Interworking SS7 with IP and H.323.pdf

Tampere University of Technology

Telecommunications Laboratory

83390 Advanced Topics in Broadband Networks

Interworking SS7 with IP and H.323

Bilhanan Silverajan

Abbreviations

ACM

Address Complete Message

ANM

Answer Message

ASE

Application Service Element

ASP

Access Signaling Protocol

COT

Continuity Test

DNS

Domain Name System

HDLC

High level Data Link Control

IAM

Initial Address Message

Intelligent Network

ISDN

Integrated Services Digital Network

ISUP

ISDN User Part

Media gateway Controller

Media Gateway

MGCP

Media Gateway Control Protocol

MTP

Message Transfer Port

NAS

Network Access Server

OAM

Operations, Administration and Maintenance

RSGP

Reliable Signaling Gateway Protocol

Routing Tables

Signaling Agent

SCAL

Signaling TCP Connection/IP Adaptation Layer

SCCP

Signaling Connection Control Part

SCP

Service Control Point

Signaling Gateway

SS7

Signaling System Number 7

SSP

Service Switching Point

STP

Signaling Transfer Point

SUAL

Signaling UDP/IP Adaptation Layer

TCAP

Transaction Capabilities Application Part

VoIP

Voice Over IP

1. Introduction

This paper discusses the interworking of SS7 with IP and H.323 to support telephony

traffic in IP networks. A number of SS7 vendors such as Lucent and Nortel also

appear to be developing products that are aimed at such a level of interworking.

Several task forces have been involved in this area, and consequently a number of

draft specifications have been published for consideration and standardisation. Some

of the more notable efforts include the creation of a new protocol for SS7-IP signaling

called Reliable Signaling Gateway Protocol (RSGP) whose rules and messages are

derived from the ITU-T Q.931 developed for call control in ISDN. Bay Networks also

has a proposal for a similar protocol called Access Signaling Protocol (ASP), but as

RSGP includes ASP, this paper will only attempt to discuss RSGP.

Another draft specification discusses the relationships of H.323 and SS7 and

consequently recommends an SS7 gateway residing between the H.323 Gatekeeper

and the SS7 exchange, whose function will be to translate the messages coming from

one side into a format the other can understand.

An adaptation layer which supports the transfer of SS7 messages atop IP while taking

advantage of MTP is also discussed briefly.

An SS7-Internet Interworking Architectural Framework is also described which aims

to unify the various proposals and drafts into an overall model framework that would

provide a frame of reference for future standardisation.

2. Motivation

The biggest motivation for combining the closed environment, circuit switched

networks of SS7 with the more open, packet switched IP world appears to be an

increased market competition and revenue possibility.

Today’s telephone networks are largely based on SS7 technology, which apart from

supporting basic customer calls, also provide advanced Intelligent Network services.

Examples of such services include call forwarding, call screening, caller ID and toll-

free numbers. Currently IP telephony with VoIP has no provision for such services. It

would be both expensive and redundant for the Internet task forces to specify and

reimplement such services separately for VoIP, as the IP protocol suite was not

designed to offer these services. By resolving such issues using an interworking

approach, two major advantages can be immediately perceived.

Firstly, apart from the obvious benefit of saving cost and effort, the IP network will

have an increased value to companies carrying voice traffic that can offer advanced

services. This will certainly help in a more rapid adoption of IP as a core public

network and aid in reducing overall costs of investing in legacy telecommunications

infrastructure.

Secondly, companies offering traditional SS7 services will now be able to reach far

wider groups of subscriber bases via interworking and interconnecting with public

internet-based networks. This also creates a higher level of flexibility for SS7 users

who might not have the SS7 protocol stack, but instead rely on IP for transporting

SS7 traffic as well. Other SS7 network nodes will already be on the standard SS7

network and have a full SS7 protocol stack. These can then act as gateways having

dual SS7/IP stacks to open up access to the SS7 nodes on the large installed base of

SS7 networks which have no knowledge of the IP network interconnection.

3. Possible End-to-end SS7 Configurations using IP

At the moment, many parts of the IP-SS7 interworking is yet to be standardised. It is

foreseen that the many existing specification drafts will be unified in the final set of

standards. One of these drafts is the interworking configuration which will allow end

users to speak SS7 via public packet-switched networks like the Internet. This type of

full topology configuration is shown in Figure 1.

SCP

STP

SSP

IP/SS7 GW

STP

internets

SCP

STP

SSP

IP/SS7 GW

STP

Figure 1. Full SS7 topology configuration

As the figure shows, the end users are communicating via SS7, so the traffic being

transferred between them is ISUP, SCCP, TCAP and so on. The Signaling Transfer

Points (STPs) offer the full features of MTP-3 such as point code routing and some

level of fault tolerance with failed nodes and links.

A more scaled down approach from Figure 1 is also possible, in which the STPs will

not exist as separate nodes but will instead function with the Service Switching Points

(SSPs) to provide only a simple point-to-point operations, using only addressing

which is provided with IP. This type of configuration is shown in Figure 2.

SCP

SSP

STP

IP/SS7 GW

internets

IP/SS7 GW

SSP

STP

Figure 2. Scaled-down point-to-point configuration

4. The SS7-IP Architectural Framework

With the wide variety of SS7-IP interworking proposals that were put forward, it was

necessary for a unifying framework to be put forth that could provide a reference for

all the ongoing work, components and entities that were being described. As such, the

SS7-IP Architectural Framework was developed to reflect the overall situation. This

framework is shown in Figure 3.

Telco

Exchanges

SCP

SS7-IP

Gatew ay/

Controller

Telco

networks

SSP

STP

SS7 links

RT &

DNS

POTS

Phones

internets

IP Phones

Figure 3. The SS7-IP Architectural Framework

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