Saturday, March 13, 2010

CCNA Voice 1

In traditional PSTNs, calls from analogue telephones are routed through CO switches. CO switches use the SS7 signaling protocol to do call setup. SS7 is a standard used by carriers to set up calls between CO switches, even across multiple vendors. ISDN places calls on the PSTN using the Q.931 protocol.
A PBX system refers to one or more private switch connected to the edge of the PSTN. PBX systems have a single trunk link to the CO switch. Unlike PBX systems, key system phones have multiple lines connected to the phones. Because of this, key systems have less advanced features and are not scalable (typically for smaller organizations <20 br="" employees="">
Analogue phones use DTMF for call dialing (also known as "address signaling"). Before DTMF was used (aka "tone"), the "pulse" was used. DTMF allows creation of 12 unique numbers through a combination of a high and low frequency tone:
      1209Hz 1336Hz 1477Hz
697Hz 1      2      3
770Hz 4      5      6
852Hz 7      8      9
941Hz *      0      #
A numbering plan is a set of rules used to construct numbers. A numbering plan has an authority that regulates number distribution in its territory. An authority like NANP (North American Numbering Plan) organization governs the istribution and formatting of numbers in US and Canada.

An example of a numbering plan is the E.164 numbering plan. It is made up of three items:
1) Country code
2) National destination code
3) Subscriber number

The standard voltage and current deployed on local and ground loops directly connected to the CO is -48 volts DC and 20ma of current. When the phone is picked up, it is said to be off-hook. This closes the circuit and allows the CO switch to know that the phone is picked up.

When a call is routed to the phone, a 40 Vrms AC voltage is sent to the phone ringer. The service also provides a ringback to the caller to let him know that the call is being established.

Ringback is an example of a Supervisory call signaling. There are three types of call signaling categories:
1) Supervisory
2) Address
3) Informational

Supervisory signaling include:
1) On-hook - When the handset is on-hook, the circuit is broken.
2) Off-hook - When the circuit is closed, the CO detects the current flow and plays a dial tone for 15 seconds. When the first digit is dialed, the tone is stopped.
3) Ringing - When a subscriber receives a call, the CO sends an AC 40 Vrms signal to the phone ringer. At the same time, the caller receives a ringback which is described later.

Address signaling are:
1) DTMF - Dual tone multifrequency sine waves that make up 12 unique combinations.
2) Pulse - Pulse created by a rotary-dial telephone which produces "make" and "break" pulses to open and close the local loop circuit to convey the number dialed.

Informational signaling:
1) Dial tone - To indicate that the CO switch is ready to receive the digits
2) Busy - To indicate that the call cannot be established
3) Ringback - To indicate that the call is being established
4) Congestion - To indicate that a long-distance call is not established due to congestion
5) Reorder tone - To indicate that local lines are busy
6) Receiver off-hook - To indicate that the phone has been off-hook for more than 15 seconds without placing a call
7) No such number - To indicate that the number doesn't exist
8) Confirmation tone - To indicate that the CO or PBX is attempting to complete a call

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