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As an important application area of access mediation, access gateways allow networks based on different signaling protocols to be interconnected. For example, telephone company mergers have created the need to interconnect networks based on different signaling standards. Wireless networks based on GSM must now interconnect with networks based on IS–41. The access gateway provides the protocol conversion needed to interconnect these networks. In addition, providers are using TCP/IP networks in place of SS7 for certain signaling (see Figure 3). In Figure 3, a TCP/IP–based network interconnects two distant IS–41–based wireless networks. The TCP/IP–based network is used to transport short messages between the two wireless networks. Since the short messages are not time-critical, the TCP/IP network is utilized to interconnect the distant networks which is a cost-efficient solution.
Figure 3. Access Gateway Example
In terms of message flow, a short message can originate in the distant IS–41–based network (shown in the upper left-hand corner of Figure 3). An access gateway in this distant network would protocol convert (i.e., encapsulate) this short message so it can flow over the TCP/IP network. Upon reaching the second IS–41 network, a corresponding access gateway converts the short message back to an IS–41 message. The short message then flows to the mobile switching center (MSC) where it is routed to the wireless base station (BS). At the BS, the short message is transmitted to the targeted subscriber's mobile station (MS), where the short message is displayed.
In addition to the fact that TCP/IP–based networks are less costly to implement and maintain than IS–41 networks, expanded capabilities also make the TCP/IP–based network desirable in this application scenario. For example, instead of requiring that the short message be inputted by a wireless subscriber based on an IS–41 network, the TCP/IP network can support Web site–based short message entry (see Figure 4). This not only makes the actual entry of the short message easier than entry on a mobile handset, but it also allows many more short-message senders. Anyone with Web access could enter the mobile subscriber's telephone number and a short text message at the short-message Web site. After the user enters the short message, it is sent over the TCP/IP network to the appropriate IS–41 network. The access gateway converts the short message into an IS–41 message and sends it to the mobile switching center, which routes the message to the base station. From there, the short message is transmitted to the mobile station where it is displayed.
Figure 4. Enhanced Access Gateway Example
