FXS and FXO question

Unanswered Question
Aug 23rd, 2007

What is the expected behavior if an FXS port is connected to the CO?

I have this problem too.
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rob.huffman Thu, 08/23/2007 - 10:10

Hi Aaron,

My guess is that connecting an FXS port to the CO would do either nothing (no Off Hook detection) or perhaps Busy out the CO circuit (permanent Off Hook). As a Station card the FXS will not have the ability to communicate properly with the Central Office Exchange :)

Analog telephony signaling, the original signaling protocol, provides the method for connecting or disconnecting calls on analog trunks. By using direct current (DC) over two-wire or four-wire circuits to signal on-hook and off-hook conditions, each analog trunk connects analog endpoints or devices such as a PBX or analog phone.

To provide connections to legacy analog central offices and PBXs, Cisco CallManager uses analog signaling protocols over analog trunks that connect voice gateways to analog endpoints and devices . Cisco CallManager supports these types of analog trunk interfaces:

Foreign Exchange Office (FXO) Analog trunks that connect a gateway to a central office (CO) or private branch exchange (PBX).

Foreign Exchange Station (FXS) Analog trunks that connect a gateway to plain old telephone service (POTS) device such as analog phones, fax machines, and legacy voice-mail systems.

From this good doc;


FXS and FXO Interfaces

An FXS interface connects the router or access server to end-user equipment such as telephones, fax machines, or modems. The FXS interface supplies ring, voltage, and dial tone to the station and includes an RJ-11 connector for basic telephone equipment, keysets, and PBXs.

An FXO interface is used for trunk, or tie line, connections to a PSTN CO or to a PBX that does not support E&M signaling (when local telecommunications authority permits). This interface is of value for off-premise station applications. A standard RJ-11 modular telephone cable connects the FXO voice interface card to the PSTN or PBX through a telephone wall outlet.

FXO and FXS interfaces indicate on-hook or off-hook status and the seizure of telephone lines by one of two access signaling methods: loop start or ground start. The type of access signaling is determined by the type of service from the CO; standard home telephone lines use loop start, but business telephones can order ground start lines instead.

From this very descriptive doc;


Hope this helps!


Aaron Dhiman Thu, 08/23/2007 - 10:15

Hi Rob, thanks for the info. The problem we have is a remote location of our has no FXO ports and has connected FXS ports to the CO there. They claim that it all works, but we are seeing problems with duplicate DTMF digits being generated across those lines. Is it possible that the cause of the duplication DTMF digits is b/c of the incorrect FXS-CO configuration?


rob.huffman Thu, 08/23/2007 - 10:45

Hi Aaron,

Wow! Are they sure this is a FXS port? I can't ever see this working and I guess it really isn't from you explanation. These Duplicate DTMF Digits are usually due to a problem with the Inter-Digit Timers between the CO and Router. I guess if this really is an FXS port maybe this would explain the problem.

I'm not sure what else I can tell you my friend :(


j.sillers Mon, 09/17/2007 - 08:45

Perhaps they are actually connecting to the DID/FXS cards in which case they are in DID mode. Then they could connect to the CO.

I would verify with them which module they have exactly.

Aaron Dhiman Mon, 09/17/2007 - 08:48

The situation was actually a Panasonic PBX that had FXO to a Cisco Router gateway and connections to their LEC that they told me were FXS. After prying deeper into it, it turned out that they were mistaken, and FXO ports were actually being used.


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