Lab 15c: Configuring Voice over ATM-Part I

Introduction to Analog Telephony

This section describes the telephone network with a focus on analog technology and includes the following topics:

• Telephone call components

• Telephone set components

• Telephone signaling

• Local loops

• Voice switches

• Trunks

• Trunk/line seizure signaling types

• Telephone call procedure

Telephone Call Components

Basic telephone call components consist of a telephone, a local loop, a voice switch (CO/PBX), and trunks, as shown in Figure 6-1. Everyone is familiar with the telephone, but there are components within the telephone set that you might not be familiar with. The handset is the part of the phone that you hold in your hand to speak (transmit) and listen (receive) to a voice conversation. The switch hook is the lever that is pushed down while the handset is resting on the cradle (on-hook). When you lift the handset to place a call, the switch hook pops up and is in a state (off-hook) that allows current to flow through the phone.

Figure 6-1. Basic Telephone Components

In the handset are four wires split into two pairs, one for transmitting and one for receiving. In each phone there is a hybrid two-to-four-wire converter that acts as a communications bridge and provides the conversion between the four-wire handset and the two-wire local loop. Sidetone is another element that emanates from the hybrid, allowing a portion of speech to bleed over into the earpiece so that you can determine how loud you are talking. The dialer, either touch-tone or rotary , signals the telephone company that you are making a call. Pushing buttons on a touch-tone phone or spinning the dialer on a rotary phone sends a signal to the telephone company specifying the location from which you are calling.

Finally, the last component of the telephone is the ringer. When someone is calling you, the telephone company sends voltage through the wires to your telephone, which triggers the ringer alerting you that there is an incoming call.

Telephone Signaling

When you use your phone to place or receive a call, you must communicate your intentions to your telephone company. Signaling does this. The two types of signaling discussed in this chapter are supervisory and address. Supervisory signaling is the means by which you and your telephone company notify each other of call status. The three different types of supervisory signaling are on-hook, off-hook, and ringing.

As discussed previously, allowing the handset to rest in the cradle (on-hook) opens the switch hook and prevents the current from flowing through your phone. Only the ringer is active when the handset is on-hook. On-hook signaling is illustrated in Figure 6-2. Lifting the handset off the cradle allows current to flow through the phone, alerting the phone company that you are requesting to make a call. The phone company, in turn , returns a dial tone to the phone to indicate that it is ready. Figure 6-3 illustrates off-hook signaling. When someone is calling you, the telephone sends voltage to the ringer. The phone company also sends a ringback tone to the caller, alerting the caller that it is sending ringing voltage to the recipient's phone. Ringing is illustrated in Figure 6-4.

Figure 6-2. Supervisory Signaling ”On-Hook

Figure 6-3. Supervisory Signaling ”Off-Hook

Figure 6-4. Supervisory Signaling ”Ringing

Address signaling can be one of two types, pulse or dual-tone multifrequency ( DTMF ). Rotary-dial phones, while somewhat outdated , are still in use today and use pulse address signaling. Each pulse consists of a make and a break. The make is the period during which the circuit is closed. The break segment is the time that the circuit is open. The cycle should correspond to the following ratio: 60 percent break, 40 percent make. A governor inside the dial controls the rate at which the digits are pulsed. Figure 6-5 illustrates the process for pulse address signaling.

Figure 6-5. Address Signaling ”Pulse

Touch-tone phones use DTMF signaling. When you look at the keypad, each row of keys is identified by a low-frequency tone, and each column is associated with a high-frequency tone. The combination of both tones notifies the phone company of the number you are calling (hence the term DTMF).

Figure 6-6 illustrates the combination of tones that you can generate for each button on the keypad.

Figure 6-6. Address Signaling ”Dual-Tone Multifrequency (DTMF)

Local Loops

Your telephone is connected to the phone company by a local loop, sometimes referred to as "the last mile." The local loop contains an electrical communication path of two wires, one for transmitting and one for receiving voice signals (see Figure 6-7). The two-wire circuit is referred to as tip and ring. The ring is tied to the negative side of the battery at the phone company, and the tip is tied to the ground. Again, when you lift the handset off the cradle and go off-hook, the current flows down the wires and your phone company provides service to you.

Figure 6-7. Local Loop

Voice Switches

The next piece of the telephony puzzle is the voice switch. The three types of switches discussed in this section are central office (CO) switches, intermediate switches, and private branch exchange (PBX) switches. A voice switch is a mechanical or electrical device that directs your voice calls to the proper destination. Voice switches typically are located on your premises or at the phone company. The voice switch selectively establishes and releases connections between transmission facilities to provide dedicated paths for the exchange of messages between two calling parties. Paths are established before the information exchanges begin, and, until the callers terminate the sessions, these paths are maintained for the switch's exclusive use.

Private phones are connected directly to a central office switch. When you place a telephone call to a central office switch, it forwards the call to one of the following:

• Another central office switch

• An end user 's phone (if it is connected to the same CO)

• An intermediate switch

The CO switch provides all the components to make your phone work ”for example, battery, current detector, dial-tone generator, dial register, and ring generator. The battery is the source of power to both the circuit and the phone. The current detector monitors the status of the circuit by detecting whether the circuit is open or closed. The dial-tone generator generates a dial tone to acknowledge the request for service. When the PBX detects current flow on the interface, the dial register receives the dialed digit. The ring generator alerts the called party by sending a ring signal to the called party.

Intermediate switches primarily act as a go between for the switch that forwards a call to other switches in the network, and they also connect trunks.

PBX switches are used in the private sector. It would be inefficient at your place of business to run individual phone lines to the CO for each telephone in your office, for example. Instead, you would install a PBX at your office to provide connectivity between your phones and the CO through trunk lines.

Figure 6-8 shows each of the different types of voice switches.

Figure 6-8. Voice Switches ”Central Office (CO), Intermediate, and Private Branch Exchange (PBX) Switches

Trunks

The trunk's primary function is to provide the path between switches. Many different subscribers share a trunk, although only one uses it at any given time. There might be many trunks between two switches. A few of the more common trunk types are private trunk lines, central office trunks, foreign exchange trunks, and direct inward/direct outward dialing trunks, as shown in Figure 6-9.

Figure 6-9. Figure 6-9 Trunks ”Private, Central Office (CO), and Foreign Exchange

Generally, private-line trunks connect PBXs together, whereas CO trunks are direct connections between the local CO and a PBX. Foreign exchange trunks enable you to fool a switch into thinking that a remote telephone is directly attached to the switch. A foreign exchange office (FXO) and foreign exchange station (FXS) are needed to fool the switch into thinking that your telephone is directly connected to the switch.

The FXO sits on the switch end of the connection. It plugs directly into the line side of the switch, so the switch thinks that the FXO interface is a telephone. The switch notifies the FXO of an incoming call by sending ringing voltage to the FXO. Likewise, the FXO answers a call by closing the loop to let current flow. When current is flowing, the FXO interface uses any current technology to transport the signal to the FXS.

The FXS sits at the remote site and looks to the telephone like a switch. It provides the dial tone and battery to the telephone. The telephone thinks that it is the switch.

Direct inward dial (DID) trunks are one-way trunks that allow you to dial into a PBX without operator intervention. The CO knows which calls to pass through a DID trunk because it associates a block of numbers with each DID trunk.

Direct outward dial (DOD) trunks are also one-way trunks that allow you to connect directly to the CO. For example, if you want to place a call outside your company's network, you simply dial an access code such as 9, and the PBX forwards your call out to the CO. At that time, the CO provides a second dial tone and uses the remaining dialed digits to forward the call to the final destination.

Figure 6-10 illustrates DID and DOD trunks.

Figure 6-10. Trunks-Direct Inward/Direct Outward Dialing Trunks

Trunk Line Seizure Signaling Types

This section covers trunk/line seizure signaling types, which are signaling standards that exist between lines and trunks in the telephone network. We focus on loop start signaling, ground start signaling, and E&M signaling. This information is critical to the successful configuration and implementation of your Cisco voice-over solution.

NOTE

E&M signaling commonly is referred to as ear and mouth or recEive and transMit, but its origin comes from the terms earth and magnet . Earth represents electrical ground, and magnet represents the electromagnet used to generate tones.

Loop start signaling allows you or the telephone company to seize a line or trunk when a call is being initiated, as shown in Figure 6-11. It primarily is used on local loops rather than on trunks. Remember that when the line is in the idle state, it is said to be on-hook. If you lift the handset off the cradle, you cause the switch hook to go off-hook and close the loop. Current can now flow through the circuit, and the CO will detect the current and return a dial tone. Finally, if your telephone is ringing to alert you of an incoming call, the CO is applying AC ring voltage superimposed over the “48 VDC battery, causing the ring generator to notify you of a telephone call. When the PBX or telephone answers the call, the CO removes the ring voltage.

Figure 6-11. Loop Start Signaling

Loop start signaling is a poor signaling solution for high-volume trunks because it is possible to seize the trunk simultaneously from both ends. This problem is known as glare. Have you ever picked up the telephone to place an outbound call only to find the person you were calling already on the other end of the connection? Both parties seized the loop simultaneously, and you experienced glare. This might not be a significant problem in your home, but imagine the implications at your place of business, with several times the phone usage. Signaling methods that detect loop or trunk seizure at both ends solve the problem.

Ground start signaling is a modified version of loop start signaling that corrects the probability of glare by providing current detection at both ends of the connection (see Figure 6-12). Ground start signaling is preferable when high-volume trunks are involved.

Figure 6-12. Ground Start Signaling

The following summarizes the ground start signaling process:

1. When the line is idle or on-hook, the PBX monitors for ground on the tip lead.

2. The battery from the CO appears on the ring lead.

3. When you go-off hook, the PBX grounds the ring lead.

4. The CO senses the ring ground and grounds the tip lead.

5. The PBX detects the tip ground and closes the circuit to complete the call.

E&M signaling supports tie-line “type facilities or signals between voice switches. Instead of superimposing both voice and signaling over the same wire, E&M uses separate leads for each. As you probably guessed, the M lead sends the signal and the E lead receives the signal. For example, if you want to place a call to someone at a remote office, your PBX routes this request over its signal leads for use of the trunk between the two sites. Your PBX makes this request by raising its M lead. The remote PBX detects the request when it detects current on its E lead. The remote PBX attaches a dial register to the trunk and your PBX. Your PBX sends the dialed digits, and the remote PBX raises its M lead to notify you that the call is complete. Five types of E&M signaling exist, as detailed in Table 6-1.

Table 6-1. E&M Signaling

PBX to Intermediate Device
Type	Lead	On-hook	Off-hook
I	M	Ground	Battery ( “48 VDC)
II	M	Open	Battery ( “48 VDC)
III	M	Ground	Battery ( “48 VDC)
IV	M	Open
V	M	Open
Intermediate Device to PIX
Type	Lead	On-hook	Off-hook
I	E	Open	Ground
II	E	Open	Ground
III	E	Open	Ground
IV	E	Open	Ground
V	E	Open	Ground

E&M Type I signaling (see Figure 6-13) is a two-wire signaling type common in North America. One wire is the E lead, and the other is the M lead. Approximately 75 percent of the PBXs in North America are E&M Type I. With the Type I interface, the tie line equipment generates the E signal to the PBX by grounding the E lead. The PBX detects the E signal by sensing the increase in current through a resistive load. Similarly, the PBX generates the M signal by sourcing a current to the tie line equipment, which detects it with a resistive load. The Type I interface requires that the PBX and tie line equipment share a common signaling ground reference.

Figure 6-13. E&M Type I

E&M Type V (see Figure 6-14) is also a two-wire E&M signaling type most common outside North America. As with Type I signaling, Type V uses one wire for the E lead and one wire for the M lead. Type V requires a common ground between the PBX and the tie line equipment. This is provided by the signal ground leads.

Figure 6-14. E&M Type V

E&M Type II is a four-wire interface (see Figure 6-15). Like Type I and V, Type II uses two wires for the E and M leads. Type II, however, uses the remaining two wires for signal ground and signal battery. These two wires also are used for the return path for the E and M leads. A Type II interface does not require a common ground; instead, each of the two signals has its own return path.

Figure 6-15. E&M Type II

E&M Type III is similar to Type II, but it uses the signal ground lead as the common ground (see Figure 6-16). The E lead operates similar to Type I. In this configuration, the PBX drops the M lead by grounding it rather than by opening a current loop. This is not a common signaling type.

Figure 6-16. E&M Type III

E&M Type IV is symmetric and requires no common ground (see Figure 6-17). Each side closes a current loop to signal; the flow of current is detected by a resistive load to indicate the presence of a signal. E&M Type IV is not supported by Cisco.

Figure 6-17. E&M Type IV

Trunk Supervision

This section covers start protocols for trunk supervision. We will discuss wink start, delay start, and immediate start. Tie trunks have bidirectional supervisory signaling that allows either end to seize the trunk. One PBX seizes the trunk and expects an acknowledgment reply from the other end. The local end needs to differentiate between a return acknowledgment and a remote end request for service.

The most common E&M trunk seizure signal type is wink start signaling. With this particular type of signaling, the office initiating the call seizes the line by going off-hook. The remote office does not immediately return an off-hook acknowledgment when it detects the line seizure of the calling office. Instead, the on-hook state is maintained until the receive digit register is attached. The called office toggles the off-hook lead for a specific amount of time (hence the term wink ). When the calling office receives the wink, it forwards the digits to the remote office. The called office answers the telephone, and the PBX raises the M lead. Figure 6-18 illustrates wink start signaling.

Figure 6-18. Trunk Supervision Signaling: Wink Start

With delay start signaling, the originating side goes off-hook and waits for about 200 ms; then it checks to see if the far end is on-hook. If the far end is on-hook, it then outputs dial digits. If the far end is off-hook, it waits until it goes on-hook and then outputs dial digits. The delay signal says, in effect, "Wait ”I'm not ready to receive digits." Figure 6-19 shows the delay start signaling process.

Figure 6-19. Trunk Supervision Signaling: Delay Start

Now, let's run through the process that takes place with immediate start signaling. With immediate start signaling, the calling office PBX seizes the line by going off-hook (see Figure 6-20). Rather than waiting for the double acknowledgment from the remote office, the local PBX waits a predetermined amount of time (say, 150 ms) and then blindly forwards digits. The remote PBX acknowledges the call only after the called party answers the telephone.

Figure 6-20. Trunk Supervision Signaling: Immediate Start

Two-to-Four-Wire Conversion and Echo

Recall that the local loop is made up of two wires. When the local loop reaches the central office switch, it is changed to four wires with a two-to-four-wire hybrid converter. This is necessary for your signal to be transported across the trunks in the network. If there is a good impedance match between the lines, the hybrid is said to be balanced with little or no reflected energy. However, if there is an impedance mismatch or inadequate balance between the lines, a portion of the transmitted voice is reflected back toward the receiver side, resulting in echo. Some degree of echo is always present. When the magnitude or loudness of the echo becomes too high, it becomes a problem.

Two common types of echo are talker echo, in which your voice reflects back to you (above and beyond side tone, which we discussed earlier) and you hear yourself twice. The other is listener echo, which gives the effect of hearing the speaker's voice twice. Echo is caused by delay.

Two ways that you can attempt to solve this problem are echo suppression and echo cancellation. Echo suppression basically suppresses your voice on the return path. The echo suppressor determines which signals match you and which signals match the person you are talking to. If the echo suppressor determines that the echo is on the return path, it either attenuates or breaks the transmission path. However, if it is determined that both speech and echo are present at the same time from a combination of both parties, the echo cannot be attenuated without affecting the voice level. A more sophisticated method of dealing with echo is echo cancellation. Rather than breaking or attenuating the transmit path, as is the case with echo suppression, echo cancellation uses an echo canceller to build a mathematical model of the speech pattern and subtracts it from the transmit path.

NOTE

An echo canceller removes echo only from one end of the circuit. If echo exists on both ends of the circuit, another echo canceller would need to be applied at the other end.

To summarize, here are the basic elements involved in placing a telephone call. When your telephone is resting on the cradle (on-hook), the line is in an idle state and the telephone or PBX opens the two-wire loop. Lifting the handset off the cradle causes the switch hook to go off-hook, closing the loop and allowing current to flow through the circuit. The switch detects current and returns a dial tone. When a dial tone is received, you request a particular connection by dialing specific numbers. The switch signals the called party by sending ringing voltage. At the same time, the switch also sends an audible ring signal back to the caller notifying the caller that the call is in progress. The analog connection is complete when the called party lifts the handset, closing the loop and allowing current to flow through the circuit.