Trình duyệt Internet là một ví dụ về các mô hình dựa trên công nghệ kéo. Ngược lại, công nghệ khác đã được giới thiệu trong mô hình WAP và được biết đến như công nghệ đẩy. Với công nghệ đẩy, một máy chủ có thể đẩy một số dữ liệu đến các thiết bị WAP không có yêu cầu rõ ràng trước từ khách hàng. | 16 Mobile Messaging Technologies and Services Figure Generic WAP architecture Push Technology In a typical client server model a client retrieves the selected information from a server by explicitly requesting the download of information from the server. This retrieval method is also known as the pull technology since the client pulls some data from a server. Internet browsing is an example of models based on pull technology. In contrast another technology has been introduced in the WAP model and is known as push technology. With push technology a server is able to push some data to the WAP device with no prior explicit request from the client. In other words the pull of information is always initiated by the client whereas the push of information is always initiated by the server. The push framework defined by the WAP Forum in WAP-250 is shown in Figure . In the push framework a push transaction is initiated by a Push Initiator PI . The Push Initiator usually a web server transmits the content to be pushed along with delivery instructions formatted with XML to a Push Proxy Gateway PPG . The PPG then delivers the push Figure The push framework Basic Concepts 17 content to the WAP device according to the delivery instructions. The Push Initiator interacts with the Push Proxy Gateway using the Push Access Protocol PAP . On the other side the Push Proxy Gateway uses the Push Over-The-Air OTA Protocol based on WSP or HTTP to deliver the push content to the WAP device. The Push Proxy Gateway may implement network-access-control policies indicating whether or not push initiators are allowed to push content to WAP devices. The Push Proxy Gateway can send back a notification to the Push Initiator to indicate the status of a push request delivered cancelled expired etc. . In addition to application-specific push contents MMS provisioning and WTA three generic types of contents can be pushed in the WAP environment Service Indication SI Service Loading SL and
