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Loop start vs. ground start

BASIC TELECOMMUNICATIONS

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Analog Loop Start Signaling is commonly associated with POTS lines and PBX systems

A loop start is a supervisory signal given by a telephone or PBX in response to the completion of the loop circuit while in the off hook condition.

In the on hook or “idle” condition the tip line is grounded at the CO and -48VDC steady state voltage from the CO is present on the ring line

BASIC TELECOMMUNICATIONS

3

Remember that Tip (GREEN) and Ring (RED) are the voice pairs for a POTS line.

On hook status means that the telephone handset is hung up and off hook status means that we’re seizing the line to make a call

So while the phone is not being used (on hook) there is a line voltage present on the line (-48VDC)

BASIC TELECOMMUNICATIONS

4

In the circuit below (idle condition) when the handset is lifted the hook switch closes and -48VDC is connected back through the tip wire to ground at the CO/PBX causing current flow.

BASIC TELECOMMUNICATIONS

ANALOG LOOP START SIGNALING

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This action seizes the line and the current flow is detected at the CO/PBX which opens the ground path and applies a nominal -20VDC voltage followed by the connection of a digit receiver and a dial tone generator to the tip wire.

BASIC TELECOMMUNICATIONS

ANALOG LOOP START SIGNALING

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The nominal -20VDC on the tip wire reduces the nominal -48VDC on the ring wire to about -28VDC. This results in a 6 to 8VDC voltage difference between the two wires at the telephone and a lower current flow

BASIC TELECOMMUNICATIONS

ANALOG LOOP START SIGNALING

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ON HOOK TO O OFF HOOK

BASIC TELECOMMUNICATIONS

8

With a required internal DC resistance of 200 to 300 ohms across the telephone in the off hook condition, current flow is approximately 25 milliamps

The telephone AC impedance must match the AC line impedance of 600 ohms

What is Impedance and what is the unit of measurement for Impedance?

BASIC TELECOMMUNICATIONS

9

After dial tone is present, the number can be dialed and at the start of the first digit the dial tone is removed.

At the end of the last digit dialed the digit receiver is disconnected and the voice (audio) circuit is established at the caller

The call is switched through the CO/PBX and through the PSTN to the next CO/PBX and then to the called phone number

BASIC TELECOMMUNICATIONS

10

At the called telephone number a ring generator is connected to the ring line and a train of nominal -90VAC ringing voltage pulses is periodically applied

The resulting AC current passes through the ringer coil and back through tip wire to ground, causing a periodic ringing sound to occur at the called telephone

BASIC TELECOMMUNICATIONS

11

While the called telephone is ringing a ring back tone is sent by the CO to the caller to indicate the phone is ringing

If the telephone is busy, then a busy tone is sent to the caller

If the handset is lifted then the ring generator is disconnected and the hook switch completes the circuit for a talk path

BASIC TELECOMMUNICATIONS

12

Analog ground start signaling is utilized by pay phones

BASIC TELECOMMUNICATIONS

13

Analog Ground Start Signaling or GST (Ground Start Trunk) is typically associated with pay phones and PBX systems.

A ground start is a supervisory signal given by a telephone or PBX in which the loop is grounded to request dial tone

In the “idle” condition both the tip and ring lines are disconnected from ground and -48VDC is applied to the ring line

BASIC TELECOMMUNICATIONS

14

In the circuit below both tip and ring are disconnected from ground and -48VDC is present on the ring side, the tip is monitored by the PBX for a ground and the ring is monitored for a ring ground by the CO.

BASIC TELECOMMUNICATIONS

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When a call is originated from the pay phone/PBX to the CO the PP/PBX grounds the ring line to signal the CO, the CO senses the line current and it grounds the tip line to signal the PP/PBX that it is ready for the call

BASIC TELECOMMUNICATIONS

16

The PBX senses the tip ground, removes the ring ground and closes the loop between the ring ground and tip lines for the PBX voice circuitry, the CO senses the reduced ring current, disconnects the tip ground and connects the CO voice circuitry to establish the call.

BASIC TELECOMMUNICATIONS

17

Analog ground start signaling from the PBX to the CO or visa versa must be completed within 100 milliseconds or the circuit times out

Most PBX systems are equipped with both loop start and ground start circuitry, though ground start is the preferred signaling type

PBXs work best on ground start trunks because it can give them an on hook signal allowing for timely clearing

BASIC TELECOMMUNICATIONS

18

How does NFPA 72 pertain to loop start or ground start trunks?

BASIC TELECOMMUNICATIONS

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In the NFPA 72 National Fire Alarm Code book article 8.6.3.2.1 (B) states:

When connecting a DACT (Digital Alarm Communicator Transmitter) to the PSTN special attention shall be require to ensure that this connection is make only to a loop start telephone circuit and not to a ground start telephone circuit.

BASIC TELECOMMUNICATIONS

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Loop start trunk boards Ground start trunk boards

BASIC TELECOMMUNICATIONS