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PART 2:PART 2:

5.3: WIRED LANS5.3: WIRED LANS

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5.3 WIRED LANS: ETHERNET

In 1985, the Computer Society of the IEEE In 1985, the Computer Society of the IEEE started a project, called Project 802, to set started a project, called Project 802, to set standards to enable intercommunication among standards to enable intercommunication among equipment from a variety of manufacturers. equipment from a variety of manufacturers. Project 802 is a way of specifying functions of Project 802 is a way of specifying functions of the physical layer and the data link layer of the physical layer and the data link layer of major LAN protocols.major LAN protocols.

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The original Ethernet was created in 1976 at Xerox’s Palo Alto Research Center (PARC). Since then, it has gone through four generations.

Ethernet evolution through four generations

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802.3 MAC frame

Preamble DA SA Length/Type LLC data FCS

SFD

Pad

7 octets 1 6 6 2 ³ 0 ³ 0 4

46 to 1500 octets

A 32-bit cyclic redundancy check, based on all fields excepts preamble, SFD and FCS

FCS [Frame check sequence]

Octets added to ensure that the frame is long enough for proper CD operation

Pad

Data unit supplied by LLCLLC data

Length of LLC data field in octets, the maximum frame size, excluding the Preamble and SFD, is 1518 octets

Length/Type

Source MAC addressSA [Source address]

Destination MAC addressDA [Destination address]

The sequence 10101011, indicating the start of the frame SFD [Start frame delimiter ]

A 7-octet pattern of 10101010 for bit synchronizationPreamble

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Example of an Ethernet address in hexadecimal notation

MAC address ⇒ global address assigned centrally by IEEE to ensure that no two stations anyway in the world have the same global address

Vendor code=24 Serial number=24

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Unicast and multicast addresses

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.

Note

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The broadcast destination address is a special case of the multicast address in which all bits are

1s.

Note

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Define the type of the following destination addresses:a. 4A:30:10:21:10:1A b. 47:20:1B:2E:08:EEc. FF:FF:FF:FF:FF:FF

Solution

To 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 6

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Show how the address 47:20:1B:2E:08:EEis sent out on line.

SolutionThe address is sent left-to-right, byte by byte; for each byte, it is sent right-to-left, bit by bit, as shown below:

Example 7

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Categories of Standard Ethernet

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IEEE 802.3 Notations

10Base5 10Base2 10Base-T 10Base-FPTransmissionmedium

Coax Coax UTP Optical fiberpair

Signaling technique Baseband[Manchester]

Baseband[Manchester]

Baseband[Manchester]

Manchester/on-off

Topology Bus Bus Star StarMaximum segmentlength (m)

500 185 100 500

Nodes per segment 100 30 - 33Cable diameter(mm)

10 5 0.4 to 0.6 62.5/125µm

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10Base5 implementation

- Maximum segment length: 500m- Maximum number of stations per segment: 100- Minimum distance between 2 stations: 2.5m- Maximum network distance between 2 stations: 2.5km (up to

5 segments)

vampire tap

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10Base2 implementation

- Maximum segment length: 185m- Maximum number of stations per segment: 30- Minimum distance between 2 stations: 0.5m- Maximum network distance between 2 stations: 925m

BNC connector

BNC connector

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10Base-T implementation

- Maximum segment length: 100m- Using star topology with center multi-port repeater (hub)- Physical star, but logical bus (all transmissions are repeated)

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10Base-F implementation

- Maximum segment length: 2000m- Using a star topology to connect stations to a hub- The stations are connected to the hub using two fiber-optic

cables

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Summary of Standard Ethernet implementations

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Fast Ethernet

� Fast Ethernet (IEEE 802.3u): High-speed standard [100Mbps] follow-on the IEEE 802.3 10Base-T standard (June 1995)

� Use the same shared, half-duplex transmission mode as traditional Ethernet but to obtain a 10-fold increase in operational bit rate over 10-BaseT

� Designed for star topology and not using coaxial cable� Retain the same wiring systems, MAC method (CSMA/CD) and frame

format (minimum size of 64 bytes)� If the maximum length is reduced, then the CSMA/CD access method

can be used with higher bit rates � the basis of the Fast Ethernet standard (maximum 200m between two DTEs)

� The standard also calls for just one to two hubs (1 class I repeater or 2 class II repeaters with 5m link)

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In practice, there are three physical implementation:

Full duplex at 100Mbps; long runs

2000mFiber optics100Base-FX

Full duplex at 100Mbps (Cat-5 UTP)

100mTwisted pairs100Base-TX

Uses Cat-3 UTP100mTwisted pairs100Base-T4

AdvantagesMax. segmentCableName

� The 100Base-TX runs in half-duplex mode if hub is used and in full-duplex mode if a switch is used

� No longer clock encoding schemes [Manchester, etc.], but bit encoding scheme [4B5B for TX & FX, 8B6T for T4]

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Gigabit Ethernet

� As more organizations move to 100Base-T, putting huge traffic loads on backbone network, there is demand for faster Ethernet

� As a result, Gigabit Ethernet was ratified by IEEE in 1998 under the name 802.3z

� A set of Gbps standards have been issued, which defines a new medium and transmission specification, while retaining the CSMA/CD protocol and frame format of its 10-Mbps and 100-Mbps predecessors

� All configurations of gigabit Ethernet are point-to-point

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� Gigabit Ethernet supports two different modes of operation: full-duplex and half-duplex

� The normal mode is full-duplex, allowing traffic in both directions at the same time

� This mode is used when there is a central switch connected to computers (or other switches) on the periphery

� All lines are buffered so each computer and switch is free to send frames whenever it wants to

� The sender does not have to sense the channel to see if anybody else is using it because contention is impossible

� On the line between a computer and a switch, the computer is theonly possible sender on that line to the switch and the transmission succeeds even if the switch is currently sending a frame to the computer (because the line is full duplex)

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� Since no contention is possible, the CSMA/CD protocol is not used � The half-duplex mode is used when the computers are connected to a

hub rather than a switch. � A hub does not buffer incoming frames. � Instead, it electrically connects all the lines internally, simulating the

multidrop cable used in traditional Ethernet. � In this mode, collisions are possible, so the standard CSMA/CD

protocol is required.� a minimum (i.e., 64-byte) frame can now be transmitted 100 times

faster than in classic Ethernet, the maximum distance is 100 times less, or 25 meters, to maintain the essential property that the sender is still transmitting when the noise burst gets back to it,

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� The 802.3z committee considered 25 meters to be unacceptable andadded two features to the standard to increase the distance

� Carrier extension - the hardware adds its own padding after the normal MAC

frame to extend the frame to 512 bytes. - Using 512 bytes worth of bandwidth to transmit 46 bytes of

user data (the payload of a 64-byte frame) has a line efficiency of9%

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� Frame bursting- A sender may transmit a concatenated sequence of multiple

frames in a single transmission - If the total burst is less than 512 bytes, the hardware pads it

again - If enough frames are waiting for transmission, this scheme is

highly efficient and preferred over carrier extension

� These new features extend the radius of the network to 200 meters, which is enough for most offices

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Physical implementations of Gigabit Ethernet:

Standard Cat-5 UTP100m2 pairs of UTP1000Base-T

Shielded twisted pair25m2 pairs of STP1000Base-CX

Single or multimode5000mFiber optics1000Base-LX

Multimode fiber550mFiber optics1000Base-SX

RemarksMax. segmentCableName

� Different encoding schemes are used:- 8B/10B for 1000Base-SX, LX and CX (collectively known as

1000Base-X) - PAM5, Trellis and Viterbi are used for 1000Base-T

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A typical application of Gigabit Ethernet is shown below