Historic Voice : Understanding Analogue Connectivity Analogue signaling: loop and ground start Call Signaling Supervisory signaling Information signaling Address signaling

1. CCNA Voice 640-461
Part 2- Historic Voice: Understanding Analogue Connectivity
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2. Historic Voice : Understanding Analogue Connectivity
•What is analogue connectivity?
•Electric wave forms: more than you ever knew
•Understanding analogue signaling

3. What is analogue connectivity?
Analogue Transmission:
Using some property of the transmission
media 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 to
store information
(Mechanically, electrically...) and other
devices read that information.
Replica of Edison’s phonograph

4. 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.

5. 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.

6. 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.

7. 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

8. 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.

9. 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

10. 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.

11. 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

12. 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.

13. 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.

14. 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

15. Address Signaling
Address Signaling
-Pulse 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. The
number of consecutive opens and
closes, or breaks and
makes, represents the dialled digit.

16. 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 doesn't 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 #

17. 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