Most wireless WANs are cellular based, but some make use of space. Take a closer look at both of these.
As shown in Figure 7-6, a cellular system consists of cell towers, concentrators, voice switches, and data gateways. The cell tower receives signals from user devices and transmits information back to the user. The voice switch connects the user device to another wireless, or wired, user through the telephone distribution system. This part of the system supports customary phones calls between users.
The component that makes the system a wireless WAN is the data gateway. In this case, the gateway is able to interface with data protocols in a way that makes it possible for users to surf the Internet, send and receive e-mails, and utilize corporate applications.
Text messaging is a popular application of cellular-based wireless WANs. Users converse by typing in short text messages and sending them to other users, similar to instant-messenger applications available for PCs. With smaller wireless WAN devices, however, it's important that users can save canned messages such as, "I'm traveling today and I'll call you later," which can be sent at the press of single button. Some wireless WAN devices also capture digital pictures and video that is sent across the network.
When mobile phones first became available, wireless communications used only analog signals. This initial cell phone system is known as first-generation cellular (1G cellular). When someone speaks through a 1G system, his voice is sent using frequency modulation (FM), which merely changes the frequency of carrier wave according to the audio signal. 1G systems make use of a limited number of channels that use FSK to send control signals necessary to set up and maintain the calls.
1G systems work well for voice phone calls, despite occasional crackles and pops, but they are not sufficient for sending computer data. As with the voice, analog signals must represent data. Users must interface PCs to the cellular system using a modem that converts the digital signals from the computing device into an analog form (such as FSK or PSK) that is suitable for transmission through a small, 4-KHz voice channel. This results in slow 20- to 30-kbps data rates.
1G systems also lack capacity to support an authentication and encryption mechanism. The digital FSK control channel only has enough capacity to support telephone calls. There is not enough room for sending usernames and passwords to an authentication service or coordinating encryption processes. It's quite apparent that 1G cellular was designed to carry voice, not data.
1G systems at one time covered most of the U.S. Today, however, they exist only in areas having low population density, where it's not feasible to upgrade the infrastructure to newer digital systems.
Not too long ago, digital cellular became available, allowing both the voice and control channels to make use of digital signing. The first phase of this totally digital system is referred to as second-generation cellular (2G cellular). Most of the telecommunications operators today have 2G systems, with various enhancements occurring periodically.
The use of digital signaling for the voice channels allows for more efficient modulation. This makes it possible to support more phone calls and data over a lower frequency spectrum. In fact, 2G systems enable enhanced services?such as short messaging, authentication, and phone software updates?to be accessed wirelessly.
Enhanced versions of 2G systems (sometimes referred to as 2.5G) include even better modulation, which increases data rates and spectrum efficiency. For example, the General Packet Radio Service (GPRS) offers high-speed data rates over a global system for mobile communications (GSM) network. Maximum data rates over GPRS are 171.2 kbps. The use of GPRS, however, requires a specialized mobile phone. Also, the Enhanced Data Rate for Global Evolution (EDGE) enhances GSM using 8-level PSK, where each transmitted symbol represents 3 data bits. This results in a maximum data rate of 474 kbps.
Many of the telecommunications operators are now beginning to offer what's known as third-generation cellular (3G cellular), with even better support for data communications. The Universal Mobile Telecommunications System (UMTS) is capable of 2-Mbps data rates for in-building implementations, up to 384 kbps in urban areas, and 144 kbps in rural areas. As a result, 3G is able to support multimedia applications.
There has been considerable argument in the wireless industry on whether 3G will replace 802.11 (Wi-Fi) wireless LAN technology. With higher data rates for indoor use, 3G is an alternative to wireless LANs. 802.11 continues, however, to have performance upgrades that significantly exceed 3G. For example, the 802.11a standard specifies data rates of 54 Mbps, which is much higher than 3G. Also, wireless LANs are much less expensive to deploy.
Wireless LANs, however, are not practical for providing coverage over wide areas. There would be too much infrastructure. 3G makes use of existing cell tower sites and distribution systems. Expenses of modifying 1G and 2G cellular systems to 3G are still high, but it's the most feasible method for providing wireless networking over wide areas.
Thus, both 3G and wireless LAN systems complement each other. This has prompted standards groups and manufacturers to find ways to seamlessly integrate 3G and wireless LANs. In fact, mobile phones and PDAs are available today that implement both technologies. With this capability, a user can roam outside the range of a wireless LAN and automatically associate with a cellular system. The problem is that standards that define this form of roaming are not yet available, which requires the user to carefully choose service providers that support the phone or PDA of choice.
Short Message Service (SMS) Applications
One of the most common services for wireless WANs is short message service (SMS), which is a text messaging system capable of sending a couple hundred characters at a time. SMS is a wireless form of the familiar instant messaging that is available from many of the ISPs. The following are additional applications of SMS for use with wireless WANs:
Many web sites use Wireless Markup Language (WML) to transform regular web pages into a format that is more easily read on a small device, such as a PDA or cell phone. WML also reduces the graphics on the page to compensate for the slower data rates of wireless WAN technologies.
For more information on instant messaging applications, check out the Instant Messaging Planet at http://www.instantmessagingplanet.com.
In addition to the land-based cellular systems, the use of space-based systems provides a means for networking users over wide areas.
The use of satellites for broadcasting television and other communications has been around for several decades. Not until recently, however, did satellite systems provide users with connections to the Internet. (See Figure 7-7.) Data rates are appreciable, with up to 1.5-Mbps downloads.
Some satellite systems support two-way exchange of data, allowing a user to send data up to the satellite (and vice versa). For example, a user's mobile device can transmit a web page request up to the satellite, and the satellite retransmits it down to the appropriate Earth station. The Earth station then sends the web page through the satellite and back down to the user. Other satellite systems, however, only support a downlink. A user's device must request the web page through another network, such as a telephone link, and the satellite broadcasts the page to the user.
By incorporating active radio repeaters in man-made, Earth-orbiting satellites, it is possible to provide broadcast and point-to-point communications over large areas of the Earth's surface. The broadcast capability of the satellite repeater is unique and, by suitable selection of satellite antenna patterns, it can be arranged to cover a well-defined area.
Satellites are located at various points in the geostationary orbit depending on the system mission requirement. To obtain global coverage, a minimum of three satellites is required. To obtain reasonably constant RF signal levels, however, four satellites are employed. This also provides some freedom in positioning.
With satellite communications, favorable frequencies are used: power efficiency, minimal propagation distortions, and minimal susceptibility to noise and interference. Unfortunately, terrestrial systems tend to favor these frequencies as well. Space is an international domain, and the International Telecommunications Union (ITU) controls satellite frequencies.
The band of frequencies between 450 MHz and 20 GHz is the most suitable for an Earth-space-Earth radio link. It is not practical to establish links to an Earth terminal located in a climatic region of heavy rainfall at frequencies higher than 20 GHz if consistent availability is expected.
For all operating bands, the lowest-frequency spectrum is used for the downlink because it has the most severe power constraints. Lower frequencies are less sensitive to free-space attenuation when compared to the higher-uplink frequencies. Losses are easier to overcome in the uplink with the higher transmit power available at the Earth station.
The satellite acts as a signal repeater. Signals sent to it on the uplink are rebroadcast back to Earth on the downlink. The device that handles this action is referred as a transponder. The satellite transponder is analogous to a repeater in a terrestrial communications link; it must receive, amplify, and retransmit signals from Earth terminals. A satellite transponder is capable of acting as a transponder for one or more RF communications links.
Low-altitude satellites, which can have circular, polar, or inclined orbits, have orbital periods of fewer than 24 hours. Therefore, they appear to move when seen from the Earth's surface. These orbits are useful for surveillance purposes, and can be used to provide communications at extreme north and south latitudes.
One type of special interest to public data communications is the geostationary orbit. A satellite in such an orbit has a 24-hour period at an altitude of 22,300 miles and remains over a fixed location on the equator. As a result, the satellite appears motionless to an observer on Earth.
Actually, the satellite does not remain truly fixed. Even if the orbit were perfectly circular and at precisely the right altitude, natural phenomena (because of low-level lunar and planetary-gravitational fields and solar-radiation pressure) introduce slight drifts in the orbit. This slow and minor drift is corrected from time-to-time by small onboard thrusters activated by ground stations.
Because of the long RF path involved (approximately 22,300 statute miles from an Earth terminal to a satellite in geostationary orbit), a transmission delay of approximately 100 ms is experienced between an Earth terminal and the satellite. This results in an approximate Earth-to-Earth-delay of 200 ms. This causes the system to be inefficient for use with protocols, such as 802.11, that require a response after each packet of information is transmitted before transmitting the next packet. In fact, most networking protocols do not work efficiently over satellite links because the protocols expect timely acknowledgments from the destination.
Billions of tiny microscopic meteors enter the Earth's atmosphere. In fact, meteors fall often throughout the day over all parts of the world. As these meteors penetrate the atmosphere, at a high altitude, they ionize into a gas. This gas is seen as a shooting star, which is an uncommon and large meteor as compared to most others.
Known as a poor man's satellite system, meteor burst communications (see Figure 7-8) bounce RF signals off meteor trails. This enables a long-haul (1,500 mile) wireless- data transmission link without the expense of launching and maintaining a satellite.
Meteor burst communications direct a 40 to 50 MHz radio wave? modulated with a data signal? at the ionized gas. The radio signal then reflects off the gas and is directed back to Earth. The availability of meteor trails is good but they are present only often enough to rely on 300 to 2400 bps. This is extremely slow, even compared to telephone modems.
However, the cost of deploying meteor burst equipment is so low compared to satellite systems that low-performance applications, such as telemetry, are feasible. Meteor burst, for example, works well for transmitting snow levels from remote mountainous areas to monitoring centers.