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Technology
The IEEE 802.11b standard in brief
Telia HomeRun uses the 2.4 GHz ISM band and is based on the IEEE 802.11b
Wireless LAN Standard, which is the first standard to have been developed
for WLANs, Wireless LAN. Work on this standard began in 1990, and it was
approved in June 1997. The purpose was to create a global standard for radio
equipment and networks which use the unlicensed ISM (Industrial, Scientific
and Medical) band, 2.4-2.5GHz, for computer speeds of 11 Mb/s. The standard
encompasses specifications for the physical layer as well as the MAC (Media
Access Control) layer.
The maximum output of RLANs in Europe is 100 mW EIRP (Effective Isotropic
Radiated Power), i.e. the power radiated from the antenna.
The indoor range varies from just over 20 metres up to 120 metres, while
outdoors it varies from around 100 to 450 metres. Considerably greater distances
can be achieved outdoors using LOS (Line Of Sight) with directional antennas.
In order for W-LAN to be used in the unlicensed band, some form of Spread
Spectrum' modulation is required. With 802.11b Direct Sequence Spread Spectrum
(DSSS) are defined for the physical layer. In addition to the transmission
method mentioned above, a specification for IR has also been produced.
The specifications for 802.11b are reminiscent of 802.3, which applies for
Ethernet. The protocol which 802.11 uses is CSMA/CA (Carrier Sense Multiple
Access/Collision Avoidance). Consequently, the protocol avoids collisions
instead of discovering collisions, which is what 802.3 does. The physical
layer uses an algorithm in order to check whether a radio channel is available.
If a channel is free, a radio connection can be established following a
'handshake'. In the MAC layer, there is also a form of Power Management
that ensures that the power of the output station is regulated according
to its condition.
NThe network elements, which are covered, are an AP (Access Point) and some
form of 'output station', which could be a PCMCIA card, an ISA card or another
type of terminal.
The standard applies to two types of network, Ad-hoc and client/server networks.
An ad-hoc network is a basic, stand-alone network that provides coverage
between different output stations without having to use an AP or another
server. A client/server network can better be described as a type of infrastructure
where one or more APs are used to handle traffic from and between the output
stations to the underlying network. It is consequently the AP that handles
roaming between the elements in the network.
Direct Sequence Spread Spectrum, DSSS
This method involves every bit, before it is transmitted, being modulated
with an 11 bit code (chipping code) which the receiver then receives with
the same code. The longer the code, the greater the likelihood that the
original data will be retrieved, although admittedly more bandwidth is also
required.
Future development of 802.11
number of suppliers have joined forces in their endeavour to make products
from different suppliers interoperable, i.e. so that products can be mixed
in the same network.
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