Integrated Terrestrial-Satellite Mobile Networks

7

Integrated Terrestrial-Satellite

Mobile Networks

7.1 Introduction

Satellite integration into terrestrial mobile networks may take two paths: the evolutionary or

the revolutionary approach. In the evolutionary scenario, existing standards are gradually

evolved towards a new technical standard. In contrast, the revolutionary policy necessitates a

completely new approach to the problem, disregarding existing standards and consequently

new standards will emerge. At the start of the last decade, it was not clear which path the

future development of mobile communications would take. However, by the end of the

decade, it became clear that any new system would have to take into account the investments

made by industry and the technological developments that had taken place. Moreover, the

level of market take-up requires the need to ensure that there is some form of backward

compatibility with existing systems.

We have already seen how GSM is evolving towards GPRS and EDGE, while cdmaOne is

following a similar path towards cdma2000. With this in mind, it is clear that there would be

very little support for introducing a revolutionary new system at this stage of the mobile

communications development. It could be argued that W-CDMA is indeed a revolutionary

new system, however, it is important to realise that the underpinning core network technology

is still that of GSM. However, with the introduction of an all-IP core network, certainly the

move from circuit to packet-oriented delivery can be considered to be a significant change in

the mode of delivery.

As far as satellites are concerned, the future requirement to inter-work seamlessly with

terrestrial mobile networks is paramount. Hence, there is a need to be able to adapt the

terrestrial mobile standards to those of the space segment. With this in mind, this chapter

will concentrate on the evolutionary approach and in particular, issues in relation to satellite￾personal communication network (S-PCN) integration with the fixed public switched tele￾phone network (PSTN), GSM and GPRS will be discussed.

In order to determine the level of integration between the S-PCN and different terrestrial

networks, the requirements imposed by the mobile users, the service providers and the

network operators have to be identified. Such requirements will enable the identification of

Mobile Satellite Communication Networks. Ray E. Sheriff and Y. Fun Hu

Copyright q 2001 John Wiley & Sons Ltd

ISBNs: 0-471-72047-X (Hardback); 0-470-845562 (Electronic)

the required modifications or adaptation of terrestrial mobile network functions for the

support of interworking between space and terrestrial networks.

This chapter is partly based on the work carried out in the EU RACE II SAINT project,

which paved the way for satellite-UMTS studies in Europe and ETSI’s GMR specifications,

which define the requirements for integration between a geostationary satellite and the GSM

network.

7.2 Integration with PSTN

7.2.1 Introduction

Mobile satellite networks are required to interwork with both fixed and mobile networks,

including PSTN and GSM. Due to different national PSTN implementation standards, inter￾operability between PSTN and S-PCN relies on the use of ITU-T Signalling System No. 7

(SSN7) both within national networks and at the international interconnection point between

national networks. This is achieved using SSN7 signalling at the international switching

centre (ISC) of the PSTN, and an interworking function at the fixed Earth station (FES), or

the gateway, of the satellite network to adapt SSN7 to an S-PCN compatible format, as shown

in Figure 7.1 [CEC-95]. A two level coupling between the PSTN and S-PCN has been

proposed.

† Gateway functions: the gateway functions ensure end-to-end interoperability between the

PSTN and S-PCN subscribers and vice versa.

† Access functions: the access functions allow S-PCN subscribers to access the S-PCN via

an FES using a transit link between the FES and the S-PCN.

The signalling configuration as shown in Figure 7.1 has the advantage that little or no

modifications to existing SSN7 procedures implemented in the PSTNs are required.

7.2.2 Gateway Functions and Operations

Three functional components are required to support interworking functions at the gateway:

the PSTN gateway switching centre (PGSC), the S-PCN gateway cell site switch (SGCSS)

and the S-PCN database (SDB) as shown in Figure 7.2. Their functions are briefly described

below.

† PGSC: the PGSC provides PSTN access to the S-PCN network. SSN7 can be used to

248 Mobile Satellite Communication Networks

Figure 7.1 S-PCN-PSTN signalling connection.

perform any signalling exchange between the PGSC and the SGCSS. All calls destined for

the S-PCN must be routed through this gateway.

† SGCSS: in analogy to PGSC, the SGCSS provides S-PCN access to PSTN. Signalling

conversion between the S-PCN and SSN7 is also performed in this gateway. In addition,

interworking functions such as voice encoding, decoding and bit rate adaptation are also

supported in order to ensure end-to-end interoperability between the PSTN and the S-PCN

gateway.

† SDB: this database contains information on the S-PCN mobile terminals such as the

location, terminal characteristics, service profiles, authentication parameters and so on.

Figure 7.2 depicts a scenario whereby a PSTN interfaces with other external networks. It is

also similar to that employed by GSM to interface with PSTNs. This approach has the

advantage that little or no modification is required in the PSTN.

Upon receipt of a call request from a PSTN subscriber, the PGSC informs the SGCSS of

such a request. The SGCSS then interrogates the SDB to check whether the called S-PCN

subscriber is attached to the network. If the S-PCN subscriber is attached to the network, the

called party’s location will be identified. Signalling conversion operation is then performed

by the SGCSS to translate the SSN7 call request from the PSTN to a S-PCN call request. The

call is then routed to the appropriate FES. If the addressed S-PCN user is unavailable, the

PSTN subscriber receives a notification, informing of the situation.

In the case when a call request is from an S-PCN subscriber, the SGCSS translates the

request to an SSN7 call request and signals the PGSC of such a request. The PGSC then

carries out relevant procedures to check if the called PSTN user is available. Appropriate

signalling is translated and exchanged between the two networks to inform the S-PCN caller

of the status of the call request through the two gateways.

7.2.3 Protocol Architecture of SSN7

7.2.3.1 Constituents

Before addressing the interworking features for signalling to interconnect the S-PCN and

PSTN, the call set-up and release procedures between the two networks need to be analysed.

Based on the assumption that the PSTN part will adopt the SSN7 signalling system, issues on

how the message transfer part (MTP), the signalling connection control part (SCCP) and the

Integrated Terrestrial-Satellite Mobile Networks 249

Figure 7.2 S-PCN-PSTN gateway function.

telephone user part1 (TUP) of this signalling can support the S-PCN call establishment

procedures need to be investigated. As shown in Figure 7.3 [STA-95], the SSN7 signalling

system has been specified in four functional levels.

† The MTP provides a reliable but connectionless service for routing messages through the

SSN7 network. It is made up of the lowest three layers of the open systems interconnection

(OSI) reference model. The lowest level, the signalling data link, corresponds to the

physical level of the OSI model and is concerned with the physical and electrical char￾acteristics of the signalling links. The signalling link level, a data link control protocol,

250 Mobile Satellite Communication Networks

1 The TUP layer may not appear in some reference books. It is used here for discussions on plain old telephone

(POT) networks.

Figure 7.3 SSN7 signalling architecture.