Wireless services: A freedom from location
An examination of the types of wireless services and how they work
In SunWorld's continuing series on local and wide area network access systems, we've been taking a look at the various forms of wireless communications both on the personal level and the business-to-business level. This month, connectivity expert Rawn Shah introduces several land-based wireless communications systems. This specifically excludes satellite-based systems, which may use similar technology but are in another category altogether. We'll narrow this broad topic down by focusing on a few key concepts and types of service. (2,300 words, including a glossary of terms)
Fact and fiction
It's commonly assumed that wireless services are intended for mobile users only. You lose a large segment of the potential market by thinking this way. In some service areas it's too difficult to maintain a wireline connection. In Hawaii, for example, radio-based data communications provide a much simpler alternative to laying undersea cabling between the various islands.
In other areas, it's simply cheaper to implement wireless services. For example, take the wide open and relatively flat desert areas of Arizona. A clear view across much of the terrain makes it easy to set up direct point-to-point wireless links between two locations without having to hassle with the issues of wireline services from a provider.
Another common misconception is that wireless services are all related or similar. You might as well say that the engine in the Geo Metro is the same as the engine in the Harrier JumpJet. Some of the principles may be similar in theory but in implementation they're worlds apart. Because wireless networks are pretty much restricted to one media -- air -- the differences lie mostly in the encoding process and the allotted frequency range.
Security is an obvious concern when your data is going over an open media such as the atmosphere. The different types of wireless services provide varying levels of security. For example, CDMA digital cell phones create a unique code for each user generated by the phone, and the packets are effectively encrypted from other CDMA cell phone users. Other cell phone systems (TDMA and GSM), however, simply send the packets over open air separated by pre-arranged time slots; anyone clever enough can listen in on a conversation with the right equipment. Wide area commercial wireless networks sometimes come with encryption devices built-in or attached, to secure communications between the points.
Categories of wireless services
You can think of wireless services grouped into three areas:
PCS is intended for home or mobile users and works over a public infrastructure network in areas served. Typically, it supports voice and low-bandwidth data services. End-user units are usually quite small -- like the cell phones some of us use. The base stations are owned by the service providers and are typically placed in "cells" of slightly overlapping service areas. The antennas are very visible from 50 to 200 feet, often with a triangular structure at the top containing the transmitter and receiver units pointed in all directions. A typical antenna can serve up to a 10-kilometer radius. By combining multiple cells and a wired infrastructure, you can achieve practically unlimited land-based coverage.
WLANs are intended for business and mobile users within the service area of an office, a building, or a campus. End-user units are typically PC card devices for portables, with a short antenna jutting out from the side. The base station sizes depend on the range of service: A single office will generally find a small box with an antenna mounted on the wall or in a prominent location sufficient. Buildings require several of these boxes -- or more powerful versions of them -- on each floor to cover the service area. Campuses utilize rooftop antennas varying from three- to fifty-feet in height. Service areas are typically in the two- to four-kilometer range.
WWANs and WLLs are intended for high-end commercial networks to cover large areas or long distances. Very often they are point-to-point links or are based on line of sight. The frequencies used in WWANs are fairly high, between 28 gigahertz and 32 gigahertz, providing a lot of bandwidth for transmission. In some scenarios, such as wireless cable, most of that bandwidth is unidirectional from the base station to a receiver with only a small uplink channel. With others, such as point-to-point spread-spectrum radio, the full frequency range is bidirectional. The transmitters and receivers come in many sizes but are typically fairly large and focused according to line of sight.
|Name||Sub-types or Other Names||Frequency||Service Type||Data Bandwidth|
|Analog Cellular||FDMA, AMPS||800 MHz - 850 MHz||analog voice, analog data||7.2 - 14.4 Kbps|
|Radio||Spread-spectrum Radio||2400 - 2483.5MHz (ISM band)||digital data||1 Mbps - 2 Mbps|
|Enhanced Specialized Mobile Radio||806 - 821 MHz, 851 - 866 MHz||digital data, paging||4.8 Kbps|
|Wireless LAN||IEEE 802.11||2400 - 2483.5 MHz (ISM band)||digital data||1 Mbps - 2 Mbps|
|Metricom Richochet||902 - 928 MHz||digital data||4.8 - 28.8 Kbps|
|Motorola ARDIS||900 - 925 MHz||digital data||4.8 - 19.2 Kbps|
|RAM Mobile Data||900 - 925 MHz||digital data||8 Kbps|
|Digital Cellular, PCS Digital||TDMA||800 - 890 MHz, 1800 - 1990 MHz||digital voice, digital data||up to 19.2 Kbps|
||806 - 890 MHz, 1850 - 1990 MHz||digital voice, digital data||up to 19.2 Kbps|
|GSM, DCS 1800||890 - 915 MHz, 935 - 960 MHz, 1800 - 1850 MHz, 1900 - 1990 MHz||digital voice, digital data||0.3 - 9.6 Kbps|
|CDPD, "Wireless IP"||800 - 890 MHz, 1800 - 1990 MHz||analog voice*, digital data||4.8 - 19.2 Kbps|
|Multipoint distribution services||Local MDS||27500 - 28350 MHz, 29100 - 29250 MHz, 31000 - 31300 MHz||digital data||1 Mbps - 45 Mbps|
|Multichannel/ MDS, Wireless Cable||2150 - 2680 MHz||digital data||500 Kbps - 2 Mbps|
* CDPD allows you to reserve part of the frequency spectrum for analog voice AMPS. CDPD is primarily intended for data or "bursty" messages like those sent by pagers.
Personal services for voice and data have been the biggest push for wireless services to the mass market audience. Billions of dollars have gone into developing products and allocating service provider areas in this specific market. In particular, the push to all-digital services over the past few years has brought in a lot of new vendors and service providers. This is perhaps one of the fastest growing communications services in the world, even beyond that of the Internet, in terms of number of users. In the short space of 15 years it has almost reached 200 million users worldwide; the Internet took almost twice that amount of time to reach between 100 and 200 million users.
Most people are familiar with the concept of cellular phone services. In a service area, a single cell covers several kilometers. All cell phone calls contact the cell station for that area and establish a call service link. As the mobile cell phone moves out of a given area, the call is automatically transferred to another cell station responsible for the new area. If the user moves outside the cell service areas or into an underground tunnel, the connection is cut off. Although a cell phone can transmit through most buildings, it can be stopped by several feet of rock or concrete.
Analog cellular services developed in the 1980s have since been replaced by the popularity of various digital services. The past two years have seen cellular services extend beyond the 800- to 900-megahertz block. With the growth of digital services, new 1800- to 2000-megahertz blocks have been set aside around the world. In the U.S., the two main contenders for digital service, time division multiple access (TDMA) and code division multiple access (CDMA), are now fighting it out for market stakes. The Global Standard for Mobiles (GSM) standard is available all across Western Europe, and mobile users in the European community can easily move from country to country with the same GSM phone service. In Asia, GSM has also had wide acceptance, particularly in India, Southeast Asia, Japan, Korea, and Taiwan.
Technically, CDMA is considered the best of all these systems. GSM and TDMA rely on partitioning end-user conversations into time slots. This separates data and voice transmissions according to a time clock and by the local cell station. CDMA works by stamping each call with a code and separating users by keeping separate unique codes. This improves security as the codes are dynamically assigned and have a large degree of variation, such that it's quite hard to pinpoint a particular conversation. Almost all the major cellular services are now advertising some form of PCS. Sprint PCS, for example, uses CDMA in its service while MCI uses TDMA.
Data invades voice
Data rates are not very high for any of these digital services. In fact, they are all below 19.2 kilobits per second (Kbps) -- the same as V.32bis analog modem technology. Although analog modems have scaled as high as 33.6 Kbps (remember that 56-Kbps modems are hybrid analog/digital systems), digital cellular has no place to go beyond 19.2 Kbps because the frequency is strictly controlled, and current technology limits have been reached.
There is another type of cellular service specifically for data networks called cellular digital packet data (CDPD). This system employs packet switching rather than circuit switching. While a normal cell phone call continues sending audio signals through even when you aren't speaking, a packet-switched call transfers individual data packets to the local cell station, then disconnects. This requires a fast connect/disconnect cycle but saves a lot of "airtime" that is normally wasted when a circuit-switched conversation is idle.
Although CDPD doesn't go any faster than the other digital cellular systems, it saves you money by saving airtime. What's more, CDPD's packet format is the Internet Protocol. When you send information using this system it goes out as TCP or UDP packets. CDPD is most prominent in AT&T's PocketNet service across the U.S.
With these systems you typically still need a TCP/IP protocol stack to reach the Internet. The phones can still work as regular phone lines, so modems that hook up to these systems work just like any regular analog modem. In some services, such as AT&T PocketNet, the stack is included for Windows 95 systems. More advanced users may want to try other stacks from WRQ or FTP Software.
Source: Yankee Group
We have only covered voice and data PCS so far. Preceding PCS were earlier proprietary systems using similar portions of the radio spectrum such as Metricom Richochet, Motorola ARDIS, and RAM Mobile Data. Most provide similar services but do not have the same nationwide coverage in the U.S. as the newer PCS providers do. Users are being wooed away from these services with offers of free phones, free air time, two-way paging, etc., so we can expect that they will slowly fade from view.
Next month we'll discuss WLANs and WWANs and the types of service they can provide in more depth. Until then, imagine what you could do with a service capable of delivering data to your home at 45 megabits per second for $500 a month.
About the author
Rawn Shah is chief analyst for Razor Research Group covering WAN and MAN networking technology and network-centric computing. He has expertise in a wide range of technologies including ATM, DSL, PC-to-Unix connectivity, PC network programming, Unix software development, and systems integration. He helped found NC World magazine in December 1996, and has led the charge to the deployment of network-centric computing in the corporate world. Reach Rawn at email@example.com.
If you have technical problems with this magazine, contact firstname.lastname@example.org
GSM: Groupe Spéciale Mobile or Global Standard for Mobiles, an international digital cellular service available in over 100 countries. Available in some areas in the U.S.
FDMA: Frequency division multiple access, the method used to separate different analog cellular calls by allotting each caller a different 30-kilohertz frequency range
TDMA: Time division multiple access, the encoding method used to separate different digital cellular calls by allotting each caller a different reference time slot
CDMA: Code division multiple access, the encoding method used to separate different digital cellular calls by designating each with a unique code on a per-call basis
CDPD: Cellular digital packet data, a form of encoding digital data in packets based upon TCP/IP
LMDS: Local multipoint distribution system, a form of point-to-point or point-to-multipoint communications for very high-bandwidth
MMDS: Multichannel/multipoint distribution system, a form of high-bandwidth, point-to-point or point-to-multipoint communications primarily for wireless cable applications
ISM: Industrial, scientific, and medical; frequency range 2.4 to 2.483.5 gigahertz
PCS: Personal communications services; frequency range 1.8 to 2.0 gigahertz
WCS: Wireless communications services; frequency range 2305 to 2360 megahertz
IEEE: Institute for Electrical and Electronic Engineers
DCS 1800: Digital cellular service, an older name for GSM 1800 that works in the 1.8-gigahertz range