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Lawful analogue telephone tapping unit

ATR, which is short for Automatische Telefoon Registratie (Automatic Telephone Recording), was an automated electronic system for lawful interception of analogue telephone lines, manufactured by the Centrale Werkplaats (Central Workshop) of the Dutch state-owned PTT (now: KPN). It was in use from 7 April 1971 to 15 December 2003, after which it was succeeded by ETSI-NL 1 [1].

The ATR system was developed in-house by the Dutch telecom provider PTT 2 at its prestigious Dr. Neher Lab in Leidschendam (Netherlands). The equipment was built by the PTT's central workshop, which was located in The Hague.

The system generally consisted of a 19" rack that was placed inside the target telephone exchange building, close to the wiring rack of the analogue subscriber lines. The image on the right shows a KOZE interface 3 of the ATR system, as it was used to intercept a single subscriber line. It was connected in parallel to the twisted wire pair.
ATR intercept board

Each interface card was implemented as a 10 x 16 cm Eurocard printed circuit board (PCB), that was fitted inside the 19" rack. Some cards were suitable for the interception of traffic information (metadata) only, whilst other versions were also capable of recording the actual call content (CC).

Depending on the warrant that was issued for a specific intercept job, the appropriate interface was installed, so that the intercepting police officer could not (accidentally) break the law.

As in most other countries, interception of tele­communications in the Netherlands is subject to the Code of Criminal Procedure 4 . At the time, tapping was covered by article 125 sub F (meta­data) and sub G (call content) of the Code. For this reason the board is also known as a 125 interface, and the letters F and G are printed on the front panel to show which variant it is.
ATR interface card front panel

The red front panel of the board shown here is marked F+G, which means that it was capable of intercepting both metadata and call content, in accordance with article 125 F and G of the Dutch Code of Criminal Procedure. The board shown here is mounted on a wooden panel. It was issued as a gift to former users of the ATR system, such as district attorneys and police interceptors, at the event of the introduction of the new ETSI-NL system that replaced ATR on 15 december 2003. The board was commonly tilted, to provide space for a metal plate with the name of the owner.

  1. ETSI = European Telecommunications Standards Institute. In this context it is used as the name of a common telecommunications intercept standard. ETSI-NL is the Dutch implementation of this standard.
  2. At the time, PTT was a state-owned telecom monopolist. In 1989, it was privatized and renamed to KPN.
  3. KOZE = Koppel Overdrager Zender (Link Interface Transmitter).
  4. Dutch: Wetboek van Strafvordering.

Tilted KOZE board on wooden panel ATR intercept board ATR interface card front panel Connection to the rack-mount backplane Top view of the KOZE board Label at the bottom Markings on the bottom side of the board IC that was purposely damaged when the board was decommissioned
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Tilted KOZE board on wooden panel
2 / 8
ATR intercept board
3 / 8
ATR interface card front panel
4 / 8
Connection to the rack-mount backplane
5 / 8
Top view of the KOZE board
6 / 8
Label at the bottom
7 / 8
Markings on the bottom side of the board
8 / 8
IC that was purposely damaged when the board was decommissioned

System setup
The ATR system was developed by the PTT during the course of 1970 and 1971, at a time when there was an increasing demand for lawful interception. Before this time, interception had been possible, but only at an ad-hoc – improvised – scale, commonly inside the telephone exchange.

The system was developed at the Dr. Neher Lab in Leidschendam (Netherlands) [4], at the time one of the most prestigious laboratories in the country. It was part of the state-owned PTT and was responsible for research and development.

Once the ATR development was completed, the actual boards were manufactured at the central workshop (CWP) of the PTT, which was located in The Hague. The image on the right shows the bottom side of the KOZE interface board, which carries the marking CWP. It was designed on 30 July 1981 and was built on 29 May 1985 [1].
Markings on the bottom side of the board

The introduction of the ATR system made it possible to intercept a line without revealing its presence (no mysterious 'clicks' or 'tones' on the line). The intercepted line was routed (covertly) to the KOZE-unit, from which it was despatched to the intercepting party, which was commonly the tapping room of a police station somewhere in the Netherlands, as shown in this diagram:

KOZE-unit placed at the tapping point in the telephone exchange

At the top right is the regular telephone exchange, with the subscriber lines coming it from the bottom right. All subscriber lines are first connected to a wiring rack, or patch panel, from which they are wired to the exchange. At the top left is a single KOZE-unit. Apart from the exchange, the targeted line is also connected to the input of the KOZE-unit. After processing, the metadata and (optionally) the call content, are passed via a rented telephone line to the tapping room.

In practice, the KOZE-unit was placed outside the view of the average PTT engineer, and the intercepting wires were hidden carefully inside the existing wiring bundles, so that it would not be spotted by a casual (or malicious) observer. Exposure could jeopardize a tapping mission.

In the tapping room at the police station, the rented line (carrying the intercepted call) was connected to an ATR-unit that was connected to an external tone-decoder and audio-switcher (the so-called ISRA-unit) that passed the signal onto two UHER Report 5000 tape recorders. 1
UHER 4000 Report-S

The intercepted audio was recorded onto one track of the stereo UHER tape recorders. On the other channel, the female voice of the PTT's time information system, also known as Tante Cor, 2 was recorded. The time came from a central PTT facility and was guaranteed to be correct. In practice, the signals were recorded simultaneously on two UHER recorders connected in parallel.

ATR-unit placed at the (remote) tapping room
Typical analogue police tapping room setup

When the targeted line was in rest, a guard tone was sent from the KOZE-unit at the exchange, to the ATR-unit at the tapping room. The (secret) guard tone was randomly chosen 3 between 1950 and 3050 Hz. In addition a 3150 Hz signal was used as a ringer identification tone. As soon as the line became active, the guard tone disappeared and the ATR-unit activated the recorders. At the same time, a line with date, time and the dialled number was printed on an external printer.

ATR/ISDN2-unit placed at the (remote) tapping room
GSM intercept room via ISDN2 line

Initially, the link between the KOZE-unit and the ATR in the tapping room, was a rented analogue line with randomly selected 3 guard tones. Some analogue lines were later replaced by ISDN lines when tapping digital communications such as GSM. Although the ISDN lines were not protected by encryption, malicious eavesdropping was much harder than with analogue PSTN lines. In the ISDN-variant, the KOZE-unit at the telephone exchange is replaced by a so-called Decentralized Tapping Interface (DTI) that is linked digitally to the provider's digital Ericsson (GSM) exchange, 4 whilst the ATR in the tapping room was swapped for an ATR/ISDN2 unit, with 425 Hz 5 signalling.

  1. The UHER Report 5000 (introduced in 1963) was later replaced by the UHER Report 6000.
  2. Tante Cor (aunt Cornelia), was the nickname of a dial-in information service, exploited by the PTT at the time. When dialled, a female voice would read the current time: bij de volgende toon is het... (at the next tone, the time is...).
  3. If a fixed (known) guard tone had been used, all the targeted party had to do to avoid interception, is send the same tone along with the conversion. As the tone was randomly chosen, this was practically impossible. However if the 425 Hz end-of-call tone was detected for a duration of 100 ms, the recording was stopped.
  4. At the time, the two major telecom providers in the Netherlands, KPN Telecom (formerly: PTT) and Libertel (now: Vodafone), both used the same digital Ericsson exchange for their GSM networks. The DTI-unit was not suitable for intercepting regular analogue telephone calls, for which the KOZE-unit remained in use.
  5. In practice, the recording was sometimes unwittingly stopped if an intermittend 425 Hz tone accidentally appeared during a conversation. This could happen, for example, when violin music was played [5].

A good description of the ATR system is given in a court ruling that was released on 4 July 2017. In this 1730 page document, in which the possibility of manipulation of the various tapping systems are investigated, the ATR and the analogue tapping room are thoroughly described.

Symilar systems
Over the years, various systems with similar features were developed and used in other countries. A good example is the RT-2000 that was made from 1982 onwards by Radio Trevisan in Italy. It is based on an UHER Report 4400, and can record up to four lines simultaneously.

The RT-2000 had a built-in LED display that showed the dialled number, and a built-in thermal printer for evidence registration.

 More information

RT-2000 wire tapping recorder

ATR   Automatische Telefoon Registratie
Autmatic Telephone Registration (or Recording).
ETSI   European Telecommunications Standards Institute
In this context, ETSI is used as the name for a telecommunications intercept protocol. ETSI-NL is the Dutch variant of this protocol, adapted to be in line with Dutch laws [2].
CC   Call Content
The actual conversation in a telecommunications session.
CWP   Centrale Werkplaats
Central workshop of the PTT (now: KPN), located in The Hague (Netherlands).
DTI   Decentrale Tap Interface
Decentralized Tapping Interface. Also known as mediation unit.
GSM   Global System for Mobile communications
World-wide standard for (digital) mobile voice and data communications, developed by the European Telecommunications Standards Institute (ETSI). Originally known as Groupe Spécial Mobile, but later renamed to the above.  Wikipedia
ISDN   Integrated Services Digital Network
Common name for the digital telephone network that replaced the older analogue PSTN service. Currently being superceeded by IP-based switched networks.
ISRA   Infra-Structuur en Rand-Apparatuur
Point of interface (Dutch: koppelvlak) between infrastructure (the line) and peripheral equipment, such as (tape) recorders.
JTS   Justitiële Tapinterface Specificatie
Department of Justice Intercept Interface Specification.
KOZE   Koppel Overdrager Zender
Link Interface Transmitter. Placed at the telephone exchange and connected to the targeted line as well as to a rented line to the tapping room.
KPN   Koninklijke PTT Nederland
Royal PTT Netherlands. New name for the former state-owned PTT, after its privatization in 1989.
LEMF   Law Enforcement Monitoring Facility
Common name for the intercept facilities in the telephone exchange.
MSC   Mobile Switching Center
Telephone exchange for digital mobile communications.
PSTN   Public Switched Telephone Network
Common name for the (old) analogue telephone network. Also known as POTS (Plain Old Telephone System).
PTT   Staatsbedrijf der Posterijen, Telegrafie en Telefonie
Dutch state-owned telecom monopolist from 1915 to 1989. Privatized in 1989 and split into Postbank (bank), KPN (telecom) and TPG (post).
TIIT   Transport of Intercepted IP Traffic
Protocol for interception of e-mail and internet traffic, developed in the Netherlands.
  1. Anonymous, ATR interface board - THANKS !
    Crypto Museum, August 2017.

  2. Bert-Jaap Koop et al., Aftapbaarheid van telecommunicatie
    Interceptability of telecommunications (Dutch).
    Tilburg Universiteit, November 2005.

  3. Wikipedia (NL), Telefoontap
    Retrieved August 2017.  English

  4. Wikipedia (NL), Neherlaboratorium
    Retrieved August 2017.

  5. Conclusie van de advocaat-generaal bij de Hoge Raad der Nederlanden,
    mr. D. Aben, naar aanleiding van het herzieningsverzoek van H. Baybasin.
    Nr. 11/02065 H, 4 July 2014.  pp. 263-271.
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Crypto Museum. Created: Monday 21 August 2017. Last changed: Tuesday, 22 August 2017 - 12:13 CET.
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