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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.
FIGURE 8
Loop-Start Signaling
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!
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
© 2008 Cisco Systems Inc. All rights reserved. This publication is protected by copyright. Please see page 147 for more details.
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.
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