The CDPD system was created with seamless mobile communications in mind. To this end, a method of efficient and effective data relay to the mobile unit is required. This mobility management mechanism must also allow transparent communications to the user. In other words, the user should not be inconvenienced just because he or she is in an area not operated by their home service provider. Given this requirement, the user's location should always be accessible from the home relay service.
If we revisit our chapter 1 road warrior Gary who is attending a conference in a distant city, most of his time will be spent in the hotel housing the conference. He could phone his administrative assistant each and every time he changes conference rooms. That way the administrative assistant could always redirect his calls to that particular conference room. However, this involves a lot of long distance calls and is inefficient!
In another mode of operation, Gary could simply inform the hotel concierge of every room change. Now, when there is a message for Gary, the administrative assistant simply relays the message to the hotel concierge, who then relays it to the appropriate conference meeting room. This more localized form of tracking Gary's whereabouts is clearly more efficient than the previous mechanism.
With this administrative redirection arrangement, the hotel concierge is responsible for tracking the local whereabouts of Gary. The home administrative assistant only needs to know that Gary is still in the same hotel. Within the CDPD Network, the serving area entity is equivalent to the hotel concierge while the home area administration entity performs the role of the administrative assistant. The serving area entity is the serving MD-IS4.5 . The home entity is the home MD-IS.
The following sub-sections present the operation of CDPD mobility management. Through this discussion, we will clarify how the mobile units announce their presence to the network and how the infrastructure components cooperate to route data to the mobile units. In addition, we shall define the home MD-IS and the serving MD-IS.
The first event necessary for a successful mobile data connection is for the mobile unit to announce its current location to the network. This is achieved through several message transactions between CDPD network components, as depicted in Figure 4.5.
The M-ES initiates the process by transmitting a simple registration message to the CDPD network. The network uses this transmission to update the data base it uses to track the mobile's location. The CDPD network must ensure that it is tracking the genuine mobile. This requires that the M-ES send both its identification and its associated authentication credentials.
This exchange of information is accomplished through the transmission of an End System Hello (ESH) message (see Figure 4.6), which is part of MNRP. This message contains the mobile's "permanent" network address, called the Network Entity Identifier (NEI), and the associated authentication credentials. In most cases, the NEI is simply an IP address assigned to the mobile.
The authentication credentials consist or a randomly generated number, called the Authentication Random Number (ARN), and a sequence number, called the Authentication Sequence Number (ASN). The ARN is generated by the CDPD network and is assigned to the NEI associated with the M-ES. The ARN must be supplied by the M-ES on succeeding registration attempts of that NEI.4.6 The authentication process is generally under the control of the home MD-IS, and is described in Chapter 6.
Both the mobile unit and the network maintain two sets of ARN/ASN tuples for each NEI. The purpose of a second set of authentication credentials is to handle the rare situations when the network and the mobile fall out of synchronization with respect to authentication data.
This can happen if, for example, at the moment the network issues a new ARN to an M-ES, it becomes unreachable due to poor radio coverage. Shortly afterwards, the mobile unit may enter a good coverage area in another routing domain which causes it to send in a new registration attempt. In this instance, the ESH sent by the mobile will contain authentication credentials that are older than those expected by the network. By ensuring that the network maintains the two most recent sets of authentication credentials, the network can always recognize the older credentials, validate them and resynchronize with the mobile.
The MD-IS that is operating in the mobile serving area receives the ESH from the M-ES. On receipt of the ESH message, this serving MD-IS records the radio coverage location of the M-ES and provides the M-ES identification information to the appropriate home network's MD-IS. The primary purpose of this data transfer is to instruct the home MD-IS to redirect data destined for the M-ES through this serving area. Thus, this serving MD-IS to home MD-IS notification is called the Redirect Request (RDR) message.
The data carried in the RDR message (see Figure 4.7)includes all registration information provided by the M-ES. This data allows the home MD-IS to confirm that the M-ES is a valid device. Other information, such as the serving area location is also sent in the RDR message to the home MD-IS.
Since CDPD is a public data network, other considerations may determine whether a unit is to be authorized to use the network. Things such as the subscriber's account status, the usage limits defined by the user at subscription time, etc., provide the basis for the home MD-IS decision to permit access. Rejection of the M-ES may also be based on geographic parameters, such as an M-ES which should not receive service outside of its home city or a serving area which is also covered by a more preferred service provider. This control by the home MD-IS of access to the CDPD network is called access control.4.7
Once the M-ES authentication and access control decisions have been made, the home MD-IS must indicate this status to the serving MD-IS. Again, we emphasize, the registration, authentication and access control mechanisms operate on a per-NEI basis. If the M-ES uses more than one NEI, each must be separately registered and authenticated.
The home MD-IS uses the Redirect Confirm (RDC) message to relay the decision on whether it is willing to redirect data traffic for the indicated M-ES. The home MD-IS sends the RDC message to the serving MD-IS that requested data redirection on behalf of the indicated M-ES.
The RDC message (see Figure 4.8) contains the information which identifies the M-ES being tracked by the network, the decision of the network to grant or deny support for that M-ES, and updated authentication credentials, if appropriate. The updated authentication credentials are not mandatory and depend on the security policies of the home service provider.
If the CDPD network has agreed to service the M-ES, then the home MD-IS updates its location data base entry for that mobile. The data maintained for each mobile NEI include the following:
¥ Mobile Network Entity Identifier
¥ Mobile Serving Function address (serving MD-IS address)
¥ Registration counter
The purpose of the first two entries are obvious. The registration counter is a monotonically incrementing value4.8 used to detect duplicate or delayed RDR messages. The table of such tuples for all M-ESs managed by a single home MD-IS is the Location Directory.
On receipt of the RDC message from the home MD-IS, the serving MD-IS examines the result indication. If the home MD-IS grants support for the M-ES, the serving MD-IS updates its local data base and allocates resources to service the M-ES. If the home MD-IS denies support for the M-ES, the serving MD-IS discards information about the M-ES and frees resources reserved for that device.
Once the serving MD-IS receives the RDC message from the home MD-IS indicating whether or not to provide service to the mobile user, it must relay that decision to the M-ES. The serving MD-IS achieves this through the transmission of the MD-IS Confirm (ISC) message.
The ISC message (see Figure 4.9) contains the mobile address (NEI) that is being acknowledged and the results indication, an optionally, updated authentication credentials.
If the received RDC indicates an agreement by the home MD-IS to provide service to the mobile unit, the serving MD-IS must update its local data base regarding the mobile. The data maintained for each mobile within the serving MD-IS's area include the following:
¥ Mobile Network Entity Identifier
¥ Subnetwork Point of Attachment (the identifier for the radio channel the mobile is currently using)
¥ Registration counter
The purpose of the first two entries is obvious. The registration counter is a monotonically incrementing value used to detect duplicate or delayed RDC messages. The table of such tuples for all M-ESs currently hosted by a single serving MD-IS is called the Registration Directory.
On receipt of a successful ISC, the M-ES is aware that the CDPD network recognizes its identity and location. The M-ES is also assured that data packets addressed to it will be delivered through the CDPD network. This constitutes a successful mobile registration.
On the other hand, the M-ES may receive an ISC indicating an unwillingness or inability by the CDPD network to serve its data routing needs. This would typically require the mobile user to intervene in resolving any outstanding network or subscription issues. Some of the reasons for refusal to service a registration request are associated with authentication failure or lack of a business agreements between the two CDPD service providers.