Specifications

Sierra Wireless, Inc. CDPD Primer

2130006 Rev 1.0 Page 27

M-ES (A)

MDBS MD-IS (A)

IS (Router)

M-ES (B)

MD-IS (B)

Internet

Figure 5-2: CDPD mobility management scenario 2

The difference between scenarios 1 and 2 is transparent to the notebook users, except for the

delays introduced by the longer transmission in scenario 2.

If either notebook were communicating with a fixed system on the Internet, the paths would be

similar. Traffic destined for Notebook B always has to travel first to MD-IS (B), the home of its

NEI, where the traffic is re-directed via the serving MD-IS (A) to the roaming ME-S (B).

5.2.2. Functions of a Modem on a CDPD Network

The CDPD modem performs a number of specific functions. In order to connect, it must:

• Find an available CDPD channel by doing a wide-area scan

• Verify that the channel is usable; that the block error rate (BLER) is acceptable; and that the

SPNI of the channel is authorized for use (if configured to check)

• Establish operation of the data link (channel)

• Establish encryption between the modem and the MD-IS

• Register by providing appropriate NEI and authentication credentials

• Choose a channel whenever required from the available list broadcast by the local MDBS:

• Whenever the received signal strength indication (RSSI) or block error rate (BLER)

parameters are exceeded

• Whenever CDPD synchronization is lost

• When the scan timer expires

• Detect voice signals and hop channels as appropriate

• If in a “sniff and hop” environment (see 3.1.2), change channels as directed by the MDBS,

either as planned channel hops or by being forced off by an impending voice call on the

current channel

When transmitting data on the reverse (transmit) channel to the MDBS, it must:

• Accept commands and digital data from the end-user application equipment (for instance,

software running on a notebook computer, or a credit-card authorization system in a taxi)

• Encrypt the data

• Assemble the encrypted data into packets

• Add forward error correction (FEC)

• Adjust its transmission power according to received signal strength and parameters set by the

CDPD carrier (see section 5.6.2)

• Check for reverse (transmit) channel availability—congestion or busy

• Transmit the encrypted data packets to the CDPD network

• Retransmit packets depending on error correction performance and packet collisions