SatSig topic:

TPC Block Size and Time slot

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Nov 10^th, 2009 at 10:13am

Can any body let me know , why the TCP 0.66 has higher time slots as compared to 0.793

What is the realtionship between Block size and time slot , and how to calculate time slots if we dnt have iNPT tool

Regards,
Danish

Nov 10^th, 2009 at 12:12pm

In iNFINITI .66 TPC (like .431 and .533) is considered small block...or 1k block size, so there are going to be more of them as they are smaller than large block (.495 or .793).

The disadvantage of using small block is its IP payload (in)efficiency. A .66 timeslot is capable of 72Bytes and that is including 2-9 bytes of encryption header per IP packet (if used), 2 bytes of VLAN header per IP packet (again, if used) and 2 bytes of SAR header per SAR block (if Segmentation and Reassembly-SAR is used). For comparison purposes, a .793 slot is capable of 394Bytes. (with all headers, enc, VLAN and SAR considered)

You really need the iNPT tool to calculated FEC blocks and timeslots with any kind of accuracy. You can get "in the ballpark" without the tool, but if you want accuracy, use iBuilder (to build a test network) or use the iNPT if you have access to it.

Nov 11^th, 2009 at 7:10am

Hi Mike ,

Thanks for the update , but what i understand is that a block size use typically for error correction , this is actually oppiste to convolutional coding which uses a constraint length , so i was confused that if one has smaller block size then the number of time slots should be less , but it is not the case with iDirect , so i just want to know how they manuplate the frame in that way which gives more time slots in smaller block

Nov 12^th, 2009 at 2:31pm

iDirect do not use convolutional coding, they use Turbo product coding in the older releases, moving to 2D16 in the very latest - which by the way is awesome.
Anyway, the selection of the FEC rate, automatically sets the block size as Mike has mentioned to either Large block - 512k or small block 128k. iDirect uses a standard frame size, or rather a recommended frame size of 125ms.
It stands to reason then that, if the frame size is a constant as you increase the block size you can fit fewer in the frame.

So...lower block size = larger number of blocks (slots) per frame.
The time slots directly relate to the block size in that each time slot will carry the 128k block (0.66) or the 512k block (0.793). so smaller block requires less time to burst which means you can fit more bursts (slots) into the same length of time (frame)
One note on Mikes comment above, just to be sure on this, if you enable SAR you have to take the 2Bytes for SAR header from the payload of 72 Bytes, this means if you are using SAR on a 0.66 TDMA stream you have effectively 70 bytes per burst of usable payload.
I think that was meant but not sure it read that way to me.

Hope this helps and doesn't confuse more.........