Once the M-ES has announced its location and the CDPD network has validated it, the network can forward data packets to the M-ES. In the following pages, we shall describe the process by following the activities necessary to route a network packet from an external host to the mobile device4.9 .
Prior to the providing its packet forwarding function, the home MD-IS for the mobile unit must announce its function to external networks. The intent is to ensure that all reachable hosts direct data traffic for the mobile unit towards the home MD-IS. To achieve this, the home MD-IS advertises itself as the shortest path to the mobile's network layer address. This is typically accomplished by the home MD-IS participating in conventional routing information exchanges with its nearest neighboring routers.
Once this has been accomplished and this routing information has been propagated to the external networks, the external host can successfully send data to the mobile unit. The external host proceeds according to standard networking operation. It sends a network packet with the mobile unit's address as the destination and its own address as the source.
This data packet is directed through the intervening networks to reach the home MD-IS in the conventional manner. Once received by the home MD-IS, it must be redirected to the correct serving MD-IS. Unfortunately, the home MD-IS cannot act as a simple router and transmit the original data packet. If the data packet were to be transmitted without modification, it would be looped back towards the home MD-IS since all other routers have been informed that the home MD-IS is the best next node for the message.
To circumvent this loop, the home MD-IS alters the data packet by encapsulating the original data packet within a new data packet. The new data packet is addressed to the mobile serving function at the appropriate serving MD-IS. The address of the mobile serving function associated with this particular mobile NEI is retrieved from the Location Directory, which is maintained through the mobile registration process described in Section 4.8.1. This encapsulation process is sometimes referred to astunneling4.10 and is depicted in Figure 4.10.
Before closing on our discussion of the home MD-IS, we should note that this functionality is further abstracted in the CDPD specification. To avoid too much association between functionality and implementation, the CDPD specification define the mobility management operation at the home MD-IS as the Mobile Home Function or MHF. Although we use home MD-IS and MHF almost interchangeably in our discussion, it should be noted that there are nuances to each of these terms.
Once the data packet arrives at the serving MD-IS, it must be transmitted over the radio network to the M-ES. However, the M-ES will not respond to the address of the encapsulation packet. The serving MD-IS must de-encapsulate the data packet from the home MD-IS prior to relaying it over the radio network. This is accomplished through the packet redirection function within the serving MD-IS.
The serving MD-IS reconstructs the original data packet through decapsulation and examines the ultimate destination address. This address is matched against the entries in its Registration Directory and the appropriated subnetwork point of attachment is determined. The original data packet is then transmitted to the radio coverage cell area identified via one or more data link frames.
The mobile unit receives the data packet over the radio connection. The data packet it receives is the unaltered original network packet sent by its peer host. Figure 4.11 illustrates the protocol events over time.
We have just presented an overview of the fundamentals of the data redirection and forwarding operation used to support data mobility. However, the CDPD network must also be able to handle seamless dynamic relocation of the mobile device to a new subnetwork point of attachment.
As in the case of the home MD-IS, so goes the serving MD-IS. In the serving MD-IS, the mobility management function is referred to as the Mobile Serving Function or MSF.