Acquiring Channel in CSMA/CD

CSMA/CD stands for Carrier Sense Multiple Access / Collision Detection. In CSMA/CD, the range of collision occurrence is 0 to PT (Propagation Time) and the range of collision detection is 0 to 2xPT.

Key points:

  • If the collisions are detected at less than 2xPT or in the worst case of 2xPT, the station stops transmitting dummy data and applies exponential back off algorithm. This algorithm is basically applied on failure situation.
  • If the collisions are not detected at less than 2xPT or in the worst case of 2xPT, then at 2xPT the station confirms that it has acquired the channel or captured the channel.
  • the maximum time where a collision is detected is 2xPT.
  • The minimum time to acquire the channel is 2xPT.

As we know that 2xPT is the minimum time to acquire any channel. Now, in the following example we will find the minimum frame size to acquire or capture the channel.


  • Step-1: Firstly calculate the transmission time(TT). Here TT will be 2xPT
  • Step-2: For the calculation of PT, length of cable and velocity of medium is given and PT is calculated using the basic formula.
  • Step-3: Now, we will find frame size using TT and band width(BW) using the basic formula of transmission time.

Above procedure can be illustrated by following example.

In CSMA/CD or Ethernet or Bus Topology
Band width (BW) = 10 Mbps
Length of cable (l)= 200 m
Velocity of medium (v) = 2 x 10^8 m/s

Here, TT = 2 x PT
Formula for propagation time is: PT = length of cable/velocity of medium
Hence, PT = 200 / (2 x 10^8) = 1/10^6 sec.
Then, TT = 2 x (1/10^6) = 2/10^6 sec
We know that, TT = Frame Size / Band width
Then, Frame Size = TT x Band width
                 = 2 / 10^6 x 10 x 10 ^6
                 = 20 bits

Hence, Min. frame size to acquire channel is 20 bits.

Previous year GATE questions:

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