Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 30
5.6.1. The Radio Resource Management Entity (RRME)
The forward and reverse channels of a CDPD data conversation, like AMPS voice channels, are
different radio frequencies, widely separated in the assigned spectrum. The M-ES selects these
channels according to a list made regularly available by the local MDBS, but the overall selection
process is managed by an entity in the CDPD network known as the Radio Resource
Management Entity (RRME).
The RRME manages and provides many pieces of information about the radio frequency in each
cell of a CDPD network, such as:
• The carrier’s SPNI
• The number of the current cell
• The number of channels allocated to the call
• How many channels are currently in use and their channel numbers
• Data to enable M-ESs and MDBSs to handle handoffs smoothly
• A reference channel to enable M-ESs to evaluate received signal strength (RSSI)
• Information to enable M-ESs to set their transmission power
• The maximum transmission power allowed for M-ESs in the cell
• Channel management diagnostics
• Error control information
As an M-ES moves about the CDPD network coverage area, it may move from one sector to
another within a cell, and between different cells in the CDPD network.
If in a new sector where the MDBS is connected to the same MD-IS as the MDBS in the previous
sector, and the handoff is called an Intra-Area Cell Transfer. In such a transfer, the link
between the virtual data connection and the physical radio connection to the M-ES changes. The
serving MD-IS updates this link, and the data link resumes from the point of interruption on a new
radio channel in the new sector. This procedure occurs very rapidly and is transparent to the user.
When moving to a new cell where the M-ES is served by an MDBS connected to a different MD-
IS from the MDBS in the previous cell, the handoff is called an Inter-Area Cell Transfer. In this
case, a new data link connection is established between the M-ES and the new MD-IS. Once the
data link connection is re-established in the new serving area, the M-ES registers with the new
MD-IS.
Based on the channel stream information (see section 4.1.2) and the Subscriber Location Service
(see section 4.2.2), the M-ES is always aware of its location. When it moves to another cell, it
notifies the CDPD network of the change. The network uses this location information, the
modem’s unique identifier (the TEI), and the CDPD’s mobility management features (see section
5.2.1) to ensure that data continues to move smoothly.
5.6.2. Power Level Issues
The CDPD specification for controlling transmission power is complex, but in essence permits
any M-ES to transmit at one of eight power levels. Devices known as Class I can transmit from
Level 7 (lowest power, usually 6 mW) to Level 0 (highest power, usually 4000 mW). However,
many modems are not capable of transmitting at the two highest power levels, 1 and 0. Such
Class III devices, like most cellular handsets, have maximum transmission power of Level 2
(600 mW).
Unlike a cellular handset, which has its transmit power controlled by the cellular base station, the
M-ES sets its own transmit power by applying a formula defined in the CDPD specification. That
formula uses the power product parameters obtained from the MDBS and the current measured
received signal strength (RSSI) measured by the modem. Using the algorithm, the modem
computes the required transmit power, which is then checked against the maximum power level
allowed, a parameter also obtained from the MDBS. If the computed power exceeds the allowable
power, the modem lowers the transmit level to allowable limit before it begins transmitting.