unit 6 -ethernet

8/13/2019 Unit 6 -Ethernet

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BehrouzA. Forouzan Data communication and Networking 1

Wired LANs: Ethernet

Chapter 13

Dr. Gihan NAGUIB


1313--11 IEEE STANDARDSIEEE STANDARDS

In In 1985 1985, , the the Computer Computer Society Society of of the the IEEE IEEE started started a a project, project, called called Project Project 802 802, , to to set set standards standards to to enable enable intercommunication intercommunication among among equipment equipment from from a a

variety variety of of manufacturers manufacturers. . Project Project 802 802 is is a a way way of of specifying specifying functions functions of of the the physical physical layer layer and and the the data data link link layer layer of of major major LAN LAN protocols protocols. .

Data Link LayerPhysical Layer

Topics discussed in this section: Topics discussed in this section:

Dr. Gihan NAGUIB


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IEEE standard for LANs

Dr. Gihan NAGUIB

IEEE divided the Data link layer into two sublayer :upper layer : logical link control ( LLC ); flow and

error control.Lower sublayer : Multiple access ( MAC ); media

access control.

Multiple access ( MAC ) for resolving access to theshared media.

If channel is dedicated ( point to point) we do not needthe ( MAC ); sublayer.


IEEE standard for LANs

Dr. Gihan NAGUIB

Token passingCSMA/CD


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LLC (Logical link control) and MAC (Media Access Control)

In IEEE project 802, flow control , error control, and part of theframing duties are collected into one sublayer called the logicallink control (LLC )LLC provides one single data link control for all IEEE LANs.

IEEE project 802 has created a sublayer MAC that defines thespecific access method for each LAN. In contrast to the LLC, MACcontains a number of distinct modules: each defines the access methodand the framing format specific to the corresponding LAN protocol

For example:

CSMA/CD as media access method for Ethernet LANs.Token passing method for Token Ring and Token Bus LANs.

Framing is handled in both the LLC and MAC sublayer.

Dr. Gihan NAGUIB

BehrouzA. Forouzan Data communication and Networking 6Dr. Gihan NAGUIB

Physical layer

Physical layer is dependent on the implementationand type of the physical media used.IEEE define detailed specifications for each LANimplementation.For example, although there is only one MACsublayer for Standard Ethernet( CSMA/CD), there isa different physical layer specifications for eachEthernet implementations .


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DescriptionName

EthernetIEEE 802.3

Token busIEEE 802.4

Token RingIEEE 802.5

Wireless LANIEEE 802.11a/b/g/n

BluetoothIEEE 802.15.1

IEEE 802 Series of LAN Standards


ETHERNET IEEEETHERNET IEEE 802 802. .33

It is the dominant LAN technology.

Cheap

First widely used LAN technology

Simpler and cheaper than token LANs

Kept up with speed race: 10, 100, 1000 Mbps


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ETHERNET EvolutionETHERNET Evolution

The The original original Ethernet Ethernet was was created created in in 1976 1976 at at Xeroxs Xeroxs Palo Palo Alto Alto Research Research Center Center (PARC) (PARC). . Since Since then,then, it it has has

gone gone through through four four generations generations. .


ETHERNETETHERNET

The MAC sublayer governs the operation of the random

access methodStandard Ethernet uses CSMA/CD with 1-persistentEthernet dose not provide any mechanism foracknowledging received frames( unreliable medium ).

Acknowledgments must be implemented at the higher layer.It also frames data received from the upper layer andpasses them to the physical layer.

MAC Sublayer


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Ethernet (IEEE 802.3) MAC frame

Preamble :7bytes (56 bits); Alternating 0s and 1s , used for synchronizing

Start Frame Delimiter (SFD):

10101011 indicates the start of the frame.Last two bits (11) alerts that the next field is destination address.

preamble and SFD are added at the physical layerand is not formally part of the frame

The Ethernet frame contains seven fields :


Ethernet (IEEE 802.3) MAC frame

DA : Destination address:SA: Source Address :Length/Type:

Define the upper-layer protocol using the MACframe. OR define the number of bytes in the data filed.

Data: minumum: 46 and maximum : 1500 bytes

CRC: error detection information:CRC-32


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Frame length:Minimum: 64 bytes (512 bits)

Maximum: 1518 bytes (12,144 bits)

Note


Minimum and maximum frame lengths

Minimum frame length restriction (64 bytes) is required for the correctoperation of CSMA/CD.

Min data length =64 - 18 (6+-6+2+4) = 46 bytesIf the upper- layer packet is less than 46 bytes, padding is added to

make up the difference.Maximum length restriction; two historical reasons:

Memory was very expensive when Ethernet was designed.Prevents one station from monopolizing the shared medium,

blocking other stations that have data to sent.

Max data length =1518- 18= 1500 bytes.


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Ethernet address in hexadecimal notation

Each station (PC or printer) has a networkinterface card (NIC) which provides the station witha 6-byte [48 bits] physical address (MAC adress)

It is written in hexadecimal notation, with acolon between the bytes.


Unicast and multicast addresses

Source address is always a unicast address the framescomes from only one station.

Destination address can be:unicast: defines only one recipient; one to onemulticast: a group of addresses; one to many

Broadcast: the recipients are all the stations onthe LAN


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The least significant bit of the first bytedefines the type of address.

If the bit is 0, the address is unicast;otherwise, it is multicast.

The broadcast destination address is aspecial case of the multicast address in whichall bits are 1s .

Note


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 secondhexadecimal digit from the left. If it is even , the address isunicast. If it is odd , the address is multicast. If all digits are Fs,the address is broadcast . Therefore, we have the following: a . This is a unicast address because A in binary is 1010.b. This is a multicast address because 7 in binary is 0111.c. This is a broadcast address because all digits are Fs.

Example 1


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Show how the address 47:20:1B:2E:08:EE is sent out online.

Solution The address is sent left-to-right , byte by byte; for eachbyte, it is sent right-to-left( LSB first), bit by bit, asshown below:

Example 2

left-to-right : 47 20 1 B 2 E 08 EE47 is 0100 0111 right -to-left 1110 0010


Slot TimeSlot time defined in bits:

It is the time required for a station to send 512 bits(min frame size).

Slot Time = round trip time + time required tosend the jam sequenceIt depends on the data rate, for traditional 10-Mbps

Ethernet it is 51.2 microseconds (512/10Mbps)

The choice of 512-bit Slot Time to allow the properfunction of CSMA/CD.


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Categories of Standard Ethernet

The Standard Ethernet defines several physical layer implementation,four of the most common:

Limitation of 10Base5 and 10Base2 is that communicationis half-duplex.

Why half deplux??


Encoding in a Standard Ethernet implementation

All standard implementations use digital signaling ( baseband) at10 Mbps.

At the sender, data are converted to a digital signal using theManchester scheme .

At the receiver, the received signal is interpreted as Manchester anddecoded into data.

Uses CSMA/CD with 1-persistent


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10Base5:Thick Ethernet (Thicknet)

Uses coaxial cable and Bus topology

With an external transceiver ( transmitter/receiver) connected via a tap.

Transceiver is responsible for:

transmitting, receiving and

detecting collisions.

The length of each segment cannot exceed 500 mIf cable > 500 m ,degradation in the signal, using repeaters to connect

multiple segments of cable.No two stations can be separated by more than 2500m( max length of the

bus) and 4 repeaters.


10Base2:Thin Ethernet (Cheapernet)

Uses Bus topology with thinner and more flexible cables.Transceiver part of a NIC cardThe implementation is more cost effective than 10Base5 because:

Thin coaxial cable is less expensive than the thick tee connections are cheaper than taps

Installation is simpler because thin coaxial cable is very flexible.The length of each segment under 200 (cannot exceed 185 m) due to the

high level of attenuationRepeaters are used to connect multiple segments


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10 Base-T: Twisted-Pair Ethernet

Physical star topologyStations are connected to a hub via two pairs of twisted

cable( one for sending and one for receiving|).

Any collisions happens in the hubCompared to others, the hub replaces the coaxial cable as

far as a collision is concerned.Max length = 100 m to minimize attenuation.


10Base-F: Fiber Ethernet

Uses star topology to connect stations to a hub

Stations is connected to the hub by using twopairs of fiber-optic cables.


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Summary of Standard Ethernetimplementations

10Base 5 10Base2 10Base-T 10Base-F

Media Thick coaxialcable

Thin coaxialcable

Two UTP 2 Fiber

Maximumlength

500 m 185 m 100 m 2000 m

Topology Bus Bus Star star

Data rate 10Mbps 10Mbps 10Mbps 10Mbps

Line coding Manchester Manchester Manchester Manchester


1313--33 CHANGES IN THE STANDARDCHANGES IN THE STANDARD

The The 10 10- -Mbps Mbps Standard Standard Ethernet Ethernet has has gone gone through through several several changes changes before before moving moving to to the the higher higher data data rates rates. . These These changes changes actually actually opened opened the the road road to to the the evolution evolution of of the the Ethernet Ethernet to to become become compatible compatible with with other other high high- -data data- -rate rate LANs LANs..

Bridged EthernetSwitched EthernetFull-Duplex Ethernet

Topics discussed in this section: Topics discussed in this section:


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Bridged Ethernet

Have two effects on an Ethernet LAN:1. They rise the bandwidth2. They separate collision domains.Without bridges, all the stations share the bandwidth of the

network.


Bridged Ethernet: Raising the Bandwidth

Bridges divide the network into two.Each network is independent.With bridges, 10 Mbps network is shared only by 6 [actually

7 as bridge acts as one station] stations.


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Using bridges:Collision domain becomes much smaller and the

probability of collision is reducedRaising the B-W

Bridged Ethernet: Separate Collision domains


Switched Ethernet

A layer 2 switch is an N-port bridge withadditional sophistication that allows fasterhandling of the packets.


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Full-duplex switched Ethernet

In full duplex switch there are two links, one for sendingand one for receiving , we dont need CSMA/CD here|(no collision).

Increases the capacity of each domain from 10 to20 Mbps.


FAST ETHERNETFAST ETHERNET

Fast Fast Ethernet Ethernet was was designed designed to to compete compete with with LAN LAN protocols protocols such such as as FDDI FDDI or or Fiber Fiber Channel Channel. . IEEE IEEE created created Fast Fast Ethernet Ethernet under under the the name name 802 802..3 3u u.. Fast Fast Ethernet Ethernet is is backward backward- -compatible compatible with with Standard Standard Ethernet,Ethernet, but but it it can can transmit transmit data data 10 10 times times faster faster at at a a rate rate of of 100 100 Mbps Mbps. .


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MAC Sublayer

Main consideration in the evolution of Ethernet from 10 to100 Mbps was to keep the MAC sublayer untouched.Drop bus topologies and keep only the star topology.

Why??Star topology have two choices :

Half-duplex approach:

The stations are connected via a hub.The access method is CSMA/CD

Full-duplex approach.(Fast )The connection is made via a switch with buffers ateach port.

No need for CSMA/CD


Fast Ethernet topology

Topology:Two stations: point-to-pointThree or more stations: star topology with a switchat the center


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1. Upgrade the data rate to 100 Mbps2. Make it compatible with Standard Ethernet3. Keep the same frame format4. Keep the same minimum and maximum frame length

Autonegotiation:Allows two devices to negotiate the mode (half duplex

or full duplex) or data rate of operation.Ex: Allow incompatible devices to connect to one another. A

device with a maximum capacity of 10 Mbps cancommunicate with device with a 100 Mbps capacity (butcan work at a lower rate)

The goals of Fast Ethernet : The goals of Fast Ethernet :


Fast Ethernet implementations

Can be categorized as

Two wire :Category 5 UTP (100Base-TX)Fiber-optic cable(100Base-FX)

four wire:Category 3 or higher UTP( 100Base-T4)


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Fast Ethernet implementationsEncoding:

Manchester encoding needs a 200- Mbaud bandwidth fora data rate of 100 Mbps.(unsuitable for a medium such asTwisted Pair cable).

In Fast Ethernet , three different encoding schemes werechosen depending on the implementation:

100 Base-TX100Base-FX 100BAse-T4


100Base-TX

Uses two pairs of twisted-paircable (either category 5 UTP orSTP).

Provide data rate of 100 MbpsMLT-3 scheme was selected

since it has good bandwidthperformance. (not self-synchronization line coding)

So 4B/5B block coding is usedto provide bit synchronization.


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100Base-FX

Uses two pairs of fiber-opticcables.

Uses NRZ-I encodingscheme ( bit synchronizationproblem.)

To overcome this problem,

4B/5B block coding is used.


100Base-T 4

Uses four pairs of

category 3 or higherUTP .(not cost efficientcompared to Category 5)

Transmit 100 Mbps.Uses 8B/6T encoding


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Summary of Fast Ethernet implementations

100Base -TX 10Base-FX 10Base-T4

Media Cat 5 UTP orSTP

Fiber Cat 3 UTP orhigher

Number of wires 2 2 4

Maximum length 100 m 400 m: half duplex2000m: full duplex

100 m

Topology Star Star Star

Data rate 100 Mbps 100 Mbps 100 Mbps

Block encoding 4B/5B 4B/5B 8B/6T

Line encoding MLT-3 NRZ-I


1313--55 GIGABIT ETHERNETGIGABIT ETHERNET

The The need need for for an an even even higher higher data data rate rate resulted resulted in in the the design design of of the the Gigabit Gigabit Ethernet Ethernet protocol protocol ( (1000 1000 Mbps) Mbps). .The The IEEE IEEE committee committee calls calls the the standard standard 802 802..3 3z z..

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