6 Introduction and Layered Network Architecture Chap.1 even though software is a major cost of a new computer system,the increasing market decreases its unit cost.Each advance in solid-state technology decreases cost and in- creases the performance of computer systems;this leads to an increase in market,thus generating decreased unit software costs,leading,in a feedback loop,to further increases in market.Each new application,however,requires new specialized software which is initially expensive (until a market develops)and which requires a user learning curve. Thus,it is difficult to forecast the details of the growth of both the computer market and the data network market. 1.1.2 Communication Technology The communication links for wide area networks are usually leased from the facilities of the voice telephone network.In Section 2.2 we explain how these physical links are used to transmit a fixed-rate stream of binary digits.The rate at which a link transmits binary digits is usually referred to as the data rate,capacity,or speed of the link,and these data rates come in standard sizes.Early networks typically used link data rates of 2.4,4.8,9.6, and 56 kilobits/sec,whereas newer networks often use 64 kilobits/sec,1.5 megabits/sec, and even 45 megabits/sec.There are major economies of scale associated with higher link speeds;for example,the cost of a 1.5 megabit/sec link is about six times that of a 64 kilobit/sec link,but the data rate is 24 times higher.This makes it economically advantageous to concentrate network traffic on a relatively small set of high-speed links (This effect is seen in Fig.1.2 with the use of multiplexers or concentrators to share communication costs.) One result of sharing high-speed (i.e.,high data rate)communication links is that the cost of sending data from one point to another increases less than linearly with the geographic separation of the points.This occurs because a user with a long communi- cation path can share one or more high-speed links with other users(thus achieving low cost per unit data)over the bulk of the path,and use low-speed links(which have high cost per unit data)only for local access to the high-speed links. Estimating the cost of transmission facilities is highly specialized and complex. The cost of a communication link depends on whether one owns the facility or leases it:with leasing,the cost depends on the current competitive and regulatory situation. The details of communication cost will be ignored in what follows,but there are several overall effects of these costs that are important. First,for wide area data networks,cost has until recently been dominated by transmission costs.Thus,it has been desirable to use the communication links efficiently, perhaps at added computational costs.As will be shown in Section 1.3,the sporadic nature of most data communication,along with the high cost of idle communication links,led to the development of packet data networks. Second,because of the gradual maturing of optical fiber technology,transmission costs,particularly for high data rate links,are dropping at an accelerating rate which is expected to continue well into the future.The capacity of a single optical fiber using today's technology is 109to 1010 bits/sec,and in the future this could rise to 1014 or more.In contrast,all the voice and data traffic in the United States amounts to about6 Introduction and Layered Network Architecture Chap. 1 even though software is a major cost of a new computer system, the increasing market decreases its unit cost. Each advance in solid-state technology decreases cost and in￾creases the performance of computer systems; this leads to an increase in market, thus generating decreased unit software costs, leading, in a feedback loop, to further increases in market. Each new application, however, requires new specialized software which is initially expensive (until a market develops) and which requires a user learning curve. Thus, it is difficult to forecast the details of the growth of both the computer market and the data network market. 1.1.2 Communication Technology The communication links for wide area networks are usually leased from the facilities of the voice telephone network. In Section 2.2 we explain how these physical links are used to transmit a fixed-rate stream of binary digits. The rate at which a link transmits binary digits is usually referred to as the data rate, capacity, or speed of the link, and these data rates come in standard sizes. Early networks typically used link data rates of 2.4, 4.8, 9.6, and 56 kilobits/sec, whereas newer networks often use 64 kilobits/sec, 1.5 megabits/sec, and even 45 megabits/sec. There are major economies of scale associated with higher link speeds; for example, the cost of a 1.5 megabit/sec link is about six times that of a 64 kilobit/sec link, but the data rate is 24 times higher. This makes it economically advantageous to concentrate network traffic on a relatively small set of high-speed links. (This effect is seen in Fig. 1.2 with the use of multiplexers or concentrators to share communication costs.) One result of sharing high-speed (i.e., high data rate) communication links is that the cost of sending data from one point to another increases less than linearly with the geographic separation of the points. This occurs because a user with a long communi￾cation path can share one or more high-speed links with other users (thus achieving low cost per unit data) over the bulk of the path, and use low-speed links (which have high cost per unit data) only for local access to the high-speed links. Estimating the cost of transmission facilities is highly specialized and complex. The cost of a communication link depends on whether one owns the facility or leases it; with leasing, the cost depends on the current competitive and regulatory situation. The details of communication cost will be ignored in what follows, but there are several overall effects of these costs that are important. First, for wide area data networks, cost has until recently been dominated by transmission costs. Thus, it has been desirable to use the communication links efficiently, perhaps at added computational costs. As will be shown in Section 1.3, the sporadic nature of most data communication, along with the high cost of idle communication links, led to the development of packet data networks. Second, because of the gradual maturing of optical fiber technology, transmission costs, particularly for high data rate links, are dropping at an accelerating rate which is expected to continue well into the future. The capacity of a single optical fiber using today's technology is 109 to 1010 bits/sec, and in the future this could rise to 1014 or more. In contrast, all the voice and data traffic in the United States amounts to about