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Explanation of TDMA ?

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mrninni
Ex Member


Nov 6th, 2015 at 2:43pm  
Hi all,
I have some questions.

First: the link http://www.satsig.net/vsat-equipment/tdma-explanation.htm says
The burst lasts 180 mS, so at a rate of 250kbit/s site 1 sends 45,000 bits per burst, or 45,000 bits per second.
It is a typo? At a rate of 250kbit/s it doesn't send 45,000 bits per second, but indeed 250,000 bits per second.

Anyway, immagine that the a site has a configured MIR. For example the MIR of site1 is 128kbps or lower. Is there an inefficiency? Could someone explain how work the TDMA in this scenario?
Thanks
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« Last Edit: Nov 6th, 2015 at 5:50pm by Admin1 »  
 
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Reply #1 - Nov 6th, 2015 at 3:22pm  
No, it is not a typo. The site only transmits 45,000 bits in each whole second, even though the transmission rate is 250,000 bits per second.

It is rather like having a 10 Mbit/s or 100 Mbit/s Ethernet port on your computer and you only sending or receiving only 2 Mbit/s. A lot of the time the Ethernet is idle.

...
In TDMA, other sites will be transmitting when you are inactive,. e.g. you are active for 180 mS, other sites are active during the remaining 820 mS, of each whole second.

In the case of VSAT transmit MIR (minimum information rate), the hub controller guarantees you a minimum of one 180ms burst per second provided you are actually needing it at the time. These figures are an illustrative example only.

In the case shown, Site 1 is doing 45 kbit/s. The burst is repeated in the same place in second TDMA frame, making a total transmission of 90 kbit in 2 seconds. If Site 1 finishes the transmission of a 90 kbit file, the 180 mS slot for Site 1 becomes empty and another site may be assigned to use that time slot.

If you want to transmit at a slower rate you need occasional smaller, shorter, burst lengths.  The burst acquisition process becomes a significant overhead and the burst preamble becomes relatively large portion of the whole burst length and thus inefficient.
...
Above User data = 89% of the burst.

...
Above User data = 66% of the burst.

iDirect has some clever method of dealing with occasional short bursts  for SCADA applications where there may be very many sites transmitting small amounts of data, such as remote weather stations, pipeline monitoring etc.

There is inefficiently in TDMA due to:
. guard time gaps between the bursts, to allow for burst timing error
. the burst preamble, to allow the hub burst receiver time to lock up
. inability of the system to fully fill the frame, without unacceptable congestion at times.
. necessarily gaps to allow burst acquisition, without unacceptable burst collision and congestion at times.

Hope that helps, Eric.
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mrninni
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Reply #2 - Nov 6th, 2015 at 3:40pm  
Thanks,
You answer helps a lot.
I have not explained what I mean with MIR: Maximum Information Rate.
It is the SLA of the terminal.

If the minimum burst size is 80ms, it allows the transmission of 20 kbps.
If the terminal has a SLA (MaxIR) of 16 kbps, are we losing bandwidth?

Thanks again for your support
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Reply #3 - Nov 6th, 2015 at 3:51pm  
The maximum possible information rate is with 1 site per uplink carrier, in the above case 125000 bit/s, transmitting almost continually all of the time, using the longest configurable burst length.

In multi user systems the hub may set a limit to each customer, to limit the maximum bit rate per customer.  This will comprise a rule like "up to 3 x 180mS bursts per frame" or "No more than 1 x 540mS burst per frame" These numbers are just examples.

Regarding your question, is sounds as though you are losing bandwidth, as you say.  It seems improbable that they give you 20 kbit/s for say 8 seconds and then block you completely for next 2 seconds.
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mrninni
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Reply #4 - Nov 6th, 2015 at 4:22pm  
Ok, I think now it is clear.
Just to summarize:

If a Network contains a siteX with SLA 20kbps (it is the max contractual rate), the hub creates a burst of 64 ms and assigns it to the terminal (one per frame).
"The burst lasts 64 ms, so at a rate of 250kbit/s siteX sends 16,000 bits per burst, or 16,000 bits per second."
Of course in the burst decrease the %overhead increase, but the final question is: how much may be little a burst? I think it is the SCADA scenario and you already explained this point.

For me is ok, many thanks

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