Section 1.c

An Introduction to ISDN Powering Considerations


1.C.1 Introduction

This section describes powering considerations for ISDN equipment. Power requirements are dependent upon the installed equipment and a number of personal preferences. Subsequent sections will contain specific powering details.

1.C.2 Powering Requirements

ISDN equipment has different powering requirements than traditional POTS equipment. With POTS, the Telco provides limited power to the customer's equipment. With ISDN, the Telco does not provide the supporting power. However, powering for ISDN equipment is similar to some types of POTS equipment.

POTS equipment can be divided into two categories: basic telephone and High Tech equipment. The basic POTS telephone requires no power when in stand-by mode (on-hook), and very little power when in normal operation (off-hook).

High Tech POTS equipment requires more power than the Telco can provide. Examples of this category of equipment include cordless telephones, fax machines, computer modems, feature phones, speaker phones, and answering machines. This equipment is typically powered from an electrical outlet.

WARNING: Loss of commercial power will require the use of backup power to maintain ISDN service; example: if an answering machine does not work then neither will the ISDN set.

A typical ISDN system of components is shown in figure 1.C.1. The service from the Telco is a single pair U interface which enters the customer location through the demarc. The demarc is then connected to the NT1. Here it is transformed into a 4 pair S/T interface (2 signal, 1 power, and 1 spare). There can be as many as eight pieces of terminal equipment connected to the S/T interface.

1.C.3 Powering Options Specifications

ISDN powering standards define three different powering options on the S/T interface. These are called PS1, PS2, and PS3. Consult the manufacturer's equipment manual for the powering option the selected equipment is using. It is recommended that 4 pair (8 wire) UTP cable, Category 3 or better be used for all new ISDN installations.

1.C.3.1 PS1

For PS1 cable wire assignments see figure 1.C.2. PS1 provides terminal power over the ISDN signaling pairs (4&5, 3&6). To use this option, the user should make sure that all ISDN TE will operate from PS1, the NT1 supports PS1, and the NT1 power supply (see 1.C.3.4) provides enough capacity for all ISDN equipment. The PS1 option is sometimes referred to as phantom powering.

1.C.3.2 PS2

For PS2 cable wire assignments see figure 1.C.3. The PS2 option for powering ISDN is the most widely used of the powering options. PS2 is considered the primary power source to ISDN TE. Under this powering configuration, power is applied to a separate pair of wires. PS2 powering uses pins 7 and 8 of the S/T interface. The signaling pairs use pins 3,4,5 and 6. Pins 1 and 2 are not used. It is recommended that the unused wires be reserved as spares.

1.C.3.3 PS3

For PS3 cable wire assignments see figure 1.C.4. PS3 is specified to power the ISDN TE through pins 1 and 2 of the S/T interface. The PS3 option is not presently used by the majority of CPE vendors, however, pins 1 and 2 should be connected at both the TE and NT1. Pins 7 and 8 are not used. It is recommended that the unused wires be reserved as spares.

1.C.3.4 Powering the NT1

The standards permit that the power to the NT1 can be supplied on pins 7 and 8 of the U interface connector from an external power source. There may be additional powering options available. The NT1 may pass power to the ISDN terminals using PS1 and PS2, as discussed above.

1.C.4 Powering Equipment Location

1.C.4.1 Local, Remote, and Distributed Power

These guidelines describe three locations for powering equipment; local, remote and distributed. Local power is defined as a power supply co-located with the equipment utilizing that supply. In this type of powering arrangement, the NT1 and TE will be co-located in the same room or possibly integrated in the same unit. The power supply will be sized to power the NT1 and TE (see figure 1.C.5.)

Remote Central or Bulk Power is defined as a power supply which is found in some location other than where the equipment is located (see figure 1.C.6.)

Distributed power is any combination of local and remote power sources. In figure 1.C.7, distributed power is provided by one remote supply and two local supplies.

1.C.5 Factors Affecting Power Equipment Sizing

1.C.5.1 Distribution Devices

The location and type of distribution devices will vary with the installation and age of installation. The most important issue regarding power is not the type of distribution device, but the number of wires it supports. If the device can not handle 8 wires then a new distribution device should be installed. See section 1.A.2.4 for information on types of distribution devices.

1.C.5.2 Power Supply Cable Length Considerations

Provided that the wire used is 24 gauge, all of the wiring configurations recommended in this document are compatible with power options described in this section.

1.C.5.3 Location and Environment

Choose a location which is easily accessible and has sufficient room for all of the equipment. The area should be cool, dry, and well ventilated. Consult the vendor's manual for recommended operational ranges and local codes for safety issues.

1.C.6 Powering and Wiring Variations

1.C.6.1 Powering of Terminal Equipment Using Existing Quad Wiring.

With existing quad wiring, local powering must be used. Bulk powering and distributed powering options require more wires than are currently available. Local powering is defined in section 1.C.4.1.

1.C.6.2 New Wiring to a Single Target Room

Wiring to a single target room can use either local or bulk power. There are no special considerations here for powering as wire installed for ISDN phone systems will provide an adequate powering source.

1.C.6.3 New Wiring to Multiple Target Rooms

Wiring to multiple target rooms could consist of local power in each room, bulk power at one central location, or distributive power at different locations.

1.C.6.4 Terminal Equipment with Internal NT1 Functionality

Some equipment have NT1s embedded inside the equipment. Equipment of this type provide a U-Interface jack and may also supply an S/T interface jack on the equipment.

1.C.6.5 Backup Powering Alternatives

1.C.6.5.1 Remote NT1 Powering

Figure 1.C.8 shows a situation where the NT1 and the power supply with backup power source (UPS or battery) are located in different parts of the house. In this case it is necessary to backfeed power from the TE location (Den) to the NT1 and the other ISDN sets, to achieve emergency power operation. Power is supplied to pins 7 and 8 of the S/T interface in the Den, using a 2:1 adapter, to power the NT1, located near the Demarc. For this application to work the NT1 must support PS2 and be able to accept power input on pins 7 and 8 of its S/T interface; check with your vendor.

1.C.6.5.2 Selectively Powering Phones

In Figure 1.C.9, only the ISDN set in the den is required to have backup power. The other sets are locally powered. The considerations are identical to those described in 1.C.6.5.1 with the exception that pins 7 and 8 going to the kitchen and bedroom are disconnected at the distribution device. Using the distribution device, you can selectively apply power to any desired destination, and all combinations of local and remote powering are possible.

1.C.7 Power Supply Alternatives

1.C.7.1 Equipment Descriptions

There are three types of equipment which may be used for powering ISDN equipment. The first type of equipment is a power supply without battery backup. The second is a power supply with battery backup. The third type is an Uninterruptable Power Supply (UPS).

1.C.7.1.1 Power Supply without Battery Backup

A power supply is defined as a unit which will take commercial power and provide DC power out to run ISDN equipment.

1.C.7.1.2. Power Supply with Battery Backup

Battery backup may be added to a power supply to maintain equipment operation in the event of commercial power failure.
1.C.7.1.3 UPS

A UPS is a powering system that takes in 115 VAC and outputs 115 VAC. When the original input power is disrupted and is no longer being provided by the local utility, the UPS will continue to provide 115 VAC to the load. Typically, internal batteries power an inverter to provide the 115 VAC, uninterrupted power source.

1.C.8 How to Select the Power Source

1.C.8.1 Determining Power Source Type

Each user must choose power to fit the system needs.
Three options are available.
€ A DC power supply only. This provides no backup. The ISDN equipment will not work during a commercial power outage unless the NT1 and the TE contain internal reserve power (battery).
€ A DC power supply with backup power. Backup may be in the DC power supply or added by an optional external battery. You need to size the power supply and the battery.
€ A DC power supply and a UPS. The UPS provides backup AC power to the DC power supply. You may have a UPS for your computer. The UPS may have extra capacity to power the DC power supply.
In all cases, you need a DC power supply.

1.C.8.2 Sizing the DC Power Source

The sizing of the power supply needs to be known so that the appropriate power supply can be purchased. This is found by determining the powering requirements of all the equipment (ISDN phone, NT1) used in the system. These powering requirements can be found by consulting the individual owner's manuals of the ISDN phone and NT1. See appendix 1.C.1 for an example on how to size the power supply. The DC power source may have an optional battery backup; see section 1.C.8.3.

1.C.8.3 Backup Power Source

1.C.8.3.1 Battery Selection for DC Power Source

We will help the user make a first order estimate of the battery size for his application. The following variables will effect the real performance of the battery. Batteries are not linear with time vs. current draw (see graph 1.C.1). Batteries are affected by ambient temperature. Usage will also affect the battery time performance. The amount of reserve capacity available is also dependent upon time since last power outage. For battery selection consult the DC power source's manual.

1.C.8.3.2 UPS Power Source

There are other possible powering configurations that can be explored. In some installations, a UPS system may be on site for the purpose of providing backup power for computer systems which are connected to ISDN communication lines and/or other systems. If this is the case, the UPS should be checked for proper rating and load prior to connecting the ISDN communication equipment.

The use of a UPS will not change the ISDN powering specification mentioned above. PS1, PS2, or PS3 powering options would still power the communication equipment. The UPS would maintain the AC power to the communication equipment power supplies.
Appendix 1.C.1

Sizing DC Power Source

To size a DC Power Source simply add together all of the loads in order to select a commercially available ISDN power supply.

Example 1.C.1: Our system will have one line entering the building. We will also have one ISDN telephone. The owner's manual for the telephone states the phone requires 3.75 watts of power. The owners manual for the required NT1 states the NT1 requires 1 watt of power. By adding these together we obtain 4.75 watts.


Eq 1.C.1.1:

Power Phone 1 + Power NT1 = Nominal Power Required

3.75 w + 1 w = 4.75 w

You must now add 10% to the above figure to arrive at the minimum capacity required of your power supply. In the above case, the supply must provide at least 5.225 w. Select a supply that delivers at least this amount of power.
Work Sheet

Sizing DC Power

How many NT1s are required? ________________

How many terminals are required? ________________


Power Power Total
NT1 Terminal Power

______ + ___________ = _______________

______ + ___________ = _______________

______ + ___________ = _______________

______ + ___________ = _______________

______ + ___________ = _______________

______ + ___________ = _______________

______ + ___________ = _______________

______ + ___________ = _______________

______ + ___________ = _______________

______ + ___________ = _______________

______ + ___________ = _______________

______ + ___________ = _______________

______ + ___________ = _______________

______ + ___________ = _______________


Nominal System Power Total _______________


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Saturday, December 22, 2001