Understanding Cisco Voice Gateways - Part 3. Loop-start and Ground-start Signaling (FXS and FXO Ports)

Now you already know what Cisco Voice Gateways are and which types of analog voice ports they can use. It is time to discuss signaling processes on FXO and FXS ports. As it was shown earlier, voice ports on routers and gateways physically connect them to telephony devices such as telephones, fax machines, PBXs, and PSTN CO switches. These devices may use any of several types of signaling interfaces to generate information about on-hook status, ringing, and line seizure. Traditional two wired FXS and FXO voice ports typically use so called Loop-Start and Ground-Start supervisory signaling. I am going to explain how these types signaling work in the in the next few paragraphs.

In telephony signaling techniques can be placed into one of three categories:

• Supervisory: provides information on line or circuit condition. It informs a switch whether a circuit (internal to the switch) or a trunk (external to the switch) is busy or idle, when a called party is off-hook or on-hook, and when a calling party is on-hook or off-hook.
• Addressing (Routing): Involves passing dialed digits (pulsed or tone) to a PBX or CO. These dialed digits provide the switch with a connection path to another phone or another telephone system. In other words it directs and routes a telephone call to the called subscriber.
• Informational: Provides audible tones, which indicate certain conditions such as an incoming call or a busy phone, to the user.

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. Both of them belongs to supervisory signaling. The type of access signaling is determined by the type of service from the telephone switch; standard home telephone lines use loop-start, but business telephone connections can order ground-start lines instead.

Loop-start is the more common access signaling technique. The Loop-start signaling process is as follows:

1. In the idle state, the telephone (or FXO module), which is connected to a telephone switch (CO - Central Office), has an open two-wire loop (the tip and ring lines are open). The CO supplies the -48VDC power to the open line and waits for a closed loop that generates a current flow. The CO has a ring generator connected to the tip line and –48 VDC on the ring line.

2. A telephone set or FXO module closes the loop between the tip and ring lines. The telephone handset goes off hook or FXO module closes a circuit connection. The CO detects current flow and then generates a dial tone, which is sent to the telephone set or FXO module. The dial tone indicates that the customer can start to dial. After analyzing dialed digits, the CO finds a destination port and seizes the ring line of the called telephone by superimposing a 20-Hz, 90-VAC signal over the –48VDC ring line. This procedure rings the called party telephone and signals that there is an incoming call. 

3. The CO removes this ring once the called-party goes off-hook and closes the circuit between the tip and ring lines. Then the CO connects both sides and the call goes to conversation.

There are two disadvantages with Loop-start signaling:

• There is no way to prevent the CO and the user from seizing the same line at the same time. This condition is known as glare. Glare can sometimes be experienced as trying to make a call out on an FXO port, and instead being connected to an incoming call. This can happen at home too, if a call out is made at the exact second that a call is about to start ringing in. 
• There is no far-end disconnect supervision. When the remote handset/line hangs-up, there is no provision for the CO switch to notify the voice gateway (FXO) of the disconnection (not a problem with a telephone). The CO expects the phone on line, right? And therefore when far-end side disconnects, the CO just sends the acoustic signal known as Busy Tone to the line. The line will be disconnected only if near-end party hangs up. 

These disadvantages explain why loop-start signaling is used primarily when a connection is made from the telephone set to a switch. The best way to prevent glare on FXOs is to use Ground-start signaling.

Ground-start signaling is another supervisory signaling technique, like loop-start, that provides a way to indicate on-hook and off-hook conditions in a voice network. Ground-start signaling is used primarily in switch-to-switch connections. The main difference between ground-start and loop-start signaling is that ground-start requires ground detection to occur in both ends of a connection before the tip and ring loop can be closed.

Let's see how Ground-start signaling works in details: 

1. In the idle state, both the tip and ring lines are disconnected from ground. The FXO constantly monitor the tip line for ground, and the CO switch constantly monitor the ring line for ground. Battery (–48 VDC) is still connected to the ring line just as in Loop-start signaling.

2. An FXO port grounds the ring line to indicate to the CO that there is an outgoing call (from FXO prospective). The CO senses the ring ground and then grounds the tip lead to let the FXO know that it is ready to receive this call.

3. The FXO senses the tip ground and closes the loop between the tip and ring lines in response. It also removes the ring ground. The CO senses the loop and provides its dial tone. The user now can dial a number for his outgoing call. 

4. To setup a call to FXO port (an incoming one from FXO prospective), the CO grounds tip and put the ringing 20-Hz, 90-VAC signal on the line. To avoid glare, before the FXO originates its outgoing call, it must first verify that the CO has not already applied ground to tip. The FXO has 100ms to sense this condition.

5. At the end of either an incoming or outgoing call, the FXO initiates disconnect by going on hook, or the central office initiates disconnect by opening tip. When the other end detects the loss of loop current, it also goes on hook and the call clears normally.

As you see, Ground-start signaling 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. Because Ground-start signaling uses a request or confirm switch, or both, at both ends of the interface, it is preferable over FXOs and other signaling methods on high-usage trunks.

Loop-start and Ground-start signaling are not compatible. So when you configure your FXS or FXO ports on the gateway, be sure to select appropriate type of signaling, which is supported by the far-end side!!!

To be continued...