Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 13
3.2.4. Transmission Rate and Peak Throughput
CDPD offers raw transmission rates of 19.2 kbps. Error control overhead means that actual
throughput of useful data can be up to 12 kbps on a clean, lightly-loaded channel. Data transfer
rates may be lower on congested networks with many voice or CDPD transmissions underway.
Data compression can, of course, increase the effective throughput.
CDPD protocols have low overhead, especially since they require no protocol conversion to
TCP/IP for Internet connectivity. CDPD also compresses the IP protocol overhead, increasing
throughput and using less radio bandwidth.
3.2.5. Number of Users Supported
Like any data communications system, each CDPD data link has a maximum capacity it can
support. In the case of CDPD, the airlink is the limiting resource of the network. The user data
transmitted over the air is a frame (a data block that includes header and error detection
information) with a maximum length. The maximum 19.2 kbps throughput of CDPD limits the
number of frames that can be sent over the channel. On average, if a user has an application that
requires 5% of this maximum channel capacity, then one radio channel data link can support 20
users. On average, if a user has an application that requires 1% of this maximum channel capacity
(not unusual, especially for light Internet connectivity), then a single radio channel data link can
support 100 users.
The nature of most user data applications is that the amount of data sent is small (a few frames)
and the rate at which the data is sent is bursty (short periods of activity, followed by long periods
of idle time). As a result, a CDPD radio channel data link can support a large number of users at
one time. The maximum number of users that can be supported on a single radio channel data link
depends on the nature of the data traffic that the users’ applications send.
3.2.6. Coverage and Availability
CDPD operates as an overlay on top of the existing analog AMPS cellular infrastructure. Thus, to
implement CDPD, carriers need only add Mobile Data Base Stations (MDBSs), Mobile Data
Intermediate Systems (MD-ISs), and other Intermediate Systems (ISs) to quickly deploy
CDPD (see section 4.1 for details on these components). Although no single nationwide CDPD
service exists across the U.S., most major metropolitan areas are covered, and roaming
agreements between most CDPD carriers—in which a CDPD device activated in one area can
connect through base stations linked to MD-ISs in another region, similar to voice roaming
arrangements—generally allow a single account to be used across the broad CDPD network.
Currently, CDPD coverage is available for the majority of the population of North America.
However, even carriers that provide CDPD service may not make it available over their entire
analog cell coverage area. Subscribers need to obtain coverage information for their region from
their carriers.
The strength of the radio signal received by the modem indicates the airlink quality, and can be
measured using the Received Signal Strength Indication (RSSI). RSSI is expressed on a
logarithmic scale, in decibels relative to one milliwatt (dBm). A strong signal level has a less
negative number; a weak signal level has a more negative number. For example, a signal level of
-60 dBm is stronger than one of -100 dBm.
CDPD modems are designed to measure the RSSI and adjust their transmitted power output
accordingly, within the limits permitted by telecommunications regulations and how the local
carrier has set the serving MDBS (see section 5.6.2). However, connections are lost for signals
below a certain strength level. For example, many devices lose their CDPD connections when the
signal strength falls below the -105 to -110 dBm range.
A CDPD carrier attempts to ensure that the signal level is fairly strong (-80 dBm or stronger)
throughout the coverage area. Shielding of the signal by artificial structures or natural obstacles
may weaken the signal, sometimes cutting off the connection.