Alarm Receiving Centre (ARC) Connection: Signalling Formats, IP vs PSTN, BS 5979 and URN Application

Summary

When a customer says they want their alarm "monitored," they want a connection to an Alarm Receiving Centre. The ARC is the human and technical infrastructure that sits between the alarm at the premises and whatever response action follows — police call, keyholding company dispatch, or simply a call to the customer.

For the installer, the ARC connection has become significantly more complex since PSTN (BT analogue) began its phased withdrawal, which accelerated sharply from 2023 onwards. New installations should not rely on PSTN as a primary or sole signalling path. IP with cellular backup is the current standard, and many ARCs are now refusing to accept new PSTN connections.

Understanding the ARC's requirements before installation saves significant rework. Different ARCs have different approved communicator lists, different response protocols, and different requirements for confirmation before dispatching a keyholding company or police. Always contact the ARC at design stage, not after commissioning.

Key Facts

• BS EN 50518:2019 — European standard for ARCs; replaced BS 5979 as the specification for UK ARCs; covers staffing, physical security, operational procedures
• BS 5979:2007 — withdrawn UK standard, superseded by BS EN 50518; some references still use this number but EN 50518 is current
• NPCC-approved ARC — police will only respond to confirmed alarms from ARCs listed on the NPCC approved list; verify before connecting
• NSI-listed ARC — NSI maintains a list of ARCs that meet BS EN 50518 and are approved for police URN applications
• SSAIB-listed ARC — SSAIB also maintains an equivalent list
• Signalling path types — IP (broadband), cellular/GPRS/4G, PSTN (analogue — being withdrawn), ISDN (withdrawn)
• Dual-path — IP primary, cellular secondary; required for Grade 3 and Grade 4 systems
• Polling interval — the ARC system polls the communicator at regular intervals; typical 30 seconds to 3 minutes depending on ARC agreement; if no response, generates a fault
• SIA DC-09 — industry standard IP signalling protocol used by most modern communicators; allows encrypted transmission over broadband
• Alarm Signalling Equipment (ASE) — device at premises that formats and transmits alarm signals; formerly called "digital communicator" or "auto-dialler"
• ATS Category — BS EN 50131-5-3 classifies Alarm Transmission System performance from ATS0 (basic) to ATS6 (fully redundant dual-path encrypted); Grade 3 requires ATS3+ for police response
• URN (Unique Reference Number) — issued by local police force to an ARC for a specific premises; required for police response; tied to the ARC, not the installer
• Hold-up alarm (PA) — personal attack/panic alarm signals are transmitted differently to intruder alarms; ARC must treat them as unconfirmed immediate threat
• Response time SLA — NSI Code of Practice requires ARC to answer 80% of calls within 60 seconds

Quick Reference Table

Detailed Guidance

How ARC Monitoring Works

The installed communicator (ASE) maintains a live connection with the ARC's receiver via the signalling path. The ARC's Alarm Handling Software (AHS) monitors this connection continuously.

Polling: The communicator sends a regular "heartbeat" to the ARC receiver. If the heartbeat stops (path failure, communicator fault, or deliberate attack), the ARC generates a "line fault" alarm. The faster the polling, the sooner a path failure is detected — Grade 3 systems typically poll at 30-second intervals.

Event transmission: When the alarm system generates an event (alarm, restore, tamper, zone isolate), the communicator formats it as an SIA DC-09 or Contact ID packet and transmits it to the ARC receiver. The receiver decodes it and passes it to the AHS.

ARC operator response: The AHS displays the event with the premises details, response procedure, and keyholder contacts. The operator follows the procedure — typically:

1. Wait for confirmation (second detector or audio/visual check)
2. Attempt to contact keyholders
3. Dispatch keyholding company or, for confirmed alarms, notify police

PSTN Withdrawal and the Impact on Legacy Systems

BT's PSTN/ISDN switch-off programme will complete by January 2027, with most exchanges already migrated to IP (Openreach Digital Voice). Analogue PSTN lines are being converted to Voice over IP (VoIP) or simply ceased.

The critical issue for alarm systems: traditional SIA protocol digital diallers transmit audio-frequency tones over the PSTN. When PSTN is replaced by VoIP, tone signalling is degraded by codec compression and jitter. Many SIA diallers will fail on VoIP lines even when the customer's voice calls work perfectly.

Practical implications:

• Do not connect new installations via PSTN — any system requiring monitoring should use IP or cellular from installation
• Legacy systems on PSTN — assess at each service visit; plan client migration; the switch-off is a service opportunity
• Openreach Digital Voice is not the same as SIP trunk/ISDN replacement for alarm diallers — dedicated alarm communicators with cellular or IP paths are required

Applying for a Police URN

A URN (Unique Reference Number) grants the police response entitlement. The application process is:

1. Install a compliant system — NSI or SSAIB approved installer, compliant grade (usually Grade 2 minimum), monitored by an NPCC-approved ARC
2. Confirmed alarm protocol — the system must use confirmed alarm (sequential: two detectors in the same incident) before the ARC requests police response
3. ARC applies for URN — the ARC (not the installer) applies to the local police force for a URN for that premises
4. Police issue URN — the URN is recorded at the ARC against the premises; it is not transferable if the customer changes ARC or installer
5. Ongoing compliance — false alarms above the NPCC threshold trigger a warning and ultimately withdrawal of the URN

NPCC false alarm thresholds (current policy):

• 3 false alarms in 3 months → First warning letter
• Further false alarms → Final warning
• Continued false alarms → URN withdrawal; police will not respond

ARC Selection Considerations

Not all ARCs are equal. Key factors when selecting an ARC for customer referral:

• NSI or SSAIB listed — essential for police URN
• BS EN 50518 certification — mandatory
• Approved communicator list — verify your preferred communicators are on their approved list before specifying
• Monitoring protocols — some ARCs use audio verification before dispatch; some require visual verification for Grade 3
• Keyholder management — how well the ARC manages and updates keyholder lists
• Response time SLA — ask for their performance data on alarm answer time
• Pricing structure — monitoring charges vary significantly; factor this into the customer's total cost of ownership

Grade 3 Dual-Path Requirements in Practice

For Grade 3 monitored systems, the communicator must:

1. Use IP as the primary path (SIA DC-09 over TCP/IP)
2. Have a cellular (4G/GPRS) backup path that activates within 20 seconds of IP failure
3. Both paths should be encrypted (AES-128 minimum)
4. Polling intervals should be 30 seconds on each path
5. The ARC receiver must support dual-path and confirm both paths are healthy during commissioning

At commissioning, test the failover explicitly:

• Disconnect the broadband router and confirm the communicator switches to cellular
• Confirm the ARC operator receives the path failure and changeover notification
• Reconnect broadband and confirm switchback

Frequently Asked Questions

What is the difference between "monitored" and "registered" CCTV?

CCTV can be "registered" under BS 8418:2015 without monitoring — it means the system meets the image quality and technical standard for use with an ARC. "Monitored CCTV" means the ARC receives video analytics alerts and a human reviews the video before responding. Most residential CCTV is neither registered nor monitored. See bs 8418 registered cctv for the full BS 8418 requirements.

Does every alarm system need ARC monitoring?

No. A bells-only or self-monitored system (app notification) is a valid choice for lower-risk premises. ARC monitoring is mandatory for police URN applications and is usually required by commercial insurers for significant premises. Discuss with the customer — ARC monitoring adds ongoing cost (£15–£60/month depending on grade and service level).

What happens if the ARC cannot reach any keyholders?

Under most ARC protocols, if all keyholders are unreachable and a confirmed alarm is active, the ARC will notify the police if a URN is held, or log the event as unattended if no URN. Some keyholder companies offer guaranteed response where they will attend even without reaching a keyholder. Document this scenario in the customer's monitoring contract so expectations are clear.

How long does ARC monitoring data need to be retained?

BS EN 50518 requires ARCs to retain alarm event records for a minimum of 12 months. The ARC holds this data as data processor; the premises owner is the data controller under UK GDPR. In practice most ARCs retain 3–5 years of event logs.

Can a business use a self-hosted app instead of an ARC?

App-based monitoring (push notifications to a smartphone) is not equivalent to ARC monitoring for insurance or police URN purposes. It relies on the customer being awake, having a charged phone, and being able to respond. It is acceptable for very low-risk applications where the customer understands the limitation. It does not satisfy insurer requirements for monitored systems.

Regulations & Standards

• BS EN 50518:2019 — Alarm receiving centres; location and construction, technical requirements, procedures and requirements for services; replaces BS 5979

• BS EN 50131-5-3:2017 — Alarm systems; intruder and hold-up systems; alarm transmission systems; requirements and test methods

• PD 6662:2017 — UK application document; ATS category requirements by grade

• NPCC Security Systems Policy — police response conditions, URN application process, false alarm thresholds

• BS 8418:2015+A1:2019 — Installation and remote monitoring of detector-activated CCTV systems; ARC connection requirements for registered CCTV

• Data Protection Act 2018 / UK GDPR — governs personal data in alarm event logs held by ARCs

• NSI ARC Listed Companies — Current NSI list of BS EN 50518 certified ARCs

• NPCC Security Systems Policy — Police policy on alarm response and URN applications

• BSIA ARC Guidance — British Security Industry Association technical guidance on alarm transmission

• Openreach PSTN Switch-Off Information — Timeline and technical implications for alarm systems

• Pyronix/Hikvision IP Communicator Documentation — Example communicator technical guide

• intruder alarm grades — Grade 3/4 requirements that mandate dual-path ARC connection

• bs 8418 registered cctv — ARC requirements for registered CCTV systems

• security system commissioning — ARC commissioning test and handover requirements

• security system maintenance contracts — Ongoing ARC connection verification at annual service