Wireless Networks: Where We Are, Where We're Headed

This chapter is from the book

Principles of Wireless Networks: A Unified Approach

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This chapter is from the book

This chapter is from the book 

Principles of Wireless Networks: A Unified Approach

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Appendix 1A Backbone Networks for Wireless Access

Table 1A.1 provides a brief history of important developments in the telecommunications industry. The telecommunications industry started with the simple wired telegraph that used Morse code for digital data communication over long distance wires between the two neighboring cities of Washington, D.C., and Baltimore in 1839 [COU01]. It took 27 years, until 1866, for engineers to communicate over the ocean. In 1900, 34 years after the challenging task of deploying cables in the ocean and three years after the first trial of wireless telegraph, Marconi demonstrated wireless transoceanic telegraph as the first wireless data application. In 1867, Bell started the telephone industry, the first wired analog voice telecommunication service. It took 47 years for the telephone to become a transoceanic service in 1915, and it took almost 100 years for this industry to flourish into the wireless cellular telephone. The wireless telegraph was a point-to-point solution that eliminated the tedious task of laying very long wires in harsh environments. The wireless telephone was a network that had to support numerous mobile users. The challenge for wireless point-to-point communications is the design of a radio; the challenge for a wireless network is the design of a system that allows many mobile radios to work together. The telegraph was a manual SMS that needed a skilled worker to decode the transmitted message.

Table 1A.1 A Brief History of Telecommunications

The first computer communication networks started after the Second World War by using voiceband modems operating over the PSTN infrastructure to exchange large amounts of data among computers located a far distance from one another [PAH88]. Approximately two decades after the era of circuit switched computer communication networks around 1970, wide area packet switched networks (DARPAnet) and wideband local area networks, which were tailored for bursty data applications, were invented.

By the end of the 20th century, multimedia wireless networks emerged to integrate all networks and provide wireless and mobile access to them. SMS services, E- and M-commerce are becoming very popular. It is interesting to note that SMS provides a similar service as the wireless telegraph using the telephone keypad as the terminal. Finally, after more than 100 years, when a terminal with easy user interface became available, the market for the same service started to explode. In the late 1990s, income from SMS in Finland, the current leader in development and consumption of wireless services, is 20% of the income from cellular telephones in that country. This phenomenon also reflects the trends in change of habits, as user-friendly terminals become available for an old application.

Connection to the wired infrastructure is a very important issue for implementation of a wireless network. In the rest of this appendix, we give a brief description of the evolution of the three major existing wired telecommunications network infrastructures: PSTN, the Internet, and cable TV.

1A.1 Evolution of PSTN and Cellular Telephony

The invention of the telegraph in 1834 started wired data communication, and the invention of telephone in 1876 was the start of analog telephone networking. At that time, operators were used to manually switch or route a session from one terminal to another. At the beginning of 20th century, the telecommunications industry had already been exposed to a number of important issues, which played different roles, culminating in the emergence of modern wireless networks. Among these important issues were analog versus digital, voice versus data, wireless versus wired, local versus long haul communications, and personal versus group services.

By the 1950s, the PSTN had more than 10 million customers in the United States, and those interested in long-haul communication issues also needed PSTN services to solve their problems. Although end users are still mostly connected to the PSTN with twisted-pair analog lines, to provide flexibility and ease of maintenance and operation of the PSTN, the core network gradually changed to digital switches and digital wired lines connecting switches together. A hierarchy of digital lines (the T-carriers in the United States) evolved as trunks to connect switches of different sizes together.

Another advancement in the PSTN was the development of private branch exchanges (PBXs) as privately owned local telephone networks for large offices. A PBX is a voice-oriented local area network owned by the end organization itself, rather than the telephone service provider. This small switch allows the telephone company to reduce the number of wires that are needed to connect all the lines in an office to the local office of the PSTN. This way, the service provider reduces the number of wires to be laid to a small area where large offices with many subscribers are located. The end user also pays less to the telephone company. The organization thus has an opportunity to enhance services to the end users connected to the PBX.

In the 1920s, Bell Laboratories conducted studies to use the PSTN facilities for data communications. In this experiment the possibility of using analog telephone lines for transferring transoceanic telegrams was examined. Researchers involved in this project discovered several key issues that included the sampling theorem and effects of phase distortion on digital communications. However, these discoveries did not affect applications until after World War II when Bell Laboratories developed voice band modems for communication among air force computers in air bases that were geographically separated by large distances [PAH98]. These modems soon found their way into commercial airlines and banking industries, resulting in the associated private long-haul data networks. These pioneering computer communications networks consisted of a central computer and a bank of modems operating over four-wire commercial grade leased telephone lines to connect several terminals to the computer. In late 1960s, the highest data rate for commercial modems was 4,800 bps. By the early 1970s, with the invention of quadrature amplitude modulation (QAM), the data rate of four-wire voice band modems reached 9,600 bps. In the early 1980s, trellis-coded modulation (TCM) was invented which increased data rates to 19.2 kbps and beyond.

In parallel with the commercial four-wire modems used in early long haul computer networks, two-wire modems emerged for distance connection of computer terminals. The two-wire modems operate over standard two-wire telephone lines, and they are equipped with dialing procedures to initiate a call and establish a POTS line during the session. These modems started at data rates of 300 bps. By the early 1970s, they reached 1,200 bps, and by the mid 1980s, they were running at 9,600 bps. These two-wire voice band modems would allow users in the home and office to have access to a regular telephone to develop a data link connection with a distant modem also having access to the PSTN. Voice band modems using two-wire telephone connections soon found a large market in residential and small office remote computer access (telnet), and the technology soon spread to a number of popular applications such as operating a facsimile machine or credit card verification device. With the popularity of Internet access, a new gold rush for higher speed modems began, which resulted in 33.6 kbps full-duplex modems in 1995 and 56 kbps asymmetric modems by 1998. The 56 kbps modems use dialing procedures and operate within the 4 kHz voice band, but they directly connect to the core pulse code modulated (PCM) digital network of the PSTN that is similar to digital subscriber lines (DSLs). DSLs use the frequency band between 2.4 kHz and 1.1 MHz to support data rates up to 10 Mbps over two-wire telephone lines.

More recently cellular telephone services evolved. To connect a cellular telephone to the PSTN, the cellular operators developed their own infrastructure to support mobility. This infrastructure was connected to the PSTN to allow mobile-to-fixed telephone conversations. The addition of new services to the PSTN demanded increases in the intelligence of the core network to support these services. As this intelligence advanced, the telephone service provider added value features such as voice mail, autodialing through network operators, call forwarding, and caller identification to the basic POTS service traditionally supported. Figure 1A.1 shows a simplified representation of today's PSTN network.

Figure 1A.1 Simplified representation of PSTN and surrounding voice networks.

1A.2 Emergence of Internet

Data networks that evolved around voice band modems connected a variety of applications in a semiprivate manner. The core of the network was still the PSTN, but the application was for specific corporate use and was not offered privately to individual users. These networks were private data networks designed for specific applications, and they did not have standard transport protocols to allow them to interconnect with one another. Another irony of this operation was that the digital data was first converted to analog to be transmitted over the telephone network; then within the telephone network, it was again converted to digital format for transmission over long distances using the digital subcarrier system. To avoid this situation, starting in the mid 1970s, telephone companies started to introduce digital data services (DDS) which provided a 56 kbps digital service directly delivered to the end user. The idea was great because at that time the maximum data rate for voice band modems was 9,600 bps. However, like many other good and new ideas in telecommunications, this idea did not become popular. A large amount of capital was already invested in the existing voice band based data networks. It was not practical to replace them at once, and DDS services were not interoperable with the analog modems. The DDS services later emerged as integrated services digital network (ISDN) services providing two 64 kbps voice channels and a 16 kbps data channel to individual users. Penetration rates of ISDN services were not as expected, but laid a foundation for digital cellular services. Digital cellular systems can be viewed as a sort of wireless ISDN technology that integrates basic digital voice with a number of data services at the terminal.

The major cost for operation of a computer network over the four-wire lines was the cost of leasing lines from the telephone company. To reduce the operation cost, multiplexers were used to connect several lower speed modems and carry all of them at once over a higher speed modem operating over a long distance line. The next generation of multiplexers consisted of statistical multiplexers that multiplexed flows of data rather than multiplexing individual modem connections. Statistical multiplexer technology later evolved into router technology, which are generalized packet data switches.

In the early 1970s, the rapid increase in the number of terminals at the offices and manufacturing floors was the force behind the emergence of LANs. LANs provided high-speed connections (greater than 1 Mbps) among terminals facilitating the sharing of printers or mainframes from different locations. LANs provided a local medium specifically designed for data communication that was completely independent from the PSTN. By the mid 1980s, several successful LAN topologies and protocols were standardized, and LANs were installed in most large offices and manufacturing floors connecting their computing facilities. However, the income of the data communication industry, both LANs and public data networks (PDNs), was far below that of the PSTN still leaving the PSTN as the dominant economical force in the information networking industry.

Another important and innovative event in the 1970s was the implementation of ARPANET, the first packet-switched data network connecting 50 cities in the United States. This experimental network used routers rather than the PSTN switches to interconnect data terminals. The routers were originally connected via 56 kbps digital leased lines from the telephone company. This way, ARPANET interconnected several universities and government computers around a large geographic area. This network was the first packet-switched network supporting end-to-end digital services. This basic network later upgraded to higher speed lines and numerous additional networks. To facilitate a uniform communication protocol to interconnect these disparate networks, transmission control protocol/Internet protocol (TCP/IP) evolved that allowed LANs, as well as a number of other PDNs, to interconnect with one another and form the Internet. In the mid 1990s with the introduction of popular applications such as Telnet, FTP, email, and Web browsing, the Internet industry was created. Soon, the Internet penetrated the home market, and the number of Internet users became comparable with that of the PSTN, creating another economical power, namely, computer communications applications, which compete with the traditional PSTN. The IP-based Internet provides a cheaper solution than circuit switched operations, and today people are thinking of employing IP to capture a large share of the traditional telephony market served by the PSTN. The Internet provides a much lower-cost alternative to PSTN for support of multimedia applications. With the growth of the wireless industry in the past two decades, wireless LANs, wireless WANs, and wireless access to the Internet have become very popular.

In a manner similar to cellular telephony, wireless data network infrastructures have evolved around the existing wired data network infrastructures. Wireless LANs are designed mostly for in-building applications to cover a small area, and the network has a minimal infrastructure. Wireless LANs are usually connected to the existing wired LANs as an extension. Mobile data services are designed for low-speed wireless data applications with metropolitan, national, and, global coverage. These networks sometimes have their own infrastructure (e.g., ARDIS, Mobitex), sometimes use the existing cellular infrastructure but their own radio interface (e.g., CDPD), and sometimes use the infrastructure, as well as air-interface, of a cellular telephone service (e.g., GPRS). In all cases ultimately they connect to the Internet and run its popular applications. The 3G systems also provide competing packet-switched service. Figure 1A.2 provides a simplified sketch of the overall data networks surrounding the Internet. As compared with the PSTN, the Internet provides a cheaper and easier means to connect and expand networks. However, the telephone company owns the wires connecting the Internet and the telephone wires that can, among other alternatives, bring the Internet to the home or office.

Figure 1A.2 Simplified representation of the Internet and surrounding data networks.

1A.3 The Cable TV Infrastructure

Another competing wired infrastructure that evolved in the last few decades of the 20th century was the cable television network. Installation of cable TV distribution networks in the United States started in 1968 and has penetrated more than 60 percent of the residential homes. This penetration rate is getting close to that of the PSTN. The cable TV network consists of three basic element: a regional hub, a distribution cable bus, and a fiber ring to connect the hubs to one another. Because of the hybrid usage of fiber and cable, this network is also referred to as the hybrid fiber coax (HFC) network. The signals containing all channels at the hub are distributed through the cable bus in a residential area, and each home taps the signal off the bus. This is radically different in many ways from home access through twisted pair wires provided by the PSTN. The bandwidth of the coaxial cable supports about 100 TV channels, each around 6 MHz, whereas the basic telephone channel is around 4 kHz. The extended telephone channel using DSL uses about 1 MHz of bandwidth. The cable access is via a long bus originally designed for a one-way multicast that has a number (up to 500) of taps, creating a less controllable medium. The twisted pair star access for the PSTN is designed for two-way operation and is easier to control. The HFC channel is noisier than the telephone channel, and despite its wider bandwidth, its current supported broadband data rate is at the same range as the digital subscriber line (DSL) services operating on telephone wiring.

Figure 1A.3 shows a general picture of a futuristic HFC and the way it connects to the PSTN and Internet. The cable TV network was also considered a backbone for wireless PCS systems, and it is considered the leading method for broadband home access to support the evolving home networks. Some of the cable TV providers in the United States also offer telephone services over this medium. In the late 1990s, success of cable in broadband access encouraged some of the PSTN providers, such as AT&T, to acquire cable companies such as MediaOne.

Figure 1A.3 General overview of HFC networks.