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HYBRID NETWORKS
7.1 TERRESTRIAL PACKET/SATELLITE
Trucking company Boyle Transport makes it crystal clear: you should be able to pick
a radio or cellular based system for regional service and satellite for long haul, but be
able to use the same provider.
1
This is the goal of most hybrid offerings, though
satellite may well be avoided in favor of an alternative technique such as circuit
switched cellular.
7.1.1 AMSC/ARDIS Multimode System
In March 1990 Rockwell announced that it was developing a satellite-based vehicle
location and two-way data system.
2
Rockwells plan was to lease L-band capacity from
AMSC, which was leasing time from InMarSat. Rockwell concentrated on the vehicular
terminals and communications hardware. It began to develop a new control center for
switching data between customers headquarters and their vehicles. Rockwell saw its
competition as OmniTRACS (then only
∼
7500 units) and Geostar (now defunct).
Rockwells perceived edge were its customers, who had already installed 80,000
TripMasters. Since Rockwells product was not yet ready (indeed, it was still debating
whether or not Loran-C was the best way to go), it purchased the first two years
devices from Canadian Aeronautics (CAL). The plan was to resell each unit for $3500;
its own device was estimated to be
∼
$2500. This was markedly cheaper than the
$4000$5000 charged by Geostar and OmniTRACS. However, the Rockwell airtime
costs were estimated to be $50$60 per month, quite a bit more than the $35$45

charged by Geostar and OmniTRACS.
3
The Wireless Data Handbook, Fourth Edition. James F. DeRose
Copyright © 1999 John Wiley & Sons, Inc.
ISBNs: 0-471-31651-2 (Hardback); 0-471-22458-8 (Electronic)
As with many acts of bravado, the world did not accommodate itself to Rockwells
goals. After long delays, Rockwell finally had the Pro2000 product in April 1993for
$4000. The airtime price was $60 for 15,000 bytes ($4 per kilobyte).
4
Rockwells device sold slowly: 3000 units over six months. Meanwhile,
OmniTRACS acceptances began to accelerate, climbing a steep hill. In October 1994
after OmniTRACS had reached 90,000 subscribers, Rockwell announced the first
multimode device: the Pro2000SL. This unit contained a Motorola radio modem for
communication over ARDIS. The price was $4495. There were also airtime pricing
adjustments: $40 per 20,000 bytes per month. Each additional kilobyte was $1.65
during peak times (7:00
AM
to 7:00
PM
Central Time)
5
; the nonpeak kilobyte charge
was $0.75.
Prior to the multimode system announcement, Rockwell had tested in a 100 mile
radius of its Cedar Rapids plant using an ARDIS hand-held device. The conclusion
was that about half the traffic would flow via satellite. This guess permitted new
customers such as Umthun Trucking (400 units) to calculate an 18-month payback.
But when using an external antenna mounted high in a truck, the device began
picking up ARDIS well outside the expected range of coverage. Instead of half the
traffic to ARDIS, 70

±
5% of the messages flowed via the terrestrial source, which
must have made Umthun happy! However, it did not please Rockwell; the business
was sold to AMSC at year-end 1996. AMSC, in turn, purchased ARDIS on March
31, 1998, and announced its intent to leverage an integrated terrestrial/satellite
network to advance its leadership in wireless data services.
6
7.1.2 BSWD/Norcom
On November 10, 1992, ARDIS and Sears began a field test with 150 service
technicians. After years of the most grueling network-independent application
development work, the number of technicians had expanded to 7000 by December
1997.
Sears, like most major corporations, is loath to be dependent upon a single
vendor. In September 1996 BSWD approached Sears with a proposal to provide
service in areas thinly covered by ARDIS. Unfortunately, BSWDs terrestrial
coverage was not that hot in those sectors either. Its strategic network approach,
which uses circuit switched cellular when the packet coverage is unavailable, was
rejected by Sears as too expensive, insufficiently reliable, and lacking adequate
nationwide coverage.
7
BSWD, taking a leaf from ARDIS book, then began work on terrestrial/satellite
hybrid device. The network chosen was Norcom, a subsidiary of Telenor, which
leases Ku/L-band time on AMSCs satellites. The key advantage is that the Norcom
design uses an X.25 call setup protocol, resulting in a short response time, typically
35 seconds. ARDIS bid the Rockwell multimode solution, of course, but in its 1996
incarnation it was a polled device. This produced a design that could have a 510
minutes latency, unacceptable to Sears. BSWD/Norcom are installing the last 5000
Sears devices using their hybrid approach.
7.1 TERRESTRIAL PACKET/SATELLITE
81

Note that the technician does not walk around interchanging data with a GEOS.
When the hybrid device is out of the cradle, there is no wireless connectivity. Only
when it is returned to the cradle can communication with the satellite begin.
7.2 TERRESTRIAL PACKET/CIRCUIT SWITCHED CELLULAR
This combination depends upon having a modem capable of operating on either
packet or circuit switched networks, which is not so unusual. However, the modem
must also be capable of automatically detecting when one of the services is
unavailable. Typically the modems are designed to listen for packet first. The modem
will then select the alternative service, sometimes in the middle of a transaction. The
key advantages for the customer are increased coverage (often at higher airtime cost),
without manual intervention or changes to the user application. Application transparency
requires network enhancement to hide the message source from the user host. The notion
is wonderfully attractive; customer acceptance has been disappointing.
7.2.1 CS-CDPD
The CS-CDPD design was pioneered by GTE Mobile, Hughes Electronics, and the
now-departed PCSI in early 1995. In this implementation a device operating on CDPD
continually tests the field strength of the CDPD signals. If they begin to fade, a special
circuit switched uplink protocol is dialed that can make a connection in about 3
secondsa delay often not noticed by the user. The dialing plan can be complex,
including the use of 800 numbers when out of range of GTE Mobile coverage.
In the spring of 1996, in cooperation with GTE Mobile, TranSettlements Network
Services announced a wireless communications system for less-than-truckload (LTL)
carriers.
8
The Tracker was intended to serve the pickup and delivery application; all
emphasis is on telephone handset functions. TranSettlements began testing the PCSI
PalPhone in the fourth quarter of 1996 with Silver Eagle (30 units) of Portland and
Cal Pak of Hayward.
9
Beta testing was essentially complete on January 31

10
and the
price list was placed on the Web.
11
However, the production roll-out failed. The many problems, not the least of which
was the demise of PCSI, were best summarized in May 1998 by TranSettlements: our
initial customers (in a decision made concurrently with TranSettlements) have had to
discontinue use of the Tracker P&D system, as problems with CDPD reliability,
interoperability, and lack of warranty for the hardware have greatly increased the
overhead necessary to support the service.
12
7.2.2 BSWD Strategic Network
On October 31, 1995, BSWD (then RAM) announced
13
the first segment of its
strategic network: connection to the circuit switched cellular and public switched
telephone (PSTN) network.
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HYBRID NETWORKS
CS-CDPD operates on the same 800-MHz cellular channels whether in packet or
circuit mode. BSWDs Mobitex network operates in the 900-MHz band. Not only is
the band allocation different, so is the channel width. CS-CDPD operates in 30-kHz
wide channels; BSWD operates in 12.5-kHz wide channels. A single, inexpensive
radio simply will not bridge this gap.
Thus, the BSWD implementation requires two radios that operate on two
completely independent networks. A single, special-purpose modeminitially, the
NovaLink GoAnywherewas the bridging unit. Conceptually, the system looked as
shown in Figure 7-1. Note that the modem was capable of dialing in over conventional
landline networks, bypassing radio altogether. A small problem: The GoAnywhere
modem never shipped.

REFERENCES
1. M. Boyle, Director of Marketing & Information Resources, T.F. Boyle Transponation,
Traffic World
, 7-22-96.
2.
Mobile Data Report
, 3-12-90.
3.
Mobile Data Report
, 3-26-90.
4.
En Route Technology
, 3-15-93.
5.
En Route Technology
, 3-27-95.
6. AMSC press release, 3-31-98.
7.
Wireless & Mobility
, Mar. 1998.
8.
Logistics Technology News
, 11-8-96.
Figure 7-1
BSWD strategic network.
REFERENCES
83
9.
Traffic World
, 12-9-96.

10.
Communications Today
, 1-31-97.
11. www.itsc.state.md.us/info/EDI/chart/trans.html.
12. E. Fannin, TranSettlements, correspondence to J. F. DeRose, 5-5-98.
13.
RAM Analyst & Media Conference
, 10-31-95.
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HYBRID NETWORKS