Basic Components of Telephony Networks

A number of components must be in place for an end-to-end call to succeed. These components are listed here and shown in Figure 1-1:

  • Edge devices

  • Local loops

  • Private or central office (CO) switches

  • Trunks

Figure 1-1. Basic Components of a Telephony Network

The next sections describe each of the basic components in more detail.

Edge Devices

The two types of edge devices used in a telephony network include:

  • Analog telephones Analog telephones are most common in home, small office/home office (SOHO), and small business environments. A direct connection to the public switched telephone network (PSTN) is usually made by using analog telephones. Proprietary analog telephones are occasionally used in conjunction with a PBX. These telephones provide additional functions such as speakerphone, volume control, PBX message-waiting indicator, call on hold, and personalized ringing.

  • Digital telephones Digital telephones contain hardware to convert analog voice into a digitized stream. Larger corporate environments with PBXs generally use digital telephones. Digital telephones are typically proprietary, meaning that they work with the PBX or key system of that vendor only.

Local Loops

A local loop is the interface to the telephone company network. Typically, it is a single pair of wires that carry a single conversation. A home or small business may have multiple local loops. You learn more about local loops in Chapter 2, "Analog and Digital Voice Connections."

Private or CO Switches

The CO switch terminates the local loop and handles signaling, digit collection, call routing, call setup, and call teardown. You learn more about these switches in the upcoming section "CO Switches and Switching Systems."

A PBX switch is a privately owned switch located at the customer site. A PBX typically interfaces with other components to provide additional services, such as voice mail. You learn more about PBXs in the section "Privately Owned Switches" later in this chapter.


The primary function of a trunk is to provide the path between two switches. There are several common trunk types, as shown in Figure 1-2, including the following:

  • Tie trunk A dedicated circuit that connects PBXs directly

  • CO trunk A direct connection between a local CO and a PBX

  • Interoffice trunk A circuit that connects two local telephone company COs

Figure 1-2. Trunk Types


The telephone installed in your home is considered an edge device because it terminates the service provided by your local telephone company. The local loop is the pair of wires that come to your house and provide residential telephone service. Trunks are the interconnections between telephone switches. They can be between private switches or telephone company switches.

CO Switches and Switching Systems

Figure 1-3 shows a typical CO switch environment. The CO switch terminates the local loop and makes the initial call-routing decision.

Figure 1-3. CO Switches

The call-routing function forwards the call to one of the following:

  • Another end-user telephone, if it is connected to the same CO

  • Another CO switch

  • A tandem switch (that is, an intermediary switch between the source and destination switch)

The CO switch makes the telephone work with the following components:

  • Battery The battery is the source of power to both the circuit and the telephone. It determines the status of the circuit. When the handset is lifted to let current flow, the telephone company provides the source that powers the circuit and the telephone. Because the telephone company powers the telephone from the CO, electrical power outages should not affect the basic telephone, also known as a POTS (plain old telephone service) phone.

  • Current detector The current detector monitors the status of a circuit by detecting whether it is open or closed. Table 1-1 describes current flow in a typical telephone.

    Table 1-1. Current Flow in a Typical Telephone



    Current Flow

    On cradle

    On hook/open circuit


    Off cradle

    Off hook/closed circuit


  • Dial-tone generator When the digit register is ready, the dial-tone generator produces a dial tone to acknowledge the request for service.

  • Dial register The digit register receives the dialed digits.

  • Ring generator When the switch detects a call for a specific subscriber, the ring generator alerts the called party by sending a ring signal to that subscriber.


Some telephones on the market offer additional features that require a supplementary power source that the subscriber supplies; for example, cordless telephones. Some cordless telephones may lose functionality during a power outage.

When configuring a PBX connection to a CO switch, the signaling should match that of the CO switch. This configuration ensures that the switch and the PBX can detect on hook, off hook, and dialed digits coming from either direction.

Switching systems provide three primary functions:

  • Call setup, routing, and teardown

  • Call supervision

  • Customer ID and telephone numbers

CO switches switch calls between locally terminated telephones. If a call recipient is not locally connected, the CO switch decides where to send the call based on its own call routing information, which is stored in a call-routing table. The call then travels over a trunk to another CO or to an intermediate switch that may belong to an inter-exchange carrier (IXC). Although intermediate switches do not provide dial tone, they act as hubs to connect other switches and provide interswitch call routing.

PSTN calls are traditionally circuit-switched, which guarantees end-to-end path and resources. Therefore, as the PSTN sends a call from one switch to another, the same resource is associated with the call until the call is terminated.


CO switches provide local service to residential telephones. The CO switch provides dial tone, indicating that the switch is ready to receive digits. When you dial your phone, the CO switch receives the digits, then routes your call. The call routing may involve more than one switch as the call progresses through the network.

Privately Owned Switches

In a corporate environment, where large numbers of staff need access to each other and the outside, individual telephone lines are not economically viable. A PBX is a smaller, privately owned version of the CO switches used by telephone companies, as illustrated in Figure 1-4.

Figure 1-4. PBX

Most businesses have a PBX telephone system, a key telephone system, or a Centrex service. Large offices with more than 50 telephones or handsets choose a PBX to connect users, both in-house and to the PSTN.

PBXs come in a variety of sizes, from 20 to 20,000 stations. The selection of a PBX is important to most companies, because a PBX has a typical life span of seven to ten years.

All PBXs offer a standard, basic set of calling features. Optional software provides additional capabilities.

A PBX connects to telephone handsets using line cards and to the local exchange using trunk cards.

A PBX has three major components:

  • Terminal interface The terminal interface provides the connection between terminals and PBX features that reside in the control complex. Terminals can include telephone handsets, trunks, and lines. Common PBX features include dial tone and ringing.

  • Switching network The switching network provides the transmission path between two or more terminals in a conversation. For example, two telephones within an office communicate over the switching network.

  • Control complex The control complex provides the logic, memory, and processing for call setup, call supervision, and call disconnection.

PBX Installations

PBX switches are installed in large business campuses to relieve the public telephone company switches from having to switch local calls. When you call a coworker locally in your office campus, the PBX switches the call locally instead of having to rely on the public CO switch. The existence of PBX switches also limits the number of trunks needed to connect to the telephone company's CO switch. With a PBX installed, not every office desktop telephone needs its own trunk to the CO switch. Rather, the trunks are shared among all users.

Small organizations and branch offices often use a key telephone system, as shown in Figure 1-5, because a PBX offers functionality and extra features that they may not require. A key system offers small businesses distributed answering from any telephone, unlike the central answering position required for a PBX. Notice in Figure 1-5 that telephones interconnect to a key system via connector blocks, while trunks coming in from the local exchange interconnect to the key system via termination blocks.

Figure 1-5. Key Telephone System

Today, key telephone systems are either analog or digital and are microprocessor based. Key systems are typically used in offices with 30 to 40 users, but can be scaled to support over 100 users.

A key system has three major components:

  • Key service unit A key service unit (KSU) holds the system switching components, power supply, intercom, line and station cards, and the system logic.

  • System software System software provides the operating system and calling-feature software.

  • Telephones (instruments or handsets) Telephones allow the user to choose a free line and dial out, usually by pressing a button on the telephone.

Larger companies use proprietary telephone networks with PBXs. In a key telephone system, each telephone has multiple lines that allow users to access outside lines to their CO. When a call comes into the company, a line or a key lights up on the telephone and indicates that a particular line is in use. Users can call another extension or let another person know where to pick up a call by using an intercom function, such as an overhead paging system or speakerphone.

Key telephone system functionality has evolved over time to include a class called hybrid telephone systems. The hybrid system adds many features that were previously available only in PBXs. There is no single definition of the functions and features that are classified as a hybrid system because all vendors provide a mix that they believe gives them a competitive advantage.

The main difference between a key telephone system and a hybrid telephone system is whether a single-line telephone can access a single CO local loop or trunk only (key telephone system) or whether the single-line telephone can access a pool of CO local loops or trunks (hybrid telephone system).

Call Signaling

Call signaling, in its most basic form, is the ability of a device to communicate a need for service to a network. The call-signaling process requires the network to detect a request for service and termination of service, send addressing information, and provide progress reports to the initiating party. This functionality corresponds to the three call-signaling types:

  • Supervisory signaling

  • Address signaling

  • Informational signaling

A basic call setup, as illustrated in Figure 1-6, includes supervisory, address, and information signaling components. The supervisory signaling is used, for example, to detect that a phone went off hook. Address signaling occurs when a caller dials digits, and information signaling is represented by the dial tone heard by the caller.

Figure 1-6. Basic Call Setup

This call setup can be broken down into three major steps. These steps include:

  1. Local signaling: originating side The user signals the switch by going off hook and sending dialed digits through the local loop.

  2. Network signaling The switch makes a routing decision and signals the next, or terminating, switch through the use of setup messages sent across a trunk.

  3. Local signaling: terminating side The terminating switch signals the call recipient by sending ringing voltage through the local loop to the recipient telephone.

Supervisory Signaling

A subscriber and telephone company notify each other of call status with audible tones and an exchange of electrical current. This exchange of information is called supervisory signaling, as shown in Figure 1-7.

Figure 1-7. Supervisory Signaling

There are three different types of supervisory signaling:

  • On hook When the handset rests on the cradle, the circuit is on hook. The switch prevents current from flowing through the telephone. Regardless of the signaling type, a circuit goes on hook when the handset is placed on the telephone cradle, and the switch hook is toggled to an open state. This prevents the current from flowing through the telephone. Only the ringer is active when the telephone is in this position.

  • Off hook When the handset is removed from the telephone cradle, the circuit is off hook. The switch hook toggles to a closed state, causing circuit current to flow through the electrical loop. The current notifies the telephone company equipment that someone is requesting to place a telephone call. When the telephone network senses the off-hook connection by the flow of current, it provides a signal in the form of a dial tone to indicate that it is ready.

  • Ringing When a subscriber makes a call, the telephone sends voltage to the ringer to notify the other subscriber of an inbound call. The telephone company also sends a ringback tone to the caller, alerting the caller that it is sending ringing voltage to the recipient telephone. Although the ringback tone sounds similar to ringing, it is a call-progress tone and not part of supervisory signaling.


The ringing pattern in the United States is 2 seconds of ringing tone followed by 4 seconds of silence. Europe uses a double ring followed by 2 seconds of silence.

Address Signaling

There are two types of telephones, as shown in Figure 1-8: a push-button (tone) telephone and a rotary-dial telephone.

Figure 1-8. Address Signaling

These telephones use two different types of address signaling to notify the telephone company where a subscriber is calling:

  • Dual-tone multifrequency (DTMF) Each button on the keypad of a touch-tone pad or push-button telephone is associated with a pair of high and low frequencies. On 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 telephone company of the number being called, thus the term dual-tone multifrequency (DTMF).

  • Pulse The large numeric dial-wheel on a rotary-dial telephone spins to send digits to place a call. These digits must be produced at a specific rate and within a certain level of tolerance. Each pulse consists of a "break" and a "make," which are achieved by opening and closing the local loop circuit. The break segment is the time during which the circuit is open. The make segment is the time during which the circuit is closed. The break-and-make cycle must correspond to a ratio of 60 percent break to 40 percent make.

    A governor inside the dial controls the rate at which the digits are pulsed. For example, when a subscriber calls someone by dialing a digit on the rotary dial, a spring winds. When the dial is released, the spring rotates the dial back to its original position. While the spring rotates the dial back to its original position, a cam-driven switch opens and closes the connection to the telephone company. The number of consecutive opens and closes, or breaks and makes, represents the dialed digit.

Information Signaling

Tone combinations indicate call progress and are used to notify subscribers of call status. Each combination of tones represents a different event in the call process. These events, whose frequencies and patterns are listed in Table 1-2, include the following:

  • Dial tone Indicates that the telephone company is ready to receive digits from the user telephone.

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

  • Ringback (line or PBX) Indicates that the telephone company 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.

  • Reorder tone Indicates that all the local telephone circuits are busy, thus preventing a telephone call from being processed.

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

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

Table 1-2. Information Signaling Tone Combinations


Frequency (Hz)

On Time (sec)

Off Time (sec)


350 + 440




480 + 620



Ringback, line

440 + 480



Ringback, PBX

440 + 480



Congestion (toll)

480 + 620



Reorder (local)

480 + 620



Receiver off hook

1400 + 2060 + 2450 + 2600



No such number

200 to 400




A call placed from your residential telephone uses all three types of call signaling. When you lift the handset, a switch in your telephone closes to start current flow and notifies the telephone company that you want to make a call (supervisory signaling). The telephone company then sends dial tone to indicate that it is ready to receive your dialed digits (informational signaling). You then dial your digits by pressing numbers on the keypad (address signaling).

Digital versus Analog Connections

Supervisory, address, and informational signaling must be carried across both analog and digital connections. Depending on your connection to the network, you must configure specific signaling to match the type of signaling required by the service provider. Figure 1-9 illustrates digital and analog connections coexisting in the same network.

Figure 1-9. Address Signaling

Digital PBX connections to the network are common in many countries. They may be T1 or E1 lines carrying channel associated signaling (CAS) or PRI lines using common channel signaling (CCS).

CAS is a signaling method that allows passing on-hook or off-hook status by setting bits that are associated with each specific voice channel. These bits are carried in band for T1 and out of band for E1.

An ISDN connection uses the D channel as the common channel to carry signaling messages for all other channels. CCS carries the signaling out of band, meaning that the signaling and the voice path do not share the same channel.

Analog interfaces require configuration of a specific signaling type to match the provider requirement. For interfaces that connect to the PSTN or to a telephone or similar edge device, the signaling is configured for either loop start or ground start, the functions of which are discussed in Chapter 2. For analog trunk interfaces that connect two PBXs to each other (that is, E&M interfaces), or a PBX to a CO switch, the signaling is either wink- start, immediate-start, or delay-start, with the signaling type set to 1, 2, 3, 4, or 5.


A two-wire analog local loop typically carries one call at a time. To make better use of wiring facilities, different multiplexing techniques have been implemented to enable two-wire or four-wire connections to carry multiple conversations at the same time.

Time-division multiplexing (TDM) is used extensively in telephony networks to carry multiple conversations concurrently across a four-wire path, as shown in Figure 1-10. TDM involves simultaneously transmitting multiple separate voice signals over one communications medium by quickly interleaving pieces of each signal, one after another. Information from each data channel is allocated bandwidth based on preassigned timeslots, regardless of whether there is data to transmit.

Figure 1-10. Time-Division Multiplexing

Frequency-division multiplexing (FDM), as illustrated in Figure 1-11, involves carrying multiple voice signals by allocating an individual frequency range to each call. FDM is typically used in analog connections, although its functionality is similar to that of TDM in digital connections. FDM is used in cable or digital subscriber line (DSL) connections to allow the simultaneous use of multiple channels over the same wire.

Figure 1-11. Frequency-Division Multiplexing


If you have cable television service at your home, the television channels are all carried (and multiplexed) over a single pair of wires. This includes both the audio signals and the video signals. All the channels are present on the cable wires all the time. When you select the channel you want to watch, your set-top cable tuner determines which channel is sent to your television.

Cisco Voice over IP Cvoice (c) Authorized Self-study Guide
Cisco Voice over IP (CVoice) (Authorized Self-Study Guide) (2nd Edition)
ISBN: 1587052628
EAN: 2147483647
Year: 2006
Pages: 111
Authors: Kevin Wallace

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