Telephone Network

Although most of us take it completely for granted, the telephone you have in your house is one of the most amazing devices ever created. If you want to talk to someone, all you have to do is pick up the phone and dial a few digits. You are instantly connected to that person, and you can have a two-way conversation.

The telephone network extends worldwide, so you can reach nearly anyone on the planet. When you compare that to the state of the world just 100 years ago, when it might have taken several weeks to get a one-way written message to someone, you realize just how amazing the telephone is!


Major Components of a Telephone Network

Local loop is a twisted-pair cable that connects the subscriber telephone to the nearest end office or local central office. Local loop has a bandwidth of 4000 Hz (4KHz) when used for voice. For a long distance calls to be transmitted, the frequencies transmitted are limited to a bandwidth of about 3000 Hz (3Hz). All the frequencies of the voice below 400 Hz and above 3400 Hz are eliminated. (howstuffworks.com)

Trunks are transmission media that handle the communication between offices. A trunk normally handles hundreds or thousands of connections through multiplexing. Transmission is usually through optical fibers or satellite links.

Switching office is a component of a telephone network that handles the connection of several local loops or trunks and allows a connection between a different subscribers. The switch may operate in local, regional, national or international level. It also avoids a permanent link between two subscribers.

ILEC vs. CLEC

Incumbent Local Exchange Carriers (ILEC) is the only carrier that provides intra-LATA services before 1996 (before the Telecommunications Act of 1996). This was found out to be a monopoly and it also owns cabling system and so it opens opportunity for other local exchange carriers inside a LATA.
Competitive Local Exchange Carriers (CLEC) is the new carriers that can provide services after the Telecommunications Act of 1996. It was agreed that the ILECs would continue to provide their main services, and the CLECs would provide the other services such as mobile telephone service, toll calls inside a LATA, and so on. (For more info about the ILEC vs. CLEC visit howstuffworks.com)

Points of Presence (POP)
In a telephone network POP is the interface point that lets carriers interact with each other. let us say that in intra-LATA services, it can be provided by several LECs (may it be one ILEC and possibly more than one CLEC). Also in the inter-LATA services, it can be provided by several IXCs (interexchange carriers). Each IXCs that wants to provide inter-LATA services in LATA must have POP in that LATA. The LECs that provides services inside the LATA must provide connections so that every suscriber can have access to all POPs.

Rotary Dialing vs. Touch-tone Dialing

Rotary dialing or pulse dialing is the thing in the past, in which a digital signal was sent to the end office for each number dialed. This type of dialing was prone to errors due to inconsistency of humans during the dialing process.

Touch-tone dialing is the technique use nowadays. instead of sending a digital signal, the user sens two small bursts of analog signals, called the dual tone. The frequency of the signals sent depends on the row and the column of the pressed pad.

800 Service vs. 900 Service

800 Service is used if a subscriber (normally an organization) needs to provide free connections for other subscribers (normally customers). It can request an 800 service also 888, 877, 866 because 800 numbers are already exhausted. The call in this case is free for the caller, but it is paid by the callee. An organization uses this service to encourage the subscribers to call. The rate is less expensive than a normal long distance call.

900 Service are like 800/888 services, but the call is paid by the caller and is normally much more expensive than a normal long-distance call. Is is more expensive because the carrier charges two fees; the first is the long-distance toll, and the second is the fee paid to the callee for each call. This service is used by an organization that needs to charge their customers for its services (eg. a software company may charge a customer for technical support).

Digital Service Unit (DSU)

It is used in a switched/56 service where it changes the rate of the digital data created by the subscriber's device to 56 Kbps and encodes them in the format used by the service provider.


Sources: Data Communications and Networking 3rd ed. by Behrouz A. Foruozan
www.howstuffworks.com