The SS7 gateways, which are used for Internet offloading in conjunction with the remote access servers, also serve as building blocks for delivering voice and fax over IP.
Figure 1 depicts the interconnection of an SS7 gateway with the rest of the PSTN and IP entities. On the PSTN side, local switches and service control point (SCP) are interconnected, as far as signaling is concerned, through signaling transfer points (STPs). The SS7 gateway is connected (via SS No. 7 A links, depicted by dotted lines) to STPs, too, thus becoming an SS No. 7 network element. In addition to direct connection to the signaling network, an SS7 gateway is connected to a dedicated IP network, which it shares with the remote access servers it supports. When Internet users make data calls, central office switches route these calls to appropriate RASs’ ports. The SS7 gateway (which acts on behalf of the RASs it supports) emulates a telephone switch on the switch-to-switch trunk setup interface exchanging ISUP messages with the real switches. The SS7 gateway also emulates a switch on the switch-to-SCP interface, exchanging INAP messages with the SCP. Thus, only the originating switches need to keep their lines occupied with the Internet calls; the rest of the PSTN is bypassed, so the valuable voice circuits are left for what they were intended for in the first place—voice. As we mentioned before, not only the PSTN benefits from this solution. ISPs can choose to leave the access service business to telephone carriers and concentrate on e-mail, World Wide Web hosting, and other services relevant to their customers’ needs; ditto for enterprise networks.
Figure 1: SS7 Gateway Interconnections.
The leading SS7 gateway products that combine both hardware and software are available in configurations that range from 10,000 to 200,000 ports and can work in the enterprise, ISP, and central office environments. The key requirement of such products—essentially high-powered (0.5 to 1 GB of memory) fault-tolerant computers—is effectively 100 percent availability (advertised as 99.999 percent). Typically, duplication of hardware components (so that all computations, including those that support all I/O operations, run in parallel) is sufficient for achieving total availability, but it is also important that hardware have self-checking logic to monitor the components in order to detect and remove the faulty one (while passing control to its double) at once. Self-monitoring is an important feature to look for in high-end products. Important software differentiating features are as follows:
§ Multistack SS No. 7 option. Allows several processes, each executing the SS No. 7 stack with a different point code (that is, the SS No. 7 endpoint address), to execute concurrently on a single system. Thus a single system can emulate several different switches (in support of the same number of remote access servers or VoIP gateways) in ISUP dialogues.
§ Support of IN. Enables communication of SS7 gateways with external service control. Top-end products support both the U.S. (DP-specific) and European (DP-generic) options of the ITU-T Intelligent Network Application Part (INAP).
§ Support of ISDN access signaling. Allows processing of calls originating on the ISDN BRI and PRI lines.
§ Online provisioning and management. Allows a customer to monitor (typically, via SNMP) and dynamically reconfigure an SS7 gateway.
In some implementations, the SS7 gateway has features of the so-called soft switch. In other words, the SS7 gateway not only acts as a telecommunications switch on the line and trunk setup interface, but also provides for the operational interface functions (such as circuit queries and blocking) as well as a full call state model, which maintains for each call the state of the path between voice and IP networks. To this end, call events are recorded automatically and call event records are generated for billing.
As you may have noticed, there is a striking similarity—as far as the interconnection architecture is concerned—between the carrier-grade VoIP gateways and remote access servers: Both need to support SS No. 7—best achieved today by pairing with the SS7 gateways. Both also advocate the need for IP trunking so as to interconnect with LECs’ telephone switches. This subject is discussed in the section that follows.
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