Chapter 3: Routing Calls over Analog Voice Ports

Cisco Press

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Table 3-4  E&M Signaling Types

Type

M-Lead Off-Hook

M-Lead On-Hook

E-Lead Off-Hook

E-Lead On-Hook

I

Battery

Ground

Ground

Open

II

Battery

Open

Ground

Open

III

Loop Current

Ground

Ground

Open

IV

Ground

Open

Ground

Open

V

Ground

Open

Ground

Open

SSDC5

Earth On

Earth Off

Earth On

Earth Off

The following list details the characteristics of each E&M signaling type introduced in Table 3-4:

  • Type I: Type I signaling is the most common E&M signaling method used in North America. One wire is the E lead. The second wire is the M lead, and the remaining two pairs of wires serve as the audio path. In this arrangement, the PBX supplies power, or battery, for both E and M leads. In the idle (on-hook) state, both the E and M leads are open. The PBX indicates an off-hook by connecting the M lead to the battery. The line side indicates an off-hook by connecting the E lead to ground.

  • Type II: Type II signaling is typically used in sensitive environments because it produces very little interference. This type uses four wires for signaling. One wire is the E lead. Another wire is the M lead, and the two other wires are signal ground (SG) and signal battery (SB). In Type II, SG and SB are the return paths for the E lead and M lead, respectively. The PBX side indicates an off-hook by connecting the M lead to the SB lead. The line side indicates an off-hook by connecting the E lead to SG lead.

  • Type III: Type III signaling is not commonly used. Type III also uses four wires for signaling. In the idle state (on-hook), the E lead is open and the M lead is connected to the SG lead, which is grounded. The PBX side indicates an off-hook by moving the M lead from the SG lead to the SB lead. The line side indicates an off-hook by grounding the E lead.

  • Type IV: Type IV also uses four wires for signaling. In the idle state (on-hook), the E and M leads are both open. The PBX side indicates an off-hook by connecting the M lead to the SB lead, which is grounded on the line side. The line side indicates an off-hook by connecting the E lead to the SG lead, which is grounded on the PBX side.


  • Note - E&M Type IV is not supported on Cisco voice gateways. However, Type IV operates similarly to Type II except for the M-lead operation. On Type IV, the M-lead states are open/ground, compared to Type II, which is open/battery. Type IV can interface with Type II. To use Type IV you can set the E&M voice port to Type II and perform the necessary M-lead rewiring.


  • Type V: Type V is the most common E&M signaling form used outside of North America. Type V is similar to Type I because two wires are used for signaling (one wire is the E lead and the other wire is the M lead). In the idle (on-hook) state, both the E and M leads are open as in the preceding diagram. The PBX indicates an off-hook by grounding the M lead. The line side indicates an off-hook by grounding the E lead.

  • SSDC5: Similar to Type V, SSDC5 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). SSDC5 is most often found in England.

E&M Physical Interface

The physical E&M interface is an RJ-48 connector that connects to PBX trunk lines, which are classified as either two-wire or four-wire.


Note - Two-wire and four-wire refer to the voice wires. A connection might be called a four-wire E&M circuit although it actually has six to eight physical wires.


Two or four wires are used for signaling, and the remaining two pairs of wires serve as the audio path. This refers to whether the audio path is full duplex on one pair of wires (two-wire) or on two pairs of wires (four-wire).

E&M Address Signaling

PBXs built by different manufacturers can indicate on-hook/off-hook status and telephone line seizure on the E&M interface by using any of three types of access signaling:

  • Immediate-start: Immediate-start, as illustrated in Figure 3-12, is the simplest method of E&M access signaling. The calling side seizes the line by going off-hook on its E lead, waits for a minimum of 150 ms and then sends address information as DTMF digits or as dialed pulses. This signaling approach is used for E&M tie trunk interfaces.

  • Figure 3-12

    Immediate-Start Signaling

  • Wink-start: Wink-start, as shown in Figure 3-13, is the most commonly used method for E&M access signaling and is the default for E&M voice ports. Wink-start was developed to minimize glare, a condition found in immediate-start E&M, in which both ends attempt to seize a trunk at the same time. In wink-start, the calling side seizes the line by going off-hook on its E lead; it then waits for a short temporary off-hook pulse, or “wink,” from the other end on its M lead before sending address information as DTMF digits. The switch interprets the pulse as an indication to proceed and then sends the dialed digits as DTMF or dialed pulses. This signaling is used for E&M tie trunk interfaces. This is the default setting for E&M voice ports.

  • Figure 3-13

    Wink-Start Signaling

  • Delay-start: With delay-start signaling, as depicted in Figure 3-14, the calling station seizes the line by going off-hook on its E lead. After a timed interval, the calling side looks at the status of the called side. If the called side is on-hook, the calling side starts sending information as DTMF digits. Otherwise, the calling side waits until the called side goes on-hook and then starts sending address information. This signaling approach is used for E&M tie trunk interfaces.

  • Figure 3-14

    Delay-Start Signaling

Configuring Analog Voice Ports

The three types of analog ports that you will learn to configure are

  • FXS

  • FXO

  • E&M

FXS Voice Port Configuration

In North America, the FXS port connection functions with default settings most of the time. The same cannot be said for other countries and continents. Remember, FXS ports look like switches to the edge devices that are connected to them. Therefore, the configuration of the FXS port should emulate the switch configuration of the local PSTN.

For example, consider an international company that has offices in the United States and England. Each PSTN provides signaling that is standard for its own country. In the United States, the PSTN provides a dial tone that is different from the dial tone in England. The signals that ring incoming calls are different in England. Another instance where the default configuration might be changed is when the connection is a trunk to a PBX or key system. In each of these cases, the FXS port must be configured to match the settings of the device to which it is connected.

In this example, you have been assigned to configure a voice gateway to route calls to a plain old telephone service (POTS) phone connected to a FXS port on a remote router in Great Britain. Figure 3-15 shows how the British office is configured to enable ground-start signaling on FXS voice port 0/2/0. The call-progress tones are set for Great Britain, and the ring cadence is set for pattern 1.

Figure 3-15

FXS Configuration Topology

The requirements for your configuration are the following:

  • Configure the voice port to use ground-start signaling.

  • Configure the call-progress tones for Great Britain.

You would then complete the following steps to accomplish the stated objectives:

Step 1. Enter voice-port configuration mode.

Router(config)#voice-port slot/port

Step 2. Select the access signaling type to match the telephony connection you are making.

Router(config-voiceport)#signal {loopstart | groundstart}

Note - If you change signal type, you must execute a shutdown and no shutdown command on the voice port.


Step 3. Select the two-letter locale for the voice call progress tones and other locale-specific parameters to be used on this voice port.

Router(config-voiceport)#cptone locale

Step 4. Specify a ring pattern. Each pattern specifies a ring-pulse time and a ring-interval time.

Router(config-voiceport)#ring cadence {pattern-number | define pulse interval}

Note - The patternXX keyword provides preset ring-cadence patterns for use on any platform. The define keyword allows you to create a custom ring cadence.


Step 5. Activate the voice port.

Router(config-voiceport)#no shutdown

Example 3-1 shows the complete FXS voice port configuration.

Example 3-1  FXS Voice Port Configuration

Router(config)#voice-port 0/2/0Router(config-voiceport)#signal groundstartRouter(config-voiceport)#cptone GBRouter(config-voiceport)#ring cadence pattern01Router(config-voiceport)#no shutdown
FXO Voice Port Configuration

An FXO trunk is one of the simplest analog trunks available. Because Dialed Number Information Service (DNIS) information can only be sent out to the PSTN, no direct inward dialing (DID) is possible. ANI is supported for inbound calls. Two signaling types exist, loopstart and groundstart, with groundstart being the preferred method.

For example, consider the topology shown in Figure 3-16. Imagine you have been assigned to configure a voice gateway to route calls to and from the PSTN through an FXO port on the router.

Figure 3-16

FXO Configuration Topology

In this scenario, you must set up a PLAR connection using an FXO port connected to the PSTN.

The configuration requirements are the following:

  • Configure the voice port to use ground-start signaling.

  • Configure a PLAR connection from a remote location to extension 4001 in Austin.

  • Configure a standard dial peer for inbound and outbound PSTN calls.

Because an FXO trunk does not support DID, two-stage dialing is required for all inbound calls. If all inbound calls should be routed to a specific extension, (for example, a front desk), you can use the connection plar opx command. In this example, all inbound calls are routed to extension 4001.

You could then complete the following steps to configure the FXO voice port:

Step 1. Enter voice-port configuration mode.

Router(config)#voice-port 0/0/0

Step 2. Select the access signaling type to match the telephony connection you are making.

Router(config-voiceport)#signal ground-start

Step 3. Specify a PLAR off-premises extension (OPX) connection.

Router(config-voiceport)#connection plar opx 4001

Note - PLAR is an autodialing mechanism that permanently associates a voice interface with a far-end voice interface, allowing call completion to a specific telephone number or PBX without dialing. When the calling telephone goes off-hook, a predefined network dial peer is automatically matched. This sets up a call to the destination telephone or PBX.

Using the opx option, the local voice port provides a local response before the remote voice port receives an answer. On FXO interfaces, the voice port does not answer until the remote side has answered.


Step 4. Activate the voice port.

Router(config-voiceport)#no shutdown

Step 5. Exit voice port configuration mode.

Router(config-voiceport)#exit

Step 6. Create a standard dial peer for inbound and outbound PSTN calls.

Router(config)#dial-peer voice 90 pots

Step 7. Specify the destination pattern.

Router(config-dialpeer)#destination-pattern 9T

Note - The T control character indicates that the destination-pattern value is a variable-length dial string. Using this control character enables the router to wait until all digits are received before routing the call.

Dial-peer configuration is covered in the section, “Introducing Dial Peers.”


Step 8. Specify the voice port associated with this dial peer.

Router(config-dialpeer)#port 0/0/0

Example 3-2 shows the complete FXO voice port configuration.

Example 3-2  FXO Voice Port Configuration

Router(config)#voice-port 0/0/0Router(config-voiceport)#signal groundstartRouter(config-voiceport)#connection plar opx 4001Router(config)#dial-peer voice 90 potsRouter(config-dialpeer)#destination-pattern 9TRouter(config-dialpeer)#port 0/0/0
E&M Voice Port Configuration

Configuring an E&M analog trunk is straightforward. Three key options have to be set:

  • The signaling E&M signaling type

  • Two- or four-wire operation

  • The E&M type

As an example, consider the topology shown in Figure 3-17.

Figure 3-17

E&M Configuration Topology

In this example, you have been assigned to configure a voice gateway to work with an existing PBX system according to network requirements. You must set up a voice gateway to interface with a PBX to allow the IP phones to call the POTS phones using a four-digit extension.

The configuration requirements are the following:

  • Configure the voice port to use wink-start signaling.

  • Configure the voice port to use 2-wire operation mode.

  • Configure the voice port to use Type I E&M signaling.

  • Configure a standard dial peer for the POTS phones behind the PBX.

Both sides of the trunk need to have a matching configuration. The following example configuration shows an E&M trunk using wink-start signaling, E&M Type I, and two-wire operation. Because E&M supports inbound and outbound DNIS, DID support is also configured on the corresponding dial peer.

You could then complete the following steps to configure the E&M voice port:

Step 1. Enter voice-port configuration mode.

Step 2. Select the access signaling type to match the telephony connection you are making.

Router(config-voiceport)#signal wink-start

Step 3. Select a specific cabling scheme for the E&M port.

Router(config-voiceport)#operation 2-wire
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