From: Levesque@scsd.gte.com (Allen Levesque)
Newsgroups: comp.dcom.telecom
Subject: Data Services for Digital Cellular
Date: 14 Dec 92 22:03:07 GMT
Organization: GTE Government Systems
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 905, Message 1 of 5

"Interoperable Data Services For Evolving North American Cellular
Networks,"
by David Weissman, Allen H. Levesque, and Richard A. Dean.  

Following is the introductory section of an article scheduled to
appear in the {IEEE Communications Magazine}, Vol. 31, No. 2, February
1993.  This article describes the progress of work being done by the
Data Services Task Group of the Telecommunications Industry
Association (TIA) Subcommittee TR45.3 on Digital Cellular Standards.
The Task Group is currently defining circuit-switched digital data
bearer services to be provided by the North American TDMA Digital
Cellular system.  The services will include Group 3 Facsimile,
Asynchronous Data and Synchronous Data.  Both Transparent and
Non-Transparent services will be supported.  Each bearer service will
support interoperation between mobile data devices in the cellular
network and standard Facsimile machines and data modems in the Public
Switched Telephone Network.

The authors wish to thank members of the TR45.3 Data Services Task
Group, including Richard Ejzak, Jay Jayapalan, John Michaelides, Simon
Mizikovsky, Hans Petter Naper, and Al Sacuta for their support and
inputs to the standards activities.

INTRODUCTION

We are all being exposed to a revolution in communications.  A
revolution that is taking us from a world where telephone subscribers
were constrained to communicate over fixed telephone lines, to one
where a tetherless and mobile environment has become a reality.
Mobile communications, which include cellular telephones, mobile
satellite, land mobile radio, mobile data networks and personal
communication systems (PCS), have experienced enormous growth over the
last decade.  

The Cellular Telecommunications Industry Association (CTIA), for
instance, has stated that there are currently over 8.8 million
cellular subscribers in the United States, which compares with
approximately 4.4 million in June 1990 and 90,000 subscribers in 1984
[1].  This trend is apparent across the industry and with the demand
for mobile communications continuing to grow at a rapid pace, problems
with high levels of communication traffic are expected to become
severe in many regions.

As a result, maximizing the utilization of limited radio frequency
resources is now a primary concern to commercial service providers and
communication regulatory committees.  In conjunction with this
revolution, we are witnessing a transition in our communication
network's infrastructure.  After more than a century of reliance on
analog-based technology for telecommunications, we now live in a mixed
analog and digital world and are rapidly moving toward all-digital
networks.  The mobile communications industry is one of many that will
continue to benefit from the introduction of digital technology.

As the demand for communication services continues to increase,
cellular carriers are looking towards digital implementations for
larger capacity and a wider offering of services to mobile users.
Digital systems have also been advocated for better performance in a
wireless environment.  Channel coding, interleaving, and other digital
techniques can be used to provide additional robustness for radio
channels affected by shadowing, fading, and other forms of
perturbation.  Quite independent of the use of digital technology in
mobile networks is the increasing reliance on data communications in
the commercial and government sectors, as well as for personal and
cultural usage.  Today's modem-based, circuit-switched data
communications connect an ever growing number of users for PC
(personal computer), facsimile and special data applications, such as
the U.S. Government's secure voice terminal (STU-III).

While mobile networks transition to digital, an environment
susceptible to a number of interoperability problems will exist for a
wide range of modem-based data services, since simplicity and
transparency of end-to-end analog communications between mobile
networks and the analog Public Switch Telephone Network (PSTN) are
lost. These issues are currently being addressed so that connectivity
between mobile environments and the PSTN can be maintained without
severely impacting the present community of PSTN data users.  With the
near coincidence of schedules for development of many second
generation digital cellular, PCS, and mobile satellite systems, an
opportunity exists to resolve interoperability problems with
commonality in mind by defining interfaces, control procedures, and
mobile data protocols that are compatible with an expandable array of
common data services.

Another growing part of today's land mobile data transfer is of the
dispatch type, which involves communication between dispatch centers
and field personnel with mobile units.  One method of using the
networks effectively for this category of applications is to subdivide
information into small packets and transmit them at allocated
intervals on shared packetized radio channels.  Several advantages of
packet-switched data include the utilization of the channels only when
sending or receiving bursts of information, and the elimination of
frequent call setup (tear-down) procedures that are often associated
with establishing (terminating) dedicated circuit-switched
connections.

The packet approach also promotes a structure supporting intelligent
data protocols, such as flow control and retransmission, which can
provide highly reliable transfer in degraded channel conditions.
Benefits to the users are often reflected by reduced transmission
costs and virtual error-free transmission.  Examples of currently-used
packet data systems for mobile applications include, ARDIS, formulated
by IBM and Motorola, and RAM Mobile Data Network, which uses Ericsson
Mobitex mobile data technology.

Mobile Satellite Systems will also offer packet-switched data, as
illustrated with Telesat Mobile Inc.'s Mobile Data System (MDS) [2].
Other implementations, such as Cellular Digital Packet Data (CDPD),
are being defined to transport data as a supplement to the services
offered by today's North American analog cellular network [3].
Unfortunately, with much publicity being given to the aforementioned
dedicated packet data systems, less has been directed at the
standardization of data transmission over next generation cellular
networks.

For several years, two standards organizations, the SMG (Special
Mobile Group) of the European Telecommunications Standards Institute
(ETSI), and the TR45.3 Digital Cellular Standards Subcommittee of the
North American Telecommunications Industry Association (TIA), have
been committed to the successful definition of both circuit-switched
and packet-switched mobile data technology for next generation
cellular telephones.  The Pan-European Digital Cellular System,
referred to as GSM (Global Systems for Mobile), already has a common
array of digital data transmission channels defined [4, 5].

Within the next year, the data services standard for the North
American TDMA (Time Division Multiple Access) Digital Cellular System
is expected to be in place.  Once implemented, both will support a
multiplicity of circuit- and packet-switched data applications,
including the latest facsimile transfer technology.  Other features,
which at one time may have been too futuristic to discuss, such as
compressed video image transfer (e.g., maps) and facsimile mailboxes,
are expected to proliferate and a larger market will be reached with
the superposition of digital data capabilities on an existing network
infrastructure that was designed primarily for voice communications.
Digital cellular data will further promote accommodations for
integrated voice, data and paging solutions with the need for only one
mobile radio unit, and it will reach a subscriber base that is
expected to dominate the mobile telephone market over the next
decades.

Defining standards for data transmission over digital cellular
requires a new look at the technologies and disciplines emerging for
not only radio, but also for data and networking.  It does not
represent a direct translation of techniques used for today's data
communications over the landline networks.  Instead, successful data
services will depend on the merger of technologies developed for
digital radios, data protocols, and networking for efficient
adaptation of data communications to the wireless channel environment.

In the following sections, we focus on data services over cellular.
We briefly discuss the limitations of data transmission over analog
cellular networks and then address the main challenges to be met in
defining a standard for effective data transmission over digital
cellular channels, devoting particular attention to North American
developments.  Table 1 briefly compares some of the technical
characteristics of the analog and TDMA digital cellular systems in
North America.  Given the expectation that next generation cellular
networks will soon become widely deployed, digital cellular data
service should develop into a large market since it will make mobile
data as accessible as voice service in the ubiquitous cellular
systems, and will offer significant advantages over other systems.


END OF EXCERPT- 12/14/92


ALLEN LEVESQUE, GTE GOVERNMENT SYSTEMS CORP., WALTHAM, MASS. USA,
TEL: (617)466-3729, FAX: (617)466-3720, e-mail: Levesque@scsd.gte.com


[Moderator's Note: I encourage interested parties to get the February
issue of the magazine to continue this interesting report which is
excerpted here due to space considerations.   PAT]