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(*)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%.
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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