only referred to as centrex service. Network switching systems include the local serving central office and any other systems facili- tating 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. Circuit switching in functional terms is nothing more than con- nection 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. Voice 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 sub- scriber line (DSL) access is available and growing. Transmission bandwidth available in circuit switched facilities varies from below 64 Kbs (rarely more than 49 Kbs) to 1.536 Mbs. The limi- tation 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 PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS much matter to voice grade service. After all most, if not all, tele- phone equipment is bandwidth limited to around 3.5 kHz, which fits easily into 8-kHz sampled PCM. ISDN and DSL access provide higher capabilities though. ISDN Access is either 144 Kbs, called basic rate interface (BRI), or 1.544 Mbs, called primary rate interface (P