April 1999

Integrating IP And SS7 Technologies
Achieving Distributed Intelligence, Realizing The Public Network's Potential

JEFF LAWRENCE

H.323 was the start, although clearly not the end, of what will be a long road to the deployment and integration of multimedia and Internet telephony services in the public network. Indeed, new standards and specifications for multimedia and associated gateway functions continue to appear, forming a long (and even daunting) list of acronyms. In addition to H.323, we have SIP, ASP, SGCP, RSGP, IPDC, and MGCP.

The H.323 family of standards specifies the components, protocols, and procedures required to provide multimedia communications over packet-based networks including, most important, Internet Protocol (IP)-based networks. H.323 supports mechanisms for point-to-point or multi-point communication for audio, video, and/or data using the following four types of components: terminals, gateways, gatekeepers, and multi-point control units.

The ITU specifies the H.323 standards. Version 1 of H.323 was accepted in 1996 and Version 2 was accepted in 1998. Version 3 is currently under discussion and will include features for fax-over-packet networks, gatekeeper-to-gatekeeper communications, and fast-connection mechanisms.

H.323 AND THE SIP ALTERNATIVE
Even as products supporting H.323 are being announced and deployed, there is a feeling throughout the industry that H.323, a full-featured protocol with a high degree of complexity and a large code size, will not be the only signaling protocol used for multimedia and Internet telephony.

H.323 has its place, but another technology, called Session Initiation Protocol (SIP), can provide a less complex solution for connection setup and media mapping with a correspondingly smaller code size, making it more attractive for future terminal and device designs. Nevertheless, because of its widespread deployment and relative maturity, H.323 will continue to play an important role for some time.

H.323 SIGNALING IN THE INTERNET, AND SS7 SIGNALING IN THE PSTN
The public telephone network and the Internet are fundamentally different. In the public telephone network, signaling and media streams (consisting primarily of voice) are carried physically on different networks. Signaling information is carried in the form of packets on a Signaling System 7 (SS7) network and media streams are typically carried on a circuit-switched or cell-based network.

In the Internet, however, signaling information and media streams are carried on the same network. Here, H.323 or SIP currently carries the signaling information, and the Real-Time Transport Protocol (RTP) carries the media stream.

USING GATEWAYS TO ACCOMMODATE DIFFERENCES BETWEEN THE INTERNET AND THE PSTN
Gateways perform the interworking function between the different signaling protocols and media streams carried on the Internet and public telephone network. Even as gateway equipment is being deployed, the industry is looking for ways to improve today's gateway solutions.

Today's gateways operate as a single piece of equipment. However, several recent announcements regarding Gateway Control Protocols (GCPs) can be seen as the telecom industry's attempts to define standards and specifications that will allow the decomposition of this single piece of equipment (called a "composed" gateway) into multiple pieces of specialized equipment (called a "decomposed" gateway).

A decomposed gateway typically consists of a signaling gateway with an internal or external call agent and a media gateway with some communication between the various pieces. The reasons for creating "decomposed" gateways include the following:

• A decomposed gateway is architecturally consistent with the public network. SS7 signaling and media networks of the public network are independent of each other.
  
  
• A single decomposed signaling gateway can manage many decomposed media gateways. SS7 signaling links on the public network side of the gateway may be underutilized in their current usage. These SS7 signaling links are designed to handle many calls and can probably handle many more calls than the media portion of the composed gateway.
  
  
• A single decomposed signaling gateway can manage many decomposed media gateways without the use of any additional SS7 point codes. SS7 point codes are used to address the different SS7 network elements. SS7 point codes are scarce and must be rationed carefully within the public network. Each composed gateway with signaling capability requires a point code. As the number of gateways with signaling capabilities increases so does the number of point codes required.
  
  
• A decomposed signaling gateway can be designed to continue and meet strict performance, reliability, and availability requirements. However, the decomposed media gateways associated with it can be designed to meet less stringent requirements resulting in lower design complexity and reduced equipment cost The SS7 signaling network has very high performance, reliability and availability requirements. The requirements imposed by the signaling side of the public network are otherwise, by default, imposed on a composed gateway.
  
  
• A decomposed signaling gateway can be divided even further into signaling and call agent functionalities, which provide service providers and/or users with the ability to offer new services and capabilities traditionally referred to as computer telephony and intelligent networking.

HISTORY OF GATEWAY CONTROL PROTOCOLS
The history of GCPs has been short but productive. In a very short time, there have been numerous announcements and varying levels of activity within industry coalitions and standards organizations. The announcements began with Bay Network's Access Signaling Protocol (ASP) and was followed by Ascend's Signaling Gateway Control Protocol, Bellcore's Simple Gateway Control Protocol (SGCP), Bay & Ascend's Reliable Signaling Gateway Protocol (RSGP) and followed up by the Level3 Technical Advisory Committee's Internet Protocol for Device Control (IPDC). More recently, SGCP and IPDC have merged to become the Media Gateway Control Protocol (MGCP).

MGCP is a simple protocol that operates between "intelligent" call agents and "dumb" media gateways. MGCP has been designed (1) to allow the call agent to control the media gateway, and (2) to use a fairly small set of messages and procedures. The messages are transmitted in ASCII text and using the IP Security (IPSEC) protocol ensures security.

A key advantage of separating the call agent and media gateway is that new services can be introduced easily without the need to change existing media gateways. A draft proposal of the MGCP has been submitted for consideration to the (International Engineering Task Force (IETF) and the European Telecommunications Standards Institute (ETSI) Project TIPHON (Telecommunications and Internet Protocol Harmonization Over Networks). MGCP appears to be the front runner as the Gateway Control Protocol of choice.

RUNNING THE SS7 PROTOCOL
In addition to the H.323 and/or SIP protocols, the signaling gateway runs the SS7 protocol and resides between the SS7 signaling network and the Internet. The media gateway performs the media mapping between circuit or cell-based public network and the packet-based Internet. One signaling gateway can control one or more media gateways and communicates with these media gateways via simple proprietary or standards-based protocols (for example, MGCP).

The presence of an internal call agent in the signaling gateway can effectively restrict the call setup, maintenance, and tear-down intelligence to the boundary between the SS7 signaling and IP networks. However, moving the call agent outside of the signaling gateway into its own network element allows the intelligence to reside anywhere in the Internet or in the public network controlled by either the service provider and/or the user. In this case the signaling gateway effectively becomes a relay of the SS7 User Part protocols (for example, INAP, TCAP, ISUP, and/or B-ISUP) between the public network and the call agent.

SS7 OVER IP
Within an SS7 network, the Message Transfer Part protocols are responsible for the reliable transport and routing of user part messages, such as the ISDN User Part (ISUP). An equivalent functionality must be achieved on the Internet side of the signaling gateway by transporting the SS7 User Part protocols over IP, between the signaling gateway and the call agent. Standards for the transport of other protocols such as TCAP over IP already exist. A draft standard for the transport of ISUP over IP has been submitted to the IETF for consideration.

DEPLOYMENT SCENARIOS
The media gateway stays the same and a call agent manages the media gateway via simple proprietary or standards-based protocols (for example, MGCP). This architecture allows one signaling gateway to support multiple call agents and one call agent to support multiple media gateways.

The potential configuration and interactions between the call agent, signaling gateway, media gateway, Internet, and public network enables a number of deployment scenarios, including:

• Trunking gateways that enable the interconnection of the public telephony network via the Internet.
• Business gateways that enable the connection of LAN PBXs in businesses and organizations to the Internet.
• Residential gateways that enable the connection homes to the Internet.
• Access gateways that enable the connection of businesses, organizations, and homes on the local loop to the Internet.
• Network access servers that enable dial up access to the Internet.

CONCLUSION
The effective and widespread deployment of multimedia and Internet Telephony services will depend on the successful implementation of the H.323, SIP, MGCP, and SS7-over-IP protocols. H.323 and SIP will provide the mechanisms for connection setup and media mapping. MGCP and SS7-over-IP will manage the call agents and gateways, which provide the interworking functions between the Internet and the public telephone network.

Pulling the call agent function out of the gateways is continues the trend toward distributed network intelligence and provides opportunities to create new services that integrate the Web, e-mail, telephony, and the mobile network. The integration of the disparate technologies of SS7 and IP is a critical step along the path of realizing the full potential of the network.

Jeff Lawrence is president and CEO of Trillium Digital Systems, Inc., a leading provider of communications software solutions for computer and communications equipment manufacturers. Trillium develops, licenses, and supports standards-based communications software solutions for SS7, ATM, ISDN, frame relay, V5, IP, and X.25/X.75 technologies. For more information, visit the company's Web site at www.trillium.com.