|
Beijing Telecommunications Planning & Dimensioning Institute
1 The Background of WLAN
Also termed as the "last mile" or "last 100-meter"
fixed-wireless access solution, the so-called Wireless LAN (WLAN)
is a key technology in realizing the mobile computer network owing
to its pivotal role in performing physical layer and link layer
functions and providing physical interfaces for the network where
necessary. Generally speaking, WLAN is born out of the marriage
between the computer network and the wireless communications technology.
From the technical perspective, WLAN has made use of one muscle
of the wireless multiple access technology to support communications
between computers, thus enabling the emergence of personalized multimedia
mobile applications. To put it simple, WLAN has made it possible
to get rid of traditional cables while bringing up features of Ethernet
or Token network.
As the information technology is speeding up its development
pace, people are requiring the communications network to do more.
Communications with anybody anytime anywhere in whatever forms -
data, voice or video - is being envisaged, and automatic roaming
of hosts in the network also seems not so distant. According to
the 10th "Statistics Report of Internet Development in China"
of CNNIC, as of June 30, 2002, China boasted 45.8 million Internet
users. Number is astounding, but more eye-catching may be the economic
benefit that is hidden behind the Internet boom. An industry expert
claimed that 2002 would be the year for Wireless LAN, given its
unique advantages of secrecy, anti-interference, ease of rollout
and maintenance as well as its particular assistance to achieving
the mobile office.
In 2001, the Ministry of Information Industry allocated the 5.8
GHz spectrum to boost the growth of new wireless applications like
WLAN - a move that served to show green lights to tremendous wireless
broad plans brewed on the minds of telcos. In a sense, the spectrum
was regarded as a de facto and timely "birth permit".
Last year, China Netcom (CNC) embarked on offering WLAN access
services in such hot business spots as Beijing, Shanghai, Shenzhen
and Guangzhou in the name of "Infinite Trip Companion",
putting into place more than 40 service branches. What a user needs
is a built-in network card in the notebook and a CNC billing card
that bears the user name and the password for him to logon. The
card also supports roaming service, meaning you can surf the Web
nationwide wirelessly with only one certification required.
But more competitive should be the debut of the bundled GPRS+WLAN
wireless data service of China Mobile. Though regarded as a mainstream
technology in 3G evolution, GPRS appears too slow to put notebooks
on line in a wireless sense. That is why China Mobile, at an appropriate
time, chose to introduce WLAN as a supplement. With a custom built-in
chip, GPRS handsets can detect indoors WLAN signals and automatically
switch to WLAN for 11M high-speed Internet access. In outdoor areas
beyond the reach of WLAN, the user may leverage the GPRS network
for Internet access. By making full use of the advantages of both
technologies, such a double-edged approach is conducive to the formation
of a "win-win" network of strong unique features.
Whether it is CNC's "Infinite Trip Companion" solution,
or China Mobile's GPRS+WLAN package, both is heralding the advent
of a whole new era of Wireless LAN (WLAN).
2 The Standard of WLAN
Wireless LAN is a computer network using wireless transmission
medium, with IEEE802.11 as its standard. As early as 1990, the IEEE802
Standardization Committee established the IEEE802.11 Wireless LAN
(WLAN) Standard Working Group, which undertook to develop wireless
communications equipment and supportive network development standards
that is able to provide data transmission rates of up to 1Mbit/
s and 2Mbits/s and operate at 2.4GHzs open spectrum. The standard
was published in June 1997. As one of the first generation of WLAN
standards, the standard defines the specification for both the Physical
Layer and the Media Access Layer (MAC).
In the Physical Layer part of the standard, signal characteristics
and modulation methods in data transmission are defined. Two RF
transmission methods and one infrared transmission method are also
defined. Spectrum spreading technology is adopted for RF transmission
to satisfy the secure operating specification allowed in most countries.
Spectrum spreading modulation is divided into Direct Sequence Spectrum
Spreading (DSSS) and Frequency Hopping Spectrum Spreading (FHSS),
with 2.4000゛2.4835GHz as the operating spectrum. DSSS adopts BPSK
and DQPSK in the modulation process. Spectrum spreading sequence
of high bit rate is employed at the transmission side to scramble
the signal. At the reception side, the same spreading sequence is
also applied to de-scramble the signal to the original form. Supporting
1Mb/s and 2Mb/s data rates, the modulation uses 11-bit Barker sequence,
with a processing gain of 10.4dB. By contrast, FHSS is a completely
different spectrum spreading technology. It is a 2 to 4-level GFSK
modulation, in which the carrier is subject to a pseudorandom code,
meaning that the frequency changes in an irregular way within its
operating bandwidth. The frequency at the receiving side also changes
in keeping with the rule of change at the transmission side. The
hopping rate is an immediate reflection of the system performance.
The higher the hopping rate, the better the anti-interference feature.
In the military system, the hopping system can hit tens of thousands
of hops per second. For cost considerations, commercial systems
by and large exhibit a relatively slow hopping rates, normally below
50 hops per second. Given its simplicity in construction, slow hopping
systems are widely applied in low-rate WLANs. FHSS supports 1Mb/s
data rate and incorporates 22 sets of hopping patterns, including
79 channels. The infrared transmission system operates at a wavelength
of 850 to 950 nm, with a peak power of 2W. For modulation, it adopts
4 or 16-level pulse-positioning technology, supporting data rates
of 1Mb/s and 2Mb/s.
The Media Access Control (MAC) layer uses Carrier Sense Multiple
Access/Collision Avoidance (CSMA/CA) protocol. Since it is usually
hard to detect conflicts in the RF transmission network, the protocol
replaces the Collision Detection mechanism used in the 802.3 protocol
with the Collision Avoidance mechanism, and adopts the Channel Congestion
Assessment (CCA) algorithm to decide if the channel is idle or not.
In addition, the antenna port energy is tested and the reception
signal strength (RSSI) tested to wrap up the whole process. CSMA/CA
uses RTS, CTS and ACK frames to slash the occurrence of collisions.
In terms of data encryption, the algorithm is the same as the WEP
as adopted in ordinary LANs, with 64-bit keys and the RC4 encryption
algorithm being applied.
With the IEEE 802.11b standard, WLAN can contribute a bandwidth
of up to 11 Mbps, 5 times faster than the 802.11 standard approved
just two years ago, thus expanding the application scope. IEEE 802.11b
employs the open 2.4GHz spectrum, which may be put into use without
any form of application. Theoretically, the IEEE 802.11b Wireless
LAN resembles the common IEEE 802.3 Ethernet. Both adopt the Carrier
Sense method in controlling the delivery of information throughout
the network. What is different is that Ethernet adopts the CSMA/CD
(Carrier Sense/ Collision Detection) technology. All terminals in
the network can sense if or not there is any information being delivered.
When the network is found idle, all terminals send their own messages
as if in a "vie-for-answer" game. At one time only one
terminal is allowed the floor, and all the others have to wait until
next chance takes place. If there are more than two terminals sending
messages simultaneously, then collision occurs in the network. After
the collision, all the collision messages will be lost, and terminals
will continue fight for the floor. By contrast, the 802.11b WLAN
introduces the collision avoidance technology to improve the network
efficiency by a great margin.
802.11b operates in two modes: point-to-point mode and basic
mode. The former refers to the way of communications between wireless
network cards. As long as a wireless network card is inserted, the
PC will then be able to connect another PC installed with the same
wireless network card. For small wireless networks, it is a handy
approach to link PCs - at most 256. The latter is applied when the
wireless network is expanded in size or when wireless and wireline
networks coexist. It is a most commonly used method in the 802.11b
case. Here, a wireless card-enabled PC needs to go through an Access
Point (AP) to connect another PC. AP plays a monitoring role in
spectrum management and roaming. One AP may connect as many as 1024
PCs (with wireless cards). When more wireless nodes are introduced
in the network, the access speed will grow slower as the network
size expands and nodes increase. To that end, the introduction of
APs can help achieve an effective control of bandwidth and spectrum.
When the wireless network needs to interconnect with the wireline
network, or the wireless node needs to connect and access resources
and servers of the fixed network, AP can serve as a bridge between
the wireless and the wireline networks.
With the wireless IEEE 802.11 standard gaining more and more
footing, IC manufacturers are beginning to look for more rapid protocols
and configurations. Among all the candidates, the mixed 802.11g
standard appears to be the latest favorite suited to WLAN. While
accommodating the traditional 802.11b standard to provide a data
transmission rate of 11Mbps under 2.4 GHz, it also complies with
the 802.11a standard to provide 54Mbps data rate under 5GHz. Followers
claim that once 802.11g is approved, it will lend a fresh boost
to the already strong growth of 802.11 WLAN.
3 Characteristics of WLAN
Below is a description of WLAN in terms of transmission method,
network topology and network interface:
1 Transmission Method
The transmission method involves the transmission medium, the
spectrum and the modulation mode applied in WLAN. Currently, two
major mediums are adopted, i.e., microwave and infrared rays. WLAN
with microwave as the transmission medium may be divided into two
modes: the spectrum-spread mode and the narrowband mode. Most of
the WLAN products have adopted the spectrum-spread mode, which was
initially applied in military broadband wireless applications. The
spreading technology guarantees the integrity and reliability of
wireless data during transmission and also prevents data on different
spectrums from interfering with each other.
In the spreading mode, the spectrum of baseband signals is spread
several or even dozens of times before being transmitted at the
RF side. At the cost of bandwidth, though, the system nevertheless
becomes more interference-resistant and secure. As the power per
bandwidth is reduced, the interference out of it is dwindled. WLANs
adopting the spreading technology normally select the so-called
ISM spectrum. Here ISM represents the initials of Industrial, Scientific
and Medical, since the energy radiation from lots of industrial,
scientific and medical facilities fall in this spectrum. Radio administrations
in Europe, the US and Japan set their own ISM spectrums respectively,
e.g., in the States, the ISM spectrum comprises three spectral parts:
902MHZ to 928MHZ, 2.4GHz to 2.484GHz and 5.725GHz to 5.850GHz. If
the emission power and the outband radiation satisfy the requirements
of the Federal Communications Commission (FCC), then no application
needs to be filed before using the ISM spectrum.
In the narrowband modulation, the spectrum of baseband signals
requires no spreading before transmission. Compared to the spectrum-spread
mode, narrowband modulation occupies fewer frequencies and thus
contributes higher spectrum efficiency. However, WLANs with narrowband
modulation normally have to choose dedicated spectrums, for which
approval from the state radio administration is a must. Certainly,
the ISM spectrum is also applicable without the need to file for
approval from the administration. But one pending problems looms:
if interferences occur from adjacent or the same frequency, the
quality and reliability of communications will be badly affected.
In recent years, infrared-based transmission technology has seen
marked growth, as evidenced by the massive adoption in nearly all
remote controls currently deployed in electrical appliances. As
for WLAN, infrared is most advantageous for being kept freed from
radio signals, and the usage of infrared rays is subject to no restrictions
from the State Radio Regulatory Commission. However, infrared is
inferior in passing non-transparent objects, and poses fatal limitations
on the transmission distance of WLAN.
2. Network Topology
The topology of WLAN is classified into two kinds: PEER-TO-PEER
and HUB-BASED topologies.
In the PEER-TO-PEER scenario, any wireless station in the network
should be able to communicate directly with another point. Networks
adopting such a topology generally use public broadcasting channels.
All stations can compete for the public channel. With regard to
the Medium Access Control (MAC) protocol, protocols like Carrier
Sense Multiple Access (CSMA) are most often used. The topology is
strong in anti-destruction, ease of construction and cost effectiveness.
However, when there are too many users (stations) in the wireless
network, channel competition may appear a bottleneck impeding the
network performance. So the topology is heavily dependent on network
layout and environmental factors, and is suited only to workgroups
of a small population.
In the HUB-BASED scenario, one wireless station works as the
hub to control the access of all stations to the network. When the
traffic rises, the throughput and delay performances of the network
won't deteriorate so drastically. Since each station may communicate
with other ones as long as it is within the coverage of the hub,
the layout of the hub is thus subject to very few environmental
factors. Moreover, the hub provides a logical access point for accessing
the wireline backbone network. The weakness of HUB-BASED topology
is poor anti-destruction capability. One failure from the HUB may
lead to a breakdown of the entire network. What's more, the introduction
of hub also adds to the network cost.
In practical applications, WLAN is often coupled with the wireline
backbone. In this case, the hub will act as the adaptor between
WLAN and the wireline network.
3. Network Interface
The network interface is where stations in WLAN access the network
system. Generally, the interface is located at the Physical Layer
or Data Link Layer of the OSI reference model. By selecting Physical
Layer interfaces, wireless channels will be used in place of the
common wireline transmission, while all structural layers above
the Physical Layer keep unchanged. The most evident advantage of
such an interface is that no modification is needed for the network
operating system and driver programs on top layers. It is often
applied in wireline hubs and wireless transponders to achieve interconnection
between fixed LANs or expand the coverage of LANs.
Another interfacing mode is to access the network through the
Data Link Layer. Here the protocol to be applied is not the wireline
MAC, but a MAC that is more geared to the wireless transmission
environment. In a practical scenario, the MAC layer and layers beneath
it are transparent to top layers. Driver programs are configured
to complete the interface with top layers, thus ensuring a sound
operation of the existing wireline OS or applications on the wireless
LAN. Currently, most of WLAN vendors have adopted Data Link Layer
interfaces.
4 The Network Structure and Application of WLAN
WLAN comprises wireless network cards, the wireless access points,
PCs and related hardware. The key difference from LAN lies in the
transmission medium and the MAC protocol. It may be interconnected
with wireline networks or deployed independently to form Ad-hoc
Network, Infrastructure Network or LAN-interconnection.
Based on the network organization characteristics of WLAN, its
application is divided into two types:
1. Independent WLAN
In this case, communications in the whole network are carried
out wirelessly. Access Point (AP) may be applied or passed over.
Without AP, users will be directly interconnected by wireless means.
The shortage of this type of WLAN is that users fall close to each
other; when there are too many users, the performance will get worse.
2. Non-independent WLAN
In most cases, wireless communications is a supplement to and
extension of wireline means. In that sense we call it non-independent
WLAN. Here multiple APs are connected to the wireline network through
cables to enable users to access all parts of the network.
Based on the application environment of WLAN, its application
can be divided into two types: indoor and outdoor applications.
1. Indoor Application: As a supplement to LAN, WLAN coexists
with LAN. Since the cost of WLAN is higher, in the indoor environment,
it can give play to its wireless strength in the following application
scenarios: large offices, workshops, supermarkets, intelligent warehouses,
ad-hoc offices, meeting rooms, securities markets, etc.
2. Outdoor Application: In the outdoor environment where wiring
is hard to implement, WLAN can give full play to its high data rate
and flexible organization benefits, particularly in places beyond
the reach of public networks where WLAN may serve as the Regional
Network (with coverage of dozens of kms). Application examples are
given as follows: communications among urban building blocks; campus
networks; factory compound automatic control and management networks;
banks; urban financial and securities networks; urban communications
information networks; mines, water conservancy, oil-field (regional)
networks; port, harbor, river, lake and dam networks; outdoor exploratory
survey and experiment (mobile) networks; military and public security
(mobile) networks and so on.
In the actual network organization work, WLAN may be connected
via different structures to adapt to different application environments
and usage requirements.
1.Bridge Connection Pattern: Due to physical restrictions, if
it is not appropriate to adopt wireline in connecting LANs, wireless
bridge can be leveraged to realize point-to-point LAN connection.
Not only can wireless bridge offer connections between Physical
Layers and Data Link Layers of LANs, but also provide routing and
protocol conversions at top layers for users of two interconnected
LANs.
2. BS Access Pattern: when cellular network is employed to establish
WLAN, the communications between stations will be through BS access
and data interchanges. Mobile stations can not only set up ad-hoc
networks via the switching center, but also set up operating networks
through WAN and remote stations.
3. Hub Access Pattern: Here wireless Hubs is used to set up a
stellar WLAN, with advantages similar to the Hub networking in the
wireline case. WLAN based on such as structure may adopt the switching
Ethernet operational mode, thereby requiring a simple internal switching
function for the Hub.
4 Peer-to-Peer Structure: Here any two stations should be able
to communicate with each other in a direct manner. WLAN of this
structure normally uses public broadcasting channels, and CSMA-like
protocols are adopted at the MAC layer.
After a full command of WLAN network structure and characteristics,
to design WLAN, one should first decide the number and location
of APs, as well as the location of the coverage of each interconnected
AP, and try to prevent "blind spots" caused by gaps between
covered areas. Field survey may be necessary to decide the location
and number of APs and understand the actual environment and user
demand, including coverage frequency, channel efficiency and throughput
demand. The last step is to decide the network structure and organization
plan. Like the reception of broadcast programs, as mobile users
go increasingly far from the AP, his communications with other APs
will become more and more difficult, with the throughput on the
decline. WLAN may resort to less reliability to improve the transmission
rate, so it may as well reduce the rate to guarantee the reliability.
A lot of plans have chosen to adopt multi-rate solutions to maintain
a good reliability. Therefore it is seen as a key feature for carrier-class
WLAN.
5 The Prospect of WLAN
The early 802.11 WLAN technology already saw successes on the
Europe and American marketplace. In 1999, the sales turnover amounted
to 4 hundred million US dollars. As the price performance ratio
of 802.11 scores substantial improvement, a whole new WLAN primetime
is on the horizon. Enterprises may apply WLAN to extend existing
LANs. Likewise, venues where business people tend to gather such
as airport, hotel, conference center and Cafes will also become
hot spots for further WLAN extension. As one survey shows, each
day there are some 150,000 people becoming new WLAN subscribers
in the world. Up until now, WLAN subscribers worldwide have hit
2 hundred million. As a new application, we may say WLAN has successfully
knocked open the door to market. According to expert forecast, the
global WLAN sale is about to top 2.2 billion US dollars in 2004,
with an average annual growth rate of around 25%. At the same time,
the application scope of WLAN is also being continually expanded,
and it may even take the place of LAN in some cases.
Today, WLAN is well received by North American and European markets.
Is it the Chinese market also calling for the same technology? Some
people may say that at present, 64Kbps is enough for Internet access
and 11Mbps does not mean much. However, as insiders point out, WLAN
offers high bandwidth and large throughput, which will decide its
wide take-up in high-end business segments such as airports, hotels,
conference centers and cafes, rather than by individual users. A
demand of this kind is becoming increasingly urgent in China's business
market segment.
WLAN with its promising market viability may greatly improve
the economic benefit of an enterprise. According to a survey of
the WLAN Association, WLAN can improve the productivity of an enterprise
by 48%, increase the corporate efficiency by 6%, better the corporate
profit margin by 6% and lower down the enterprise cost by 40%. WLAN
can not only slash some expenditure on wiring but also enable users
to access the information in a more flexible and mobile way.
Given the huge market potential of WLAN, in May 2001, at a seminar
held at Beijing World Hotel entitled "World New Economy, High-tech
& Finance Forum", CNC launched its trial "Infinite
Trip Companion" service for wireless broadband access, the
first ever commercial trial of its kind in China. Later, CNC's WLAN
became a bright point at Shenzhen South China Internet Exhibition.
On October 10, 2001, the APEC Summit informal meeting took place
in Shanghai, where CNC's "Infinite Trip Companion" was
among Guaranteed Communications Services for the Summit. During
the event, people showed great enthusiasm towards the service and
applied for it in queues. It is evident that the market potential
for WLAN is immense.
In the domestic market, WLAN technology and product appear new
in the actual application area. However, due to the irreplaceable
nature of wireless means, WLAN will be rapidly applied in scenarios
where interconnection in mobility or inter-network roaming is required.
In places where wiring is difficult and at data processing nodes
lying afar, WLAN will also give strong network support. In particular,
WLAN will see robust growth in the following business sectors:
Petrol industry: wireless connection can provide data link from
the drilling platform to the compressor house to display or input
key data obtained by the artesian well. Due to the separation of
broad waters, it is usually very difficult to transmit data and
information on the offshore drilling platform, and it would be quite
expensive and challenging to lay fiber cables. With WLAN, the expense
would be only less than one tenth of fiber laying, while with higher
efficiency and better quality.
Medical care: a lot of hospitals are now equipped with computerized
patient monitoring device, medical treatment instrument and medicine
stocking system. With WLAN, doctors and nurses may have consultations
or inspect wards at clinic or emergency rooms equipped with dedicated
PC lines. In operations, it is not necessary for surgeons to bring
heavy medical records. Doctor's advice may be recorded real-time
in notebooks or PDAs. Also, treatment opinions may be transmitted,
and the patient's medical record or drugs can be inquired on-line.
Factory workshops: in a factory, it is normally impossible to
lay cables under concrete floors for PC connection. Clumsy cranes
make overhead wiring a fancy dream. Meanwhile, it is also not easy
to do floor wiring at spare part or cargo passages. In this case,
WLAN can enable technicians to check and repair products, modify
designs or discuss the project plan anywhere. They may even inquire
about technical files, send technical instructions, ask for technical
support or discuss with outside experts via WLAN.
Stock control: with radio links, the sending and warehouse registration
of spare parts and cargo may be directly processed by connecting
OCRs, notebooks and the Central Processing Computer for cargo checking,
storage record updating and list printing.
Exhibition and conference: for temporary occasions like conferences
or exhibitions, WLAN can help the working staff to access handy
Internet service in a short time and get the required information.
Also, mobile PC may be used to exchange information, deliver transcripts
or make reports.
Financial service: backed by the wireless network, banking, securities
and future dealing branches can be interconnected. Even if the cable
PC network is in place, to avoid link failures, it is still necessary
to set up a WLAN as backup. In securities and future deals, price
and buy/sell information is fickle, changing up to the minute. With
handheld devices, information may be input and delivered to PCs,
quotation service system and the display board in the transaction
hall through the wireless network for managers, brokers and dealers
to manage their business and engage in direct dealing. By doing
so, losses caused by inaccurate information and time delay due to
gesture, telephone transmitter or manual input mistakes would be
minimized.
Travel service: With WLAN, hotels can offer anytime anywhere service
for its guests. Once registration and accounting systems are set
up, the guest may hang around at any place within the region, e.g.,
bars, fitness club, function hall or restaurants. Attendants may
leverage the handheld device to communicate with each other and
update the charging record, without the need to wait for the result
of a complex accounting system.
Mobile office system: WLAN can empower office PCs with mobility
to realize PC roaming within the network. For business staff, department
managers, engineers or technicians, provided that they have mobile
terminals or notebooks at hand, they may inquire about and access
information as necessary in offices, archive room, meeting room
or even dorms. Executives can issue directions, notifications or
contact business people at any point of the network. That is to
say, mobile office anytime anywhere has become a reality.
It is foreseeable that, with the increasing popularity of open
office and handheld devices, people will pose more and more demand
for mobile information access and storage. Therefore, WLAN will
enjoy a broad application in offices, production sites and households.
A bright future for WLAN is right ahead.
Author biography:
Mr. Li Wei graduated from Changchun University of Posts and Telecommunications
in 1996. He is currently Director of No. 1 Department of Radio Communications
of Beijing Telecommunications Planning & Dimensioning Institute.
|
