5-13-2 Trunking Operational Theory

Trunking is the sharing of all available channels in a repeater system by remote or mobile units with access to that repeater. A trunking system is composed of up to 20 repeaters, with each repeater assigned up to 250 separate ID codes. Trunking is based on the theory that an individual mobile unit, with access to several communication channels at once, has a low probability of being blocked from transmitting since the other subscribers to the system will use it only a small percentage of the time and that a large number of other subscribers will not use it at exactly the same instant. Hence, at least one channel will be free at any given time.

Each mobile unit in a trunking system is assigned a "Home Repeater" from which it receives digital control information. A listening mobile unit monitors data from its home repeater to determine on which channel it is being called by another mobile unit or which channel it may use if it wants to transmit.

Trunking systems are allocated 999 channel pairs in two separate bands by the FCC. In the lower band of 600 channel pairs, 806 MHz to 821 MHz is used for mobile transmit and 851 MHz to 866 MHz is used for mobile receive, with a 45 MHz offset between the transmit and receive frequency in each channel pair. In the upper band of 399 channel pairs, 896 MHz to 901 MHz is used for mobile transmit and 935 MHz to 940 MHz is used for mobile receive with a 39 MHz offset maintained between transmit and receive in each channel pair.

Each repeater in a trunking system uses only one channel pair to communicate with each of the mobile units assigned to it. If a mobile unit is instructed to use a different repeater, another channel pair is used.

Home repeaters can only exchange data with mobile units that have ID numbers assigned to that repeater. Other mobile units are ignored by a repeater unless the mobile unit has received permissions from a separate repeater to access the first repeater. This is known as a "Handoff". All repeaters in a trunking system are physically connected by coax so all repeaters in the community can be informed of accesses and handoffs.

Mobile units in the trunking system work in a simplex or push-to-talk manner. Some radios automatically handle push-to-talk and receive functions in a manner that makes them appear to be duplex in nature, yet these units are only half duplex. Mobile units can only hear messages transmitted through a repeater from mobiles in the ID number group assigned to that repeater. Likewise, mobile units can only transmit to other mobiles in the ID number group assigned to the same home repeater. Through a logic lock-out in the units, only one mobile at a time is allowed to transmit through a home repeater to other units assigned to that repeater.

Mobile units can change their ID numbers to either listen or talk to other mobiles, groups of mobiles or subgroups. These mobile units change their ID numbers with either manual controls or by scanning. The organization of mobile units is arbitrary and done differently by different mobile transmitter-receiver companies.

Signaling between mobile units and repeaters occurs at subaudible frequencies and occurs continuously and concurrently with the audio portion of the transmission. Updates are transmitted by repeaters to keep mobile units informed as to which channel is available to them or when other mobile units are calling.

5-13-3 Accessing The Trunking System

Before a mobile unit can access the system, a data "handshake" with a repeater must be accomplished. When the mobile unit's push-to-talk is p. ressed, the mobile uses a repeater's data packet to determine if there is a free repeater channel available. If there is, that repeater becomes that mobile unit's "home" repeater. The mobile transmitter is enabled and the mobile unit transmits a data message to the repeater containing the ID code for broadcast. The mobile unit then listens for a response from the repeater for the go-ahead to transmit. When this handshake is received, transmission is enabled. Also, all listening mobiles with that repeater's ID number detect their ID, select the directed channel and unsquelch to receive voice transmission.

 

A. The Trunking Data Packet

The trunking data packet is a 40-bit word formatted as' SYNC AREA GOTO : HOME ID CODE FREE : CHECK These data are defined as-

SYNC (9 bits)- A bit pattern used to initialize the receive data circuitry. These bits are compared by the trunking hardware to a known bit pattern. If the pattern matches, the remaining bits of the word are checked for acceptance. Patterns used by the industry at the present time are'

101011000 101010000 001010000

AREA (1 bit)' Repeater systems in close proximity to others may use the same channel frequencies. These systems use this area or intermod bit as an "us-them" comparison for data acceptance purposes. This bit is usually "0" unless two systems are in close proximity to one another.

GOTO CHANNEL (5 bits)' These bits represent either the repeater being used or the repeater to which a mobile unit is being transferred for a transmission. These bits must be the same number (1 thru 20) as that assigned to the repeater for data acceptance in a mobile-to-repeater message. In a repeater-to-mobile message, this is the number of the repeater to be used. The mobile translates this number through a look-up table into an FCC channel pair.

A "31" code in this location indicates a "turn-off" code for the end of transmission.

HOME CHANNEL (5 bits)- In a mobile-to-repeater data message, this slot is the home repeater assigned to that mobile for the current group selection and is used for system accounting. In a repeater-to-mobile message, this contains the home repeater number of the mobile making the call, no matter which repeater is being used--the home repeater or another repeater.

 

ID CODE (8 BITS): In a mobile-to-repeater message, this slot con-
tains the ID code (1-250) being used for broadcast. In a
repeater-to-mobile message, the same ID is reflected so a mobile
can decide if it needs to unsquelch its receiver to listen to the
voice transmission.

FREE CHANNEL (5 bits): In a repeater-to-mobile message, this
slot tells a mobile which repeater (1-20) to use if it decides to
transmit. This can be the same number as the home repeater. If
all repeaters are busy, this field will contain a zero (0). In a
mobile-to-repeater message, this contains a 31--used simply to
fill the slot since mobiles do not use this piece of information.

CHECK (7 bits): This field contains error check bits that are
derived from all the message after (but not including) the sync.
When the message is sent, a special parity operation is performed
on the data and "shoe-horned" into these seven bits. A receiver
(mobile or repeater) performs its own computation to produce what
it thinks the check should be. If the two separately derived
check codes do not agree, the message is rejected by the receiver.

B. Repeater Channel Numbers

A mobile radio is programmed with a set of valid repeater numbers
from i to 20. An RF channel number is associated with each of
these repeater numbers in a mobile radio look-up table. One of
these repeaters is designated the home repeater for the mobile
radio.

For Example:

REPEATER CHANNEL MOBILE RECEIVE MOBILE TRANSMIT
LIST NUMBER FREQUENCY* FREQUENCY*

39 851.9625 806.9625
79 852.9625 807.9625
11 119 853 . 9625 808. 9625
15 159 854. 9625 809. 9625
19 199 855. 9625 810. 9625

*These channel numbers and frequencies reflect those used in the lower Trunking band. For a complete list of channel numbers and frequencies, see Appendix F.

 

Given a channel number, the mobile unit can compute the transmit and receive frequencies according to the following formulae-

Low Frequency Band

Transmit RF = 806.0125 + [.025 x (Channel # -1)MHz]

Receive RF = Transmit RF + 45 MHz

High Frequency Band

Transmit RF = 896.0000 + (.0125 x Channel #) MHz

Receive RF = Transmit RF + 39 MHz

Radios operating in the Low Trunking Frequency Band within range of Mexico or Canada must employ a -12.5 kHz offset as required by the FCC.

Repeaters, in turn, are programmed to recognize a set of ID codes. Any received data stream from mobiles with IDs not in repeater's table are discarded.

C. Mobile Calling a Free Repeater

The following illustrates a simple repeater/mobile handshake. The bit transmissions are formatted as-

SYNC (S) : AREA : GOTO : HOME : ID CODE : FREE : CHECK (C)

The following illustrates a Handshake sequence:

View image

D. Mobile Handoff

When a home repeater is busy, it will instruct a mobile unit to change its channel to the next free repeater in the system. A handoff will occur in this manner:

View image