FXO and FXS Ports
Given that FXO and FXS ports have a lot of similarities, they are going to be described together. In addition to their similarities, the most common differences between FXO and FXS ports will also be highlighted and described in this section.
Foreign Exchange Station (FXS) ports are the ports that you plug a telephone, fax machine, or modem into. These ports provide the telephony service for these analog devices. The FXS port has the ability to provide ring voltage, dial tone, and other basic signaling to the end station. The FXS port connects with a standard RJ-11 connection. The standard RJ-11 pinout and pinout signals are illustrated in the following diagram:
Like FXS ports, FXO ports also connect with a standard RJ-11 connection. However, rather than supplying the signaling and voltage needed for basic telephony equipment, FXO ports are used to connect subscriber devices to the CO or customer PBX to receive subscriber services.
When using FXO ports, it is important to use only an FXO port that is approved for the specific country or region to connect to the PSTN. If this is not possible, then the FXO port should be connected to a PBX, which would then be connected to the PSTN instead. This is because FXO ports emulate the operation of a telephone handset waiting for ring voltage from the CO switch.
When deciding on FXO and FXS ports, it is important to keep in mind that FXS ports should never be connected to the PSTN because the FXS interface emulates the CO switch to the endpoint device by providing dial tone and ring voltage. Do not connect endpoints to an FXO port. If an FXO was inadvertently used in place of the FXS, the endpoint may receive signaling; however, it will be unable to ring the phone to the alert the user of the incoming connection. Additionally, if an FXS port is used in place of the FXO, it will not anticipate the ring voltage on the line, thus it will be unable to close the loop to complete the connection.
Cisco FXO ports support a feature referred to as FXO Power Failover. The FXO Power Failure feature is a hardware feature built into the FXO cards that allows connectivity to an analog phone patched into the right pair of wires to be activated by a relay if power to the Cisco router containing the High-Density Analog Network Module (NM-HDA) module fails. The High-Density Analog Network Module supports an RJ-21 connector and both FXS and FXO traffic.
This allows PSTN calls to be made via the FXO line normally connected to the router from a designated emergency phone in the office while power is out. For an FXO Expansion Module (EM) in slot 0 of the NM-HDA, the analog phone must be connected to pair 14 on the RJ-21 connector to take advantage of the FXO Power Failover feature. For an FXO EM in slot 1 of the NM-HDA, the analog phone must be connected to pair 24 on the RJ-21 connector to take advantage of the FXO Power Failover feature. If both EMs are populated with FXO modules, two emergency phones can be used.
FXO and FXS ports share the same signaling types. These signaling types are used to 'seize' the line in preparation for a call. This line seizure is also referred to as 'starting' since it completes the circuit, enabling voltage to flow, thus 'starting' the call. The two signaling types used by FXO and FXS ports are ground-start and loop-start signaling. Both of these signaling types will be described in detail shortly.
Both ground-start and loop-start have three signaling states, which are the idle, seizure and ring states. In the idle state, the handset is on-hook and no voltage is being applied to indicate an incoming call. The seizure state occurs when the handset goes off-hook. This state completes the loop and allows current to flow. In the ring state, voltage is passed to the ring generator to indicate an incoming call. The same wires that carry voice are used by loop-start and ground-start to supply signaling, i.e. the signaling is performed in-band. This technique enables telephone companies to reduce the number of wires terminated at the subscriber location.
Loop-start signaling is used on local loops to initiate a call. It involves the breaking and connecting of the 48V circuit loop originating from the CO. In the on-hook state, there is a break in the loop so no voltage is passing. When the telephone set goes off-hook, the loop is closed and current flows.
On the remote end, i.e. the called party, when the incoming call is detected, the CO or PBX equipment supplies AC current to the ring line, causing the ring generator to active, thus ringing the telephone. When the call is answered, the loop completes and voltage ceases to flow to the ring generator and the call is connected. The diagram illustrated below shows how loop-start signaling operates on a telephone connected to a Cisco router FXS port:
Loop-start signaling works quite well for simple, single-line connections such as homes. It also has the advantage of not requiring a common ground between the CO or FXS port and the subscriber connection. However, it does have some disadvantages as well.
The first disadvantage is that loop-start signaling is prone to glare, which is a condition that occurs when two parties want to speak to each other and call each other simultaneously, thus getting a busy signal and getting a busy tone. The second disadvantage is that loop-start signaling does not provide remote-end disconnect monitoring. In other words, there is no mechanism to detect that the remote end has returned to the on-hook state. It is for these reasons that ground-start signaling was developed.
Ground-start signaling works in an almost similar manner to loop-start signaling. However, ground-start signaling can detect when loops have been seized at both ends and does not have the inherent glare weaknesses of loop-start signaling.
Ground-start signaling requires that the 48V loop be grounded on both sides of the connection. When the line is idle, the subscriber has a break in the ring and the CO or FXS port has a break in the tip and no voltage is supplied. When the subscriber equipment goes off-hook, the ring is grounded, allowing voltage to pass through and when the CO detects this, it grounds the tip. When the subscriber equipment detects the tip ground, it closes the loop and removes the ring ground, which closes the circuit.
As is the case with loop-start signaling, when a call comes in, AC voltage flows over the ring wire, causing the ring generator to produce a ring tone. The CO or FXS senses the current flowing from the tip and ring loop, and then removes the ringing tone. The PBX or FXO must sense the tip ground and ringing within 100 ms or the circuit times out and the caller has to reorder the call. This 100-ms timeout helps prevent glare. The figure that follows illustrates the basic operation of ground-start signaling:
FXO and FXS ports use analog address signaling, which can either be pulse dialing or DTMF tones. This address signaling is transmitted only from the FXO port to the FXS port and is used to indicate the final destination of the call. This is often confusing a confusing concept; however, as you may recall, earlier in this chapter we learned that FXO ports are the ports on subscriber devices, such as analog telephones, fax machines and modems that connect to the PSTN or PBX by way of an FXS port. This means that when you dial digits on a telephone set, for example, they leave the FXO port on the telephone set and are sent to the FXS port connected to the PBX, PSTN, or Cisco voice gateway with FXS modules.
This means that informational signaling is provided by only an FXS port, which emulates the Central Office. FXS ports use call progress (CP) tones, i.e. network progress or alerting tones, to indicate the status of calls. Functions are determined by the frequency of the tone that is sent, as well as by the cadence, i.e. the tone-on and tone-off durations, of that tone.
For example, dial-tone is comprised of two interfering tones between 350 Hz and 440 Hz in the U.S. with the busy signal being 480Hz and 620 Hz, whereas it comprises a constant single tone of 425 Hz in most of Europe. The default cadence in the US is 2 seconds of ringing tone followed by 4 seconds of silence and ½ a second on and off for a busy signal; however, the European standard uses a double ring followed by 2 seconds of silence.
It is important to remember that CP tones are country specific. Keep this in mind because Cisco voice gateways default to U.S. CP tones, which are not used worldwide. CP tones can be adjusted by using the cptone [locale] command under the voice port. The following output illustrates the options available with this configuration command:
|locale 2 letter ISO-3166 country code|
|C2||Custom2||KR||Korea Republic||ZA||South Africa|
|DE||Germany||NZ||New Zealand||GB||United Kingdom|
The selected tone can then be viewed in the gateway running configuration as follows:
R1#show running-config | begin voice-port 0/0/1
The show voice port command can also be used to view the selected tone as illustrated in the Cisco voice gateway output printed below:
R1#show voice port 0/0/1 | include Tone
Region Tone is set for ZW