In order to describe mobility in WAN environments, we need a terminology describing range of motion. Mobility consists of receiving service anywhere; movement to another region should result in receiving service in the new region. Although the application running on the mobile host should be oblivious to the regional boundaries (transparency of service), the mobile host itself will likely need to be aware of the boundary crossing and actively participate in it.
The region types we define are a hybrid of conventional data network and cellular terminology; this reflects our need to define mobility in both (network) topological and geographic terms. Since WANs are hierarchical in nature, so are these regional definitions. This is depicted in Figure 1.14.
A channel is the means by which a mobile host receives service; it is the subnet used by the mobile to access the network. The channel is both a physical media and a logical location (i.e., network access point) of the mobile host. In many cases, such as CDPD, the channel is shared by multiple mobile hosts.
A cell is defined by the geographical area covered by a channel. This means that a mobile host could potentially continue to receive service from the channel even while moving about the cell.
A cell can be either large or small, depending on the system and the situation; media-specific physical limitations and capacity constraints determine the size of a cell. In a wireless system, a cell is defined by the RF coverage area of a single channel. Depending on the mobile system, the cell might be identical to the mobility area; in this case, geography and network topology would be essentially identical.
Multiple channels could cover a single cell. This would be akin to multiple Ethernet LANs being available to office workers in a building; which Ethernet is used is based on need for logical interconnectivity (e.g., a group of users who work together a lot) or traffic (load balancing)1.18 .
Sometimes, depending on the medium used, cells overlap. In such a situation, a mobile could receive service from any of a number of channels. This could be significant from the standpoint of mobility management if the channels covering the mobile's geographic location are based in different network topological locations. An example of this is the A-side and B-side systems in the cellular industry.
In some systems, the mobile hosts in a cell can directly communicate with one another in a peer-to-peer fashion. In other systems, the mobile hosts in a cell must communicate with one another via some kind of hub. The requirement for a hub is typically due to the limitations of the physical media in use.
A region controlled (from a mobility perspective) by a single mobility-supporting router is called a mobility area; this is analogous to a routing area in conventional data networks [PERL92]. The mobility router is responsible for accepting network layer packets destined for mobile hosts in its area of control (its mobility area) and forwarding those packets to their destinations.
From a WAN perspective, the mobility router is directly connected to all of the mobile hosts in its mobility area; this is a network layer topological concept. The mobility router is the last entity to handle the NPDUs (in their Layer 3 form) before the destination host; any intervening entity operates at a strictly lower layer. The mobility router is an assisting entity in any mobility scenario.
A mobility area will generally consist of one or more cells. Each cell is contained in its entirety within a single mobility area.
An administrative domain in a mobile network is the same as in a conventional data network: a region under the control of a single authority. This is important from the standpoint of accounting, directory services, security and authorization. The administrative domain is the highest-level region for mobility.
An adminstrative domain is under the jurisdiction of a single authoritative body; this body either accepts or rejects a mobile entity requesting services in the domain. The idea is that someone has to pay for and run a network (or subnetwork); the collection of subnets under control of one organization is an administrative domain. An administrative domain is a logical concept and is sometimes referred to as an autonomous system.
An administrative domain will generally consist of one or more mobility areas. Each mobility area is contained in its entirety within a single administrative domain.