Introduction to Analog Circuits

Analog (in contrast to digital) circuits are still the most common telephone connections worldwide. The phone line to a North American home is most commonly an analog loop circuit, although more and more digital phone services are being installed. Cisco gateways must connect to various analog services to place calls to the PSTN; the analog circuits that Cisco supports are Foreign Exchange Station (FXS), Foreign Exchange Office (FXO), and Earth and Magneto (E&M). This section examines the components of an analog telephone and the signaling methods used by analog circuits.


Components of an Analog Phone

An analog phone includes the following components:

• Receiver: The handset speaker

• Transmitter: The handset microphone

• 2-wire/4-wire hybrid: Converts 2-wire from the CO to 4-wire in the phone

• Dialer (tone/pulse): The dialing keypad or rotary dial

• Switch hook: The switch that closes/opens the circuit (off-hook/on-hook)

• Ringer: Sounds to indicate inbound call

Foreign Exchange Station

An FXS port connects directly to an analog phone or fax machine. Switches (including CO switches and PBXs) and Cisco gateways will have FXS ports to connect an analog phone. The switch or FXS gateway port must provide power, call progress tones, and dial tone to the analog device. An FXS port on a gateway is also the direct connection to the VoIP network and consequently also contains a coder-decoder (Codec) to convert the analog signal to digital for packetization. Alternatively, a Cisco Analog Telephony Adapter can serve as a remote FXS-to-Ethernet converter to connect an analog station to the VoIP network.

Foreign Exchange Office

An FXO port connects to the PSTN CO switch. If you want to connect your gateway router to the phone company over standard analog lines (that you could plug your analog phone into), you use FXO ports. These ports allow the gateway to place and receive calls to/from the PSTN. FXO ports also include a codec.


Loop-start signaling is commonly associated with local loop circuits (such as an analog line to the PSTN); it is seldom seen on trunk connections. A local loop is a two-wire service that uses very simple electrical signaling; remember that this technology has been in use and substantially unchanged for 100 years!

Following is the loop-start process:

1. A phone that is on-hook breaks the electrical circuit; we say opens the circuit. No electricity can flow because of the open circuit.

2. When the receiver is lifted, the circuit closes and electricity flows. This current is -48V DC. The CO switch that is connected to the local loop detects the current flow and interprets this as an attempt to place a call—we say "seize a circuit." The CO switch plays dial tone down the line to the phone as an indication that it is prepared to collect digits.

3. If the phone is on-hook and the CO switch has a call inbound for it, the CO switch applies 90V AC current to the open circuit; because it is AC, the current can be applied even on the open circuit. By the way, this is why you should not have an analog phone near the bath. The DC voltage won't do much, but you will definitely know it if the phone rings and you get zapped by the AC voltage.

Loop-start works very well for homes or other lightly used circuits, but if it is in constant use, a problem known as glare can occur; this refers to both ends of the circuit being seized at the same time, so that you pick up the phone and there is a caller on the line at the same moment, by coincidence.


Ground-Start Signaling

Ground-start signaling is an adaptation of loop-start. Instead of the circuit being closed only at the phone end, both ends of the circuit have the capability to detect current, and both ends can request and confirm the use of the circuit. This is achieved by both ends being able to ground one of the wires in the circuit. These wires (or leads) are referred to as Tip and Ring. These terms date back to the use of 1/4-inch jacks with a positive contact at the tip and a negative conductor in the ring. The advantage is that it makes glare much less likely, and consequently ground-start is appropriate for trunk connections that are heavily used. However, it is very rare to see a ground-start trunk in a VoIP network or indeed in any new trunk deployment.

The ground-start process as it occurs on a trunk between a PBX and the CO switch is described next; refer to the diagram for each step:

1. The PBX has a call that it must send to the PSTN. It signals to the CO switch that there is an inbound call by grounding the ring lead.

2. The CO senses the ring lead as grounded and grounds the tip lead to signal the PBX that it is ready to receive the call.

3. The PBX senses the tip ground and closes the loop between tip and ring in response; the PBX also removes the ring ground.

4. The voice circuit is complete, and communication can begin.


E&M Signaling

Variously called "Ear and Mouth," "RecEive and TransMit," and "Earth and Magneto," E&M analog trunks were typically used to interconnect PBXs (tie-trunks). E&M connections have separate leads for signaling and voice; the signaling leads are known as the E and M leads.

In an E&M connection, one side is called the trunk side; this is usually the PBX side. The other side is called the signaling-unit side; this is the CO, channel-bank, or Cisco gateway E&M interface. The E lead is used to indicate to the trunk side that the signaling-unit side has gone off-hook; conversely, the M lead is used to indicate to the signaling-unit side that the trunk side has gone off-hook.

Five types of E&M signaling exist, numbered Type I through Type V. In a Cisco Gateway application, Types II and V can be connected back-to-back and Type I cannot be. Cisco does not support Type IV.

Three main techniques are employed in E&M circuit signaling:

• Wink Start: The terminating side (for example, a Cisco Gateway) uses a brief off-hook-on-hook "wink" to acknowledge that the originating side (for example, a PBX) has gone off-hook. Upon receipt of the wink, the originating side begins sending digits. When the far-end device answers the call, the terminating side goes off-hook and the voice circuit is then set up.

• Immediate Start: The originating side goes off-hook, waits a set time (perhaps 200ms), and then begins sending digits whether or not the terminating side is ready.

• Delay Dial: Assume that a PBX is placing a call outbound to the PSTN: First, the PBX goes off-hook. The CO then goes off-hook until it is ready to receive digits; it then goes on-hook. (This time period is the delay dial signal.) The PBX sends digits. When the far-end device answers the call, the CO goes off-hook (called Answer Supervision), and the voice circuit is then set up. The advantage of Delay Dial is that some equipment is not ready to receive digits instantly, even though it has sent the wink; the delay compensates for this.