The ability of a CDPD mobile host to freely roam within a potentially vast coverage area requires continuous tracking of the mobile host's location by the network. Although different tracking techniques may be fashioned, each impacts applications wishing to contact the mobile host in its own way.
As we described in Chapter 1, two dimensions of mobility are defined by frequency of movement and span of movement. From the mobile user's perspective, these are geographic concepts. One user may consider how frequently he or she travels from one business district of a city to another. A different user may view frequency and span in terms of their weekly need to operate from two office locations within a campus. The road warrior may think in terms of daily movement between cities or weekly movement between continents!
On the other hand, the network designer considers frequency and span of mobile movement in network infrastructure terms. Whether the movement is from city to city or from room to room, its system impact depends on how the coverage area is configured; this is a topological consideration.
If the entire city area is effectively covered by a single radio site, then all physical movement of mobile users within the city is invisible from a network perspective. The only noticeable relocation would be movement of mobile users from one city to another. However, if each building in a city commands its own radio coverage sector, then any movement into and out of a building would constitute a relocation event that must be tracked by the network.
Given that each mobile relocation that is visible to the network introduces management overhead, why would the network designer not construct a single all encompassing radio coverage area? Well, unfortunately, we live in a physical world where physical laws apply. Current technology does not allow effective radio coverage of a large area while providing low error, high speed, high capacity data communications with low power/low cost devices. So to satisfy a target of system capacity and performance, geographic areas of typical modern cities must be divided and served by distinct radio coverage zones.
In the cellular telephone system, these zones were originally designed to approximate the honeycomb structure of hexagonal cells. This layout permitted non-interfering re-use of a small set of radio frequencies. The standard configuration relied on a re-use pattern of 7 or 12 frequencies as described in Chapter 2. This approach allowed each frequency to be re-deployed at a radio site that is close by but far enough away such that the two transmitters would not cause insurmountable interference.
This approach served the cellular carriers well-at least in the early years. As time went by and users began to appreciate the value of being constantly in contact, more users signed on to the cellular network. To handle the increasing traffic, cellular carriers subdivided cells into smaller units. These smaller coverage areas are typically one third or one sixth of the original cell and are called sectors.
The CDPD system was designed as a data network transparently overlaid on the cellular system, as depicted in Figure 4.1. As such, it adheres to the radio constraints of the cellular telephone network. The CDPD network infrastructure uses radio coverage areas that are equivalent to the cellular sectors. Therefore, in CDPD, the mobility of importance is the relocation of users from one radio coverage sector to another.
Beyond cellular sectors, the CDPD system also approximates the cellular telephone system in aggregating the radio coverage sectors into regions that are served by a single Mobile Switching Center (MSC). In CDPD parlance, this is equivalent to a single routing area.
At a higher level, various cellular service providers offer service to each city. Movement of a cellular telephone user between cities is handled through "hand-off" of the user from one carrier in one city to the carrier in the next city. An analogous situation exists within CDPD. The CDPD network routing domain approximates the city to city hand-off.