Interworking SS7 with IP and H.323, VOIP 1
[ Pobierz całość w formacie 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
IN
Intelligent Network
ISDN
Integrated Services Digital Network
ISUP
ISDN User Part
MC
Media gateway Controller
MG
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
RT
Routing Tables
SA
Signaling Agent
SCAL
Signaling TCP Connection/IP Adaptation Layer
SCCP
Signaling Connection Control Part
SCP
Service Control Point
SG
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
SCP
STP
STP
SSP
IP/SS7 GW
STP
STP
internets
SCP
SCP
STP
STP
SSP
IP/SS7 GW
STP
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
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
SG
RT &
DNS
MC
POTS
Phones
internets
MG
IP Phones
Figure 3. The SS7-IP Architectural Framework
[ Pobierz całość w formacie PDF ]