IEEE 802.11 standards

Wireless connectivity for computers is now well established and virtually all new laptops contain a Wi-Fi capability. Of the WLAN solutions that are available the IEEE 802.11 standard, often termed Wi-Fi has become the de-facto standard. With operating speeds of systems using the IEEE 802.11 standards of around 54 Mbps being commonplace, Wi-Fi is able to compete well with wired systems. As a result of the flexibility and performance of the system, Wi-Fi "hotpots" are widespread and in common use. These enable people to use their laptop computers as they wait in hotels, airport lounges, cafes, and many other places using a wire-less link rather than needing to use a cable.

In addition to the 802.11 standards being used for temporary connections, and for temporary Wireless Local Area Network, WLAN applications, they may also be used for more permanent installations. In offices WLAN equipment may be used to provide semi-permanent WLAN solutions. Here the use of WLAN equipment enables offices to be set up without the need for permanent wiring, and this can provide a considerable cost saving. The use of WLAN equipment allows changes to be made around the office without the need to re-wiring.

As a result the Wi-Fi, IEEE 802.11 standard is widely used to provide WLAN solutions both for temporary connections in hotspots in cafes, airports, hotels and similar places as well as within office scenarios.

IEEE 802.11 Standards

There is a plethora of standards under the IEEE 802 LMSC (LAN / MAN Standards Committee). Of these even 802.11 has a variety of standards, each with a letter suffix. These cover everything from the wireless standards themselves, to standards for security aspects, quality of service and the like:

·      802.11a - Wireless network bearer operating in the 5 GHz ISM band with data rate up to 54 Mbps

·      802.11b - Wireless network bearer operating in the 2.4 GHz ISM band with data rates up to 11 Mbps

·      802.11e - Quality of service and prioritisation

·      802.11f - Handover

·      802.11g - Wireless network bearer operating in 2.4 GHz ISM band with data rates up to 54 Mbps

·      802.11h - Power control

·      802.11i - Authentication and encryption

·      802.11j - Interworking

·      802.11k - Measurement reporting

·      802.11n - Wireless network bearer operating in the 2.4 and 5 GHz ISM bands with data rates up to 600 Mbps

·      802.11s - Mesh networking

·      802.11ac - Wireless network bearer operating below 6GHz to provide data rates of at least 1Gbps per second for   

                      multi-station operation and 500 Mbps on a single link

·      802.11ad - Wireless network bearer providing very high throughput at frequencies up to 60GHz

·      802.11af - Wi-Fi in TV spectrum white spaces (often called White-Fi)

Of these the standards that are most widely known are the network bearer standards, 802.11a, 802.11b, 802.11g and now 802.11n.

802.11 Network bearer standards

All the 802.11 Wi-Fi standards operate within the ISM (Industrial, Scientific and Medical) frequency bands. These are shared by a variety of other users, but no license is required for operation within these frequencies. This makes them ideal for a general system for widespread use.

There are a number of bearer standards that are in common use. These are the 802.11a, 802.11b, and 802.11g standards. The 802.11n standard is the latest providing raw data rates of up to 600 Mbps.

Each of the different standards has different features and they were launched at different times. The first accepted 802.11 WLAN standard was 802.11b. This used frequencies in the 2.4 GHz Industrial Scientific and Medial (ISM) frequency band, this offered raw, over the air data rates of 11 Mbps using a modulation scheme known as Complementary Code Keying (CCK) as well as supporting Direct-Sequence Spread Spectrum, or DSSS, from the original 802.11 specification. Almost in parallel with this a second standard was defined. This was 802.11a which used a different modulation technique, Orthogonal Frequency Division Multiplexing (OFDM) and used the 5 GHz ISM band. Of the two standards it was the 802.11b variant that caught on. This was primarily because the chips for the lower 2.4 GHz band were easier and cheaper to manufacture.

The 802.11b standard became the main Wi-Fi standard. Looking to increase the speeds, another standard, 802.11g was introduced and ratified in June 2003. Using the more popular 2.4 GHz band and OFDM, it offered raw data rates of 54 Mbps, the same as 802.11b. In addition to this, it offered backward compatibility to 802.11b. Even before the standard was ratified, many vendors were offering chipsets for the new standard, and today the vast majority of computer networking that is shipped uses 802.11g.

Then in January 2004, the IEEE announced it had formed a new committee to develop an even higher speed standard. With much of the work now complete, 802.11n is beginning to establish itself in the same way as 802.11g. The industry came to a substantive agreement about the features for 802.11n in early 2006. This gave many chip manufacturers sufficient information to get their developments under way. As a result it is anticipated that before long, with ratification of 802.11n expected in 2007, that some cards and routers will find their way into the stores.

	802.11A	802.11B	802.11G	802.11N
Date of standard approval	July 1999	July 1999	June 2003	Oct 2009
Maximum data rate (Mbps)	54	11	54	~600
Modulation	OFDM	CCK or DSSS	CCK, DSSS, or OFDM	CCK, DSSS, or OFDM
RF Band (GHz)	5	2.4	2.4	2.4 or 5
Number of spatial streams	1	1	1	1, 2, 3, or 4
Channel width (MHz) nominal	20	20	20	20, or 40

                           Summary of major 802.11 Wi-Fi Standards

Bandwidths of nominal 20 MHz are usually quoted, although the actual bandwidth allowed is generally 22 MHz.

802.11 Networks

There are two types of WLAN network that can be formed: infrastructure networks; and ad-hoc networks.

The infrastructure application is aimed at office areas or to provide a "hotspot". The WLAN equipment can be installed instead of a wired system, and can provide considerable cost savings, especially when used in established offices. A backbone wired network is still required and is connected to a server. The wireless network is then split up into a number of cells, each serviced by a base station or Access Point (AP) which acts as a controller for the cell. Each Access Point may have a range of between 30 and 300 metres dependent upon the environment and the location of the Access Point.

The other type of network that may be used is termed an Ad-Hoc network. These are formed when a number of computers and peripherals are brought together. They may be needed when several people come together and need to share data or if they need to access a printer without the need for having to use wire connections. In this situation the users only communicate with each other and not with a larger wired network. As a result there is no Access Point and special algorithms within the protocols are used to enable one of the peripherals to take over the role of master to control the network with the others acting as slaves.

The IEEE 802.11a standard is capable of producing a high level of performance, and being in a band which is used less than the levels of interference are less allowing high levels of performance.

The 802.11a standard is alphabetically the first of the variety of 802.11 standards that are in widespread use today. Although 802.11a was ratified at the same time as 802.11b, it never caught on in the same way despite the fact that it offered a much higher data transfer rate. The reason for this was that it operated in the 5 GHz ISM band rather than the 2.4 GHz band, and this made chips more expensive. 802.11 was also, possibly ahead of its time. With the introduction of wireless LAN technology, people were happier to settle for any connection, and even one with a lower speed. Nevertheless 802.11 did achieve a significant amount of use and it also forced up the speed of other 802.11 technologies running at 2.4 GHz.

802.11a specification

802.11a boasts an impressive performance. It is able to transfer data with raw data rates up to 54 Mbps, and has a good range, although not when operating at its full data rate.

PARAMETER	VALUE
Date of standard approval	July 1999
Maximum data rate (Mbps)	54
Typical data rate (Mbps)	25
Typical range indoors (Metres)	~30
Modulation	OFDM
RF Band (GHz)	5
Number of spatial streams	1
Channel width (MHz)	20

Summary of 802.11 Wi-Fi Standards

The 802.11a standard uses basic 802.11 concepts as its base, and it operates within the 5GHz Industrial, Scientific and Medical (ISM) band enabling it to be used worldwide in a licence free band. The modulation is Orthogonal Frequency Division Multiplexing (OFDM) to enable it to transfer raw data at a maximum rate of 54 Mbps, although a more realistic practical level is in the region of the mid 20 Mbps region. The data rate can be 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.

Note on OFDM:

Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multi-path effects..

802.11a RF signal

The OFDM signal used for 802.11 comprises 52 subcarriers. Of these 48 are used for the data transmission and four are sued as pilot subcarriers. The separation between the individual subcarriers is 0.3125 MHz. This results from the fact that the 20 MHz bandwidth is divided by 64. Although only 52 subcarriers are used, occupying a total of 16.6 MHz, the remaining space is used as a guard band between the different channels.

A variety of forms of modulation can be used on each of the 802.11a subcarriers. BPSK, QPSK, 16-QAM, and 64 QAM can be used as the conditions permit. For each set data rate there is a corresponding form of modulation that is used. Within the signal itself the symbol duration is 4 microseconds, and there is a guard interval of 0.8 microseconds.

DATA RATE (MBPS)	MODULATION	CODING RATE
6	BPSK	1/2
9	BPSK	3/4
12	QPSK	1/2
18	QPSK	3/4
24	16-QAM	1/2
36	16-QAM	3/4
48	64-QAM	1/2
54	64-QAM	3/4

As with many data transmission systems, the generation of the signal is performed using digital signal processing techniques and a baseband signal is generated. This is then upconverted to the final frequency. Similarly for signal reception, the incoming 802.11a signal is converted down to baseband and converted to its digital format after which it can be processed digitally.

Although the use of OFDM for a mass produced systems such as 802.11a may appear to be particularly complicated, it offers many advantages. The use of OFDM provides a significant reduction in the problems iof interference caused by multipath effects. The use of OFDM also ensures that there is efficient use of the radio spectrum.

IEEE 802.11b was the first wireless LAN standard to be widely adopted and built in to many laptop computers and other forms of equipment. The standard for 802.11b was ratified by the IEEE in July 1999 and the idea for wireless networking quickly caught on with many W-Fi hotspots being set up so that business people could access their emails and surf the Internet as required when they were travelling.

It was only after 802.11 was ratified and products became available that W-Fi took off in a large way. Wi-Fi hotspots were set up in many offices, hotels and airports and the idea of using portable laptop computers while travelling became far easier.

Although the IEEE 802.11a standard was introduced at the same time, it did not catch on in the same way even though it was capable of higher speeds. The main reason for this was that it operated in the 5 GHz ISM band rather than the 2.4 GHz of 802.11b, and this made it more expensive.

802.11b specification

802.11b boasts an impressive performance. It is able to transfer data with raw data rates up to 11 Mbps, and has a good range, although not when operating at its full data rate.

PARAMETER	VALUE
Date of standard approval	July 1999
Maximum data rate (Mbps)	11
Typical data rate (Mbps)	5
Typical range indoors (Metres)	~30
Modulation	CCK (DSSS)
RF Band (GHz)	2.4
Channel width (MHz)	20

Summary of 802.11b Wi-Fi Standard Specification

When transmitting data 802.11b uses the CSMA/CA technique that was defined in the original 802.11 base standard and retained for 802.11b. Using this technique, when a node wants to make a transmission it listens for a clear channel and then transmits. It then listens for an acknowledgement and if it does not receive one it backs off a random amount of time, assuming another transmission caused interference, and then listens for a clear channel and then retransmits the data.

RF modulation for 802.11b

The RF signal format used for 802.11b is CCK or complementary Code Keying. This is a slight variation on CDMA (Code Division Multiple Access) that uses the basic DSSS (Direct Sequence Spread Spectrum) as its basis. In view of the fact that the original 802.11 specification use CDMA / DSSS, it was easy to upgrade any existing chipset and other investment to provide the new 802.11b standard. As a result 802.11b chipsets appeared relatively quickly onto the market.

802.11b data rates

Although 802.11b cards are specified to operate at a basic rate of 11 Mbps, the system monitors the signal quality. If the signal falls or interference levels rise, then it is possible for the system to adopt a slower data rate with more error correction that is more resilient. Under these conditions the system will first fall back to a rate of 5.5 Mbps, then 2, and finally 1 Mbps. This scheme is known as Adaptive rate Selection (ARS).

Although the basic raw data rates for transmitting data seem very good, in reality the actual data rates achieved over a real time network are much smaller. Even under reasonably good radio conditions, i.e. good signal and low interference the maximum data rate that might be expected when the system uses TCP is about 5.9 Mbps. This results from a number of factors. One is the use of CSMA/CA where the system has to wait for clear times on a channel to transmit and another is associated with the use of TCP and the additional overhead required. If UDP is used rather than TCP then the data rate can increase to around 7.1 Mbps.

Some 802.11b systems advertise that they support much higher data rates than the basic 802.11b standard specifies. While more recent versions of the 802.11 standard, namely 802.11g, and 802.11n specify much higher speeds, some proprietary improvements were made to 802.11b. These proprietary improvements offered speeds of 22, 33, or 44 Mbps and were sometimes labelled as "802.11b+". These schemes were not endorsed by the IEEE and in any case they have been superseded by later versions of the 802.11 standard.

Wi-Fi technology based on the 802.11 standard is now widespread in its use. Not only is it used to provide real wireless LAN (WLAN) functionality, but it is also widely used to provide localised mobile connectivity in terms of "hotspots". A variety of flavours of the IEEE 802.11 are available: 802.11a, 802.11b, 802.11g, and these different standards provide different data throughput speeds and operate on different bands.

One of the major shortfalls for the developing applications for Wi-Fi is that it is not possible to allocate a required quality of service for the particular application. Now with IEEE 802.11e the Quality of Service or QoS problem is being addressed.

The need for QoS

The issue of Quality of Service, QoS on 802.11 Wi-Fi is of particular importance in some applications, and accordingly 802.11e is addressing it. For surfing applications such as internet web browsing of sending emails, delays in receiving responses or sending data does not have a major impact. It results in slow downloads, or small delays in emails being sent. While it may have a small annoyance to the user, there is no real operational impact on the service being provided. However for applications such as voice or video transmission such as Voice over IP, VoIP, there is a far greater impact and this creates a much greater need for 802.11e. Delays, jitter and missing packets result in the system loosing the data and the service quality becoming poor. Accordingly for these time sensitive applications it is necessary to be able to prioritise the traffic. This can only be done by allocating a service priority level to the packets being sent, and this is now all being addressed by IEEE standard 802.11e.

MAC layer

The way in which data is transmitted and controlled has a major impact on the way that QoS is achieved. This is largely determined by the way the Medium Access Control (MAC) layer operates. Within 802.11 there are two options for the MAC layer. The first is a centralised control scheme that is referred to as the Point Coordination Function (PCF), and the second is a contention based approach called Distributed Coordination Function (DCF). Of these few manufacturers of chips and equipment have implemented PCF and the industry seems to have adopted the DCF approach.

The PCF mode supports time sensitive traffic flows to some degree. Wireless Access Points periodically send beacon frames to communicate network management and identification which is specific to that WLAN. Between the sending of these frames, PCF splits the time frame into a contention free period and a contention period. If PCF is enabled on the remote station, it can transmit data during the contention free polling periods. However the main reason why this approach has not been widely adopted is because the transmission times are not predictable.

The other scheme, DCF uses a scheme called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Within this scheme the MAC layer sends instructions for the receiver to look for other carriers transmitting. If it sees none then it sends its packet after a given interval and awaits an acknowledgement. If one is not received it then it knows its packet was not successfully received. It then waits for a given time interval and also checks the channel before retrying to send its data packet.

In more exact terms the transmitter uses a variety of methods to determine whether the channel is in use, monitoring the activity looking for real signals and also determining whether any signals may be expected. This can be achieved because every packet that is transmitted includes a value indicating the length of time that transmitting station expects to occupy the channel. This is noted by any stations that receive the signal, and only when this time has expired may they consider transmitting.

Once the channel appears to be idle the prospective transmitting station must wait for a period equal to the DCF Inter-Frame Space (DIFS). If the channel has been active it must first wait for a time consisting of the DIFS plus a random number of back off slot times. This is to ensure that if two stations are waiting to transmit, then they do not both transmit together, and then repeatedly transmit together.

A time known as a Contention Window (CW) is used for this. This is a random number of back-off slots. If a transmitter intending to transmit senses that the channel becomes active, it must wait until the channel comes free, waiting a random period for the channel to come free, but this time allowing a longer CW.

While the system works well in preventing stations transmitting together, the result of using this access system is that if the network usage level is high, then the time that it takes for data to be successfully transferred increases. This results in the system appearing to become slower for the users. In view of this WLANs may not provide a suitable QoS in their current form for systems where real time data transfer is required.

Introducing QoS

The problem can be addressed by introducing a Quality of Service, QoS identifier into the system. In this way those applications where a high quality of service is required can tag their transmissions and take priority over the transmissions carrying data that does not require immediate transmission and response. In this way the level of delay and jitter on data such as that used for VoIP and video may be reduced.

To introduce the QoS identifier, it has been necessary to develop a new MAC layer and this has been undertaken under the standard IEEE 802.11e. In this the traffic is assigned a priority level prior to transmission. These are termed User Priority (UP) levels and there are eight in total. Having done this, the transmitter then prioritises all the data it has to waiting to be sent by assigning it one of four Access Categories (AC).

In order to achieve the required functions, the re-developed MAC layer takes on aspects of both the DCF and PCF from the previous MAC layer alternatives and is termed the Hybrid Coordination Function (HCF). In this the modified elements of the DCF are termed the Enhanced Distributed Channel Access (EDCA), while the elements of the PCF are termed the HCF Controlled Channel Access (HCCA).

EDCA

Of these the EDCA provides a mechanism whereby traffic can be prioritised but it remains a contention based system and therefore it cannot guarantee a give QoS. In view of this it is still possible that transmitters with data of a lower importance could still pre-empt data from another transmitter with data of a higher importance.

When using EDCA, a new class of interframe space called an Arbitration Inter Frame Space (AIFS) has been introduced. This is chosen such that the higher the priority the message, the shorter the AIFS and associated with this there is also a shorter contention window. The transmitter then gains access to the channel in the normal way, but in view of the shorter AIFS and shorter contention window, this means that the higher the chance of it gaining access to the channel. Although, statistically a higher priority message will usually gain the channel, this will not always be the case.

HCCA

The HCCA adopts a different technique, using a polling mechanism. Accordingly it can provide guarantees about the level of service it can provide, and thereby providing a true Quality of Service level. Using this the transmitter is able to gain access to a radio channel for a given number of packets, and only after these have been sent is the channel released.

The control station which is normally the Access Point is known as the Hybrid Coordinator (HC). It takes control of the channel. Although it has an IFS, it has what is termed a Point Coordination IFS. As this is shorter than the DIFS mentioned earlier, it will always gain control of the channel. Once it has taken control it polls all the stations or transmitters in the network. To do this it broadcasts as particular frame indicating the start of polling, and it will poll each station in turn to determine the highest priority. It will then enable the transmitter with the highest priority data to transmit, although it will result in longer delays for traffic that has a lower priority.

After the introduction of Wi-Fi with the 802.11a and 802.11b standards, the 802.11b standard became the most popular operating in the 2.4 GHz ISM band. This standard proved to be the most popular despite the faster operating speed of the a variant of the standard because the cost of producing chips to operate at 2.4 GHz were much less than ones to run at 5 GHz.

In order to provide the higher speeds of 802.11a while operating on the 2.4 GHz ISM band, a new standard was introduced. Known as 802.11g, it soon took over from the b standard. Even before the standard was ratified, 802.11g products were available on the market, and before long it became the dominant Wi-Fi technology.

802.11g specifications

The 802.11g standard provided a number of improvements over the 802.11b standard which was its predecessor. The highlights of its performance are given in the table below.

IEEE 802.11G WI-FI FEATURES
FEATURE	802.11G
Date of standard approval	June 2003
Maximum data rate (Mbps)	54
Modulation	CCK, DSSS, or OFDM
RF Band (GHz)	2.4
Channel width (MHz)	20

802.11g physical layer

Like 802.11b, its predecessor, 802.11g operates in the 2.4 GHz ISM band. It provides a maximum raw data throughput of 54 Mbps, although this translates to a real maximum throughput of just over 24 Mbps.

Although the system is compatible with 802.11b, the presence of an 802.11b participant in a network significantly reduces the speed of a net. In fact it was compatibility issues that took up much of the working time of the IEEE 802.11g committee.

In order to provide resilience against multipath effects while also being able to carry the high data rates, the main modulation method chosen for 802.11g was that of OFDM - orthogonal frequency division multiplex, although other schemes are used to maintain compatibility, etc..

Note on OFDM:

Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multi-path effects..

In addition to the use of OFDM, DSSS - direct sequence spread spectrum is also used.

To provide the maximum capability while maintaining backward compatibility, four different physical layers are used - three of which are defined as Extended Rate Physicals, ERPs.These coexist during the frame exchange so that the sender can use any one of the four, provided they are supported at each end of the link.

The four layer options defined in the 802.11g specification are:

·      ERP-DSSS-CCK: This layer is that used with 11b. Direct sequence spread spectrum is used along with CCK –

    complementary code keying. The performance is that of the legacy 802.11b systems.

·      ERP-OFDM: This physical layer is a new one introduced for 802.11g where OFDM is used to enable the

    provision of the data rates at 2.4 GHz that were achieved by 11a at 5.8 GHz.

·      ERP-DSSS/PBCC: This physical layer was introduced for use with 802.11b and initially provided the same data

     rates as the DSS/CCK layer, but with 802.11g, the data rates have been extended to provide 22 and 33 Mbps. As

    indicated by the title, it uses DSSS technology for the modulation  combined with PBCC coding for the data.

·      DSSS-OFDM: This layer is new to 11g and uses a combination of DSSS and OFDM - the packet header is transmitted using DSSS while the payload is transmitted using OFDM

802.11g occupies a nominal 22 MHz channel bandwidth, making it possible to accommodate up to three non-overlapping signals within the 2.4 GHz band. Despite this, the separation between different Wi-Fi access points means that interference is not normally too much of an issue.

IEEE 802.11G WI-FI PHYSICAL LAYER SUMMARY
PHYSICAL LAYER	USE	DATA RATES (MBPS)
ERP-DSSS	Mandatory	1, 2, 5.5, 11
ERP-OFDM	Mandatory	6, 9, 12, 18, 24, 36, 48, 54
ERP-PBCC	Optional	1, 2, 5.5, 11, 22, 33
DSSS-OFDM	Optional	6, 9, 12, 18, 24, 36, 48, 54

802.11g packet structure

It is customary for data packets to be split into different elements. For Wi-Fi systems the data packets sent over the radio interface can be thought of as consisting of two main parts:

·      Preamble / Header: As with any other preamble / header, it serves to alert receivers, in this case radios, that a transmission is to start, and then it enables them to synchronise. The preamble consists of a known series of '1's and '0's that enable the receivers to synchronise with the incoming transmission. The Header element immediately follows the pre-amble and contains information about the data to follow including the length of the payload.

·      Payload: This is the actual data that is sent across the radio network and can range from 64 bytes up to 1500 bytes. In most cases the preamble/header are sent using the same modulation format as the payload, but this is not always the case. When using the DSSS-OFDM format, the header is sent using DSSS, while the payload uses OFDM.

The initial 802.11 standard defined a long preamble PLCP frame set. In the later 802.11b standard, an optional short preamble was defined. Then for 802.11g the short preamble PPDU was defined as mandatory.

802.11g ERP-DSSS/CCK PPDU frame

Abbreviations
  PPDU: This is the format into which data is converted by the PLCP for transmission. 
    PLCP: This is the PHY Layer Convergence Procedure and it transforms each 802.11 frame that a station wishes  

               to send into a PLCP protocol data unit, PPDU.

   PDSU: This is the Physical Layer Service Data Unit, it represents the contents of the PPDU, i.e., the actual data to

               be sent.

 Service: This field is always set to 00000000. The802.11 standard reserves its data and format for future use.

For the ERP-OFDM PHY option an ERP packet must be followed by a 6 µs period of no transmission called the signal extension period. The reason for this that for a 16 µs period was allowed in 802.11a to enable convolutional decode processing to finish before the next packet arrived.

Within 802.11g, the ERP-OFDM modulation scheme still requires 16 µs to ensure that the convolutional decoding process is able to be completed within the overall process timing. To enable this to occur, a signal extension of 6 µs is included. This enables the transmitting station to compute the Duration field in the MAC header. In turn this ensures that the NAV value of 802.11b stations is set correctly and compatibility is maintained.

IEEE 802.11i Wi-Fi Security: WEP & WPA / WPA2

Wi-Fi security is an issue of importance to all Wi-Fi users. It is defined under IEEE802.11i and systems such as WEP, WPA and WPA2 are widely mentioned, with keys or codes being provided for the various wi-fi hotspots in use.

Wi-Fi security is of significant importance because very many people use it: at home, in the office and when they are on the move. As the wireless signal can be picked up by non-authorised users, it is imperative to ensure that they cannot access the system.

Even users who legitimately gain access to a system could the try to hack other computers on the same hotspot.

Wi-Fi Security background

Wi-Fi access points advertise their presence by periodically sending out a beacon signal that contains the SSID. This allows prospective users to identify the access point and to try to connect to it.

Once detected, it is possible to try to connect to the access point, and the Wi-Fi authentication procedure starts. To achieve access, a key is generally required.

Since the introduction of Wi-Fi a variety of keys have been used:

·      WEP: WEP or Wired Equivalent Privacy was the first form of authentication used with Wi-Fi. Unfortunately it was easy to crack, and other systems are now more widely used.

·      WPA: Wi-Fi Protected Access WPA is a software / firmware improvement over WEP. The first version of this is also known as WPA1 or WPAv1.

·      WPA2: WPA2 or WPAv2 is the next update to WPAv1 and provides significant improvement in the level of security.

WEP - wired-equivalent privacy key

The aim for this key was to make wireless networks such as Wi-Fi as safe as wired communications. Unfortunately this form of security did not live up to its name because it was soon hacked, and now there are many open source applications that can easily break into it in a matter of seconds.

In terms of its operation, the Wi-Fi WEP key uses a clear text message sent from the client. This is then encrypted and returned using a pre-shared key.

A WEP comes in different key sizes. The common key lengths are normally 128 or 256 bits.

The security of the WEP system is seriously flawed. Primarily it does not address the issue of key management and this is a primary consideration to any security system. Normally keys are distributed manually or via another secure route. The Wi-Fi WEP system uses shared keys - i.e. the access point uses the same key for all clients, and therefore this means that if the key is accessed then all users are compromised. It only takes listening to the returned authentication frames to be able to determine the key.

Obviously Wi-Fi WEP is better than nothing because not all people listening to a Wi-Fi access point will be hackers. It is still widely used and provides some level of security. However if it is used then higher layer encryption (SSL, TLS, etc.) should also be used when possible.

WPA Wi-Fi Protected Access

In order to provide a workable improvement to the flawed WEP system, the WPA access methodology was devised. The scheme was developed under the auspices of the Wi-Fi Alliance and utilised a portion of the IEEE 802.11i security standard - in turn the IEEE 802.11i standard had been developed to replace the WEP protocol.

One of the key elements of the WPA scheme is the use of the TKIP - Temporal Key Integrity Protocol. TKIP is part of the IEEE802.11i standard and operates by performing per-packet key mixing with re-keying.

In addition to this the WPA, Wi-Fi Protected Access scheme also provides optional support for AES-CCMP algorithm. This provides a significantly improved level of security.

WPA2 / WPAv2

The WPA2 scheme has now superseded WPA. It implements the mandatory elements of IEEE 802.11i. In particular, it introduces CCMP, a new AES-based encryption mode with strong security.

Certification for WPA2 began in September, 2004 and now it is mandatory for all new devices that bear the Wi-Fi trademark.

IEEE 802.11n Standard

Once Wi-Fi standards including 802.11a, 802.11b, and 802.11g were established, work commenced on looking at how the raw data speeds provided by Wi-Fi, 802.11 networks could be increased still further. The result was that in January 2004, the IEEE announced that it had formed a new committee to develop the new high speed, IEEE 802.11 n standard.

The industry came to a substantive agreement about the features for 802.11n in early 2006. This gave many chip manufacturers sufficient information to get their developments under way. The draft is expected to be finalized in November 2008 with its formal publication in July 2009. However many products are already available on the market. Manufacturers are now releasing products based on the early or draft versions of the specifications assuming that the changes will only be minor in their scope.

With the improved performance offered by 802.11n, the standard soon became widespread with many products offered for sale and use. Although initially few Wi-Fi hotspots offered the standard, 802.11n devices were compatible and able to work with the 802.11b and 802.11g based hotspots.

Basic specification for the IEEE 802.11n standard

The idea behind the IEEE 802.11n standard was that it would be able to provide much better performance and be able to keep pace with the rapidly growing speeds provided by technologies such as Ethernet. The new 802.11n standard boasts an impressive performance, the main points of which are summarized below:

IEEE 802.11N SALIENT FEATURES
PARAMETER	IEEE 802.11N STANDARD
Maximum data rate (Mbps)	600
RF Band (GHz)	2.4 or 5
Modulation	CCK, DSSS, or OFDM
Number of spatial streams	1, 2, 3, or 4
Channel width (MHz)	20, or 40

To achieve this a number of new features that have been incorporated into the IEEE 802.11n standard to enable the higher performance. The major innovations are summarized below:

·         Changes to implementation of OFDM

·         Introduction of MIMO

·         MIMO power saving

·         Wider channel bandwidth

·         Antenna technology

·         Reduced support for backward compatibility under special circumstances to improve data throughput

Although each of these new innovations adds complexity to the system, much of this can be incorporated into the chipsets, enabling a large amount of the cost increase to be absorbed by the large production runs of the chipsets.

Backward compatibility switching

802.11n provides backward compatibility for devices in a net using earlier versions of Wi-Fi, this adds a significant overhead to any exchanges, thereby reducing the data transfer capacity. To provide the maximum data transfer speeds when all devices in the net at to the 802.11n standard, the backwards compatibility feature can be removed. When earlier devices enter the net, the backward compatibility overhead and features are re-introduced. As with 802.11g, when earlier devices enter a net, the operation of the whole net is considerably slowed. Therefore operating a net in 802.11n only mode offers considerable advantages.

In view of the features associated with backward compatibility, there are three modes in which an 802.11n access point can operate:

·         Legacy (only 802.11 a, b, and g)

·         Mixed (both 802.11 a, b, g, and n)

·         Greenfield (only 802.11 n) - maximum performance

By implementing these modes, 802.11n is able to provide complete backward compatibility while maintaining the highest data rates. These modes have a significant impact on the physical layer, PHY and the way the signal is structured.

 

 

 

 

802.11n signal / OFDM implementation

This version of the Wi-Fi standard uses OFDM to provide the various parameters required.

Note on OFDM:

Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multi-path effects..

The way the OFDM has been used has been tailored to enable it to fulfil the various requirements for 802.11n.

To achieve this, two new formats are defined for the PHY Layer Convergence Protocol, PLCP, i.e. the Mixed Mode and the Green Field. These are called High Throughput, HT formats. In addition to these HT formats, there is also a legacy duplicate format. This duplicates the 20MHz legacy packet in two 20MHz halves of the overall 40MHz channel.

The signal formats are changed according to the mode in which the system is operating:

·         Legacy Mode: This may occur as either a 20 MHz signal or a 40 MHz signal:

o    20 MHz: In this mode the 802.11n signal is divided into 64 sub-carriers. 4 pilot signals are inserted in sub-carriers -21, -7, 7 and 21. In the legacy mode, signal is transmitted on sub-carriers -26 to -1 and 1 to 26, with 0 being the centre carrier. In the HT modes signal is transmitted on sub-carriers -28 to -1 and 1 to 28.

o    40 MHz: For this transmission two adjacent 20MHz channels are used and in this instance the channel is divided into 128 sub-carriers. 6 pilot signals are inserted in sub-carriers -53, -25, -11, 11, 25, 53. Signal is transmitted on sub-carriers -58 to -2 and 2 to 58.

In terms of the frames that are transmitted conform to the legacy 802.11a/g OFDM format.

·         Mixed Mode: In this 802.11n mode, packets are transmitted with a preamble compatible with the legacy 802.11a/g. The rest of the packet has a new MIMO training sequence format.

·         Greenfield Mode: In the Greenfield mode, high throughput packets are transmitted without a legacy compatible part. As this form of packet does not have any legacy elements, the maximum data throughput is much higher.

802.11n MIMO

In order to be able to carry very high data rates, often within an office or domestic environment, 802.11n has utilised MIMO. This gives the maximum use of the available bandwidth.

Note on MIMO:

Two major limitations in communications channels can be multipath interference, and the data throughput limitations as a result of Shannon's Law. MIMO provides a way of utilising the multiple signal paths that exist between a transmitter and receiver to significantly improve the data throughput available on a given channel with its defined bandwidth. By using multiple antennas at the transmitter and receiver along with some complex digital signal processing, MIMO technology enables the system to set up multiple data streams on the same channel, thereby increasing the data capacity of a channel.

The 802.11n standard allows for up to four spatial streams to give a significant improvement in the available data rate available as it allows a number of different data streams to be carried over the same channel.

As might be expected, the number of data streams and hence the overall data capacity is limited by the number of spatial streams that can be carried - one of the limits for this is the number of antennas that are available at either end.

To give a quick indication of the capability of a given system or radio a simple notation may be used. It is of the form: a x b : c. Where a is the maximum number of transmit antennas or RF chains at the transmitter; b is the maximum of receive antennas or receive RF chains; and c is the maximum number of data spatial streams. An example might be 2 x 4 : 2 would be for a radio that can transmit on two antennas and receive on four, but can only send or receive two data streams.

The 802.11n standard allows for systems with a capability of up to 4 x 4 : 4. However common configurations that are in use include 2 x 2 : 2;   2 x 3 : 2;   3 x 2 : 2. These configurations all have the same data throughput capability and only differ by the level of diversity provided by the antennas. A further configuration of, 3 x 3 : 3 is becoming more widespread because it has a higher throughput, because of the extra data stream that is present.

Power saving

One of the problems with using MIMO is that it increases the power of the hardware circuitry. More transmitters and receivers need to be supported and this entails the use of more current. While it is not possible to eliminate the power increase resulting from the use of MIMO in 802.11n, it is possible to make the most efficient use of it. Data is normally transmitted in a "bursty" fashion. This means that there are long periods when the system remains idle or running at a very slow speed. During these periods when MIMO is not required, the circuitry can be held inactive so that it does not consume power.

Increased bandwidth

An optional mode for the new 802.11n chips is to run using a double sized channel bandwidth. Previous systems used 20 MHz bandwidth, but the new ones have the option of using 40 MHz. The main trade-off for this is that there are less channels that can be used for other devices. There is sufficient room at 2.4 GHz for three 20 MHz channels, but only one 40 MHz channel can be accommodated. Thus the choice of whether to use 20 or 40 MHz has to be made dynamically by the devices in the net.

Antenna technology

For 802.11n, the antenna associated technologies have been significantly improved by the introduction of beam forming and diversity.

Beam forming focuses the radio signals directly along the path for the receiving antenna to improve the range and overall performance. A higher signal level and better signal to noise ratio will mean that the full use can be made of the channel.

Diversity uses the multiple antennas available and combines or selects the best subset from a larger number of antennas to obtain the optimum signal conditions. This can be achieved because there are often surplus antennas in a MIMO system. As 802.11n supports any number of antennas between one and four, it is possible that one device may have three antennas while another with which it is communicating will only have two. The supposedly surplus antenna can be used to provide diversity reception or transmission as appropriate.

The IEEE 802.11n standard provides a major improvement in the speed at which data can be transferred over a wireless network. While this may not be needed for many small networks where small files are being transferred, the amount of data being passed over most networks is increasing with many more large files, including photos, video clips (and videos), etc. being transferred. With the levels of data only set to increase, the new 802.11n standard will be able to meet the challenge of providing the required capacity for wireless or Wi-Fi networks.

IEEE 802.11ac Gigabit Wi-Fi

The IEEE802.11ac Wi-Fi standard has been developed to raise the data throughput rates attainable on Wi-Fi networks up to a minimum of around 1 Gbps with speeds up to nearly 7 Gbps possible. As a result of these speeds, one manufacturer is marketing the products as 5G WiFi.

The implementation of Gigabit Wi-Fi is needed to ensure that Wi-Fi standards keep up with the requirements of users.

With users requiring ever higher data rates, the IEEE developed their 802.11ac Gigabit standard also known as VHT, Very High Throughput the system enables absolute maximum data rates of nearly 7 Gbps with all options running.This will enable those wanting to stream high definition video and many other files to be able to achieve this at the speeds they require.

802.11ac VHT key features

Some of the key or highlight features are tabulated below:

IEEE 802.11AC SALIENT FEATURES
PARAMETER	DETAILS
Frequency band	5.8 GHz ISM (unlicensed) band
Max data rate	6.93 Gbps
Transmission bandwidth	20, 40, & 80 MHz 160 & 80 + 80 MHz optional
Modulation formats	BPSK, QPSK, 16-QAM, 64-QAM 256-QAM optional
FEC coding	Convolutional or LPDC (optional) with coding rates of 1/2, 2/3, 3/4, or 5/6
MIMO	Both single and multi-user MIMO with up to 8 spatial streams.
Beam-forming	Optional

IEEE 802.11ac Gigabit Wi-Fi technologies

The IEEE 802.11ac Gigabit Wi-Fi standard utilises a number of techniques that have been utilised within previous IEEE 802.11 standards and builds on these technologies, while adding some new techniques to ensure that the required throughput can be attained.

·      OFDM: The IEEE 802.11ac standard utilises OFDM that has been very successfully used in previous forms of 802.11.The use of OFDM is particularly applicable to wideband data transmission as it combats some of the problems with selective fading.

Note on OFDM:

Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multi-path effects.

·      MIMO and MU-MIMO:   In order to achieve the required spectral usage figures to attain the data throughput within the available space, the spectral usage figure of 7.5 bps/Hz is required. To achieve this, MIMO is required, and in the case of IEEE 802.11ac Wi-Fi, a form known as Multi-User MIMO, or MU MIMO is implemented.

Note on MIMO:

Two major limitations in communications channels can be multipath interference, and the data throughput limitations as a result of Shannon's Law. MIMO provides a way of utilising the multiple signal paths that exist between a transmitter and receiver to significantly improve the data throughput available on a given channel with its defined bandwidth. By using multiple antennas at the transmitter and receiver along with some complex digital signal processing, MIMO technology enables the system to set up multiple data streams on the same channel, thereby increasing the data capacity of a channel.

MU-MIMO enables the simultaneous transmission of different data frames to different clients. The use of MU-MIMO requires that equipment is able to utilise the spatial awareness of the different remote users. It also needs sophisticated queuing systems that can take advantage of opportunities to transmit to multiple clients when conditions are right.

·      Error correction coding:   The advances in chip manufacturing technology have enabled designers to take advantage of additional levels of processing power when compared to previous implementations of the 802.11 standards. This has enabled the use more sensitive coding techniques that depend on finer distinctions in the received signal. In addition to this more aggressive error correction codes that use fewer check bits for the same amount of data have been utilised within the 802.11ac format

·      Increased channel bandwidth:   The previous versions of 802.11 standards have typically used 20 MHz channels, although 802.11n used up to 40 MHz wide channels. The 802.11ac standard uses channel bandwidths up to 80 MHz as standard with options of 160MHz or two 80MHz blocks. To achieve this it is necessary to adapt automatic radio tuning capabilities so that higher-bandwidth channels are only used where necessary to conserve spectrum

The IEEE 802.11ac VHT Wi-Fi offers significant advantages over the previous incarnations of the 802.11 standard. It offers backwards compatibility with previous versions and this will enable it to be introduced in the existing Wi-Fi ecosystem with the minimum of disruption.

Physical layer RF

The key RF features of the 802.11ac physical layer are tabulated below::

IEEE 802.11AC PHYSICAL LAYER
FEATURE	MANDATORY	OPTIONAL
Channel bandwidth	20MHz, 40 MHz, 80 MHz	160 MHz, 80+80 MHz
FFT size	64, 128, 256	512
Data subcarriers / Pilots	52 / 4, 108 / 6, 234 / 8	468 / 16
Modulation types	BPSK, QPSK, 16-QAM, 64-QAM	256-QAM
Spatial streams & MIMO	1	2 to 8 TX beamforming, STBC Multi-user-MIMO

In terms of the use of MIMO, this standard is able to use up to eight spatial streams as well as using multiple user MIMO where the different streams can be sued to support a number of different users and provides a form of multiple access scheme.

As an example, consider a situation where the transmitter has four antennas. Four streams from each can pass data to the four receive antennas. Alternatively two users can be supported, each with two streams.

It is also worth noting that the top data rate is only achieved using 256-QAM, 160 MHz bandwidth and MIMO with all eight spatial streams..

However, to achieve the highest data rates, reduces the number of channels that are available, even at 5.8 GHz. As a comparison, when using 802.11a, a total of 24 non-overlapping channels is available, but when 802.11ac is used in its high data rates mode, it is only possible to accommodate two 80MHz channels or just one 160 MHz channel.

Spectral mask

Like other transmission standards, 802.11ac is given a spectral mask into which the emitted signals must fall. This spectral mask details the maximum level of spurious signals and noise that are permissible.

The spectral mask differs between the various bandwidths and also according to the offset from the centre frequency.

40 MHz 802.11ac Spectral Mask

80 MHz 802.11ac Spectral Mask

It can be seen that the roll-off from the 0dBr to the -20dBr points still occurs over a 2 MHz bandwidth, the same bandwidth for the 40 MHz mask. This means that in terms of the percentage of the signal bandwidth the roll-off is twice as steep over these points.

Measured with 100 kHz resolution bandwidth and 30 kHz video bandwidth. dBr = dB relative to the maximum spectral density of the signal.

Physical layer frame

As with other 802.11 standards, there is a Physical Layer Convergence Protocol, PLCP and this defines a PLCP Protocol Data Unit, PPDU. For 802.11ac, this has been defined to be backward compatible with 802.11a and 802.11n which may also use the 5.8 GHz unlicensed ISM band.

IEEE 802.11ac PPDU

There are various fields within the frame structure:

·      L-STF: This short training field is two symbols in length and it is transmitted for backwards compatibility with previous versions of 802.11. The field is duplicated over each 20 MHz sub-band with phase rotation. The subcarriers are rotated by 90° or 180° in some sub-bands to reduce the peak to average power ratio.

·      L-LTF: This is a legacy long training field, and is two symbols long. It has many of the same properties as the L-STF including the transmission criteria, being transmitted in sub-bands and those of phase rotation.

·      L-SIG: This field is one symbol long and it is transmitted in BPSK. Like the L-STF and L-LTF it is a legacy field.

·      VHT-SIG-A: This is an 802.11ac field and consists of one symbol transmitted in BPSK and a second in QBPSK, i.e. BPSK rotated by 90°. This mode of transmission enables auto-detection of a VHT transmission. The filed contains information to enable the receiver to correctly interpret the later data packets. Information including he bandwidth, number of MIMO streams, STBC used, guard interval, BCC or LDPC coding, MCS, and beam-forming information.

·      VHT-STF - VHT Short Training Field : This 802.11ac field is one symbol long and is used to improve the gain control estimation for MIMO operation.

·      VHT-LTF - VHT Long Training Field: The long training fields may include 1, 2, 4, 6, or 8 VHT-LTFs. The mapping matrix for 1, 2, or 4 VHT-LTFs is the same as in 802.11n whereas the 6 and 8 VHT-LTF combinations have been added for 802.11ac.

·      VHT-SIG-B: This field details payload data including the length of data and modulation coding scheme for the multi-user mode. Bits are repeated for each 20 MHz sub-band

Applications

With need to increased data in many areas of communications 802.11ac is expected to find many application in a variety of areas. It is likely to be required as more data hungry applications including highly interactive video gaming, video conferencing, high definition video streaming and many more applications need data at rates that push the boundaries of exiting Wi-Fi systems. In view of the speeds attainable, the system is being marketed by one manufacturer as 5G WiFi.

IEEE 802.11ad Microwave Wi-Fi / WiGig Tutorial

The IEEE 802.11ad standard is aimed at providing data throughput speeds of up to 7 Gbps. To achieve these speeds the technology uses the 60 GHz ISM band to achieve the levels of bandwidth needed and ensure reduced interference levels.

Using frequencies in the millimetre range IEEE 802.11ad microwave Wi-Fi has a range that is measured of a few metres. The aim is that it will be used for very short range (across a room) high volume data transfers such as HD video transfers. When longer ranges are needed standards such as 802.11ac can be used.

As part of the marketing, the scheme will be known by the name WiGig after the Wireless Gigabit Alliance that endorses the system.

Wireless Gigabit Alliance

In order to provide the industry support and an easy marketing name, the IEEE and Wireless Gigabit Alliance have worked together on developing the IEEE 802.11ad WiGig standard.

To this end, the WiGig MAC/PHY specification aligns exactly with the 802.11ad standard. This provides industry standardisation, industry recognition, input from industry to ensure that the standard is realisable and also meets the industry needs, and it also provides an easy marketing name.

The Wireless Gigabit Alliance was formed to provide a single multi-gigabit wireless communications standard among consumer electronics, handheld devices and PCs, and drives industry convergence using unlicensed ISM (industrial, scientific and medical) 60 GHz spectrum.

802.11ad salient features

The table below gives a summary of the salient features of 802.11ad.

802.11AD CHARACTERISTIC	DESCRIPTION
Operating frequency range	60 GHz ISM band
Maximum data rate	7 Gbps
Typical distances	1 - 10 m
Antenna technology	Uses beamforming
Modulation formats	Various: single carrier and OFDM

In addition to the tabulated details, the system uses a MAC layer standard that is shared with current 802.11 standards to enable session switching between 802.11 Wi-Fi networks operating in the 2.4 GHz, and 5 GHz bands with those using the 60 GHz WiGig bands. In this way, seamless transition can occur between the systems.

However the 802.11ad MAC layer has been updated to address aspects of channel access, synchronization, association, and authentication required for the 60 GHz operation.

Physical layer

The WLAN system uses frequencies in the 60GHz unlicensed spectrum. Dependent upon geography these are located between 57 GHz and 66GHz. 

60 GHZ GLOBAL ALLOCATIONS
REGION	ALLOCATION (GHZ)
European Union	57.00 - 66.00
USA & Canada	57.05 - 64.00
South Korea	57.00 - 64.00
Japan	59.00 - 66.00
Australia	59.4 - 62.90

The ITU-R then recommends the use of four channels, each 2.16 GHz wide with centre frequencies of 58,32, 60.48, 62.64, and 64.80 GHz. It can therefore be seen that only channel 2 with its centre frequency of 60.48 GHz is globally available. This is recommended to be the default channel.

The signal spectrum and spectral mask needs to ensure that the signal is maintained within a certain bandwidth. The spectral mask shows the mask for the spectrum.

802.11ad Spectral Mask

One of the main forms of modulation used is OFDM. This is a key element of the overall modulation and RF signal format, providing the capability for high data rates while supplying good resilience against multiple paths.

Note on OFDM:

Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multi-path effects.

The 802.11ad PHY supports three main signals with different modulation.

·      Control PHY, CPHY:   Providing control, this signal has high levels of error correction and detection. Accordingly it has a relatively low throughput. As it does not carry the main payload, this is not an issue. It exclusively carriers control channel messages.

The CPHY uses differential encoding, code spreading and BPSK modulation.

·      Single Carrier PHY:   The SCPHY employs single carrier modulation techniques: BPSK, QPSK or 16-QAM on a suppressed carrier located on the channel centre frequency. This single has a fixed symbol rate of 1.76 Gsym/sec. A variety of error coding and error coding modes are available according to the requirements.

·      Orthogonal Frequency Division Multiplex PHY, OFDMPHY:   As with any OFDM scheme, the OFDMPHY uses multicarrier modulation to provide high modulation densities and higher data throughput levels than the single carriermodes.
The modulation format SQPSK is Spread QPSK and involves using paired OFDM carriers onto which the data is modulated. The two carriers are maximally separated to improve the robustness of the signal in the presence of frequency selective fading.

·      Low Power Single Carrier PHY, LPSCPHY:   This 802.11ad signal uses a single carrier as the name implies, and this is to minimise the power consumption. It is intended for small battery devices that may not be able to support the processing required for the OFDM format.

802.11AD MODULATION AND CODING SUMMARY
CONTROL PHY
CODING	MODULATION	IDEAL RAW BIT RATE
1/2 LDPC 32X Spreading	Π/2 DBPSK	27.5 Mbps
SINGLE CARRIER PHY
CODING	MODULATION	IDEAL RAW BIT RATE
1/2 LDPC, 2X repetition 1/2 LDPC 5/8 LDPC, 3/4 LDPC 13/16 LDPC	Π/2 BPSK Π/2 QPSK Π/2 16-QAM	385 Mpbs  to 4620 Mbps
OFDM PHY
CODING	MODULATION	IDEAL RAW BIT RATE
1/2 LDPC 5/8 LDPC, 3/4 LDPC 13/16 LDPC-	OFDM-SQPSK OFDM-QPSK OFDM-16-QAM OFDM-64-QAM	693 Mpbs  to 6756.75 Mbps
LOW POWER SINGLE CARRIER PHY
CODING	MODULATION	IDEAL RAW BIT RATE
RS(224,208) + Block Code (16/12/9/8.8)	Π/2 BPSK Π/2 QPSK	625.6 Mpbs  to 2503 Mbps

802.11ad beam management

One of the features of WiGig microwave Wi-Fi is the aspect of antenna beam management. The very high frequencies used means that the antennas are very small and this makes the development, manufacture and use of the phased arrays required for this a feasible proposition.

The beam-forming is accomplished using a bi-directional training sequence that is appended to each transmission. This enables the system to shape the transmit and / or the receive beams to achieve the optimum link properties. This enables the system to overcome any movement of the transmitter, receiver, or objects between them that might alter the path characteristics.

IEEE 802.11af White-Fi

White-fi is a term being used to describe the use of a Wi-Fi technology within the TV unused spectrum, or TV white space. The IEEE 802.11af working group has been set up to define a standard to implement this.

With a number of administrations around the globe taking a more flexible approach to spectrum allocations, the idea of low power systems that are able to work within portions of spectrum that may need to be kept clear of high power transmitters to ensure coverage areas do not overlap is being seriously investigated.

When using systems like white-fi, IEEE 802.11af that use TV white space, the overall system must not cause interference to the primary users. With processing technology developing further, this is now becoming more of a possibility.

Benefits of IEEE 802.11af, White-Fi

There are many benefits for a system such as IEEE 802.11af from using TV white space. While the exact nature of the IEEE 802.11af system has not been fully defined, it is still possible to see many of the benefits that can be gained:

·         Propagation characteristics:   In view of the fact that the 802.11af white-fi system operating the TV white spaces would use frequencies below 1 GHz, this would allow for greater distances to be achieved. Current Wi-Fi systems use frequencies in the ISM bands - the lowest band is 2.4 GHz and here signals are easily absorbed.

·         Additional bandwidth:   One of the advantages of using TV white space is that additional otherwise unused frequencies can be accessed. However, it will be necessary to aggregate several TV channels to provide the bandwidths that Wi-Fi uses on 2.4 and 5.6 GHz, to achieve the required data throughput rates. It is possible that vacant channels in any given area will vary widely in frequency and this presents some challenges in managing the data sharing across the different channels, although this has been successfully achieved in technologies such as LTE.

Looking at these benefits, it is believed that the White-Fi system offers sufficient advantages to enable development to be undertaken.

IEEE 802.1af white-fi technologies

In order for white-fi 802.11af to be able to operate, it is necessary to ensure that the system does not create any undue interference with existing television transmissions. To achieve this there are a number of technologies and rules that may be utilised.

·         Cognitive radio:   One way in which a white-fi system would be able to operate is to use cognitive radio technology;

Note on Cognitive Radio:

With pressure on radio spectrum increasing all the time, it is necessary to utilise the available spectrum as efficently as possible. One method of helping to achieve this is utlise radio technology that is able to sense the environment and configure itself accordingly - Cognitive Radio. The technology is heavily dependent upon Software Defined Radio technology as the radio needs to be configurable according to the previaling radio environment.

Using this technology, it will be possible for the white-fi, IEEE 802.11af system to detect transmissions and move to alternative channels.

·         Geographic sensing:   Another method that is favoured by many is geographic sensing. Although details are not fully defined, having a geographic database and a knowledge of what channels are available there is another way of allowing the system to avoid used channels.

Salient features

The table below gives a summary of the salient features of 802.11af.

802.11AF CHARACTERISTIC	DESCRIPTION
Operating frequency range	470 - 710MHz
Channel bandwidth	6MHz
Transmission power	20dBm
Modulation format	BPSK
Antenna gain	0dBi

The proposal for the implementation of White-Fi si sill in its draft or development stages. However it provides an effective way or accessing more radio spectrum in an area where available bandwidth is at a premium, and utilising the resource more effectively.

Wi-Fi / WLAN Channels, Frequencies, Bands & Bandwidths

The IEEE 802.11 Wi-Fi / WLAN standards set the attributes for the different channels that may be used.

These attributes enable different Wi-Fi modules to talk to each other and effectively set up a WLAN.

To ensure that WLAN solutions operate satisfactorily, parameters such as the RF signal centre frequencies, channel numbers and the bandwidths must all be set..

ISM bands

Wi-Fi is aimed at use within unlicensed spectrum. This enables users to access the radio spectrum without the need for the regulations and restrictions that might be applicable elsewhere. The downside is that this spectrum is also shared by many other users and as a result the system has to be resilient to interference.

There are a number of unlicensed spectrum bands in a variety of areas of the radio spectrum. Often these are referred to as ISM bands - Industrial, Scientific and Medical, and they carry everything from microwave ovens to radio communications. Many of these bands, including he two used for Wi-Fi are global allocations, although local restrictions may apply for some aspects of their use.

The main bands used for carrying Wi-Fi are those in the table below:

LOWER FREQUENCY MHZ	UPPER FREQUENCY MHZ	COMMENTS
2400	2500	Often referred to as the 2.4 GHz band, this spectrum is the most widely used of the bands available for Wi-Fi. Used by 802.11b, g, & n. It can carry a maximum of three non-overlapping channels.
5725	5875	This 5 GHz band or 5.8 GHz band provides additional bandwidth, and being at a higher frequency, equipment costs are slightly higher, although usage, and hence interference is less.It can be used by 802.11a & n. It can carry up to 23 non-overlapping channels, but gives a shorter range than 2.4 GHz.

2.4 GHz 802.11 channels

There is a total of fourteen channels defined for use by Wi-Fi 802.11 for the 2.4 GHz ISM band. Not all of the channels are allowed in all countries: 11 are allowed by the FCC and used in what is often termed the North American domain, and 13 are allowed in Europe where channels have been defined by ETSI. The WLAN / Wi-Fi channels are spaced 5 MHz apart (with the exception of a 12 MHz spacing between the last two channels).

The 802.11 WLAN standards specify a bandwidth of 22 MHz and a 25 MHz channel separation, although nominal figures for the bandwidth of 20 MHz are often given. The 20 / 22 MHz bandwidth and channel separation of 5 MHz means that adjacent channels overlap and signals on adjacent channels will interfere with each other.

The 22 MHz channel bandwidth holds for all standards even though 802.11b WLAN standard can run at variety of speeds: 1, 2, 5.5, or 11 Mbps and the newer 802.11g standard can run at speeds up to 54 Mbps. The differences occur in the RF modulation scheme used, but the WLAN channels are identical across all of the applicable 802.11 standards.

When using 802.11 Wi-Fi to provide WLAN solutions for offices, general use hotspots, or for any WLAN applications, it is necessary to ensure that parameters such as the channels are correctly set to ensure the required performance is achieved.

2.4 GHz Wi-Fi channel frequencies

The table given below provides the frequencies for the total of fourteen 802.11 Wi-Fi channels that are available around the globe. Not all of these channels are available for use in all countries.

CHANNEL NUMBER	LOWER FREQUENCY MHZ	CENTER FREQUENCY MHZ	UPPER FREQUENCY MHZ
1	2401	2412	2423
2	2404	2417	2428
3	2411	2422	2433
4	2416	2427	2438
5	2421	2432	2443
6	2426	2437	2448
7	2431	2442	2453
8	2436	2447	2458
9	2441	2452	2463
10	2451	2457	2468
11	2451	2462	2473
12	2456	2467	2478
13	2461	2472	2483
14	2473	2484	2495

2.4 GHz WiFi channel overlap and selection

The channels used for WiFi are separated by 5 MHz in most cases but have a bandwidth of 22 MHz. As a result channels overlap and it can be seen that it is possible to find a maximum of three non-overlapping channels. Therefore if there are adjacent pieces of WLAN equipment that need to work on non-interfering channels, there is only a possibility of three. There are five combinations of available non overlapping channels are given below:

Wi-Fi Channel overlap and which ones can be used as sets.

From the diagram above, it can be seen that Wi-Fi channels 1, 6, 11, or 2, 7, 12, or 3, 8, 13 or 4, 9, 14 (if allowed) or 5, 10 (and possibly 14 if allowed) can be used together as sets. Often WiFi routers are set to channel 6 as the default, and therefore the set of channels 1, 6 and 11 is possibly the most widely used.

As some energy spreads out further outside the nominal bandwidth, if only two channels are used, then the further away from each other the better the performance.

It is found that when interference exists, the throughput of the system is reduced. It therefore pays to reduce the levels of interference to improve the overall performance of the WLAN equipment.

With the use of IEEE 802.11n, there is the possibility of using signal bandwidths of either 20 MHz or 40 MHz. When 40 MHz bandwidth is used to gain the higher data throughput, this obviously reduces the number of channels that can be used.

802.11n 40 MHz channel capacity

The diagram above shows the 802.11n 40 MHz signals. These signals are designated with their equivalent centre channel numbers.

2.4 GHz WLAN / Wi-Fi Channel availability

In view of the differences in spectrum allocations around the globe and different requirements for the regulatory authorities, not all the WLAN channels are available in every country. The table below provides a broad indication of the availability of the different Wi-Fi channels in different parts of the world.

CHANNEL NUMBER	EUROPE (ETSI)	NORTH AMERICA  (FCC)	JAPAN
1	✔	✔	✔
2	✔	✔	✔
3	✔	✔	✔
4	✔	✔	✔
5	✔	✔	✔
6	✔	✔	✔
7	✔	✔	✔
8	✔	✔	✔
9	✔	✔	✔
10	✔	✔	✔
11	✔	✔	✔
12	✔	No	✔
13	✔	No	✔
14	No	No	802.11b only

This chart is only provides a general view, and there may be variations between different countries. For example some countries within the European zone Spain have restrictions on the channels that may be used (France: channels 10 - 13 and Spain channels 10 and 11) use of Wi-Fi and do not allow many of the channels that might be thought to be available, although the position is likely to change.

3.6 GHz WiFi band

This band of frequencies is only allowed for use within the USA under a scheme known as 802.11y. Here high powered stations can be used as backhaul for networks, etc.

CHANNEL NUMBER	FREQUENCY (MHZ)	5 MHZ BANDWIDTH	10 MHZ BANDWIDTH	20 MHZ BANDWIDTH
131	3657.5	✔
132	36622.5	✔
132	3660.0		✔
133	3667.5	✔
133	3665.0			✔
134	3672.5	✔
134	3670.0		✔
135	3677.5	✔
136	3682.5	✔
136	3680.0		✔
137	3687.5	✔
137	3685.0			✔
138	3689.5	✔
138	3690.0		✔

Note: the channel centre frequency depends upon the bandwidth used. This accounts for the fact that the centre frequency for various channels is different if different signal bandwidths are used.

5 GHz WiFi channels & frequencies

As the 2.4 GHz band becomes more crowded, many users are opting to use the 5 GHz ISM band. This not only provides more spectrum, but it is not as widely used by Wi-Fi as well as many other appliances including items such as microwave ovens, etc.

It will be seen that many of the 5 GHz Wi-Fi channels fall outside the accepted ISM unlicensed band and as a result various restrictions are placed on operation at these frequencies.

CHANNEL NUMBER	FREQUENCY MHZ	EUROPE (ETSI)	NORTH AMERICA  (FCC)	JAPAN
36	5180	Indoors	✔	✔
40	5200	Indoors	✔	✔
44	5220	Indoors	✔	✔
48	5240	Indoors	✔	✔
52	5260	Indoors / DFS / TPC	DFS	DFS / TPC
56	5280	Indoors / DFS / TPC	DFS	DFS / TPC
60	5300	Indoors / DFS / TPC	DFS	DFS / TPC
64	5320	Indoors / DFS / TPC	DFS	DFS / TPC
100	5500	DFS / TPC	DFS	DFS / TPC
104	5520	DFS / TPC	DFS	DFS / TPC
108	5540	DFS / TPC	DFS	DFS / TPC
112	5560	DFS / TPC	DFS	DFS / TPC
116	5580	DFS / TPC	DFS	DFS / TPC
120	5600	DFS / TPC	No Access	DFS / TPC
124	5620	DFS / TPC	No Access	DFS / TPC
128	5640	DFS / TPC	No Access	DFS / TPC
132	5660	DFS / TPC	DFS	DFS / TPC
136	5680	DFS / TPC	DFS	DFS / TPC
140	5700	DFS / TPC	DFS	DFS / TPC
149	5745	SRD	✔	No Access
153	5765	SRD	✔	No Access
157	5785	SRD	✔	No Access
161	5805	SRD	✔	No Access
165	5825	SRD	✔	No Access

Note 1: there are additional regional variations for countries including Australia, Brazil, China, Israel, Korea, Singapore, South Africa, Turkey, etc. Additionally Japan has access to some channels below 5180 MHz.

Note 2: DFS = Dynamic Frequency Selection; TPC = Transmit Power Control; SRD = Short Range Devices 25 mW max power.

Additional bands and frequencies

In addition to the more established forms of Wi-Fi, new formats are being developed that will use new frequencies and bands. Technologies employing white space usage, etc. and also new standards using bands that are well into the microwave region and will deliver gigabit transfer speeds are being developed and introduced. These technologies will require the use of new spectrum for Wi-Fi.

WI-FI TECHNOLOGY	STANDARD	FREQUENCIES BANDS
White-Fi	802.11af	470 - 710MHz
Microwave Wi-Fi	802.11ad	57.0 - 64.0 GHz ISM band (Regional variations apply) Channels: 58,32, 60.48, 62.64, and 64.80 GHz

As WLAN and Wi-Fi technology develops further new bands will be added to enable sufficient interference bandwidth to be available to ensure the ever increasing requirement for the transfer of high speed data.