Ethernet or IEEE802.3

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Ethernet or IEEE802.3

• Format of lecture
• Introduction to Ethernet
• CSMA/CD
• Carrier sense multiple access/collision detect
• Various types and names
• How it works
• Collisions
• Packet structure

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Lecture in a nutshell

• Token ring networks have no collisions
• Ethernet does have collisions
• It works on contention it allows only one
  station to transmit at a time on a shared medium
  they are called shared-media broadcast networks
• Managing these collisions is important
• Ethernet attempts to provide a best effort
  delivery service

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Ethernet

• Ethernet is a standard protocol
• It uses Carrier Sense Multiple Access /
  Collision Detect
• Or CSMA/CD not a snappy name!
• Several other protocols exist
• A committee of the IEEE selected some of them as
  standards

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Access Methods
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Why Ethernet ?

• It provided the best solution from the CSMA pool
• It could not do everything and was expensive at
  the time
• Token ring and Token bus were also accepted as
  standards

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How it Works

• It makes the assumption that if no station
  attempts to use the network for a short time the
  network will remain free
• So a CSMA station wishing to transmit first
  listens for a short time
• It listens for the amplitude of the signal
• If all is quiet it transmits the packet
• The packet will take time to transmit
• If busy then waits

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Once underway

• Once transmitted the packet will travel towards
  its destination
• The transmitter will listen
• It is possible that two or more stations may try
  to transmit together, if this happens then all
  the data would become corrupted
• To see if it is corrupted the sender listens
  throughout the transmission

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What happens if they transmit together?

• If two (or more) Stations transmit together, we
  have a collision - data will invariably be
  corrupted
• The packet will be transmitted for long enough to
  ensure that if a collision occurs it will be
  detected by all stations
• anything being transmitted at that time will be
  cancelled
• Back off algorithms determine when colliding
  stations can retransmit

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What happens then?

• If the same stations transmitted again there
  would be another collision!
• We need to resolve the problem

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Who goes First?

• The stations involved in the collision will wait
  a short time
• The time they wait will be one or maybe two time
  periods and try again
• If there is another collision they will wait
  1,2,3 or 4 time periods and try again
• Next time they will wait up to 8
• And if they try again, up to 16 (maximum)

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Do Collisions happen often?

• Yes and no
• They happen but are resolved
• The more users the more likely they become
• The more likely they are the longer the queues
  become and this can make the problem worse!

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The specification

• The network is divided up into segments there
  are rules as to segment length
• each segment may be up to 100m long(10BaseT)
• You can have design for the maximum length of the
  network using segments and repeaters
• Some cabling options give less distance

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The Packet

• The packet will be a minimum length
• This ensures that it will still be being
  transmitted when it has reached the end of the
  network
• In fact it will be transmitted for twice the time
  it will need to go from one end to the other
• To ensure this, dummy data known as the pad
  (padding) may need to be sent

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Ethernet Packet Structure

• If data is small, padding is large
• If data is large, padding is zero
• Padding ensures a minimum packet size
• Note that the address are not IP numbers we are
  at level 2 not level 3

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Generic Frame Format
Frame check sequence
Destination
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MAC Addressing

• Without a name computers cannot be accessed
• At the data link layer, a header, and possibly a
  trailer, is added to upper layer data.
• Header and trailer contain control information
  intended for the data link layer entity in the
  destination system.
• Data from upper layer entities is encapsulated in
  the data link layer header and trailer.

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Data Encapsulation
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Analogy for Encapsulation

• A shrink-wrapped pallet is the last step before
  shipping
• Framing is the last step before data is
  transmitted over the medium

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Network Interface Card (NIC)
A layer two device plugs into motherboard A NIC
has a MAC address
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MAC Addresses
Organisational Unique Identifier OUI
Vendor assigned (NIC Cards, Interfaces)
24 bits
24 bits
6 hex digits
6 hex digits
00 60 2F
3A 07 BC
Cisco
Particular device
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Flat structure

• MAC addresses provide a way for computers to
  identify themselves.
• They give hosts a permanent, unique name.
• The number of possible addresses is extremely
  large 1612 (over 2 trillion!) possible MAC
  addresses.
• One major disadvantage, they have no structure,
  and are considered flat address spaces.
• Different vendors have different OUIs, but
  they're like personal identification numbers.
• As your network grows to more than a few
  computers, this disadvantage becomes a real
  problem.

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3 Analogies for Media Access Control

• Stopping at a tollbooth
• Waiting in a ticket queue
• Speaking in a meeting

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Deterministic MAC
Taking turns
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Question

• Which does Ethernet use?
• Non-deterministic

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Summary

• This has been a broad introduction to Ethernet
• In the next two weeks we will looking at other
  aspects of Ethernet