In the early 1980's, leading telecommunications manufacturers pioneered work on Protocol 1327 under the auspices of the United Kingdom Department of Trade and Industry.

MPT1327 was developed over a two-year period and was the first of the Protocol 1327 family of standards.

This is the signaling standard incorporated into U.K. trunked radio networks as the basic operating system.

The Protocol 1327 family defines a trunking system for private mobile radio (PMR/SMR). It can be used on both VHF (136-174 MHz) and UHF (403-470MHz) spectrum. These standards were first adopted in the U.K. in 1987. Since then, they have been in successful operation in public and private trunking systems in an increasing number of countries worldwide.

A conventional PMR service offers the user a single channel. When the user wishes to make a call, this channel may well be occupied by another user. The prospective user must wait until the end of the conversation and then compete with others to obtain the vacant channel.

In a trunking system, a set of channels share the communication demands of the users. If no channel is free at the time a user - makes a call, the call will be placed on hold for a few seconds until any channel becomes available. As a result, the user has less time to wait and enjoys a better quality service.

To arrive at the optimum working implementation of the MPT1327 standard, network operators were consulted and the following additional standards were developed:

- MPT1343 - Defining the behavior of radio units on public networks

- MPT1347 - The fixed network specification to complement MF'Tl343

- MIP'Tl352 - The test schedule that radio units pass before being accepted onto radio networks.

Trunked Radio Technology

Trunking was developed because radio spectrum throughout the world was congested. Dynamic growth in mobile communications has made channel availability very difficult.

The increased demand for mobile communications means that more users access the same number of channels without any loss of quality of service.

At the same time, users are demanding service in a wider geographical area than is economically viable for most conventional PMR systems. Advances in technology have allowed trunking techniques, previously only associated with hard wire communications, to be applied to radio networks. These allow more efficient use of spectrum.

In trunking, each radio station transmits a control signal on a radio channel. Radio units have a number of "traffic" channels at their disposal for communication. When not in use, the radio unit is automatically tuned to the control signal and the unit's microprocessor can communicate with the system computer on this channel at any time. If the user wishes to make a call, the unit transmits the request in the form of a data signal to the system computer. The computer finds the caller's desired correspondent and, by means of the control channel, checks for willingness to receive a call. When both parties are ready to communicate, the computer allocates the first available traffic channel.

When the call is terminated by either party, the radio unit sends a data-signal releasing the channel. Radio stations may be interconnected to increase the service area to provide any size network up to a national or international level. Calls may also be made into other fixed-line networks such as telephone systems.

User Features

Features include two-way conversation with a variety of types of calls, as well as data transmission of status message instructions, short text messages and bulk data print-outs. Conference calls, call transfer, automatic call holding, and priority and urgent calls are also features. Users have the ability to call individual radio units, a group of units, or all the units on the system.

Other features include: calling PABX or PSTN numbers, a "call-me-back" facility when the unit is unattended, and the ability to roam across boundaries.

System features include Protocol capacity-one million addresses per system code. The three channel types available are dedicated control channel, floating control channel and traffic channel.

Traffic jam prevention controls, interference prevention, automatic user location and registration, automatic release of traffic channels at call termination, and periodic checking of serial numbers for subscriber security are also system features. The standards are designed to support any size of system, from local to international networks.

The U.K. Experience

The development of Protocol 1327 has led to an immediate improvement in the grade of service and has also allowed a large increasing traffic capacity compared with single-channel, non-trunked systems.

Wide area networks have been set up throughout the U.K. for shared system users in the Band III frequencies and cover much of mainland Britain.

Two national networks were established, although they subsequently merged into one, and several companies have been licensed to set up and operate local single and multiple-site systems. They offer operators of commercial vehicle fleets a communications system that has major benefits over private mobile radio technology.

The networks provide excellent clarity and call security. Users can make rapid, dedicated voice and data communications to other units, and connect to their company's private telephone system and main computer. Dialing into the public switched telephone network is also possible.

Leading electronics manufacturers worldwide have developed radio units designed to meet the Protocol 1327 standards. They provide a choice of features such as handsets or fist microphones, a range of dialing facilities and hands-free operation for safe communications on the move.

Tens of thousands of users are already operating on the new networks. Among those benefiting are a number of U.K. organizations ranging from local small businesses to the large public utility companies that prefer to operate their own closed system.

With a need to stay in constant communication with a large number of service vehicles over a wide area, the use of Protocol 1327 allows better vehicle use, greater manpower efficiencies and significant operating economies.

Worldwide Application

Widespread adoption of Protocol 1327 could provide great opportunities for manufacturers by establishing a worldwide market for their products. Systems can be set up quickly since network and user systems already exist. This eliminates troublesome delays while standards are devised and equipment is designed and produced. Systems can be set up immediately while home manufacturers develop their own products.

Because the Protocol 1327 standards are flexible, they can be easily implemented to suit specific requirements. If universally adopted, the transfer of radio units conforming to similar frequencies between networks and countries will no longer be a problem. Users will benefit too. The larger the market and economies of scale, the less expensive the subscriber equipment.

2 MPT1327 Protocol

MPT1327 is a signaling standard for Trunked private land mobile radio systems. It defines the protocol rules for communication between a trunking system controller (TSC) and users' radio units.

The standard can be used to implement a wide variety of systems, from small systems with only a few radio channels (even single-channel systems) through to large networks which may be formed by-the interconnection of TSCS.

The protocol offers a broad range of user facilities and system options. However, it is not necessary to implement all of the facilities available; an appropriate subset of the protocol could-be implemented, according to the user requirements. Also, there is scope for customization for special requirements, and provision has been made for further standardized facilities to be added to the protocol in the future.

The standard defines only the over-air signaling and imposes only minimum constraints on system design. Additional specifications will be required for specific implementations, for example, to define:

- the facilities that must be implemented

- parameter values

- a channel plan

- for a network, criteria for when a radio unit should register.

3 Types of Calls

The standard protocol enables radio units to make the following types of calls:

a. Speech call.

Speech calls (RF Dispatch call) may be requested with normal or high priority. For group calls, the-calling party may opt for a conversational mode, where all parties are able to speak, or for an announcement mode where only the caller may speak.

b. Data call. (Data on Traffic Channel) for the transmission of non-prescribed signaling. Parameters are available to specify either normal or high priority and, for a group call, whether the called group members can reply. (Provision has been made for specifying a standard method of data communication in the future).

c. Emergency call.

Parameters are available to specify either a speech or a data call and, for a group call, whether the called group members can reply. Also, a radio unit may request a special mode of emergency service previously arranged with the system; the TSC determines the required action by reference to the calling unit's address.

d. Include call.

During a call, a unit may request that another party joins the call. This facility may be used to implement a Conference Call or Call Transfer.

e. Status message.

Thirty-two different status messages may be conveyed between units. The meanings of two of these messages are prescribed as a "call-me-back request" and "cancel previous call-me-back request". The remaining thirty messages have user-defined meanings. (Status messages can also be sent between radio units and the TSC.)

f. Short Data Message.

Messages of up to 184 bits of free format data can be sent between units, or between units and the TSC.

4 Making Calls

A radio unit may request a call to any of the following called parties (except for status message, which cannot be addressed to PABX or PSTN destinations or to groups):

- an individual radio unit or line-connected unit

- a group, or all units in the system

- a PABX number, up to nine digits

- a PSTN number, up to 31 digits.

A call request may be cancelled at any time.

5 Receiving Calls

A radio unit may receive calls from a radio unit or line unit, or (except for status messages) from a PABX extension or the PSTN. In addition, status messages and short data messages may be received from the TSC. For a call from a radio unit, a line unit or the TSC, the calling address may be supplied to the called unit. For a call from a PA-BX extension or from the PSTN, the calling gateway is indicated as the source of the call but the caller's number is not conveyed to the called unit.

Incoming calls may be addressed to the unit individually or to a group to which it belongs. A radio unit may be a member of an arbitrary number of groups; its group addresses can be chosen independently of its individual address.

A radio unit may refuse to accept all incoming calls, for example by means of a "busy" or "out of-vehicle" control, or incoming calls could be refused selectively, depending on the source of the call. If a user does not wish to proceed with an incoming call immediately, he can indicate that he will call back later. Systems may be configured to alert a called individual and require him to indicate that he is ready, before a traffic channel is allocated for a call.

6 Diverting Calls

If a radio unit does not wish to receive calls, it may request that future calls addressed to it be redirected to a specified alternative destination. A radio unit may also request redirection on behalf of a third party, for example, for a unit which is not equipped for call diversion. A radio unit calling a diverted party will be informed of the alternative destination to try; it may then remake the call automatically, or it may give the user the option of deciding whether to call the alternative destination.

System Features and Facilities

The protocol uses signaling at 1200 bits with Fast Frequency Shift Keying (FFSK) subcarrier modulation. It is designed for use by two frequency half-duplex radio units and a duplex TSC. The signaling for Setting up calls is transmitted on a "control" channel. A TSC can be operated using either of two control channel strategies: dedicated or non-dedicated.

A dedicated system has a control channel permanently available for signaling, whereas a non-dedicated system may assign the control channel for traffic (speech or data communication) if all the other channels are in use. The use of a dedicated control channel is appropriate for a system with many channels, whereas a non-dedicated control channel may be more appropriate for a system with only a few channels. The protocol allows the use of either strategy.

Broadcast (Public Announcements) messages are available to inform radio units of system information, such as the channels which the system may use for control signaling.

One of the problems of mobile radio signaling systems is the clashing (collision) of messages from different radio units transmitting at the same time. The problems of clashing are controlled by an access protocol which offers high efficiency, stability and flexibility.

Protection against interference is provided by labeling the signaling with a system identity code and, in some messages, the channel number. If heavy interference is encountered, control can be changed to a different channel. To cope with system malfunction, a customized fall-back (fail-safe) mode of operation may be defined by the system designer.

Call Handling

The protocol is designed for use by systems which queue calls that cannot be set up immediately, for example, if no channel is currently available for traffic.

Before a traffic channel is assigned for a call to an individual radio unit, the TSC checks that the called unit is in radio contact, in order to avoid wasted channel assignments. It may also check that the radio unit's operator is ready for the call, to avoid a traffic channel being assigned to an unmanned unit.

Call maintenance signaling is defined for prompt release of traffic channels at the end of a conversation, or in case communication is lost during a call.

As a precaution against fraudulent use of a system by an unauthorized radio unit, the TSC may at any time instruct a radio unit to transmit its unique electronic serial number; comparison of the received electronic serial number with the expected value in the database will assist in the detection of fraudulent users.

7 Multi-site Systems

The standard leaves scope for various multi-site wide-area coverage techniques to be for example:

-synchronous/quasi-synchronous operation

-a separate control channel at each site

-a single control channel shared by time division.

The protocol includes a registration facility to assist the implementation of multi-site systems and networks of TSCS: a radio unit can inform the TSC of its location,4s it roams between sites or systems. (The system identity code distinguishes the signaling from different sites and systems). The standard defines signaling procedures for registration (section 8), but the criteria for registration will be system-dependent.

A TSC can broadcast information to assist radio units hunting for a control channel when they roam; for example, it can announce the channels which may be used for control by itself or by TSCs on adjacent sites.

8 Guide to Some Key Protocol Aspects

This section provides an introduction to the operation of the protocol which, because of its scope and flexibility, is necessarily complex. The section outlines the control channel structure, the random access protocol and some message exchange procedures for call set-up.

This section is intended only as a guide: it should not be regarded as a protocol specification. Readers should refer to the main body of the standard for the complete and precise definition.

8.1 Control Channel Signaling Structure

The signaling for setting up calls is transmitted on a "control" channel. Time on the control channel is divided into slots of duration 106.7 ms (128 bits), and one signaling message can be sent in each slot. Signaling on the forward-channel (base station transmit frequency ) is nominally continuous, with each slot comprising two 64-bit codewords, usually:

i) A Control Channel System Codeword (CCSC).

The CCSC identifies the system to radio units and provides synchronization for the following "address" codeword.

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

Both the CCSC and address codewords are displaced when the Trunking System Controller (TSC) transmits longer messages, with "data" codewords appended to an address codeword.

A radio unit can receive a message from the TSC in one slot, transmit a response in the next slot and then retune to the forward channel in time to decode the following message from the TSC.

8.2 Control Channel Signaling Messages

The messages sent on a control channel may be classified as follows:

Aloha messages - Sent by the TSC to invite and control random access.

Requests - Sent by radio units to request calls/transactions.

"Ahoy" messages - Sent by the TSC to demand a response from an addressed radio unit.

Acknowledgements - Sent by the TSC and by radio units.

Go To Channel messages - Sent by the TSC to allocate traffic channels.

Single address messages - Currently sent only by radio units.

Short data messages - Sent by the TSC and by radio unite.

Miscellaneous;messages - Sent by the TSC for system control.

Some uses of these messages are illustrated in the following sections.

8.3 Random Access Protocol

Principle of operation

One of the problems of mobile radio signaling schemes is the clash of messages from different radio units transmitting at the same time. In this standard, the problems of clashing are controlled by a random access protocol which is based on slotted Aloha, with a superimposed framing structure. The access protocol can be used to minimize access delays, ensure stability and maintain peak throughput under heavy traffic loads.

The TSC transmits a synchronization message to invite radio units to send random access messages.

The ALH message contains a parameter (N) which indicates the number of following timeslots, constituting a frame, that are available for access. If a frame is already in progress when a user initiates a call, the radio unit may send its random access message in the next slot. Otherwise the unit waits for a frame to be started and then chooses a random slot from the frame for its message. A unit wishing to send a repeat transmission after an unsuccessful message (corrupted by fading or clashing) chooses again from a new frame.

9 Achieving Spectral Efficiency

In the U.K. Trunking systems were promoted from the start as a means of improving spectral efficiency. This could be taken as an indication that the Trunking system will do it all, and that there are no other issues to consider.

Nothing could be further from the truth.

When driving any car, the best improvements in fuel consumption are obtained through careful attention by the driver, not by the sophisticated engine management system. Likewise, Trunking will only give of its best if the usage of the system is carefully managed.

There are of course many pressures on network operators, and these will vary in nature and severity around the world. Nevertheless, it may be helpful to highlight two basic factors which have been shown to have a dramatic effect on system load:

The amount of idle chit-chat

and ...

The level of Intersite traffic

In the first case, the solution is to minimize all those features which allow unsupervised conversations. Group calls and mobile to mobile calls can foster a "CB" type environment. The solution is to avoid offering the group call facilities and try to promote RQQ (status call) between mobile and dispatcher, with only the dispatcher allowed to make speech calls. This is very efficient for the network and also for the dispatcher who, after all, is probably picking up the bill for the whole fleet.

Reducing unnecessary intersite calls may be achieved by very careful site planning, judicious selling policy which seeks not to oversell coverage, and the inclusion of "HOME HUNT" in any mobiles which use the network.

At first sight, adoption of these philosophies might seem harsh on the end user. Indeed, in countries where cellular and PCN networks are well established and customer expectations are therefore high, a well thought-out tariff structure may be required.