The Integration of UMTS and B - ISDN


In the future, existing fixed networks will be complemented
by mobile networks with similar numbers of users. These
mobile users will have identical requirements and
expectations to the fixed users, for on-demand applications
of telecommunications requiring high bit-rate channels. It
will be necessary for these fixed and mobile networks to
interoperate in order to pass data, in real time and at
high speeds, between their users. But how far must this
interoperation be taken? How much integration of the fixed
and mobile network structures is needed? Here, a fixed
network, B-ISDN, and a mobile network, UMTS, under
development at the same time, are examined to see how well
and closely they should work together in order to meet
expected user needs. Work already taking place on this is

The Universal Mobile Telecommunication System (UMTS), the
third generation of mobile networks, is presently being
specified as part of the European RACE technology
initiative. The aim of UMTS is to implement terminal
mobility and personal mobility within its systems,
providing a single world mobile standard. Outside Europe,
UMTS is now known as International Mobile
Telecommunications 2000 (IMT2000), which replaces its
previous name of Future Public Land Mobile
Telecommunication System (FPLMTS). [BUIT95] UMTS is
envisaged as providing the infrastructure needed to support
a wide range of multimedia digital services, or
teleservices [CHEU94], requiring channel bit-rates of less
than the UMTS upper ceiling of 2 Mbits/second, as allocated
to it in the World Administrative Radio Conference (WARC)
'92 bands. UMTS must also support the traditional mobile
services presently offered by separate networks, including
cordless, cellular, paging, wireless local loop, and
satellite services. [BUIT95] Mobile teleservices requiring
higher bit rates, from 2 to 155 Mbits/second, are expected
to be catered for by Mobile Broadband Services (MBS), the
eventual successor to UMTS, which is still under study.
[RACED732] Broadband Integrated Services Digital Network
(B-ISDN), conceived as an all-purpose digital network that
will supersede Narrowband ISDN (N-ISDN or ISDN), is also
still being specified. B-ISDN, with its transport layer of
Asynchronous Transfer Mode (ATM) is expected to be the
backbone of future fixed digital networks. [MINZ89] It is
anticipated that, by the year 2005, up to 50% of all
communication terminals will be mobile. [CHEU94] The Mobile
Green Paper, issued by the European Commission in 1994,
predicts 40 million mobile users in the European Union by
2000, rising to 80 million by 2010. This gives mobile users
an importance ranking alongside fixed-network users.
[BUIT95] One result of this growth in mobile
telecommunications will be the increase in teleservice
operations that originate in either the fixed or mobile
network, but terminate in the other, crossing the boundary
between the two. UMTS is expected to be introduced within
the next ten years, and integration with narrowband and
broadband ISDN is possible in this time. Interoperability
between UMTS and ISDN in some fashion will be necessary to
support the interoperability between the fixed and mobile
networks that users have already come to expect with
existing mobile networks, and to meet the expectation of
consistency of fixed/mobile service provision laid out in
the initial RACE vision. [SWAI94] One way of making UMTS
attractive to potential customers is to offer the same
range of services that B-ISDN will offer, within the bounds
of the lower 2 Mbits/second ceiling of UMTS. [BUIT95] So,
with the twin goals of meeting existing expectations and
making UMTS as flexible as possible to attract customers,
how closely integrated must UMTS be with B-ISDN to achieve
The UMTS network could be developed along one of the
following alternative integration paths: 1. Developing an
'optimised' network structure and signalling protocols
tailored for the special mobile requirements of UMTS. This
would be incompatible with anything else. Services from all
fixed networks would be passed through via gateways. This
design-from-scratch method would result in highly efficient
intra-network operation, at the expense of highly
inefficient inter-network operation, high development cost,
scepticism relating to non-standard technology, and slow
market take-up. True integration with fixed networks is not
possible in this scenario. Given the drawbacks, this is not
a realistic option, and it has not been considered in
depth. One of the RACE goals was to design UMTS not as a
separate overlay network, but to allow integration with a
fixed network; this option is undesirable. [BUIT95] 2.
Integration with and evolution from the existing Global
System for Mobile telecommunication. (GSM, formerly
standing for Group Special Mobil during early French-led
specification, is now taken as meaning Global System for
Mobile communications by the non-French-speaking world.)
GSM is currently being introduced on the European market.
This option has the advantage of using already-existing
mobile infrastructure with a ready and captive market, but
at the expense of limiting channel bit-rate considerably,
which in turn limits the services that can be made
available over UMTS. Some of the technical assumptions of
UMTS, such as advanced security algorithms and distributed
databases, would require new protocols to implement over
GSM. GSM would be limiting the capabilities of UMTS.
[BROE93a] 3. Integration with N-ISDN. Like the GSM option
above, this initially limits UMTS's channel bit-rate for
services, but has a distinct advantage over integration
with B-ISDN - N-ISDN is widely available, right now.
However, integrating UMTS and N-ISDN would require
effective use of the intelligent network concept for the
implementation of mobile functions, and modification to
existing fixed network protocols to support mobile access.
Integrating UMTS with N-ISDN makes possible widespread
early introduction and interoperability of UMTS in areas
that do not yet have B-ISDN available. This allows wider
market penetration, as investment in new B-ISDN equipment
is not required, and removes the dependency of UMTS on
successful uptake of B-ISDN for interoperability with fixed
networks. Eventual interoperability with B-ISDN, albeit
with constrictions imposed on UMTS by the initial N-ISDN
compatibility, is not prevented. [BROE93a] 4. Integration
with B-ISDN. This scenario was the target of MONET (MObile
NETwork), or RACE Project R2066. Unlike the above options,
B-ISDN's high available bandwidth and feature set does not
impose limitations on the service provisioning in UMTS.
Fewer restrictions are placed on the possible uses and
marketability of UMTS as a result. Development of B-ISDN is
taking place at the same time as UMTS, making smooth
integration and adaptation of the standards to each other
possible. For these reasons, integration of UMTS with
B-ISDN has been accepted as the eventual goal for
interoperability of future fixed and mobile networks using
these standards, and this integration has been discussed in
depth. [BROE93a, BROE93b, BUIT95, NORP94] At present,
existing B-ISDN standards cannot support the
mobile-specific functions required by a mobile system like
UMTS. Enhancements supporting mobile functions, such as
call handover between cells, are needed before B-ISDN can
act as the core network of UMTS. Flexible support of fixed,
multi-party calls, to allow B-ISDN to be used in
conferencing and broadcasting applications, has many of the
same requirements as support for mobile switching, so
providing common solutions to allow both could minimise the
number of mobile-specific extensions that B-ISDN needs. As
an example of how B-ISDN can be adjusted to meet UMTS's
needs, let's look at that mobile requirement for support
for call handover. Within RACE a multiparty-capable
enhancement of B-ISDN, upwardly compatible with Q.2931, has
already been developed, and implementing UMTS with this has
been studied. For example, a UMTS handover can be handled
as a multi-party call, where the cell the mobile is moving
to is added to the call as a new party, and the old cell is
dropped as a party leaving the call, using ADD(_party) and
DROP (_party) primitives. Other mobile functions can be
handled by similar adaptations to the B-ISDN protocols. The
enhancements to B-ISDN Release 2 and 3 that are required
for UMTS support are minimal enough to be able to form an
integral part of future B-ISDN standards, without impacting
on existing B-ISDN work. [BUIT95] These modifications only
concern high-level B-ISDN signalling protocols, and do not
alter the transport mechanisms. The underlying ATM layers,
including the ATM adaptation layer (AAL) are unaffected by
The Intelligent Network (IN) is a means for service
providers to create new services and rapidly introduce them
on existing networks. As the IN was considered useful for
implementing mobility procedures in UMTS, it was studied as
part of MONET, and is now specified in the Q.1200 series of
the ITU-T recommendations. The intelligent network
separates service control and service data from basic call
control. Service control is then activated by 'trigger
points' in the basic call. This means that services can be
developed on computers independent of the network switches
responsible for basic call and connection control. This
gives flexibility to the network operators and service
providers, as well as the potential to support the services
on any network that supports the trigger points.
Eventually, IN can be expanded to control the network
itself, such as handling all UMTS mobile functions.
[BROE93a] Any network supporting the intelligent network
service set will be able to support new services using that
service set easily, making integration of networks easier
and transparent to the user of those services. The
intelligent network is thus an important factor in the
integration of B-ISDN and UMTS. UMTS, B-ISDN and the
intelligent network set are all being developed at the same
time, allowing each to influence the others in producing a
coherent, integrated whole. [BUIT95] 
In order to be accepted by users as useful and to provide
as wide a variety of services as possible, UMTS needs some
form of interoperabilty or integration with a fixed
network. Integration of UMTS with B-ISDN offers the most
flexibility in providing services when compared to other
network integration options, and constrains UMTS the least.
With the increase in the number of services that will be
made available in UMTS and B-ISDN over present standalone
services, it is unrealistic to develop two separate, and
incompatible, versions of each service for the fixed and
mobile networks. Integrating UMTS and B-ISDN makes the same
service set available to both sets of users in the same
timescale, reducing development costs for the services, and
promoting uptake and use in the market. The intelligent
network concept allows the easy provision of additional
services with little extra development cost. Integrating
UMTS with B-ISDN, and with the intelligent network set, is
therefore desirable. Work on this integration indicates
that the mobile requirements of UMTS can be met by
extending existing B-ISDN signalling to handle them,
without significantly modifying B-ISDN. Integration of UMTS
with B-ISDN is therefore technically feasible. 
W. van den Broek, A. N. Brydon, J. M. Cullen, S. Kukkonen,
A. Lensink, P. C. Mason, A. Tuoriniemi,
"RACE 2066: Functional models of UMTS and integration into
future networks",
IEE Electronics and Communication Engineering Journal, June
W. van den Broek and A. Lensink,
"A UMTS architecture based on IN and B-ISDN developments",
Proceedings of the Mobile and Personal Communications
Conference, 13-15 December 1993.
IEE Conference Publication 387.
E. Buitenwerf, G. Colombo, H. Mitts, P. Wright,
"UMTS: Fixed network issues and design options",
IEEE Personal Communications, February 1995.
J. C. S. Cheung, M. A. Beach and J. P. McGeehan,
"Network planning for third-generation mobile radio
IEEE Communications Magazine, November 1994.
S. E. Minzer, 

"Broadband ISDN and Asynchronous Transfer Mode (ATM)",
IEE Communications Magazine, September 1989.
T. Norp and A. J. M. Roovers,
"UMTS integrated with B-ISDN",
IEEE Communications Magazine, November 1994.
IBC Common Functional Specification, Issue D.
Race D732: Service Aspects.
R. S. Swain,
"UMTS - a 21st century system: a RACE mobile project line
assembly vision"


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