Informational Signaling

DTMF tones are used not just for address signaling but also for informational signaling. Specifically, call-progress indicators in the form of tone combinations are used to notify subscribers of call status. Each combination of tones represents a different event in the call process, as follows:

• Dial tone Indicates that the telephone company is ready to receive digits from the user telephone. Cisco routers provide dial tone as a method of showing that the hardware is installed. In a PBX or key telephone system, the dial tone indicates the system is ready to receive digits.

• Busy Indicates that a call cannot be completed because the telephone at the remote end is already in use.

• Ringback (CO or PBX) Indicates that the telephone switch is attempting to complete a call on behalf of a subscriber.

• Congestion Indicates that congestion in the long-distance telephone network is preventing a telephone call from being processed. The congestion tone is sometimes known as the all-circuits-busy tone.

• Reorder Indicates that all of the local telephone circuits are busy, thus preventing a telephone call from being processed. The reorder tone is known to the user as fast-busy and is familiar to anyone who operates a telephone from a PBX.

• Receiver off hook Indicates that the receiver has been off hook for an extended period without placing a call.

• No such number Indicates that a subscriber placed a call to a nonexistent number.

Trunk Connections

Before a telephone call terminates at its final destination, the call is routed through multiple switches. When a switch receives a call, it determines whether the destination telephone number is within a local switch or if the call needs to go through another switch to a remote destination. Trunks interconnect the telephone company and PBX switches, as shown in Figure 2-9.

The primary function of the trunk is to provide the path between switches. The switch must route the call to the correct trunk or telephone line. Although many different subscribers share a trunk, only one subscriber uses it at any given time. As telephone calls end, they release trunks and make them available to the switch for subsequent calls. There can be several trunks between two switches.

The following are examples of the more common trunk types:

• Private trunk lines (tie-lines) Companies with multiple PBXs often connect them with tie trunk lines. Generally, tie trunk lines serve as dedicated circuits that connect PBXs. On a monthly basis, subscribers lease trunks from the telephone company to avoid the expense of using telephone lines on a per-extension basis. These types of connections, known as tie-lines, typically use special interfaces called recEive and transMit, or E&M interfaces.

• CO trunks A CO trunk serves as a direct connection between a PBX and the local CO that routes calls; for example, the connection from a private office network to the public switched telephone network (PSTN). When users dial 9, they are connecting through their PBX to the CO trunk to access the PSTN. CO trunks typically use Foreign Exchange Office interfaces. Certain specialized CO trunks are frequently used on the telephony network. A direct inward dial trunk, for example, allows outside callers to reach specific internal destinations without having to be connected via an operator.

• Interoffice trunks An interoffice trunk is a circuit that connects two local telephone company COs.

• Foreign exchange (FX) trunks FX trunks are interfaces that are connected to switches supporting connections to either office equipment or station equipment. Office equipment includes other switches (to extend the connection) and Cisco devices. Station equipment includes telephones, fax machines, and modems.

Trunk Signaling

Lines and trunks must adhere to signaling standards just as telephony networks and telephone companies do. Trunk signaling serves to initiate the connection between the switch and the network. There are five different types of trunk signaling, and each applies to different kinds of interfaces, such as FXS, FXO, and E&M:

• Loop-start signaling

• Ground-start signaling

• E&M wink-start signaling

• E&M immediate-start signaling

• E&M delay-start signaling

The following sections explain these signaling types.


Loop-Start Signaling

Loop-start signaling allows a user or the telephone company to seize a line or trunk when a subscriber is initiating a call. It is primarily used on local loops connecting to residences rather than on trunks interconnecting telephone switches.

A telephone connection exists in one of the following states, as illustrated in Figure 2-10:

• Idle (on hook)
• Telephone seizure (off hook)
• CO seizure (ringing)

A summary of the loop-start signaling process is as follows:

1. When the line is in the idle state, or on hook, the telephone or PBX opens the two-wire loop. The CO or FXS has battery on ring and ground on tip.

2. If a user lifts the handset off the cradle to place a call, the switch hook goes off hook and closes the loop (line seizure). The current can now flow through the telephone circuit. The CO or FXS module detects the current and returns a dial tone.

3. When the CO or FXS module detects an incoming call, it applies AC ring voltage superimposed over the 48 VDC battery, causing the ring generator to notify the recipient of a telephone call. When the telephone or PBX answers the call, thus closing the loop, the CO or FXS module removes the ring voltage.

Loop-start signaling is a poor solution for high-volume trunks because it leads to glare, which is the simultaneous seizure of the trunk from both ends. Glare occurs, for example, when you pick up your home telephone and find that someone is already at the other end.

Glare is not a significant problem at home. It is, however, a major problem when it occurs between switches at high-volume switching centers, such as long-distance carriers or large PBX systems.


Ground-Start Signaling

Ground-start signaling, illustrated in Figure 2-11, is a modification of loop-start signaling that corrects for the probability of glare. It solves the problem by providing current detection at both ends.

Although loop-start signaling works when you use your telephone at home, ground-start signaling is preferable when there are high-volume trunks involved at telephone switching centers. Because ground-start signaling uses a request or confirm switch at both ends of the interface, it is preferable over other signaling methods on high-usage trunks, such as FXOs. FXOs require implementation of answer supervision (reversal or absence of current) on the interface for the confirmation of on hook or off hook.


E&M Signaling

E&M signaling supports tie-line type facilities or signals between voice switches. Instead of superimposing both voice and signaling on the same wire, E&M uses separate paths, or leads, for each.

To call a remote office, your PBX must route a request for use of the trunk over its signal leads between the two sites. Your PBX makes the request by activating its M-lead. The other PBX detects the request when it detects current flowing on its E-lead. It then attaches a dial register to the trunk and your PBX, which sends the dialed digits. The remote PBX activates its M-lead to notify the local PBX that the call has been answered.

There are five types of E&M signaling: Type I, Type II, Type III, Type IV, and Type V. The E&M leads operate differently with each wiring scheme, as shown in Table 2-1 and Table 2-2. Keep in mind that any of the E&M supervisory signaling types (that is, wink-start, immediate-start, and delay-start) can operate over any of the following wiring schemes.