Connecting a Router to a Phone Line


As voice travels from a LAN to a PBX or to the public switched telephone network (PSTN), it needs a "translator" to convert back and forth between those two environments. Voice on a LAN-based VoIP network takes the form of packets, whereas voice traveling to the PBX or PSTN might be analog waves or digital signals. The job of this translator can be performed by a voice-enabled router. When a router acts in this capacity, it is called a gateway.

A gateway typically has at least one interface that connects to the LAN (for example, an Ethernet or Fast Ethernet interface) and at least one interface that connects to the PBX/PSTN environment. These PBX/PSTN interfaces might be either analog or digital, as shown here:

Analog Interfaces:

  • FXS (Foreign Exchange Station)

  • FXO (Foreign Exchange Office)

  • E&M

Digital Interfaces:

  • T1

  • E1

  • Basic rate interface (BRI) /primary rate interface (PRI) (which use Integrated Services Digital Network [ISDN] technology)

First, let's consider what connects to an FXS port. A FXS port connects to a station, such as an analog phone, fax machine, or speaker phone, as shown in Figure 3-6. Consider the analog phone you have in your home. Just as you can connect that analog phone into your RJ-11 wall jack (which goes back to the telephone company), you can also connect that phone into an FXS port. The FXS port can provide the attached device with -48 VDC to power the phone. Ringing voltage can be sent from the FXS port to the device, and the FXS port can recognize digits dialed by the attached device.

Figure 3-6. FXS Connections


A Cisco router is configured using the Cisco Internetwork Operating System (IOS). Using the IOS, we can configure the characteristics of an FXS port, including the following parameters:

  • Signal type An FXS port on a Cisco router defaults to loop start signaling. However, for some applications, such as connecting a PBX trunk port into an FXS port, we might prefer to use ground start signaling.

  • Call progress tones A call progress tone gives the caller an idea of how the call is progressing. For example, if you call your friend, before your friend answers the phone, you hear ring back in your ear, indicating that your friend's phone is indeed ringing. If your friend is already on the phone when you call, you might instead hear a busy signal. Both ring back and a busy signal are examples of call progress tones. However, these call progress tones might vary from country to country. A Cisco router defaults to call progress tones heard in the United States. However, we can alternately configure the FXS to ports to use call progress tones common to other countries.

  • Ringing pattern If you live in the United States, chances are, when your home phone rings, the ringing lasts for two seconds, followed by a four-second pause, followed by two seconds of ringing, and so on. This ring pattern (sometimes called the ring cadence) might vary in different countries. Fortunately, the Cisco IOS allows us to configure a predefined ringing pattern to be sent out of an FXS port, or we can define a unique ringing pattern.

    If you are configuring several phones in an office (for example, in a cubicle environment), it might be wise to configure different ringing patterns for different phones. Then, when all of the employees are gathered around the water cooler, and a phone rings, an employee will be able to know that it is his phone ringing due to the distinctive ringing pattern you configured.

  • Ringing frequency When I was five years old, my family's home phone was on a party line. (No, not one of those 900 number party lines.) The party line allowed more than one home to share the tip and ring wires going back to the central office (CO). As a result, only one home could use its phone at any one time (unless the homes were talking with each other). But the question is, "If we have more than one phone on the same tip and ring circuit, how can we make only one phone (the phone that was called) on that party line ring?" Back in those days, phones belonged to the phone company. We could not just go down to the local Wal-Mart and buy one like we can today. Because the telephone company controlled who got which phone, it could give a phone with one ring frequency to one party line member and a phone with another ring frequency to another party line member. These phones had a mechanical ringer, and these ringers were tuned to only ring at a specific frequency. If any of these phones are still being used in your VoIP environment, you might need to adjust the ringing frequency used by an FXS port to make the phone ring. However, this is not a concern for most modern phones, which use piezoelectric speakers. These ringers sound the same, regardless of the ringing frequency we specify.

  • Caller-ID information A popular feature on many home telephones today is caller-ID, which allows a called party to see who is calling them. On an FXS port, we can configure the caller-ID information that the router transmits over the VoIP network to the destination phone.

An FXO port connects to an office (that is, a phone switch such as a PBX or a switch in the local CO). For example, you could connect a router's FXO port to the RJ-11 wall jack in your home (which goes back to the telephone company). Or, you could connect an FXO port into the station side of a PBX, as shown in Figure 3-7. Therefore, we could say that an FXO port acts like a phone. It can place calls, receive calls, and dial digits (using either dual tone multifrequency [DTMF] or pulse dialing).

Figure 3-7. FXO Connections


Using the IOS on a Cisco router, we can configure the characteristics of an FXO port including:

  • Signal type Just as we can select between loop start and ground start signaling on an FXS port, we can also select the signal type of an FXO port. Like the FXS port, the FXO port defaults to loop start signaling.

  • Ring number When I was first learning about VoIP, I connected a router's FXO port to my home telephone line, and set the router up so that a phone connected to an FXS port could call out to the PSTN. What I didn't consider, however, was that an FXO port can answer a call, and, by default, an FXO port answers a call after only one ring. So, when I had that FXO port connected into the phone wall jack in my home, if someone called my home, the FXO port on the router answered. As soon as the router answered, the caller would hear the dial tone, which was understandably confusing for the caller. The reason a caller would hear the dial tone was the FXO's ring number (that is, the number of rings received on an FXO port before the port answers the call) was at the default value of one. By default, an FXO port plays the dial tone when it answers a call, allowing the caller to call another number known to the router. However, the FXO port supports other options. For example, an FXO port can be configured to forward a call to a predetermined number after it answers, or the FXO port could look at the dialed digits and forward the incoming call based on those dialed digits.

  • Dial type On your home phone, you probably have an option of selecting either pulse dialing or DTMF dialing. Most locations in the United States now have COs that support DTMF dialing. However, in some parts of the world, we might need to use pulse dialing (that is, the type of dialing used by older rotary phones); and we have the option of changing the dial type on our Cisco FXO port from the default of DTMF dialing to pulse dialing.




Voice over IP First-Step
Voice over IP First-Step
ISBN: 1587201569
EAN: 2147483647
Year: 2005
Pages: 138
Authors: Kevin Wallace

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