Circuit-switched networks are the oldest and simplest approach to MAN and WAN circuits. These services operate over the public switched telephone network (PSTN); that is, the telephone networks operated by the common carriers such as AT&T, BellSouth, and so on. When you telephone someone, you are using the PSTN. The first service we discuss is the dial-up service you use when you call an ISP with a dial-up modem—but first we need to discuss the basic architecture shared by all circuit-switched services.
At this point, you’re probably wondering who uses dial-up any more? Most of my students use broadband Internet, which we discuss in the next topic. However, statistics show that less than 25 percent of the U.S. and Canadian population has broadband access; believe it or not, most of us still use dial-up—you just won’t find many dial-up users on college campuses.
Basic Architecture
Circuit-switched services use a cloud architecture. The users lease connection points (e.g., telephone lines) into the common carrier’s network, which is called the cloud1 (Figure 9.1). A person (or computer) dials the telephone number of the destination computer and establishes a temporary circuit between the two computers. The computers exchange data, and when the task is complete, the circuit is disconnected (e.g., by hanging up the phone).
Figure 9.1 Dialed circuit services. LAN = local area network
This architecture is very flexible. Circuits can be established as needed between any computers attached to the cloud at any point. However, data can be transmitted only while a circuit is established, and only to the one location it connects to. If a computer needs to send data to a number of other locations, a series of temporary circuits must be established with and later disconnected from each location, one after another. In general, only a limited number of circuits can be established from or to any one location at a time (e.g., each location has only so many telephone lines).
Cloud-based designs are simpler for the organization because they move the burden of network design and management inside the cloud from the organization to the common carrier. Network managers do not need to worry about the amount of traffic sent between each computer; they just need to specify the amount of traffic entering and leaving each computer and buy the appropriate size and number of connections into the PSTN. However, this comes at a price. Cloud-based designs can be more expensive because users must pay for each connection into the network and pay on the basis of the amount of time each circuit is used. Cloud-based designs are often used when network managers are uncertain of network demand, particularly in a new or rapidly growing network.
There are two basic types of switched-circuit services in use today: POTS and ISDN.
Plain Old Telephone Service
Plain old telephone service (POTS) is the name for the dial-up services you or your parents used at one time. To use POTS, you need to lease a circuit into the network (i.e., a telephone line) and install special equipment (i.e., a modem) to enable your computer to talk to the PSTN. To transfer data to and from another computer on the network, you instruct your modem to dial the other computer’s telephone. Once the modem in your computer connects to the modem at the other end, you can transfer data back and forth. When you are done, you hang up and can then call another computer if you wish. Today, POTS is most commonly used to connect to the Internet, but you can also use it to communicate directly with a private non-Internet server.
POTS may use different circuit paths between the two computers each time a number is dialed. Some circuits have more noise and distortion than others, so the quality and maximum data transmission rate can vary. Typical data rates are between 33 kbps and 56 kbps.
Charges for direct dialing are based on the distance between the two telephones (in miles) and the number of minutes the connection is used. Data communications users pay the same rate as voice communications users. In general, most local calls are free, but this depends on the type of local telephone service you have purchased. Long-distance calls are charged at the rate for which you have contracted with your long-distance carrier.
Wide area telephone services (WATSs) are special-rate services that allows calls for both voice communications and data transmission to be purchased in large quantities. For example, you might purchase 100 hours of usage per month for one fixed rate and be charged so many dollars per hour thereafter.
ISDN
The first generation of integrated services digital network (ISDN) combines voice, video, and data over the same digital circuit. Because there is a newer version of ISDN, the original version is occasionally called narrowband ISDN, but we will just use the term ISDN. ISDN is widely available from a number of common carriers in North America.
To use ISDN, users first need to lease connection points in the PSTN, which are telephone lines just like POTS. Next, they must have special equipment to connect their computers (or networks) into the PSTN. Users need an ISDN network terminator (NT-1 or NT-2) that functions much like a hub, and a NIC (called a terminal adapter [TA] or even an "ISDN modem") in all computers attached to the NT-1/NT-2. In most cases, the ISDN service appears identical to the regular dialed telephone service, with the exception that usually (but not always) each device attached to the NT-1/NT-2 needs a unique service profile identifier (SPID) to identify it. To connect to another computer using ISDN, you dial that computer’s telephone number using the ISDN NIC in much the same way as you would with a modem on a regular telephone line.
ISDN has long been more of a concept than a reliable service in North America. It has been available since the late 1970s, although it has not been widely adopted. Its largest problems are a lack of standards and a lack of interest from common carriers. Acceptance of ISDN has also been slowed because equipment vendors and common carriers have conflicting interpretations of the ISDN standards and because the data rates it offers are low compared with newer services. Skeptics claim that ISDN actually stands for "I still don’t know," "I still don’t need it" or "It still does nothing." ISDN offers two types of "normal" or narrowband service, plus one higher-speed broadband service.
Basic Rate Interface Basic rate interface (BRI) (sometimes called basic access service or 2B+D) provides a communication circuit with two 64-Kbps digital transmission channels (called B channels) and one 16-Kbps control signaling channel (called a D channel). The two B channels handle digitized voice, data, and image transmissions, providing a total of 128 Kbps. The D channel is used for control messages such as acknowledgments, call setup and termination, and other functions such as automatic number identification. Some common carriers sell just one single 64-Kbps channel to those customers needing less capacity than full BRI.
One advantage of BRI is that it can be installed in many existing telephone locations without adding any new cable. If the connection from the customer’s telephone to the common carrier’s end office is less than 3.5 miles, the ISDN line can use the existing two pairs of twisted-pair wires. The only changes are the end connections at the customer’s location and at the carrier’s end office. If the connection is longer than 3.5 miles, then new cable will have to be laid.
Primary Rate Interface Primary rate interface (PRI) (also called primary access service or 23B+D) is typically offered to commercial customers. It consists of 23 64-Kbps B channels plus 1 64-Kbps D channel. PRI has almost the same capacity as a T1 circuit (1.544 Mbps). In Europe, PRI is defined as 30 B channels plus 1 D channel, making interconnection between America and Europe difficult.
Broadband Integrated Services Digital Network Broadband ISDN (B-ISDN) is very different from narrowband ISDN—so different, in fact, that it really is not ISDN. It is a circuit-switched service, but B-ISDN uses ATM to move data from one end point to the other (more on ATM later in this topic). B-ISDN is backward-compatible with narrowband ISDN, which means it can accept narrowband BRI and PRI transmissions. B-ISDN currently defines three services. The first is a full-duplex channel that operates at 155.52 Mbps; the second provides a full-duplex channel that operates at 622.08 Mbps; and the third is an asymmetrical service with two simplex channels, one from the subscriber at 155.52 Mbps and one from the host to the subscriber at 622.08 Mbps. The first two services are intended for normal bidirectional information exchange. The third (asymmetrical) service is intended to be used for information distribution services such as digital broadcast television.
