This chapter describes the major internal components of the gateway and their interaction in general terms.

The gateway will divide the processing load on several hosts, which are of three different types: Bearer box This host connects to the SMS centers and CSD routers, and provides a unified interface to them for the other boxes. It implements the WDP layer of the WAP stack. WAP box These hosts run the upper layers of the WAP stack. Each session and the transactions that belong to that session are handled by the same WAP box. Sessions and transactions are not migrated between WAP boxes. SMS box These hosts run the SMS gateway. They can't connect directly to the SMS centers, because the same SMS center connection can be used for both SMS services and WAP. There can, in this design, only be one bearer box, but any number of the other boxes. (This is a potential performance bottleneck, and will be dealt with later.) The bearer box receives SMS messages from SMS centers, inspects them to see whether they are plain SMS messages or contain WAP packets, and then routes them to the appropriate kind of box. It keeps track of the WAP and SMS boxes that are currently running, and routes all messages from a particular terminal to the same box. Similarly, it receives UDP packets from CSD routers and routes them to the boxes based on the sender. This simple design allows the bearer box to do as little processing as possible per packet, which reduces its bottleneck karma. The bearer box also sends the SMS messages and UDP packets that the other boxes generate. This adds some more routing processing. The WAP boxes implement the WTP (transaction) and WSP (session) layers (and the WTSL security layer in the future). These take HTTP-like requests from the phones, and make the actual HTTP requests to content servers, compress the response, and send it back to the terminals. (Sessions are needed to make as much use of the limited radio bandwidth as possible.) The SMS boxes implement traditional SMS services: SMS messages with keywords and arguments are translated into URLs and the corresponding HTML pages are stripped of markup and sent to the phone.