Sierra Wireless DART 200 CDPD Modem - Manual (Page 117)

DART 200 CDPD Modem User’s Guide 8 Application Programming
PN1197-00 Revision 1.0 8-23
If the DART 200 is attached to a device that operates at a different speed
or data format, an appropriate AT&L command, followed by an AT&W
(to save the change) must be issued. Executing the AT&L command
disables autobaud and fixes (locks) the DART 200 to settings in the
AT&L command. Available bauds are 300, 1200, 2400, 4800, 9600, and
19.2K. Regardless of the baud selected for the communications port, the
CDPD airlink speed is fixed at 19,200 bps.
Autobaud can be reactivated with an AT&L1 command.
Maximum block size considerations
To maximize data throughput, most communications application
designers use the maximum blocksize compatible with good
performance. On land lines this number is usually 1024 bytes or larger. If
data blocks are too large, the likelihood of one dropping bits and having
to retransmit increases. If this occurs, the overall message transmission
times increase proportionately. If blocks are too small, the likelihood of
dropping bits is much smaller, but delays associated with transmitting
many more blocks increases overall transmission time. Better quality
lines decrease the likelihood of dropping bits, and faster lines decrease
delay times; however, the tradeoff always remains.
In a wireless environment the same tradeoffs apply. However, the speed
and quality is not high by landline standards, so maximum blocksizes are
necessarily smaller. The real advantage of wireless is simply in being
wireless.
Another consideration in the CDPD environment, is that there is a charge
for the number of bytes transferred. Since the headers are a fixed size and
relatively large (40 for TCP, 28 for UDP), there is customer pressure to
minimize the number of blocks (make them larger) to reduce the cost.
This can be self-defeating because larger blocks increase the likelihood
of occasionally retransmitting a large block. Again, what is gained by
having fewer blocks is lost by retransmitting some of them due to the
increased likelihood of errors.
Data packets above 536 bytes are likely to be fragmented; a process that
on average adds to the overhead associated with larger blocks. Refer to
Maximum packet size transmit control, p. 8-9, for a discussion of
problems resulting from a packet size that was too large.
Start with a blocksize of 256 bytes. However, this parameter is a function
of both the application and the local CDPD environment. To optimize
data transfer a trial and error approach varying the blocksize is
recommended, for example: