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A Brief History of Wireless Networking
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 ALOHAnet 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:
Ad hoc – An ad hoc 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 its own timer and if the received value is greater, then it updates its 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.
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