User's Guide

The Operational View SB300 Series
Page 48 Proprietary and Confidential 2110059 Rev 1.1
• In the case of SLIP, the disposition of the broadcast and multicast messages is a function of
the support provided by the software stack and end-user applications (one must be UDP)
operating on the Mobile Application Subsystem (MAS). If a UDP session is not active, the
messages will be lost.
Broadcast
The Broadcast function is used on a geographic coverage basis. To support this function the
MDIS is set up with a Broadcast NEI that includes a list of the Mobile Data Base Stations
(MDBSs) in the desired broadcast area in its definition. There can be multiple broadcast areas
within the coverage area of a single MDIS. A message is transmitted to a Broadcast NEI at the
MDIS that replicates the message, puts a special broadcast header on it, and sends it to the cells
indicated by the broadcast list.
For most applications broadcast is not appropriate, because broadcast messages are sent to all the
CDPD modems in the covered geography. It is more useful to be able to specify that messages
only go to all, or a subset of the IP Addresses associated with a specific customer; a capability
offered by multicast.
Multicast
Multicast is set up similarly to broadcast, except that the MDIS is set up with a Multicast NEI and
a list of the terminals that belong to that specific group rather than a list of cells, as used by the
broadcast function. The message to be multicast is sent to the Multicast NEI at the MDIS, which
replicates the message and sends individual messages to the terminals indicated in its multicast
group list.
A terminal can be a member of any number of multicast groups, however it can only be active in
one at any given time. A terminal wishing to receive multicast messages indicates its readiness by
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.
5.8.3. 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.