DB>Actually both LAPM and MNP 4 have the same 7 byte/packet
DB>overhead in standard implementations.  What get's resent
DB>without SREJ is all the packets in the pipeline.  On a
DB>Courier, that would be 15 128 byte packets or 8 244 byte
DB>packets using LAPM.  Using MNP 4, it would be 8 256 byte
DB>packets -- ouch!
I'm writing an article on high-speed comms. I wrote one in the
days of v32bis and it's time for another go. Perhaps you could
help me with some info.
1. Could you describe the LAPM and MNP4 packet structure.
2. Which is the correct term for this data structure:
Frame, Packet or Block?
3. I seem to remember that MNP4 gives slightly higher throughput
compared to LAPM but that LAPM hangs on better in difficult
circumstances. Since the packet sizes appear to be the same I
would expect the throughput to be the same also. Comments?
4. LAPM still uses 16-bit CRC, doesn't it. If you believe it
hangs on better than MNP4 do you know what increases its
performance in this area?
5. Beside 128/15 and 244/8, my Courier v34+ also uses 244/15 to
another Courier V34+. I don't know how common this is with other
v34 and v34+ modems.
6. With reference to SREJ, the way the quote above is written it
appears that the throwaway without SREJ *is* either 8 or 15
packets but shouldn't this be up to a *maximum* of 8 or 15
packets (i.e. it depends on roundtrip delay + 2 passes through
the data pumps).
7. At 28,800bps, (244+7) = 9ms and (128+7) = 4.8ms. (I assume
that the 244 or 128 values are for the data octets only, not the
packet extras.)  On a intra-city connection I get 2ms roundtrip
delay reported so I presume that a short (less than 9ms) burst of
noise occuring at the beginning of a packet would produce a NAK
(or whathever LAPM uses) at my end within 41ms (9ms+2ms+30ms
double-pass through the data pumps (source for 30ms figure:
20-50ms datapump range mentioned in "What is Selective Reject" at
http://www.geocities.com/SiliconValley/Park/2432/modem.htm)).
This would be received during the transmission of the 5th 244
frame (or 8th 128 frame). I presume without SREJ this 5th frame
would be aborted shortly afterwards, not at the end of the 5th
frame.
Without SREJ, frames 1, 2, 3, 4, 5, would be retransmitted. With
SREJ only frame 1 would be resent.
So in an intra-city situation the difference between no SREJ and
SREJ present would be 4 244 frames or 7 128 frames, best case
(short noise burst at start of frame) or 5 244 frames or 8 128
frames, worsecase (if the short noise burst overlapped the end of
one frame and the beginning of the next). This would produce a
throughput degradation difference with/outout SREJ of 3.6% of the
normal cps, with a 244-byte frame size, for each whole-number
increment in the avg_number_of_errors/sec rate, assuming that
most noise only affected one frame at a time. (3.3% for 128-byte
frames.) Of course, a longer turnaround delay would increase the
difference further. Comments?
8. Have you nay idea of the fallback/fallforward times for v32,
v32bis, vfc and v34?
Cheers, Dan Bridges, Brisbug PCUG
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