The first true Wireless Network was the ALOHAnet, developed
within Hawaii University in the early 1970s. This led
to the development of wireless networks that are in common use today, such as
the 802.11 WLAN standards and 802.15 Bluetooth PAN standards.
ALOHA used a random access method for packet data over UHF
frequencies and this system of sending packet data became know as the ALOHA
channel method. The ALONAnet was used to link a number of computers over 4 of
the Hawaiian islands. Adoption of this method
of communication spread into the satellite world and was even used in some
early first and second generation mobile phone systems.
The ALOHA experiment prompted much research into packet
radio networks using spread spectrum techniques, and in 1985 experimental
frequency bands were allocated by the FCC for the use of spread spectrum
techniques for commercial purposes. These bands became know as the ISM
(Industrial, Scientific and Medical) bands, originally for use with
non-communication devices such as Microwave Ovens and hospital equipment such
as diathermy machines used as a muscle relaxant by creating heat.
Devices used for communications could use these ISM bands,
but on the understanding that ISM equipment could be a source of interference.
For this reason, communications equipment operating in these bands had to be
designed to operate in error prone environments. Good error detection methods
had to be developed to ensure that communications was not disrupted due to a
nearby diathermy machine, for example.
The first standards for Wireless LANs were born out of
discussions and workshops held in the early 1990s, and the IEEE eventually
announced the first 802.11 standards. The 802.11b standard operates within the
2.4Ghz band at speeds up to 11Mbps , while the 802.11a and 802.11g standards
operate at 54Mbps in the 2.4Ghz and 5Ghz bands respectively. In 2008 the 802.11
committee approved a draft 802.11n standard with data rates of 300Mbps. This
draft standard used MIMO (Multiple-input Multiple-output) through the use of
multiple transmit and receive antennas and a technique called spatial
diversity. Some modern wireless network equipment is able to utilise two
separate bands (2.4Ghz and 5Ghz) for increased reliability and performance.
Modulation techniques used for WiFi had to include methods
which would combat interference in the error prone ISM Bands. IEEE 802.11b uses
a modulation technique called direct sequence spread spectrum with
Complementary Code Keying (CCK), which utilises 64 eight-bit codewords for
encoding the data at 5.5 and 11Mbps and finally modulated using QPSK
(Quadrature Phase Shift Keying). The IEEE 802.11a and 802.11g standards use
OFDM (Orthogonal Frequency Division Multiplexing) where the radio band is
divided into 64 sub-channels running in parallel. Each sub-carrier is modulated
by means of BPSK, QPSK or Quadratue Amplitude Modulation. Some of the
sub-carriers carry redundant, duplicate information, so if interference affects
a number of sub-carriers then the data can normally still be received and
re-constructed.
WiFi, as it is widely referred to can be configured in 3
main topologies:
Adhoc - An adhoc network is otherwise known as an
IBSS (Independent Basic Service Set), where all stations communicate with each
other in a peer-to-peer configuration. There is no need for a Wireless Access
Point as all stations communicate directly with each other. There is not normally any planning and
certainly no site survey prior to an ‘ad hoc’ network being formed. Stations
can only talk to other stations that are in range of each other. This is an
issue known as the ‘hidden node, whereby a station may be able to hear two
other stations but the two stations may not be able to hear each other because
of their geographical locations. The station in the middle has no means of
relaying information between the other two.
There is no access point to act as the source of timing information so
timing has to be achieved in a distributed manner. The first station to transmit
sets the ‘beacon interval’ and creates a set of Target Beacon Transmission
Times (TBTT). Once the TBTT has been reached by a client, a client will:
•
Suspend any pending backoff timers from a
previous TBTT.
•
Determine a new random delay.
•
If another beacon signal arrives before the
end of the random delay, suspend the random backoff timers. If no beacon
arrives then send a beacon and resume the suspended backoff timers.
Within the
beacon is an embedded Timer Sychronisation Function (TSF) where each client
compares the TSF in a received beacon with it’s own timer and if the received
value is greater then it updates it’s own timer. This has the effect that
eventually every client will synchronise with the station that has the fastest
timer. The time it takes for the timing to distribute will depend on the number
of clients within the network.
BSS (Basic Service Set)
- Stations all communicate
through a wireless access point and must associate with that wireless access
point by means of a SSID (Service Set Identifier). Within a BSS, an Access Point will act as the
central point for all communications within the BSS network. In effect, the AP
relays frames between clients and so is in receipt of all data traffic as well
as management traffic. Additionally, the AP may well be connected to a wired
network, providing the clients with communications access across a wider
audience.
ESS (Extended Service Set)
- A number of BSSs connected via their uplink
interfaces, via a wired or wireless connection. The BSSs are connected to what
is known as the Distribution System (DS) which in most cases are wired
networks. An ESS is sometimes known as a Multiple Infrastructure BSS due to a
number of BSSs being used to form it. Once again, clients must communicate with
an AP in order to pass traffic to other clients within a BSS or in an adjacent
BSS connected to the same DS.
Wireless
Networks have become increasingly popular for both business and home users,
mainly due to the mobility that they allow. Less cabling infrastructure is
required and users can roam within the area covered by the WLAN. Many devices
are now wireless enabled including Wireless Access Points, Wireless Adapters, Wireless
Routers, and of course many Notebook computers come with onboard wireless.
This article on Wireless
was written by David Christie, MD at NSTUK Ltd, Website
http://www.ipexpress.co.uk