Circuit switching establishes a circuit (a circuit being all the phone lines used to connect the sending and receiving devices) during the communication session between sender and receiver. This means that, temporarily, the lines are dedicated to that communication session. Then, when the session is over, the circuit is immediately "torn down" and becomes available for another communication session.
Every time you pick up the telephone and make a phone call, you are taking advantage of circuit-switching technology. A circuit is established during your phone call. However, the circuit might be different (meaning different phone lines are used) each time you call the very same person. Two important WAN technologies that are circuit-switched are ISDN and DSL. Let's take a closer look at each.
ISDN is available from local phone providers and takes advantage of digital phone switching systems. The cost of an ISDN connection will be dictated by how often the line is used for data transfer. Your usage charge is determined by the connection charge (and there is also often a recurring monthly charge for being connected to the service). ISDN moves data on different channels like the T-Carrier system. ISDN comes in two flavors: Basic Rate ISDN (BRI) and Primary Rate ISDN (PRI).
Basic Rate ISDN uses three channels. Two data-carrying channels, called B channels , each provide 64Kbps of bandwidth for data transfer. The third channel, the D channel, is not used for data transfer and, operating at 16Kbps, is used exclusively for setup and control information used during the communication session. BRI can be used for both voice and data communications by dedicating a B channel for each. Typically, however, the two B channels are combined in BRI to provide a data transfer speed of 128Kbps.
For businesses with a need for more bandwidth, there is Primary Rate ISDN . PRI uses a T1 line and provides 23 B channels (each operating at 64Kbps). One D channel is also necessary (as with BRI) to handle setup and to control the connection. All or any number of the 23 B channels available can be combined for data transfer. Some channels can also be used for voice communication.
Digital Subscriber Line
ISDN is rapidly being eclipsed (particularly in the BRI arena) by DSL (Digital Subscriber Line) connections. DSL offers voice and data communication over the regular phone lines with speeds of up to 7Mbps (in research situations, connections of 50Mbps have been realized). The great thing about DSL in the small office environment is that the data and voice communication can take place simultaneously over the very same phone line. DSL shares the line with the analog voice signal by using a different frequency for the digital data (that is sent and received using the line). This means you can make a phone call while sending data over the line.
One of the problems with DSL is that there are several different flavors: ADSL, HDSL, IDSL, SDSL, and others. They are often referred to as xDSL . The most common DSL service for home and small business use is Asymmetric Digital Subscriber Line (ADSL) . ADSL gets its "asymmetry" from the fact that it supplies three different avenues (or pipes) on a single phone line. Each pipe provides a different level of bandwidth (and is therefore asymmetrical ). One of the pipes is called the POTS pipe , and it takes care of the analog traffic on the phone line, such as telephone calls. The second pipe provides a medium-size upstream pipe, and the third pipe provides a large downstream pipe.
The fact that a standard has not been chosen by the Baby Bells and other providers of DSL means that the equipment used to connect a home or company to the provider's network and the bandwidth available will vary from place to place. Figure 13.3 shows a Cisco DSL router used to connect to the QWEST (formerly U.S. West) DSL network. As already mentioned in this section, DSL providers typically offer more bandwidth downstream (meaning you get a very fast connection to another network, such as the Internet) and less bandwidth upstream (it will take longer for you to send data than receive it). This, again, depends on the service provider you use.
As with the other WAN technologies we've discussed, DSL requires a device (such as the Cisco ADSL router, as shown previously in Figure 13.3) to connect your LAN to the provider's switched network. This device is considered customer premise equipment (this means you have to buy and maintain this device, although some providers have been giving away these devices to get customers to subscribe to their DSL services). The availability of DSL is also determined by the distance between the provider's nearest switching station (also called the central office or CO ) and your home or company's location. In many cases, the limit is currently two to three miles (although in some markets five miles is the limit).
Figure 13.3. A customer premise device, such as the Cisco 675 ADSL Router, is used to connect the LAN to the DSL provider's network.
The big question is this: How does the DSL provider take the numerous client connections and move that data on the provider's switched network? This task is handled by a Digital Subscriber Line Access Multiplexer (DSLAM) . Typically located at the provider's central office, it receives signals from multiple customer DSL connections and puts the signals on a high-speed backbone using multiplexing techniques (meaning that all the data is multiplexed into a single signal, just as we discussed in relation to T-Carrier channels). The high-speed backbone can use technology such as ATM (Asynchronous Transfer Mode), which is discussed later in this chapter.
Although DSL is currently marketed as an avenue for high-speed Internet connection, it can be used to connect telecommuters and different corporate sites by using Virtual Private Networking (VPN). We will discuss both remote access and VPN in Chapter 18.