User's Guide

The Operational View SB300 Series
Page 46 Proprietary and Confidential 2110059 Rev 1.0
registering a Multicast NEI and by being ready to receive UDP datagrams. The latter can be
accomplished by having the UDP Server function set to active, or if in SLIP mode, by having a
UDP application set to ready. In all of the preceding cases the MAS is assumed to be ready to
process any incoming multicast datagrams.
Contact the service provider for information on the availability and use of multicast service in
your area.
5.7.3. Transmission Control Protocol (TCP)
Transmission Control Protocol (TCP) is an advanced transmission protocol that adds substantial
functionality to the underlying Internet Protocol it is built on. Because of this structure it is often
referred to as TCP/IP. The advantage of TCP is reliability of data transmission; achieved by using
positive acknowledgements with retransmission if required. The main disadvantage is the
overhead needed to provide this reliability under various conditions.
Due to the positive acknowledgements used in TCP, it is not recommended that sleep modes be
enabled in the modem. If the modem goes to sleep, the possible delays in sending and picking up
acknowledgements can result in excessive re-transmissions and even lost connections.
Unlike UDP, TCP offers both reliable and connected data transmission service. Lost, delayed,
duplicated, or out of sequence packets are detected and automatically corrected. A logical point-
to-point connection is established and maintained throughout the communications session. It
requires very little other than session establishment, sending data over the serial port, and session
termination to get a basic application operational.
The price for this reliability is the number of extra data packets used and the time required to
process them. Both need to be considered when selecting an application protocol.
5.7.3.1. Configuring a TCP Client Session
Client sessions are those in which your modem initiates a conversation with a server.
A detailed discussion of this type of session follows the outline of steps to open a TCP Client
Session. (AT is omitted for brevity):
1. +WS46=4 to place the modem in CDPD mode (only required on the SB320).
2. +S31=0 to disable logic sleep.
3. +WS175=0 to disable CDPD sleep.
4. +WS45=1 to select TCP packet service.
5. Optionally assign a channel with +WPCHAN=n where n is the channel to use.
6. Optionally verify the channel acquisition with +WPRSSI looking for the second value in the
return string to be 3.
If the modem is using auto-registration (+WS173=1) skip to step 9.
7. +WS197=n where n is the index (1-4) of the NEI to register, to select the NEI
8. +WPREG to register on the CDPD network. The modem will reply as noted in the section
above on manual registration.
9. +WS56? to query the registration status. Repeat this command until it returns a value of
1 (registered) or until the host chooses to timeout, failing the connection.
10. +WS212=<port> where <port> is the port number (1025-4999) that the modem will be
calling on and listening to. The modem will only accept packets addressed to this port. The
factory default is 2100.