In 1875, while working on the use of sinusoidal signals for multiplexing in telegraphy, Alexander Graham Bell recognized that direct transmission of a voice signal over wires was possible. In 1876 Bell developed a system that could transmit the entire voice signal and could form the basis for voice communication, which we now know as the telephone. The modern telephone network was developed to provide basic telephone service, which involve the two-way, real-time transmission of voice signals across a network.
The telephone and telegraph provided services that were fundamentally different. The telegraph required an expert operator with knowledge of Morse code, while the telephone terminal was very simple and did not require any expertise. Consequently the telephone was targeted as a direct service to end users, first in the business and later in residential markets.
Connectivity in the original telephone system was provided by an analog transmission system. The transmitted electrical signal is analogous to the original voice signal; that is, the signal is proportional to the sound pressure in speech. It was quickly recognized in the early days of telephony that providing dedicated lines between each pair of users is very costly. Switches were introduced shortly after the invention of the telephone to minimize the cost of providing connectivity between communities of users.
In its simplest form, the switch consists of a patch cord panel and a human operator as shown in Figure 1.2-5a. The originating user picks up the telephone and in the process activates a signal in the circuit that connects it to the telephone office. The signal alerts the operator that a connection is requested. The operator takes the requested name and checks to see whether the desired user is available. If so, the operator establishes a connection by inserting the two ends of a cord into the sockets that terminate the lines of the two users as shown in Figure 1.2-5b. This connection allows electrical current, and the associated voice signal, to now between the two users. This end-to-end connection is maintained for the duration of the call. When the users are done with their conversation, they "hang up" their telephones, which generates a signal indicating that the call is complete. The two telephone lines are then available to make new connections. We say that telephone networks are connection-oriented because they require the setting up of a connection before the actual transfer of information can take place. The transfer mode of a network that involves setting up a dedicated end-to-end connection is called circuit switching.
(a) A switch in the form of an operator with a path cord patch (not shown)
(b) Cords interconnect user sockets providing end-to-end connection Fig. 1.2-5 Switching
Note that in circuit switching the routing decision is made when the path is set up across the network. After the call has been set up, information is "forwarded", continuously across each switch in the path. No additional address information is required after the call is set up.
The telephone network has undergone a gradual transition to its present state, where it is almost completely based on digital transmission and computer technology. This transition began with the invention of the transistor in 1948 and accelerated with the invention of integrated circuits in the 1960s, leading to the development of digital transmission system that could carry voice in a more cost-effective manner. Digital transmission systems were designed to carry binary information in the form of 0s and 1s. Thus these systems required that the analog voice signal be converted into a binary sequence, using a technique called pulse code modulation (PCM). The T-1digita1 transmission system was first deployed in 1962 to carry voice traffic between telephone offices. This multiplexing system could handle 24 voice calls for a total transmission rate of 1.5 Mbps.
Even as digital transmission systems were being deployed, the new digital transmission segments had to interface to existing analog switches. Upon arrival at an analog switch, the digital signal would be reconverted into analog voice signals for switching and then reconverted to digital form for transmission in the next hop. This situation eventually led to the invention of digital switches that could switch the voice signals in digital form. Thus a voice call would need to be digitized only once upon entering the network; then it would be transmitted and switched in digital form until it reached the other end of the backbone network. The call would then be converted to analog form for transmission over the pair of wires that connects the user to the network.
As the number of users and geographical extent of the user population increased, telephone networks grew along the lines shown in Figure 1.2-6. Users are served by an access network that connects them to a local central office (CO) switch. The switches themselves are interconnected with higher-speed communication lines through tandem switches. Multiplexing is used to combine many calls in these high-speed lines. Tandem switches, in turn, connect to toll switches that are used to provide long-distance connections. The result is hierarchical network topology, such as the one shown in Figure 1.2-6. The transmission rates that have been deployed in backbone transmission lines steadily increase over the years. The telephone digital multiplexing systems have advanced tremendously with the introduction of optical fiber transmission. In 2000 we are seeing the introduction of 1600 gigabit/second dense wavelength-division multiplexing (DWDM) systems.
The hierarchical network topology of the telephone network is complemented by a hierarchical decimal numbering system for dialing connections in the telephone network. For example in the 10-number system used in many countries, the area code specifies a subarea that has been assigned a three-digit number. The next three numbers are the exchange code, which identifies specific switching facilities in a central office within the subarea. The final four digits specify a specific line that connects the user to the central office.
Another advance in telephone networks involved the introduction of computer control for the setting up of connections in a switch. These computers would examine a request for a call as it came in, check to see whether the destination was available, and if so, make the appropriate connection. The use of computers to control the switch provided great flexibility in modifying the control and in introducing new features. It also led to the introduction of a separate signaling network to carry the messages between the switch computers.
When telephone companies realize that the No.7 signaling network (and its computer control) could be used to introduce enhanced telephone services. Credit-card calls, long-distance calls, 800 calls, and other services can all be implemented using this more capable signaling network. In the case of credit-card calls, a recorded message can request the credit-card number. The digits will be collected, and a message will be sent to a database to check the credit-card number; if authorized, the call will then be set up. The signaling network also enables mobility, which is the capability of a network to direct calls to users as they roam away from their home network.
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