Definition of Local Area Network LAN

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Definition of Local Area Network (LAN)

• LAN is a data communication network, which is
• usually confined to a small geographic area
• has a moderate to high data rate
• owned and used by a single organization
• low error rate
• low delay

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IEEE 802
1. IEEE 802.1 Architecture 2. IEEE
802.2 Logical Link Control (LLC) 3.
IEEE 802.3 CSMA/CD (Ethernet) 4. IEEE
802.4 Token Passing Bus 5. IEEE
802.5 Token Ring 6. IEEE 802.6 MAN
(Metropolitan Area)
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• 7. IEEE 802.7 Broadband Technology
• 8. IEEE 802.8 Fiber Optics
• 9. IEEE 802.9 Integrated Voice and Data
  
• 10. IEEE 802.10 LAN Security
• 11. IEEE 802.11 Wireless Communication
• 12. IEEE 802.12 Fast Ethernet (100Mbps)

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Pure-ALOHA

• Users are not synchronized.
• Each user transmits a data packet when ready.
• In the event of two or more packets collide
  (overlap in time), each user involved realized
  this and retransmit the packet after a randomized
  delay.

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Slotted ALOHA

• Like Pure-ALOHA with additional requirements
• The channel is slotted in time
• Each user is required to synchronize the start of
  packet transmission to coincide with the slot
  boundary (only complete collision would occur,
  avoid partial collision)

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Carrier Sense Multiple Access (CSMA)

• Used in radio network.
• Propagation delay is small compared to packet
  transmission time.
• Avoid collision by listening to the carrier
  before transmission.

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• If there is a collision, then reschedule
  according to some random delay.
• If channel is free then transmit else wait and
  operate
• 1-persistent -gt Listen persistently to the
  channel
• non-persistent -gt Wait a random period of time
• p-persistent (where 0 lt p lt 1) -gt access with
  prob. p

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IEEE 802.3 CSMA/CD

• Uses 1-persistent CSMA algorithm.
• Rules
• if the channel is idle then transmit
• if the channel is busy, then continue to listen
  until idle then transmit immediately

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• if a collision is detected during the
  transmission, immediately cease transmitting the
  frame and transmit a jamming signal to ensure
  everyone knows the collision, hence the name
  collision detection (CD)
• After transmitting the jamming signal, then wait
  a random time and attempt to transmit again.

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Binary Backoff Scheme

• After each repeated collision, the mean value of
  random delay is doubled. After first collision,
  each station will wait 0 or 1 slot.
• After nth collision, each station waits 0 to 2n-1
  slot.
• After 16th time of unsuccessful attempts, the
  computer will give up and report error to the
  user.

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IEEE 802.3 Frame Format
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IEEE 802.4 Token Bus

• Although the physical topology is a bus, the
  stations on the bus form a logical ring.
• Each station knows TS (this station), PS
  (previous station) and NS (next station).

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• A control packet (token) regulates the right of
  access to the shared bus. The token frame
  contains the destination address.
• The station receiving the token is granted
  control of the median (bus) for a specified time.
• When the station completes its transmission or
  time has expired, it passes on the token to the
  NS in the logical sequence.

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Priority Scheme

• As an option, the token bus system may include
  four classes of services priority class 6, 4, 2
  and 0 (descending order of priority).

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IEEE 802.4 Frame Structure

• Preamble byte pattern used by the receiver
  to establish bit synchronization and
  locate first bit of a frame
• Start delimiter indicates start of frame

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• Frame control indicates whether this is LLC data
  frame. If not, bit in this field indicates
  it is a control frame (e.g.,token frame)
• DA destination address
• SA source address
• Data unit up to 8182 or 8174 bytes
• CRC Frame Check sequence
• End delimiter end of frame

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IEEE 802.5 Token Ring

• Employs a ring topology.
• A station wishing to transmit must wait until it
  detects a passing by token. It changes the token
  from a free token to busy token by inverting
  a bit (4-bit of the access control byte) of the
  token.

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• The frame on the ring will make a round trip and
  then it will be purged by the sending station.
• The sending station will insert a free token on
  the ring.

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IEEE 802.5

• A station having a higher priority (message) than
  the current busy token can reserve the next token
  for its priority level as the busy token passes
  by.
• When the current transmitting station completes,
  it issues a free token at the higher priority
  level.

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• Station with lower priority level cannot use the
  token, so it passes it to the requesting station,
  or an intermediate station of equal or higher
  priority (message) to send.
• When the station that upgraded the priority level
  sees a free token at the higher priority level,
  it assumes no higher priority traffic is waiting
  and downgrades the token.

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IEEE 802.5
(a) Token format
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(b) Data frame format