Daily Learning: Computer Networks – Access Control Methods – TDM, Polling, Token Passing, Aloha

Types of Communication Links

  • Point to Point Link
  • Broadcast Link – The connection is shared between all the stations.

Need Of Access Control

In the broadcast link if all stations are sending data simultaneously then there will be collision that’s why we implement the Access Control.

Types Of Access Control Method

1. TDM(Time Division Multiplexing) –

Divide the time into slots and assign each slot to one station.

Efficiency(η) = 1 / 1 + a [ a = Tp / Tt ]

2. Polling –

When a station wants to transmit the data then only we give the chance to that station to transmit the data.

Efficiency(η) = Tt / Tpoll + Tt + Tp [ Tt = Time taken for transmission, Tp = Time taken for propagation]

3. Token passing-

Token – A token is a small message composed of a special bit pattern.

Ring Latency – It is time taken by a bit to cover the entire ring and come back to the same point.

RL = d / v + N * b

[ d = length of the ring, v = velocity of data in ring, N = no. of stations in ring, b = time taken by each station to hold the bit before transmitting it (bit delay)]

Cycle Time – The time taken by the token to complete one revolution of the ring is known as cycle time.

CL – d / v + N * (THT)

[ d = length of the ring, v = velocity of data in ring, N = no. of stations in ring, THT = Token Holding Time ]

Strategies –

Delayed Token Reinsertion (DTR) –

Station keeps holding the token until the last bit of the data packet transmitted by it takes the complete revolution of the ring and comes back to it.

Working –

After a station acquires the token,

  • It transmits its data packet.
  • It holds the token until the data packet reaches back to it.
  • After data packet reaches to it, it discards its data packet as its journey is completed.
  • It releases the token.

Token Holding Time (THT) = Transmission delay + Ring Latency = Tt + Tp [ Tt = Transmission time, Tp = Propagation time ]

Ring Latency = Tp + N x bit delay = 0 [ bit delay = 0 ]

Early Token Reinsertion (ETR) –

Station releases the token immediately after putting its data packet to be transmitted on the ring.

Token Holding Time (THT) = Transmission delay of data packet = Tt

4. Aloha

Rules –

  1. Any station can transmit data to a channel at any time.
  2. No carrier sensing.
  3. There is no collision detection.
  4. It re-transmit the data after some time.(If acknowledgement don’t come)

There are mainly two type of aloha –

  • Pure Aloha –
  1. The total vulnerable time = 2 * Tfr [ Tfr = Average time required to send a packet ]
  2. Maximum throughput occurs when G = 1/ 2 that is 18.4%.
  3. Successful transmission of data frame is S = G * e ^ – 2 G.
  • Slotted Aloha –

We divide the process into slots and a host can only send packets at the beginning of any slot. If it comes after then it has to wait till next slot.

  1. Maximum throughput occurs in the slotted Aloha when G = 1 that is 37%.
  2. The probability of successfully transmitting the data frame in the slotted Aloha is S = G * e ^ – 2 G.
  3. The total vulnerable time required in slotted Aloha is Tfr.



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