Let us begin by discussing the conceptual entities which define the routing hierarchy and architecture used in the CDPD network. These elements are illustrated in Figure 4.2.
In order to provide a viable structure to the routing problem, it is necessary to partition the network into manageable sections. On the physical level, we rely on radio coverage limitations to define the smallest unit of control. Each radio channel in a cell coverage area is a single distinct entity, much as an Ethernet LAN is a distinct subnet.
Multiple M-ESs may be sharing such a channel stream. A channel stream represents a single subnetwork point of attachment (SNPA) to the CDPD network. The M-ESs sharing it are much the same as hosts on an Ethernet LAN, from both an operational and a networking perspective.
Multiple channel streams may operate over a single radio coverage area called a cell. In the cellular telephony world, a CDPD cell may be equivalent to a sector.4.2 Each CDPD cell is controlled by a Mobile Data Base Station or MDBS4.3 .
Typically multiple MDBSs are controlled by a single Mobile Data Intermediate System or MD-IS. An MD-IS managing the data links over the radio channel to M-ESs is a serving MD-IS. All cell coverage areas under the control of a single serving MD-IS is called a routing area sub-domain.
The collection of routing area sub-domains under the management of a single operating authority is a CDPD service provider network. CDPD service provider networks may be interconnected through the specified I-Interface. The collection of all interconnected CDPD service provider networks is referred to as the CDPD network.