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Document:	Wireless Data Communications - A Technology Primer
Order Number:	XX-36DD4-DA
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Pages:	25
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Wireless Data Communications
A Technology Primer

By
Bob Egan
Digital Equipment Corporation

Mobile and Wireless Technologies Primer

Wireless
Communications

Welcome to the Age of Wireless Computing
With advances in telecommunications technology over the past 10 years, computer users
now enjoy virtually unlimited access to any kind of information they need. Data of every
possible type is truly at our fingertips. Until recently, however, such access to information
has been available only while we were within the walls which house our hard-wired
infrastructure.
Now, a new communications revolution is underway. Over the next decade we will see an
explosion in what the industry calls the Wireless Evolution.
The New Mobile Workforce
Businesses of all kinds are discovering a powerful new way to compete: the mobile
workforce. The "virtual office" is fast becoming a commonplace reality. Corporate
professionals are beginning to exercise the same control over information on the road as
they enjoy in their offices. Wireless technology is being used to link portable computer
equipment to corporate databases and other sources of necessary information.
The Shift to Wireless Technology
A definite shift is taking place -- from systems based on hard-wired, location-dependent
infrastructures serving stationary clients to systems using wireless, location-independent
technology to serve clients who are constantly on the move.
Industry analysis confirms this shift from the office-based worker to the mobile professional.
During 1993 alone, sales of portable computing equipment such as laptops exceeded unit
purchases of desktop equipment by 20% or more.
The Wireless Evolution will enable mobile professionals to send and receive data anytime,
anywhere, so they can do business more efficiently and effectively than ever before. Crucial
3

Mobile and Wireless Technologies Primer

to the success of this evolution will be technology that provides mobile professionals with
the same ease of use, reliability, and robust service they have come to expect in the office.
Directions Wireless Technology Is Taking
The wireless marketplace can be divided into two general methods of wireless service. The
first is lower-speed, wide-area access which is known as Mobile Data or Wireless WideArea Networks (WWANs). The second method is higher-speed, local-area access which is
referred to as Wireless Local Area Networks (WLANs).

Wireless Wide-Area Communications (WWANs) -- Mobile Data
Much has been written about the various forms of two-way, wireless, wide-area services. A
potential user of wireless technology, seeking to understand whether a particular offering
best suits his application, can easily be frustrated when attempting to sort out the different
offerings.
Wireless data services can provide benefits to three types of mobile user applications:
1. Transaction Processing - Includes applications such as taxi dispatch and credit
card verification.
2. Information Distribution - Includes providing advisory services for traffic,
weather, public safety and multicast sales updates.
3. Interactive Access - Provides connectivity from remote clients to central host
computers such as connecting field service personnel to headquarters.
WWAN services require the use of a large infrastructure which can provide wireless radio
coverage throughout the service area. The mobile professional does not want to have to
worry about being within a defined area of service, but rather wishes to be productive
regardless of geographic location.
There are two ways one can approach this problem. The first, and more complex from an
software perspective, is to provide the capability of disconnected operations. Using this
approach, the mobile professional works on files which are later synchronized with the
central host database. The mobile professional may also download optionally filtered files
such as mail, and generate responses for transmission when wireless coverage has been
established. Digital Equipment’s Mobile User Services provide this type of solution.
The second solution to this problem is to install wireless base stations which can provide
continuous radio coverage as well as seamless connectivity throughout a service area. This
requires significant infrastructure costs as well as innovative network software to handle
billing, roaming, and authentication issues. For mobile professionals in the United States this
would imply seamless coverage throughout the contiguous 48 states. However, in each of
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Mobile and Wireless Technologies Primer

the public service areas there are at least two competitors who provide a wireless
infrastructure which can be used for mobile data. And in many areas there are packet radio
systems, such as ARDIS and MOBITEX, which are also available for mobile data. This
suggests another problem for the mobile professional: inter-system protocol compatibility.
Digital’s RoamAbout Transporter product provides an application-independent solution
for the inter-system communication problem.
WWAN Technology
In this section we will explore the major WWAN infrastructures available for data
communication for the mobile professional: analog cellular systems and packet radio
systems. We will also discuss some technologies just on the horizon.
Analog Cellular Systems
Potential data subscribers view existing analog cellular telephone systems as having an
advantage over specialized packet radio systems -- ARDIS, MOBITEX, or other private
Special Mobile Radio (SMR) services -- in providing future WWAN data service. The
advantages of the existing cellular system are that it provides the widest geographic
coverage, reliability of existing service, and fundamental network intelligence. These
advantages are perceived by many industry analysts as reducing the cost of deployment in
utilizing this service as a reliable, integrated voice/data system in the future.
At the same time, analog cellular systems have certain shortcomings. These include limited
system capacity and inefficient deployment of fast, reliable, and available data services.
Furthermore, these systems were designed primarily to provide voice service only. Such
shortcomings will continue to exist, regardless of the variations or extensions that are
implemented in this system technology.
For example, the primary advances in cellular infrastructure technology -- N-AMPS and
CDPD -- are designed to increase overall system capacity. Data applications capability for
the mobile professional remains inherently limited.
Providing an effective, integrated voice and data wireless wide-area network depends upon
the development and deployment of digital radio techniques. As will be seen later, the Group
Special Mobile (GSM) service, a digital cellular system which has been widely accepted in
Europe and Asia, will provide significant benefits to the mobile data professional. In the
United States, however, we are faced with multiple variant expansions of the existing analog
technology, and at least two adopted digital systems which are referred to as Digital-TDMA
(NTIA IS-54) and Digital-CDMA (NTIA IS-95).

Mobile and Wireless Technologies Primer

Analog Cellular Radiotelephone

Existing analog cellular standards in use worldwide include the following:
1. NMT - The Nordic Mobile Telephone service. This system has two variants
based on the frequency of allocation. NMT450 operates on 450 MHz, while
NMT900 operates on 900 MHz. Due to its standardization throughout
the Nordic countries, this system is the only analog system which allows
roaming through multiple European countries.
2. TACS - Total Access Communications Systems. This is a cellular system based
in the U.K. The TACS system has several variants in use: JTACS in Japan
(which is similar to theU.S. AMP system), E-TACS, or Expanded TACS of
the U.K. system, and J-TACS, a Japanese cellular specification similar
to N-AMPS in the U.S.
3. N-AMPS - Narrowband Advanced Mobile System. This provides three times
the current capacity of the AMPS system in the U.S. by using 10KHz channel
bandwidths instead of the standard 30 KHz channel bandwidths used in
the AMPS system.
4. Intersitial cellular, proprietary to Cellular Data Inc. This is a technique for
transmitting data on unused cellular frequency allocations, known as guard
bands or buffer zones, that separate the regular voice channels. The result is
a 2400 bit/s, X.25 data network which is optimized for short duration
data transmissions.

Mobile and Wireless Technologies Primer

Some of the specifics of these analog cellular systems are shown in Table 1.
Table 1 - Analog Cellular Systems
AMPS
Frequency
824-894
Band (MHz)
Allocated BW 50
(MHz)
Base to Mobile 869-894
Mobile to Base 824-849
Access
FDMA
Scheme
Duplex
FDD
Method
Channel BW 30
(KHz)
Total
832
Channels
Voice
PM
Modulation
Trans.
Rate 10
(Kbps)
notes: BW means bandwidth

N-AMPS

NMT-450

NMT-900

TACS

JTACS

824-894

453-467.5

890-960

890-950

870-940

869-894
824-849
FDMA

463-467.5
453-457.5
FDMA

935-960
890-915
FDMA

935-950
890-905
FDMA

870-885
925-940
FDMA

FDD

12.5

2456

180

2000

600

1.2

All these systems were designed primarily to support voice communications via analog
technology. As such, they all share fundamental characteristics with the U.S. Advanced
Mobile Phone Services (AMPS) system with regard to implementing mobile data. For this
reason the U.S. AMPS system will be used as a model for exploring the use of cellular
telephone infrastructures for mobile data.
Cellular Infrastructures for Mobile Data
The AMPS system employs analog technology and was designed for point-to-point, circuitswitched, radio/telephone voice communications for mobile professionals. The emergence
and deployment of one standard system in the U.S. has allowed easier development of both
intra-system and inter-system roaming around the country for voice-based mobile
professionals. However, this service convenience may be short-lived due to the recent
deployments of incompatible cellular telephone variants such as N-AMPS, CDPD, and
Intersitial Cellular.
European analog cellular system subscribers have not enjoyed a similar robust roaming
service advantage. The European system has been composed of several standards as shown
in Table #1, usually divided by country borders. As we will see later, the European Group
Special Mobile (GSM) standard, a digital cellular technology, provides a major step towards
7

Mobile and Wireless Technologies Primer

a total solution of this cellular system shortcoming for both the voice and data mobile
professional.
In the United States the Federal Communications Commission (FCC) has allocated 50 MHz
in the 800 MHz band which is divided among two different providers for each service area
of the AMPS system. The 25 MHz allocated to each provider is divided into 832, 30-KHz
wide channels.
Cellular communications techniques are based on dividing the coverage areas of a central
radio base station control center into terrestrial zones known as cells. The size of a cell is
determined by several factors, including:
• Local propagation environment
• Mobile traffic distribution and management
• Geographic topology

In Figure 1 cells are represented as idealized areas in the form of hexagons with no overlaps
or gaps. Each cell uses at least two frequencies or channels to communicate with mobile
stations. Cell channel allocations are arranged in regular patterns to maximize frequency
reuse of the radio spectrum and minimize interference between adjacent cells.

Mobile Cells

Mobile Telephone Switching Office
(MTSO)

PSTN
Central Office
(CO)
Figure 1 - Cellular Infrastructure Overview
Cell sizes can vary, providing coverage from less than a square mile to over 50 square miles
in sparsely populated areas. To increase capacity, cells can be further divided into sectors
(three sectors are common) using directional antennas.
8

Mobile and Wireless Technologies Primer

The base station control radio is connected to a mobile telephone switching office (MTSO)
using wired or point-to-point wireless technology such as microwave radio. The MTSO is
responsible for supervision, signaling, switching, and allocating Radio Frequency (RF)
channels to the cells.
The MTSO maintains a list and status of connected customers, including the signal-to- noise
ratio of the established connection. As a subscriber signal becomes weak, the system noise
becomes a more appreciable percentage of the total signal received at the central base
station. When the connection approaches a minimum acceptance level, the customer is
"handed-off" to an adjoining cell. The subscriber unit, referred to as a mobile end station,
may also be handed off to another cell for traffic handling management.
Cellular Data Transmission
The analog cellular network can be used for data transmission; however, the radio link
performance for data is considered marginal. This is due to the limitations imposed by this
technology. Radio channel dynamics such as dropouts, signal fades, and multipaths, which
can be tolerated during a voice connection, can be disastrous to a mobile data subscriber.
For example, "clicks" which occur during cell hand-off -- caused by the analog system using
a technique known as "block and burst mode" -- can cause a complete disconnect or garbled
data. These problems will become much more manageable when digital cellular transmission
in the United States and GSM in Europe (both digital radio transmission standards) become
widely available. Digital Signal Processing (DSP) techniques, which are used as part of the
process of transceiving information in digital form, will eliminate much of the noise-based
interference in the data channel and will enhance signal recovery.
Subscriber data rates of 2400 baud or less can be sustained using standard modems -similar to those used for the commonplace wireline connection -- with some adaptation for
connection to the cellular network. Using special, high-level, error-correcting modems and
data transmission techniques targeted for use in the cellular radio infrastructure, data
throughputs have approached >20K bits from a stationary user.
Cellular systems charge mobile professionals by "air" time used, rather than by the amount
of data (packets) transmitted, as is the case with packet radio systems. For relatively lowduty-cycle data communications -- which do not require a dedicated full-time connection
-- subscribers pay for real-time, dedicated, two-way, point-to-point connections. Connection
set-up times can be long, especially when compared to the actual time required for the
mobile professional to send only a few kilobytes of data.
Short cellular data messages can be very expensive to send, as compared to those sent
through packet radio systems. The bottom line is that current analog cellular infrastructure
systems are generally not an efficient means of sending data due to limited capacities
available, limitations of data recovery, low security, and the high cost of use for many
applications.
9

Mobile and Wireless Technologies Primer

Cellular Digital Packet Data (CDPD)
Developed by a consortium of eight U.S. companies, CDPD is a technique which allows
data transmission to be overlaid onto the existing AMPS analog cellular channels. CDPD
provides two significant enhancements to the existing analog cellular radio/telephone
system: increased total system capacity and specifications for implementing data.
CDPD uses a channel-hopping technique to transmit data in short bursts during idle times in
the existing 30 KHz-wide analog cellular channels (AMPS channels). Given sufficient
capacity, no degradation in system capacity for voice calls should occur, since voice calls on
the system still maintain priority. Voice and data are digitized and sent through the
infrastructure. Digitalization also allows for encryption, which increases data and voice
security.
The CDPD Airlink Interface specification defines all procedures and protocols necessary to
allow effective use of existing analog cellular channels for data communications.
The CDPD architecture provides for mobility management and inter-networking between
separate CDPD network providers. This will result in seamless access for the mobile
professional when moving between cells and between different providers. Mobile subscribers
will be handed off gracefully using soft transfer techniques, reducing the chance of corrupted
data or lost connection as is common in the AMPS system.
The CDPD standard specifies support for CLNP, the Connectionless Network Protocol
defined in ISO-8473, and the Internet Protocol as defined in RFC-791. CDPD does not
provide protocol translation; therefore, each station must use the same network layer
protocol. Application compatibility between the wireless mobile professional and standard
wired data network topologies are made easier with the decision of CDPD to adapt the
Internet Protocol (IP).

Mobile and Wireless Technologies Primer

According to a report from the Yankee Group, the key advantages of CDPD include the
following :
• Efficiency: The specifications of CDPD will allow the use of hardware components

found in standard cellular telephones.
• Open protocol: CDPD will contribute to the system's low cost and wide availability.
• Speed: CDPD will support 19.2Kb/s transmission rates. Overhead for coding and
channel management to handle frequency hopping, however, will reduce actual
throughput.
• Capacity: Cellular carriers control more than 800 channels nationwide as compared
with ARDIS, which has an average of one-to-five channels per city.
• Preservation of voice transmission quality: Use of idle channels will
prevent degradation of voice quality and reduction of network capacity.
• Feature enhancements to regular cellular service: These include encryption,
message broadcast, roaming, compression, and authentication.
CDPD availability is dependent upon the aggressiveness and financial commitment of its
provider companies. In the short term CDPD may become a serious competitor to packet
radio systems such as ARDIS or MOBITEX in the United States. More than likely,
however, CDPD will prove to be merely an interim solution until digital cellular radio is
available.
Air Interfaces

Mobile
End System

External Network
Interfaces

CDPD Service
Provider X Network

External
Networks

Inter-Service
Provider Interfaces
CDPD Service
Provider Y Network

Figure 2 - CDPD Network Interfaces

Packet Radio Systems
A packet radio system is a type of specialized mobile radio system which functions as a
wireless wide-area data network for the mobile professional. Two packet radio systems are
gaining popularity and acceptance for reliable data transmission. These are ARDIS,
provided by Motorola/IBM, and MOBITEX, provided by RAM Mobile Data/Bell South
Industries in the U.S. and by Ericsson in Europe.
11

Mobile and Wireless Technologies Primer

Like cellular radio/telephone systems, these packet radio systems use analog radio
technology. Unlike cellular systems, however, these networks offer connectionless support.
Subscribers do not maintain a dedicated, point-to-point connection to the destination
station, but share radio channel access by time, transceiving data between source and
destinations.
Subscribers using the packet radio system are billed a monthly fee, plus a usage fee based on
the amount of information ( packets) transmitted through the system.
From a subscriber point of view, ARDIS and MOBITEX have more similarities than
disparities. Some commonalities are:
• Analog radio channel technology, with narrowband modulation
• Low data rate throughput (<2400 bit/s)
•Smaller infrastructure than public cellular systems; therefore,

limited service
availability
• Special radio channel protocol (applications must know how to "talk" to each
system)
• Packet format data transmission
• Usage fee based on packets transmitted
• Requirement for special radios dedicated to each service
We will now take a closer look at these systems and describe some of their specifics.
ARDIS
Using 1,100 radio base stations, ARDIS operates as many as 600 two-way radio data
channels in about 8,000 cities in the U.S. The system widely utilizes a raw data rate of 4800
bits/s, resulting in a theoretical subscriber data throughput of 2080 bit/s. The ARDIS system
operates in 30 MHz of spectrum: between 806-821 MHz for receiving and 851-866 MHz
for transmitting, with 25 KHz channel spacing. ARDIS is currently working on a system
upgrade to 19,200 bit/s without overhead. Due to overhead burdens associated with the
radio channel protocol and error correction, subscriber data throughputs will theoretically be
something less than 8000 bit/s. In actuality, subscriber data rates will be much lower, due to
packet collisions which occur at the base stations. These collisions necessitate data retries,
which cause added system overhead.

Mobile and Wireless Technologies Primer

ARDIS base
station
ARDIS base
station
ARDIS base
station
Building in Service Area

Figure 3 - ARDIS Base Stations are positioned around metropolitan areas
Customer host systems or remote data sites connect to the ARDIS system via leased lines or
X.25 packet services. The ARDIS radio base stations are generally positioned in a triangular
fashion, with multiple cell coverage areas, as shown in Figure 3. Positioned in this way
around business centers, they provide reasonable in-building coverage.
Originally deployed in support of IBM field personnel, the ARDIS system has been in use
for many years. It was not commercialized until recently. ARDIS is now being deployed in
Europe, beginning in Germany.
MOBITEX
MOBITEX is available in many cities throughout the United States, Canada, and
Scandinavia. It operates in 10 channels at 934 MHz, using a Time Division Multiple Access
(TDMA) technique. MOBITEX base stations have been widely upgraded to support 19.2
kbit/s. However, subscriber radios largely continue to support a nominal raw data rate of
8000 bit/s. Similar to ARDIS, the customer data rate is much less than the raw data rate due
to data transmission overhead. Subscriber data throughputs of 800 baud are common.
MOBITEX base station radios are generally positioned at locations with above-average
terrain characteristics throughout the service area. Triangularity of the base stations around
a coverage area, seen in the ARDIS system, is generally absent in the MOBITEX
installations, which indicates that the in-building coverage may not be as good as with the
ARDIS system.
Both the ARDIS and MOBITEX systems use portable radio transceivers, which are
generally expensive and heavy. The transceivers connect to a portable data host, such as a
laptop computer, to access the systems. This approach is an artifact of long deployment and
low volumes and is not inherent in the technology. Several vendors are working to integrate
13

Mobile and Wireless Technologies Primer

radio technologies into smaller form factors, such as the industry standard for credit card
sized devices, PCMCIA.
As mentioned earlier, the ARDIS and MOBITEX systems each have their own specific
radio channel protocol. This requires that subscriber client and host stations -- and more
specifically, the applications -- know how to talk to client radios. Two solutions to this
problem are currently available.
The first solution uses programs written by vendors who have developed specific
applications to support mobile professionals on each of these networks. This limits the
flexibility of the mobile professional by constraining the use of these systems to that which is
specific, both to the network and to the application in use by the mobile professional.
Digital Equipment’s RoamAbout Transporter product provides a more flexible, costeffective solution to this network compatibility issue. Transporter facilitates the selection of
the best carrier for the mobile professional, and quickly enables the change of a carrier as
needed. This capability provides both investment protection and flexibility. Transporter also
supports TCP/IP-based applications written to industry-standard NetBIOS and WINsockets
APIs. This saves development resources, since applications do not have to be rewritten for
mobile use or modified for each service carrier.
tm

PC Applications RoamAbout Transport
NetBIOS

WINsockets

Sockets

TCP/IP Services
For
OpenVMS

TCP/IP
Cellular ARDIS
Mobitex Mobitex
Dial-in
Dial-in Cellular
Ardis

Applications

RoamAbout
Transport
for MS-DOS/
MS-Windows

Enet

RoamAbout
Transport
For Open VMS

TCP/IP
Mobitex

ARDIS

Ethernet

VAX P.S.I.

Phone
Network

ARDIS
Network
Mobitex
Network

X.25

DS700 SLIP

Mobile and Wireless Technologies Primer

Digital Cellular Technology = The Future
During the next decade the current AMPS cellular infrastructure, an analog technology, will
be replaced by an all-digital system. Digital cellular technology provides many advantages
for both the voice- and data-based mobile professional.

Some of these advantages are:
•
•
•
•
•

Increased security against casual eavesdropping
Increased system capacity
Better recovery of the radio signal under all conditions, including high noise
environments
Superior cell hand-off and roaming of mobile end-stations
Significant increase in reliable data rates for mobile data professionals

Table 2 - Technical Summary of Digital Cellular Techniques for WWANs
|-----Europe -----|---------------------United States ---------------------|

Frequency Band
Base to Mobile
Mobile to Base
Allocated BW
Access Scheme
Duplex Method
Channel BW
# of voice
(full/half rate)
Channel/Freq.Ch.
Total Traffic Ch.
Channel
Bit
Rate
(kbps)
Voice Coding
(kbps)

GSM

TDMA

E-TDMA

CDMA

890-960 Mhz
935-960
890-915
50
TDMA
FDD
200 KHz
8/16

824-894 Mhz
869-894
824-849
50
TDMA
FDD
30 KHz
3/6

824-894 Mhz
869-894
824-849
50
TDMA
FDD
30 KHz
11/15

824-894 Mhz
869-894
824-849
50
CDMA
TDD
1250 KHz
?/?

1000 / 2000
270.833

2496 / 4992
48.6

9152 / 12480
48.6

?/?
vendor dependent

22.8

8/4.5

4.5

Two digital cellular standards have been adopted in the U.S.: IS54-TDMA and IS95CDMA. The IS54-TDMA standard employs Time Division Multiple Access techniques,
which provide for packetized voice and data to be transmitted on a time-shared channel.
IS95-CDMA uses Code Division Multiple Access techniques. These two digital
technologies are not compatible, and will not interoperate with each other.
One difference between the two technologies is that in using CDMA all subscribers share an
entire channel simultaneously. Each subscriber’s data is transmitted using orthogonal data
15

Mobile and Wireless Technologies Primer

patterns, providing unique discrimination between each subscriber’s data in the radio
channel. Subscriber radios "see" other data patterns in the shared radio channel as simply
another form of ambient noise, and therefore cannot process that data.
TDMA, on the other hand, allocates time-slots for each subscriber to use the channel. The
radio channel carrier is subdivided into time-slots, which are then accessed sequentially by a
number of different subscribers.
Because two incompatible U.S. digital standards have been developed, the U.S. consumer
may soon face some of the frustrations that existed before the adoption of the GSM singular
standard throughout Europe. These frustrations include incompatible systems across service
areas, as well as the transition phase noted above in which customers will be required to use
dual-mode phones. In Europe a decision has been made that no dual-mode phones will be
offered.
The move to all-digital in the United States will be gradual. This means customers will be
using an analog/digital hybrid infrastructure, known as D-AMPS. Subscriber capacities of
three-to-six times are envisioned, with more effectiveness for the data customer than with
the AMPS system.
Wireless Local Area Networks
Office professionals should not be tied to their desks in order to have robust computing
power at their fingertips. Wireless local area networks (WLANs) will probably never replace
the classical wired LAN; however, a WLAN can be a very effective means of extending the
flexibility of a wired LAN. A WLAN can offer reasonable performance and reliability while
providing mobility in the work environment.
RoamAbout
PCMCIA Network Adapter
x
RoamAbout
Access Point

Corporate Backbone
Coverage Area

File Server or
Workstation with
WaveLAN NIC
Laptops with
RoamAbout
PCMCIA
Network Adapters

Figure 5 - Example of a Wireless LAN network
Additionally, WLANs can provide benefits for temporary work environments or hard-towire buildings. A WLAN can be operational in a matter of minutes at lower costs. Using
16

Mobile and Wireless Technologies Primer

WLAN technology, reliable, collaborative workgroups can be set up and made operational
with little effort or planning time.

RoamAbout
PCMCIA
Network
Adapter

Antenna 1

Antenna 2

RoamAbout
Access Point

Coverage Area
RemoteConnect
Solution
(antenna,
WaveLAN NIC,
and software)
File Server or
Workstation with
WaveLAN NIC

RoamAbout
Access Point

File Server or
Workstation
Laptops with
RoamAbout
PCMCIA
Network Adapters

x
x

RoamAbout
PCMCIA
Network
Adapter

Wired Infrastructures

Figure 6 - Wireless LAN configuration for connecting two Buildings
Many people are using WLANs for building-to-building or campus-bridge configurations.
The building-to-building configuration has been used both as the primary link between two
facilities, and as a back-up to a primary wired connection. In the figure above, a wired LAN
infrastructure in one building is connected to a second building by directional antennas. The
second building uses both wired and wireless LAN systems, demonstrating the true
versatility of WLAN technology.
Technology Types
Two types of technologies are available for Wireless Local Area Networks: Radio
Frequency (RF) and infrared. RF offers high speed, along with reasonably secure
communication data links. These links can penetrate, to some degree, walls, floors and
ceilings.
Infrared technologies can provide high-speed wireless services, although the operational
distance is limited to a single room. This can be a security advantage over radio frequency
technology.
17

Mobile and Wireless Technologies Primer

Security
Most RF WLANs employ a radio technology know as “spread spectrum”. Spread spectrum
is a method of communications technology which was developed by the military to guard
against casual eavesdropping and jamming by non-allied forces. RF WLANs that employ
spread spectrum modulation techniques have some inherent security, but can still be
penetrated by a intruder using a compatible device. Unlike infrared intrusion, the RF
intruder need not be physically inside the service area of the RF WLAN. To guard against
illegal access to the network, high-quality RF WLAN manufacturers provide additional
scrambling of data using techniques like DES.
Other WLAN technologies are available which use narrowband modulation techniques, or
low-power, wideband FM technology. These methods tend to be low-cost, and provide low
data speeds. They are deficient because other devices -- walkie-talkies, cellular phones, and
even nearby computers, cabling and various industrial machines -- can easily interfere with
them.. These systems also generally lack any sophisticated security capabilities.

LKG-8880-94f

Figure 7- Digital’s RoamAbout Wireless Network Adapter Solution.
Digital Equipment Corporation’s RoamAbout wireless network adapter is
shown in the figure above. The RoamAbout network adapter is a type 2,
PCMCIA-compatible card set which operates at 900 Mhz. It uses direct
sequence spread spectrum technology, and provides a raw data rate of 2 Mb/s.

Mobile and Wireless Technologies Primer

Licensing and Safety
Manufacturers of RF WLANs must meet stringent emission levels, specified in the U.S. by
the FCC, Part 15.247. European technical specifications are set forth in ETSI 300-328.
Specific country regulations must be met where the product is to be marketed. Some
countries, such as the U.K., require users to mail away for an operating permit in order to
deploy a WLAN. Except for a system marketed by Motorola, most RF WLANs today do
not require a user license.
RF WLAN devices emit very low levels electromagnetic energy, and generally are exempt
from local and federal health guidelines. However, RF WLAN manufacturers should ensure
that the devices meet the specifications set forth in ANSI C95.1-1992. It should be noted
that there is very little comparison between the levels of energy and methods of use of RF
WLANs and portable cellular telephones.
Infrared systems must meet certification specifications for non-intentional emissions, as must
all computing devices. They also do not require operating licenses.
Radio Wave Technology
The Wireless Evolution will see the development of radio technology aimed at high-speed
data, voice, and video communications. Robust connectivity, high system availability,
reliability, and system security are the requirements for a WLAN. Customers are demanding
response times which rival those they are accustomed to receiving from wire technologies.
However, higher speeds of raw data demand wider bandwidths in the radio spectrum. In
theory, the radio spectrum is a limitless resource. However, in reality, the availability and
use of the spectrum is not limitless. This is due to the following factors:
•

The availability of technology at reasonable costs in radio devices that
operate beyond 5 GHz. Advancements such as Gallium Arsnide
technology will make this a less severe obstacle over time.
• The difficulty of propagating through walls, around corners, over tall radiopath obstructions, and over office partitions. Improvements in digital signal processing
will continue to improve these limitations.
• Clear spectrum allocations. The regulatory environments continue to evolve
rapidly worldwide.
The fastest WLANs marketed today provide mobility, offer raw data rates of 2mb/s, and can
support a user moving at speeds of up to 4 mph. People exploring the possibility of using
this technology are sometimes concerned about this data rate, due to their belief that they
have been using 10mb/s for their wired LAN connections. (In reality, customers in wired
LAN topologies often have access to much less bandwidth than 10mb/s.) WLAN topologies
can be designed to provide Ethernet-like response time, even though the shared bandwidth
is less. One way to do this is to provide access points through a coverage area, operating
with a different radio channel and NET-ID.
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Mobile and Wireless Technologies Primer

Faster data rates will become increasingly necessary as applications for real-time voice and
video increase and the number of deployed WLAN data clients increases. For this reason it
is paramount that clear spectrum be made available by the appropriate legislative bodies
worldwide.
Workstation
with
WaveLAN NIC

Laptops
with
RoamAbout
PCMCIA
Network
Adapters

RoamAbout
Access Point

LKG-8871-94C-94F

Figure 8 - Digital’s RoamAbout Access Point provides wireless connectivity to
portable laptops as well as desktop systems.
Radio Technology Types
Two types of radio technologies are approved for WLAN applications in the 915 Mhz, 2.4
Ghz, and 5.7 Mhz ISM bands in the U.S., and in the 2.4 Ghz band in Europe and Japan.
They are both digital techniques and are known as:
•
•

Direct Sequence Spread Spectrum (DSSS)
Frequency Hopped Spread Spectrum (FHSS)

Direct Sequence Spread Spectrum Radio Technology
Direct Sequence Spread Spectrum technology combines the data with a higher rate bit
sequence, known as a chip sequence. This higher rate bit sequence, divided by the user data
rate, determines a figure of merit for DSSS systems known as processing gain, or spreading
ratio. (The minimum specified processing gain in the United States and Japan is ten.) Since
the spreading process is applied to each data bit, this method is known as direct sequence.
The combination of required processing gain and allocated bandwidth are two major factors
which limit the possible raw data rates which can be used in an allocated frequency band.
Because the total transmitted power is spread across a wide frequency band as a result of
this spreading process, power spectral density is much lower than with narrowband
transmitters such as cellular telephones. One of the effects of a lower spectral density is that
since other radios in near proximity will be subjected to less interference, some level of coexistence results. Futhermore, “normal” narrowband radios, which could be used to
20

Mobile and Wireless Technologies Primer

eavesdrop, cannot detect signals transmitted using this process. The reverse is also true.
Narrowband radios will not interfere with a WLAN unless the WLAN receiver becomes
overloaded.

Figure 9 - Some examples of detected signals using spread spectrum radio techniques.
The receiver uses the same chip sequence, and is synchronized to the transmitter in order to
despread the desired signal. As shown in the figure above, other DSSS signals with different
chip sequences, and other narrowband signals, become suppressed relative to the desired
signal during the detection process by the digital signal processing circuitry.
Frequency Hopped Spread Spectrum
Instead of chopping each data bit into small pieces of the frequency domain, frequency
hopped technology spreads transmitted energy in the time domain. One or more of the raw
data bits are transmitted on a signal narrowband frequency. The transmitter then "jumps" to
another frequency, where another few bits are transmitted. This is similar to using the
frequency scan feature of an automobile radio, where the radio automatically jumps from
one frequency to another and pauses only for a short time when a signal is detected. FHSS
systems hop in a random, but known, sequence, and pause for a very short time to detect
and process data.
The group of frequencies among which the transmitter jumps are known as hop sequences.
A hop sequence is also referred to as a channel. The receiver must have knowledge of the
hop sequence, and, as in DSSS, must be able to synchronize. A narrowband interferer such
as a police radio transmitter may cause data transmission on a hop frequency to be
corrupted. The data will then be re-sent on a different channel, or the corrupted frequency
will be omitted from the hopping sequence.

Mobile and Wireless Technologies Primer

5khz

Power

Frequency

Figure 10a.

5khz

Frequency

Figure 10b.

The figure above shows a relative comparison between the transmitted power of a
narrowband system, such as a walkie-talkie, and a WLAN.
FHSS and DSSS Compared
The performance of DSSS systems is generally acknowledged to be better in terms of
multipath interference. In-building WLANs must overcome deep channel fades that can
occur between the source and destinations on the wireless data path. For example, a
transmitted signal can have multiple paths between the source and destination. This results
in the same signal arriving at the destination with different signal levels, and with slight shifts
in time. Sometimes these signals add together; sometime they subtract.
FHSS systems tend to be less costly to implement, and are more easily designed to run on
lower power consumption. From a performance viewpoint, FHSS is generally considered to
be less tolerant of multipath and other interference sources. If data is corrupted on one
frequency hop, the system must re-send the data on another frequency. In DSSS systems,
interference within a small portion of the channel (resulting in only partial data corruption)
can often be recovered in the DSP circuitry.
The price paid for the sophistication of DSSS technology comes in the form of increased
power consumption. DSSS systems tend to consume two-to-three times the power of an
equivalent FHSS system. DSSS systems, while generally able to provide good resistance to
interference such as multipath and lower level interference sources, tend to be weaker than
FHSS systems when the interferer has significant power bandwidth. This condition can
occur when an interferer is in near proximity to the WLAN. Should the interference occupy
a significant portion of the band on which a WLAN is operating and be very strong (e.g.,
nearby), neither FHSS nor DSSS systems will be able to operate.
Standards Activities
Most devices for high-speed, local-area, wireless data communication on the market today,
and those in development for the foreseeable future, are designed to operate in the
Industrial, Scientific and Medical (ISM) Bands. Devices designed to operate in these bands
in North America require no user licenses from regulatory officials. Manufacturers of these
22

Mobile and Wireless Technologies Primer

devices are granted type certification from the regulatory officials by supplying test data to
the appropriate regulatory agency.
In the United States the technical regulations for wireless LAN technology which uses the
ISM bands as noted earlier is specified under Part 15, Section !5.247. In Europe the
technical specifications are set forth in ETS-300-328. The ETSI standard seems to be
favored in many countries in the Pacific Rim.
United States
In the United States the IEEE 802.11 committee is hard at work generating a standard
which will provide interoperable capability between WLANs sold by different vendors.
During the last six months the group has adopted a MAC foundation, and PHY modulations
specification for both DSSS and FHSS systems. The DSSS system provides for 2 Mb/s
capability, while the FHSS PHY is standardized for 1Mb/s operation and 2 Mb/s as an
option. Some formidable challenges lie ahead in this standards group, such as providing for
a standardized MAC/PHY interface. The document is due for balloting in the November
1994 timeframe.
In the fall of 1993, the FCC, acting on a directive from Congress, allocated 60 Mhz of
spectrum in the 2 GHz area for the use and deployment of what is known as the Personal
Communications Services (PCS). These systems tend to be targeted as new versions of the
cordless telephone. They use digital technology, and are planned to provide data capability.
There are currently several licensed incumbents of this band, who must be moved before this
clean spectrum can be used for PCS. Potential manufacturers through a CBEMA task force
known as UTAM are coordinating the fees and administration of the spectrum clearing. It
may be more than two years before this clean spectrum becomes available for use by PCS.
Europe
As mentioned earlier, Europe already has a technical standard in place for WLANs operating
in the 2.4 Ghz band. The most important work being done in Europe, which in some
respects is a parallel to the U.S. effort, is being done in Hiperlan. This group has just
released their architectural specification. While the U.S. standard is focused on 1-2Mb/s data
rates, the Hiperlan work is targeting speeds in excess of 10 Mb/s.

Mobile and Wireless Technologies Primer

Summary
Mobile and wireless technology has certainly become a reality. Wireless LAN technology is
solving real customer needs for extending wired LANs; enabling untethered computing for
office, warehouse, and production environments; and providing inexpensive, reliable
solutions for hard-to-wire buildings, and inter-building data links.
Wide-area wireless systems such as those using the existing cellular infrastructure can
achieve data rates to 9600 bit/s. Bursty in nature, they provide a mobile professional with
perhaps an average of 1200 bit/s performance. Cellular systems technology is almost
impossible to use when in motion and can be very expensive due to point-to-point
connection configurations which monopolize air time. Packet radio systems and other
SMR-type systems offer an alternative to cellular for the mobile professional; however, the
smaller infrastructure may leave a person without coverage. While the data throughputs tend
to be below 1200 bit/s at this time, professionals are charged on a per-packet basis.
System architects must view the wireless technologies of radio, air data protocols, and
networking as integrated entities. All of these technologies must merge to create an
effective, integrated wireless communication system. Many of the challenges are best
overcome by cognizant design within the physical media, data link, and transport layers of
the IEEE Open Standards Interface (OSI) model. This will enable the higher level network
protocol and application layers to be used with little or no modification in the new wireless
environment.
Advances in technology and evolving customer needs will help drive these technologies to
provide higher speeds, and lower cost of deployment and utilization in the not too distant
future.

Mobile and Wireless Technologies Primer

References
1. Wireless Nets Not Yet Unplug And Play - Datamation - September 1, 1993.
2. Wireless and Mobile Data Networks: From In-Building to Wide-Area Solutions
- The Yankee Group - 1993.
3. Digital Cellular Radio - George Calhoun - Artech House - 1988.
4. Wireless Access and the Local Telephone Network - George Calhoun
- Artech House - 1992.
5. Mobile Communication Technologies, Applications and Regulations - Ira Brodsky
- Datacomm Research Company -1993.
6 - Cellular Digital Packet Data System Specification - Release 1.0, July 19,1993.
Biography
Bob Egan is Technical Director for Mobile and Wireless Technologies at Digital Equipment
Corporation in Littleton, Massachusetts. Bob is responsible for RF technology, and is
technical consultant for business strategy and development within Digital’s wireless
program. Prior to joining the Wireless group, Bob was responsible for EMC compliance and
design for Digital’s Network Communications Division and was technical manager for
Digital’s EMC facilities. Bob is a voting member of the IEEE 802.11 Wireless Standard
group, as well as ANSI C63.4, where he currently serves as a member of the interpretation
subcommittee for the FCC. He is a certified NARTE Engineer and is an active amateur
radio operator. Bob obtained a degree in Electrical Engineering from Wentworth Institute in
1974 and a degree in Business Management from Bryant College in 1987. Bob Egan can be
reached at Digital at 508-486-5746, FAX 508-486-5554 or via Internet at
Bob.Egan@lkg.mts.dec.com.
Trademarks
Digital believes the information in this publication is accurate as of its publication date; such
information is subject to change without notice. Digital is not responsible for any
inadvertent errors.
Digital will conduct its business in a manner that conserves the environment.
The following are trademarks of Digital Equipment Corporation: DEC, Digital, the
DIGITAL logo, and RoamAbout Transporter.
Third-party trademarks: ARDIS is a trademark of Communication Software Applications of
Ardis Company. MOBITEX is a registered trademark of Televerket. NetBIOS is a
trademark of Micro Computer Systems, Inc.

EC-N3596-42
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Mobile and Wireless Technologies Primer