ISO/IEC 22535:2006

INTERNATIONAL ISO/IEC
STANDARD 22535
Second edition
2006-04-15


Information technology —
Telecommunications and information
exchange between systems — Corporate
Telecommunication Networks —
Tunnelling of QSIG over SIP
Technologies de l'information — Télécommunications et échange
d'information entre systèmes — Réseaux de télécommunications
d'entreprise — Tunnellisation de QSIG sur SIP




Reference number
ISO/IEC 22535:2006(E)
©
ISO/IEC 2006

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ISO/IEC 22535:2006(E)
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ISO/IEC 22535:2006(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 2
3.1 External definitions. 2
3.2 Other definitions . 2
4 Abbreviations and acronyms . 3
5 Background and architecture. 4
6 Procedures . 6
6.1 General. 6
6.2 Encapsulation of QSIG messages in SIP messages. 6
6.3 QSIG SETUP message handling at an ingress gateway. 6
6.3.1 Sending a SIP INVITE request . 6
6.3.2 Receipt of responses to the INVITE request.7
6.4 QSIG SETUP message handling at an egress gateway. 7
6.4.1 Receiving a SIP INVITE request . 7
6.4.2 Rejecting a QSIG message in an INVITE request . 8
6.5 Subsequent QSIG messages. 8
6.6 Terminating the SIP dialog . 8
6.7 QSIG connectionless message handling at an ingress gateway . 8
6.7.1 Sending a SIP INVITE request . 8
6.7.2 Receipt of responses to the INVITE request.9
6.8 QSIG connectionless message handling at an egress gateway. 9
7 Example message sequences . 10
7.1 Call establishment . 10
7.2 Call clearing. 11
7.3 QSIG connectionless message . 12
7.4 Call establishment with m=0 in first SDP answer. 13
8 Security considerations . 13

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ISO/IEC 22535:2006(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/IEC 22535 was prepared by Ecma International (as ECMA-355) and was adopted, under a special
“fast-track procedure”, by Joint Technical Committee ISO/IEC JTC 1, Information technology, in parallel with
its approval by national bodies of ISO and IEC.
This second edition cancels and replaces the first edition (ISO/IEC 22535:2005), which has been technically
revised.
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ISO/IEC 22535:2006(E)
Introduction
This International Standard is one of a series defining the interworking of services and signalling protocols
deployed in corporate telecommunication networks (CNs) (also known as enterprise networks). The series
uses telecommunication concepts as developed by ITU-T and conforms to the framework of International
Standards on Open Systems Interconnection as defined by ISO/IEC.
This particular International Standard specifies tunnelling of QSIG over the Session Initiation Protocol (SIP).
This enables calls between “islands” of circuit switched networks that use QSIG signalling to be
interconnected by an IP network that uses SIP signalling without loss of QSIG functionality.
This International Standard is based upon the practical experience of Ecma member companies and the
results of their active and continuous participation in the work of ISO/IEC JTC1, ITU-T, IETF, ETSI and other
international and national standardization bodies. It represents a pragmatic and widely based consensus.

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INTERNATIONAL STANDARD ISO/IEC 22535:2006(E)

Information technology — Telecommunications and information
exchange between systems — Corporate Telecommunication
Networks — Tunnelling of QSIG over SIP
1 Scope
This International Standard specifies tunnelling of “QSIG” over the Session Initiation Protocol (SIP) within a
corporate telecommunication network (CN).
QSIG is a signalling protocol that operates between Private Integrated services Network eXchanges (PINX)
within a Private Integrated Services Network (PISN). A PISN provides circuit-switched basic services and
supplementary services to its users. QSIG is specified in International Standards, in particular ISO/IEC 11572
(call control in support of basic services), ISO/IEC 11582 (generic functional protocol for the support of
supplementary services) and a number of Standards specifying individual supplementary services.
NOTE The name QSIG was derived from the fact that it is used for signalling at the Q reference point. The Q
reference point is a point of demarcation between two PINXs.
SIP is an application layer protocol for establishing, terminating and modifying multimedia sessions. It is
typically carried over IP (RFC 791, RFC 2460). Telephone calls are considered as a type of multimedia
session where just audio is exchanged. SIP is defined in RFC 3261.
Often a CN comprises both PISNs employing QSIG and IP networks employing SIP. A call or call independent
signalling can originate at a user connected to a PISN and terminate at a user connected to an IP network or
vice versa. In either case, a gateway provides interworking between QSIG and SIP at the boundary between
the PISN and the IP network. Basic call interworking at a gateway is specified in ISO/IEC 17343. Another case
is where a call or call independent signalling originates at a user connected to a PISN, traverses an IP
network using SIP, and terminates at a user connected to another (or another part of the same) PISN. This
International Standard addresses this last case in a way that preserves all QSIG capabilities across the IP
network. It achieves this by tunnelling QSIG messages within SIP requests and responses in the context of a
SIP dialog.
The tunnelling of QSIG through a public IP network employing SIP is outside the scope of this International
Standard. However, the functionality specified in this International Standard is in principle applicable to such a
scenario when deployed in conjunction with other relevant functionality (e.g., address translation, security
functions, etc.).
This specification is applicable to any interworking unit that can act as a gateway between a PISN employing
QSIG and a corporate IP network employing SIP, with QSIG tunnelled within SIP requests and responses.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO/IEC 11572, Information technology — Telecommunications and information exchange between
systems — Private Integrated Services Network — Circuit mode bearer services — Inter-exchange signalling
procedures and protocol (also published by Ecma as ECMA-143)
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ISO/IEC 22535:2006(E)
ISO/IEC 11582, Information technology — Telecommunications and information exchange between
systems — Private Integrated Services Network — Generic functional protocol for the support of
supplementary services — Inter-exchange signalling procedures and protocol (also published by Ecma as
ECMA-165)
ISO/IEC 17343, Information technology — Telecommunications and information exchange between
systems — Corporate telecommunication networks — Signalling interworking between QSIG and SIP —
Basic services (also published by Ecma as ECMA-339)
RFC 791, J. Postel, “Internet Protocol”
RFC 2119, Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14
RFC 2460, S. Deering, R. Hinden, “Internet Protocol, Version 6 (IPv6)”
RFC 2976, Donovan, S., “The SIP INFO Method”
RFC 3204, Zimmerer, E., Peterson, J., Vemuri, A., Ong, L., Audet, F., Watson, M. and M. Zonoun, “MIME
media types for ISUP and QSIG objects”
RFC 3261, J. Rosenberg, H. Schulzrinne, et al., “SIP: Session initiation protocol”
RFC 3264, J. Rosenberg, H. Schulzrinne, et al., “An Offer/Answer Model with the Session Description
Protocol (SDP)”
RFC 3311, Rosenberg, J., “The Session Initiation Protocol (SIP) UPDATE message”
3 Terms and definitions
In this document, the key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”,
“SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” are to be interpreted as described in
RFC 2119 and indicate requirement levels for compliant SIP implementations.
For the purposes of this document, the following definitions apply.
3.1 External definitions
The definitions in ISO/IEC 11572 and RFC 3261 apply as appropriate.
3.2 Other definitions
3.2.1
Corporate telecommunication Network
CN
sets of privately-owned or carrier-provided equipment that are located at geographically dispersed locations
and are interconnected to provide telecommunication services to a defined group of users
NOTE A CN can comprise a PISN, a private IP network (intranet) or a combination of the two.
3.2.2
egress gateway
gateway handling a QSIG call or call-independent signalling connection established in the direction IP network
to PISN
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ISO/IEC 22535:2006(E)
3.2.3
gateway
entity that behaves as a QSIG Transit PINX with QSIG carried over a circuit-switched link within a PISN on
one side and QSIG tunnelled over SIP within an IP network on the other side
3.2.4
ingress gateway
gateway handling a QSIG call or call-independent signalling connection established in the direction PISN to IP
network
3.2.5
IP network
network, unless otherwise stated a corporate network, offering connectionless packet-mode services based
on the Internet Protocol (IP) as the network layer protocol
3.2.6
media stream
audio or other user information transmitted in UDP packets, typically containing RTP, in a single direction
between the gateway and a peer entity participating in a session established using SIP
NOTE Normally a SIP session establishes a pair of media streams, one in each direction.
3.2.7
Private Integrated Services Network
PISN
CN or part of a CN that employs circuit-switched technology and QSIG signalling
3.2.8
Private Integrated services Network eXchange
PINX
PISN nodal entity comprising switching and call handling functions and supporting QSIG signalling in
accordance with ISO/IEC 11572
4 Abbreviations and acronyms
CN Corporate telecommunication Network
IP Internet Protocol
PINX Private Integrated services Network eXchange
PISN Private Integrated Services Network
QSIG Signalling system for the Q reference point
RTP Real-time Transport Protocol
SDP Session Description Protocol
SIP Session Initiation Protocol
TCP Transmission Control Protocol
TLS Transport Layer Security
UA User Agent
UAC User Agent Client
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ISO/IEC 22535:2006(E)
UAS User Agent Server
UDP User Datagram Protocol
URI Universal Resource Identifier
5 Background and architecture
This document concerns the case of a call or call independent signalling that originates at a user connected to
a PISN employing QSIG, traverses an IP network employing SIP, and terminates at a user connected to
another (or another part of the same) PISN. This can be achieved by employing a gateway at each boundary
between a PISN employing QSIG and an IP network employing SIP, as shown in Figure 1.

Figure 1 — Call from QSIG via SIP to QSIG
Each gateway can provide interworking as specified in ISO/IEC 17343. This provides a basic call capability.
However, ISO/IEC 17343 only specifies interworking for QSIG basic call, as specified in ISO/IEC 11572. Many
of the other capabilities of QSIG (support for supplementary services and additional network features) as
specified in other standards and in vendor-specific specifications are not covered. Some of these additional
capabilities of QSIG are suitable for interworking with SIP and might be the subject of future standards or
other specifications. Other capabilities of QSIG are unsuitable for interworking with SIP because
corresponding capabilities do not exist in SIP or are achieved in ways that are incompatible with QSIG.
Therefore interworking at a gateway between QSIG and SIP will be limited to those QSIG capabilities that
have sufficiently compatible equivalents in SIP. Each capability requires special implementation in the
gateway, and therefore a typical gateway might provide interworking for only a subset of capabilities for which
interworking is feasible.
The result of this is that there will be a loss of capability on a call or call independent signalling from QSIG to
SIP or vice versa. For a case similar to that shown in Figure 1 there will likewise be a loss of capability. This
can be compounded if the two gateways are of different types, since only those capabilities common to both
gateways will survive end-to-end.
The solution is to tunnel QSIG messages through the IP network within SIP messages so that no end-to-end
QSIG capabilities are lost. One of the two gateways originates a SIP dialog to the other gateway. SIP
messages within the dialog are used to tunnel QSIG messages. Through the use of SDP RFC 3264, the
dialog also establishes a session in which media streams carry user information (e.g., speech) between the
two QSIG gateways, if required. The two gateways act as QSIG Transit PINXs, which relay QSIG messages
with little or no modification.
In a conventional PISN employing QSIG, two PINXs are connected by means of an inter-PINX link, which
comprises a signalling channel (carrying QSIG messages) and one or more user information channels
carrying speech, modem information or data. With the tunnelling solution, the IP network provides the inter-
PINX link between the two gateways acting as Transit PINXs. The tunnel provided by SIP for QSIG messages
acts as the signalling channel and the media streams act as the user information channels.
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ISO/IEC 22535:2006(E)
This document covers the case where a single dialog between two gateways is used for a single QSIG call or
call independent signalling connection, as specified in ISO/IEC 11582. This means that the dialog is
established when the QSIG call or call independent signalling connection is established and cleared down
when the QSIG call or call independent signalling connection is cleared down.
An enhanced scenario in which a single SIP dialog is maintained long term and used to tunnel a multiplicity of
QSIG calls or call independent signalling connections, with the possibility of multiple QSIG calls or call
independent signalling
...