A summary is given of the government's perspective on evolving digital communications as they affect secure voice users and approaches for operating during a transition period to an all digital world. An integrated architecture and a mobile satellite interface are discussed

Dean, Richard A.

English

NASA Technical Reports Server

N92-24094
Transitioning from Analog to Digital
Communications: An Information
Security Perspective
By Richard A. Dean
Department of Defense,
Fort Meade, Md 20755
ABSTRACT
This is an age of revolution in
communications technology and
systems. While this revolution
might simply be described as a
transition to digital
communications technologies,
these fragmented (post Bell
System), market driven
developments create a totally new
environment to plan, influence
and develop systems for the U.S.
Government's secure voice users.
Secure communications depend on
the availability of either end-
to-end analog connections or end-
to-end digital connections. The
uncontrolled mixture of analog
and digital links in the public
switched network (and its
extensions), as will happen in
this digital transition, can
inhibit end-to-end encryption.
The introduction of security into
digital systems such as Mobile
Satellite, Digital Cellular
Telephone, Integrated Services
Digital Network (ISDN), Land
Mobile Radio and other systems
represents both a challenge to
our existing infrastructure and
an opportunity to capitalize on
new technology.
This paper is the author's
attempt to summarize the
governments perspective on
evolving digital communications
as they affect secure voice users
and approaches for operating
during a transition period to an
all digital world.
NEW DIGITAL COMMUNICATIONS
Virtually every component of
the communications infrastructure
will be influenced by digital
technology. The critical systems
affected by this transition are
summarized below.
Integrated Services Digital
Network (ISDN) represents the
largest and most visible digital
service on the horizon. ISDN will
offer many new features for data
and secure voice users on the
Public Switched Network (PSN).
The menu of services will allow
backward compatibility with
analog POTS service for data
modems, facsimile, and secure
voice users. In fact the
flexibility of ISDN will
facilitate some of the connection
problems for other networks as
described later.
Mobile communications which
includes Cellular, Satellite,
Land Mobile Radio, and portable
phones represents today's fastest
growing communications market
[i]. The growth of this market
and the pressure for bandwidth
conservation has led to the
introduction of low rate
compressed voice in the range of
4-13 kbps. Because these low
rate digital voice links will
not support a conventional modem,
and the networks do not carry the
digital signal to the far end,
this voice compression will
challenge our ability to secure
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International Mobile Satellite Conference, Ottawa, 1990
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these links. Mobile
communications also represents a
potential threat to sensitive
U.S. communications as these
radio communications will be
readily available in the clear.
Backward compatibility to the
government's STU-III secure
telephone and other secure
systems will likewise be a
challenge. Several systems are
in various stages of development
including Digital Cellular
telephone, Land Mobile Radio,
Mobile Satellite, and INMARSAT.
Each of these systems has the
potential of becoming a valuable
secure communications link but
each will require inclusion of
features to enable secure
interoperability.
INTEGRATED ARCHITECTURE
The recent explosion of new
digital systems has forced a more
comprehensive look at at
interoperability on these
systems. While each system has a
unique protocol set, developing
custom solutions for each new
network is neither wise nor
feasible. It is clear that the
challenge is to conceive a plan
that will enable transparent,
communications among the dominant
future digital systems. The
interoperability challenge is to
facilitate a smooth transition by
maintaining backward
compatibility with existing
secure analog equipments. These
existing and future systems
consist of STU-III on existing
analog POTS, Digital Cellular
Telephone, Mobile Satellite, and
ISDN (with a potential new
terminal for that media).
Managing the transition from a
STU-III based (analog end-to-end)
system to an ISDN based system
with a large mobile user
population represents a major
challenge. One must either make
all new terminals backward
compatible at some expense,
replace existing secure voice
equipment with new equipment, or
face the problem of building high
volume gateways for
interoperability. In this paper
it was decided by the author to
try to move forward to embrace
new digital techniques for future
compatibility rather attempt to
maintain continued analog
operation. While a centralized
Gateway solution is presented,
various decentralized solutions
will be practical and desirable
to implement in specific
instances. A family of
compatible solutions are in fact
possible in concept and such a
set of solutions might be
necessary to resolve the
anticipated bulge that will
appear in the 1994-98 timeframe
when we are in the midst of the
transition from analog to
digital. Key to the success of
such an architecture, however, is
to develop a common interface
with mixed digital systems that
will allow transparent end to end
digital operation.
In looking at alternatives for
a global connection architecture,
there is an option to resolve the
interface problem with a
distributed solution or with a
centralized solution. A
distributed solution deals with
the interface problem by a
translation for compatibility at
each end terminal or as close to
that terminal as possible. A
centralized solution performs the
translation at one central
location. Both approaches have
advantages. The approach
presented here shows an evolution
from a distributed solution to a
centralized solution using a
Gateway.
The architecture is presented
in three phases consistent with
developments in the associated
usage and availability of the
International Mobile Satellite Conference, Ottawa, 1990
143
Cellular, Mobile and ISDN
services.
The architecture for Phase 1
is shown in Figure I. This is a
distributed solution using STU-
III modem pools (shown as M-3) to
perform analog to digital
transition for Mobile Satellite
and Cellular. The STU-IIIB uses a
direct (black) digital output for
mobile applications. It is shown
for the 1990-93 time frame where
the extent and geographic
distribution of this solution is
limited. This approach maintains
the digital nature of the mobile
network and its interface is the
same as most other digital users.
An architecture for Phase 2
is shown in Figure 2 for the
timeframe 1994-1998. This
represents the peak of the analog
to digital transition where it is
expected that there will be large
numbers of both digital and
analog secure systems connected.
In this case the Gateway is the
primary place where the mobile
systems transition to analog STU-
III terminals. This is performed
by establishing a digital
connection (switched or
dedicated) from the Mobile and
Cellular interfaces to the
Gateway. The M-3 modems shown in
figure 1 are relocated at the
Gateway. The digital interface
shown as G is a simple (assumed
standard) digital interface to
the Mobile and Cellular
interfaces. Figure 2 also shows
an early ISDN network. The early
ISDN terminals are assumed to be
STU-III interoperable (direct or
virtual) and include a STU-III
modem. This is provided so that
the Gateway traffic and resulting
blockage is limited. ISDN will
also provide a simple and cheap
switched connection from the
Mobile and Cellular interfaces
and the Gateway. Figure 2 also
shows a STU-IIIB (a STU-III with
BLACK digital output) coupled
directly into ISDN with an ISDN
adapter. The incorporation of
this adapter allows direct
digital communication of these
STU-III's over ISDN with mobile
STU-III's and with ISDN
terminals. The use of analog
modems in the ISDN terminal and
the use of ISDN adapters on STU-
III'S provide users options to
enhance their grade of service
and limit traffic through the
Digital Gateway.
An architecture for Phase 3
is shown in Figure 3. This is the
architecture for the period 1999
and beyond. In this era ISDN is
almost universally available.
Mobile and Cellular operation is
supported by direct end-to-end
BLACK connections directly to
ISDN terminals via a simple
(assumed standard) digital
interface. The Digital Gateway
continues to operate, however, as
an interface to the remnant of
analog STU-III users. These may
be people at remote locations or
international service where end-
to-end digital is unavailable.
The M-3 modems are removed from
the ISDN terminals and can be
used at the Gateway.
The centralized solution has
several inherent advantages. This
solution lumps most of the
interoperability problems into a
single solution where it is
assumed that economies of scale
will reduce overall costs. Our
requirements to each of the
numerous communications carriers
is uniform and standard. We
simply want to pull out the
embedded 4.8 kbps stream from
their system and port it to the
Gateway on a standard digital
link. We eliminate the need for
a custom, government owned M-3
modem pool at geographically and
organizationally diverse
facilities. We trade off this
widely dispersed logistics
problem against the
International Mobile Satellite Conference, Ottawa, 1990
communications costs to connect
to a centralized, more efficient
facility. Transition to an all
digital environment is clear.
When direct end-to-end digital
connections can be made from the
mobile interface to the ISDN
user, the central Gateway is
bypassed. Finally, when the bulk
of the users are connected to
ISDN on either an ISDN terminal
or a STU-III with a terminal
adapter, the gateway can be used
to extend service to the remnant
of analog STU-III users.
MOBILE SATELLITE INTERFACE
The proposed interoperability
scheme requires a common set of
features be provided by mobile
cellular and satellite systems. A
preliminary evaluation of the
systems function reduces these to
the following:
i. Recognition by the host system
of the STU-III capability at the
mobile unit.
2. Enable the insertion of an
encrypted data stream in place of
the 4.8 kbps speech data within
the packet structure and removal
at the other end (mobile or PSN
interface) with transparency.
3. Include the control signaling
to allow either the mobile STU-
III or the PSN STU-III to switch
from clear voice to secure voice
and back at will. In the case of
the PSN interface, a 2100 Hz tone
is the signal used to bring in
the STU-III modem.(Phase i)
4. Support a STU-III modem pool
at the PSN entry.(Phase I)
5. Include the interface and
control necessary for direct
transfer of the 4.8 kbps data to
a remote facility by either a
dedicated or switched fractional
T1 link or ISDN when
available. (Phase 2)
6. Perform all system signaling
out of band after the call has
been established.
7. Maintain synchronization and
bit integrity for the 4.8 kbps
stream after the connection is
established.
8. Incorporate the Fed. Std. 1016
4.8 kbps CELP voice coder as the
standard speech algorithm for
both clear and secure.
These functional requirements are
judged to be straightforward to
implement if identified in the
early design phase of the mobile
system. These features are also
similar to the requirement of
many other data a facsimile users
who need to interoperate on
mobile systems.
CONCLUSIONS
A concept for secure voice
interoperability on emerging
digital systems was presented.
The transition period from analog
to digital systems is handled by
modem pools and a central
Gateway. Long term compatibility
is satisfied by an end to end
connection via ISDN. The
interface requirements for
digital systems such as Mobile
Satellite were presented and are
straightforward and similar in
nature to the needs of other data
and facsimile users.
REFERENCES
I. M. Bonatti et al.,
"Telecommunications Network
Design and Planning", _E£E J0urDal
on Selected Areas of
Communications, vol 7, Number 8
Oct 1989
International Mobile Satellite Conference, Ottawa, t990
145
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146
International Mobile Satellite Conference, Ottawa, 1990


Transitioning from analog to digital communications: An information security perspective

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