Understanding Basic VoIP with PBX Trunking

>emUnderstanding Basic VoIP with PBX Trunking: “The voice network module on the Cisco 3600 series supports up to two voice interface cards (VICs). These VICs come in three types, each having two ports. Each type provides a slightly different interface for connecting to different types of equipment. The basic types of voice interfaces are: foreign exchange service (FXS), foreign exchange office (FXO), and E&M.


The FXO interface is an RJ-11 connector that connects local calls to a public switched telephone network (PSTN) central office (CO), or to a PBX that does not support E&M signaling. This is the interface a standard telephone provides. This is the only voice interface card approved to connect to off-premise lines, and this is its primary use. This interface may be used to provide backup over the PSTN or for Centrex-type operations. The FXO ports on the Cisco 3600 series router can support both loop-start and ground-start modes. (In certain situations, a hardware jumper must be used to make an FXO port operate in ground-start mode.) The ground-start signaling method is used primarily on trunk lines or tie lines between private branch exchanges (PBXs).


The FXS interface is also an RJ-11 connector that connects directly to a standard telephone, fax machine, or similar device. The FXS interface supplies ringing voltage, dial tone, and similar signals to FXO devices. As the exercises in module 1 demonstrated, you would use the FXS interface when connecting a phone directly to a router. Basically, the FXS interface mimics the PSTN. The FXS ports on the Cisco 3600 series router can provide the battery at 24-volt DC. They can also support both loop-start and ground-start modes of operation according to the software configuration of the router. Loop start, however, is the most common, because most residential telephones are analog loop-start devices.


E&M, which stands for ‘Ear and Mouth’ (or ‘recEive and transMit,’ or sometimes ‘Earth and Magnet’), is a signaling technique for two- and four-wire telephone and trunk interfaces that is used mainly between PBXs or other network-to-network telephony switches, such as Lucent 5 Electronic Switching System [5ESS], Nortel DMS-100, and so on. The E&M analog interface is an RJ-48S connector used to connect remote calls from an IP network to a PBX for local distribution and between PBX trunk lines (tie lines) or other network-to-network telephony switches. Unlike loop-start or ground-start modes of operation on the FXS/FXO ports, E&M uses separate wires for signaling and voice. There are five types of E&M signaling, as well as two wiring methods.

Exercise care when designing a voice network…

Exercise care when designing a voice network to ensure supervisory signals (discussed in module 1 of this tutorial) are transmitted when expected to avoid common problems such as spurious disconnects, undisconnected lines, and continuous rings. For example, a typical disconnect supervision problem can occur when a given line can be released only by the party who originated the phone call. This problem can be fixed by implementing either disconnect supervision or ground start.

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In the first module of this tutorial, you learned about the basics of analog voice internetworking, including the history of telephony, telephone networks and technology, and the various types of telephony signaling. You know how to configure and troubleshoot a Cisco 3640 router using loop-start access signaling across the FXS interface. In this module you will learn more about trunks, analog E&M signaling, and VoIP configuration using E&M. The Cisco VoIP implementation supports E&M types I, II, III, and V, using both two- and four-wire implementations.

Click on any highlighted term see its pop-up glossary definition. For additional information and more definitions of terms used throughout this course, see the full Glossary. You can access the Glossary at any time by clicking Help.

A basic understanding of trunks is necessary to understanding the use of E&M signaling. A line is a communications path between a customer’s telephone and a telephone switch, such as a CO switch or a PBX, whereas a trunk is an actual telephone circuit or path between two switches, at least one of which is usually a CO or a switching center. This type of trunk, where voice is carried over a telco’s network, is a CO trunk. Another example of a trunk is a private leased line that interconnects two PBXs, forming a corporate voice network. This type of connection is referred to as a PBX trunk.

Central-Office Trunks
Two-Wire Central-Office Trunks
A hunt group is…

A hunt group is a feature supported by voice-capable Cisco routers that involves the configuration of a group of dial peers on the same router with the same destination pattern. With a hunt group, if a call attempt is made to a dial peer on a specific time slot and that time slot is busy, the router hunts for another time slot on that channel until an available time slot is found.

The Cisco 3640 hunts for a group based on matching numbers.

It is possible to configure hunt groups by setting up multiple dial peers having the same destination pattern. Hunt groups are not configured in this tutorial, and the feature is not enabled in the simulation exercises in this module.

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The most common two-wire CO trunks used in PBX networks employ ground start. These trunks may be restricted such that operation may be outgoing only, incoming only, or both. In the case of outgoing calls (direct outward dialing, or DOD) operation additionally may be restricted to local calling only, versus direct distance dialing (DDD). Most incoming calls are routed to an attendant or to a messaging or answering system. The ringback tone is generated from the local exchange carrier’s (LEC’s) switch that connects to the PBX. This type of trunk is common.

Most of these trunks are assigned the same phone number within the PSTN, and operate in a ‘hunt group’ fashion.
Direct Inward Dialing Central Office Trunks

Direct inward dialing (DID) is used to determine how the called number is treated for incoming calls. Incoming calls to a PBX often first flow through an attendant position. DID trunks allow users to receive calls directly from the outside without intervention from the attendant.

Use of DID offers three main advantages:

* It allows direct access to stations from outside the PBX.
* It allows users to receive calls even when the attendant switchboard is closed.
* It takes a portion of the load off the attendants.

To accomplish DID on a trunk, the switch at the PSTN (either the local exchange carrier [LEC] or the inter-exchange carriers [IXC]) transmits the number of the called party to the PBX for routing. DID trunks often use existing two-wire circuits, and typically use ground-start signaling. Four-wire circuits using E&M signaling or digital access are sometimes used as well. To use DID, a block of PSTN telephone numbers generally must be reserved from the IXC or LEC.

DID trunk circuits usually employ either Wink Start or Delay-Dial signaling schemes, so that the network knows that the PBX is ready to accept the incoming address supervision. Wink Start and Delay Dial are discussed in more detail later in this section.

The next module in this tutorial, ‘Basic Analog-to-Digital Voice over IP,’ covers the use of DID in a VoIP configuration.
Wide-Area Telephone Service Central-Office Trunks

Wide-Area Telephone Service (WATS—sometimes referred to as OUTWATS) trunks are used for outgoing calls. They take advantage of interconnection to the IXC. These trunks require that the PBX output digits to the PSTN switch for routing. The network usually provides answer and disconnect supervision. WATS trunks can operate using either ground-start or loop-start signaling, in addition to E&M signaling. Ringback and busy signals are generated from the remote switch or PBX.

WATS trunks do not have addressable PSTN telephone numbers; therefore, these trunks are typically implemented in a hunt-group fashion within the PBX.
Central-Office 800-Service Trunks

Trunks that provide 800-service are sometimes referred to as INWATS service trunks. These trunks are used for incoming calls and take advantage of interconnection to the IXC. With an 800-service trunk, the PSTN does not output digits to the PBX; incoming calls on these trunk numbers are routed to specific individuals or groups by the PBX. These trunks may utilize loop reverse battery or E&M signaling.

Ringback and busy tones are usually generated by the PSTN switch that is interconnected to the PBX. Generally, these trunks are implemented in a hunt group fashion.
Long-Distance Trunks

Long-distance trunks interconnect to an IXC to allow both incoming and outgoing long-distance calling. They usually operate on two-wire ground-start circuits, or two- or four-wire E&M circuits. If access to the PSTN switch is digital (DS1), then conventional robbed-bit signaling (RBS) is used. (RBS is explained in greater detail in the next module of the Voice Internetworking CIM tutorial.)

Ringback and busy tones are usually generated by the IXC’s PSTN switch that is interconnected to the PBX. Generally, these types of trunks are implemented in a hunt-group fashion, and incoming calls are typically routed to an attendant or messaging system.

PBX Trunks

A PBX is a private digital or analog telephone switchboard used to originate and answer calls to and from the public network (via CO trunks). PBXs can be combined with PBX trunks between them. When a caller picks up a handset and dials from within the corporation, the PBX connects that user to an idle line or to an idle trunk in an appropriate trunk group, then returns the appropriate call status signal, such as a dial tone or a ringback. A busy or fast-busy signal is returned if the line or the trunk group is busy. An attendant may be present to answer incoming calls and for user assistance.
FX Trunks

Foreign exchange (FX) circuits may consist of either stations or trunks. An FX line is a special line that is run from a local telephone to a CO switch or PBX switch. In this case, the local telephone is assigned a number on a remote switch; for all inbound and outbound calls, the telephone behaves as if it were connected to the remote switch. Generally, an FX circuit uses a two-wire circuit with loop start signaling. The ringback and busy signals are provided by the switch connected to the CO.

An FX trunk operates in a similar fashion, except that a local trunk provides interconnectivity to a remote switch. For outbound calls, local PBX users dial an access code such as 9, receive a dial tone from the remote switch, then output digits to the remote switch for routing. Incoming calls are either barred, or are routed to a PBX attendant or message system.

Because a trunk circuit is involved, features such as call transferring and call forwarding can also be deployed. These types of activities requires disconnect supervision to release the PBX trunk. This supervision cannot be provided with loop-start signaling, so ground start signaling must be used.
Tie Trunks and Tie Lines

Tie trunks interconnect PBX switches within a customer’s network. Tie trunks typically connect both trunks and station lines through a network connection. These circuits are usually heavily used and normally support both incoming and outgoing calls.

Tie trunks are typically four-wire E&M-type circuits, but may often interconnect through DS1 facilities using RBS. The use of two-wire trunks is also allowable; these would typically use ground-start signaling.

Although tie lines are generally considered the same as tie trunks, and the terms are often used interchangeably, there are some technical differences:

* Tie trunks can interconnect both trunks and lines. In tie-trunk operation, routing of a call is totally automatic. The remote PBX usually supplies another dial tone, at which the user must enter the necessary address for the called party at that PBX.
* Tie lines connect only lines throughout a network connection. Routing of a call over tie lines is under control of the user. A user would need to dial an access code (9, for example) to gain access to the desired remote PBX. Tie lines are actually tie trunks that use tone-start type of start-dial supervision and cut-through operation. (Details about start-dial supervision can be found at the end of this section.)

Although the next module will include use of a PBX in the network topology, a complete discussion of PBXs and PBX configuration is out of the scope of this tutorial. To learn about PBX configuration, you should refer to the manuals for the particular PBX that you are using in your network. For additional PBX information, however, please see the related document The Private Branch Exchange (PBX) .

As discussed in module 1, ‘Basic Analog Voice over IP,’ the primary purpose of signaling in a voice network is to establish a connection. Signaling can be classified into four basic functions:

* Supervisory signaling — Informs the telephone or switch of the status of the local loop and any connected trunks. Supervisory signaling is used to:
o Initiate a call request on line or trunks (called line signaling on trunks).
o Indicate that the call has been answered or the call has been disconnected (disconnect and answer supervision).
o Initiate or terminate charging for calls.
o Recall an operator on an established connection.
* Address signaling — Contains information indicating the destination of a call, such as the telephone number and an area code, an access code, or a PBX tie trunk access code.
* Call progress indicators — Convey call-progress or call-failure information to subscribers or operators by the use of usually audible tones (such as the busy signals, the reorder tone, and the ringback, all of which were discussed in module 1).
* Network management signaling — Controls the bulk assignment of circuits or modifies the operating characteristics of switching systems in a network in response to overload conditions. (These signaling functions will not be covered in detail in this tutorial, but are mentioned here for completeness.)

For a more detailed discussion of signaling, including descriptions of supervision, address, call-progress, and network management signaling, read Signaling, which is included in this tutorial. Also search Cisco Connection Online (CCO) at http://www.cisco.com for more information on this specific topic as well as more available information on treansmitting voice over a data network.

Just as the local loop requires supervisory signaling, so does a trunk. Although loop-start signaling, which is generally used on everyday residential telephones, can also be used on trunks, its use can result in problems that make it an ineffective signaling method for a trunk. With loop start, only the device originating the call can release the connection. Also, loop start would not prevent a trunk from being seized by both parties to a call, a condition referred to as glare. Glare can be tolerable on a local loop, but its occurrence on a trunk would be unacceptable. Ttwo-way handshaking methods were developed to coordinate the sequence of events that occur during a call: the request by the calling end for access to the trunk, followed by the acknowledgment by the called end, then the subsequent seizure of the trunk by the calling end.

The basic types of supervisory signaling used on trunks are: ground start, E&M, and start-dial supervision.

Ground-Start Signaling

In module 1 of this course, you configured the Cisco 3640 router FXS port to use loop-start signaling, because calls were being made using a directly connected analog telephone. Ground start is the access signaling method used on trunk lines or tie lines between PBXs to indicate on-hook/off-hook status to the CO. Ground-start signaling works by using ground and current detectors. Ground start behaves like a loop start, however, the PBXs on both sides—the telco and the customer’s telephone—have to agree on the status of a line before a call can be placed. This scenario allows the network to indicate off-hook or seizure of an incoming call independent of the ringing signal.

E&M Signaling Interface Types

Analog trunk circuits connect between automated systems (a PBX) and the network (a CO). The most common form of analog trunk is the E&M interface.

E&M signaling is typically used for trunk lines. It provides native support for both disconnect and answer supervision, as well as glare avoidance. In E&M signaling, separate paths are used for voice and signaling. E&M is normally the only way that a CO switch can provide two-way dialing with DID.

The history of the semantics of E&M circuits dates back to the days of telegraphy, where the main office end had a ‘key’ that grounded the E circuit, and the other end had a sounder with an electromagnet attached to a battery. Descriptions such as ‘ear’ and ‘mouth’ were created to help provide a reference to field personnel as to the direction of a signal in a wire. Note that these terms correspond to the inbound and outbound circuits, known as the E lead and M lead, respectively.

There are five standard types of E&M interfaces, which correspond to the five signaling types:

* E&M Type I Interface Model
* E&M Type II Interface Model
* E&M Type III Interface Model
* E&M Type IV Interface Model
* E&M Type V Interface Model

With each signaling type, the PBX supplies one signal, known as the M signal (mouth), and accepts one signal, known as the E signal (ear). Conversely, the tie-line equipment accepts the M signal from the PBX and provides the E signal to the PBX. The M signal accepted by the tie-line equipment at one end of a tie circuit becomes the E signal output by the remote tie-line interface.
Remember learning about tip and ring in module 1?

* When a user tries to place a call by grounding the ‘ring’ lead, the PBX at the telco senses the flow of current and grounds the ‘tip’ lead to indicate the PBX is ready to serve.
* The user’s equipment perceives the flow of current on its ‘tip’ and knows that the PBX is ready to serve.

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The signaling part of the interface has five distinct physical configurations (Types I – V) and the audio interface has two (two- and four-wire). Note that the terms two-wire and four-wire are used to reference the communication protocol of the audio path. The difference between a two-wire and four-wire circuit is whether the audio path is full duplex on one pair or two pairs of wires.

Two full-duplex wires (called tip and ring) carry the audio on two-wire E&M. Four-wire E&M uses four half-duplex wires for the audio path (Tip, Ring, Tip1, and Ring1). Note that a four-wire E&M circuit may have from six to eight physical wires.

Signaling Types

* E — Ear or earth: Signal wire from the trunking (CO or network) side to signaling (user) side
* M — Mouth or magnet: Signal wire from signaling (user) side to trunking (CO or network) side
* SG — Signal ground: Used only on certain types of E&M; sometimes grounded, sometimes not>
* SB — Signal battery: Used on certain types of E&M; sometimes provides -48V Direct Current, sometimes ground, sometimes is not used at all
* T/R — Tip/ring: Used only on a four-wire circuit; carries audio from the signaling (user) side to the trunking (CO/network) side; not used on a two-wire circuit
* T1/R1 — Tip1/ring1: On four-wire circuits, carries audio from the trunking (CO/network) side to the signaling (user) side; on a two-wire circuit, this pair carries the full-duplex audio path

Types I and II are the most popular E&M signaling types in the Americas. Type V is used in the United States, and is very popular in Europe. (SSDC5 is most often found in the United Kingdom.) Similar to Type V, SSDC5A differs in that on- and off-hook states are backward to allow for fail-safe operation: if the line breaks, the interface defaults to off hook (busy). Of all the types, only Types II and V are symmetrical (can be back to back using a crossover cable).

The Cisco 2600 and 3600 series routers currently support types I, II, III, and V utilizing both two- and four-wire implementations. The E&M interface of the Cisco 3600 series presents the ‘channel bank,’ ‘tie equipment,’ or ‘CO’ side of a trunk interface to a PBX. Each E&M signaling type has a unique circuit model and connection diagram. All Cisco analog voice gateway routers have 24V DC designed on the FXS ports. According to EIA 464 standard, any voltage from DC 24 volts to DC 52 volts is acceptable.

Characteristics of E&M Lead Signaling


M Lead

E Lead
Outbound Direction Inbound Direction

Off Hook

On Hook

Off Hook

On Hook












Loop current















Earth on

Earth off

Earth on

Earth off

Cisco routers expect to see off-hook conditions on the M lead and signal off hook to the remote device on the E lead.

See the following sections and illustrations for individual descriptions of the different types. Each of the following illustrations shows the E&M interface of the PBX on the left, and the corresponding tie-line equipment interface on the right. The symbol V refers to battery voltage, which can be 25 VDC to 65 VDC, and is usually (nominally) -48 VDC.
E&M Type I

In E&M Type I signaling, the battery PBX provides the battery for both the E and M leads. During an off-hook condition, the PBX generates the E signal by grounding the E lead. The PBX detects the E signal by sensing the increase in current through a resistive load. Similarly, in an on-hook condition, the PBX generates the M signal by sourcing a current to the tie-line equipment, which detects it via a resistive load.

The four-wire Type I interface from the PBX has the following characteristics:

* E (pin 7) detector ‘floats’ at -48V below ground
* M (pin 2) contact has low ohms to ground on hook, and is -48V below ground when off hook
* Approximately 30-150 ohms between T/R (pins 6/3) sometimes in series with 2.2uf of capacitance
* Approximately 30-150 ohms between T1/R1 (pins 5/4) sometimes in series with 2.2uf of capacitance

E&M Type I

The Type I interface requires that the PBX and tie-line equipment share a common signaling ground reference. This setup can be achieved by connecting signal ground from the PBX to the signal-ground (SG) lead (pin 8) of the RJ-48S connector.

Note that the voltage on the E and M leads may not be the same (the E lead may have lower voltage). This asymmetrical signaling scheme is a potential source of interference, which could cause high return current through the grounding system. If two PBXs were not correctly grounded, current could flow down the M lead, causing the remote PBX to erroneously detect a current on the E lead, thus resulting in false seizure of a trunk. Despite this potential problem, E & M signaling is the most common four-wire trunk interface used in North America.
E&M Type II

The E&M Type II interface provides almost complete isolation of signaling power systems by the addition of two additional signaling leads: signal battery (SB) and signal ground (SG). In Type II, each of the two signals has its own return. For the E signal, the E lead works with the SSG lead to allow current to flow from the PBX while the M lead is strapped to the SB lead. This scenario results in the trunk being grounded at each end, eliminating the potential problem associated with Type I, discussed above.

The four-wire Type II interface from the PBX has the following characteristics:

* E lead (pin 7) detector ‘floats’ at -48V below ground
* SG lead (pin 8) has a low ohms to ground
* M lead (pin 2) contact between M and SB is open when on-hook, and closed when off hook
* M lead floats
* SB lead (pin 1) floats
* Approximately 30-150 ohms between T/R (pins 6/3) sometimes in series with 2.2uF of capacitance
* Approximately 30-150 ohms between T1/R1 (pins 5/4) sometimes in series with 2.2uF of capacitance

E&M Type II

Type II is seen only occasionally in North America, usually on Centrex trunk circuits or Nortel PBX systems.
E&M Type III

E&M Type III signaling is very similar to Type I, except that the battery and ground source for the M lead is supplied by the transmission equipment. Complete power isolation is provided with the M lead, and the facility can establish and control the amount of E lead current. Type III uses the SG lead to provide common ground. The PBX drops the M signal by grounding it, rather than by opening a current loop.

The four-wire Type III interface from the PBX has the following characteristics:

* E lead (pin 7) detector ‘floats’ at -48V below ground
* M lead (pin 2) contact between M and SG when on hook, and between M and SB when off hook
* SG lead (pin 8) floats
* M lead (pin 2) floats
* SB lead (pin 1) floats
* Approximately 30-150 ohms between T/R (pins 6/3) sometimes in series with 2.2uF of capacitance
* Approximately 30-150 ohms between T1/R1 (pins 5/4) sometimes in series with 2.2uf of capacitance

E&M Type III

There is no evidence that the unbalanced E lead of Type III has caused any interference problems, but one drawback of this interface is its inability to operate in a ‘back-to-back’ configuration.

This interface is most often used in older CO equipment such as 1/1AESS, 2/2BESS, and 3ESS switches. It is not often seen now because most of these older switches have been replaced.
E&M Type IV

Type IV is symmetric and requires no common ground. Each side closes a current loop to signal; the flow of current is detected via a resistive load to indicate the presence of the signal. The Type IV interface is similar to Type II, with the difference in the operation of the M lead—in Type II, the M lead states are ‘open’ and ‘battery;’ Type IV states are ‘ground’ and ‘open.’

The four-wire Type IV interface from the PBX has the following characteristics:

* E lead (pin 7) detector ‘floats’ at -48V below ground
* SG lead (pin 8) has low ohms to ground
* M lead (pin 2) contact between M and SB is open when on-hook, and closed when off hook
* M lead (pin 2) floats
* SB lead (pin 1) floats
* Approximately 30-150 ohms between T/R (pins 6/3) sometimes in series with 2.2uF of capacitance
* Approximately 30-150 ohms between T1/R1 (pins 5/4) sometimes in series with 2.2uf of capacitance

E&M Type IV

The advantages of Type IV include:

* Accidental shorting of the SB lead (during cable wiring, for example) will not result in an excessive current flow.
* The interface can interconnect to a Type II device.
* The interface can operate in a ‘back-to-back’ configuration.

Because it can be difficult for an external monitor to distinguish between ‘open’ and ‘ground’ states, it can be difficult to obtain test and supporting equipment for a Type IV interface. Type IV is not currently supported by the Cisco 3600 series routers.
E&M Type V

E&M Type V interface is a simplified version of Type IV. It is also a symmetric interface, using only two wires. In the Type V interface, both the switch and the transmission equipment supply battery. The battery for the M lead is located in the signaling equipment, and the battery for the E lead is located in the PBX. Type V requires a common ground between the PBX and the tie line equipment, which is provided via the SG leads.

The Type V interface from the PBX has the following characteristics:

* E lead (pin 7) detector ‘floats’ at -48V below ground
* M lead (pin 2) contact ground is open when on-hook, and closed when off hook
* Approximately 30-150 ohms between T/R (pins 6/3) sometimes in series with 2.2uF of capacitance
* Approximately 30-150 ohms between T1/R1 (pins 5/4) sometimes in series with 2.2uf of capacitance

E&M Type V

Although this interface does not provide isolation between power systems, there is minimal (or no) return currents in this symmetrical signaling scheme. Type V is the most popular interface outside North America.

E&M Start-Dial Supervision Signaling Protocol

Start-dial supervision is the line protocol used between equipment that takes place after the initial off-hook condition, up to the passing of dial digits to the connected device.

Three principal protocols are used on E&M circuits:

* E&M Immediate Start Signaling
* E&M Wink-Start Signaling
* E&M Delay-Dial Signaling

Wink start is used to notify the remote side it can send dialed number identification service (DNIS) information. Wink acknowledgment is a second wink that is sent to acknowledge the receipt of the DNIS information. Immediate start does not send any winks at all.

The following sections explain how each protocol works. It is important to understand what the protocol is supposed to do when debugging, because call progress anomalies provide clues to the cause.
Immediate Start

E&M Immediate Start is the simplest of protocols. Here, the originating switch goes off hook, waits for a finite period of time (say, 200 ms) then sends the address digits without regard to the remote. Because there is no acknowledgment, or handshaking, between switches, this type of trunk signaling should be used only when there is a dedicated physical or logical trunk between switches. Cisco routers support the use of the Immediate Start protocol.

Immediate Start

Wink Start

E&M Wink Start is the most common of protocols. Wink start is an in-band technique in which the calling switch waits from 140 to 290 ms for an off-hook wink pulse from the called switch before sending the dialed digits. Wink start was developed to minimize glare, which occurs when both ends attempt to seize the trunk at the same time. If there is no wink pulse, an error condition is caused by both ends attempting to place a call on the same trunk.

* In the original Wink Start protocol, the remote responds to an off hook from the originating device with a short wink (transition from on hook to off hook and back again). This wink tells the originating device that the remote device is ready to receive digits. After receiving the addressing digits, the remote then goes off hook for the duration of the call. The originating device maintains off hook for the duration of the call.
* In Wink Start with Wink Acknowledge protocol (sometimes referred to as double wink), the remote responds to an off hook from the originating device with a short wink (transition from on hook to off hook and back again), just as in the original Wink Start. This wink tells the originating side that the remote is ready to receive digits. After receiving the addressing digits, the remote then provides another wink (called an Acknowledgment Wink) that tells the originating side that the terminating side has received the dialed digits. The remote then goes off hook to indicate connection when the ultimate called endpoint has answered. The originating device maintains off-hook status for the duration of the call.

Wink Start normally is not used on trunks that are controlled with message-oriented signaling schemes such as ISDN or Signaling System 7 (SS7). Cisco routers support both wink-start (fgb) and wink-start with wink-acknowledge or double-wink (fgd).

Wink Start

Delay Dial

In E&M Delay Dial mode, the originating device goes off hook and waits for about 200 ms, then checks to see if the remote end is on hook. If so, it then outputs addressing digits. If the remote is off hook, the calling device waits until the remote goes back on hook before transmitting digits. The delay signal says, in effect ‘hold on, I’m not ready to receive digits.’ This protocol was invented for use with systems that have fewer digit collectors than trunk interfaces. Delay dial is not currently supported by Cisco 3600 series routers.


Start Dial Supervision Mismatches

Although both ends of a call do not need Start Dial Supervision, mismatches can be a source of problems. Sometimes a PBX has a different Start Dial Supervision protocol for inbound and outbound calls. This setup can lead to erratic behavior if the remote is not configured to properly handle this condition. The following general rule set applies:

* An Immediate-Start interface can usually originate a call to a Wink-Start interface.
* An Immediate-Start interface can usually place a call to a Delay-Dial interface if the delay pulse is shorter than the immediate-start delay. Otherwise, operation is erratic.
* A Wink-Start interface can usually originate a call into a Delay-Dial interface if there is a delay pulse. Otherwise, the call will hang, with only a 50-percent chance of working.
* A Delay-Dial interface can, for the most part, originate a call into an Immediate-Start or Wink-Start interface.

Go on to Configuration of VoIP with E&M Signaling.”

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