INTEGRATION OF UMTS AND B-ISDN - IS IT POSSIBLE OR DESIRABLE?
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
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
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.
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 this?
ALTERNATIVES FOR INTEGRATING UMTS WITH OTHER NETWORKS
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.
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
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
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,
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 this.
THE INTELLIGENT NETWORK
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
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
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