Signaling Is the Central Nervous System of the Telephone Network

TDC 364

Signaling

Dr. C.M. White

Chapter 9 Ramteke

Signaling is the central nervous system of the telephone network.

But before we jump into signaling, what does a CO look like?

Central Office Wire Center

When the local loop phone line enters the local telco, the wire enters into a wire center.

Three basic parts:

1. Central office switches

Common Control is a central processor

Data Store contains all customer feature and billing information

Service Circuits are shared circuits for ringing, tones, digit collection, etc.

Switch Matrix can interconnect line/trunk cards and Service Circuits

Line Cards perform per line processing

Trunk Cardsperform per trunk processing

2. Main distribution frame

Patch cables connect switch interfaces (Switch Side) to outside cable (Cable Side).

3. Cable vault

The Outside Plant is the collection of subscriber loops and trunks.

Back to signaling.

Placing a Call

1. Call originationAll hidden

a. Originating phone goes off-hook

b. Line Card see current, notifies Common Control

c. Common Control checks customer records

d. Common Control sets up connections to Service Circuits

Digit Decoder circuit

Dial Tone Generator circuit

e. Customer dials number

f. Dial Tone is disconnected after first digit

g. Digit Decoder deallocated after all digits dialed

h. Common Control

Consults Routing Table

Determines features of terminating line

Connects originating line to Ringback Generator

Connects terminating line to Ringing Circuit

2. Call routing - alternate route choice

3. Answer supervision - off-hook back to CO-A, billing begins

4. Disconnect - on-hook

Different Types of SignalsAll hidden

On-hook, off-hook

Address signals - phone numbers

Information signals - A...this number has been disconnected.@

Dial tone - 350 Hz + 440 Hz

Ringback - 440 Hz + 480 Hz

Busy signal - 480 Hz + 620 Hz every second

All trunks busy - same as busy signal but twice as fast

Ringing bells - 90 VAC at 20 Hz, on for 2 seconds off for 4 seconds

Flashing - very quick on-hook / off-hook

Are the signals sent over the same trunk as the data?

Per-trunk signaling

Channel associated signaling

If not - Common Channel Interoffice Signaling (CCIS)

Signals sent over a separate, common channel

Channel is not a voice channel, but a packet data network!

All STPs come in pairs for backup.

SS7 is the latest version of CCIS.

CCIS faster than no CCIS!Placing a call cross-country: was 20 sec, now 4 sec with CCIS

Fewer trunks needed.

Because of CCIS, can offer 1-800 service, calling cards, VPNs, ISDN, etc.

Address Signaling

Dial Pulsing - rotary telephone

Dial 4 - four very fast on-hook / off-hook pulses

Each pulse = 100 msec

MF Signaling (MultiFrequency)

Being replaced by SS7. See Table 9.1

DTMF Signaling - touch tone (Dual-Tone MultiFrequency)

How does a telephone / PBX / CO know when a call is being placed?

Signaling types that provide supervision

SF Signaling (Single Frequency)

Used to exchange simple on/off conditions or to provide supervisory signaling

Can also be used to provide address signals.

Loop Start Signaling

Common on residential loops

Requires one pair of wires

Note the 8v across the phone.

Ground Start Signaling

Uses DC loop currents and two wires too, but:

Used primarily on PBX to CO connections

Doesn=t rely on dial tones (explain)

Uses 3-contact switches

Where is the signaling information placed?

1. Robbed bit

Like T-1

2. Clear channel

No signaling bits in the data channel

3. Common channel

One channel carries signaling info for multiple data channels

4. In-band digital

Like T-1

5. Out-of-band digital

Same as clear channel, maybe common channel

6. In-band analog

Signals are in the 300 - 3300 Hz band; DTMF does this

7. Out-of-band analog

Signals outside 300 - 3300 Hz like outdate N1 system at 3700 Hz

Signaling Interfaces

4-wire termination set

At the CO:

E&M Signaling

E&M Signaling (Inter-PBX tie lines)

–Used for trunk-trunk connections.

–Add separate wire leads for signaling

E lead - Receive Signal (Ear)
M lead – Send Signal (Mouth)

–2 wire E&M uses one pair for both directions of speech transmission (T1,R1).

–4 wire E&M uses two speech pairs, one for each direction of transmission (T,R,T1,R1).

–Supervision uses 2 or 4 additional wires (E,M,SB,SG).

–Five “Types”

Type I
Type II
Type III
Type IV
Type V

–Two types of audio interfaces

2 Wire
4 Wire

E&M Signaling (cont.)

Definitions

E&M Interface Signals / Description
Ear (E) / Signal wire from trunking (CO) side to signaling (user) side.
Mouth (M) / Signal wire from signaling (user) side to trunking (CO) side.
Signal Ground (SG) / Applicable to certain types of E&M signaling, provides 48V or ground.
Signal Battery (SB) / Applicable to certain types of E&M signaling, provides 48V or ground.
Tip/Ring (T/R) / Used on four-wire circuits to carry audio from signaling (user) side to the trunking (CO) side. Not used on a two-wire circuit.
Tip-1/Ring-1 (T1/R1) / Used on four-wire circuits to carry audio from the trunking (CO) side to the signaling (user) side. On two-wire circuits, used to carry full-duplex audio.

Wire leads

Signaling Type / Description
I / Commonly used in North America. E,M leads.
II / E, M, SG, SB leads.
III / E, M, SG, SB leads.
IV / E, M, SG, SB leads.
V / Commonly used outside North America. E,M leads.

Type I

The battery for both the E and M leads is supplied by the PBX.

An on-hook condition results in the M lead being grounded and the E lead open.

An off-hook condition results in the M lead providing the battery and the E lead being grounded.

Type I signaling is the most commonly used four-wire trunk interface in North America.

Signaling Condition / M lead / E Lead
On-hook / Ground / Open
Off-hook / Battery / Ground

E&M Signaling (cont.)

Type II

Type II signaling adds two additional signaling leads: Battery (SB) and signal ground (SG).

The E lead works in conjunction with the SG lead.

The M lead is strapped to the SB lead.

This results in the grounding of the trunk at each end and eliminates potential grounding problems from occurring.

Signaling Condition / M lead / E Lead
On-hook / Open / Open
Off-hook / Battery / Ground

E&M Signaling (cont.)

Type III

Type III signaling is very similar to Type I.

The key difference is in the use of the transmission equipment to supply the battery and ground source, which results in loop current flowing on the M lead when an off-hook condition occurs.

Type III signaling was primarily used with older central office equipment and is now in very limited use as most older CO switches have been replaced.

Signaling Condition / M lead / SB / E Lead / SG
On-hook / Ground / Open
Off-hook / Loop Current / Ground

E&M Signaling (cont.)

Type IV

Type IV signaling is similar to Type II, but the operation of the M lead differs.

In Type II signaling, the M lead states are “open” and “battery.”

Under Type IV signaling, the states are “ground” and “open.”

The key advantage of Type IV signaling is the fact that an accidental shorting of the SB lead will not result in an excessive current flow.

Signaling Condition / M lead / SB / E Lead / SG
On-hook / Open / Open
Off-hook / Ground / Ground

E&M Signaling (cont.)

Type V

Under Type V signaling, both the switch and the transmission equipment supply a battery.

The battery for the M lead is located in the signaling equipment.

The battery for the E lead is located in the PBX.

The PBX side grounds its M lead to seize the trunk.

A voice port on the transmission equipment would ground its E lead to seize the trunk.

Type V signaling is the most popular interface outside of North America.

Signaling Condition / M lead / E Lead
On-hook / Open / Open
Off-hook / Ground / Ground

E&M Signaling (cont.)

Example:

The talk path is on a set of wires separate from the –48 VDC signaling lead wires. The voice signal is modulated onto the tip and ring when operating in a 2-wire full duplex configuration. In the 4-wire configuration, the voice is transmitted onto the T R pair and received on the T1, R1 pair.

The signaling (on hook, off hook) is still done with 48 VDC but it rides on the E&M leads.

The PBX is an "M lead originate" device and the E&M card is an "E lead originate" device.

When PBX A wants to place a call, it raises the M lead. The PBX does this by applying -48 VDC to the M lead.

The E&M card sees -48 VDC and converts that -48 VDC into signal bits (TX 11 RX 00 ).

The remote E&M card sees the 11 bits coming from the other side and converts the signal back to analog by grounding the E lead.

This causes current to flow from the PBX to the E&M card on the E lead.

When PBX B sees that the E lead has gone high, it knows a call is coming in and rings the appropriate phone.

When the ringing phone goes off hook, PBX B then signals PBX A by applying -48 VDC to the M lead.

The E&M card sees the M lead now has voltage and transmits 11 back to PBX A.

The E&M card at site A converts the RX 11 back into an analog state by grounding the E lead.

Now current is flowing from PBX A into the E&M card A.

The originating PBX A has now seen the E lead return high and the connection is complete.

Used on tie trunks between PBXs

Used for conversion between DC loop currents and in-band analog signaling

Digital Signaling Interfaces