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Behrouz A. Forouzan “Data communication and Networking 1
Wired LANs: Ethernet
Chapter 13
Dr. Gihan NAGUIB
“Behrouz A. Forouzan “Data communication and Networking” 2
1313--1 1 IEEE STANDARDSIEEE STANDARDS
InIn 19851985,, thethe ComputerComputer SocietySociety ofof thethe IEEEIEEE startedstarted aa
project,project, calledcalled ProjectProject 802802,, toto setset standardsstandards toto enableenable
intercommunicationintercommunication amongamong equipmentequipment fromfrom aa
varietyvariety ofof manufacturersmanufacturers.. ProjectProject 802802 isis aa wayway ofof
specifyingspecifying functionsfunctions ofof thethe physicalphysical layerlayer andand thethe
datadata linklink layerlayer ofof majormajor LANLAN protocolsprotocols..
Data Link Layer
Physical 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 the
shared media.
If channel is dedicated ( point to point) we do not need
the (MAC); sublayer.
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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 the
framing duties are collected into one sublayer called the logical
link control (LLC )
LLC provides one single data link control for all IEEE LANs.
IEEE project 802 has created a sublayer MAC that defines the
specific access method for each LAN. In contrast to the LLC, MAC
contains a number of distinct modules: each defines the access method
and 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
“Behrouz A. Forouzan “Data communication and Networking” 6Dr. Gihan NAGUIB
Physical layer
Physical layer is dependent on the implementation
and type of the physical media used.
IEEE define detailed specifications for each LAN
implementation.
For example, although there is only one MAC
sublayer for Standard Ethernet( CSMA/CD), there is
a different physical layer specifications for each
Ethernet implementations.
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DescriptionName
EthernetIEEE 802.3
Token busIEEE 802.4
Token RingIEEE 802.5
Wireless LAN IEEE 802.11a/b/g/n
BluetoothIEEE 802.15.1
IEEE 802 Series of LAN Standards
“Behrouz A. Forouzan “Data communication and Networking” 8Dr. Gihan NAGUIB
ETHERNET IEEE ETHERNET IEEE 802802..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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“Behrouz A. Forouzan “Data communication and Networking” 9Dr. Gihan NAGUIB
ETHERNET EvolutionETHERNET Evolution
TheThe originaloriginal EthernetEthernet waswas createdcreated inin 19761976 atat Xerox’sXerox’s
PaloPalo AltoAlto ResearchResearch CenterCenter (PARC)(PARC).. SinceSince then,then, itit hashas
gonegone throughthrough fourfour generationsgenerations..
“Behrouz A. Forouzan “Data communication and Networking” 10Dr. Gihan NAGUIB
ETHERNETETHERNET
The MAC sublayer governs the operation of the random
access method
Standard Ethernet uses CSMA/CD with 1-persistent
Ethernet dose not provide any mechanism for
acknowledging received frames( unreliable medium).
Acknowledgments must be implemented at the higher layer.
It also frames data received from the upper layer and
passes 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 layer
and is not formally part of the frame
The Ethernet frame contains seven fields:
“Behrouz A. Forouzan “Data communication and Networking” 12Dr. Gihan NAGUIB
Ethernet (IEEE 802.3) MAC frame
DA : Destination address:
SA: Source Address:
Length/Type:
Define the upper-layer protocol using the MAC
frame. 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
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Minimum and maximum frame lengths
Minimum frame length restriction (64 bytes) is required for the correct
operation of CSMA/CD.
Min data length =64 - 18 (6+-6+2+4) = 46 bytes
If 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 network
interface card (NIC) which provides the station with
a 6-byte [48 bits] physical address (MAC adress)
It is written in hexadecimal notation, with a
colon between the bytes.
“Behrouz A. Forouzan “Data communication and Networking” 16Dr. Gihan NAGUIB
Unicast and multicast addresses
Source address is always a unicast address – the frames
comes from only one station.
Destination address can be:
unicast: defines only one recipient; one to one
multicast: a group of addresses; one to many
Broadcast: the recipients are all the stations on
the LAN
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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.
The broadcast destination address is a
special case of the multicast address in which
all bits are 1s.
Note
“Behrouz A. Forouzan “Data communication and Networking” 18Dr. Gihan NAGUIB
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
SolutionTo 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.
b. This is a multicast address because 7 in binary is 0111.
c. This is a broadcast address because all digits are F’s.
Example 1
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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( LSB first), bit by bit, as
shown below:
Example 2
left-to-right :47→20→1B→2E→08→ EE 47 is 0100 0111 right -to-left 1110 0010
“Behrouz A. Forouzan “Data communication and Networking” 20Dr. Gihan NAGUIB
Slot Time
Slot 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 to
send the jam sequence
It 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 proper
function of CSMA/CD.
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Slot Time
If the sender sends a frame larger than the
minimum size ( 512 to 1518 bits)
If the station has sent out the first 512 bits and has not
heard a collision, it is guaranteed that the collision will
never occur during the transmission of the frame.
The reason is that all stations sensed the existence of the
signal and refrained from sending .
Collisions can only occur during the first half of the slot
time (slot time/2), and if it dose, it can be sensed by the
sender during the slot time.
“Behrouz A. Forouzan “Data communication and Networking” 22Dr. Gihan NAGUIB
Slot Time and Maximum network length
Slot time = 2 x Tp ( neglecting time required to send jam signal)
= 2 x Max Length/propagation speed
Max.Length = propagation speed x (SlotTime/2)
Let propagation speed = 2x108 m/s
Max.Length = (2 x 108) x (51.2 x10-6/2) = 5120 m
Consider the delay times in repeaters and interfaces, and
the time required to send the jam sequence.
Max.Length = 2500 m ( = 48 % of the theoretical)
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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 communication
is half-duplex.
Why half deplux??
“Behrouz A. Forouzan “Data communication and Networking” 24Dr. Gihan NAGUIB
Encoding in a Standard Ethernet implementation
All standard implementations use digital signaling( baseband) at
10 Mbps.
At the sender, data are converted to a digital signal using the
Manchester scheme.
At the receiver, the received signal is interpreted as Manchester and
decoded 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 m
If 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.
“Behrouz A. Forouzan “Data communication and Networking” 26Dr. Gihan NAGUIB
10Base2:Thin Ethernet (Cheapernet)
Uses Bus topology with thinner and more flexible cables.
Transceiver part of a NIC card
The 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 attenuation
Repeaters are used to connect multiple segments
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10 Base-T: Twisted-Pair Ethernet
Physical star topology
Stations are connected to a hub via two pairs of twisted
cable( one for sending and one for receiving|).
Any collisions happens in the hub
Compared to others, the hub replaces the coaxial cable as
far as a collision is concerned.
Max length = 100 m to minimize attenuation.
“Behrouz A. Forouzan “Data communication and Networking” 28Dr. Gihan NAGUIB
10Base-F: Fiber Ethernet
Uses star topology to connect stations to a hub
Stations is connected to the hub by using two
pairs of fiber-optic cables.
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Summary of Standard Ethernet
implementations
10Base 5 10Base2 10Base-T 10Base-F
Media Thick coaxial cable
Thin coaxial cable
Two UTP 2 Fiber
Maximum length
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
“Behrouz A. Forouzan “Data communication and Networking” 30Dr. Gihan NAGUIB
1313--3 3 CHANGES IN THE STANDARDCHANGES IN THE STANDARD
TheThe 1010--MbpsMbps StandardStandard EthernetEthernet hashas gonegone throughthrough
severalseveral changeschanges beforebefore movingmoving toto thethe higherhigher datadata
ratesrates.. TheseThese changeschanges actuallyactually openedopened thethe roadroad toto thethe
evolutionevolution ofof thethe EthernetEthernet toto becomebecome compatiblecompatible withwith
otherother highhigh--datadata--raterate LANsLANs..
Bridged Ethernet
Switched Ethernet
Full-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 bandwidth
2. They separate collision domains.
Without bridges, all the stations share the bandwidth of the
network.
“Behrouz A. Forouzan “Data communication and Networking” 32Dr. Gihan NAGUIB
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 reduced
Raising the B-W
Bridged Ethernet: Separate Collision domains
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Switched Ethernet
A layer 2 switch is an N-port bridge with
additional sophistication that allows faster
handling of the packets.
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Full-duplex switched Ethernet
In full duplex switch there are two links, one for sending
and one for receiving, we don’t need CSMA/CD here|(
no collision).
Increases the capacity of each domain from 10 to
20 Mbps.
“Behrouz A. Forouzan “Data communication and Networking” 36Dr. Gihan NAGUIB
FAST ETHERNETFAST ETHERNET
FastFast EthernetEthernet waswas designeddesigned toto competecompete withwith LANLAN
protocolsprotocols suchsuch asas FDDIFDDI oror FiberFiber ChannelChannel.. IEEEIEEE
createdcreated FastFast EthernetEthernet underunder thethe namename 802802..33uu.. FastFast
EthernetEthernet isis backwardbackward--compatiblecompatible withwith StandardStandard
Ethernet,Ethernet, butbut itit cancan transmittransmit datadata 1010 timestimes fasterfaster atat
aa raterate ofof 100100 MbpsMbps..
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MAC Sublayer
Main consideration in the evolution of Ethernet from 10 to
100 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 at
each port.
No need for CSMA/CD
“Behrouz A. Forouzan “Data communication and Networking” 38Dr. Gihan NAGUIB
Fast Ethernet topology
Topology:
Two stations: point-to-point
Three or more stations: star topology with a switch
at the center
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1. Upgrade the data rate to 100 Mbps
2. Make it compatible with Standard Ethernet
3. Keep the same frame format
4. 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 can
communicate with device with a 100 Mbps capacity (but
can work at a lower rate)
The goals of Fast Ethernet :The goals of Fast Ethernet :
“Behrouz A. Forouzan “Data communication and Networking” 40Dr. Gihan NAGUIB
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 implementations
Encoding:Manchester encoding needs a 200- Mbaud bandwidth for
a data rate of 100 Mbps.(unsuitable for a medium such as
Twisted Pair cable).
In Fast Ethernet , three different encoding schemes were
chosen depending on the implementation:
100 Base-TX
100Base-FX
100BAse-T4
“Behrouz A. Forouzan “Data communication and Networking” 42Dr. Gihan NAGUIB
100Base-TX
Uses two pairs of twisted-pair
cable (either category 5 UTP or
STP).
Provide data rate of 100 Mbps
MLT-3 scheme was selected
since it has good bandwidth
performance. (not self-
synchronization line coding)
So 4B/5B block coding is used
to provide bit synchronization.
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100Base-FX
Uses two pairs of fiber-optic
cables.
Uses NRZ-I encoding
scheme ( bit synchronization
problem.)
To overcome this problem,
4B/5B block coding is used.
“Behrouz A. Forouzan “Data communication and Networking” 44Dr. Gihan NAGUIB
100Base-T4
Uses four pairs of
category 3 or higher
UTP.(not cost efficient
compared to Category 5)
Transmit 100 Mbps.
Uses 8B/6T encoding
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“Behrouz A. Forouzan “Data communication and Networking” 45Dr. Gihan NAGUIB
Summary of Fast Ethernet implementations
100Base -TX 10Base-FX 10Base-T4
Media Cat 5 UTP or STP
Fiber Cat 3 UTP or higher
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
“Behrouz A. Forouzan “Data communication and Networking” 46Dr. Gihan NAGUIB
1313--5 5 GIGABIT ETHERNETGIGABIT ETHERNET
TheThe needneed forfor anan eveneven higherhigher datadata raterate resultedresulted inin thethe
designdesign ofof thethe GigabitGigabit EthernetEthernet protocolprotocol ((10001000 Mbps)Mbps)..
TheThe IEEEIEEE committeecommittee callscalls thethe standardstandard 802802..33zz..