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Ch 4- Medium Access Control sub layer

  • Channel Allocation problem
  • Media Access Protocol – ALOHA
  • CSMA
  • CSMA/CD Protocols
  • Collision free protocols
  • Contention protocols
  • WDM access protocols, Wireless LAN protocols
  • Ethernet- types of Ethernet, IEEE 802.2
  • Bus-token ring,
  • DQDB, FDDI, Bridges

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Channel: multiple access

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Channel: multiple access

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Channel Allocation problem

  • how to allocate a single broadcast channel among competing users?
  • Channel - wireless spectrum/single wire/ optical fiber

By which / where multiple nodes are connected

  • Static channel allocation / (inefficient) performance?=>poor
    • Allocating a single channel, telephone trunk
  • FDM, if N users, the bandwidth is divided into N equal-sized portions
  • FM, Each station gets a portion of the FM band and uses
    • it most of the time to broadcast its signal.
  • Problems:-
    • If the spectrum is cut up into N regions and fewer than N users are currently interested in communicating, a large piece of valuable spectrum will be wasted.
    • if more than N users want to communicate, some of them will be denied permission for lack of bandwidth
    • Some of the users who have been assigned a frequency band hardly ever transmit or receive anything. (idle user)

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Key Assumption for Dynamic Allocation

  1. Independent Traffic
    • stations(e.g., computers, telephones), Δt(frame generated)
    • Once a frame has been generated, the station is blocked and does nothing until the frame has been successfully transmitted.
  2. Single Channel
    • Single channel for all communication, all can transmit/ receive
    • stations are assumed to be equally capable
  3. Observable Collisions
    • Transmission simultaneously, overlap in time and the resulting signal is garbled. This event is called a collision.
  4. Continuous or Slotted Time

Time may be assumed, slot may have 0,1, more frames , idle slots

Successful transmission or Collision

  • Carrier Sense or No Carrier Sense

sense, if busy=> wait for free channel

No sense, go ahead and transmit

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Aloha (Random access Method)

  • ALOHA mainly designed for radio (wireless) LAN (1970)

it’s also used for shared medium

  • The main problem in shared media is Collision, when more than one station want to transmit simultaneously
  • Aloha can be of two variations
    1. Pure ALOHA
      • Message get overlap on the shared media (collision)
    2. Slotted ALOHA

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Pure Aloha

any station can send frame at any time

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Vulnerable time for pure ALOHA Protocol

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  • Efficiency of pure Aloha

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Slotted Aloha

any station can send frame according to the time slot

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Vulnerable time for slotted ALOHA Protocol

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  • Efficiency of Slotted Aloha

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Slotted ALOHA

Assumptions

  • all frames same size
  • time is divided into equal size slots, time to transmit 1 frame
  • nodes start to transmit frames only at beginning of slots
  • nodes are synchronized
  • if 2 or more nodes transmit in slot, all nodes detect collision

Operation

  • when node obtains fresh frame, it transmits in next slot
  • no collision, node can send new frame in next slot
  • if collision, node retransmits frame in each subsequent slot with prob. p until success

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Basis for Comparison

Pure ALOHA

Slotted ALOHA

Frame Transmission

The user can transmit the data frame whenever the station has the data to be transmitted.

The user has to wait till the next time slot start, to transmit the data frame.

Time

In Pure ALOHA the time is continuous.

In Slotted ALOHA the time is discrete.

Successful Transmission

The probability of successful transmission of the data frame is:�S= G* e^-2G

The probability of successful transmission of the data frame is:�S= G*e^-G

Synchronization

The time is not globally synchronized.

The time here is globally synchronized.

Throughput

The maximum throughput occurs at G = 1/2 which is 18%.

The maximum throughput occurs at G = 1 which is 37%.

Introduced

1970

1972

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End of Part-1 (Ch-4 MAC layer)

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Thanks.

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