CPE power supply methods
Customer premises equipment connected to a PSTN line may be powered by two alternative methods as follows:-

(a) Line powered, which relies entirely on the d.c. supplied from the line for all its electrical functions.

(b) Locally powered, which has its own power source for the main functions

(i.e. speech transmission, etc.), but relies on the line feed for supervisory functions.

* 'Locally powered' includes battery powered devices such as some types of Caller Display Units. Return to Contents

Function of d.c. line feed
(1) For line powered devices, the d.c. line feed from the exchange performs two basic functions by providing power for the following:-

(a) Supervisory and signalling functions.

(b) Speech signals and general operation of customer equipment.

(c) Maintenance of memory facilities in customer equipment.

* (a) and (b) are dependent on d.c. line current in the off-hook condition. (c) however is normally maintained by the very small current flowing when the customer equipment is on-hook.

(2) For both line and locally powered equipment, satisfactory performance is dependent on the d.c. characteristics of that equipment satisfying the off-hook requirements of clause 6.3.

(3) With certain exceptions (see sub-clause (4) below), all customer equipment shall continue to operate normally, and the requirements of this Specification shall be met independently of changes to the d.c. line conditions described in Section 4 of Technical Document TNA 102. Typical changes are short breaks, transient votages, line polarity, etc.

* Changes to line polarity may occur as follows and customer equipment shall not be affected by such changes:-

(a) Rearrangements within the network may result in reversal of the line polarity. Customer wiring also does not necessarily maintain polarity.

(b) Line reversals can occur before during or after the application of ringing and at other times during a call.

(4) Some locally powered devices are dependent on a particular line polarity (e.g. some PABX's, etc) for correct operation. This requirement shall be made clear to both customers and installers, and the matter will need to be discussed between installers and local Telecom staff at the time of installation.

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Off-hook d.c. line characteristics
(1) For all devices designed for connection to a PSTN line, to ensure satisfactory operation, the d.c. voltage versus current characteristics in the off-hook condition shall be no greater than the upper limit curve shown in Fig. 6-1. This curve is formed by straight lines joining the following coordinates:-


(2) In order to improve the chances of call handover where applicable, the off-hook d.c. characteristics should also not be less than the lower limit of the curve shown in Fig. 6-1. This curve is formed by straight lines joining the following coordinates:-


* Although not mandatory, this requirement is strongly recommended for all devices with the possible exception of facsimile equipment and data modems, where handover is unlikely to occur.


(3) For voice equipment with d.c. characteristics that lie beneath the lower limit curve shown in Fig. 6-1, the user instructions shall contain a warning as follows:-

"This equipment may not provide for the effective hand-over of a call to another device connected to the same line."

(4) All products shall be capable of full operation down to a line current of 20 mA.

* Satisfactory operation at line currents down to 18 mA is desirable as this may occur on occasions, particularly from derived circuit systems during mains power failure.

* Notwithstanding compliance with these requirements, no guarantee can be given that effective transmission will be achieved between or by two or more items of equipment which are connected to the same line. Return to Contents

Supervisory conditions
(1) Line seizure: Customer equipment shall initiate an outgoing call (seize an exchange line) by going off-hook and drawing d.c. line current from the PSTN line (see also clause 8.1 for devices with automatic line seizure facilities).

(2) Ring trip: Similarly, customer equipment shall answer an incoming call (trip the incoming ring), by going off-hook and drawing d.c. line current from the PSTN line.

(3) Call release: The equipment shall terminate a call by going on-hook and breaking the line current. At this point it shall be ready to receive a subsequent incoming call.

(4) It is strongly recommended that hookswitch contacts are 'debounced' to avoid spurious line seizure following call release.

(5) To ensure satisfactory performance of each of these supervisory functions, the device shall satisfy the mandatory requirements given in clause 6.3.

* An increasing number of line feeds have limited current sources (such as those provided by "derived circuit" equipment and many PABX's). For this reason it is not permitted to have customer equipment incorporating constant current sinks as a means of holding the line in a seized condition. This is reflected in the d.c. characteristic upper limit curve having a finite slope from the 20 mA minimum rather than a vertical constant current characteristic.

(6) Any CPE device that relies entirely on a local power source for its operation (either battery or mains) shall automatically release any call in progress within 5 seconds of failure of that power source.

* This does not necessarily apply if some functions are line powered. For example, a mains powered answerphone with a line powered handset does not have to release a call while the handset is being used on a call in progress.

* See also Section 8 for requirements for automatic calling and automatic answering devices.
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Hold Condition
* This facility typically places a condition on the line to maintain a call in progress while the user makes a call on another line, or wishes to transfer to another device to retrieve the first call.

(1) Where a device has a manually operated 'hold' (commonly used for call 'handover') facility, such a facility shall be subject to at least one of the following conditions:-

(a) some part of the equipment, such as a control switch, etc, shall be clearly seen to be in other than the normal idle condition, or

(b) some form of lamp or signal shall be provided to clearly indicate that the terminal equipment is in the 'hold' condition, or

(c) the terminal equipment shall time out the 'hold' condition and release within 3 minutes.

(2) When 'hold' is activated, the device shall maintain the line current at not less than 15 mA when connected to a nominal 24 V, 400 ohm source.

(3) Following activation of the 'hold' condition as described in sub-clause (2) above, full release to the on-hook condition shall occur within 500 ms of separate application of each of the following:-

(a) A 450 ohm resistive shunt.

(b) The same type of terminal device, connected in parallel to the same source, going off-hook.

(c) Disconnection of the line current source.

(4) It is strongly recommended that the 'hold' condition should be inoperable if another device on the same line is off-hook.

(5) On series connected automatic dialling devices, the dialling feature shall release from the line and hand over control to the associated telephone within 1 second of sending the last digit of the called number.

* The Telecom network now makes extensive use of No. 7 signalling, such that the call may be switched through immediately the last digit is received. Should the dialling function hold across the line, it may prevent the associated telephone receiving sufficient line current to turn on. If the called party answers immediately, any delay in completing the speech path may result in that party dropping the call.

(6) The dialling feature may initially drop back to a hold condition, in accordance with sub-clause (3). In such cases, the hold condition may be indicated by a message on the associated display to the effect "Pick up phone".

* Because of their varying characteristics, not all telephones and automatic diallers can be relied upon to work in combination or provide an effective handover from one to the other. Return to Contents

Line breaks
The duration of any undefined interruption or random break in the d.c. path caused by the equipment shall not exceed 9 ms. Return to Contents

On-hook line current
(1) The total current drawn by all equipment connected to a PSTN line within a customer's premises in the idle condition shall not exceed 1 mA.

(2) In the on-hook state the direct current drawn from a nominal 50 V supply by any single item of equipment shall be not greater than 120 µA.

* The on-hook current drain should be as low as possible to allow customers the freedom to install several telephones without undue risk of exceeding the maximum value for the line.
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Recall and call waiting
(1) The recall facility is in the form of a timed break in the holding loop condition during a call, and it is known as "timed-break recall" or "TBR". It is strongly recommended that this facility is provided on telephony devices.

(2) To activate call waiting, it is necessary to signal a break equal to the TBR signal. If a separate 'call waiting' key is provided, it shall either provide the same facility as the TBR or be rendered inoperable and suitable mention made in the User Instructions.

(3) Where provided, the TBR of customer products shall be within the range 300 ms to 800 ms. If a product cannot be set to operate within this range then the TBR function shall be disabled. Also, a suitable explanation shall be inserted in the User Instructions advising users that the facility is not available.

(4) For the highest probability of correct operation in all situations, TBR should be within the range 500 - 650 ms.

* Following investigations, it has been found that there are a number of earlier exchanges which require TBR to be within 500 - 800 ms to ensure consistent and reliable operation.

* While most modern PABX's will accept a lower TBR on their extension ports, some earlier PABX products, many of which are still in service, require that TBR be in the range 300 - 650 ms.

(5) Since 100 % certainty of operation cannot be guaranteed for products with TBR of less than the preferred minimum, the following warning notice shall be included in the User Instructions for all products providing a TBR of less than 500 ms:-

"The recall facility of this product may not always operate correctly on some parts of the Telecom network or on some PABX's available on the market. If this is the case then 'recall' can be activated by 'flashing' the switch-hook or repeat use of the recall button.


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Series connected equipment
6.9.1 D.C. line characteristics
(1) Some series connected devices provide a continuous d.c. path between ports at some stage of operation. On such devices, the sum of the total voltage drops between ports via each leg shall, for the off-hook condition, not exceed the limit curve shown in Fig. 6-2, which is formed by straight lines joining the following coordinates:-

0 - 30 mA3 V
100 mA6 V


(2) Some series connected devices perform a switching function during ringing and it is necessary to restrict any 'in-rush' current that may occur. In such cases, the current drawn at the line port shall fall to a level not exceeding 5 mA within a period of 10 ms following the connection of a call to a terminal equipment port. The following test set-up shall be used (see Fig. 6-3):-

(a) With a 50 V dc source in series with a 400 ohm resistor connected directly to the line port, and,

(b) a series network consisting of a 5 µF capacitor and a 1800 ohm resistor connected to the terminal equipment port.


(3) For connections between the line port and each terminal port of multiple port devices, the requirements of sub-clauses (1) and (2) above shall apply (see Fig. 6-3).

* This ensures that the local exchange will not detect any 'in-rush' current caused by the capacitive loads as an off-hook condition. The time at which a connection is made to the terminating equipment cannot be clearly defined, and may be outside the period during which the local exchange equipment is desensitised to in-rush current.

(4) All series connected equipment shall, in conjunction with any terminating device, satisfy the requirements of clauses 6.3 to 6.8.

* For 2-wire - 4-wire - 2-wire devices (see Fig. 4-5 in Section 4), the 2-wire PSTN line port provides termination of the d.c. path and it is important that it satisfies all d.c. requirements, even though it is not strictly functioning as a terminating device.

(5) For some series connected automatic dialling devices when dialling, it is necessary for both the device and an associated telephone to terminate the line at the same time. Depending on the characteristics of the telephone, this may present difficulties as there may be insufficient current to serve both devices.

* See also clause 5.6.4 and the associated mandatory warning notices.
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Call pick-up
Where appropriate, provision may be included on series connected devices for incoming calls to be picked up by terminal CPE other than that receiving an incoming ring. This can be achieved by going off-hook and sending a suitable signal back to the series connected device to divert the incoming call.

* This is a common feature on PABX's and may soon be regarded as normal for multiple port Distinctive Alert decoders. Return to Contents

Call collision
(1) A common problem for series connected switching devices is for an outgoing call attempt to occur while an incoming call is being processed, i.e. call collision. To avoid this possibility, it is recommended that the switching device returns busy tone to all ports during the processing period.

(2) The normal network busy tone is defined in Document TNA 102, clause 7.1. It may however be desirable to consider use of a more urgent "busy" for this application.

* This recommendation applies particularly to multiple port Distinctive Alert decoders.

* Care must be taken to make sure no lockup situations arise from solutions to call pickup (clause 6.9.2), and call collision.
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Line grabbing equipment
(1) Some series connected devices incorporate a line grabbing function whereby all other equipment normally connected is isolated from the line (see clause 10.7.1). The following conditions shall apply:-

(a) Where the line grabbing function is designed to seize the line even when another call is in progress, the equipment shall terminate the existing call by opening the loop for at least 2 seconds before initiating the next call.

(b) To initiate a subsequent call, the equipment shall comply with the call initiation requirements of clause 8.1.2(1).

* It should be noted that a few older generation Telecom exchanges provide for a calling party to hold a for up to 8 seconds when the called party breaks the connection. In such cases, a line grabbing device may not be able to release an existing call and initiate a new call unless a suitable delay is incorporated, or several attempts are made and dial tone is detected before signalling commences.

(2) Line grabbing equipment designed to make automatic test calls shall incorporate in the User Instructions suitable information advising the customer of the conditions to be experienced if they are using the line when a test call occurs. Routine test calls should preferably be delayed until the line is free.

* Disconnecting, or dialling over, a call in progress inconveniences customers and is only really necessary in an emergency.

(3) The following warning notices shall be inserted in the User Instructions for all line grabbing equipment:-

"This equipment is set up to carry out test calls at pre-determined times. Such test calls will interrupt any other calls that may be set up on the line at the same time. The timing set for such test calls should be discussed with the installer"

"The timing set for test calls from this equipment may be subject to 'drift'. If this proves to be inconvenient and your calls are interrupted, then the problem of timing should be discussed with the equipment installer. The matter should NOT be reported as a fault to Telecom Faults Service"

* A typical example of such equipment is an alarm system designed to make calls in an emergency. Return to Contents

D.C. line feed to other devices
(1) For series connected equipment, all ports for connection to other items of CPE shall provide a d.c. line feed. This may be sourced from the PSTN as specified in clause 6.9.1 or locally powered.

(2) There are two alternative recommended methods of locally powered line feed as follows:-

(a) 50 V d.c., fed via a constant impedance source (commonly 400 ohm), with current usually limited at around 60 - 80 mA.

(b) Constant or restricted current in the range 18 mA to 45 mA.

* It is expected that, for most applications, clause (2)(b) will apply. Method (2)(a) is that normally used in Telecom exchanges.

* The line feed is normally applied as negative battery (relative to earth) on one wire and earth on the other. This is not however mandatory for this application. CPE devices are required to be polarity insensitive so it is not necessary to specify a particular polarity for line feed.

(3) The User Instructions shall clearly state any restrictions on the loop resistance of line between the series device and the CPE connected to it such that sufficient current can be drawn to establish the off-hook condition.

* Equipment covered by this clause, though series connected, is normally expected to be installed within the same premises as the terminal CPE. As a result the line length will normally be extremely short.

* For the small number of cases involving devices which are designed to be installed remote from the terminal CPE, consideration must be given to the Telecom local line limits, i.e. the limit will apply to the total of the length of line between exchange and device plus the length of line between the device and the terminal CPE. For such devices, each application will be considered on its individual merits.

(4) Ripple components shall not exceed 2 mV psophometric.

(5) The line feed shall be such that the equipment recognises the CPE state as follows:-

(a) Off-hook, when the d.c. feed current is greater than 15 mA for a period not less than 10 ms for a load of 1000 ohm.

* When ringing is being sent to CPE, the ring trip d.c. would normally have to be maintained for 40 ms or more.

(b) On-hook, when the d.c. feed current is less than 5 mA for a period not less than 1000 ms for a load of 10 kohm.

(6) Many CPE devices have features such as 'last number redial' or 'memory dial' which depend on a small on-hook line current for the maintenance of memory information. The line feed provided by series connected switching devices shall either:-

(a) provide a d.c. power source capable of supplying a continuous on-hook current of at least 150 µA in order to maintain such memories, or,

(b) have a clear warning inserted in the User Instructions advising users that such terminal devices used with the series connected device may lose their memory functions.

* Series connected devices without switching facilities will normally apply feed voltage to all terminal CPE ports in the on-hook condition anyway.

(7) The User Instructions shall give some indication of the maximum number of parallel CPE devices that can satisfactorily be operated on one port. This number of devices, however, is subject to the limitations of clause 7.7.2 which means the maximum will never exceed 4 or 5.

* Individual items of CPE are normally restricted to drawing no more than 120 µA from the line in the on-hook condition. This limit may be used as a means of establishing the number of items likely to operate. Return to Contents

Fail-safe requirements
All customer equipment
(1) On restoration of power after a power failure, all equipment powered by a.c. mains, shall comply with the following:-

(a) Revert to or continue in the on-hook state without seizing the line, and

(b) Remain in the on-hook state until another incoming or outgoing call is initiated.

(2) All battery powered equipment shall fail safe and not cause any harm to the Telecom network under the following conditions:-

(a) If the life of its batteries is approaching a critical point.

(b) If the battery voltage falls for any reason to that level which could cause the device to malfunction. (3) If, under any power failure conditions, the basic telephony functions of any device are lost then the following warning notice shall be included in the user instructions:-


Under power failure conditions, this telephone may not operate. Please ensure that a separate telephone, not dependent on local power, is available for emergency use" Return to Contents

Additional requirements for series connected equipment
(1) Other than on 4-wire devices for which fail-safe arrangements are difficult to achieve, every possible step shall be taken to maintain basic telephone service when a failure of series connected equipment occurs. To this end, the fail-safe mechanisms stated in the following sub-clauses shall be provided:-

(2) Multiple port devices (see Fig. 6-3):-

(a) In the event of a power failure, the line port shall be directly connected to at least one of the terminal equipment ports. The port or ports shall be clearly marked and shall also be clearly indicated in the User Manual. * On Distinctive Alert decoders, it would be normal to connect the DA1 port directly to the network.

(b) In the event of a logic or component failure, the device shall be designed to fail to a predetermined state, details of which shall be given in the User Manual.

* Techniques such as 'watch dog' timers and relays which connect through in their power off state are typical solutions which would achieve the above requirements.

* In the event of failure, it is normally expected that all telephones are connected, but not automatic answering devices.

(3) Single port devices:-

(a) It is strongly recommended that the failsafe mechanisms necessary for multiple devices be provided on single port devices.

(b) User Manuals associated with single port devices shall include details of whether or not there is any failsafe connection provided in the event of any type of failure. Return to Contents

PABX call waiting
On some PABX's, the call/message waiting indicator on extension devices is activated by a 100 V d.c. signal from the PABX. CPE items with this facility shall satisfy all electrical safety requirements of Specification PTC 101.

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