CIRCUIT SWITCHING | Network Technology And Methodology

Circuit switching and routing is the basis for all domestic and international telephone or voice grade, dial-up traffic. The circuit switching function is distributed between end-office switching systems and network switching systems. End office switching systems may be a private branch exchange (PBX) physically located on subscriber premises, or a partition in the telephone company’s nearest office, commonly referred to as centrex service. Network switching systems include the local serving central office and any other systems facilitating a path for a telephone call. Nowadays, these systems range in size from a few thousand to hundreds of thousands of ports capable of handling millions of calls per hour.
Add a note hereCircuit switching in functional terms is nothing more than connection of one transmit-receive pair on one side of a switch to a transmit-receive pair on another port on the same path or route, or a different path or route, sometimes called the other side of the switch. Tandem switches are nothing more than transit points that link up network or inter-network transmission facilities. For example, each of the 200+ local access and transport areas (LATA) in the United States has a minimum of one tandem switch, which acts as the transit point between the access and transport networks used by long distance carriers to carry calls from one LATA to another.
Add a note hereVoice grade dial-up service is almost all digital in the United States. However, many analog switches remain in other parts of the world. Where digital switches provide the service, integrated services digital network (ISDN) services—really an access method, not a service—is available. In highly populated areas of many countries, digital subscriber line (DSL) access is available and growing.
Add a note hereTransmission bandwidth available in circuit switched facilities varies from below 64 Kbs (rarely more than 49 Kbs) to 1.536 Mbs. The limitation in analog service is a matter of the ability of a modem to talk to another modem over a local telephone loop. Of course, it doesn’t much matter to voice grade service. After all most, if not all, telephone equipment is bandwidth limited to around 3.5 kHz, which fits easily into 8-kHz sampled PCM.
Add a note hereISDN and DSL access provide higher capabilities though. ISDN Access is either 144 Kbs, called basic rate interface (BRI), or1.544 Mbs, called primary rate interface (PRI). BRI is channelized into three channels, two bearer or B channels at 64 Kbs, and one delta or data or D channel (16 Kbs) used for signaling and control purposes. PRI access is facilitated with T1 transmission facilities and is channelized into 23 to 64 Kbs B channels and 1 to 64 Kbs D channel. It should be emphasized that the previous explanation is purely in terms of technical capability. Leveraging the bandwidth into variable amounts and getting charged for it on a case-by-case, service-by-service basis is an entirely different matter.
Add a note hereFor example, ISDN-based Internet access never achieved large usage because the equipment used by ISPs and their users was limited to BRI rates—64 Kbs at best. And because the ISPs are not the telephone company and have no capability, such as a big digital circuit switch, and have no funds available to buy a big digital circuit switch and therefore no interest in competing with the telephone company, they only offer Internet access service. From the telephone company viewpoint, they simply are prevented from being in the data services—Internet access, or Internet service provider (ISP) business—by current FCC rules and legislation. The telephone company can only sell POTS, ISDN, or private line service. It cannot offer any type of switching other than these services. Some of the independent non–regional Bell operating telephone companies have purchased and operate ATM equipment, but basically they are quite limited in the service they can provide using these or other non-voice service, frame relay, and IP-based switching and routing systems.
Add a note hereDSL access varies according to several factors, the main one of which is the distance between the subscriber premises equipment and nearest central office or wire center. Conceptually and technically, DSL access is intended to be capable of multiple service types such as voice and data. However, implementation reality has driven most service providers to offer only Internet access without any voice service initially. It remains to be seen how long this is likely to continue. The classical telephone companies don’t want to cannibalize their bread and butter—lucrative voice services—and they desperately want to tap into new revenue streams of their up and coming competitors—cable modems and DSL-capable ISPs. Therefore, initial DSL service is limited to Internet access. As the Internet matures—achieves a grade and quality of service capable of supporting voice-over IP—this situation will change. Who knows when, but someday in the future it may be possible to call up the telephone company and ask them to discontinue POTS.
Add a note hereKeep in mind that the main purpose of the switching function is to share use of the transmission function. Also, keep in mind the fact that change in the network is more a direct result of economic pressure than technological or regulatory forces.

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