As voice-over-packet technology gains ground on multiple fronts, IP is tackling the nitty-gritty details of voice telephony - the SS7 protocols. SS7 is the behind-the-scenes data network that supports today's voice telephone network, both nationally and globally. The SS7 network carries signaling and control information for basic calling services and intelligent network services. And as an ITU standard, SS7 is widely deployed throughout the world.

Today's SS7 network connects telephone switches and databases with redundant data links (typically 56 to 64 kb/s) and redundant routers, called signal transfer points (STPs).

Like other data networks, SS7 is packet-based, has multiple protocol layers and is extensible. Message Transfer Parts 1, 2 and 3 (MTP1, MTP2 and MTP3) are the lower protocol layers for SS7. Above these basic protocols, ISDN user part (ISUP) carries call control information; transaction capabilities application part (TCAP) handles database access by transporting queries and responses. And to abstract the location of specific databases, an extra layer called signaling connection control part is inserted between TCAP and MTP3.

As data networks go, SS7 is a bit unusual. The SS7 protocol stack contains four or five layers vs. the seven layers of the traditional open systems interconnection model. And although SS7 provides only connectionless transport, some messaging procedures emulate virtual circuits. But if a link in such a virtual circuit fails, network management will reroute that message, thus providing the best of both worlds.

Finally, even the idle codes for SS7 are special. They contain data and a frame check sequence that helps verify link integrity.

Not only is SS7 widely deployed, reliable and well-suited for voice telephone call control, but it also can support intelligent network applications such as caller ID, local number portability (LNP) and mobile telephony. For example, SS7 provides access to home location register (HLR) and visitor location register (VLR) databases that contain information about where a subscriber currently is located.

SS7 has some disadvantages, however. It's a very telecom-specific protocol running on a very telecom-specific infrastructure. It shares little with the rest of the computer industry, so it has been slow to take advantage of widely available, higher bandwidth technology. The majority of the SS7 network still runs at 56 or 64 kb/s. And as a specialized technology, SS7 also is fairly expensive. It can't leverage the enormous R&D investments being made in computers and the Internet.

However, none of these disadvantages would really matter without the impact of the Internet and IP on the communications industry.

Forces of change

Several factors are driving the desire for SS7 and IP integration. These include Internet offload, voice over IP and wireless data access.

With millions of dial-up Web surfers using voice lines for long Internet sessions, there are immediate benefits to putting remote access servers (RASs) close to the customer and routing customer calls to the nearest appropriate RAS box.

Because one SS7 connection can support many distributed RAS boxes, it makes sense to transport SS7 messages over the back-end IP infrastructure that goes to each RAS system. While approaches differ, each major vendor has found a solution for transporting SS7 information over IP.

The migration of voice to IP networks is well under way, driven by the enormous investments in Internet technology. Internet traffic already is comparable to or greater than voice telephone traffic in the U.S., and Internet traffic is growing at a dramatic rate. Within three to five years, voice traffic will represent only a small portion of the total bits transported, so a new, converged infrastructure makes economic sense.

But how this plays out for individual telephone users is a matter of debate. There are 800 million conventional landline phones in the world and more than 400 million wireless phones. The converged network needs to set up calls to people on the existing telephone network and to access the 800 number translation, caller ID, and HLR/VLR databases. This happens through a signaling gateway (Figure 1).

As currently defined, this gateway does not provide protocol mediation; it does not attempt to translate protocols. It transfers SS7 application protocols - such as ISUP for call setup and TCAP for carrying data access messages - from the SS7 network and transports their messages over the IP network to the applications server that manages calls in the converged network.

The third force adding momentum to the integration of SS7 and IP is mobile Internet access. Today, most wireless telephony is voice-only, consisting of cellular and PCS phone service.

However, wireless data access is hot - whether it's short message services that integrate pagers and cell phones or Internet access using the cellular phone's display to get traffic information, weather and stock quotes. The wireless industry has a road map for migrating from today's limited text messages to third generation wireless networks with substantial IP bandwidth - up to 384 kb/s for mobile users and up to 2 Mb/s for stationary users.

While this will take some years to be widely deployed, wireless equipment vendors have realized that they need an IP infrastructure to connect their wireless base stations as well as - or in place of - their current voice infrastructure.

And while they are designing an IP-centric infrastructure, the opportunity arises to rethink how signaling information is being transported in the backhaul from the base station. The emerging decision is to send wireless signaling such as IS-41 or GSM MAP messages over IP instead of MTPs.

Proper protocol

The telephony industry and the Internet community have been working together within the SIGTRAN working group of the Internet Engineering Task Force to develop the stream control transmission protocol (SCTP) for reliably transporting signaling messages - especially SS7 messages - over IP. SCTP development has been going on for more than a year and is nearing completion. This effort is being supported by major equipment vendors such as Nortel Networks, Ericsson, Siemens, Nokia, Lucent Technologies, Motorola and Alcatel.

SCTP is based on IP but can provide the redundancy and reliability that telecom applications demand - and that the traditional SS7 network has historically provided. SCTP operates at the same level as TCP and user datagram protocol, which means it operates directly on top of IP. SCTP-on-IP directly replaces MTP1, MTP2 and MTP3 (Figure 2).

Messages that are passed through the traditional SS7 network from STPs or signaling endpoints are transferred in the signaling gateway to run over SCTP to the application server in the new network. Application servers that already have emerged include IP versions of traditional telephony databases - such as 800 number translation, LNP and HLR/VLR - and the new functions of media gateway controllers or softswitches.

SCTP provides all the functions of TCP, namely reliable data transfer, guaranteed message delivery and end-to-end connection control. It also supports multiple individual message streams over the same connection, eliminating head-of-line blocking. In addition, SCTP can support redundant transmission paths and redundant equipment with separate IP addresses participating in a single, logical transmission path. When this is combined with Internet security solutions developed in other areas of the IETF, SCTP can support systems as reliable as the current SS7 network.

Besides defining SCTP, the working group also has defined adaptation layers such as the MTP3 User Adaptation, or M3UA, layer. Adaptation layers preserve the same interface as the pre-IP equivalent layer, facilitating software migration. And because one signaling gateway can serve multiple application servers, M3UA also handles address translation and mapping between the routing information in MTP3 user messages and the IP addresses of application server processes.

Of course, no service provider would expose its call control network to the Internet. The good news is that multiple security protocols are defined for use with IP.

But as a practical matter, private IP networks make sense, both to avoid congestion and for security. Today private IP networks are implemented on shared ATM infrastructure, and in the future, private IP networks will be implemented on multiprotocol label switched (MPLS) paths in shared IP networks.

SS7 in the new world

As voice transport migrates to packet technology, new signaling protocols are emerging, but all the functions now supported by SS7 messaging will remain and will continue to use established SS7 application level messages. Existing SS7 applications and databases remain critical to telephone service. Meanwhile, SS7 and IP integration will lead to unique new services.

The industry already is seeing combinations of voice portals and wireless Internet access providing new information services for mobile users. Here, mobility information is combined with business directories to provide gas station, restaurant or travel information to mobile users in voice or data formats.

New overlay networks are providing rich roaming services for GSM mobile users. Now the features and functions in a home network can be available while users are roaming. This is implemented with an overlay IP network that connects to existing GSM operators using SS7 signaling gateways. Once connected, the actual control logic can be located anywhere, with SS7 messages backhauled using IP over cost-effective computer networking facilities.

Core SS7 functionality and the SS7 application layer messages will be with us for a long time, even while we're busy changing out the SS7 lower layers and the specialized SS7 hardware that has outlived its usefulness.

As new services are deployed over an IP backbone, service providers can directly access SS7 messages and transport them over IP networks to signaling gateways, where they can go back into the traditional telephone network. Access to the intelligent network will no longer require specialized SS7 hardware; it will only require knowledge of specific SS7 application layer protocols. This will substantially simplify the development of new network applications.