At each layer of the OSI reference model there are protocol entities communicating with each other. They are the sources and destinations of PDUs at that layer. Because this book is about mobility in WANs, the entities of greatest interest are Layer 3 entities, commonly called hosts, nodes or end systems. Layer 3 PDUs, commonly called packets, are exchanged between hosts via Layer 3 entities commonly called routers.
Adding the capability of mobility to a wide area data network creates a need for defining additional entities, as depicted in Figure 0.10.
A mobile host or mobile is a host which can receive network services regardless of its location. The extent to which this host enjoys transparent location-independence is a key concern. Different systems use different terminology for the mobile; CDPD adopts ISO terminology by calling it a Mobile End System (M-ES). The mobile is an occasionally-connected entity, which means it may or may not be connected at any given moment to a subnet somewhere in the mobile WAN.
The second role necessary in any communications is the opposite side of the correspondence. In this book we refer to this as the correspondent host or correspondent. The correspondent is the location of the opposite side of a mobile's application association; it could be the ultimate source of data destined for the mobile or another entity such as a store-and-forward device. The correspondent could itself be mobile or fixed in location, but this is generally not material to our analysis. CDPD refers to the correspondent as the Fixed End System (F-ES) when it is fixed in location.
In circuit-switched systems, there will be a maximum of one correspondent per mobile host. However, in the packet-switched systems of greatest interest to us, there can always be multiple correspondents per mobile host.
Associated with the mobile communications is the assisting entity or assistant. The assistant is an enabler of mobility. It could be a network store-and-forward device or mobility-supporting intermediate system (router). Most likely, it consists of multiple entities in a mobile network infrastructure which collectively support host mobility. In CDPD this role is largely filled by a combination of the Mobile Serving Function and the Mobile Home Function; the Mobile IP Task Force calls this combination the foreign agent and the mobile router or home agent.
As we shall see, the essential problem of mobility is getting data to the mobile. Thus, the mobile will generally be the destination host in any mobile system scenario of interest. Consistent with this, we will adopt a system-centric viewpoint by defining the flow of data traffic to a mobile as moving in the forward direction (i.e., mobile network to mobile host). Likewise, the flow of data traffic from a mobile is said to move in the reverse direction (i.e., mobile host to mobile network). This terminology is consistent with the cellular industry, which is the origin of CDPD, and is displayed in Figure 0.11.
