Mobility can exist in many forms. To assist in analyzing different mobile systems, we offer the following simple taxonomy of mobility. In this taxonomy, levels of mobility depend on the degree of location-independence and in-motion capability provided by the mobile system.
We should point out, before beginning our mobile taxonomy, that mobility could be considered to be just one of the dimensions of a mobile data system. As pictured in Figure 1.13 the medium in use and the protocols supported are additional dimensions to consider. Depending on the situation at hand, different mobility solutions are more or less appropriate.
Type 0 Mobility describes systems which do not support mobility. Network entities are limited to essentially fixed locations for communication with others. This is the level of mobility provided by conventional data networking technology.
In conventional networks, the network layer address indicates the topological location of the host. The netid part of the address identifies the subnet on which a host can be found and is used by routers to forward a packet to its destination. Once the packet reaches the "last router" which connects the destination subnet to the rest of the world, the hostid part of the address is used to identify the destination host on that subnet1.17 . In general, the network address (netid + hostid) defines the host's location relative to well-known places in the internetwork.
Typically, human intervention is required to configure and administer network addresses of hosts in Type 0 systems. In these systems, moving a host from one location (subnet) to another amounts to creating a new entity (i.e., network address). [RICH95] states that "the static addresses of traditional network architectures bind a computer to a specific LAN or subnet. Current versions of IP, for example, assume that an IP node has a fixed point of connection to its network."
Type 0 systems are location-dependent and are thus not mobile. Examples of Type 0 systems include not only traditional networks, but also wireless LANs and campus networks such as Metricom Ricochet. The limitations to mobility in such systems are often due to the technology not being scalable to the magnitude required for ubiquitous mobile operations. In any case, the mobility is limited to the subnet or immediate (LAN) medium containing the host.
Type 1 Mobility describes systems in which hosts enjoy mobility regardless of location. We make no distinction between systems which could be deployed everywhere and those which are deployed everywhere. Rather than discuss the current state of deployment, which is itself a moving target, we assume that a system capable of global deployment is globally deployed; this makes it location-independent. Examples of Type 1 systems include Mobile IP, RAM and Ardis.
As mentioned earlier, transparency is a key aspect of mobile systems. If a peer application needs to know about the mobility of a host, then the benefit of that mobility is limited. In particular, systems requiring effort to port applications to them, while supporting mobility, are limited by the lack of mobile transparency.
Typically these systems require gateways to "spoof" protocols on one side of their networks and use proprietary protocols to convey data over radio channels. The horizontal market acceptance of these systems has been limited due to the effort required to port applications to nonstandard APIs and mobile devices.
In many cases, Type 1 systems have been designed from the bottom up, with an initial design focus on the Physical Layer, which is typically RF-based. Early mobile systems were developed from an RF perspective and customized to minimize radio traffic, using techniques such as canned messages and proprietary protocols at all layers. This effort to optimize the Physical Layer resulted in systems with physical (RF) protocols visible to applications. These systems are poor examples of protocol layering.
A very recent mobile technology development is that of Mobile IP, which is media-independent. Regardless of the underlying physical and MAC layers, Mobile IP provides the capability for two mobile hosts to directly communicate regardless of location. CDPD's mobility management paradigm is based on that of Mobile IP.
Type 2 Mobility describes systems in which in-motion operation is supported in a transparent and location-independent fashion. Type 2 mobility is the ultimate form of anywhere anytime communications capability. An example of such a system is CDPD, which was originally conceived of as a cellular overlay. In this system, the efficiency of mobility management (and radio resource management) is optimized to support active moving hosts which are capable of automobile speeds.