Datacommchapter 5 Part2

PART 2: 5.3: WIRED LANS

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

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Ethernet evolution through four generations

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

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802.3 MAC frame 46 to 1500 octets 7 octets

Preamble

1 S F D

6

6

2

³0

DA

SA

Length/ Type

LLC data

³0 P a d

4

FCS

Preamble

A 7-octet pattern of 10101010 for bit synchronization

SFD [Start frame delimiter ]

The sequence 10101011, indicating the start of the frame

DA [Destination address]

Destination MAC address

SA [Source address]

Source MAC address

Length/Type

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

LLC data

Data unit supplied by LLC

Pad

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

FCS [Frame check sequence]

A 32-bit cyclic redundancy check, based on all fields excepts preamble, SFD and FCS 4

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

Note 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. 6

Note The broadcast destination address is a special case of the multicast address in which all bits are 1s.

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Example 6 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 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. 8

Example 7 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, bit by bit, as shown below:

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Categories of Standard Ethernet

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IEEE 802.3 Notations

10Base5 Coax

Transmission medium Signaling technique Baseband [Manchester] Topology Bus Maximum segment 500 length (m) Nodes per segment 100 Cable diameter 10 (mm)

10Base2 Coax

10Base-T UTP

Baseband [Manchester] Bus 185

Baseband [Manchester] Star 100

10Base-FP Optical fiber pair Manchester/ on-off Star 500

30 5

0.4 to 0.6

33 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] followon 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 10BaseT 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: Name

Cable

100Base-T4

Twisted pairs

100m

Uses Cat-3 UTP

100Base-TX

Twisted pairs

100m

Full duplex at 100Mbps (Cat-5 UTP)

100Base-FX

Fiber optics

2000m

Full duplex at 100Mbps; long runs





Max. segment Advantages

The 100Base-TX runs in half-duplex mode if hub is used and in fullduplex 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: fullduplex 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 the only 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 and added 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 of 9%

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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: Name

Cable

1000Base-SX

Fiber optics

550m

Multimode fiber

1000Base-LX

Fiber optics

5000m

Single or multimode

1000Base-CX

2 pairs of STP

25m

Shielded twisted pair

1000Base-T

2 pairs of UTP

100m

Standard Cat-5 UTP




Max. segment

Remarks

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

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