Historic Voice : Understanding Analogue Connectivity •What is analogue connectivity? •Electric wave forms: more than you ever knew •Understanding analogue signaling
What is analogue connectivity?Analogue Transmission:Using some property of the transmissionmedia to convey a signal•Thomas Edison’s phonograph in 1877•Record player•Braille for the blind•Typical home telephony lines-In other words, there are many ways tostore information(Mechanically, electrically...) and otherdevices read that information. Replica of Edison’s phonograph
What is analogue connectivity? - An analogue signal uses a property of the device that captures the audio signal to convey audio information - Digital communication do not use the property of media, instead it use ones and zeros.
Properties of electricity When you speak into an analogue phone, the sounds that come out of your mouth are converted into electricity. Voltage Time Electrical Analogue Waveform of Human Speech -Analogue phone lines use the properties of electricity to convey changes in voice over cabling. -The analogue phones convey many different types of signaling such as dial tone, dialed digits, busy signals, and so on.
Analogue signaling: loop and ground start Each analogue circuit is composed of a pair of wires. Anytime the phone is on hook, the phone separates the two wires, preventing electric signal from flowing through the phone.
Analogue signaling: loop and ground start -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 signalling. -Loop start signaling is the typical signaling type used in home environments Handset Circuit Current Flow On cradle On hook/open circuit No Off cradle Off hook/closed circuit Yes
Analogue signaling: loop and ground start -Loop start signaling is susceptible to a problem known as glare. -Glare occurs when you pick up the phone to make an outgoing call at the same time as a call comes in on the phone line before the phone has a chance to ring. -Glare may not be a major concern in a home environment, but in business environments, glare can become a significant problem because of the large number of employees and high call volume. There is another type of signaling used on Key systems, and also on pay phones. That other type of signalling is ground start, and the good news is that ground start signalling prevents glare.
Analogue signaling: loop and ground start Loop start signaling - In a home environment, the phone switch in the local CO can determine whether a phone is on-hook or off-hook based on whether current is flowing over the local loop connecting back to that phone. -Because an on-hook phone mechanically has its trip and ring circuit open, the -48 volts of DC applied across the tip and ring wire isn’t doing anything. The voltage is just sitting there, waiting for the circuits to close. - After the handset goes off-hook, however, the tip and the ring circuit is closed, and the current can begin to flow through that circuit. When the telephone switch at the CO sees this current begin to flow, it knows that the phone has gone off-hook, and the telephone switch sends a dial tone to the caller, indicating that they can begin dialling digits
Analogue signaling: loop and ground start Ground start Signaling -With ground start signaling, the phone switch monitors the voltage potential on the “ring” lead of a line, and when the ring lead has a ground potential, the line seized. -Ground start signaling originated from its implementation in pay phone systems. when a person lifted the handset of a pay phone, he did not receive a dial tone until he dropped in a coin. The coin would brush past the tip and ring wires and temporarily ground them. The grounding of the wires signaled the phone company to send a dial tone on the line. -Using this type of signaling in key systems allows the key systems to separate an answering phone from an incoming phone line, reducing the problem of glare. To receive a dial tone from the CO, the key system must send a ground signal on the wires. This intentionally signals to the telephone CO that an outgoing call is going to happen, whereas using the loop start method of signaling just connects the wires to receive an incoming call or place an outgoing call.
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 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. Information signaling Tone combinations indicate call progress and are used to notify subscribers of call status Address signaling Telephones use address signaling to notify the telephone company where a subscriber is calling
Supervisory signaling Supervisory signaling •On-hook signaling When the handset rests on the cradle, the circuit is on hook. The switch prevents current from flowing through the telephone. •Of-hook signaling 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. 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 signaling -When the phone company would like to send a ringing signal to ring a phone, it sends AC current rather than DC. There is an AC receiving chip Inside a phone which receives the ringing signal and rings. -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.
Information Signaling Information Signaling Information signaling uses combinations of frequencies, in this case to indicate the status of a call. •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.
Address Signaling Address Signaling There are two types of telephones: Push-button (tone) telephone Rotary-dial telephone These telephones use two different types of address signaling to notify the telephone company where a subscriber is calling
Address SignalingAddress Signaling-Pulse These digits must beproduced at a specific rate andwithin a certain level of tolerance.-Each pulse consists of a "break"and a "make," which are achievedby opening and closing the localloop circuit. The break segment isthe time during which the circuit isopen. The make segment is the timeduring which the circuit is closed.-The break-and-make cycle mustcorrespond to a ratio of 60 percentbreak to 40 percent make. Thenumber of consecutive opens andcloses, or breaks andmakes, represents the dialled digit.
Information Signaling Address Signaling -Dual-tone multifrequency (DTMF) -With DTMF, two simultaneous frequencies are generated, and a phone switch interprets this combination of frequencies as a dialed digit. For example, the combination of a 697 Hz tone and a 1209 Hz tone indicates a dialed digit of 1. - You might be curious as to why "dual" tones are used instead of just a single tone; the answer is background noise. The phone company doesnt want the radio or your kids playing in the background to make a sound that may be interpreted as a dialed digit. So, specific combinations of two simultaneous frequencies are used to represent a dialed digit. Frequency 1209 Hz 1336 Hz 1477 Hz 697 Hz 1 2 3 770 Hz 4 5 6 852 Hz 7 8 9 941 Hz * 0 #
References Cioara, J., Valentine, M. (2012). CCNA Voice 640-461 Official Cert Guide, Cisco Press, USA Davidson, J., Peters, J., Bhatia, M., Kalidindi, S., Mukherjee, S. (2006). Voice over IP Fundamentals, Second Edition, Cisco Press, USA Froehlich, A. (2010). CCNA Voice Study Guide, Wiley Publishing, Inc., Indianapolis, Indiana Kaza, R., Asadullah, S. (2005). Cisco IP Telephony: Planning, Design, Implementation, Operation, and Optimization, Cisco Press, USA Wallace, K. (2005). Voice over IP First-Step, Cisco Press, USA Wallace, K. (2006). Authorized Self-Study Guide Cisco Voice over IP (CVoice), Cisco Press, USA