Ethernet frame format

transcript

Ethernet Frame Format

9EC606A.35 2

Objectives

• Ethernet characteristics

• Ethernet Frame Format

Ethernet

• Trade mark of Xerox Corporation

• Invented at Xerox PARC in 1976

• Standardised by IEEE in 1978

• Popular for LAN

• Uses Packet Switching Technology

Ethernet

• Xerox PARC

• CSMA / CD

• 2.94 Mbps

• No length field in Packet Header

• IEEE

• 1 – Persistent CSMA / CD

• 1 – 10 Mbps

• Length field in Packet Header

Ethernet

• Ether refers to the ‘Cable’. Its variants are

• 10 base 5

• 10 base 2

• 10 base T

• 10 base F

10 Base 5 – Called Thick Ethernet4

• Resembles yellow garden hose

• Marking for every 2.5 meters

• Connections are made using vampire taps

• Can run for 500 meters

• Can handle 100 machines

9EC606A.35 7

10 Base 2 – Called Thin Ethernet

• Bends easily

• Connections can be made using BNC to for

T-junction

• Can run over 100 meters

• Cheaper and Easier to install

10 Base T – Twisted Pair

• Easy maintenance

• Can run over 100 meters

10 Base F – Fibre Optics

• Best choice of using between two buildings or widely separated Hubs

• Run over 2000 meters

• Excellent Noise Immunity

• More Expensive

ETHERNET CONNECTIONS

Encoding

• Is a way to determine the start, end, or middle of each bit

• There are two approaches

• Manchester encoding

• Differential Manchester encoding

Manchester Encoding

• Each bit period is divided into two equal intervals

• Binary 1 is sent by having the voltage set high during the first interval & low in the second interval

• Binary 0 is just reverse

Disadvantage– Requires twice the Band width than straight binary encoding

Differential Manchester Encoding

• A binary 1 is indicated by the absence of a transition at the start of the interval

• A binary 0 is indicated by the presence of transition at the start of the interval

• Advantage– Better noise immunity

• Disadvantage– Requires more complex equipment

IEEE 802.3 Frame Format

Preamble

• Length of the field is 7 bytes.

• Each byte contain the bit pattern of 10101010.

• Manchester encoding produces a square

wave for this bit pattern with.

• Frequency of 10 MHz.

• Time period of 5.6 sec.

• Start of Frame delimiter– Contains 10101011 to indicate the start of Frame

• Source Address– Contains either 2 bytes or 6 bytes

• Destination Address– Contains either 2 bytes or 6 bytes– For ordinary addressing the higher order bit is 0– For group addressing the higher order bit is 1 i.e Multicasting– For Broadcasting of the frame in the network all the bits are made as 1’s

SFD D Address S AddressPreamble

Length – Tells how many bytes are present in the data field 0 to a maximum of 1500

– A data field of 0 bytes is legal, it causes a problem

– When a computer detects a collision, it truncates the current frame which means that corrupted frames appear on the cable all the time

– To make it easier to distinguish valid frames from corrupted frames (due to collisions), 802.3 states that valid frames to be at least 64 bytes long from destination address to checksum

Preamble SFD D Address S Address Length

• Pad field

If the data portion is less than 46 bytes, the pad

field is used to fill out the frame to the minimum

size of 64 bytes

• Checksum

The Checksum is used to detect if any data bits

have been corrupted during transmission

Preamble SFD D Address S Address Length Data Pad Checksum

IEEE 802.3 Ethernet LANs

When a computer wants to receive a frame it

• Listens to all frames traveling on the cable

• If the frame address is the same as the computer’s address or the same as the group address of the computers of which it is a member, it copies the frame from the cable

• Otherwise it just ignores the frame

Summary

In this class, you have learnt about

• Different types of Ethernet cables

• Encoding

• All the fields of Frame Format

• Unicast a frame from one computer to any other computer connected to the same cable

• Broadcast a frame from one computer to all other computers connected to the same cable

• Multicast a frame from one computer to a subset of the computers connected to the same cable

Summary

1. In Ethernet the valid frame size is

(a) 64 bytes

(b) 16 bytes

(c) 46 bytes

(d) 32 bytes

2. A frame containing all 1’s in the destination field is for

(a) Unicasting

(b) Broadcasting

(c) Multicasting

Frequently Asked Questions

1. Explain Ethernet frame format?

2. List the various types of Ethernet cables