The operation of a network such as the CDPD network requires cooperation of multiple network components. This cooperation is achieved through exchanges of administration information and routing update information. Most of the protocol exchanges were described in Section 4.8, however other exchanges are necessary for complete management of the routing data. These protocol exchanges and procedures are described here, while other exchanges relating to lower layer functionality are detailed in the Chapter 5.
When a mobile device relocates from one CDPD serving area to another serving area, the network infrastructure is made aware of the movement through the re-registration of the mobile unit in the new serving area. In this instance, the serving MD-IS in the new routing domain is informed of the entry of the mobile device. However, the serving MD-IS in the previous routing domain is not informed by the mobile unit of its departure.
In this instance, it is possible that the previous serving MD-IS is unnecessarily holding network resources for the departed M-ES. These resources may include the Temporary Equipment Identifier (TEI)4.15 , data buffer allocation, timer resources, etc. While these resources are usually not excessively taxing on the network, it is preferable to ensure that the most efficient use of the infrastructure is maintained.
To support resource efficiency, the home MD-IS, on receipt of a successful registration attempt from the serving MD-IS in the new routing domain, may notify the serving MD-IS in the previous routing domain of this event. This is achieved via a Redirect Flush (RDF) message (see Figure 4.16), which informs the previous serving MD-IS that the home MD-IS will no longer route data packets destined to the identified NEI towards it. Since there is no longer any requirement to reserve resources for that NEI, the previous serving MD-IS may then release all resources held for the it. This is illustrated in Figure 4.15 as an optional transmission.
When a mobile device registers its NEI, it must provide the associated authentication credentials. These credentials are verified against the NEI to credentials association maintained by the network. If the network maintained credentials agree with those provided by the M-ES, the network will authorize access by the mobile device.
This mechanism works well in most instances. Since the mobile device and the network must share a common understanding of the credentials, the network is protected from use by fraudulent devices. However, even with the mandatory data encryption4.16 over the airlink, the RF nature of the CDPD system means that the authentication credentials can be intercepted by other devices within the RF coverage area.
To address this concern, the CDPD system additionally allows the network operator concerned with security attacks by fraudulent devices to periodically update the authentication credentials. The network may issue new authentication credentials in the response to any registration attempt. Once issued, the mobile device is required to provide the new credentials on subsequent registration attempts. By instituting this type of authentication credentials updates, the probability of fraudulent unit discovery is increased.
In addition, the CDPD system specifications defines that any mobile device must periodically provide its authentication credentials to the network. The maximum amount of time a mobile is allowed to operate without execution of the registration process is defined as the Configuration Timer. This value is conveyed from the network to the mobile device by an optional field in the ISC message (see Figure 4.9). If a mobile fails to issue an ESH message prior to expiry of its Configuration Timer, the serving MD-IS may consider the M-ES as unreachable and sends a Redirect Expiry (RDE) message (see Figure 4.17) to the home MD-IS.
If the home MD-IS wishes to confirm that a particular M-ES4.17 is still reachable, it may send a Redirect Query (RDQ) (Figure 4.18) to the serving MD-IS identified in its Location Directory.
If the serving MD-IS determines from its Registration Directory that the NEI is no longer active, it returns an RDE message to the home MD-IS. If the Registration Directory indicates that the NEI specified is registered, the serving MD-IS in turn, sends an End System Query (ESQ) to the M-ES.
When the M-ES receives an ESQ message for an active NEI4.18 , it must respond with an ESH message. The serving MD-IS processes the ESH message using the normal mechanism by issuing the corresponding RDR message to the home MD-IS. If the home MD-IS does not receive any response to the RDQ message, it may assess the NEI as unreachable and remove its entry from the Location Directory. It optionally may send a RDF message to the serving MD-IS to release unneeded resources.