Mobile Messaging Technologies and Services: SMS, EMS and MMS ...

20 Mobile Messaging Technologies and Services
gateways. These proxies/gateways may belong to the mobile network operator or alternatively
to service providers. The primary function of proxies/gateways is to optimize the
transport of content from servers to WAP devices. Supporting servers, as defined by the WAP
Forum, include Public Key Infrastructure (PKI) portals, content adaptation servers, and
provisioning servers.
1.6.3 Push Technology
Figure 1.9 Generic WAP architecture
In a typical client/server model, a client retrieves the selected information from an
application 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. Web 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 the
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 1.10.
In the push framework, the push initiator initiates the push transaction. The push initiator,
usually an application server (e.g., web server, MMS center, etc.) transmits the content to be
pushed along with XML-formatted delivery instructions to a Push Proxy Gateway (PPG).
The PPG then delivers the push content to the WAP device according to the delivery
instructions. The push initiator interacts with the PPG using the Push Access Protocol (PAP).
On the other side, the PPG uses the Push Over The Air (OTA) protocol (based on WSP or
HTTP) to deliver the push content to the WAP device.