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In the normal client/server model, a client requests information or a service from a server. The server responds by transmitting information or performing a service to the client. This is known as pull technology—the client pulls information from the server.
In contrast to this, there is also push technology. The WAP push framework transmits information to a device without a previous user action. This technology is also based on the client/server model, but there is no explicit request from the client before the server transmits its content.
To perform a push operation in WAP, a Push Initiator (PI) transmits content to a client. However, the Push Initiator protocol is not fully compatible with the WAP Client—the Push Initiator is on the Internet, and the WAP Client is in the WAP domain. Therefore, we need to insert a translating gateway to serve as an intermediary between the Push Initiator and the WAP Client. The translating gateway is known as the Push Proxy Gateway (PPG).
The access protocol on the Internet side is called the Push Access Protocol (PAP).
The protocol on the WAP end is called the Push Over-The-Air (OTA) protocol.
The Push Initiator contacts the Push Proxy Gateway (PPG) over the Internet using the PAP Internet protocol. PAP uses XML messages that may be tunneled through various well-known Internet protocols such as HTTP. The PPG forwards the pushed content to the WAP domain. The content is then transmitted using the OTA protocol over the mobile network to the destination client. The OTA protocol is based on WSP services.
In addition to providing basic proxy gateway services, the PPG is capable of notifying the Push Initiator about the final status of the push operation. In two-way mobile networks, it can also wait for the client to accept or reject the content.
Push services can be of the SL or SI type:
SL. The Service Loading (SL) content type provides the ability to cause a user agent on a mobile client to load and execute a service—for example, a WML deck. The SL contains a URI indicating the service to be loaded by the user agent without user intervention when appropriate.
SI. The Service Indication (SI) content type provides the ability to send notifications to end-users in an asynchronous manner. For example, the notifications may be about new emails, changes in stock price, news headlines, and advertising.
In its most basic form, an SI contains a short message and a URI indicating a service. The message is presented to the end-user upon reception, and the user is given the choice to either start the service indicated by the URI immediately, or postpone the SI for later handling. If the SI is postponed, the client stores it and the end-user is given the ability to act upon it at a later point of time.