Chapter 26 - Multimedia Telephony Services: PSTN/ISDN Simulation Services pps

Chapter 26

Multimedia Telephony Services:

PSTN/ISDN Simulation Services

We described earlier that the core IMS enables multimedia services and traditional telephony

services. In this section we focus on the traditional telephony services that, in the

context of the PSTN and ISDN, are globally known as PSTN/ISDN supplementary services.

Traditional supplementary services are: Call Forwarding, Call Hold/Resume, Connected Line

Identification Presentation/Restriction, etc.

In IMS, Applications Servers (ASes) are also able to provide PSTN/ISDN simulation

services. These are telephony services similar in nature to the PSTN/ISDN supplementary

services, but different in the realization since SIP is the call control protocol and is quite

different to the PSTN/ISDN protocols. PSTN/ISDN simulation services are a simulated

version of the PSTN/ISDN supplementary services. However, the aim of the service remains.

A number of these PSTN/ISDN simulation services are specified. Most of them assume

an AS that is able to provide the service, either to the originating or to the terminating party.

It is perfectly valid to combine several of these services into a single AS, a decision that is

left to the implementors.

The PSTN/ISDN simulation services were initially developed by ETSI TISPAN in the

context of services for fixed access to IMS. Later, especially when TISPAN IMS merged with

3GPP IMS, the TISPAN-created PSTN/ISDN simulation services were slightly improved for

mobile access devices and renamed as IMS multimedia telephony services. Through this

chapter we used both terms, IMS multimedia telephony services and PSTN/ISDN simulation

services in an interchangeable manner. As for documentation aspects, 3GPP combines all

of these services in a single specification, 3GPP TS 24.173 [36], which is an umbrella that

contains pointers to each of the specifications that describe one or more related services.

However, for historical reasons, ETSI published the set of PSTN/ISDN simulation services in

the ETSI namespace of specifications. Each service or group of related services is described

as a separate specification. Later, those ETSI specifications were contributed to 3GPP, and

published under the 3GPP namespace. In general, the reader who is interested in more

information should be looking at 3GPP TS 24.173 [36].

PSTN/ISDN simulation services were developed with a clear focus on compatibility with

existing PSTN/ISDN supplementary services. Although PSTN/ISDN simulation services

were originally created in the context of fixed broadband access to IMS, they have been

embraced and are nowadays part of the full IMS, including wireless devices.

ıa- ´ Martın´

The 3G IP Multimedia Subsystem (IMS): Merging the Internet and the Cellular Worlds Third Edition

Gonzalo Camarillo and Miguel A. Garc

© 2008 John Wiley & Sons, Ltd. ISBN: 978- 0- 470- 51662- 1

530 CHAPTER 26. MULTIMEDIA TELEPHONY SERVICES

Each PSTN/ISDN simulation service is based on the corresponding supplementary

service in the PSTN/ISDN. However, in many cases the name of the service is changed

in IMS, since in SIP the concept of a call can be broadened with messages, subscriptions,

notifications, or multimedia sessions. Therefore, most of the PSTN/ISDN simulation services

refer to communication rather than calls.

In Table 26.1 we take a brief look at each of these PSTN/ISDN simulation services. The

O/T column indicates whether the service is provided to the originating side of a call (i.e., the

caller) or the terminating side (i.e., the callee).

26.1 Providing Audible Announcements

Before we start looking into the different Multimedia Telephony Services, we ought to

describe a common building block that is used by a few of these services: providing

an audible announcement. In effect, this is inherited from the PSTN/ISDN services,

where the only mechanism to provide feedback to the user was to provide an audible

announcement. In IMS, one could use other mechanisms to provide feedback, such

as sending an instant message, rendering a web page in the display of the phone, etc.

However, the common minimum denominator, device-independent mechanism, is to provide

an audible announcement. This section describes the different ways to provide an audible

announcement. These feature can be used by most of the Multimedia Telephony Services

that we describe in the following sections.

The procedures for audible announcements are specified in ETSI TS 183 028 [142] and

its equivalent 3GPP TS 24.628 [60]. Audible announcements can be provided at the time a

session is being established, during an established session, or at the time the session is being

released. The procedures are slightly different in each case. Audible announcements can also

be sent when a session attempt is rejected.

26.1.1 Announcement at the Time a Session is Being Established

In the case of an audible announcement that is provided at the time the session is being

established there are four different ways to implement it.

(i) If the announcement is to be rendered at the callee, then the Call-Info header field

in the INVITE request carries the SIP or HTTP URI that identifies the source of the

announcement. This makes the callee’s terminal fetch the URL included in the header

and render it to the user.

(ii) If the announcement is to be rendered at the caller, then an Alert-Info header field

is included in a 180 (Ringing) response. The header contains a SIP or HTTP URI

that identifies the source of the announcement. The caller’s terminal fetches the URL

included in the header and renders it to the user.

(iii) Using the Early Media mechanisms for the gateway model specified in RFC 3960 [111]

in conjunction with the P-Early-Media header field (specified in RFC 5009 [128]).

(iv) Using the Early Media mechanisms in an early dialog, as specified in RFC 3960 [111],

in conjunction with the P-Early-Media header field (specified in RFC 5009 [128]).

The overall effect consists of the caller having two early SIP dialogs at the time the

session is being established. One dialog is established between the caller and the

26.1. PROVIDING AUDIBLE ANNOUNCEMENTS 531

Table 26.1: PSTN/ISDN simulation services in IMS

PSTN/ISDN simulation PSTN/ISDN supplementary

Abbreviation service service O/T

CDIV Communication Diversion Call Diversion T

CFU Communication Forwarding

Unconditional

Call Forwarding

Unconditional

T

CFB Communication Forwarding on

Busy user

Call Forwarding Busy T

CFNR Communication Forwarding on

No Reply

Call Forwarding No Reply T

CFNL Communication Forwarding on

Not Logged-in

— T

CONF Conference Conference Calling O/T

MWI Message Waiting Indication Message Waiting Indication T

OIP Originating Identification

Presentation

Calling Line Identification

Presentation

T

OIR Originating Identification

Restriction

Calling Line Identification

Restriction

O

TIP Terminating Identification

Presentation

Connected Line Identification

Presentation

O

TIR Terminating Identification

Restriction

Connected Line Identification

Restriction

T

CW Communication Waiting Call Waiting T

HOLD Communication Hold Call Hold O/T

ACR Anonymous Communication

Rejection

Anonymous Call Rejection T

CB Communication Barring Call Barring O/T

ICB Incoming Communication Barring Incoming Call Barring T

OCB Outgoing Communication Barring Outgoing Call Barring O

AoC Advice of Charge Advice of Charge O

CCBS Completion of Communications

to Busy Subscriber

Completion of Calls to

Busy Subscriber

O

CCNR Completion of Communications

on No Reply

Completion of Calls on

No Reply

O

MCID Malicious Communication

Identification

Malicious Call Identification T

ECT Explicit Communication

Transfer

Explicit Call Transfer O/T