Chapter 3: Wireless Networks

Wireless network technology is one of the hottest topics in mobile computing. Everyone has an opinion on the state of the third-generation (3G) wireless networks, the effect of Bluetooth for personal networks, and which wireless local area network technology will dominate the market. Even though not all mobile applications require wireless connectivity, there is no doubt that wireless technology is one of the main driving forces behind mobile computing.

This chapter provides an overview of the four main categories of wireless networks: wireless personal area networks (WPANs), wireless local area networks (WLANs), wireless wide area networks (WWANs), and satellite networks. For each category we summarize the prevalent technologies and discuss what the future holds. By reading this chapter you will gain an understanding of which wireless network protocols are being used and for what types of applications. This knowledge will be valuable as you continue through the book to learn more about the design and development of mobile and wireless applications.

Overview of Wireless Networks

Wireless networks serve many purposes. In some cases they are used as cable replacements, while in other cases they are used to provide access to corporate data from remote locations. Much of the industry hype surrounds third-generation wide area networks that provide broadband wireless connectivity to users on a national basis. These networks are now commercially available (in larger urban centers) in most first-world regions. At the same time, breakthroughs in short-range networks are also generating excitement. As users carry around multiple devices, a need arises for an easy, effective way for them to communicate; and what is easier than wireless?

For the purpose of our discussion, wireless networks will be divided into two broad segments: short-range and long-range. Short-range wireless pertains to networks that are confined to a limited area. This applies to local area networks (LANs), such as corporate buildings, school campuses, manufacturing plants or homes, as well as to personal area networks (PANs) where portable computers within close proximity to one another need to communicate. These networks typically operate over unlicensed spectrum reserved for industrial, scientific, medical (ISM) usage. The available frequencies differ from country to country. The most common frequency band is at 2.4 GHz, which is available across most of the globe. Other bands at 5 GHz and 40 GHz are also often used. The availability of these frequencies allows users to operate wireless networks without obtaining a license, and without charge.

Long-range networks continue where LANs end. Connectivity is typically provided by companies that sell the wireless connectivity as a service. These networks span large areas such as a metropolitan area, a state or province, or an entire country. The goal of long-range networks is to provide wireless coverage globally. The most common longrange network is wireless wide area network (WWAN). When true global coverage is required, satellite networks are also available.

Four Categories of Wireless Networks

Table 3.1 provides more detail about the four wireless networks categories. Information such as coverage area, function, relative cost, and throughput are some of the main areas where these networks differ. As mentioned earlier, short-range networks operate on unlicensed frequency bands, therefore, there are no airtime fees associated with their usage. The same is not true of WWANs and satellite networks, which charge either by the minute or by the amount of data transferred.

Table 3.1: High-Level Differences between WPANs, WLANs, WWANs, and Satellite

TYPE OF NETWORK	COVERAGE AREA	FUNCTION	ASSOCIATED COST	TYPICAL THROUGHPUT	STANDARDS
Wireless personal area network (WPAN)	Personal operating space; typically 10 meters	Cable replacement technology, personal networks	Very low	0.1-4 Mbps	IrDA, Bluetooth, 802.15
Wireless local area network (WLAN)	In buildings or campuses; typically 100 meters	Extension or alternative to wired LAN	Low-medium	1-54 Mbps	802.11a, b, g, HIPERLAN/2
Wireless wide area network (WWAN)	Coverage provided on national basis from multiple carriers	Extension of LAN	Medium-high	8 Kbps-2 Mbps	GSM, TDMA, CDMA, GPRS, EDGE, WCDMA
Satellite networks	Global coverage	Extension of LAN	Very high	2 Kbps-19.2 Kbps	TDMA, CDMA, FDMA

Application developers do not have to know the internal workings of wireless networks to be successful, but having knowledge of how they work helps them understand why certain wireless technologies behave the way they do. This knowledge also lays a foundation on which application design architectures can be based. For example, if an application architect knows that the wireless network will only provide 9.6 Kbps of throughput, he or she will want to limit the frequency and amount of data transfer. Similar rationale applies to coverage and cost issues.

For each category of wireless networks, we will provide a summary of the leading standards as well as some insight into where they are being used.

Frequency Fundamentals

Before we can delve too deeply into wireless technologies, a brief primer on radio frequencies is required. Many of the wireless technologies in the WPAN, WLAN, and WWAN categories transmit information using radio waves. For this to take place, the data is superimposed onto the radio wave, which is also known as the carrier wave, since it carries the data. This process is called modulation. There are many modulation techniques available, all with certain advantages and disadvantages in terms of efficiency and power requirements. This summary of the various mechanisms will be helpful as you read this chapter. The modulation techniques are as follows:

• Narrowband technology. Narrowband radio systems transmit and receive data on a specific radio frequency. The frequency band is kept as narrow as possible to allow the information to be passed. Interference is avoided by coordinating different users on different frequencies. The radio receiver filters out all signals except those on the designated frequency. For a company to use narrowband technology, it requires a license issued by the government. Examples of such companies include many of the wide area network providers discussed later in this chapter.

• Spread spectrum. By design, spread spectrum trades off bandwidth efficiency for reliability, integrity, and security. It consumes more bandwidth than narrow-band technology, but produces a signal that is louder and easier to detect by receivers that know the parameters of the signal being broadcast. To everyone else, the spread-spectrum signal looks like background noise. Two variations of spread-spectrum radio exist: frequency-hopping and direct-sequence.

  • Frequency-hopping spread spectrum (FHSS). FHSS uses a narrowband carrier that rapidly cycles through frequencies. Both the sender and receiver know the frequency pattern being used. The idea is that even if one frequency is blocked, another should be available. If this is not the case, then the data is re-sent. When properly synchronized, the result is a single logical channel over which the information is transmitted. To everyone else, it appears as short bursts of noise. The maximum data rate using FHSS is typically around 1 Mbps.

  • Direct-sequence spread spectrum (DSSS). DSSS spreads the signal across a broad band of radio frequencies simultaneously. Each bit transmitted has a redundant bit pattern called a chip. The longer the chip, the more likely the original data can be recovered. Longer bits also require more bandwidth. To receivers not expecting the signal, DSSS appears as low-power broadband noise and is rejected. DSSS requires more power than FHSS, but data rates can be increased to a maximum of 2 Mbps.

• Orthogonal Frequency Division Multiplexing (OFDM). OFDM transmits data in a parallel method, as opposed to the hopping technique used by FHSS and the spreading technique used by DSSS. This protects it from interference since the signal is being sent over parallel frequencies. OFDM has ultrahigh spectrum efficiency, meaning that more data can travel over a smaller amount of bandwidth. This makes it effective for high-data-rate transmissions. The drawbacks of OFDM are that it is more difficult to implement than either FHSS or DSSS, and consumes greater amounts of power.