Howto:Analog 2-wire telephony

From innovaphone-wiki

Jump to: navigation, search

This article describes few general topics that are helpful for understanding analog telephony.

To avoid duplication with our product specific articles here are explained just the principles, for possible setups and further innovaphone features see related articles.

How analog telephony works is, as any technology if observed precisely, a complex topic. In order to understand our analog gateways, however, it is not necessary to know all the details.

Note: In the innovaphone documentation, you will usually find the term “analog device” or “analog endpoint”; this is the technical correct expression for telephone, fax machine, dialer, modem etc. However, because the typical analog device is an analog telephone and for simplicity, this documentation always mention "telephone" or “phone” meaning any “analog endpoint”.


The FX interface

Some innovaphone gateways have FXS interface for connecting analog phones while some models have an FXO interface for connecting an analog trunk line.

"FX" stands for "Foreign eXchange", “S” for Source and “O” for Office.

The FXS interface supplies voltage (is a “voltage source”) while the FXO interface expects a voltage like a telephone. An FXS and an FXO can thus functionally connected to one another, while FXS with FXS or FXO with FXO is a senseless connection and will not work.

An analog telephone set is connected to an FXS interface and the phone must be supplied with power at least during conversation and ringing, but electronic could even require (small) power in idle mode.

An FXO interface is connected to an analog exchange line, which supplies voltage and ringing AC current; the FXO is more like an analog telephone, passive.

At idle, a FXS interface shows typically 24V, 32V or 48V, in the public telephone network up to 70 volts direct current (DC). You can measure it with a simple multimeter and so e.g. quickly determine whether "something" arrives at all.

Ringing on FXS interfaces is done with AC voltage, typically 25Hz or 50Hz with voltages around 70 volts, which you can “feel” with your fingers, uncomfortable but harmless because there is a current limitation.

a/b Interface

Analog telephony works with 2 wires, whereby the "a” wire and the "b” wire give the name. While in the IP-world, signaling and speech are separated, and even direction for voice and data is separated, in analog telephony, all this is together in any time on just 2 wires.

The “b” wire is grounded (i.e. 0 volts) and the “a” wire carries voltage, in older telephone systems this is negative (e.g. -48V) and in some country it is different again (as always). With modern endpoint devices it is irrelevant whether the voltage of the “a” wire is negative or positive and also there is no polarity required, so “a“ and “b” wire can be switched (but not always…). Depending on the system, the ringing current can be symmetrical (i.e. on the “a” and “b” wire) or, as is more usual, only on the “a” wire.

The a/b wires are twisted, but not necessarily with short connections like with a telephone where usually flat wire cables are used. In cables with several pairs, the pairs are twisted between each other. Twisting the wire pairs prevents the cables from running parallel to each other over a longer distant. Parallel lines act like a capacitor and the longer the line becomes the larger the size of this capacitor is. Capacitors block DC voltage but conduct AC voltage; the conduction is better the higher the frequency or size of the capacitor is. A long non-twisted line (or an equivalent capacitor) will attenuate high frequencies and cause other problems like crossover talk from one line to the other.

RJ Interface

The a/b lines can be connected in dozens of variants, the most common, which can also be found in the innovaphone gateways and analog phones, are RJ-11 connectors.

"RJ" stands for "Registered Jack", i.e. standardized socket. There are "contact positions" for the plugs, which are, so to speak, possible contacts (which are sometimes not electrically present) and actually used contacts. So there is "Position" and "Contact" and an RJ-11 is e.g. a 6P2C connector, can theoretically have 6 contacts in width, but in fact only 2 (the two middle ones, for the a/b) are used. The IP29-20, on the other hand, has an RJ-45 8P8C interface, i.e. 8 positions and all positions are wired contacts. This setup is identical in construction to an Ethernet interface.

The contacts are numbered, if you look to the connector (not the plug) with the contacts up (the “nose” is down) numbering starts from the left to the right. A RJ-11 has the a/b in the middle contacts (3-4). With the RJ-45 the numbering goes from 1-8, the a/b pairs are located on the two central contacts (4-5) as with RJ-11, the two outer contacts (1-2 and 7-8) and on pin 3-6. This corresponds to the Ethernet pinning.


Each telephone has a loudspeaker and a microphone, i.e. 4 lines that lead from the handset to the device. In a digital Phone those pairs end up once to a digital to analog converter and the other pair to an analog to digital converter following then digital elaborations. With analog telephones, the four lines must be mixed into a single pair and vice versa. A bridge transformer (Hybrid coil) does this.


X and Z are the two pair to speaker and microphone, voice on W is in both directions on just one pair.

If you pick up the receiver, the hook switch close the loop to the transformer.

a/b interfaces are short-circuit and no-load proof. Therefore if you shortcut the a/b interface from a logical point of view this is like picking up the receiver, the shortcut is not threated different in the PBX.

Pick-up and release

When you pick up the receiver, the hook switch close the circuit between “a” and “b” by the transformer and a direct current begins to flow. The electronics in the ATA recognize that there is DC flow on the port and interprets this as "off-hook". Conversely, if you hang up the current flow is interrupted and this is detected as “on-hook”. As you can imagine for all that there are thresholds in term of current value and timing.

The bridge transformer or better “the telephone” has a DC resistance of approx. 300-600 Ohms. One problem, ignoring the complex real parameters such as capacities and coils, is the line resistance; Phone lines are quite thin and therefore have a relatively high resistance. The line resistance is in series with the phone. Therefore, not only a part of the available power is consumed in it, there is also little voltage on the phone when current starts to flow because the line is a Voltage divider. If for example the line resistant is equal to the phone resistant just 50% of the voltage is on the phone in off-hook. However, analog telephones are usually surprisingly robust and economical in power consumption and the phone electronics usually still work. The problem often begins when the phone rings and the user pick up, now the DC current begins to flow. This current is in the very first moment mostly quite high but after a couple of seconds this first current peak goes down. If the line resistance and telephone resistant together is high, the current becomes small. If that current goes below a threshold, the electronics in the ATA assume that the subscriber has hung up. In practice, the phone rings, you pick up and get busy. Surprisingly that sometimes, outgoing calls on that phone are possible. Another typical symptom is just a short ring. Long cables have a capacitive component and the hight Voltage of the AC Power in ringing cause also a DC component flow. Now the electronics in any PBX must recognize even during the ring when the phone goes off-hook and stop immediately the ringing. If not the AC would go in the transformer and to the receiver; practically you “ring” into the loudspeaker. (Long) Lines confused this detection and therefore on the ATA side innovaphone allows to setup the Loop Current in connected and in ringing mode. Consider that cables often take long distances in buildings. In practice, simple formulas such as “300 meters” could be wrong; further or shorter cable lengths may be possible. Because not just the cable quality and diameter is important; cables can also be spliced or connected via floor distributors, in such cases there are also transition resistances to consider and the possibility that not for the entire cable route the same type of cable is used.

Thicker cables, avoid splicing, installing that ATA near the phones e.g. in the floor distributor are solutions. However, this is often not possible, and especially in larger installations, customers assume that the "new" PBX must be installed in place of the previous telephone system, especially since all the cables are already nicely listed here in a main distributor.

The only thing left to do is to increase the voltage on the ATA, do an optimization of the source resistant, adjust ringing voltage, and modify thresholds. All that is possible with innovaphone analog ports; see relative help.

Set up a port in “Hight Power mode” means also more Power required for that port (not in idle mode but during ringing and speaking). While a traditional PBX has a near unlimited power supply using an AC grid Power Supply and large battery, an ATA has due to the PoE a limitation in that. A PoE can provide max. 12,5 Watt to the ATA, subtract the required Power for CPU and electronics and divide this remaining power by the number of ports. If your ATA has many ports and you put all ports on High Power mode not all Ports can deliver that power in the same moment. Therefore, you should set as few ports as possible to high power or make sure that all frequent callers are connected on one single GW. The trick is to distribute these high power ports over several ATA.


Ringing is done using AC Voltage. Old phones have a mechanical bell, modern types an electronic circuit (which we still call bell here). The bell is connected with a capacitor between a/b wire (so that no direct current flows through the bell) and this connection is interrupted when the handset is lifted off (mechanically via the hook switch or electronically blocked). This prevents the bell from ringing during dialing with pulse.

In contrast to an IP Phone the ATA can not be aware if the phone is connected or the phone is really ringing. Vice versa, the phone is not aware what is going on in the PBX. This missing reverse release on the FXS cause a well-known problem for cordless phones: even if the caller gives up the cordless handset continuous ringing for a while. When the caller hangs up the PBX recognizes this immediately and stops the ATA in calling. Therefore, the FXS stops ringing, but for the analog endpoint it is not clear whether it is just the pause between rings or whether the far party has hung up. Cordless base stations send to the handset a ring command, but the base station can only switch the handset off when it is certain that the remote subscriber is no longer calling. That is certainly only after a certain time passed after the last ring.

A similar problem has the FXO interface (that is “like” a phone). In theory, public trunk lines should set a reverse release signal (in form of a short interruption of the power) to the PBX. Nowadays, even large providers often use ATA and an ATA typically cannot deliver the reverse release. Our trunk FXO therefore adopted a similar strategy like the cordless base station.


There are two methods, the now obsolete pulse dialing and the tone or DTMF dialing. The ATA from innovaphone can handle both methods; however, features are only possible with DTMF, so with pulse dialing allows only a simple basic call, usually sufficient for the mostly simple applications such as alarm systems or automatic dialing devices.

Pulse dialing is done by interrupting (open and close) the loop current. For dialing “1” the current is interrupted for 60 ms followed by a pause of 40ms, dialing “0” requires this sequence 10 times. Between numbers, a 200ms pause follows. All that can be build also mechanically as with a rotary telephone 100 Years ago. The huge disadvantage is that dial a longer number takes a long time. Therefore at some point in the 1960s a new faster variant using tones for dialing, called multi-frequency method or Dual-Tone Multi-Frequency (DTMF) starts in the U.S. At each number two tone are assigned, and since the transmission "audio" bandwith ranges from 400 to 3.4 kHz, tones within this frequency range where selected. The two tones for a number (1 to 9, *, #, ABCD) are a mixture of a low and a higher tone. Two tones instead of a single frequency prevent electronics from interpreting language as a dialing information. It is not easy to perform a DTMF tone with the mouth (but possible).

The number "1" is e.g. a sinus tone with 676 Hz superimposed with 1209Hz, the “#” a 941Hz tone with a 1477Hz ton superimposed. The tone has to be constant for 50-100ms and then a pause of 20-50ms must follow; a pretty quickly dialing is possible.

Like all other analog signaling, these tones are transmitted over the two analog lines, i.e. "inband". In the Voip, on the other hand, the signaling is done separately, the DTMF tones are decoded in the ATA and sent separately as a signaling information in TCP/IP ("outband"). The innovaphone IP-phone where you push the number generates the DTMF sound just for for the receiver when you dial in an established connection, also the IP-Phone on the far end generate this from the received signaling information. For comfort and feedback, no real “Tone” is transmitted. However, sometimes it may be necessary to recognize or send DTMF tones in the voice channel (in the RTP stream) even in a VoIP connection, e.g. with some SIP providers. Possible but problematic because the coding and decoding do not reproduce the analog tones perfectly and, similar to FAX, transmission errors can occur.


To put a call on hold and/or activate features, also on an analog phone is an "R" key, which is often referred to as the "Flash" key. Pushing that button the loop circuit opens for 100ms. So it is practically a "briefly" hook-switch on and off, the different to impulse dialing is the time (puls dialing opens 60ms, a flash opens 100ms). Because time of pulse and flash it is quite close to each other, larger tolerances or different timing are often set. The flash button was already available in the impulse dialing times, where it was mechanically build. In many American movie, you can see how fingers are tapped on the hook. In fact, a short interruption is, when you are skillful enough to catch the correct timing, just a flash; and in the US a Flash will transfer you to the (public) operator.

In order to start a feature with analog telephones it is necessary to perform first a Flash, because this is the only way to make a clear distinction between DTMF post selection and feature invocation.


You can see the FXO interface like a Phone, the interface requires to be powered and called with AC. A FXS port can be connected to a FXO port for testing. While shortcut on an FXS Port is equivalent to an off hook and in the PBX starts a signaling on an FXO Port nothing will happen because this port expects Voltage and AC ringing to start up actions.

Some public analog switch can do a backward release. Means that when the caller hangs up there is a short interruption in the power supply to inform the called PBX that the call is released by the far end. This enables the gateway to recognize that the other party has hung up and to cancel its own internal call. However, this does not always work if e.g. an ATA is connected there is no reverse resolution. Therefore, the call cycle is monitored in the gateway, similar to cordless telephones, and if the ring sequence is interrupted, the call ends.

In some countries direct dialing in is or was possible on analog FXO lines, either via special signaling (such as polarity reversal) or with pulse dialing. This is not supported by our FXO, but a DTMF post-dialing (and thus quasi-direct dialing) is possible via a waiting queue Object.

There are some very special setup in some country, and innovaphone supports a lot of parameter.

Related Articles

Personal tools