Signaling System 7 (SS7)


A critical part of the core infrastructure for voice networks is its ability to interconnect with the existing TDM networks. Call setup, progress, and teardown are all accomplished via complex network signaling commands that traverse the network end to end. While several signaling options exist, most new telephony services depend on the power and reliability of Common Channel Signaling System 7 (SS7/C7), more commonly abbreviated and known as SS7. Differing from in-band signaling methods such as R2 or Channel Associated Signaling (CAS), an SS7 architecture dictates that call control information traverse a separate network dedicated solely to the business of transmitting the required information to successfully connect telephone calls.

Realizing the importance of SS7 call control, Cisco created a network architecture that supplements its Voice Infrastructure and Application (VIA) solution, allowing customers to introduce SS7 signaling into their new or existing H.323- or Session Initiation Protocol (SIP)-based voice-over-IP (VoIP) networks.

The Cisco SS7 Interconnect for Voice Gateways option adds the Cisco PGW 2200 PSTN Gateway to the Cisco VIA solution. There are two additional hardware elements: the Cisco Signaling Link Terminal (SLT) and the PGW 2200 hosts. "A" or "F" links from the SS7 network are physically connected to the Cisco SLT via one of several supported interface cards. The Cisco SLT terminates Message Transfer Part (MTP) Layers 1 and 2 of the SS7 protocol stack. Signaling traffic is then backhauled over an IP network to the Cisco PGW 2200, which interprets call signaling information that is used to establish and tear down calls in H.323- and SIP-based networks. The figure below illustrates the distributed architecture of the Cisco SS7 Interconnect for Voice Gateways option.

The Cisco VIA solution allows service providers to take advantage of packet network investments. With the Cisco VIA solution, service providers can quickly build multiservice networks and deploy revenue-generating services. The Cisco SS7 Interconnect for Voice Gateways option builds upon this opportunity by expanding available interconnect options and lowering operating costs. Key benefits to customers include:

  • Reduced recurring charges - In general, tariffs favor interconnection using SS7 signaling. Inter-machine trunks (IMTs) are less expensive than ISDN-based facilities, both on a one-time (installation and provisioning) and recurring (monthly charge) basis. This equates to lower monthly expenses, reduced cost of goods sold, and higher margins for service providers.
  • Opportunity to enter new markets - Many countries will not permit alternate carriers such as competitive local exchange carriers (CLECs) and Internet telephony service providers (ITSPs) to interconnect to the public switched telephone network (PSTN) with signaling methods other than SS7. The Cisco SS7 Interconnect option allows Cisco VIA customers to enter those markets.
  • Network scalability - SS7 signaling offers superior scaling over other signaling methods such as R2, Primary Rate Interface (PRI), and CAS.
  • Compatibility - Cisco SS7 Interconnect is compatible with previous versions of all Cisco VIA signaling methods. Customers can feel confident that revenue-generating services supported by their VoIP network will continue to operate and be fully supported with the addition of the SS7 signaling option. Also, upgrading to the Cisco SS7 Interconnect option is simple. With the addition of the Cisco PGW 2200 PSTN Gateway and the Cisco SLT, SS7 appears as "just another signaling method" to the VoIP network.
  • Faster customer response time - Telecommunications operations tend to be optimized around ISDN User Part (ISUP) trunk (IMT) provisioning. Therefore, IMTs can generally be provisioned more quickly than PRI, CAS, or R2 trunks. SS7 enables service providers to contract for IMTs as opposed to PRI, CAS, or R2 trunks.
  • Widespread network ubiquity - SS7-signaled trunks are usually supported at all TDM switch and point-of-presence (POP) sites. Other trunks, especially PRI-signaled trunks, may only be supported on a subset of PSTN switches.
  • The Cisco VIA solution is a proven infrastructure that enables a broad portfolio of packet-based voice services. It is designed for service providers and international carriers who seek to increase revenue, build customer loyalty, and boost profits by adding national and international call transport and a variety of other services such as prepaid and postpaid calling card services, voice mail and unified communications, and application service provider (ASP) termination services to their service portfolios. Support for industry-standard protocols such as H.323 and SIP make it easy to link a service provider's network through many interconnect options.