Integrating IP And SS7 Technologies
Achieving Distributed Intelligence, Realizing The Public Network's Potential
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
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
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
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
- 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
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
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
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
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
The potential configuration and interactions between the call agent, signaling gateway,
media gateway, Internet, and public network enables a number of deployment scenarios,
- Trunking gateways that enable the interconnection of the public telephony network via
- 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.
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.