Voice over Internet Protocol - Voip

Voice over Internet Protocol

Before we begin our discussion on transporting voice over a data networks, it is important to understand the concept and terms associated with a traditional telephone network. One of the most common piece of telecommunication equipment used today is the telephone. When you plug an ordinary analog telephone into the wall jack installed by the local telephone company, you are connected to the telephone network and are able to place calls. 1), but how is this process performed?

In early telephone networks, call completion was dependent on an operator to complete the calls. When you wanted to place a call you would pick up the receiver and be connected to an operator at the Central Office (CO). The CO is the local telephone office which all local lines connect too and where circuit switching of subscriber lines occurs. The operator would determine which two lines need to be connected for the parties to talk and connect the lines. To connect the two lines the operator would plug a cord connecting the two ports on a cord board to bridge the two ports together. The cord board used by the operator to connect the two parties was the first example of an early switching system.

In today's telephone networks a CO switch or a private branch exchange (PBX) provides the same service once performed by an operator. A PBX is a premise switching system, serving a commercial or government organization, and usually located on that organization's premise. PBX's provide telecommunication services at the location and access to public and private telecommunications network services. The PBX switch is connected to a similar switch located at the CO.

Now we will discuss some of the key components of any telephone network. These components are loops, lines and trunks. "Loop transmission facilities connect switching systems to customer premises equipment throughout the serving area. A loop is a transmission path between a customer's premises and a local exchange carrier (LEC) central office" (Loop Transmission). A line is a communication connection between your telephone and the local phone company's switch, such as a PBX or a CO switch. No that we know that a line connects a telephone to the switch, the next stage is the trunk. A trunk is a shared communication channel that connects multiple telephone switches together and has the ability to transfer a telephone call from one location to the next. It consists of wires that are twisted together to minimize the electromagnetic radiation created by the current flowing through them. "A trunk is a communications path connecting two switching systems used to establish end-to-end connections between customers (Trunks)." A trunk assigns connections case-by-case when a number is dialed. When using a PBX everyone has a telephone line connected to the PBX, but the actual connection is only established when in use. When there is a need to access an outside line, an access code is dialed, such as 9 and the PBX connects the telephone to an outside line to the Public Switched Telephone Network (PSTN).

Another component of any telephone network is the transmission media. The basic transmission media are: twisted-pair cable, coaxial cable, radio frequencies, and fiber. Twisted-pair cabling is made up of two insulated wires twisted in a spiral pattern. The wires can be shielded twisted-pair (STP) or unshielded twisted-pair (UTP). Twisted-pair cable is common in telecommunications networks to carry analog signals from the user to other parts of the network. Integrated Services Digital Network (ISDN) is an international communication standard offered by telephone companies for sending data, voice, video, and other traffic. Both ISDN and digital subscriber line (DSL) can carry digital signals at a higher rate across the same twisted-pair cabling used by a standard telephone. DSL includes a variety of technologies being used to get higher digital bandwidth out to the customer's location.

Another transmission media is cellular technology, which use radio waves for network access. Radio gives you the convenience of not having to install wiring in the ground to provide the service. Inside the network, radio is usually used with microwave technology such as Local Microwave Distribution System (LMDS) and Multipoint Microwave Distribution System (MMDS) to carry higher-rate digital systems on a line-to-site basis between points. Fiber Optics is a method of transmitting data in the form of laser light over bundles of glass fibers. This method has numerous advantages over such traditional techniques using twisted pair wires, coax, and radio waves. Fiber optics offers greater bandwidth and is virtually unsusceptible to electromagnetic interference. The North America format standard for transmission over fiber is typically Synchronous Optical Network, or SONET. SONET has the ability to deliver very high speeds, up to 2.5 Gbps.

Types and Uses of Various Transmission Media

Media User to Network Network to Network Speed

Twisted Pair Analog voice ISDN T1 / E1 Digital subscriber line (xDSL) T1 E1 1.544 Mpbs / 2.048 Mbps

Coaxial Cable Cable TV T3, T4 E3 44.736 / 34.368 Mbps

Radio Cellular WLL, LMDS, and MMDS T3, T4 E3 44.73 Mbps 34.368 Mbps

Fiber SONET Cable Television SONET 2.5 Gbps

For a telephone call to be completed, several forms of signaling must occur. In a voice network signaling is used to establish a connection. A voice connection usually begins when the phone is taken off hook, which sends a type of signal called an access signal. The access signal determines when a line is off hook or on hook. Foreign Exchange (FX) trunk signaling is another form of signaling that can be provided over analog or T1 lines. FX is a term used in a trunk network that has access to a distant CO. FX uses either loop-start or ground-start signaling methods. A loop-start is a type of access signal that is used in a PSTN. Ground-start is another access signaling method used on trunk lines or tie lines between PBXs to indicate on-hook/off-hook status to the CO. A Foreign Exchange Station (FXS) is how standard residential phone lines are configured for signaling. A FXS interface connects basic devices such as phones, modems, and faxes and must provide voltage, ring generation, off-hook detection, and call progress indicators. Foreign Exchange Office (FXO) is a type of signaling that is used primarily to communicate with CO switching equipment or PBXs.

There are two forms of technology that can be used to transmit voice across a traditional telephony network, analog and digital. In