How A Phone Call Really Works
Routing On A Dedicated 1+ (Outbound) Voice Call

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Definitions For Terms On This Page
 Carrier
Competitive Access Provider (CAP)
Competitive Local Exchange Carrier (CLEC)
Channel Bank
Channel Service Unit (CSU)
Dedicated Service
DS1 or T1
Data Services Unit (DSU)
Facilities
Interexchange
Local Exchange Carrier (LEC)
Local Loop
PBX (Private Branch Exchange)
POP (Point of Presence)
Switch

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Carrier - A long distance telephone company which uses it's own transmission facilities and network lines for the majority of it's customer's calls. Many people refer to any long distance company as a carrier, even if it does not own or operate it own transmission facilities or network. [Back to Diagram]

Competitive Access Provider (CAP) and Competitive Local Exchange Carrier (CLEC) - Companies that compete with the local telephone company (LEC). Most CAP's and CLEC's were created around the time the Telecommunications Reform Act of 1996 opened up local telephone services to competition. Prior to the 1996 Act, the LEC's generally had a monopoly on providing local exchange services (dial tone, hunt groups, local loops, etc.) [Back to Diagram]

Channel Bank - A piece of equipment that converts analog voice grade telephone lines into a digital format (usually 56Kbps or 64Kbps per voice grade line or channel) and multiplexes these digitized voice and/or data lines or channels for use on a 1.544 Mbps (T1 or DS1) digital circuit. A Channel Bank typically sits between a T1 digital circuit and a telephone system (analog PBX or switch) controlling several (up to 24 per channel bank) voice grade telephone lines. If a company utilizing a T-1 has a digital PBX, then a channel bank is not needed, but a CSU is required. Used in conjunction with Dedicated (T1) Services. Click here for more information about Dedicated Services or Dedicated Toll Free Services. [Back to Diagram]

CSU (Channel Service Unit) - A piece of equipment that provides line isolation between the customer premise equipment and a local telephone company services. Also provides alarms, loopback, signal regeneration, line build out, and surge protection. A CSU typically sits between a T1 digital circuit and a telephone system (digital PBX or switch) controlling the T1's 24 voice grade channels. Used in conjunction with Dedicated (T1) Services. Click here for more information about Dedicated Services or Dedicated Toll Free Services. [Back to Diagram]

Dedicated Service - Also called Dedicated Access. A type of service often used by large companies which have a connection (local loop) going directly to the long distance companies' Point of Presence (POP) , thereby bypassing the local telephone company's switch and reducing the long distance cost per minute. Often referred to as "T-1" service. Click here for more information about Dedicated Services or Dedicated Toll Free Services. [Back to Diagram]

DS1 or T1- "T1" is actually a speed of digital signal transmission over telephone lines. But the term is often used to describe various types of high speed and/or high volume telecommunications service including Dedicated local access to long distance facilities, point-to-point data services (private line) and for regular local service. A T1 has the capacity of 24 regular telephone lines. Sometimes also called a "T-Span".

Click here for more information about Dedicated Services or Dedicated Toll Free Services. [Back to Diagram]

DSU (Data Services Unit) - A piece of equipment that converts digital signals from data terminal equipment to a type that can be transmitted through a digital phone line. A DSU typically sits between a 56Kbps or T1 digital circuit and computer(s) to allow data communication over a point-to-point circuit. Used in conjunction with Point-to-Point Services (sometimes also called a private line). [Back to Diagram]

Facilities - Typically refers to transmission switches and equipment used by long distance carriers to handle a long distance call. [Back to Diagram]

Interexchange - The part of long distance voice and/or data transmission carried over the long distance carrier's facilities. [Back to Diagram]

Local Exchange Carrier (LEC) - Your Local Telephone company. [Back to Diagram]

Local Loop - That part of a communication circuit that connects the customer's telephone equipment to the long distance carrier's POP through the local telephone company (LEC). [Back to Diagram]

PBX (Private Branch Exchange) - A private phone system (switch) used by medium and large companies, connected to the customer's local telephone company (LEC). Performs a variety of in-house routing and switching. [Back to Diagram]

POP (Point of Presence) - 1) Physical place where a long distance carrier interfaces with the network of the local telephone company (LEC). 2) The point at which the local telephone company terminates customer's circuits for long distance calls or leased line communications. [Back to Diagram]

Switch - Guess we ought to explain what this is, even though it's not technically a "part" of a telephone call. Rather a switch is a device that handles calls. When a long distance call is made using dedicated service, the local telephone company does not "switch" the call across the dedicated local loop. Rather the dedicated local loop provides a dedicated connection between the customer's telephone equipment and their long distance carrier's Point of Presence (POP). The local telephone company on the terminating end of the call does switch the call and route it to the called party. [Back to Diagram]