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Kb/s : kilo-bit per second
Mb：meag-bit
Gb：giga-bit
Tb：Tera-bit
Pb：Peta-bit

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1.2 Services, Circuit Switching, and Packet Switching

Many types of services are offered by carriers to their customers. In many cases, these are connection-oriented services in that there is the notion of a connection between two or more parties across an underlying network. The differences lie in the bandwidth of the connection and the type of underlying network with which the connection is supported, which has a significant impact on the quality-of-service guarantees offered by the carriers to their customers. Networks can also provide connectionless service; we will discuss this type of service later in this section.
There are two fundamental types of underlying network infrastructures based on how traffic is multiplexed and switched inside the network: circuit-switched and packet-switched. Figure 1.2 illustrates some of the differences in the type of multiplexing used in these cases.
　A circuit-switched network provides circuit-switched connections to its cus-tomers. In circuit switching,a guaranteed amount of bandwidth is allocated to each connection and is available to the connection all the time, once the connection is setup. The sum of the bandwidth of all the circuits, or connections, on a link must be less than the link bandwidth. The most common example of a circuit-switched network is the public-switched telephone network (PSTN), which provides a nailed-down connection to end users with a fixed amount of bandwidth(typically around 4 kHz) once the connection is established. This circuit is converted to a digital 64 kb/s circuit at the carrier central ofice. This network was designed to support voice streams and does a fine job for this application.
　The circuit-switched services offered by carriers today include circuits at a varietyof bit rates, ranging from 64 kb/s voice circuits all the way up to several Gb/s. These connections are typically leased by a carrier to its customers and remain nailed down for fairly long periods, ranging from several days to months to years as the bandwidth on the connection goes up. These services are also called private line services. The PSTN fits into this category with one important difference-in the PSTN, users dial up and establish connections between themselves, whereas with private line services, the carrier usually sets up the connection using a management system. This situation is changing, and we will no doubt see users dialing for higher-speed private lines in the future, particularly as the connection durations come down.
　The problem with circuit switching is that it is not efficient at handling bursty data traffic. An example of a bursty trafic stream is traffic from a user typing on a keyboard. When the user is actively typing, bits are transmitted at more or less a steady rate. When the user pauses, there is no traffic. Another example is Web browsing. When a user is looking at a recently downloaded screen, there is almost no traffic. When she clicks on a hyperlink,a new page needs to be downloaded as soon as possible from the network. Thus a bursty stream requires a lot of bandwidth from the network whenever it is active and very little bandwidth when it is not active.
　It is usually characterized by an average bandwidth and a peak bandwidth, which correspond to the long-term average and the short-term burst rates, respectively. In a circuit-switched network, we would have to reserve sufficient bandwidth to deal with the peak rate, and this bandwidth would be unused a lot of the time.

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