The solution is to bypass all switches and IMTs except the ingress switch. In this solution to the congestion problem, the originating switch routes calls that are intended for ISPs to ports that are connected directly to the RAS of the ISP. In this solution, the SS7 gateway enables the RAS to accept the signaling required for call setup and call teardown.

Figure 2 shows the system overview of this solution. The major feature of this solution is bypassing the egress and tandem switches, as well as all the IMTs, and terminating all signaling associated with Internet access within a region to a regional SS7 gateway.

Figure 2. SS7 System Overview

In this scheme, network-enabled RASs—or NASs—can be co-located with the local switches or centralized in one location. Based on today's technology, each NAS can have at least 672 ports (one DS3). Some networking vendors are currently working to increase this number. Every call to an ISP is identified by the called number and is routed to one circuit within the range of circuit trunks that are connected to these modem banks (the NASs). The routing is based on one of the following techniques: IN office-based triggers, local-number portability (LNP) routing of ISP numbers, IN single number service, or *XX service code. The integrated services digital network user part (ISUP) messages associated with these calls are routed by shielded twisted pairs (STPs) to the SS7 gateway. A combination of the SS7 gateway and the NAS devices in a region suggest the image of a switch to the network and the originating switch. Each SS7 gateway can manage many NAS devices in one location or many.

Because the call model for an Internet access call is rather simple, it is possible for one SS7 gateway to manage tens of thousands of NAS ports. Each gateway has a unique point code and is connected to at least one pair of STPs via multiple A links. It is important to note that each gateway can handle many remote point codes (ingress switches). The original switch sends ISUP messages that are received by the gateway and are translated into a set of equivalent messages. These messages are transmitted via a dedicated Internet protocol (IP) wide-area network (WAN) to the proper NAS device. The NAS devices are connected to two IP WANs. The first WAN is a dedicated WAN for communication between the gateway and the NAS modules. The second WAN is the data network that carries data between the end Internet users and the ISPs.