Trunked mobile radio makes a limited number of radio channels available for a relatively large number of mobile subscribers by channel sharing and appropriate access and signaling procedures on a control channel. The software monitors control and traffic channels.

A trunked network is controlled by a fixed base station (TSC - Trunked System Controller). Wide areas requiring radio coverage are divided into cells each of which is controlled by a TSC. The TSCs are connected to a hub and are controlled by a Management Controller. The TSCs register roaming of the mobile units and route traffic to the TSC which is nearest to the subscriber. TSCs may be connected to the public telephone network.

Trunked systems may carry voice or data signals. The mobile unit uses two-channel simplex and the base station full-duplex.

The British MPT-1327 specifications for trunked private land mobile radio systems have won general and de facto acceptance in Europe. MPT-1327 specifies the general signaling features to be used on the trunked system. The system is used in the UK, France, Germany (Chekker, License A), Switzerland (Speedcom) and the Scandinavian countries with national adaptations.

The control channel is divided into the Forward Control Channel from base station to units, and the Return Control Channel from units to base.

The forward control channel may be dedicated (fixed), non-dedicated (any free channel) or the same for all TSC's, which then access the channel in TDMA (time division multiple access).

The return control channel is randomly accessed by the mobiles in timeslots of 106.7 ms (128 bits).

The forward control channel is divided into time slots each carrying two 64-bit code words:

Ø A Control Channel System Codeword (CCSC), which identifies the system to radio units and provides synchronization to the following address codeword.

Ø An address codeword, which is the first codeword of any message and defines the nature of the message.

A message consists of a codeword sync sequence, an address codeword and one or more data code words. A codeword contains 48 information bits and 16 check bits. If bit 1 is logical one the codeword is an address codeword, otherwise it is a data codeword.

The more important address code words are:

ALOHA (ALH)

TSC to mobile. Carries information on the number of time slots in the succeeding frame and the channel number of the control channel.

REQUEST (RQS)

Mobile to TSC. Requests for action by the TSC.

AHOY (AHY)

TSC to mobile. General availability check.

ACKNOWLEDGEMENT (ACK)

TSC to mobile, mobile to TSC. Response to RQS or AHY. ACK from TSC also carries ALOHA time slot information.

GO-TO-CHANNEL (GTC)

TSC to mobile. Carries information on traffic channel number allocated for requested call.

The return control channel for the mobile units is accessed randomly using a particular form of the slotted ALOHA protocol. The TSC continuously sends sync messages on the forward control channel to the mobiles inviting random access messages. These TSC messages contain a parameter indicating the number of following timeslots - a frame - available. The mobile unit finds a random slot in the frame for its message. If the messages of two or more units collide, the collision is detected by the TSC which now increases the number of timeslots for the next frame and the mobile units retransmit.

If you select Fixed stations AHL messages will be filtered out except for the two following a non-AHL message. This is to prevent the display being flooded by these messages which are the most numerous in the system.

If you select Mobile stations all messages in the control channel will be displayed, and the messages which are identical, but have different labels depending on their origin from TSC or mobile, will be labeled with their mobile label.

Error correction may be enabled or disabled by toggling the ECC menu item.

A PC generated timestamp may be inserted ahead of the decoded message. Go to the Options menu and enable the Time stamp function.

Here are a few examples of the most frequent MPT messages monitored on the Forward Control Channel transmitted by the TSC. The most common message is ALH, a general invitation to transmit any single codeword message:

ALH(P:42,I1:5461,CH:3,WT:6,M:0,N:4).

Invitation to all mobile units belonging to group 42-5461 (P:42,I1:5461) to transmit. The message is sent on control channel 3 (CH:3), the TSC responses may be delayed by 6 slots (WT:6), no subdivision of the radio population is enabled (M:0) and the next frame contains 4 random access timeslots (N:4).

A radio unit calls another unit with the same prefix:

AHY(P:79,I1:760,I2:770,D:0,P:0,CHK:1,E:0,AD:0).

General availability check on the called (D:0) unit 760 (I1:760) sent before allocating a traffic channel. The TSC is checking if the called unit is ready for a data call (CHK:1). The calling party 770 (I2:770) is requesting a non-emergency transaction. No data codeword (AD:0) is appended to this AHY message.

GTC(P:79,I1:760,CH:427,I2:770,N:0).

Calling unit 770 and called unit 760 are directed to proceed with traffic on channel 427 (CH:427). The next frame contains no timeslots (N:0).

A broadcast message:

BCAST(SYSDEF:5,SYS:20265,CH:520,SPARE:0,RSVD:00,ADJ:2).

Option for non-active radio units to determine the signal strength (SYSDEF:5) at the following time slice on control channel 520 (CH:520) of system 20265 (SYS:20265) having local serial number 2 (ADJ:2).

To select monitoring either the base station (Fixed) or the Mobiles, use Options/Station. To change the display format of Data Code Words use Options/Format.