FXS vs FXO

Answered Question
Jul 24th, 2007
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Hi all,


I'm currently studying for CCVP and i'm struggling to summarise the differences between FXS and FXO ports - ie what I can connect to what, such as phones, other PBX's, and which port can be used (via a lead) to which kind of port.


Any help on this would be appreciated!


Cheers.

Correct Answer by Rob Huffman about 9 years 9 months ago

Hi Michael,


Here is some background info;


Analog Telephony Protocols


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;


http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00801ec5cc.html#1134121



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.


Loop-start is the more common of the access signaling techniques. When a handset is picked up (the telephone goes off-hook), this action closes the circuit that draws current from the telephone company CO and indicates a change in status, which signals the CO to provide dial tone. An incoming call is signaled from the CO to the handset by sending a signal in a standard on/off pattern, which causes the telephone to ring.


Loop-start has two disadvantages, however, that usually are not a problem on residential telephones but that become significant with the higher call volume experienced on business telephones. Loop-start signaling has no means of preventing two sides from seizing the same line simultaneously, a condition known as glare. Also, loop start signaling does not provide switch-side disconnect supervision for FXO calls. The telephony switch (the connection in the PSTN, another PBX, or key system) expects the router's FXO interface, which looks like a telephone to the switch, to hang up the calls it receives through its FXO port. However, this function is not built into the router for received calls; it only operates for calls originating from the FXO port.


Another access signaling method used by FXO and FXS interfaces to indicate on-hook or off-hook status to the CO is ground start signaling. It works by using ground and current detectors that allow the network to indicate off-hook or seizure of an incoming call independent of the ringing signal and allow for positive recognition of connects and disconnects.


From this very descriptive doc;


http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a0080080afd.html


Hope this helps!

Rob

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Correct Answer
Rob Huffman Tue, 07/24/2007 - 05:18
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Hi Michael,


Here is some background info;


Analog Telephony Protocols


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;


http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00801ec5cc.html#1134121



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.


Loop-start is the more common of the access signaling techniques. When a handset is picked up (the telephone goes off-hook), this action closes the circuit that draws current from the telephone company CO and indicates a change in status, which signals the CO to provide dial tone. An incoming call is signaled from the CO to the handset by sending a signal in a standard on/off pattern, which causes the telephone to ring.


Loop-start has two disadvantages, however, that usually are not a problem on residential telephones but that become significant with the higher call volume experienced on business telephones. Loop-start signaling has no means of preventing two sides from seizing the same line simultaneously, a condition known as glare. Also, loop start signaling does not provide switch-side disconnect supervision for FXO calls. The telephony switch (the connection in the PSTN, another PBX, or key system) expects the router's FXO interface, which looks like a telephone to the switch, to hang up the calls it receives through its FXO port. However, this function is not built into the router for received calls; it only operates for calls originating from the FXO port.


Another access signaling method used by FXO and FXS interfaces to indicate on-hook or off-hook status to the CO is ground start signaling. It works by using ground and current detectors that allow the network to indicate off-hook or seizure of an incoming call independent of the ringing signal and allow for positive recognition of connects and disconnects.


From this very descriptive doc;


http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a0080080afd.html


Hope this helps!

Rob

Rob Huffman Tue, 07/24/2007 - 05:40
User Badges:
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  • Hall of Fame,

    Founding Member

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Hi Mike,


You are most welcome :) Thanks for your kind words and nice rating!


Take care,

Rob

JATINDER KUMAR Wed, 04/27/2011 - 01:35
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Dear Mike,

     i have got a small quesry may be it could sound stupid but looking for some idea..

My setup is we have 3825 router serving as IPT gateway having BRI lines./ cards attached to it. all the calls goes out via that. Now we want to use some ITSP to provide internet dialing and reduce our international calls toll. for this i got one Planet technology device  (VIP-480FO)which support SIP and i configured my account on this this device is having FXO port + LAN ports + WAN port. I want to connect this device to gateway router so that when some one calls it reached gateway and from gateway it gets routed to  VIP-480FO device. but i am getting confused how to connect

then thought of putting FXS card in router and connect FXs of router to FXO of VoIP gateway. i m not clear with the connectivity i m doing, router is very critical and i dont want to end up with something very bad in connectivity which stops everything.

Please advice if the below connectivity makes sense..


IP Phone ==> Call Manager (CUCM 61.13) ==> Gateway (Cisco 3825) + FXS Card ==> FXO port of Planet device (VIP 480-FO)

just want to understand FXS can connects to FXO or not secondly if i try to dial from IP phone which sends the call to FXS, dows my FXS routes the call to FXO... please help..

told you on the top what we try to achive.. if there is some other way please let mi know


please advice


JAtin


Mercy

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