Signalling Systems

Purpose of this information sheet

To explain the Agency’s policy on the use of signalling tones or codes on Private Business Radio Systems (PBR).

This information sheet covers the following types of signalling systems:

• Continuous Tone Controlled Signalling System (CTCSS)
• Digitally Coded Squelch (DCS)
• Selective Calling (SELCALL)
• POCSAG
• Hexadecimal Sequential Coding (HSC)
• Dual-tone Multi-frequency (DTMF)

Why are tones/codes needed?

Where different users share a radio channel, use of signalling codes/tones enables groups of users (i.e. a single business) to communicate without receiving their co-channel sharers’ (i.e. another business) transmissions.

When a radio user has to listen to not only the required, but also an unwanted transmission throughout the day, there is a tendency to mentally switch off. The wanted message may then be missed.

The simple expedient of fitting a signalling code/tone means that when a user hears a transmission, although it may not be for them, it will be for their user group and not for a channel sharer (i.e. another business).

Different types of signalling

CTCSS

This is an abbreviation of Continuous Tone Controlled Signalling System, also known as Continuously Tone Controlled Sub-Audio Squelch.

The important word being “Continuous”as the tone will be present throughout the entire transmission until the Press to Talk button (PTT) on the equipment is released. Different terms for the same function include “Tone Lock”, “Private Line” or “PL”.

One of 32 available tones is allocated to each user. Each tone is transmitted continuously during each transmission, however, this tone is below the normal speech frequencies passed by the receiver and cannot be heard in the loudspeaker of the receiving set.

The way in which the facility works can be likened to a lock for a door; only the correct “key” from the available 32 will ‘unlock’ the receiver “door” and allow the message to pass through.

CTCSS can be used with SELCALL (see page 2 ) if required as this uses its own discrete frequencies.

Digitally Coded Squelch

Digitally Coded Squelch (DCS) works in a similar way to CTCSS. It is an unaudiable digital code sent with each transmission. The code is recognised by the selected receivers enabling them to receive the transmission. Receivers that do not recognise the code remain silent.

The DCS codeword consists of a 23 bit frame which is transmitted at 134.4 bits/sec. As this frequency is below that normally used for speech on Private Business Radio (PBR), it is a fairly simple matter to filter out the tone in the receiver so the user is unaware of it. There are a total of 104 codes available but only 52 codes are normally assigned for use on PBR bands, this is because the other 52 are the inverted codes (also known as complementary codes) e.g. a codeword such as 10010 is the complement of 01101.

DCS can be used with SELCALL (see over) if required as this uses its own discrete frequencies.

Selective calling

Selective calling, also referred to as either “Selcall” or “Five tone”, is a means of generating tones for identifying individual or groups of radios. The applicable code of practice to cover the installation, adjustment and use of Selcall is MPT 1316. The actual tone system used is HSC (see overleaf for details).

Some users have had both CTCSS or DCS and Selcall fitted, but some manufacturers equipment is unable to offer full Encode and Decode for both.

HSC

Hexadecimal Sequential Coding [HSC] is the system normally used for SELCALL. A number of audio tones (usually five) are transmitted sequentially as a contiguous group. Each subsequent tone is a different frequency to the preceding one. These differing tones are easily detected by audio filters or by computer software. A tone set comprises ten different tones and an eleventh as the repeat tone. Also in the set is a group tone plus four others (usually designated as A, B, C and D).

Licensing

Assignment of CTCSS tones / DCS codes

All licensees are automatically assigned both a CTCSS tone and a DCS code whenever a licence is being issued or amended.

On issue of a PMR (Standard) Licence which authorises the licensee to operate a number of handheld radios (no base station) anywhere in the country, the Agency will automatically assign three DSC codes and CTCSS tones. This is to allow the licensee a degree of flexibility in the event of a co-channel interference problem. The Agency strongly recommends that either CTCSS or DCS is installed and functional from the outset of all new systems, and, subject to it being capable of ready installation, following a technical change to an existing system. If interference is reported by a licensee, Agency Technical Investigation Officers will not take any action until the assigned tone / code as detailed on the licence schedule has been installed; providing that is, the equipment in use is capable of having it installed.

Use of Selcall

The use of Selcall for the identification of an individual or a group of mobiles on shared PBR channels is authorised automatically on a Wide Area or On-site PBR Licence.

Equipment

Nll equipment used must comply with the United Kingdom Radio Interface Requirement published by the Radiocommunications Agency in accordance with Article 4.1 of Directive 1999/5/EC of the European Parliament and of the Council on radio equipment and telecommunications terminal equipment (RTTE) and the mutual recognition of their conformity. Equipment which was first placed on the market before 8 April 2000, must meet a current and relevant type approval specification details of which are contained in Information Sheet RA108 available from our library and information service (see page 3 for contact details).

It is the licensee’s responsibility to ensure that the system meets these requirements and complies with the technical parameters detailed in the schedules. The licensee must be capable of providing acceptable documentary evidence, on request within 28 days, for all or any part of the system to a representative of the Secretary Of State for Trade and Industry.

Common Base Stations

Common Base Station (CBS) Operators should apply in writing to the Licence Centre which normally deals with their licence.

Up to four groups of codes will be assigned from the list given below. Should you wish to request a specific code group you may do so, although no guarantee is given that it can be assigned to you. Licensees who are already using CTCSS may additionally have a DCS code groups assigned to them which they may use in conjunction with their CTCSS tone. A CBS using more than one channel on a site will have the same DCS code groups allocated to all co-sited channels.

Some manufacturers’ equipment transmits the DCS codeword by increasing the carrier frequency for a logical 1 and decreasing it for a logical 0 (positive carrier shift). Other equipment uses the inverse, ie. decrease the carrier frequency for a logical 1, increase it for a logical 0 (negative carrier shift).

This could lead to equipment operating on one system decoding both its own correct code and the logical invert of incompatible systems in adjacent areas. To avoid this, the logical inverts of allocated codes are omitted from the code groups, thus allowing operators a free choice of equipment employing either positive or negative carrier shift.

Logical inverts may be utilised on a site-by-site basis to double the codes available, provided all equipment used on a particular site employs exclusively either positive or negative carrier shift.

For example, in the case above an operator using octal code 431 may use octal code 723 without mutual interference if all equipment employed uses positive carrier shift.

Any questions you may have about the use of DCS on Common Base Stations should be directed to your radio dealer or to the Agency’s Public Telecommunication Networks Section (PTN) on telephone: 020 7211 0238

DCS code groups for Common Base Station (CBS) operators

Additional Information

For further information regarding the use of signalling codes, please contact your normal Agency Local Office, details of which can be found on information sheet RA206 available from the library (details below).

Library & Information Service
Radiocommunications Agency
Wyndham House
189 Marsh Wall
London
E14 9SX

Tel: 020 7211 0502 or 0505
Fax: 020 7211 0507
E-mail: library@ra.gsi.gov.uk

 

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RA 373 JULY 2000 FOL 436