Thanks for your reply to my query. It got me thinking...
DAVID> I'm not at home so will be doing much of this from memory
DAVID> -- I'll try to avoid screwing up but no promises.
Know what you mean. It is a pity that I stuffed up (as I'll
explain later on).
DAN> 1. Could you describe the LAPM and MNP4 packet structure.
DAVID> 
So this means that the header/ID structues is 3 bytes? (flag
byte,2-byte CRC,flag byte makes up 4 bytes). Do you know the
composition of the header/ID structure? Are a full 2 bytes used
for ID? Max frame numbering of 64K? Are the flags at the start
and the end of the frame the same value? What do they encode?
DAN> 2. Which is the correct term for this data structure:
DAN> Frame, Packet or Block?
A comms book I reread recently used "packet" to refer to the FTP
data structure and used "frame" to refer to the LAPM data
structure. Don't know whether that's correct usage though.
DAN> 3. I seem to remember that MNP4 gives slightly higher
DAN> throughput compared to LAPM
DAVID> The default LAPM packet size is 128 while the default MNP4
DAVID> packet is 256 bytes. The overhead for a 128 byte packet is
DAVID> ~6.75% while the overhead for a 256 byte packet is ~4.25%.
DAVID> Given the same (or close) packet size, the throughput will
DAVID> be virtually identical. Using the USR 244 byte LAPM packet
DAVID> gives us an overhead of 4.38% compared to an MNP4 256 byte
DAVID> packet.
Thanks, it appears the reason I thought MNP4 was more efficient
was due solely to its typically larger frame size. When USR uses
its 244+7 byte frame there would not be any apparent difference.
How did you derive these percentages? For example with (128+7) I
would assume the frame-support overhead would be 7/(128+7) = 5.2%
DAN> 4. LAPM still uses 16-bit CRC, doesn't it. If you believe it
DAN> hangs on better than MNP4 do you know what increases its
DAN> performance in this area?
DAVID> I'm not sure what you mean by hangs on better.
Less likely to loose the connection in a noisy situation. I've
heard this said before but I've not compared the two (Error
Detect + Resend) error handlers enough to be sure. Maybe LAPM has
a more refined algorithm than MNP4 in this respect.
(concluded in the next message)
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