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VSAT SECTION

--TDMA(Time Division Multiple Access):-


(*)Introduction:-
It is a star network system.Which means that all VSATS sends to the hub and the hub sends to all VSATS.
-IB channel:-All VSATS sends data on it to the hub using TDMA access method.
-OB channel:-The hub sends data on it to all VSATS using TDM (Time Division Mulitiplixing)access method

(*)VSAT access on the IB channel:-
In TDMA access method the time is divided into frames,every frame is divided into time slots(i.e a second may contain 16 frame).The data is transmitted in packets,every packet in just one time slot.
Due to more than one VSAT try to send data on one channel so a collision of data may happen.so the vsat tries to send it's data to the hub and wait for an acknoledgment on the OB channel from the hub that his data has succesfully transmitted.
If no acknoledgment came to the VSAT after fixed time,this means that it's data has collide with another VSAT's data and it is wasted.So the VSAT tries to retransmitt after a random time(i.e 2-8 time slots) and so on.

(*)The HUB access on the OB channel:-
In TDM the time is divided also into frames and time slots.But the difference here is that the channel is access only by the hub.So no possibilty for a collision and so the channel can be used with efficiency 100%.


Different access modes that TDMA offers:-

1)Random access mode:-
It is based on the first come -first served.all the remote sites compete to access the channel and wait for acknowledge on the OB channel.They send data as packets,every packet in one the slot.

2)Reservation mode:-
If any one of the remote site wants to send data more than one packet it may use reservation mode.
This is done by sending the first packet with a request to reserve numbre of time slow to send it's data.
The hub,when it recieves the request,on the OB channel to all remote sites on the spicfied IB channel to stop sending at certain time slots except the one who sent the request.
Note:-(the adv of this mode is to improve the efficiency of the channel by reducing the probability of collisions.

3)Permenant mode:-
When in your network you know that certain remote site sends data at a certain rate,you may reserve fixed time slots in every frame just for this remote site to send it's data on it.
ADV:-will improv the channel efficiency by reducing the probability of the coolisions.
DISADV:-will make a delay for the other remote sites in the same IB channel if they want to send data at the time of the reserved time slots.


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