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

Small Ethernet LANs

Chapter 7

Copyright 2001 Prentice HallRevision 2: July 2001

2Ethernet

The Most Popular LAN Technology– Carries perhaps 80% of all LAN traffic

Created at the Xerox Palo Alto Research Center (PARC)

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

3LAN Standards

Now, most LAN Standards are Developed by the IEEE– Institute for Electrical and Electronics

Engineers– Not just Ethernet LAN standards

4LAN Standards

Now, most LAN Standards are Developed by the IEEE– Developed through the IEEE’s 802 LAN MAN

Standards Committee MAN is a metropolitan area network (for a city and

its suburbs)

– IEEE LAN standards are submitted to ISO for ratification as OSI standards

5LAN Standards

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

6LANs are Subnet Standards

Only Physical and Data Link Layer standards

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)

7LANs are Subnet Standards

Implemented by the NICs– NICs on the two machines must talk to one

another

Hubs and Switches Merely Relay Transmissions– Hubs implement Physical layer only (no Data

Link layer needed)– Switches implement the Physical and Data Link

layers Wiring Implements Physical Layer

8802 Layering

802 Committee Subdivided the Data Link Layer– Media access control (MAC) layer– Logical link control (LLC) layer

OSI 802

Data LinkLLCMAC

PHY PHY

9802 Layering

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

10802 Layering

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

IP IPX Etc.

802.3 802.5 802.11

11Higher Layers

With OSI LAN standards, six-layer model– Hybrid TCP/IP-IEEE framework

Application Transport Internet Logical Link Control Media Access Control Physical

– Client and server must use same standard for each layer

12Ethernet 802.3 Physical Layer

Topology: Order in which stations receive bits

Ethernet hubs use a bus topology– Signal is broadcast– All stations receive almost simultaneously


Topology: Order in which stations receive bits 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


Topology: Order in which stations receive bits Ethernet switches use a switched topology

– Signal only goes to one station


Ethernet began as a bus network

Some question whether Ethernet switching is really Ethernet

However, hubs will be disappearing in the next few years, and almost all Ethernet will be switched


Recent Ethernet 802.3 Standards use Unshielded Twisted Pair (UTP) Wiring or Optical Fiber

For Small LANs with a Single Hub or Switch, use UTP Exclusively

17Physical Layer 802.3 UTP Standards

Ethernet 802.3 10Base-T

– Physical layer standard

– Created by the 802.3 Working Group

– 10 Mbps

– Baseband transmission Insert signal directly into wire No channels

– T means uses UTP telephone wire

10 Mbps

802.3


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

Ethernet 802.3 1000Base-T

– Gigabit Ethernet

– Overkill for small LANs


Wiring

– Unshielded Twisted Pair

– Bundle of 4 pairs (only uses 2 pairs) One pair to send One pair to receive

– Terminates in RJ-45 connector Slightly larger than RJ-11 home phone connector


Categories of UTP Wiring

For 10Base-T– Categories 3, 4, or 5 are OK– However, most installed wiring is Cat 5

For 100Base-TX, Cat 5 is required

For Gigabit Ethernet, better to use Enhanced Category 5 (Cat5e)

Cat5e is now recommended for all new LANs in the TIA/EIA-568 standard

New

21Physical Layer: 802.3 UTP Standards

Wiring– 100 meters maximum UTP distance hub-to-

station or hub-switch– 200 meters maximum distance between stations

100 m 100 m

200 m

22Physical Layer 802.3 Standards

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)

23Physical Layer 802.3 Standards

Upgrading from 10Base-T to 100Base-TX

– Need new hub or switch May have autosensing 10/100 ports that handle

either 10 Mbps or 100 Mbps NICs

– Need new NICs Only for stations that need more speed

– No need to rewire This would be expensive

24Electrical Signaling: Serial Ports

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

0 1 0 0 1

25Electrical Signaling: Loss of Synch

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

1 2 3 4 5

1 2 3 4 5 6Sender

Receiver

26Electrical Signaling: 10Base-T

Manchester Encoding– Used in 10Base-T only– Two voltage levels

High: TD+ (Pin 1) is 2.2 to 2.8 volts higher than TD- (Pin 2)

Low: TD+ is 2.2 to 2.8 volts lower than TD-

1 1 0 1

10Base-T

High

Low


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

1 1 0 1

Transitionin mid-bit 1 ends

high

10Base-T


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

1 1 0 1

8 possibletransitions

4 bits

10Base-T

29Older Ethernet Standards

Do Not Use Hubs or Switches

Daisy-Chain Layouts

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

30Ethernet 10Base2 (802.3a)

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-connectorTo next

NICTo next

NIC

Mod C

31Ethernet 10Base2 (802.3a)

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 NIC NICTerminator

Mod C

32802.3 MAC Layer: Access Control

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 Hub


Access Control in Ethernet: CSMA/CD

Carrier Sense Multiple Access (CSMA)– Carrier sense = listen to traffic– Multiple access = control multiple stations


Access Control in Ethernet: CSMA/CD CSMA Operation

– If no one else is transmitting, NIC may transmit– If anyone else is transmitting, NIC must wait

until nobody is transmitting

If IncomingTraffic, wait

If NoIncoming

Traffic, send


CSMA/CD

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


CSMA/CD

Collision Detection

– If there is another collision

– Stations back off a longer random time period

– After 16 collisions, discard the frame


How to Describe CSMA/CD

1. First describe CSMA

2. Second, describe collision detection

3. Third, describe what happens if there aremultiple collisions


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

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

39802.3 Ethernet MAC Layer Frame

MAC Standard Also Defines 802.3 Ethernet MAC Frame– Header– Data Field– Trailer

Header Has Multiple Fields– Measure size in octets (bytes)

Trailer Data Field

Header Fields

Ethernet Frame


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


Destination Address Field– Address of destination device (receiver)

Source Address Field– Address of source device (sender)

48-bit MAC Addresses– Must be unique– All NICs are sold with unique MAC addresses



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


A1-BD-23-0C-09-C3 h


Hex Symbols

Bits Hex Symbol

Bits Hex Symbol

0000 0 1000 80001 1 1001 90010 2 1010 A0011 3 1011 B0100 4 1100 C0101 5 1101 D0110 6 1110 E0111 7 1111 F


Length Field (2 Octets)

– Length of the Data Field, not of the entire frame

– Maximum data field size is 1500 octets

LenDataPAD


Data Field– Frame of next higher layer,

LLC

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


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



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



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

PreSFDDASALenDataPADFCS TPIDTCI


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)



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)


51Processing an Incoming MAC Frame

Receiving NIC reads Preamble and SFD– Synchronizes itself to the incoming bit stream

Receiving NIC reads Source and Destination Address– Discards frame if destination address is not its

own– If destination address is its own, continues


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” Places Data Field in RAM Discards PAD if Present

– Note: sender adds the PAD, not the receiver


Examines Frame Check Sequence– Recomputes the Value based on bits in other

fields

If same value as transmitted, the frame is good– Passes deencapsulated data field to LLC layer

If different value than transmitted, frame is bad– Discards the frame– There is no error correction (retransmission)

54Other LAN Standards

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

Box

55802.11 Wireless LANs

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

Box

56802.11 Wireless LAN Standards

Standards come from the 802.11 Working Group– Initially, 1 Mbps and 2 Mbps. Ignored by the

market– Now, 11 Mbps (802.11b)

Becoming popular Can only serve a few wireless stations in each area

– 802.11a is being finalized 54 Mbps, so can serve many more stations Should create an explosion in wireless LAN use Vendors building products before standard is

finalized

Box

New


Normally use an Access Point– Bridges wireless device to server on wired LAN

Box about the size of a hard cover book Devices in theory can be 100 meters from the access

point for 802.11b. Usually only 30 meters

Access Point

Box

UTP RJ-45Port

SwitchOr Hub Server


Ad Hoc Mode– Clients and servers communicate directly– Good for wireless conference rooms– Not scalable beyond one group of devices

New

Server


Media Access Control (CSMA/CA+ACK)– CSMA/CA– CSMA with Collision Avoidance

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


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

61802.11 Versus Bluetooth

802.11– Designed for site radio LANs

Bluetooth– Created by an industrial consortium– Designed to link nearby objects (within a few

meters)– Personal area networking (cellphone, computer,

printer, etc.)

Box


Bluetooth

– Only 721 kbps transmission speed Only 56 kbps back channel

– Up to 10 piconets in an area 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


Possible Interference

– 802.11 and Bluetooth use the same frequency band

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

New

64802.5 Token-Ring Networks: Topology

An alternative to Ethernet 802.3 LANs

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

65802.5 TRN Physical Layer: Topology

Access Unit Access Unit


STP linkfrom Stationto Access

UnitStations

Station

UTP Linkfrom Stationto Access

Unit

Physically, stations connect to access units which are connected in a ring

Mod C

66802.5 TRN Physical Layer: Wiring



STP link betweenAccess Units

STP linkfrom Stationto Access

UnitStations

Station

UTP Linkfrom Stationto Access

Unit

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

67802.5 TRN: Physical Speed

802.5 Speeds– Initially, 4 Mbps– Now, mostly 16 Mbps– 100 Mbps is standardized but not widely used

Box

68802.5 TRN MAC Layer: Token Passing

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

69Token-Ring Networks

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

70802.3 Ethernet versus 802.5 Token-Ring Network

802.3 802.5

LLC 802.2

MAC Access Control CSMA/CD Token Passing

PHY Topology Bus Ring

Both use 802.2 Standard at the LLC Layer

MAC Layer: CSMA/CD versus token-passing

PHY Layer Topology: Bus versus Ring

71Total Standards Picture

Client PC and Server Must be Compatible at All Six Layers

Application

Transport

Internet

LAN Subnet(NIC)

Application

Transport

Internet

LAN Subnet(NIC)

72Upper Layers

Application Layer– Standard depends on the application– File service– Print services– Electronic mail– Etc.

Transport and Internet Layers– All new servers use TCP/IP standards– Transport layer: TCP– Internet layer: IP

Application (service)

Transport (TCP)

Internet (IP)

73Upper Layers

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

74Upper Layers

Novell NetWare– IPX at the internet layer– NCP at transport & application

File service, print service, etc.– SPX sometimes at transport

Application

Transport

Internet

Other

SPX

IPX

NCP

IPX

75Upper Layers

All Servers on a LAN Use the Same Subnet Layer Standards, which are implemented by NICs

Servers can differ in upper-layer standards

AppTCPIP

802.2802.3 MAC100Base-TX

AppSPXIPX

802.2802.3 MAC100Base-TX

App

NetBEUI

802.2802.3 MAC100Base-TX

Box

76Upper Layers

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/IP IPX/SPX

77Client Communication

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 andDevice driver

Together handlePHY, MAC, LLC

Box


Client PC has Multiple Transport-Internet Layer Protocol Stacks for Different Protocols

NDIS in Windows Governs their Communication with the Single NIC

TCPIP

SPXIPX

NDIS

Box


NDIS routes incoming packets to correct stack (IPX to SPX/IPX, etc.)

Box

TCPIP

SPXIPX

NDIS


NDIS feeds outgoing packets one at a time to the NIC

TCPIP

SPXIPX

NDIS

Box

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

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