hort Messaging Service (SMS) services are becoming near-ubiquitous. Who among us has not used their cellphone to check the latest cricket score or to get the latest news? One very popular service, available through most carriers, is Yahoo's 8242 service. This service allows you to check your Yahoo Mail, use Yahoo Instant Messenger, or check scores, news, etc. When you access this type of service, you're using both the carrier's network and the service provider's network (in this case Yahoo). There are also many carrier-dependent services, in which case you make use of the carriers' network alone.

To provide you with these services, these networks (Yahoo, or any of the carriers like Hutch, AirTel, etc.) implement something known as a gateway. A gateway is nothing but—as the word suggests—an entrance to the network. In the case of mobile-based services, these gateways are called mobile-to-network gateways,or simply mobile gateways.

This article uses a low-level implementation to demonstrate how a mobile-to-Web gateway works. The example network uses the Web, but carriers and service providers may use their own networks.

What Is a Gateway?

Figure 1. A Typical Gateway: The image shows a typical gateway.

Typically, a service has many users. The gateway (depicted as a thick line in the service network in Figure 1) performs the following tasks:

• Implements a queuing mechanism to service each incoming request.
• Determines whether the the service requested is acceptable.
• Sends a response granting the service.

Thus, a gateway acts as a filter between the service network to the service requester (user's) network.

The sections that follow provide an overview of an SMS-to-Web gateway, and an MMS-to-Web gateway. For simplicity's sake, the examples assume only one user. This leaves the task of implementing mobile gateways that service incoming mobile requests (in the form of SMSes and MMSes) on a continuous basis.