Specifications

Sierra Wireless, Inc. CDPD Primer
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update the Subscriber Directory Profile (see section 5.1.3) so that the old NEI can be re-used with
the new device.
5.1.5. Service Provider Network Identifier (SPNI)
Just as each M-ES has its unique numbers, including the NEI and EID, each CDPD carrier has a
number identifying it to M-ESs connecting to its network equipment. That 16-bit number is
known as the Service Provider Network Identifier (SPNI). This number enables M-ES units
communicating over the airlink to identify equipment for their own network (as opposed to those
used by another carrier), and also to determine when they are roaming in another carrier’s CDPD
network space.
Whenever an M-ES connects to an AMPS channel to make a CDPD connection, the SPNI is
included in the information sent to that M-ES by the nearest MDBS. CDPD modems can be
configured to register only on a network operated by a carrier whose SPNI is on a list set in the
modem. If so configured, a modem will not lock onto a channel unless the SPNI matches one in
the list. Some Sierra Wireless CDPD products also permit the user to specify an exclusion list of
SPNIs to which they should never connect.
Businesses that provide CDPD devices for use only on one carrier’s network, or in a limited range,
can configure their M-ESs this way to simplify usage and billing. Others who want their devices
to connect whenever possible, as permitted by the carrier, can configure their M-ESs to connect
regardless of the SPNI provided by the network.
5.2. Moving Data Through the CDPD Network
Once registered and connected to a CDPD network, an M-ES can begin communicating. As
mentioned in section 3.2.1, a packet-switched protocol can remain connected all day—as long as
the M-ES is powered up and within radio range of an MDBS, and has performed the proper
registration and authentication procedures. It incurs charges only when data is actually moved
over the network. The M-ES may move from sector to sector and cell to cell while maintaining its
network connection.
5.2.1. CDPD Mobility Management
As mentioned in section 3.1.6, traditional Internet protocols are not designed to handle nodes that
travel, changing how they are connected to the network from moment to moment. CDPD gets
around the problem with a system similar to that used by the roaming cellular phones using the
same radio infrastructure. For CDPD, the system is known as mobility management (see section
4.2.3).
A CDPD modem (or M-ES) has a static IP address (the NEI) so, as far as the Internet is
concerned, it sits behind its home MD-IS, which acts like a normal Internet router. However, if an
M-ES changes location so that it now connects to a different MD-IS through a different set of
MDBSs, its NEI does not change—yet the same IP address cannot logically be connected to a
different router. CDPD therefore encapsulates and forwards Internet data packets so that the
M-ES appears to the rest of the Internet as though it still resides behind its home MD-IS.
Figure 5-1 and Figure 5-2 illustrate the process for a sample situation. Notebook computers
labeled M-ES A and B are the end systems communicating with each other. Notebook A’s home
is labeled MD-IS (A). Notebook B’s home, MD-IS (B) is elsewhere; B is roaming. Both
notebooks are currently being served from MD-IS (A) — the home MD-IS for Notebook A, but a
roaming MD-IS for Notebook B.