While defining the CDPD System Specification, it became apparent that an early adopter of this mobile data communications technology would be the fleet manager. Public safety organizations have been using private mobile wireless data communications systems for over a decade. Other fleet services such as package courier services have benefitted from the competitive advantage a mobile data system provides. These mobile data users would greatly benefit from a "multicast" service.
Before we proceed, we should clarify that CDPD multicast services is not the same as IP multicast. The services are different although both are forms of multicast communications.
The purpose of the CDPD multicast service is to support organizations and applications that have a need to send the same data to a group of subscribers. This group of subscribers may be dispersed across the network at separate geographic locations. The message will be of lower priority. It is not deemed necessary to guarantee delivery of the message to all members of the group.
Multicast is typically used for some informational bulletin of minimal significance, similar to paging information services. In other words, the service is geared towards inexpensive but possibly unreliable delivery. For those applications requiring reliable group delivery, higher protocol layer distribution list functionality should be utilized.
Before providing service to any multicast group, there must be a mechanism for the mobile devices that belong to that multicast group to announce their presence and service requirements to the network. In CDPD, a single network address (Network Entity Identifier or NEI) is used for all members of each multicast group. This common NEI is known to the CDPD network as a multicast NEI. Any network packet that contains the multicast NEI as the destination address is automatically routed to all members of the group. The CDPD network handles all replication of the packet as necessary to distribute to all group members.
The extensions necessary for this registration mechanism is the use of an additional parameter named the Group Member Identifier (GMID). The GMID is assigned to each device in a way that assures uniqueness among members of the same group.
When a mobile device registers a multicast NEI, it must submit an ESH message with the optional GMID parameter along with the associated authentication credentials4.19 . When the serving MD-IS receives the ESH with the GMID parameter, it builds the associated RDR and forwards the authentication information to the home MD-IS.
The main difference at this point involves the treatment of the Registration Directory. In the multicast case, the serving MD-IS allows for the possibility of multiple entries to reference a single NEI value. Under the reference of the single multicast NEI, there will be multiple subnetwork addresses, each associated with one or more unique GMID values.
On receipt of the multicast RDR from the serving MD-IS, the home MD-IS validates the registration attempt based on the NEI, GMID and authentication credentials. Each unique GMID within a single multicast NEI has its own stream of authentication credentials.Once validated, the home MD-IS responds with a RDC message containing the proper multicast NEI and GMID data.
Once again, the home MD-IS extends the Location Directory structure to allow multiple entries under the same NEI value. Each entry within the multicast NEI value is a unique GMID and associated data forwarding service address.
When the home MD-IS receives a data packet destined for the multicast NEI, it must redirect as many copies of the data as is necessary to reach all members of the multicast group.
The home MD-IS references the Location Directory. It examines the entries associated with the multicast NEI, and sends one copy of the data packet to every data forwarding service reported to have group members of the multicast NEI. Each copy of the data packet is encapsulated as per normal point-to-point data traffic.
On receipt of an encapsulated data packet for a multicast NEI, the serving MD-IS examines its Registration Directory to determine the location of the group members. The data forwarding service then sends a copy of the decapsulated packet on each subnetwork point of attachment that has a group member of the multicast network address.
The data packets are transmitted on the appropriate cells on a broadcast channel. This ensures that all mobiles belonging to the multicast group can have access to the data packet.
The actions of multicast redirection and forwarding are illustrated in Figure 4.23. In this example, MDBS 11 and 12 are within the routing domain of serving MD-IS 1. Similarly, MDBS 21 and 22 are within the routing domain of serving MD-IS 2. A multicast NEI has been assigned and some of the group members have registered from cells in MDBS 11, MDBS 12 and MDBS 22. No group member of that multicast NEI have registered from MDBS 21. When the external host sends a data packet to the multicast NEI, the home MD-IS replicates that NPDU and sends one copy to each of MD-IS 1 and MD-IS 2. MD-IS 1 decapsulates the data and sends one copy of the original NPDU to each of MDBS 11 and MDBS 12. MD-IS 2 decapsulates the data and sends a copy of the original NPDU to MDBS 22 but does not send any copy to MDBS 21. This thus shows how the network infrastructure has distributed the multicast NPDU through the most efficient distribution method. Furthermore, it shows how the external host can send identical copies of an NPDU to multiple recipients with a single transmission.
The use of broadcast services with the two tier distribution process ensures that the most efficient mechanism is used to reach all group members. Only one copy of the data packet is sent between each pair of MD-ISs. Only the serving MD-ISs that are hosting group members receive the distribution. Only one copy of the data packet is broadcast on every channel that contain one or more group members. Only the cells that have reported registered group members receive the broadcast.
The use of broadcast channels for distribution of multicast data packets disallows the return of receipt acknowledgments. There is no receipt confirmation returned to the sender. If acknowledgment of receipt is necessary from all members of the group, the application will need to institute its own application layer mechanism. If guaranteed delivery is necessary for a collection of mobile users, then message handling system distribution list mechanisms should be used instead.
The broadcast nature of the multicast data distribution also disallows data link layer encryption. The possibility of unrecoverable loss of data frames disallows useful operation of services that demand data stream synchronization such as encryption. If secure data is necessary in a multicast situation, application layer encryption may be used. The CDPD network will not interfere with such schemes.