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

Quick Answer: An Alarm Receiving Centre (ARC) is a 24/7 monitoring facility that receives signals from intruder, fire, and hold-up alarm systems and takes a defined response action. ARCs must be listed on the NSI or SSAIB schemes and meet BS EN 50518 (BS 5979 is the predecessor UK standard). For police response, the ARC must be an NPCC-approved ARC, and the system must use a confirmed alarm protocol. Dual-path IP signalling (primary) with GPRS/cellular backup is now the industry norm.

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

Quick Reference Table

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Signalling Path Protocol Grade Suitability Notes
PSTN (analogue) SIA, Contact ID Grade 1–2 only Being withdrawn; new installs not recommended
IP (broadband) SIA DC-09, SurGard Grade 1–4 Primary path; requires router and internet
Cellular/GPRS SIA DC-09 Grade 1–4 Backup or sole path in remote sites
4G LTE SIA DC-09 Grade 1–4 Faster than GPRS; preferred for primary cell
Dual-path IP+4G SIA DC-09 Grade 3–4 Both paths active; failover < 20 seconds
ATS Category Description Grade Requirement
ATS0 No supervision Grade 1 only
ATS1 Basic PSTN digital Grade 1–2
ATS2 Supervised PSTN Grade 2
ATS3 Supervised IP (single) Grade 2–3
ATS4 Supervised dual-path Grade 3
ATS5 Diverse dual-path (different providers) Grade 3–4
ATS6 Fully redundant dual-path Grade 4

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:

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):

ARC Selection Considerations

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

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:

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