Telephone Networks

The telephone network consists of your phone at home that is connected (by the Local Loop) to the Central Office. The Central Office is in turn connected to a Hierarchical Phone Network. Worldwide, there are over 300 million (300,000,000) telephones - 98% of them interconnected.

POTS - Plain Old Telephone Set

The POTS, or Plain Old Telephone Set, consists of these 5 sections:

1. Ringer Unit
   
2. Hook Switch
   
3. Dialer Unit
   
4. Hybrid/Speech Network
   
5. Hand Set
   

The connection to the CO (Central Office) comprises only 2 wires: Tip and Ring. This connection is called the "Local Loop."

The Tip is +ve and colored green.. The Ring is -ve and colored Red. If you look at a phone jack in your house, you will see that it is wired for 4 wires: Red, Green, Black and Yellow. However, black and yellow are not normally used.

The black and yellow wires can be used for a second telephone line or they can be used for running a Network Physical layer protocol called Phonenet (by Farralon). Phonenet uses the black and yellow for Network communications. It is for use with Appletalk, and is a replacement for Localtalk. It runs at the Localtalk speed of 230 Kbps, reasonable for small networks.

i. Ringer Unit

The ringer is a device that alerts you to an incoming call: it interprets the ringing voltage from the Central Office. Originally, the ringer was a electromagnetic bell. Today, though, most ringers are electronic devices.

The Central Office sends the following:

• a 90 to 120 VAC ringing voltage
  
• Frequency of 20 Hz
  
• Cadence for North America is 2 sec On/ 4 sec Off
  

ii. Hook Switch

The hook switch is activated by lifting the handset off of the cradle. The position of the hook switch determines whether the telephone is waiting for a call, or is actively using the line. The off-hook position informs the network of a request for use. The on-hook position releases the use of the network.

iii. Dialer Unit

There are two types of Dialer Units: Rotary and Touch Tone. Rotary is the old "put your finger in the hole and spin" type. The rotary dial operates by toggling the Hook Switch on and off.

Touch Tone is the modern method where 2 frequencies per push button are sent. Touch Tone is a trade name; the correct name is DTMF (Dual Tone Multi Frequency).

iv. Hybrid/Speech Network

The Hybrid/Speech Network performs these functions:

• It converts the Tx/Rx 4 wires from the Handset to the 2 wires for the Local Loop.
  
• It interfaces the signals from the Dialer Unit to the telephone line.
  
• It provides auto line compensation for line length to keep the volume constant.
  

v. Handset

The Handset contains transducers that convert mechanical energy into electrical energy. The microphone converts speech into electrical energy while the diaphragm (or speaker) converts electrical signals into audible signals.

Functions of a Telephone Set are shown below.

1. Request use of network from the CO (Central Office).
   
2. Inform you of the network status: Dial-tone, Ringing, Busy, Fast Busy (Talk Mail)
   
3. Informs CO of desired number.
   
4. Informs you when a call is incoming (phone rings).
   
5. Releases use of network when call is complete (hang-up)
   
6. Transmit speech on network & receives speech from distant caller.
   
7. Adjust power levels and compensates for line length.
   

Local Loops

The Local Loop is the connection between the Central Office and the home or business. Two wires (1 pair) are run into every home. The pair does not go directly to the Central Office. Instead, it goes to those big green boxes--that you see on the street corners--called "Serving Area Interfaces" (SIA) . Large multi-conductor bundles of wires then go from there to the Central Office.

Central Office

The Central Office provides the following functions:

1. It supplies the battery voltage for the telephone system. The On-hook voltage is 48 Vdc +/- 2V. Off-hook voltage is -6.5 Vdc.
   
2. It supplies the Ringing Generator - 90 to 120 VAC, 20 Hz, 2 sec on/ 4 sec off
   
3. It supplies the Busy signal (480 + 620 Hz, 0.5 sec On/ 0.5 sec Off), Dial Tone (350 + 440 Hz) and Fast Busy (480 + 620 Hz, 0.2 sec On/ 0.3 sec Off).
   
4. It has the digital switching gear that determines if the number is an Interoffice call (local) or an Intraoffice call (Toll - long distance).
   

Hierarchical Phone Networks

The PSTN (Public Switch Telephone Network) is divided into a hierarchical network. Here are the 5 classes of switching centers in North America:

ClassCenter	Abbreviation	Symbol	Examples
1	Regional Center	RC		2 in Canada: West - ReginaEast - Montreal
2	Sectional Center	SC		Calgary serves Alberta
3	Primary Center	PC		Edmonton
4	Toll Center	TC		Drumheller
4	Toll Point	TP		Rainbow Lake
5	Central Office(Local Loop)	CO		284-xxxx

In the following example:

The Hierarchical portion is seen as follows:

Trunk	Long distance telephone cable
Toll Trunk	Connects CO (Central Office) to TC (Toll Center)
Intertoll Trunk	Everything above TC (Toll Center) and TC to TC
Interoffice Trunk	Between CO (Central Office)
Intraoffice Trunk	Call between 2 subscribers within the same CO (284-7079 to 284-8181).

Call routing:

1. Preferred route
   
2. Second choice
   
3. Third Choice
   

Call routing is determined by network engineering and physical location. When all lines are idle, the call routing selects the preferred route. If the preferred route is busy, then the call is routed to the second choice. Because the second choice is routed through one toll center, the charge for the call is greater than the preferred route. The third choice is used when the second choice is busy. The third choice goes through 2 toll centers, and is the most expensive route.

A Central Office can have up to 10,000 subscribers (for example, 284-0000 to 284-9999). Most have 4,000 to 5,000 subscribers. The Central Office bases the loading requirements on roughly 10% of the phones that will be in use at any one time. However, the use of Internet dialup access has drastically changed this statistic!