Loop start signaling
Anytime the phone is on-hook, the phone separates the two wires, preventing electric signal from flowing through the phone. When the phone is lifted off-hook, the phone connects the two wires, causing an electrical signal (48V DC voltage) to flow from the phone company central office (CO) into the phone. This is known as loop start signaling.
Supervisory Signaling
In addition to using the of properties of electric signals to generate voice traffic, analog phones are responsible for accepting and sending multiple types of signaling. The first type of signaling the phone handles is considered supervisory signaling. Supervisory signaling handles the following:
- On-hook signal: When the phone is on-hook, the connection between the tip andring wires is broken and no electrical signal passes between them.
- Off-hook signal: When the phone is off-hook, the phone connects the tip and ring wires, completing the circuit and allowing electrical signal to pass.
- Ringing: To cause an analog phone to ring, the phone company sends an alternating current (AC) signal down one of the wires, which the phone detects and generates aring signal.
Informational Signaling
Once the analog phone generates an off-hook signal, the phone company responds by using informational signaling to generate a dial tone. Informational signaling uses specific electrical frequencies to send a plethora of information to the caller. The following events are results of informational signaling:
- Dial tone: Indicates the phone company is ready to receive digits
- Busy: Indicates the remote phone is already in use
- Ringback: Indicates the remote phone is currently ringing
- Congestion: Indicates the long-distance telephone network is not able to complete the call
- Reorder: Indicates the local telephone company is not able to complete the call
- Receiver off-hook: Indicates the local receiver has been off-hook for an extended period of time
- No such number: Indicates the dialed number is invalid
- Confirmation: Indicates the telephone company is attempting to complete the call
The phone company generates each of these informational signals using well-known electrical frequencies for the local region (country). All analog phones operating within the same country should have identical informational signals.
Address Signaling
Once the phone company has used informational signaling to generate a dial tone signal, the user can dial digits. There are two types of address signaling in use worldwide:
- Dual-tone multifrequency (DTMF): The buttons on a telephone keypad use a pair of high and low electrical frequencies (thus “dual-tone”) to generate a signal each time a caller presses a digit. DTMF is the predominant signal type used in the United States.
- Pulse: The rotary-dial wheel of a phone connects and disconnects the local loop circuit as it rotates around to signal specific digits.
The Evolution: Digital Connections
Analog signaling was a massive improvement over tin cans and string, but still posed plenty of problems of its own. First, an analog electrical signal experiences degradation (signal fading) over long distances. To increase the distance the analog signal could travel, the phone company had to install repeaters (shown in Figure 1.6) to regenerate the signal as it became weak.
Unfortunately, as the analog signal was regenerated, the repeater device was unable to differentiate between the voice traveling over the wire and line noise. Each time the repeater regenerated the voice, it amplified the line noise as well. Thus, the more times a phone company regenerated a signal, the more distorted and difficult to understand the signal became.
The second difficulty encountered with analog connections was the sheer number of wires the phone company had to run to support a large geographical area or a business with a large number of phones. Because each phone required two wires, the bundles of wire became massive and difficult to maintain (imagine the hassle of a single pair of wires in the bundle breaking). A solution to send multiple calls over a single wire was needed. A digital connection is that solution.
Sending Multiple Calls over a Single Line
Now, let’s come back to the original problems of analog connections:
- The signal degrades over long distances.
- You can’t send multiple calls over a single line (resulting in massive cabling requirements).
Digitizing voice solves the first problem because you can easily transmit a numeric valu any distance a cable can run without any degradation or line noise. Time-division multiplexing (TDM) solves the second problem.
TDM allows voice networks to carry multiple conversations at the same time over a single, four-wire path. Because the multiple conversations have been digitized, the numeric values are transmitted in specific time slots (thus the “time-division”) that differentiate the separate conversations. Figure 1.10 illustrates three separate voice conversations sent over a digital connection.
Corporations use digital voice connections to the PSTN as T1 circuits in the United States, Canada, and Japan. A T1 circuit is built from 24 separate 64-kbps channels known as a digital signal 0 (DS0). Each one of these channels is able to support a single voice call. Corporations in areas outside the United States, Canada, and Japan use E1 circuits, which allow you to use up to 32 DS0s for voice calls.
Two primary styles of signaling were created for digital circuits:
- Channel associated signaling (CAS): Signaling information is transmitted using the same bandwidth as the voice. Uses the eighth bit on every sixth sample in each T1channel (DS0)
- Common channel signaling (CCS): Signaling information is transmitted using a separate, dedicated signaling channel.
When using CCS configurations with T1 lines, the 24th time slot is always the signaling channel. When using CCS configurations with E1 lines, the 17th time slot is always the signaling channel.
The Pieces of the PSTN
- Analog telephone: Able to connect directly to the PSTN and is the most common device on the PSTN. Converts audio into electrical signals.
- Local loop: The link between the customer premises (such as a home or business) and the telecommunications service provider.
- Central office (CO) switch: Provides services to the devices on the local loop. These services include signaling, digit collection, call routing, setup, and teardown.
- Trunk: Provides a connection between switches. These switches could be CO or private.
- Private switch: Allows a business to operate a “miniature PSTN” inside its company.This provides efficiency and cost savings because each phone in the company does not require a direct connection to the CO switch.
- Digital telephone: Typically connects to a PBX system. Converts audio into binary 1s and 0s, which allows more efficient communication than analog.
Understanding PBX
Many businesses have hundreds or even thousands of phones they support in the organization. If the company purchased a direct PSTN connection for each one of these phones, the cost would be astronomical. Instead, most organizations choose to use a private branch exchange (PBX) or key system internally to manage in-house phones. These systems allow internal users to make phone calls inside the office without using any PSTN resources. Calls to the PSTN will forward out the company’s PSTN trunk link.
When you first look at a PBX system, it looks like a large box full of cards. Each one of
these cards has a specific function:
- Line cards: Provide the connection between telephone handsets and the PBX system.
- Trunk cards: Provide connections from the PBX system to the PSTN or other PBX systems.
- Control complex: Provides the intelligence behind the PBX system; all call setup, routing, and management functions are contained in the control complex.
PSTN Numbering Plans
Just as data networks use IP addressing to organize and locate resources, voice networks use a numbering plan to organize and locate telephones all around the world.
Organizations managing their own, internal telephony systems can develop any internal number scheme that best fits the company needs (similar to private IP addressing).
However, when connecting to the PSTN, you must use a valid, E.164 standard address for your telephone system. E.164 is an international numbering plan created by the International Telecommunication Union (ITU). Each number in the E.164 numbering plan contains the following components:
- Country code
- National destination code
- Subscriber number
E.164 numbers are limited to a maximum length of 15 digits
