PBX Voice SignalingPBX Voice Signaling

Nathalie Crucet+ 33 2 23 28 40 39

Agenda

This presentation include the different aspects linked to Voice signaling Main characteristics of interfaces with associated signaling Level of services that can be offered to each signaling What does it mean in terms of level of services offered in

VoIP architecture Gateway capabilities in terms of voice interface/voice

signaling

The PBX can be connected to the PSTN and to another PBX with different interfaces using different signaling

There are 2 types of interface: Digital interfaces

E1 interface : 30 B channels + 1 D-channel T1 interface : 23B channels + 1 D channel BRI interface : 2B channel + 1D channel

Analog interfaces E&M interfaces FXS/FX0 interface (not detailed in this presentation)

The normalized signaling available are the following ISDN : T1, E1 or BRI QSIG : E1 or BRI CAS (wink start, immediate start, delay start) : E&M or E1 CAS

There are also proprietary protocols available on E1/T1 interfaces

PBX signaling

Ear and Mouth (E&M) : Separate Signaling Path Two wires and four wires refer to the voice wires The Switch listens on the ear (e-lead) The Switch signals on the mouth (m-lead)

E&M voice interface

T and T1

R and R1EM

TR

EM

PBXPBXCO

Switch

2 Wire E and M 4 Wire E and M

SignalingInterfaceExchange

SignalingInterfaceExchange

The E&M signaling allows Supervision of the seizure of the line : it means that the PBX

is able to detect the seizure of the line as well as the disconnect signal

Transport of the called number in-band (using DTMF tones) : it means the PBX is able when he receives a call on E&M interface to route the call by using the information of the called number

In VoIP architecture, you have to know all the parameters in the PBX to configure the same way the gateway Timers parameters Wiring 4 wire/2wires Type of signaling : wink, immediate or delay

Due to the poor signaling, this type of connection is quite difficult to debug

E&M Signaling

E&M signaling states

E-Lead/SGM-Lead/SBConditionE and M Type

Off-Hook

Type I

Type III

Type V

Open

Battery

GroundOn-Hook

Type II

Type IV

Off-Hook

Off-Hook

On-Hook

On-Hook

Ground Ground

Ground

Ground

Battery

Open Open

Open

Open

Loop Current

On-Hook

Off-Hook

Ground

Ground

Open

Ground

Open

Ground

OpenOn-Hook

Off-Hook

E&M voice interface The PBX can send the digits in two way on E&M interface

Transparent mode : The PBX sends the digits on the line that have been dialed by the user transparently (it leads to variable digit duration and interdigit). It will lead to duplication digit issue.

Tandem mode : The PBX waits for complete dialing from the user and then sends the digits on the line (Digit duration and interdigit are fixed). No issue with this mode

Different signaling can be used with E&M interface Wink start : The terminating switch sends a wink start (momentary off-hook) signal

to the originating switch, indicating that the terminating switch is ready to receive digits

Immediate start : The originating switch sends digits immediately after seizure (off-hook) without waiting for a start-dial signal from the terminating switch

Delay start : The terminating switch sends a delay-dial signal (an off-hook signal followed by an on-hook signal) to the originating switch indicating that the terminating switch is ready to receive digits

E&M Immediate signaling

Immediate start : I am always ready to receive the digits

DTMF digits

Sending switch goes off-hook

On-Hook

Off-Hook

Sending switch Receiving switch

M E

Dialout-delay

Receiving switch goes off-hook after connection is established

On-Hook

Off-Hook

Sending switch waits a minimum of 210ms (dialout-delay timer) before sending addressing

Receving switch Sending switch

M E

E&MWink signaling

Wink start : I am ready to receive the digits now

Sending switch goes off-hook and waits the wink signal during a period of time (wait-wink timer)

On-Hook

Off-HookSending switch Receiving switchWait-wink

M E

Receiving switch goes momentarily off-hook for 140 to 200ms (wink duration timer)

On-Hook

Wink durationWink-waitincoming seizure

detection

Receiving switch Sending switch

M E

DTMF digits

Receiving switch goes off-hook after connection is established

On-Hook

Off-Hook

EM

E&MDelay signaling

On-HookOff-Hook

Sending switch goes off-hook and waits for 200ms and check if the far-end is off-hook. If the far end is off hook he waits until he goes on-hook before sending digits

Sending switch Receiving switch

Delay start : Wait I am not ready to receive digits

Receiving switch goes momentarily off-hook for 140 to 200ms (wink duration timer)

On-Hook

Delay durationDelay startincoming seizure

detection

Receiving switch Sending switch

M E

DTMF digits

Receiving switch goes off-hook after connection is established

On-Hook

Off-Hook

EM

Digital signaling - Better

Distorted Analog

Communications

Clear Digital

Communications

300–3400 Hz 4kHz2 x 4 kHz = 8 KHz

8 KHz x 8 bits = 64 kbps

T1 signaling format

Ext

ende

d Su

per

Fram

e

Time Slot #XTime Slot #X1 7 8

1

6

12

18

24

A

B

C

D

Time Slot #24Time Slot #241 7 8

1

6

12

18

24Usa

ble f

or IS

DN or P

BX Sign

aling

Channel Associated Signaling (CAS)

or Robbed Bit SignalingCommon Channel Signaling

E1 signaling format

01 2 8

1

2

3

4

56

10

15

7

8

9

1112

13

14

Sem

i-Mul

tifra

me

(SM

F) 1

Sem

i-Mul

tifra

me

(SM

F) 2

3 4 5 6 70 0 0 X X X X0

A B C D1

A B C D2

A B C D3

A B C D4

A B C D8

A B C D5

A B C D9

A B C D12

A B C D7

A B C D6

A B C D10

A B C D11

A B C D13

A B C D14

A B C D14

A B C D17

A B C D18

A B C D19

A B C D20

A B C D24

A B C D21

A B C D25

A B C D28

A B C D23

A B C D22

A B C D26

A B C D27

A B C D29

A B C D30

A B C D31

1 2 8

1

2

3

4

56

10

15

7

8

9

1112

13

14

3 4 5 6 70

Usabl

e for

ISDN or

PBX Si

gnali

ng

Channel Associated Signaling Common Channel Signaling

Time Slot #16Time Slot #16 Time Slot #16Time Slot #161 2 8

2

4

5

10

15

7

8

9

1112

14

6 70

1 2

1

2

3

4

56

10

15

7

8

9

1112

13

14

0

Channel Associated Signaling Common Channel Signaling

Digital interfaces

The signaling that can be used on digital interface (PRI or BRI) are

ISDN signaling : a user to network signaling QSIG signaling : PBX to PBX signaling E1/T1 CAS (Channel Associated signaling) : PBX to PBX

signaling with poor signaling (same level as E&M)

The ISDN signaling have been defined in order to allow the connection of PBX to the PSTN

The purpose of QSIG signaling is to extend facilities normally available between extension on a single PBX to all extensions on PBX’s that are connected together in a private network

Signaling that can be used on E1 CAS interface are Wink start, immediate start and delay start.

Digital interfaces ISDN signaling

ISDN signaling is a User to Network signaling that allows Convey information from a PBX to another Network can send information to the PBX Users on the PBX can activate services on the network

Description of some ISDN services with the example below A calls B, as B is unavailable, C Answers the call

PBXPBX

A = calling number

PSTN

Called number = B

C = connected number

Digital interfaces ISDN signaling The PBX can send the digits to the network in two ways

Enbloc sending : the complete called number is in the Setup message send by the PBX

Overlap sending : The PBX sends the setup message and then the called number is send in Information message (digit by digit)

Description of main ISDN services : Convey of number identification : called number, calling number (CLIP)

connected number (COLP) Convey of User to User information Network provides charging information to the PBX : Advice of charge

during the call and at the end of the call The PBX can activate some services provided by the Network

Call forwarding unconditional Secret of number identification : Calling restriction (CLIR), Connected restriction

(COLR)

ISDN Call completion

CallingParty Network

Setup

Setup Acknowledge

Information

Call ProceedingSetup

Call Proceeding

Alerting

Alerting Connect

ConnectConnect Acknowledge

Connect Acknowledge

CalledParty

Digital interfaces QSIG signaling QSIG signaling allows to have a good level of services in a

PBX networking. QSIG basic call offers the following services

Number identification : called number, calling number (CLIP) connected number (COLP)

Name identification : called name, calling name (CNIP) connected name (CNOP)

QSIG Generic Functionnal offers the following services that can be activate in the PBX networking Call forwarding (unconditional, on busy and no reply) Call transfer Call completion Do not disturb ...

Digital interfaces - QSIG signaling

On the router it is not possible to distinguish QSIG Basic call from QSIG Generic Functionnal.

It leads to some restrictions in case of PBX interconnexion through Gateway if one PBX support QSIG GF and the other one no, it will

not work because the PBX that support only QSIG basic call will receive some services (which it is not able to decode) from the other PBX (Services are conveyed transparently through the gateway)

If both PBXs support QSIG GF but not with the same level of services, it will lead to some issues also.

Voice signaling recommended E&M interface

Low cost solutionAvailable on all PBXs world wildOnly basic call without servicesDifficult to debugNo backup solution

E1 CAS interfaceOnly basic call without servicesDifficult to debugNo backup solution

Voice signaling recommended ISDN signaling (E1, T1 or BRI)

Provide basic call with some services (like calling party)Easy to debugGood for backup solutionGood solution for heterogeneous PBXsNo networking services

QSIG signaling (E1, T1 or BRI)A good level of services for PBX networkGood solution for homogeneous PBXsGood for backup solutionMore complicated to debug than ISDNInternetworking issues in PBX heterogeneous

Proprietary signaling When a customer asks for PBXs interconnexion using

proprietary protocol (ABC for Alcatel, MOVACS for Matra, DCS for Lucent …) on E1 interfaces, the gateway offers the TCCS (Transparent Common Channel Signaling) mode

The gateway do not decode D channel so it is like a tunnel mode for D-channel between the local and distant gateway.

This mode is bandwidth consuming : 84 kbit/s for D-channel transport (even if there is no call on the link)

The voice channels can be compressed (using G729 codec for example)

Not a standard solution (high score)

Overflow and backup

IP Network

PABX B

R1 R2

PBXPABX APBX

PSTN network

Release (failure cause)

Problem in the network

The recommended solution of overflow is described below In case of problem on the distant link or in the IP network, the

overflow strategy consists of sending a release message with a configured cause to the PBX that is able to reroute the call to the PSTN on reception of this cause.

It works only with ISDN or QSIG signaling

Thank you !!!