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New Member


hi, see if i wanted to connect a PBX with a CO switch i normally do it via trunk cards rite ? can i also do it via line cards by simple purchasing some analog lines from pstn and plugging them into the line cards ?? can i do that plz confirm it

2) the signalling types that we study like loopstart, ground and e&m, they are used when i m connecting a pbx with co switch via trunk rite ?? plz tell me i m a bit confused here

thanks in advance plz answer in detail

Hall of Fame Super Gold


Hi Shaila,

it goes like this, loopstart is used all over the world. Only in the US, and only in certain areas, seems like the telco can give provide ground-start in some case. This is not common at all in all other contries. E&M to my knowledge is never used for trunk to the CO, but only for private connections.

Hall of Fame Super Red


Hi Shaila,

Like Paolo indicated you are 99% likely to be using Loop Start to connect the Switch/Line Card to the CO. Here is some further info;

Loop start. This is the most common technique for access signaling in a standard Public Switched Telephone Network (PSTN). Most residential telephones are analog loop-start telephones, based upon the concept of the subscriber loop, or local loop. The loop is an electrical communications path consisting of two wires, one for transmitting and one for receiving voice signals. The two-wire circuit is still referred to as the tip and ring, with the tip being tied to the ground and the ring tied to the negative side of the battery. When the phone handset is picked up (goes off hook), this action closes the circuit, establishing a loop between the PBX and the phone. Current is drawn from the battery of the PBX, indicating a change in status. This change in status signals the current detector in the PBX to provide dial tone. An incoming call is signaled to the handset by a standard on/off pattern, which causes the telephone to ring.

Ground start. Ground start is another access signaling method used on on trunk lines or tie lines between PBXs to indicate on-hook/off-hook status to the CO. In ground-start signaling, one side of the two-wire trunk (typically the ring in the tip and ring configuration) is momentarily grounded to create dial tone.

In a normal loop-start circuit, when you pick up the handset, you hear a dial tone indicating that a circuit is ready. On a ground-start circuit, however, the equipment at the user?s end should sense the flow of electrical current on the "tip" lead and interpret that the PBX is ready, so a dial tone from a PBX is not necessary, and its presence is optional. This setup allows the network to indicate off-hook status, or seizure of an incoming call independent of the ringing signal.

E&M Signaling. Another analog signaling technique, used mainly between PBXs or other network-to-network telephony switches, is known as E&M, which stands for "ear and mouth" (or for "recEive and transMit"). There are five E&M signaling types, as well as two different wiring methods. Cisco's VoIP implementation supports E&M types I, II, III, and V, using both two-wire and four-wire implementations.

From this good doc;

Hope this helps!