Subject: [NEXTEL-1] Bridging devices in disparate bands

From: [email-masked]
Date: Wed, 26 May 2004 18:57:57 -0400

Bridging devices in disparate bands
By Harold Kinley

Mobile Radio Technology, May 1, 2004
Despite the steady migration of public-safety radio organizations to 800
MHz trunking systems over the past few years, many
public-safety radio systems still operate in the VHF and UHF bands.
Fortunately, the national frequency plan developed by the
National Public Safety Planning Advisory Committee and adopted by the
Federal Communications Commission in 1987 calls for
certain 800 MHz frequencies to be set aside for the purpose of
interoperability — or “mutual aid” — between public-safety entities
operating in various frequency bands.

To review the requirements and objectives, I have excerpted a portion of
Appendix C of NPSPAC's Channel Planning Analysis,
as follows in the next section. (Visit
www.pswn.gov/admin/librarydocs/800appendixc.pdf for the full document.)
The national plan requires that regional plans contain several sections
concerning the implementation of the five nationwide mutual-aid
channels. These sections typically define the use of each of these
channels and provide detailed standards and operating procedures to
govern the use of these channels.

Common Channel Implementation — The national plan sets forth the
guidelines for using and implementing the five National Common Channels
(see Figure 1). Four of these channels are dedicated as National
Tactical Channels and one channel is dedicated as the National Calling
Channel. The National Common Channels are to be available for use
throughout the region.

The National Calling Channel, channel 601, is to be implemented as a
full mobile relay, with wide area coverage transmitters to maximize
coverage. Large system users (five or more channels) are required to
monitor this channel and could be required to provide satellite receiver
feeds into the wide coverage area.

The four National Tactical Channels are to be assigned throughout the
region for use by all eligible entities. Large system users could be
required to sponsor one or two localized mobile relays to cover specific
geographic areas. The users of these channels must be eligible for
licensing on other 800 MHz public-safety channels, but no special
licensing is required.

Operation on Common Channels — The five common channels are only to be
used for activities requiring intersystem communications between
entities not already sharing communications systems and are not to be
used for any daily operations. In emergencies, the channels may be
assigned by the primary public-safety agency in that area. On all common
channels, plain English and familiar words and phrases should be used.
The calling channel is used to establish contact with other users and
determine which tactical channel to use. It is not to be used as an
ongoing working channel. Tactical channels are reserved for interagency
communications and are used as directed by the primary public-safety
agency in the area. Tactical channels can be assigned by the various
public-safety services, or they can be assigned by county or area.
Network Operating Method — A wide area network will be established on
the National Calling Channel. The tactical channel communications
systems will be implemented by volunteer entities, and each primary
geographic section of the region is covered by at least one tactical
channel.

Coded Squelch on Mutual-Aid Channels — The National Common Tone Squelch
of 156.7 Hz will be used on all equipment operating on the five common
channels.

If these frequencies are used nationally, as the plan requires, then
interoperability comes down to providing a link or bridge from
public-safety radio systems operating in bands other than 800 MHz. In
Figure 2, a conventional VHF radio is interconnected to a
conventional 800 MHz radio to form a translator. The function of the
translator is to receive radio transmissions on the 800 MHz
system and retransmit (translate) them over the VHF radio system. The
translator works bidirectionally — that is, it also picks up
radio transmissions on the VHF radio system and retransmits them through
the 800 MHz radio.

As shown in Figure 2, the audio output of the VHF transceiver in the
translator is fed to the exciter input of the 800 MHz
transceiver. The control circuitry keys the 800 MHz transceiver, thus
transmitting the signal to the 800 MHz mobile relay station.
The 800 MHz mobile relay station then retransmits the 800 MHz signal to
the 800 MHz mobile (or portable) radio transceiver.
The signal follows the path of the blue arrows. When the 800 MHz mobile
transceiver is transmitting, the reverse operation takes
place, and the signal follows the path of the red arrows.

Figure 3 shows another setup in which the translator is the only bridge
between the 800 MHz radio and the VHF radio. The 800
MHz mobile relay station is omitted from the loop. It is important to
note that the translator setups described above will only work on
conventional 800 MHz frequencies.

More elaborate methods must be employed to link a conventional system at
other bands to an 800 MHz trunking system. One
solution can be found in the SmartBridge trunked radio interconnect
systems from Melbourne, Australia-based Radio Systems
Technologies. For example, the SB100 Talk Group extender — designed for
the company's SmartNet/SmartZone trunked radio
networks — can be installed at any conventional site within radio range
of a trunked site.

When the bridge is operating in the connect state, any radio operating
within the repeater's radio range becomes an extension of
the TalkGroup. Thus, the conventional radio effectively behaves as
though it were a trunked radio itself. The connect on/off state
is controlled by using “over-the-air” dual-tone multi-frequency
signaling. A unique “trunked system busy” protocol ensures that
users of conventional radios are immediately informed of unsuccessful
attempts by audible alerts. (For more information visit
www.rstradio.com.)

Another device that has gained popularity is the TRP-1000 transportable
interconnect system from JPS Communications, a
division of Raytheon. The TRP-1000 provides interoperable communications
between organizations using different radios on
disparate frequencies. It is a self-contained, portable unit consisting
of a power source and multiple radios pre-wired to the
company's ACU-1000 solution. The ACU-1000 is designed to provide
on-scene interoperability by linking radios, SATCOM,
cellular, Wi-Fi and landline phones directly and over IP networks.

The TRP-1000, integrated with the ACU-1000, can serve as a bridge
between conventional systems as well as between conventional systems and
trunked systems. (More information on the TRP-1000 and ACU-1000 can be
found at www.jps.com.)

Although trunked radio systems (800 MHz and other bands) currently serve
a large number of public-safety agencies, many
agencies continue to operate conventional radio systems in the VHF and
UHF bands. Of these, many — if not most — are
pleased with their radio systems and don't want to move to 800 MHz
trunking systems. Consequently, until the time comes when
all public-safety agencies are on the same radio system, bridging
devices such as those described in this space will be required to
achieve interoperability.

Until next time — stay tuned!

http://iwce-mrt.com/ar/radio_bridging_devices_disparate/index.htm