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IEEE STANDARD 802.3

ETHERNET

Common Bus topology

Uses CSMA/CD

Named after “ether”, the imaginary substance many once believed occupied all of space.

Possible Ethernet connection

terminators

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Transceiver is connected to cable using a vampire clamp.

Transceiver create interface between cable and computer

Transmit bits on cable using CSMA/CD contentionControl

Transceiver cable (AUI) attachment unit interface cableConsisted of 5 twisted pair, two used to send data and control info to computer, two used for receiving data and control info, fifth pair could be used to connect power source and ground.

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Cable connects to pc using NIC, network interface card.

NIC contains logic necessary to buffer data,

Also:

error checking

creates frames

determine when to transmit following collisions

recognize frames addressed to its computer

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Steps in sending data to another computerexecute network software to put packet of info in

computer’s memory

NIC gets packet, creates frame, than waits for signal from transceiver which is monitoring line segment waiting for chance to send

When transceiver detects quite cable, it signals NIC to send frame to transceiver. Transceiver transmits and listens for collision. If one occurs, NIC is notified and NIC executes exponential backoff algorithm.

Transceiver on receiving end monitors cable traffic, copies frames and sends to NIC on that end

NIC does CRC error check, if no error, checks destination address, if correct addr. Buffers frame and generates an interrupt to inform computer that packet has arrived.

Receiving PC executes network software to determine if packet can be received.

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Segments in original standard had maximum length of 500 meters.

If greater length required, segments can be connected by repeaters.

Original standard allowed no more than four repeaters.

(related to collision detection)

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Ethernet frame format:

Preamble: 7 byte pattern, alternating 1s and 0s used for synchronization

Start of Frame Delimitor: special pattern 10101011 to indicate start of frame

Destination Address: if first bit = 0 , field specifies a specific device, if 1, group address

Source Address: where did frame come from

Data Length field: number of bytes in combined data and pad fields. (46 to 1500 bytes)

Pad field: Data field must be at least 46 bytes.

Frame Check Sequence: 32 bit CRC

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Frame must be long enough and segments short enough to allow collision detection:

Sender should still be listening to detect collision

A signal can cover about 2500 meters in 12.5 microsecondsRound trip time from most distant point would be 25 msec.Repeater delay adds another 25 msec.At 10 Mbps, 10 bits are sent every msec or 500 bits in 50 microsec.

So minimum frame length was set at 512 bits or 64 bytes.

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Physical implementation of 10 Mbps Ethernet

ThickNet Thick Wire Ethernet

10 Base 5 Ethernet used 50 ohm 10 mm diameter

Coaxial cable, difficult to bend.

ThinNet Thin Wire Ethernet 10 Base 2 still 10 Mbps but

Thinner cable which could be bent and routed more easily. Also cheaper, sometimes called Cheapernet.

Could be installed using a T-connector so transceiver was built into NIC

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Another popular configuration IEEE 802.3i

Uses Hub also called a multiport repeater.

Hubs are connected to devices using 10Base T cable (cat 3,4, or 5 UTP. The hubs, in turn could be connected used Ethernet segment.While this physically is no longer a common bus, it behaves logically as ifIt is. The hubs broadcast signals received, so all devices “see” the signalsJust as they would on a common bus. Collision domain is not changed.

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Hub configuration offers many advantages.

linear arrangements may not be practical

can often use existing wiring

simplifies diagnostics

Ethernet uses Manchester Encoding

Many other standards listed in text.

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Fast Ethernet:

10 Mbps was once considered fast, not anymore!!!

Fast Ethernet 100 Mbps IEEE 802.3u

Uses physical star topology

100 Base TX uses cat 5 UTPManchester encoding was not used because the highFrequency requirements generated too much noise.

Uses a different encoding standard 4B/5B

It replaces every 4 bits (half byte or nibble) with 5 bitsFurther it uses Multilevel Line Transmission-Three Levels MLT-3. This requires only about 25% of the frequency of Manchester and thus generates less noise, so it works with Cat 5 UTP.

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100 Base FX Fiber optic cable standard.

Another interesting standard 100 Base T4

Suppose you need to upgrade your 10 Mbps Ethernet to

Fast Ethernet.

Rewire your building with cat 5? Or try to use the existing cat 3?

100 Base T4 uses a three level encoding also, but it uses

8B/6T where every 8 data bits are represented by 6 ternary bits, called trits.

There are 256 possible 8 bit strings and 729 possible 6 trit strings, so this scheme uses only slightly more than a third of the possible encodings.

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This means to get 100 Mbps, we need on 75 million baud.

This still cannot be done with cat 3 wires, but can be done with 3 pair of cat three wires.

We actually use four cat 3 cable pairs. One permanently configured forCommunication is one direction and two bidirectional. This standard onlyOperates in half duplex mode. For any communication 3 pairs are actuallyUsed, each carrying a part of the message and operating at 25 million baudOr 33 Mbps giving a total, nominal, of 100 Mbps. Four wire is used for Control info in the other direction.

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Gigabit Ethernet:

Designed to run over both optical fiber and copper wire.

Can run in both half duplex and full duplex modes

In full duplex mode, collisions do not occur.

Gigabit Ethernet uses “frame bursting”, if operating half duplex where collisions can occur.

If full duplex, frame bursting is not needed.

CSC 31118. 10 pts. Indicate the contents of the bridge forwarding table given the following traffic. Indicate after each message, what action the bridge would take and the current contents of the routing table.

1. A sends a packet to C 2 . B sends a packet to E3. E sends a packet to C4. C sends a packet to E5. C sends a packet to D6. G sends a packet to A7. D sends a packet to F

A B C

D E

F

G