1. Ethernet Frame Format
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2. Objectives
• Ethernet characteristics
• Ethernet Frame Format
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3. Ethernet
• Trade mark of Xerox Corporation
• Invented at Xerox PARC in 1976
• Standardised by IEEE in 1978
• Popular for LAN
• Uses Packet Switching Technology
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4. 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
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5. Ethernet
• Ether refers to the ‘Cable’. Its variants are
• 10 base 5
• 10 base 2
• 10 base T
• 10 base F
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6. 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
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7. 10 Base 2 – Called Thin Ethernet
• Bends easily
• Connections can be made using BNC to for
T-junction
• Can run over 100 meters
• Can handle 30 machines
• Cheaper and Easier to install
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8. 10 Base T – Twisted Pair
• Easy maintenance
• Can run over 100 meters
• Can handle 1024 machines
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9. 10 Base F – Fibre Optics
• Best choice of using between two buildings or
widely separated Hubs
• Run over 2000 meters
• Can handle 1024 machines
• Excellent Noise Immunity
• More Expensive
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10. ETHERNET CONNECTIONS
Fig.1
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11. Encoding
• Is a way to determine the start, end, or middle of each bit
• There are two approaches
• Manchester encoding
• Differential Manchester encoding
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12. 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
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13. 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
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14. IEEE 802.3 Frame Format
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15. 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 µsec.
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16. IEEE 802.3 Frame Format
Preamble SFD D Address S Address
• 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
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17. IEEE 802.3 Frame Format
Preamble SFD D Address S Address Length
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
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18. IEEE 802.3 Frame Format
Preamble SFD D Address S Address Length Data Pad Checksum
• 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
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19. 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
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20. Summary
In this class, you have learnt about
• Different types of Ethernet cables
• Encoding
• All the fields of Frame Format
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21. Summary
• 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
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22. Quiz
1. In Ethernet the valid frame size is
(a) 64 bytes
(b) 16 bytes
(c) 46 bytes
(d) 32 bytes
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23. Quiz
2. A frame containing all 1’s in the destination field
is for
(a) Unicasting
(b) Broadcasting
(c) Multicasting
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24. Frequently Asked Questions
1. Explain Ethernet frame format?
2. List the various types of Ethernet cables
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