Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 17
4.1.2. Mobile Data Base Station (MDBS)
The Mobile Data Base Station (MDBS), together with the Mobile Data Intermediate System,
connects the traditionally non-mobile protocols of the Internet to mobile CDPD subscriber M-ESs.
The MDBS provides a relay function between the MD-IS and the M-ESs. It is responsible for the
detailed control of the airlink interface between the M-ES and the CDPD network, such as
managing forward error correction, providing M-ES transmitter power control parameters, and
controlling access of several M-ESs to a single radio channel. In some cases, the MDBS may
support more than one radio channel to provide CDPD service if there is a large enough demand in
the coverage area provided by the MDBS in question. The MDBS usually shares a location (and
an antenna) with the base station of a cell site.
The logical medium that connects the MDBS to a set of M-ESs that are receiving on a particular
radio channel at any given time, is called a channel stream (see section 3.2.9). This channel
stream may be thought of as a pair of point to multipoint connections. Each channel stream within
a cell is uniquely identified by a Channel Stream Identifier (CSI) in the messages sent by the
MDBS to all M-ESs listening to that channel stream.
CDPD, as a full-duplex protocol, supports communication in two directions simultaneously. In
the forward direction, the transmissions are sent from the MDBS to all of the M-ESs listening on
that channel. In the reverse direction, the MDBS receives the transmissions from any transmitting
M-ES. The MDBS may support one or more channel streams within a sector depending on the
data traffic demands placed by the M-ESs within that sector.
In general, the “downstream” components of a CDPD network (the modem/subscriber device, and
the MDBS at the cell site antenna) have a much greater involvement in the selection of channels
for channel hopping and in network management than their AMPS voice counterparts (the cellular
handset and the cell site base station).
4.1.3. Mobile Data Intermediate Station (MD-IS)
The Mobile Data Intermediate System (MD-IS) controls the mobile data link between the
M-ESs and the CDPD network as well as the mobility management aspect of CDPD (see section
5.2.1). The MD-IS is the only element in the CDPD network that has any knowledge of M-ES
mobility with the network—the MD-IS insulates the upstream elements of the CDPD network and
the rest of the Internet from having to track the location of any M-ES, and it also prevents the
MDBSs from needing to know anything about each other. The MD-IS is often in the same
location as a cellular carrier’s Mobile Telephone Switching Office (MTSO—see section 2.2.4).
4.1.4. Connections to Other Networks – Intermediate Systems (IS)
Since CDPD is an IP-based system, it can connect individual CDPD service provider networks to
others, and to any other network connected to the Internet. Hardware and software systems such
as routers, firewalls, and others that permit these connections fall under the general term of
Intermediate Systems (ISs). In particular, ISs connected to a CDPD MD-IS permit M-ESs
connected through the CDPD airlink to link seamlessly into other networks, potentially anywhere
in the world, using any IP-compatible technology.
The network layer functions provided within the CDPD network allow any pair of end systems in
the CDPD network to communicate with each other. These functions must determine a path
through a series of interconnected elements called Intermediate Systems (ISs) until the desired
destination end system is reached. ISs along the communication path must forward network layer
packets (datagrams) between themselves to provide the required end-to-end connectivity. They
must deal with route calculation, packet fragmentation, and congestion control within the
interconnected ISs. The IS functionality is provided through commercially available routers
within the service provider network. Their presence is not visible to the CDPD subscriber.