Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 23
5. Operations: Making a CDPD Connection
Understanding a CDPD network requires more than knowing how it is physically and logically
configured. The actual process by which a mobile end system (M-ES), such as a CDPD modem
plugged into a notebook, connects to the network and moves data through it is particularly
important, and requires the M-ES to have a verified address on the system.
5.1. The Registration Process
After a CDPD subscriber purchases a CDPD modem or other M-ES, they must contact their
preferred CDPD service provider and arrange to have it connected to the CDPD network. This
process is similar to setting up a cellular phone with a carrier network. Without activation, the
CDPD device or cellular handset is useless, because it has no network to connect to. Each time an
M-ES is turned on or reset, it acquires a channel and registers with the CDPD network, then goes
through an authentication and verification process to sign on.
5.1.1. Network Entity Identifier (NEI) and Home Subdomain
Whenever it is connected to a CDPD network, each M-ES is identified by a distinct Network
Entity Identifier (NEI) assigned by the CDPD carrier, which gives a CDPD modem a unique
address visible to the rest of the Internet. In fact, the NEI is an Internet Protocol (IP) address of
the same form as that used by other machines connected to the Internet. The CDPD network uses
the NEI (through its analog, the Temporary Equipment Identifier—see section 5.1.2) to send
messages to the M-ES via the MD-IS that is serving the M-ES at any particular time.
The NEI is 32 bits long, and like other IP addresses it is generally represented as four 8-bit
numbers, separated by periods, with each number being written in a decimal format (such as
64.114.87.11). Without the unique NEI, connecting to the CDPD network and the wider Internet
would not work.
Each NEI has a single home subdomain—its normal location in the network—that is set by the
subscriber’s CDPD carrier. However, some CDPD modems may support more than one NEI,
each of which has a home subdomain, which may or may not be the same.
For example, one CDPD modem may have three available NEIs, one each for Las Vegas, New
York, and Dallas. When in New York, the owner would use the NEI with a New York home
subdomain. Alternatively, two NEIs might belong to the same subdomain—perhaps to maintain
separate billing for business and personal use of CDPD services. The subscriber can set the NEI
from the available choices using software.
If an M-ES moves to a non-home subdomain (a roaming subdomain), the CDPD network’s
mobility management features (see section 5.2.1) handle the routing of packets appropriately,
provided that the subscriber has made appropriate roaming arrangements with the home CDPD
carrier, and that carrier has a roaming agreement with any new carrier in the roaming area.
Neither the NEI nor the behavior of the network connection needs to change, although
transmission costs may vary depending on the rate structures of the carriers involved.
5.1.2. Temporary Equipment Identifier (TEI)
Since the airlink portion of the CDPD network is encrypted (see sections 3.2.7 and 5.3), the CDPD
network can actually use a masqueraded value of the NEI for improved security. Using the
Mobile Data Link Protocol (see section 5.4), the CDPD network actually transmits a value known
as the Temporary Equipment Identifier (TEI), which is a data link layer frame address that
corresponds directly to the NEI for a particular M-ES.