Access control methods are the methods used for providing access control.
1) They prevent the collision or deal with it and ensures smooth flow of traffic on the network.
2) They are implemented at the data link layer of the OSI reference model.

1) Time Division Multiplexing: we are reserving the slots for some station bcz of this there could be the dis advantage.we assume that transmission time Ttof each station is same.
η=usefull time/cycle time
=Tt/Tt +Tp
η=1/1+a ,where a=Tp/Tt
Disadvantage of TDM :reservation ,whenever a station get a chance to transmit which means it gets the slots but it doesn't have any data to send,so that entire slot is going to be waste.
2) Polling: one station if it get access to the link,it has to participate to polling for which some time is wasting and then it get chance to transmit and propogate through the channel.
η=usefull time/cycle time
η=Tt/Tt +Tp+Tpoll
3) CSMA/CD(Carrier Sensing Multiple Access/Collision Detection):
a) any station can send data at any time and multiple station is connected to channel.
b)before sending any data every station is suppose to sense the carrier ,if the carrier is free you can send the data otherwise not.
c)There is no ack in CSMA/CD,Which means when you are sending a data and if your data is lost in collision,it is the responsibility of sender to detect that it is a collision and to retransmit the packet.
So in order to detect collision ,in worst case we should keep transmitting the data untill we get back the collision signal if any.
L ≥ 2*Tp*B
This is the min size of packet inorder to detect the collision.
now efficiency of csma/cd or ethernet both are equal and is:
η=1/1+6.44a
4) Token Passing: In token passing advantage is we are not going to have collision ,reason is:
a) transmission will be in unidirection.
b)At a time only one station can transmitt the packet and that station should have token.
η=usefull time/cycle time
cycle time in token passing is,The time taken by the token to start at a point in the ring and again come back to the same point is called cycle time.
cycle time =(d/v +N*THT)
THT=token holding time
token holding time is depend on two strategies we are going to use here.
- Delay token re insertion
- Early token re insertion
1)Delay token reinsertion:

THT = Tt+RL(ring latency)
THT =Tt+Tp
η = N*Tt /(Tp+N(Tt + Tp))
η = 1/1+((N+1)a)/N
2) Early token re insertion:

THT= Tt
because after transmission ,station will release the token.
η = N*Tt /(Tp+N(Tt ))
η = 1/1+(a/N) ,a=Tp/Tt
- In case of reliability delay is better but in case of efficiency early is better.
- If nothing is given in exam, by default go for early token reinsertion
5) Aloha:
- Any time ,any station could transmit the data.
- There is no carrier sensing.
- Collision could be possible.
- There are ack in Aloha,no need of Collision detection.
- When ever your data is lost in collision ,then we are going to re transmit the data packet.Retransmission will take place after some random amount of time , that time is called as back off time.
Aloha is of two type :
a)Pure Aloha:
Vulnuable time=2Tt ,
means before one Tt and after one Tt no one should transmit the data ,if some one is transmit the data we are going to get collision.
η = G *e-2G
η =18.4%
b)Slotted Aloha:
The main diff between pure aloha and slotted aloha is, in pure aloha any station can start transmitting data at any time But in slotted aloha time is divide in to slots and every station is forced in such a way that it should start transmitted at the beginning of the slot,if it is missed out time slot it has to wait fornnext time slot.
Vulnuable time=Tt
η = G *e-G
η =36.8%
Nice !!