3. PHYSICAL MEDIA

For Local Area Networks to function they require a physical media over which to operate, which might be seen as the first layer or physical layer in the OSI model. In the past this would have been 10Base 5, or 10 Base 2 Coax, but the most common type of cabling at the time of writing is UTP (Unshielded Twisted Pair). Alternate media that are also used are fibre optic, or wireless systems which have become more popular in recent years The following paragraphs provide an overview various media options.

Copper Cabling

Co-axial

Co-axial cable (sometimes referred to as ‘co-ax), is based on a central copper core encased in a plastic sheath which is then surrounded in a plastic coating.

The signal is carried on the central core with the outer conductor or mesh forming a screen to outside electrical noise. The most common example of co-axial is television aerial cable.

Originally this form of cable was the most common form of LAN cable due to its high capacity and resistance to interference.

Its main disadvantage is its thickness, which means it is limited in its ability to be run through small cable ducts and around tight angles. Also, the cost of co-ax is relatively high in comparison to more traditional forms of data cabling.

While coax, both thick and thin is rarely used, most of the networks, which specified this cable type, are now able to operate on other types such as unshielded twisted pair (UTP) or on fibre.

Thick Ethernet

This form of cabling, often known as “Yellow Cable” was the original co-axial cable used by most networks, with Ethernet being the main champion of such cable. Its capacity in terms of distance is great, but the cost of cabling is high and its thickness prohibitive in tight cable runs and cabling ducts, which may already be relatively full.

Thin Coax

Thin coax (RG58) was introduced to reduce the cost of cabling networks. This was mainly associated with Ethernet and became known as ‘Cheapernet’. Its main sacrifice over Ethernet is the distance that a single branch can run. However the cable is much cheaper and thinner, and therefore overcomes some of the disadvantages of the original cable.

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Twisted Pair

Today twisted pair cabling is the most common form of cabling. It originated as the method for connecting telephones to the local PABX with the same wiring used to connect data terminals to computers around a building.

The pairs are twisted to reduce the interference between adjacent pairs in the cable. Usually a series of pairs are encased in a single sheath and colour coded to reduce the numbers of physical cables, which need to be pulled through the ducting.

Twisted Pair – Unscreened

The main advantage of this type of cable is that its lower cost, easy to handle and cables already laid for other devices can often be re-used to implement a LAN.

The main drawbacks are its relatively high error rate and the short distances which can be run without signal regeneration.

Twisted Pair – Screened

Screened twisted pair has been introduced to reduce the number of errors due to outside interference. The wire is encased in a metallic braid, somewhat similar to co-axial cable. This reduces errors but also raises costs.

Categories of Twisted Pair Cable

Category 5 cable – Cat 5

Commonly known as Cat 5, this is an unshielded twisted pair cable designed for high signal integrity. The actual standard defines specific electrical properties of the wire, but it is most commonly known as being rated for its Ethernet capability of 100 Mbit/s. Its specific standard designation is EIA/TIA-568. Cat 5 cable typically has three twists per inch of each twisted pair of 24 gauge copper wires within the cable. Another important characteristic is that the wires are insulated with a plastic (FEP) that has low dispersion, that is, the dielectric constant of the plastic does not depend greatly on frequency. Special attention also has to be paid to minimizing impedance mismatches at connection points.

It is often used in structured cabling for computer networks such as Fast Ethernet, although it is often used to carry many other signals such as basic voice services, token ring, and ATM (at up to 155 Mbit/s, over short distances).

Category 5 (Cat 5) Patch Leads

Patch leads created from Cat 5 are often terminated with RJ-45 electrical connectors. Normal Cat 5 cables are wired “straight through” and connect a computer to a hub or switch. In other words, pin 1 is connected to pin 1, pin 2 to pin 2, etc. The RJ-45 pinout for a Cat 5 cable can either be TIA-568A or TIA-568B. TIA-568A is used by some phone systems and Token Ring. Most everything else, such as the Ethernet standards 10BASE-T and 100BASE-TX, use TIA-568B.

In Ethernet, “crossover” Cat 5 cables are cables in which pairs two and three are reversed. (For 100BASE-T4 a more complex connection layout is needed.) These are most often used to connect two PC’s NICs directly (with no intervening hub). They can also be used to connect two hubs or switches together. However most hubs and switches either have an uplink port, a button to change a port to uplink or one or more ports with autosense (most modern switches now have autosense on every port). These features eliminate the need for crossover cables when connecting them.

Category 5e Cable (CAT5e)

Cat 5e cable is an enhanced version of Cat 5 for use with 1000 Base-T networks, or for long-distance 100 Base-T links (350 m, compared with 100 m for Cat 5). It must meet the EIA/TIA 568A-5 specification.

Category 6 Cable (CAT 6)

Cat 6 is a cable standard for Gigabit Ethernet and other interconnect that is backward compatible with Category 5 cable, Cat-5e and Cat-3. Cat-6 features more stringent specifications for crosstalk and system noise. The cable standard is suitable for 10BASE-T/100BASE-TX and 1000BASE-T (Gigabit Ethernet) connections. It is suitable for 1000 Base-T (gigabit) Ethernet up to 100 M.

The cable contains four twisted copper wire pairs, just like earlier copper cable standards. When used as a patch cable, Cat-6 is normally terminated in RJ-45 electrical connectors. If components of the various cable standards are intermixed, the performance of the signal path will be limited to that of the lowest category.

Category 7 cable (CAT7)

Cat 7 (ISO/IEC 11801: 2002 category 7/class F), is a cable standard for Ultra Fast Ethernet and other interconnect technologies that can be made to be backwards compatible with traditional CAT5 and CAT6 Ethernet cable. CAT7 features even more stringent specifications for crosstalk and system noise than CAT6. To achieve this, shielding has been added for individual wire pairs and the cable as a whole.

The CAT7 cable standard has been created to allow 10-Gigabit Ethernet over 100M of copper cabling. The cable contains four twisted copper wire pairs, just like the earlier standards. CAT7 can be terminated in RJ-45 compatible GG45 electrical connectors which incorporate the RJ-45 standard, and a new type of connection to enable a smoother migration to the new standard. When combined with GG-45 connectors, CAT7 cable is rated for transmission frequencies of up to 600 MHz.

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EIA/TIA-568A and EIA/TIA-568B Pin Outs

EIA/TIA-568A and EIA/TIA-568B are closely related joint Electronic Industries Alliance (EIA), Telecommunications Industry Association (TIA), and International Telecommunications Union (ITU) standards for twisted pair wiring. They define the pinout, or order of connections, for wires in RJ-45 8-pin modular connector plugs and jacks used with Category 3, Category 5 and Category 6, 4-pair cables.

Both TIA-568A and TIA-568B are used by many modern computer LAN media on twisted pair cable, such as Ethernet 10BASE-T, 100BASE-TX and 1000BASE-T. They are also used by many digital telephone PBX systems.

The reason there are two conflicting standards is that the EIA/TIA produced TIA-568A long after AT&T developed its own, different convention known as 258A. By the time TIA-568A was published, AT&T 258A had become so widespread that it could not easily be discarded. So the EIA/TIA blessed the AT&T 258A convention as TIA-568B.

Some advocate that TIA-568A be preferred to TIA-568B in new installations because the mapping of pair numbers to telephone line numbers is more consistent with TIA-568A. However, the TIA-568B (AT&T 258A) convention is solidly entrenched and seems to show no signs of going away.

Pairing and colours

The eight wires in the cable are grouped into four pairs. According to telephony tradition dating from the days of manual switchboards, one wire in each pair is the tip and the other is the ring. Each wire pair is twisted within the cable to reduce crosstalk with the other pairs. The pairs must be used as such; if a cable is incorrectly wired to group wires from different pairs into a single pair, the network will almost certainly malfunction. In telephony, hum, noise and crosstalk may be present. This is known as a split pair error.

The cable pairs are assigned the first four entries in the AT&T standard for colour codes in 25-pair and larger cables. The ring wire is assigned the primary color with a stripe of the secondary color, and the tip wire is assigned the secondary color with a stripe of the primary color. In many cables, the tip wire lacks the secondary color stripe; the solid primary color is used.

The primary color of pair 1 is blue, pair 2 is orange, pair 3 is green and pair 4 is brown. The secondary color for all four pairs is white. It is important to note that because these wire color codes come from an old AT&T standard, they are the same for all 8-pin termination standards, TIA-568A, TIA-568B, and USOC-8 (RJ-61). Only the specific assignments of pairs to connector pins varies among these standards.

Wiring

Regardless of the wiring standard, RJ-45 modular jack pins are numbered 1 through 8 as shown:

Figure 4: RJ45 Modular Jack Wiring

The assignments of wire pairs to plug and jack pins are as follows:

Figure 5: Assignment of Wire Pairs

Note that the only difference between TIA-568A and TIA-568B is that pairs 2 & 3 (orange and green) are swapped. Both standards wire the pins “straight through”, i.e., pins 1 through 8 on one end are connected to pins 1 through 8 on the other end. Also, the same sets of pins are paired in both standards: pins 1&2 form a pair, as do 3&6, 4&5 and 7&8. Since electricity doesn’t care about wire insulation color, only about pin connections and pairings, cables wired to either standard are interchangeable.

In other words, the choice between TIA-568A and TIA-568B is arbitrary as long as both ends of each cable follow the same standard (except for crossover cables, see below). Different cables may follow different standards.

So if your wiring consists entirely of connectorised cables (i.e., cables that terminate directly in a RJ-45 jack or plug), especially if you buy pre-connectorised cables, then you don’t really need to choose a standard.

But if you make a lot of cables yourself, and especially if you have punch-block cross-connects or patch panels, then it becomes important to pick one standard and make it the local site convention to avoid confusion. Both standards are widespread, and neither shows signs of going away although there seems to be a trend to TIA-568A in new equipment and construction.

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Crossover wiring

10BASE-T and 100BASE-TX use one pair for transmission in each direction. The Tx+ line from each device connects to the tip conductor and the Tx- line is connected to the ring. This requires that the transmit pair of each device be connected to the receive pair of the device on the other end. When a terminal device is connected to a switch or hub, this crossover is done internally in the latter. A standard straight through cable is used for this purpose where each pin of the connector on one end is connected to the corresponding pin on the other connector. Because the connector pin pairings are the same in TIA-568A and TIA-568B, any given cable may be wired to either standard and it will work; the choice between TIA-568A and TIA-568B is arbitrary.

One terminal device may be connected directly to another without the use of a switch or hub, but in that case the crossover must be done externally in the cable. Since 10BASE-T and 100BASE-TX use pairs 2 and 3, these two pairs must be swapped in the cable. This is a crossover cable. A crossover cable must also be used to connect two internally crossed devices (e.g., two hubs or switches) as the internal crossovers cancel each other out.

Because the only difference between TIA-568A and TIA-568B are that pairs 2 and 3 are swapped, a crossover cable is just a cable with one connector following TIA-568A and the other TIA-568B.

Many newer Ethernet NICs, switches and hubs automatically apply an internal crossover when necessary. This feature is known by various vendor-specific terms, e.g., Netgear calls it Auto uplink™ and other common vendor terms include Auto-MDI/MDI-X, Universal Cable Recognition and Auto Sensing. This eliminates the need for crossover cables, obsoletes the uplink/normal ports and manual selector switches found on many older hubs and switches, and vastly reduces installation errors, especially by non-technical users.

Crossover cables are never necessary in 1000BASE-T (Gigabit)as all four pairs are used bidirectionally. All 1000BASE-T connections should be made with straight-through cables using Category 5e cable or better that provides all four pairs.

Backwards compatibility

Because pair 1 connects to the center pins (4&5) of the RJ-45 jack in both TIA-568A and TIA-568B, both standards are compatible with the first line of RJ-11, RJ-14 RJ-25 and RJ-61 connectors that all have the first pair in the center pins of these connectors.

If the second line of a RJ-14, RJ-25 or RJ-61 plug is used, it connects to pair 2 (orange/white) of jacks wired to TIA-568A but to pair 3 (green/white) in jacks wired to TIA-568B. This makes TIA-568B potentially confusing in telephone applications.

Because of different pin pairings, the RJ-25 and RJ-61 plugs cannot pick up lines 3 or 4 from either TIA-568A or TIA-568B without splitting pairs. This would most likely result in unacceptable levels of hum, crosstalk and noise.

Because 10BASE-T and 100BASE-TX use only pairs 2 and 3, pairs 1 and 4 need not even be present in the cable. It is also common in some networks to use one 4-pair Category 5 cable to provide two separate 10BASE-T or 100BASE-TX links, assigning only two pairs to each link. However, such jacks cannot be used with 1000BASE-T as it requires all four pairs for each link. They are also incompatible with direct use by single-line telephones with standard RJ-11 plugs as nothing is connected to pair 1 in the jack. However, a separate telephone line could be connected to pair 1, thus allowing a single jack to be used for either voice or Ethernet without reconfiguration.

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Power over Ethernet

Power over Ethernet or PoE technology describes any system to transmit electrical power, along with data, to remote devices over standard twisted-pair cable in an Ethernet network. This technology is useful for powering IP telephones, wireless LAN access points, webcams, hubs, computers, and other appliances where it would be inconvenient or infeasible to supply power separately. The technology is somewhat comparable to POTS telephones, which also receive power and data (although analog) through the same cable. It works with an unmodified Ethernet cabling infrastructure.

Power over Ethernet is standardized in IEEE 802.3af. There are several earlier, techniques, but the IEEE standard will probably become dominant.

IEEE 802.3af provides 48 volts DC over two pairs of a four-pair cable at a maximum current of 350 mA for a maximum load power of 16.8 watts. A “phantom” technique is used so that the powered pairs may also carry data. This permits its use not only with 10BASE-T and 100BASE-TX, which use only two of the four pairs in the cable, but also with 1000BASE-T (Gigabit Ethernet), which uses all four pairs for data transmission. This is possible because all versions of Ethernet over twisted pair cable specify differential data transmission over each pair with transformer coupling; the DC supply and load connections can be made to the transformer centre-taps at each end. Each pair thus operates in “common mode” as one side of the DC supply, so two pairs are required to complete the circuit. The polarity of the DC supply is unspecified; the powered device must operate with either polarity with the use of a bridge rectifier.

As of May 2005 there is discussion about increasing the amount of power available on the cable. This may be done by sending power through all four pairs of wire which would double the amount of power. Other discussions include increasing the amount of current.

Before applying power, an IEEE 802.3af power source first “probes” the remote device to determine if it can accept power, and if so, which pairs should be used to supply it. Two modes, A and B, are available. In mode A, pins 1&2 (pair #2 in TIA-568B wiring) form one side of the 48VDC supply, and pins 3&6 (pair #3 in TIA-568B) provide the 48VDC return. These are the same two pairs used for data transmission in 10BASE-T and 100BASE-TX, allowing the provision of both power and data over only two pairs in such networks.

In mode B, pins 4&5 (pair #1 in both TIA-568A and TIA-568B) form one side of the DC supply and pins 7&8 (pair 4 in TIA-568A and TIA-568B) provide the return; these are the “spare” pairs in 10BASE-T and 100BASE-TX. Mode B therefore requires a 4-pair cable.

A load may choose either mode A or B, but not both. It does so by connecting a nominal 25 kilo-ohm resistor between the desired pair of pairs. The supply detects this resistor and applies power in the selected mode. If the supply detects either an open or a short circuit, no power is applied, thus protecting devices that do not support IEEE 802.3af.

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Fibre Optic Cabling

Fibre optic cable is made up of one or more continous strands of glass. Each is surrounded by cladding and then re-inforcing material to protect the fibre, and the whole cable is clad in sheath.

Rather than using electrical pulses, light is uses to transmit information, which is read at the receiving end and converted into electrical pulses for th eprocessing device.

The advantages of fibre optic cable are that it can carry much higher quantities of data and it is immune to normal interference, making errors negligible. Similarly, the signals do not attentuate or weaken to any great degree and much longer distances can be run before signals need to be regenerated. Fibre also offers the advantage of security as it is almos impossible to tap in to it without being detected. The cable is light in weight and small in size for the capacity it can handle.

Its principle disadvantage is its cost. Due to the quality of glass necessary and its fagility the cost of production is high. Termination of fibre cables is a skilled task requiring care, and so installation costs will also be higher. However while it is unlikely to compete directly with coax and twisted pair as demand and and use of optical fibre increases the cost will undoubtedly drop.

Types of Fibre

Multi-Mode Fibre

Multi-mode fibre is a type of optical fibre mostly used for shorter distances, e.g. on campus. It can carry 100 Mbit/s for typical campus distances; the actual maximum speed (given the right electronics) depends upon the actual distance. It is easier to connect to than single-mode optical fibre, but its limit on speed x distance is lower. Multi-mode fibre has a larger centre core than single-mode fibre.

The earliest fibre optic cables used a technique termed multi-mode transmission. This is where the light signals from the laser are broken up into a number of paths along the length of the fibre and is reflected off the fibre wall. The amount of reflection, which occurs, dictates the quality of the signal.

Multi-mode optical fibre is less expensive than Single-mode optical fibre. Current transmission speeds and distances are 100Mb/s up to 10km and 1Gb/s for distances up to 1km. Multi-mode optical fibre has two categories. They are Step Index and Graded Index.

Single Mode Fibre

A single-mode optical fibre is an optical fibre in which only the lowest order bound mode can propagate at the wavelength of interest. Single mode fibres are best at retaining the fidelity of each light pulse over longer distances and exhibit no dispersion caused by multiple modes; thus more information can be transmitted per unit time giving single mode fibres a higher bandwidth in comparison with multi-mode fibres. A typical single mode optical fibre has a core radius of 5-10 micrometers and a cladding radius of 120 micrometers. Currently, data rates of up to 1 Gigabits/second are possible at distances of 60 km and over 6 Gigabits/second at distances of up to 10km. Typically single mode fibre is used within the Wide Area Network rather than the Local Area Network.

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Structured Wiring

Structured wiring has come to encompass many differet ideas but at best can be descried as a wiring system designed in a logical hierachy that accommodate all current data cabling and future requirements in a single system.

Structured wiring is a concept which regards the communications wiring of a building as an asset rather than a task to be undertaken when equipment is installed. In a similar way to telephone wiring, which is installed when a building is being furbished, data wiring should be installed at the outset and designed with the intention of obtaining a typical capital life-span of seven to ten years.

Wiring requirements change as equipment capabilities improve or the organisation reorganises its office or technology requirements. Structured wiring is able to accommodate these changes with very little disruption and without the cost of rewiring the building. Also, as the wiring becomes more and more important to the business’s operation, the management, control and fault detection within that wiring needs to be more sophisticated.

A simple way of achieving structured wiring may be to install a backbone network between the floors of a building. Ideally this should be fibre due to potential voltage difference between the electricity supply on the floors.

Each floor may then have twisted pair cable to connect the different desks, printers and servers to a central point on that floor, or to a local device if the floor is very large.

Originally devices were connected to Hubs, but these days its very difficult to find hubs, as Ethernet switches provide far better functionality (prevent collisions, allow speed mis-matching, provide security etc.) and are now less expensive than hubs.

Typically a large organisation may use a chassis based Ethernet switch for the backbone and then stackable or Edge devices as they get closer to the workgroups. Each network connection is then made by connecting the appropriate cable in the wiring closet for that floor. When people move, the most that will need to be done is to run a new cable from the desk to the wiring closet or to the nearest edge or workgroup switch. Figure 6 below provides a typical example of structured wiring.

Figure 6: Example of Structured Wiring

The above plan, although simplified, shows the principle of structured wiring. When people move only the wiring between the desk and the local access point should need to be changed and when a new computer is introduced to the organisation it simply needs to be connected to its local network. To gain access all the users need to do is to obtain the correct authorisation for the computer, regardless of which network they are connected to.

Typically, a system of flood wiring is employed, so that cables will be in place already whenever a device is added or moved. All that is required is an adjustment to the patch panel in the local wiring closet.

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Wireless LAN

Introduction

When this book was first published Local Area Networks used Thick cable to communicate, and within the space of 15 years, have gone from Thick Coax (10 Base 5) to thin coaxial (10 Base 2) to twisted pair (10 base T) cable, and now to wireless as a means of providing an infrastructure.

A Wireless LAN or WLAN is a wireless local area network that uses radio waves as its carrier: the last link with the users is wireless, to give a network connection to all users in the surrounding area. Areas may range from a single room to an entire campus. The backbone network usually uses cables, with one or more wireless access points connecting the wireless users to the wired network. Having said this, there are also point to point wireless systems which are used to build single links,although strictly speaking these systems tend to use Microwave or Infrared.

WLAN is expected to continue to be an important form of connection in many business areas. The market is expected to grow as the benefits of WLAN are recognized. So far WLANs have been installed in universities, airports, and other major public places. Decreasing costs of WLAN equipment has also brought it to many homes. However, in the UK the exorbitant cost of using such connections in public has so far limited use to airports’ Business Class lounges, etc. Large future markets are estimated to be in health care, corporate offices and the downtown area of major cities. New York City has even begun a pilot program to cover all five boroughs of the city with wireless Internet. BT’s Openzone is one such example of a public Wireless LAN.

Originally WLAN hardware was so expensive that it was only used as an alternative to cabled LAN in places where cabling was difficult or impossible. Such places could be old protected buildings or classrooms, although the restricted range of the 802.11b (typically 30ft.) limits its use to smaller buildings. WLAN components are now cheap enough to be used in the home, with many being set-up so that one PC (a parent’s PC, for example) can be used to share an Internet connection with the whole family (whilst retaining access control at the parents’ PC).

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802.11

802.11 legacy

The original version of the standard IEEE 802.11 released in 1997 specifies two raw data rates of 1 and 2 megabits per second (Mbit/s) to be transmitted via infrared (IR) signals or in the Industrial Scientific Medical frequency band at 2.4 GHz. IR remains a part of the standard but has no actual implementations.

The original standard also defines Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) as the media access method. A significant percentage of the available raw channel capacity is sacrificed (via the CSMA/CA mechanisms) in order to improve the reliability of data transmissions under diverse and adverse environmental conditions.

At least five different, somewhat-interoperable, commercial products appeared using the original specification, from companies like Alvarion (PRO.11 and BreezeAccess-II), Netwave Technologies (AirSurfer Plus and AirSurfer Pro) and Proxim (OpenAir). A weakness of this original specification was that it offered so many choices that interoperability was sometimes challenging to realise. It is really more of a "meta-specification" than a rigid specification, allowing individual product vendors the flexibility to differentiate their products. Legacy 802.11 was rapidly supplemented (and popularized) by 802.11b.

802.11b

The 802.11b amendment to the original standard was ratified in 1999. 802.11b has a maximum raw data rate of 11 Mbit/s and uses the same CSMA/CA media access method defined in the original standard. Due to the CSMA/CA protocol overhead, in practice the maximum 802.11b throughput that an application can achieve is about 5.9 Mbit/s over TCP and 7.1 Mbit/s over UDP.

802.11b products appeared on the market very quickly, since 802.11b is a direct extension of the DSSS modulation technique defined in the original standard. Hence, chipsets and products were easily upgraded to support the 802.11b enhancements. The dramatic increase in throughput of 802.11b (compared to the original standard) along with substantial price reductions led to the rapid acceptance of 802.11b as the definitive wireless LAN technology.

802.11b is usually used in a point-to-multipoint configuration, wherein an access point communicates via an omni-directional antenna with one or more clients that are located in a coverage area around the access point. With high-gain external antennas, the protocol can also be used in fixed point-to-point arrangements, typically at ranges up to eight kilometers (km) although some report success at ranges up to 80–120 km where line of sight can be established. This is usually done in place of costly leased lines or very cumbersome microwave communications equipment.

802.11b cards can operate at 11 Mbit/s, but will scale back to 5.5, then 2, then 1 Mbit/s, if signal quality becomes an issue. Since the lower data rates use less complex and more redundant methods of encoding the data, they are less susceptible to corruption due to interference and signal attenuation. Extensions have been made to the 802.11b protocol (e.g., channel bonding and burst transmission techniques) in order to increase speed to 22, 33, and 44 Mbit/s, but the extensions are proprietary and have not been endorsed by the IEEE. Many companies call enhanced versions "802.11b+". These extensions have been largely obviated by the development of 802.11g, which has data rates up to 54 Mbit/s and is backwards-compatible with 802.11b.

802.11a

The 802.11a amendment to the original standard was ratified in 1999. The 802.11a standard uses the same core protocol as the original standard, with a maximum raw data rate of 54 Mbit/s, which yields realistic achievable throughput in the mid-20 Mbit/s. The data rate is reduced to 48, 36, 24, 18, 12, 9 then 6 Mbit/s if required. 802.11a has 12 non-overlapping channels, 8 dedicated to indoor and 4 to point to point. It is not interoperable with 802.11b, except if using equipment that implements both standards.

Since the 2.4 GHz band is heavily used, using the 5 GHz band gives 802.11a the advantage of less interference. However, this high carrier frequency also brings disadvantages. It restricts the use of 802.11a to almost line of sight, necessitating the use of more access points; it also means that 802.11a cannot penetrate as far as 802.11b since it is absorbed more readily, other things (such as power) being equal.

802.11g

In June 2003, a third modulation standard was ratified: 802.11g. This flavour works in the 2.4 GHz band (like 802.11b) but operates at a maximum raw data rate of 54 Mbit/s, or about 24.7 Mbit/s net throughput like 802.11a. It is fully backwards compatible with “b” and uses the same frequencies. Details of making “b” and “g” work well together occupied much of the lingering technical process. In older networks, however, the presence of an 802.11b participant significantly reduces the speed of an 802.11g network.

The 802.11g standard swept the consumer world of early adopters starting in January 2003, well before ratification. The corporate users held back and Cisco and other big equipment makers waited until ratification. By summer 2003, announcements were flourishing. Most of the dual-band 802.11a/b products became dual-band/tri-mode, supporting “a”, “b”, and “g” in a single mobile adaptor card or access point.

While 802.11g held the promise of higher throughput, actual results were mitigated by a number of factors: conflict with 802.11b-only devices, exposure to the same interference sources as 802.11b, limited channelization (only 3 fully non-overlapping channels like 802.11b) and the fact that the higher data rates of 802.11g are often more susceptible to interference than 802.11b, causing the 802.11g device to reduce the data rate to effectively the same rates used by 802.11b. The move to dual-mode/tri-mode products also carries with it economies of scale (e.g. single chip manufacturing). The use of dual-band/tri-mode products ensures the best possible throughput in any given environment.

A new proprietary feature called Super G is now integrated in certain access points. These can boost network speeds up to 108 Mbit/s by using channel bonding. This feature may interfere with other networks and may not support all b and g client cards. In addition, packet bursting techniques are also available in some chipsets and products which will also considerably increase speeds. Again, they may not be compatible with some equipment.

802.11n

In January 2004 IEEE announced that it had formed a new 802.11 Task Group (TGn) to develop a new amendment to the 802.11 standard for local-area wireless networks. The real data throughput will be at least 100 Mbit/s (which may require an even higher raw data rate at the physical layer), and should be up to 4–5 times faster than 802.11a or 802.11g, and perhaps 20 times faster than 802.11b. It is projected that 802.11n will also offer a better operating distance than current networks.

There are two competing proposals of the 802.11n standard, expected to be ratified: WWiSE (World-Wide Spectrum Efficiency), backed by companies including Broadcom, and TGn Sync backed by Intel and Philips. TGnSync and WWiSE are holding discussions to determine how the proposals may be merged. The standardization process is expected to be completed by the end of 2006.

802.11n builds upon previous 802.11 standards by adding MIMO (multiple-input multiple-output).

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MIMO

MIMO stands for multiple-input multiple-output, an abstract mathematical model for some systems. In radio communications if multiple antennas are employed, the MIMO model naturally arises. MIMO exploits phenomena such as multipath propagation to increase throughput, or reduce bit error rates, rather than attempting to eliminate effects of multipath.

MIMO can also be used in conjunction with OFDM and it will be part of the IEEE 802.11n High-Throughput standard, which is expected to be finalized in early 2007.

MIMO and information theory

It has been shown that the channel capacity (a theoretical measure of throughput) for a MIMO system is increased as the number of antennas are increased, proportional to the minimum of number of transmit and receive antennas. This basic result in information theory is what lead to a spur of research in this area.

Benefits of MIMO

MIMO will offer up to eight times the coverage, and up to six times the speed, of current 802.11g networks. Most manufacturers have released "pre-n" hardware in anticipation of the eventual standard.

Wireless LAN In PCs

The use of Windows XP as the ‘standard’ in home PCs makes it very easy to setup a PC as a Wireless LAN ‘basestation’ and (using XP built in Internet Connection Sharing mode) allows all the PCs in the home to access the Internet via the ‘base’ PC. However, lack of expertise in setting up such systems often means that someone nearby, such as a next-door neighbour, may also share the Internet connection. This is typically without the wireless network owner’s knowledge; it may even be without the knowledge of the user (the neighbour) if the user’s computer automatically selects a wireless network.

The future of wireless networks

The 802.11n (MIMO) standard is still being discussed, but one prototype can offer up to (under optimal conditions) 250 Mbit/second. This is over four times the speed of existing 802.11g hardware.

Other new enhancements will include the arrival of 802.11e and 802.11i. 802.11e will prioritize important information on the network (i.e. a voice message takes precedence over email or a webpage). 802.11i will give an increase in security by using WPA2.

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