NASA Technical Reports Server (NTRS) 19940018286: Interworking and integration of the Inmarsat Standard-M with Pan-European GSM system

6 Pages·0.42 MB·English

by  NASA Technical Reports Server (NTRS)

N94- 22759
Interworking and Integration of the lnmarsat Standard-M
with the Pan-European GSM System
R. Tafazolli and Professor B.G. Evans
Centre for Satellite Engineering Research (CSER)
University of Surrey
Guildford
Surrey GU2 5XH
England
Tel: [phone]
Fax: [phone]
latmau.cxi The GSM System
The market demand on mobile tele- The idea of a Pan-European mobile
communication system was originated in
phone communications has been increasing
1981 and it is to replace the many different
since the introduction of the cellular mobile
analogue mobile telephone networks which
telephone systems about twelve years ago.
exist throughout Europe. Roaming will be
In Europe, projections indicate a demand of
about 17 million subscribers for a fully permitted within all the countries deploying
deployed Pan-European system. The the system.
The GSM offers digital voice channel to-
Pan-European GSM system is to harmonise
gether with several supplementary services
the growth of the terrestrial mobile system.
and will be ISDN compatible. Very
Studies conducted by the European Space
importantly the system has been structured
Agency (ESA) indicate that even with
60-70% of the area being covered by the to permit evolving technology to be incorpo-
rated.
GSM, a significant traffic demand (voice
and data) will still exist for areas not cov- The system adopts a TDMA structure and
makes use of the CCITT Signalling System
ered by the terrestrial systems.
Number 7. Extensive use is also made of the
This demand could be satisfied by a land
other CCITT, CEPT and ISO (for layers 1,2
mobile satellite system. The satellite system
and 3 - the physical, data link and network
is therefore seen in a complementary role
rather than in competition with the terrestrial layers) standards.
The main building blocks of the GSM net-
system in an integrated telecommunication
work are shown in figure 1.
network. One possible scenario may be that
The Mobile Switching Centre (MSC) acts
initially the satellite system provides ser-
vices to the rural areas together with areas mainly as an ISDN exchange and represents
the interface between the fixed network and
still not covered by the GSM system. This
mobile network. Each MSC has many Base
service area is then gradually diminished as
Stations (BS) under its control, typically
the terrestrial system expands until an opti-
three. The MSC also controls mobile spe-
mum point is reached where the systems co-
cific functions such as handover. In general,
exist optimally.
each MSC will have two associated location
The objective of this paper is to investigate
registers. Home Location Register (HLR) is
the possibility of integration of a space
used to store permanent and temporary in-
based system, in this case Inmarsat
formation related to the mobile subscriber or
Standard-M, with the GSM (Global System
for Mobile communication). One very im- related to the position of the MSs in each
instant. Visitor Location Register (VLR)
portant advantage of incorporating GSM ser-
contains all kinds of information both per-
vices in Standard-M is that it will be feasible
to extend the GSM services economically manent and temporary needed to handle the
MSs in the area where it is roaming.
worldwide, not only to land mobiles but also
to aeronautical and maritime mobiles. A Base Station provides radio coverage of
geographical zone containing one or several
cells. It provides the radio channels for both
subscriber services and associated network
131
signalling.It is airectlyconnected to the vide sufficient information to a MS for an
MSC associated with its coverage area. estimate to be made of the call charge
The Mobile Station enables subscribers to incurred.
access the GSM network at any point within
the service area. Various types are available Customer access [1], [2]
of different transmission power levels- the Mobile stations with have different access
higher powers being vehicle mounted rather capabilities. For example, a small compact
than handheld.
handheld unit is unlikely to offer all services
in the early 1990s. Therefore, GSM has de-
GSM se_rvices and subscribers acc¢-_ veloped a structured approach which allows
In this section, services which are for maximum evolution of products. This
supported in GSM and customer access to process is shown in figure 2. The figure
them will be described. shows that the MS consists of a Mobile
Termination (MT) unit and terminal equip-
Tele/Data and Supplementary_ Serviceslll ment. The function of the MT unit is to offer
standard interfaces to the customer and to
transfer the data on these interfaces over the
The teleservices provide complete radio interface (Um). For example, cus-
capability for two customers to communi- tomers will be able to directly connect their
cate with one another. The following generic ISDN terminal via the standard ISDN "S"
teleservices are offered. interface to the MS. The ISDN terminal
presents a 64 kbps data stream to the mobile
a) Speech transmission termination and the composite effect of the
b) Emergency call rate adaptation in the ISDN terminal and
c) Short message transmission (telegrams up mobile termination unit is to produce a flow
to 256 characters)
of controlled data stream compatible with
d) Message handling service the capacity of the digital radio channel.
e) Videotex Figure 2 also indicates the alternative which
f) Teletex is being specified so that customers with
g) Facsimile transmission group 3,4 non-ISDN terminals can either directly con-
nect to the standard "S" interface or to a "R"
Bearer Services type interface.
The bearer service provides a "trans-
port-service" which enables customers to in- There are three types of MT:
terchange data according to their specified MT0: includes functions belonging to MT,
application. The GSM services are: with support of no terminal interfaces.
a)Audio restricted 3.1 KHz
MTI: includes functions belonging to MT,
b) Data circuit duplex asynchronous with an interface that complies with the
300-9600 bits/sec GSM recommended subset of ISDN user-ne-
c) Data circuit duplex synchronous twork interface.
1200-9600 bits/sec MT2: includes functions belonging to MT,
d) PAD access circuit asynchronous with an interface that complies with non-
300-9600 bits/sec ISDN interface eg., CCITT X or V series
e) Data packet duplex synchronous interface.
2400-9600 bits/sec _
f) Alternate speech/unrestricted digital The MT plus a TE/(TE+TA) constitutes the
g) 12.6 kbits/sec unrestricted digital mobile station.
h) ISDN terminal support
lnmarsat Standard-M Land Mobile Sys-
Supplementary_ Services tem [3]
a) Call forwarding The Inmarsat Standard-M which will
b) Call barring start commercial service by 1993, provides a
c) Call waiting digital communications service between the
d) Call hold public switched terrestrial networks and mo-
e) Three party service bile users on land or sea. Standard'M sup-
f) Advice of charge. This is intended to pro- ports medium quality telephony, facsimile
and 2400 bits/sec. Links to and from mobile
installations are established via Inmarsat
132
geostationarysatellitesandtheassociated "multi-numbering scheme").
groundsegmentN. earglobalcoveragepro- The primary function of the MIU at the LES
videdbetweenlatitudes75*northandsouth is to interface with the terrestrial modems,
oftheEquator. and to convert the analogue baseband voice
ThemajorelementsofStandard-Msystem and data format originated from fixed-user
areshowninfigure3andaredescribed voice band modems to the Standard-M
briefly asfollows: 2.4Kbits/sec SCPC satellite data channel
format. The recommended protocol between
The_ capacityisprovidedby
Inmarsatsfirstgeneration(i.e,intelsat-V the LES MIU and the terrestrial fixed-user's
Maritimecommunicationssub-systemsatel- data modem is the CCITF Rec. V.22 bis.
lites)andsubsequengtenerationspaceseg-
ment(Inmarsat-3). |ntgrworking between lnmarsat
Standard-MMobileEarthStation(MES), Standard-M and GSM Systems
whichinterfacewith thespacesegmentsat The main purpose of this section is to pre-
L-band(1.5/1.6GHz)forcommunication pare the ground for integration of GSM and
withLESs. Standard-M systems which is to be the
Land Earth Stations interface with the space subject of the next section. Before consider-
ing integration an understanding of the con-
segment at C-band (4/6 GHz) and L-bands
and with the terrestrial networks for com- cept of interworking and how it can be
munications with MESs. achieved is necessary. The interworking in
Network Coordination Stations (NCS), this chapter is described on the ability of
GSM network to interwork with non-ISDN
located at the designated (co-located) LESs,
networks.
which interface with the space segment at
C-band and L-band for the purpose of sig- The concept of Bearer services was devel-
nalling with MESs and LESs, and for overall oped for the ISDN and has been extended to
network control and monitoring functions. the GSM. Here an attempt will be made to
interwork GSM Bearer services and hence
Standard-M services and customer access its Teleservices with Inmarsat Standard-M
This section describes the services network within the limitations of the two
which are recommended for Standard-M systems. Three areas of interworking were
identified and thoroughly investigated:
system. This, in no way, means that these
are the only services which Standard-M sys-
1. Service interworking
tem is capable of supporting.
Service interworking is required
when the teleservices at the calling and
Telephony Services are provided through
called terminals are different (e.g. Teletex
6.4 Kbits/sec full duplex.
CCITT Group-3 Facsimile Service is pro- interworking with Facsimile).
vided in accordance with the transmission 2. Signalling interworking
For network interworking, signalling
standard CCITT Rec.T4 regarding Group-3
facsimile service. requirements have to be defined. Existing
2.4 Kbits/sec full duplex baseband data ser- call control signalling procedures e.g SS#7,
is used in the GSM while a subset of SS#5 is
vices (with optional fall back mode capabil-
used in Standard-M.
ity of operating 1.2 Kbits/sec) are supported.
Figure 4 shows the customer access to data 3. Supplementary service interworking
services. The recommended interface re-
From the network interworking, the follow-
quirement between an MES (DCE) and its
data terminal equipment (DTE) is the ing can be deduced:
CCITT V.24, V.28 and ISO 2110 (equival- 1. Additional signalling messages "bearer
ent to EIA specification RS-232C). capability" are required to be transmitted be-
The recommended auto-calling and tween MESs and LESs to support GSM ser-
vices. Such messages will be needed to
auto-answering protocol at the MES V.24
interface is the CCITT Rec.V.25 bis. indicate things like service nature (i.e simple
Modem Interface Unit (MIU) is a logical or duplex), service type (i.e short message
service) etc. These signals can be accommo-
block allowing a fixed Hayes compatible
modem to interface with the LES and a DTE dated since both MESs and LESs will be
to interface with the MES. MIUs are useful equipped with software expansion
on MESs to allow selection to off-the shelf capabilities to accommodate new signalling
modems and DTEs by the end users (using codes for future Inmarsat service offer-
133
ings[3].Hencethiscanbeutilisedtosupport mode terminals for both non-ISDN and
newsignallingmessagersequiredfor GSM
ISDN services are shown in figures 5 and 6
services.
respectively. In ISDN compatible terminal,
2.Signallingconversionisrequiredtocon- the GSM Mobile Termination 1 (MT1) was
verttheforwardandthebackwardsignalsof
considered, as MT1 is designed to support
SS#7(ISUP)messagesandinformation
ISDN user interface, i.e "S" reference point,
elementsof Standard-Msignallingsystem
allowing TEl to be connected directly to
(subset of SS#5). MT1. As Standard-M MES is designed to
3. Appropriate interfaces at the MSSC are support non-ISDN terminals, some modifi-
required to support GSM services within cations were required to enable the MES to
Standard-M network e.g X.30 and X.31 to support ISDN terminals.
support packet data services.
4. The MSSC must support all of GSM Integration of Standard-M and GSM Net-
supplementary services. works
5. Additional interworking functions (IWFs) The important elements of the two
will be required to support various GSM separate networks which monitor the mobile
services. The IWFs will be in the following position and thus allow calls to be forwarded
form: to the mobiles are the HLR in the GSM and
NCS in the Standard-M system. The gate-
i) Echo control devices at the MSSCs way between the two networks must there-
ii) Modem ._ols and network based rate fore be a link between these two elements so
adaptation (i.e to allow for no more than that information can be exchanged as to the
2400 bits/sec and no less than 1200 bits/sec mobile location within the whole network.
user data rate) at LESs. This is shown in figure 7. The GSM Direc-
iii) Functions to select the appropriate tory Number (DN) is incorporated in the
modem and rate adaptation for each re- Integrated Network for the following
quested service. reasons:
iv) Appropriate interfaces at the MSSC 1. The GSM is designed to provide flexible
(ISDN switch) to support data services with and economic call set up procedures by in-
other specialised networks e.g X.32 access, corporating network elements such as HLR,
to interface with packet networks including VLR and Mobile Network identity such as
GSM. MSRN.
2. The main purpose of this integration is to
6. To support short message services, provi- allow Standard-M to complement the GSM
sion must be made for a use of service by providing similar services as the GSM in
centre, which acts as a store and forward the rural/maritime areas. Thus it is essential
centre for short messages. This centre can to harmonise the user access methods by
either be located at each LES or the GSM adopting the GSM numbering scheme.
PLMN centre can be shared with the home
LES. To reach this integrated scenario, careful
steps were also taken to minimise the propa-
Integration of Standard-M and GSM Ter- gation delays due to satellite links and the
mim_
terrestrial tails, by extensive use of the
In this section, the possibility of proposing a terrestrial elements. This also resulted in the
single terminal to access both networks will following advantages:
be looked at. The achievable level of inte- i) Avoided many changes in the space seg-
gration will be determined by the amount of ment due to the need to transmit many sig-
similarities between the two systems. nalling messages to support roaming in the
integrated network.
The two systems are very different at all the ii) Efficient utilization of the space segment
three lower layers of OSI model, thus there radio resources i.e roaming can be supported
is no point in trying to integrate the Mobile with very little use of the satellite radio
Termination unit (MT) in GSM and the Mo- channels.
bile Earth Station (MES)in Standard-M into
a single terminal. Hence customer interfaces
at both the MT and MES were examined in
order see how the GSM services can be sup-
ported in a dual-mode terminal. The dual-
134
_ummal"y_
In this paper two main areas were examined
in order to achieve maximum possible level
of synergy between the Inmarsat
Standard-M and the GSM systems.
In the interworking, signalling, supplemen-
tary services and network issues were ad-
dressed and many additional network
interfaces were proposed. These solutions
were aimed at incorporation of GSM ser-
vices in Standard-M system as much as
possible.
The network integration was obtained
through detailed comparisons of the two sys-
tems in accordance with OSI/RM model.
This resulted in the suggestion of a
dual-mode terminal with fully defined
user-machine interfaces to access both GSM
and the "enhanced" Standard-M services.
Rfferences
[1] Mallinder B.I.T., 1988 "The GSM digital
cellular system", GSM noyan permanent,
CEPT, Paris.
[2] 1990, "GSM PLMN Access Reference
Configuration", GSM Recommendation
04.02.
[3] 1988, "INMARSAT-M System Defini-
tion Manual", Issue 3.0, Module 1, INMAR-
SAT Satellite Organisation- London.
[4] Alverche, M., And Branes, D., 1988,
"Overview of the GSM Services and Faci-
lities", Digital Cellular Radio Conference,
Hagen, Germany.
135
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