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

2 Ethernet n Most Popular LAN Technology ( 80% of LAN’s) n Created: Xerox Palo Alto Research Center (1970s) n Standardized by Digital Equipment Corporation, Intel, and Xerox (1980s) – Ethernet II final standard of this partnership n Now, most LAN Standards are Developed by IEEE – Institute for Electrical and Electronics Engineers – Not just Ethernet LAN standards – 802 LAN MAN Standards Committee n MAN is a metropolitan area network (for a city and its suburbs) – IEEE LAN standards submitted to ISO for ratification as OSI standards

3 LAN Standards n 802 Committee has Working Groups – Working groups develop specific standards – Submit to whole 802 committee – develops priority & other general standards – development of new Ethernet standards – develops Token-Ring Network standards – develops wireless LAN standards

4 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)

5 LANs are Subnet Standards n Implemented by the NICs – NICs on two machines must talk to one another n Hubs and Switches Merely Relay Transmissions – Hubs implement Physical layer only n Bit by bit – Switches implement Physical & Data Link layers n Frames n Wiring Implements Physical Layer

6 802 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

7 802 Layering n Media Access Control (MAC) Layer – MAC layer standards ensure that only one can transmit (access the medium) at a time n Reduces collisions & unreadable transmissions – Also defines frame format n Logical Link Control (LLC) Layer – Adds optional error correction (rarely used) – Connects to next-higher-layer (internet) Logical Link Control Layer Standard IPIPXEtc

8 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

9 Ethernet Physical Layer n Topology: Order in which stations receive bits n Ethernet hubs use a bus topology – Broadcast Signal received simultaneously n Ethernet switches use a switched topology – Signal only goes to one station

10 Ethernet Physical Layer n Early Ethernet arranged stations in daisy chain – Stations broadcast on the chain in both directions & received almost simultaneously – Original idea of bus 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

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

12 Physical Layer UTP Standards n Ethernet Base-T n Ethernet Base-TX – 100Base-TX: Not just 100Base-T because other 100Mbps UTP standards were created but were not used significantly n Ethernet Base-T – Overkill for small LANs – Number represents speed – Created by the Working Group – Baseband transmission n Insert signal directly into wire, No channels – T means uses UTP telephone wire

13 Physical Layer UTP Standards n Wiring: Unshielded Twisted Pair – Bundle of 4 pairs (only 1000 Base-T uses all 4 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 n For 10Base-T: Categories 3, 4, or 5 (most use Cat 5) n For 100Base-TX, Cat 5 is required n For Gigabit Ethernet: best to use Enhanced Category 5

14 Physical Layer Standards n 10 Base-T & 100 Base-TX NIC-Hub Communication – NIC transmits on one pair (Pins 1&2) – Hub/switch transmits on another pair (Pins 3 & 6) – Other 4 wires are not used (except with 1000 Base-T) n Echo Cancellation To Hub or Switch (Pins 1&2) From Hub or Switch (Pins 3&6)

15 Physical Layer: 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

16 Physical Layer 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

17 Electrical Signaling: Serial Ports n EIA/TIA-232 Serial Ports (Chapter 4) – Zero is a low voltage (-3 to -15 volts) – One is a high voltage (+3 to +15 volts) – 300 bps to kbps – Length of clock cycle is 1/bit rate 10110

18 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 Sender Receiver

19 Electrical Signaling: 10Base-T n Manchester Encoding: Transition in middle of each bit period – Used in 10Base-T only – Two voltage levels n High: TD+(Pin 1) is volts higher than TD-(Pin 2) n Low: TD+ is volts lower than TD- – Ones end high & zeroes end low – Resynchronizes receiver’s clock every bit 1101 Transition in mid-bit

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

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

MAC Layer: Access Control n Access Control in Ethernet: CSMA/CD n Carrier Sense Multiple Access (CSMA)with Collision Detection – Carrier sensing = listening to network signal – Multiple access = control multiple stations – Collision Detection = two stations transmitting at same time

MAC Layer: Access Control n CSMA Operation – If NIC hears no signal, NIC may transmit (half-duplex) – If NIC hears transmitting signal, NIC must wait n Collision Detection (CD) – When collide all stations stop, wait random amounts of time (wait time differs with station) – After wait, implement CSMA (transmit if line is free) – If another collision, stop & wait longer random time – After 16 collisions, discard the frame

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

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

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

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

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

Ethernet MAC Layer Frame n Hex Symbols BitsHex Symbol BitsHex Symbol A B C D E F

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 n Future-Jumbo Frames: 9,000 octets LenDataPAD

Ethernet MAC Layer Frame n Data Field – Frame of next higher layer, LLC (message to be sent) 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

Ethernet MAC Layer Frame n Frame Check Sequence Field (4 Octets) – Created on basis of bit patterns in other fields (excl Preamble, Start of Field, Frame Check Seq) – Sending computer computes FCS number and places it in FCS field – Uses cyclical redundancy check (CRC) method PreSFDDASALenDataPADFCS

Ethernet MAC Layer Frame n Frame Check Sequence (4 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

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) – 802.1Q standardizes overall structure – 802.1p standardizes priority levels PreSFDDASALenDataPADFCSTPIDTCI

Ethernet MAC Layer Frame n Tag Protocol ID (TPID) (2 Octets) – Follows source address & Identifies frame as tagged – Length fields have maximum length of 1500 octets – If tagged TPID field = hex (33,024 decimal) n Tag Control Information (TCI) (2 Octets) – User Priority (3 bit): Eight priority levels (2 3 ) n Larger values indicate higher priority – Canonical form indicator (1 bit): value=0 unless frame encapsulates Token-Ring Network frame with routing information field (rarely used) – VLAN ID (12-bit): virtual LAN identifier PreSFDDASALenDataPADFCSTPIDTCI

36 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 n Reads Next 2 Octets – If Length field (<= 1500), reserves RAM for data field – If TPID, handles TCI information, then reads Length Field n Places Data Field in RAM n Discards PAD if Present

37 Processing an Incoming MAC Frame n Examines Frame Check Sequence via recomputation – If FCS equals, the frame is good – Passes deencapsulated data field to LLC layer n LLC determines which next-higher-layer program receives frame’s data contents n Optional error correction n If FCS does not equal, frame is bad – Discards the frame – There is no error correction (retransmission)

38 Other LAN Standards n There are Other Physical and MAC Layer Standards – Wireless LAN standards – Token-Ring Network standards – Etc.

Wireless LANs n Wireless Technologies for LANs – Radio or Infrared light (TV remote control) – Ideal for mobile devices – Access Point bridges wireless device to wired LAN n Box about the size of a hard cover book mounted on walls or ceilings Access Point UTP RJ-45 Port Switch Or Hub Server

Wireless LANs n Media Access Control (CSMA/CA+ACK) – CSMA with Collision Avoidance n Tries to avoid collisions – When line is clear, station may send (CSMA), – Before sending, must wait a random amount of time n This prevents waiting stations from transmitting all at once when the currently transmitting station is finished n Media Access Control – Frames received correctly, send back acknowledgement – This allows the sender to know if it needs to resend

Versus Bluetooth n Bluetooth – Designed to link nearby objects (within a few meters) – Personal area networking (cellphone, computer, printer) – Limited by: n Speed – Only 721 kbps transmission speed n Maximum number of devices connected n Distance

Token-Ring Networks: Topology n An alternative to Ethernet LANs n Physical Layer Topology: Ring – Stations connected in a loop – Signals go in one direction, station-to-station – Unlike Ethernet bus topology

TRN Physical Layer: Topology n Physically, stations connect to access units which are connected in a ring n Most connections use shielded twisted pair (STP), which has each pair and the whole cable covered with a metal shield to reduce interference

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

45 Token-Ring Networks n Token-Ring versus 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 standards do this LLC MAC Access Control CSMA/CDToken Passing PHY Topology BusRing

46 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)

47 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 MAC 100Base-TX App SPX IPX MAC 100Base-TX App NetBEUI MAC 100Base-TX

48 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

49 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

50 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 n NDIS feeds outgoing packets one at a time to the NIC n NDIS routes incoming packets to correct stack (IPX to SPX/IPX, etc.) TCP IP SPX IPX NDIS