireless LANs n Normally use an Access Point – Bridges wireless device to server on wired LAN n Box about the size of a hard cover book n Devices in theory can be 100 meters from the access point for 802.11b. Usually only 30 meters Access Point Box UTP RJ-45 Port Switch Or Hub Server

58. 58 802.11 Wireless LANs n Ad Hoc Mode – Clients and servers communicate directly – Good for wireless conference rooms – Not scalable beyond one group of devices New Server

59. 59 802.11 Wireless LANs n Media Access Control (CSMA/CA+ACK) – CSMA/CA – CSMA with Collision Avoidance n Tries to avoid collisions – When line is clear, station may send (CSMA), – but before it sends, must wait a random amount of time – This prevents stations that have been waiting to transmit from all transmitting at once when the currently transmitting station is finished Mod C

60. 60 802.11 Wireless LANs n Media Access Control – When a frame is received correctly, the receiver immediately sends back an acknowledgement – This allows the sender to know if it needs to resend Frame ACK Mod C

61. 61 802.11 Versus Bluetooth n 802.11 – Designed for site radio LANs n Bluetooth – Created by an industrial consortium – Designed to link nearby objects (within a few meters) – Personal area networking (cellphone, computer, printer, etc.) Box

62. 62 802.11 Versus Bluetooth n Bluetooth – Only 721 kbps transmission speed n Only 56 kbps back channel – Up to 10 piconets in an area n Each with a maximum of eight devices – Named for King Harald Bluetooth – Has standard for device synchronization. For instance, PC can use a printer without first loading a print driver for that printer. New

63. 63 802.11 Versus Bluetooth n Possible Interference – 802.11 and Bluetooth use the same frequency band n This is the 2.4 GHz band (2.4-2.485 GHz), which does not require each device to be licensed – May interfere if they are active in the same area – 801.15 Working Group is working on coexistence methods Box New

64. 64 802.5 Token-Ring Networks: Topology n An alternative to Ethernet 802.3 LANs n Physical Layer Topology: Ring – Stations connected in a loop – Signals go in only one direction, station-to- station – Not bus physical layer topology like Ethernet 802.3 Box

65. 65 802.5 TRN Physical Layer: Topology Access Unit STP link from Station to Access Unit Stations Station UTP Link from Station to Access Unit n Physically, stations connect to access units which are connected in a ring Mod C

66. 66 802.5 TRN Physical Layer: Wiring Access Unit STP link between Access Units STP link from Station to Access Unit Stations Station UTP Link from Station to Access Unit n Most connections use shielded twisted pair (STP), which has each pair and the whole cable covered with a metal shield to reduce interference Mod C

67. 67 802.5 TRN: Physical Speed n 802.5 Speeds – Initially, 4 Mbps – Now, mostly 16 Mbps – 100 Mbps is standardized but not widely used Box

68. 68 802.5 TRN MAC Layer: Token Passing n Media Access Control – Not CSMA/CD – Token passing – Special frame called a token circulates – Station can only transmit if it has the token Token Transmits Box

69. 69 Token-Ring Networks n 802.5 Token-Ring versus 802.3 CSMA/CD-Bus – Token-Ring is more reliable – Token-Ring is more efficient – Token-Ring is more expensive – Token-Ring has a small market share – Companies buy something good enough to meet requirements, and 802.3 standards do this

70. 70 802.3 Ethernet versus 802.5 Token-Ring Network 802.3802.5 LLC 802.2 MAC Access Control CSMA/CDToken Passing PHY Topology BusRing n Both use 802.2 Standard at the LLC Layer n MAC Layer: CSMA/CD versus token-passing n PHY Layer Topology: Bus versus Ring

71. 71 Total Standards Picture n Client PC and Server Must be Compatible at All Six Layers Application Transport Internet LAN Subnet (NIC) Application Transport Internet LAN Subnet (NIC)

72. 72 Upper Layers n Application Layer – Standard depends on the application – File service – Print services – Electronic mail – Etc. n Transport and Internet Layers – All new servers use TCP/IP standards – Transport layer: TCP – Internet layer: IP Application (service) Transport (TCP) Internet (IP)

73. 73 Upper Layers n Novell NetWare – Now uses TCP/IP – Before, used NetWare’s proprietary IPX/SPX – Many old NetWare servers still use IPX/SPX Application Transport Internet Other SPX IPX NCP IPX

74. 74 Upper Layers n Novell NetWare – IPX at the internet layer – NCP at transport & application n File service, print service, etc. – SPX sometimes at transport Application Transport Internet Other SPX IPX NCP IPX

75. 75 Upper Layers n All Servers on a LAN Use the Same Subnet Layer Standards, which are implemented by NICs n Servers can differ in upper-layer standards App TCP IP 802.2 802.3 MAC 100Base-TX App SPX IPX 802.2 802.3 MAC 100Base-TX App NetBEUI 802.2 802.3 MAC 100Base-TX Box

76. 76 Upper Layers n Client Software is Flexible – Speaks TCP/IP to Windows NT Server, UNIX, new Novell NetWare Servers – Speaks IPX/SPX to older NetWare Servers – Simultaneously! TCP/IPIPX/SPX

77. 77 Client Communication n NIC is Really – The physical hardware plus – Software: device driver – Upper-layer software talks to device driver – Together, implement subnet layer protocols Device Driver NIC NIC and Device driver Together handle PHY, MAC, LLC Box

78. 78 Client Communication n Client PC has Multiple Transport-Internet Layer Protocol Stacks for Different Protocols n NDIS in Windows Governs their Communication with the Single NIC TCP IP SPX IPX NDIS Box

79. 79 Client Communication n NDIS routes incoming packets to correct stack (IPX to SPX/IPX, etc.) Box TCP IP SPX IPX NDIS

80. 80 Client Communication n NDIS feeds outgoing packets one at a time to the NIC TCP IP SPX IPX NDIS Box