Chapter 6. Wireless LAN Standards

6.1. The 802.11 WLAN Standards

6.1.1. Origins and Evolution

The development of wireless LAN standards by the IEEE began in the late 1980s, following the opening up of the three ISM radio bands for unlicensed use by the FCC in 1985, and reached a major milestone in 1997 with the approval and publication of the 802.11 standard. This standard, which initially specified modest data rates of 1 and 2 Mbps, has been enhanced over the years, the many revisions being denoted by the addition of a suffix letter to the original 802.11, as for example in 802.11a, b and g.

The 802.11a and 802.11b extensions were ratified in July 1999, and 802.11b, offering data rates up to 11 Mbps, became the first standard with products to market under the Wi-Fi banner. The 802.11g specification was ratified in June 2003 and raised the PHY layer data rate to 54 Mbps, while offering a degree of interoperability with 802.11b equipment with which it shares the 2.4 GHz ISM band.

Table 6.1summarizes the 802.11 standard’s relentless march through the alphabet, with various revisions addressing issues such as security, local regulatory compliance and mesh networking, as well as other enhancements that will lift the PHY layer data rate to 600 Mbps.

Table 6.1. The IEEE 802.11 standard suite

Standard	Key features
802.11a	High speed WLAN standard, supporting 54 Mbps data rate using OFDM modulation in the 5 GHz ISM band.
802.11b	The original Wi-Fi standard, providing 11 Mbps using DSSS and CCK on the 2.4 GHz ISM band.
802.11d	Enables MAC level configuration of allowed frequencies, power levels and signal bandwidth to comply with local RF regulations, thereby facilitating international roaming.
802.11e	Addresses quality of service (QoS) requirements for all 802.11 radio interfaces, providing TDMA to prioritize and error-correction to enhance performance of delay sensitive applications.
802.11f	Defines recommended practices and an Inter-Access Point Protocol to enable access points to exchange the information required to support distribution system services. Ensures inter-operability of access points from multiple vendors, for example to support roaming.
802.11g	Enhances data rate to 54 Mbps using OFDM modulation on the 2.4 GHz ISM band. Interoperable in the same network with 802.11b equipment.
802.11h	Spectrum management in the 5 GHz band, using dynamic frequency selection (DFS) and transmit power control (TPC) to meet European requirements to minimise interference with military radar and satellite communications.
802.11i	Addresses the security weaknesses in user authentication and encryption protocols. The standard employs advanced encryption standard (AES) and 802.1x authentication.
802.11j	Japanese regulatory extension to 802.11a adding RF channels between 4.9 and 5.0 GHz.
802.11k	Specifies network performance optimization through channel selection, roaming and TPC. Overall network throughput is maximized by efficiently loading all access points in a network, including those with weaker signal strength.
802.11n	Provides higher data rates of 150, 350 and up to 600 Mbps using MIMO radio technology, wider RF channels and protocol stack improvements, while maintaining backward compatibility with 802.11 a, b and g.
802.11p	Wireless access for the vehicular environment (WAVE), providing communication between vehicles or from a vehicle to a roadside access point using the licensed intelligent transportation systems (ITS) band at 5.9 GHz.
802.11r	Enables fast BSS to BSS (Basic Service Set) transitions for mobile devices, to support delay sensitive services such as VoIP on stations roaming between access points.
802.11s	Extending 802.11 MAC to support ESS (Extended Service Set) mesh networking. The 802.11s protocol will enable message delivery over self-configuring multi-hop mesh topologies.
802.11T	Recommended practices on measurement methods, performance metrics and test procedures to assess the performance of 802.11 equipment and networks. The capital T denotes a recommended practice rather than a technical standard.
802.11u	Amendments to both PHY and MAC layers to provide a generic and standardized approach to inter-working with non-802.11 networks, such as Bluetooth, ZigBee and WiMAX.
802.11v	Enhancements to increase throughput, reduce interference and improve reliability through network management.
802.11w	Increased network security by extending 802.11 protection to management as well as data frames.

6.1.2. Overview of the Main Characteristics of 802.11 WLANs

The 802.11 standards cover the PHY and MAC layer definition for local area wireless networking. As shown in Figure 6.1, the upper part of the Data Link layer (OSI Layer 2)is provided by Logical Link Control (LLC) services specified in the 802.2 standard, which are also used by Ethernet (802.3) networks, and provide the link to the Network layer and higher layer protocols. 802.11 networks are composed of three basic components: stations, access points and a distribution system, as described in Table 6.2.

Figure 6.1. 802.11 logical architecture

Table 6.2. 802.11 network components

Component	Description
Station	Any device that implements the 802.11 MAC and PHY layer protocols.
Access point	A station that provides an addressable interface between a set of stations, known as a basic service set (BSS), and the distribution system.
Distribution system	A network component, commonly a wired Ethernet, that connects access points and their associated BSSs to form an extended service set (ESS).

In the 802.11 standard, WLANs are based on a cellular structure where each cell, under the control of an access point, is known as a basic service set (BSS). When a number of stations are working in a BSS it means that they all transmit and receive on the same RF channel, use a common BSSID, use the same set of data rates and are all synchronized to a common timer. These BSS parameters are included in “beacon frames” that are broadcast at regular intervals either by individual stations or by the access point.

The standard defines two modes of operation for a BSS: ad-hoc mode and infrastructure mode. An ad-hoc network is formed when a group of two or more 802.11 stations communicate directly with each other with no access point or connection to a wired network.

This operating mode (also known as peer-to-peer mode) allows wireless connections to be quickly established for data sharing among a group of wireless enabled computers (Figure 6.2). Under ad-hoc mode the service set is called an independent basic service set (IBSS), and in an IBSS all stations broadcast beacon packets, and use a randomly generated BSSID.

Figure 6.2. Ad-hoc mode topology

Infrastructure mode exists when stations are communicating with an access point rather than directly with each other. A home WLAN with an access point and several wired devices connected through an Ethernet hub or switch is a simple example of a BSS in infrastructure mode (Figure 6.3). All communication between stations in a BSS goes through the access point, even if two wireless stations in the same cell need to communicate with each other.

Figure 6.3. Infrastructure mode topology

This doubling-up of communication within a cell (first from sending station to the access point, then from the access point to the destination station) might seem like an unnecessary overhead for a simple network, but among the benefits of using a BSS rather than an IBSS is that the access point can buffer data if the receiving station is in standby mode, temporarily out of range or switched off. In infrastructure mode, the access point takes on the role of broadcasting beacon frames.

The access point will also be connected to a distribution system which will usually be a wired network, but could also be a wireless bridge to other WLAN cells. In this case the cell supported by each access point is a BSS and if two or more such cells exist on a LAN the combined set is known as an extended service set (ESS).

In an ESS, access points (APs) will use the distribution system to transfer data from one BSS to another, and also to enable stations to move from one AP to another without any interruption in service. The transport and routing protocols that operate on the external network have no concept of mobility—of the route to a device changing rapidly—and within the 802.11 architecture the ESS provides this mobility to stations while keeping it invisible to the outside network.

Prior to 802.11k, support for mobility within 802.11 networks was limited to movement of a station between BSSs within a single ESS, so-called BSS transitions. With 802.11k, which will be described further in Section 6.4.3, the roaming of stations between ESSs is supported. When a station is sensed as moving out of range, an access point is able to deliver a site report that identifies alternative access points the station can connect to for uninterrupted service.

6.2. The 802.11 MAC Layer

The MAC layer is implemented in every 802.11 station, and enables the station to establish a network or join a pre-existing network and to transmit data passed down by Logical Link Control (LLC). These functions are delivered using two classes of services, station services and distribution system services, which are implemented by the transmission of a variety of management, control and data frames between MAC layers in communicating stations.

Before these MAC services can be invoked, the MAC first needs to gain access to the wireless medium within a BSS, with potentially many other stations also competing for access to the medium. The mechanisms to efficiently share access within a BSS are described in the next section.

6.2.1. Wireless Media Access

Sharing media access among many transmitting stations in a wireless network is more complex to achieve than in a wired network. This is because a wireless network station is not able to detect a collision between its transmission and the transmission from another station, since a radio transceiver is unable both to transmit and to listen for other stations transmitting at the same time.

In a wired network a network interface is able to detect collisions by sensing the carrier, for example the Ethernet cable, during transmission and ceasing transmission if a collision is detected. This results in a medium access mechanism known as carrier sense multiple access/ collision detection (CSMA/CD).

The 802.11 standard defines a number of MAC layer coordination functions to co-ordinate media access among multiple stations. Media access can either be contention-based, as in the mandatory 802.11 distributed coordination function (DCF), when all stations essentially compete for access to the media, or contention free, as in the optional 802.11 point coordination function (PCF), when stations can be allocated specific periods during which they will have sole use of the media.

The media access method used by the distributed coordination function is carrier sense multiple access/collision avoidance (CSMA/CA), illustrated in Figure 6.4. In this mode a station that is waiting to transmit will sense the medium on the channel being used and wait until the medium is free of other transmissions. Once the medium is free, the station waits a predetermined period (the distributed inter-frame spacing or DIFS).

Figure 6.4. 802.11 CSMA/CA

If the station senses no other transmission before the end of the DIFS period, it computes a random back-off time, between parameter values Cw^min and Cw^max, and commences its transmission if the medium remains free after this time has elapsed. The contention window parameter Cw is specified in terms of a multiple of a slot time that is 20 μs for 802.11b or 9 μs for 802.11a/g networks. The back-off time is randomized so that, if many stations are waiting, they will not all try again at the same time—one will have a shorter back-off and will succeed in starting its transmission. If a station has to make repeated attempts to transmit a packet, the computed back-off period is doubled with each new attempt, up to a maximum value Cw^max defined for each station. This ensures that, when many stations are competing for access, individual attempts are spaced out more widely to minimize repeated collisions.

If another station is sensed transmitting before the end of the DIFS period, this is because a short IFS (SIFS) can be used by a station that is waiting either to transmit certain control frames (CTS or ACK—see Figure 6.5) or to continue the transmission of parts of a data packet that has been fragmented to improve transmission reliability.

Figure 6.5. DCF transmission timing

CSMA/CA is a simple media access protocol that works efficiently if there is no interference and if the data being transmitted across the network is not time critical. In the presence of interference, network throughput can be dramatically reduced as stations continually back-off to avoid collisions or wait for the medium to become idle.

CSMA/CA is a contention-based protocol, since all stations have to compete for access. With the exception of the SIFS mechanism noted above, no priorities are given and, as a result, no quality of service guarantees can be made.

The 802.11 standard also specifies an optional priority based media access mechanism, the point coordination function (PCF) which is able to provide contention free media access to stations with time critical requirements. This is achieved by allowing a station implementing PCF to use an interframe spacing (PIFS) intermediate between SIFS and DIFS, effectively giving these stations higher priority access to the medium. Once the point coordinator has control, it informs all stations of the length of the contention free period, to ensure that stations do not try to take control of the medium during this period. The coordinator then sequentially polls stations, giving any pollable station the opportunity to transmit a data frame.

Although it provides some limited capability for assuring quality of service, the PCF function has not been widely implemented in 802.11 hardware and it is only with the 802.11e enhancements, described below in the Section “Quality of Service (802.11e specification), p. 157”, that quality of service (QoS) and prioritized access are more comprehensively incorporated into the 802.11 standard.

6.2.2. Discovering and Joining a Network

The first step for a newly activated station is to determine what other stations are within range and available for association. This can be achieved by either passive or active scanning.

In passive scanning the new station listens to each channel for a predetermined period and detects beacon frames transmitted by other stations. The beacon frame will provide a time synchronization mark and other PHY layer parameters, such as frequency hopping pattern, to allow the two stations to communicate.

If the new station has been set up with a preferred SSID name for association, it can use active scanning by transmitting a Probe frame containing this SSID and waiting for a Probe Response frame to be returned by the preferred access point. A broadcast Probe frame can also be sent, requesting all access points within range to respond with a Probe Response. This will provide the new station with a full list of access points available. The process of authentication and association can then start—either with the preferred access point or with another access point selected by the new station or by the user from the response list.

6.2.3. Station Services

MAC layer station services provide functions to send and receive data units passed down by the LLC and to implement authentication and security between stations, as described in Table 6.3.

Table 6.3. 802.11 MAC layer station services

Service	Description
Authentication	This service enables a receiving station to authenticate another station prior to association. An access point can be configured for either open system or shared key authentication. Open system authentication offers minimal security and does not validate the identity of other stations—any station that attempts to authenticate will receive authentication. Shared key authentication requires both stations to have received a secret key (e.g. a passphrase) via another secure channel such as direct user input.
Deauthentication	Prior to disassociation, a station will deauthenticate from the station that it intends to stop communication with. Both deauthentication and authentication are achieved by the exchange of management frames between the MAC layers of the two communicating stations.
Privacy	This service enables data frames and shared key authentication frames to be optionally encrypted before transmission, for example using wired equivalent privacy (WEP) or Wi-Fi protected access (WPA).
MAC service data unit delivery	A MAC service data unit (MSDU) is a unit of data passed to the MAC layer by the logical link controller. The point at which the LLC accesses MAC services (at the “top” of the MAC layer) is termed the MAC service access point or SAP. This service ensures the delivery of MSDUs between these service access points. Control frames such as RTS, CTS and ACK may be used to control the flow of frames between stations, for example in 802.11b/g mixed-mode operation.

6.2.4. Distribution System Services

The functionality provided by MAC distribution system services is distinct from station services in that these services extend across the distribution system rather than just between sending and receiving stations at either end of the air interface. The 802.11 distribution system services are described in Table 6.4.

Table 6.4. 802.11 MAC layer distribution system services

Service	Description
Association	This service enables a logical connection to be made between a station and an access point. An access point cannot receive or deliver any data until a station has associated, since association provides the distribution system with the information necessary for delivery of data.
Disassociation	A station disassociates before leaving a network, for example when a wireless link is disabled, the network interface controller is manually disconnected or its host PC is powered down.
Reassociation	The reassociation service allows a station to change the attributes (such as supported data rates) of an existing association or to change its association from one BSS to another within an extended BSS. For example, a roaming station may change its association when it senses another access point transmitting a stronger beacon frame.
Distribution	The distribution service is used by a station to send frames to another station within the same BSS, or across the distribution system to a station in another BSS.
Integration	Integration is an extension of distribution when the access point is a portal to a non-802.11 network and the MSDU has to be transmitted across this network to its destination. The integration service provides the necessary address and media specific translation so that an 802.11 MSDU can be transmitted across the new medium and successfully received by the destination device’s non-802.11 MAC.

6.3. 802.11 PHY Layer

The initial 802.11 standard, as ratified in 1997, supported three alternative PHY layers; frequency hopping and direct sequence spread spectrum in the 2.4 GHz band as well as an infrared PHY. All three PHYs delivered data rates of 1 and 2 Mbps.

The infrared PHY specified a wavelength in the 800–900 nm range and used a diffuse mode of propagation rather than direct alignment of infrared transceivers, as is the case in IrDA for example (Section 10.5). A connection between stations would be made via passive ceiling reflection of the infrared beam, giving a range of 10–20 meters, depending on the height of the ceiling. Pulse position modulation was specified, 16-PPM and 4-PPM, respectively, for the 1 and 2 Mbps data rates.

Later extensions to the standard have focused on high rate DSSS (802.11b), OFDM (802.11a and g) and OFDM plus MIMO (802.11n). These PHY layers will be described in the following sections.

6.3.1. 802.11a PHY Layer

The 802.11a amendment to the original 802.11 standard was ratified in 1999 and the first 802.11a compliant chipsets were introduced by Atheros in 2001. The 802.11a standard specifies a PHY layer based on orthogonal frequency division multiplexing (OFDM) in the 5 GHz frequency range. In the US, 802.11a OFDM uses the three unlicensed national information infrastructure bands (U-NII), with each band accommodating four non-overlapping channels, each of 20-MHz bandwidth. Maximum transmit power levels are specified by the FCC for each of these bands and, in view of the higher permitted power level, the four upper band channels are reserved for outdoor applications.

Table 6.5. US FCC specified U-NII channels used in the 802.11a OFDM PHY

RF Band	Frequency Range (GHz)	Channel number	Centre frequency (GHz)	Maximum transmit power (mW)
U-NII lower band	5.150–5.250	36	5.180	50
		40	5.200
		44	5.550
		48	5.240
U-NII middle band	5.250–5.350	52	5.260	250
		56	5.280
		60	5.300
		64	5.320
U-NII upper band	5.725–5.825	149	5.745	1000
		153	5.765
		157	5.785
		162	5.805

In Europe, in addition to the 8 channels between 5.150 and 5.350 GHz, 11 channels are available between 5.470 and 5.725 GHz (channels 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140). European regulations on maximum power level and indoor versus outdoor use vary from country to country, but typically the 5.15–5.35 GHz band is reserved for indoor use with a maximum EIRP of 200 mW, while the 5.47–5.725 GHz band has an EIRP limit of 1 W and is reserved for outdoor use.

As part of the global spectrum harmonization drive following the 2003 ITU World Radio Communication Conference, the 5.470–5.725 GHz spectrum has also been available in the US since November 2003, subject to the implementation of the 802.11h spectrum management mechanisms described in Section 6.4.2.

Each of the 20 MHz wide channels accommodates 52 OFDM subcarriers, with a separation of 312.5 kHz (=20 MHz/64) between centre frequencies. Four of the subcarriers are used as pilot tones, providing a reference to compensate for phase and frequency shifts, while the remaining 48 are used to carry data.

Four different modulation methods are specified, as shown in Table 6.6, which result in a range of PHY layer data rates from 6 Mbps up to 54 Mbps.

Table 6.6. 802.11a OFDM modulation methods, coding and data rate

Modulation	Code bits per subcarrier	Code bits per OFDM symbol	Coding rate	Data bits per OFDM symbol	Data rate (Mbps)
BPSK	1	48	1/2	24	6
BPSK	1	48	3/4	36	9
QPSK	2	96	1/2	48	12
QPSK	2	96	3/4	72	18
16-QAM	4	192	1/2	96	24
16-QAM	4	192	3/4	144	36
64-QAM	6	288	2/3	192	48
64-QAM	6	288	3/4	216	54

The coding rate indicates the error-correction overhead that is added to the input data stream and is equal to m/(m+n) where n is the number of error correction bits applied to a data block of length m bits. For example, with a coding rate of 3/4 every 8 transmitted bits includes 6 bits of user data and 2 error correction bits.

The user data rate resulting from a given combination of modulation method and coding rate can be determined as follows, taking the 64-QAM, 3/4 coding rate line as an example. During one symbol period of 4 μs, which includes a guard interval of 800 ns between symbols, each carrier is encoded with a phase and amplitude represented by one point on the 64-QAM constellation. Since there are 64 such points, this encodes 6 code bits. The 48 subcarriers together therefore carry 6×48=288 code bits for each symbol period. With a 3/4 coding rate, 216 of those code bits will be user data while the remaining 72 will be error correction bits. Transmitting 216 data bits every 4 μs corresponds to a data rate of 216 data bits per OFDM symbol×250 OFDM symbols per second=54 Mbps.

The 802.11a specifies 6, 12 and 24 Mbps data rates as mandatory, corresponding to 1/2 coding rate for BPSK, QPSK and 16-QAM modulation methods. The 802.11a MAC protocol allows stations to negotiate modulation parameters in order to achieve the maximum robust data rate.

Transmitting at 5 GHz gives 802.11a the advantage of less interference compared to 802.11b, operating in the more crowded 2.4 GHz ISM band, but the higher carrier frequency is not without disadvantages. It restricts 802.11a to near line-of-sight applications and, taken together with the lower penetration at 5 GHz, means that indoors more WLAN access points are likely to be required to cover a given operating area.

6.3.2. 802.11b PHY Layer

The original 802.11 DSSS PHY used the 11-chip Barker spreading code together with DBPSK and DQPSK modulation methods to deliver PHY layer data rates of 1 and 2 Mbps respectively (Table 6.7).

Table 6.7. 802.11b DSSS modulation methods, coding and data rate

Modulation	Code length (Chips)	Code type	Symbol rate (Mbps)	Data bits per symbol	Data rate (Mbps)
BPSK	11	Barker	1	1	1
QPSK	11	Barker	1	2	2
DQPSK	8	CCK	1.375	4	5.5
DQPSK	8	CCK	1.375	8	11

The high rate DSSS PHY specified in 802.11b added complementary code keying (CCK), using 8-chip spreading codes.

The 802.11 standard supports dynamic rate shifting (DRS) or adaptive rate selection (ARS), allowing the data rate to be dynamically adjusted to compensate for interference or varying path losses. When interference is present, or if a station moves beyond the optimal range for reliable operation at the maximum data rate, access points will progressively fall back to lower rates until reliable communication is restored.

Conversely, if a station moves back within range for a higher rate, or if interference is reduced, the link will shift to a higher rate. Rate shifting is implemented in the PHY layer and is transparent to the upper layers of the protocol stack.

The 802.11 standard specifies the division of the 2.4 GHz ISM band into a number of overlapping 22 MHz channels. The FCC in the US and the ETSI in Europe have both authorized the use of spectrum from 2.400 to 2.4835 GHz, with 11 channels approved in the US and 13 in (most of) Europe. In Japan, channel 14 at 2.484 GHz is also authorized by the ARIB. Some countries in Europe have more restrictive channel allocations, notably France where only four channels (10 through 13) are approved. The available channels for 802.11b operation are summarized in Table 6.8.

Table 6.8. International channel availability for 802.11b networks in the 2.4 GHz band

Channel number	Center frequency (GHz)	Geographical usage
1	2.412	US, Canada, Europe, Japan
2	2.417	US, Canada, Europe, Japan
3	2.422	US, Canada, Europe, Japan
4	2.427	US, Canada, Europe, Japan
5	2.432	US, Canada, Europe, Japan
6	2.437	US, Canada, Europe, Japan
7	2.442	US, Canada, Europe, Japan
8	2.447	US, Canada, Europe, Japan
9	2.452	US, Canada, Europe, Japan
10	2.457	US, Canada, Europe, Japan, France
11	2.462	US, Canada, Europe, Japan, France
12	2.467	Europe, Japan, France
13	2.472	Europe, Japan, France
14	2.484	Japan

The 802.11b standard also includes a second, optional modulation and coding method, packet binary convolutional coding (PBCC™–Texas Instruments), which offers improved performance at 5.5 and 11 Mbps by achieving an additional 3 dB processing gain. Rather than the 2 or 4 phase states or phase shifts used by BPSK/DQSK, PBCC uses 8-PSK (8 phase states) giving a higher chip per symbol rate. This can be translated into either a higher data rate for a given chipping code length, or a higher processing gain for a given data rate, by using a longer chipping code.

6.3.3. 802.11g PHY Layer

The 802.11g PHY layer was the third 802.11 standard to be approved by the IEEE standards board and was ratified in June 2003. Like 802.11b, 11g operates in the 2.4 GHz band, but increases the PHY layer data rate to 54 Mbps, as for 802.11a.

The 802.11g uses OFDM to add data rates from 12 Mbps to 54 Mbps, but is fully backward compatible with 802.11b, so that hardware supporting both standards can operate in the same 2.4 GHz WLAN. The OFDM modulation and coding scheme is identical to that applied in the 802.11a standard, with each 20 MHz channel in the 2.4 GHz band (as shown in Table 6.8) divided into 52 subcarriers, with 4 pilot tones and 48 data tones. Data rates from 6 to 54 Mbps are achieved using the same modulation methods and coding rates shown for 802.11a in Table 6.6.

Although 802.11b and 11g hardware can operate in the same WLAN, throughput is reduced when 802.11b stations are associated with an 11g network (so-called mixed-mode operation) because of a number of protection mechanisms to ensure interoperability, as described Table 6.9.

Table 6.9. 802.11b/g mixed-mode interoperability mechanisms

Mechanism	Description
RTS/CTS	Before transmitting, 11b stations request access to the medium by sending a request to send (RTS) message to the access point. Transmission can commence on receipt of the clear to send (CTS) response. This avoids collisions between 11b and 11g transmissions, but the additional RTS/CTS signaling adds a significant overhead that decreases network throughput.
CTS to self	The CTS to self option dispenses with the exchange of RTS/CTS messages and just relies on the 802.11b station to check that the channel is clear before transmitting. Although this does not provide the same degree of collision avoidance, it can increase throughput significantly when there are fewer stations competing for medium access.
Backoff time	802.11g back-off timing is based on the 802.11a specification (up to a maximum of 15×9 μs slots) but in mixed-mode an 802.11g network will adopt 802.11b backoff parameters (maximum 31×20 μs slots). The longer 802.11b backoff results in reduced network throughput.

The impact of mixed mode operation on the throughput of an 802.11g network is shown in Table 6.10.

Table 6.10. PHY and MAC SAP throughput comparison for 802.11a, b and g networks

Network standard and configuration	PHY data rate (Mbps)	Effective MAC SAP throughput (Mbps)	Effective Throughput versus 802.11b (%)
802.11b network	11	6	100
802.11g network with 802.11b stations (CTS/RTS)	54	8	133
802.11g network with 802.11b stations (CTS-to-self)	54	13	217
802.11g network with no 802.11b stations	54	22	367
802.11a	54	25	417

A number of hardware manufacturers have introduced proprietary extensions to the 802.11g specification to boost the data rate above 54 Mbps. An example is D-Link’s proprietary “108G” which uses packet bursting and channel bonding to achieve a PHY layer data rate of 108 Mbps. Packet bursting, also known as frame bursting, bundles short data packets into fewer but larger packet to reduce the impact of gaps between transmitted packets.

Packet bursting as a data rate enhancement strategy runs counter to packet fragmentation as a strategy for improving transmission robustness, so packet bursting will only be effective when interference or high levels of contention between stations are absent.

Channel bonding is a method where multiple network interfaces in a single machine are used together to transmit a single data stream. In the 108G example, two non-overlapping channels in the 2.4 GHz ISM band are used simultaneously to transmit data frames.

6.3.4. Data Rates at the PHY and MAC Layer

In considering the technical requirements for a WLAN implementation, it will be important to recognize the difference between the headline data rate of a wireless networking standard and the true effective data rate as seen by the higher OSI layers when passing data packets down to the MAC layer.

Each “raw” data packet passed to the MAC service access point (MAC SAP) will acquire a MAC header and a message integrity code and additional security related header information before being passed to the PHY layer for transmission. The headline data rate, for example 54 Mbps for 802.11a or 11g networks, measures the transmission rate of this extended data stream at the PHY layer.

The effective data rate is the rate at which the underlying user data is being transmitted if all the transmitted bits relating to headers, integrity checking and other overheads are ignored. For example, on average, every 6 bits of raw data passed to the MAC SAP of an 802.11b WLAN will gain an extra 5 bits of overhead before transmission, reducing a PHY layer peak data rate of 11 Mbps to an effective rate of 6 Mbps.

Table 6.10 shows the PHY and MAC SAP data rates for 802.11 WLANs. For 802.11g networks the MAC SAP data rate depends on the presence of 802.11b stations, as a result of the mixed-mode media access control mechanisms described in the previous section.

6.4. 802.11 Enhancements

In the following sections some of the key enhancements to 802.11 network capabilities and performance will be described.

6.4.1. Quality of Service (802.11e Specification)

The 802.11e specification provides a number of enhancements to the 802.11 MAC to improve the quality of service for time sensitive applications, such as streaming media and voice over wireless IP (VoWIP), and was approved for publication by the IEEE Standards Board in September 2005.

The 802.11e specification defines two new coordination functions for controlling and prioritizing media access, which enhance the original 802.11 DCF and PCF mechanisms described previously in Section 6.2.1. Up to eight traffic classes (TC) or access categories (AC) are defined, each of which can have specific QoS requirements and receive specific priority for media access.

The simplest of the 802.11e coordination functions is enhanced DCF (EDCF) which allows several MAC parameters determining ease of media access to be specified per traffic class. An arbitrary interframe space (AIFS) is defined which is equal to DIFS for the highest priority traffic class and longer for other classes. This provides a deterministic mechanism for traffic prioritization as shown in Figure 6.6.

Figure 6.6. EDCF timing

The minimum back-off time Cw^min is also TC dependent, so that, when a collision occurs, higher priority traffic, with a lower Cw^min, will have a higher probability of accessing the medium.

Each station maintains a separate queue for each TC (Figure 6.7), and these behave as virtual stations, each with their individual MAC parameters. If two queues within a station reach the end of their back-off periods at the same time, data from the higher priority queue will be transmitted when the station gains access to the medium.

Figure 6.7. EDCF traffic class queues

Although the EDCF coordination mode does not provide a guaranteed service for any TC, it has the advantage of being simple to configure and implement as an extension of DCF.

The second enhancement defines a new hybrid coordination function (HCF) which complements the polling concept of PCF with an awareness of the QoS requirements of each station. Stations report the lengths of their queues for each traffic class and the hybrid coordinator uses this to determine which station will receive a transmit opportunity (TXOP) during a contention free transmission period. This HCF controlled channel access (HCCA) mechanism considers several factors in determining this allocation;

• The priority of the TC
• The QoS requirements of the TC (bandwidth, latency and jitter)
• Queue lengths per TC and per station
• The duration of the TXOP available to be allocated
• The past QoS given to the TC and station.

HCCA allows applications to schedule access according to their needs, and therefore enables QoS guarantees to be made. Scheduled access requires a client station to know its resource requirements in advance and scheduling concurrent traffic from multiple stations also requires the access point to make certain assumptions regarding data packet sizes, data transmission rates and the need to reserve surplus bandwidth for transmission retries.

The Wi-Fi Alliance adopted a subset of the 802.11e standard in advance of the IEEE’s September 2005 approval. This subset, called Wi-Fi multimedia (WMM), describes four access categories as shown in Table 6.11, with EDCF timings as shown in Figure 6.8.

Table 6.11. WMM access category descriptions

Access category	Description
WMM voice priority	Highest priority. Allows multiple concurrent VoWLAN calls, with low latency and quality equal to a toll voice call.
WMM video priority	Prioritizes video traffic above lower categories. One 802.11g or 802.11a channel can support 3 to 4 standard definition TV streams or 1 high definition TV stream.
WMM best effort priority	Traffic from legacy devices, from applications or devices that lack QoS capabilities, or traffic such as internet surfing that is less sensitive to latency but is affected by long delays.
WMM background priority	Low priority traffic, such as a file download or print job, that does not have strict latency or throughput requirements.

Figure 6.8. AIFS and back-off timing per WMM traffic class

The prioritization mechanism certified in WMM is equivalent to the EDCF coordination mode defined in 802.11e but did not initially include the scheduled access capability available through HCF and HCCA. This and other 802.11e capabilities are planned to be progressively included in the Wi-Fi Alliance’s WMM certification program.

6.4.2. Spectrum Management at 5 GHz (802.11h)

The 802.11h standard supplements the 802.11 MAC with two additional spectrum management services, transmit power control (TPC), which limits the transmitted power to the lowest level needed to ensure adequate signal strength at the farthest station, and dynamic frequency selection (DFS), which enables a station to switch to a new channel if it detects other non-associated stations or systems transmitting on the same channel.

These mechanisms are required for 5 GHz WLANs operating under European regulations, in order to minimize interference with satellite communications (TPC) and military radar (DFS), and support for the 802.11h extensions was required from 2005 for all 802.11a compliant systems operating in Europe.

In the US, compliance with 802.11h is also required for 802.11a products operating in the 12 channels from 5.47 to 5.725 GHz. IEEE 802.11h compliant networks therefore have access to 24 non-overlapping OFDM channels, resulting in a potential doubling of overall network capacity.

6.4.2.1. Transmit Power Control

An 802.11h compliant station indicates its transmit power capability, including minimum and maximum transmit power levels in dBm, in the association or reassociation frame sent to an access point. An access point may refuse the association request if the station’s transmit power capability is unacceptable, for example if it violates local constraints. The access point in return indicates local maximum transmit power constraints in its beacon and probe request frames.

An access point monitors signal strength within its BSS by requesting stations to report back the link margin for the frame containing the report request and the transmit power used to transmit the report frame back to the access point. This data is used by the access point to estimate the path loss to other stations and to dynamically adjust transmit power levels in its BSS in order to reduce interference with other devices while maintaining sufficient link margin for robust communication.

6.4.2.2. Dynamic Frequency Selection

When a station uses a Probe frame to identify access points in range, an access point will specify in the Probe Response frame that it uses DFS. When a station associates or re-associates with an access point that uses DFS, the station provides a list of supported channels that enables the access point to determine the best channel when a shift is required. As for TPC, an access point may reject an association request if a station’s list of supported channels is considered unacceptable, for example if it is too limited.

To determine if other radio transmissions are present, either on the channel in use or on a potential new channel, an access point sends a measurement request to one or more stations identifying the channel where activity is to be measured, and the start time and duration of the measurement period. To enable these measurements, the access point can specify a quiet period in its beacon frames to ensure that all other associated stations stop transmission during the measurement period. After performing the requested measurement, stations send back a report on the measured channel activity to the access point.

When necessary, channel switching is initiated by the access point, which sends a channel switch announcement management frame to all associated stations. This announcement identifies the new channel, specifies the number of beacon periods until the channel switch takes effect, and also specifies whether or not further transmissions are allowed on the current channel. The access point can use the short interframe spacing (SIFS—see Section 6.2.1) to gain priority access to the wireless medium in order to broadcast a channel switch announcement.

Dynamic frequency selection is more complicated in an IBSS (ad-hoc mode) as there is no association process during which supported channel information can be exchanged, and no access point to coordinate channel measurement or switching. A separate DFS owner service is defined in 802.11h to address these complications, although channel switching remains inherently less robust in an IBSS than in an infrastructure mode BSS.

6.4.3. Network Performance and Roaming (802.11k and 802.11r)

A client station may need to make a transition between WLAN access points for one of three reasons, as described in Table 6.12.

Table 6.12. Reasons for roaming in a WLAN

Roaming need	Description
Mobile client station	A mobile client station may move out of range of its current access point and need to transition to another access point with a higher signal strength.
Service availability	The QoS available at the current access point may either deteriorate or may be inadequate for a new service requirement, for example if a VoWLAN application is started.
Load balancing	An access point may redirect some associated clients to another available access point in order to maximize the use of available capacity within the network.

The 802.11 Task Groups TGk and TGr are addressing issues relating to handoffs or transitions between access points that need to be fast and reliable for applications such as VoWLAN. TGk will standardize radio measurements and reports that will enable location-based services, such as a roaming station’s choice of a new access point to connect to, while TGr aims to minimize the delay and maintain QoS during these transitions.

6.4.3.1. 802.11k; Radio Resource Measurement Enhancements

The 802.11 Task Group TGk, subtitled Radio Resource Measurement Enhancements, began meeting in early 2003 with the objective of defining radio and network information gathering and reporting mechanisms to aid the management and maintenance of wireless LANs.

The 11k supplement will be compatible with the 802.11 MAC as well as implementing all mandatory parts of the 802.11 standards and specifications, and targets improved network management for all 802.11 networks. The key measurements and reports defined by the supplement are as follows;

• Beacon reports
• Channel reports
• Hidden station reports
• Client station statistics
• Site reports.

IEEE 802.11k will also extend the 802.11h TPC to cover other regulatory requirements and frequency bands.

Stations will be able to use these reports and statistics to make intelligent roaming decisions, for example eliminating a candidate access point if a high level of non-802.11 energy is detected in the channel being used. The 802.11k supplement only addresses the measurement and reporting of this information and does not address the processes and decisions that will make use of the measurements.

The three roaming scenarios described above will be enabled by the TGk measurements and reports, summarized in Table 6.13.

Table 6.13. 802.11k measurements and reports

802.11k feature	Description
Beacon report	Access points will use a beacon request to ask a station to report all the access point beacons it detects on a specified channel. Details such as supported services, encryption types and received signal strength will be gathered.
Channel reports (noise histogram, medium sensing time histogram report and channel load report)	Access points can request stations to construct a noise histogram showing all non-802.11 energy detected on a specified channel, or to report data about channel loading (how long a channel was busy during a specified time interval as well as the histogram of channel busy and idle times).
Hidden station report	Under 802.11k, stations will maintain lists of hidden stations (stations that they can detect but are not detected by their access point). Access points can request a station to report this list and can use the information as input to roaming decisions.
Station statistic report and frame report	802.11k access points will be able to query stations to report statistics such as the link quality and network performance experienced by a station, the counts of packets transmitted and received, and transmission retries.
Site report	A station can request an access point to provide a site report—a ranked list of alternative access points based on an analysis of all the data and measurements available via the above reports.

For example, a mobile station experiencing a reduced RSSI will request a neighbor report from its current access point that will provide information on other access points in its vicinity. A smart roaming algorithm in the mobile station will then analyze channel conditions and the loading of candidate access points and select a new access point that is best able to provide the required QoS.

Once a new access point has been selected, the station will perform a BSS transition by disassociating from the current access point and associating with the new one, including authentication and establishing the required QoS.

6.4.3.2. 802.11r; Fast BSS Transitions

The speed and security of transitions between access points will be further enhanced by the 802.11r specification which is also under development and is intended to improve WLAN support for mobile telephony via VoWLAN. IEEE 802.11r will give access points and stations the ability to make fast BSS to BSS transitions through a four-step process;

• Active or passive scanning for other access points in the vicinity,
• Authentication with one or more target access points,
• Reassociation to establish a connection with the target access point, and
• Pairwise temporal key (PTK) derivation and 802.1x based authentication via a 4-way handshake, leading to re-establishment of the connection with continuous QoS through the transition.

A key element of the process of associating with the new access point will be a pre-allocation of media reservations that will assure continuity of service—a station will not be in the position of having jumped to a new access point only to find it is unable to get the slot time required to maintain a time critical service.

The 802.11k and 802.11r enhancements address roaming within 802.11 networks, and are a step towards transparent roaming between different wireless networks such as 802.11, 3G and WiMAX. The IEEE 802.21 media independent handover (MIH) function will eventually enable mobile stations to roam across these diverse wireless networks.

6.4.4. MIMO and Data Rates to 600 Mbps (802.11n)

The IEEE 802.11 Task Group TGn started work during the second half of 2003 to respond to the demand for further increase in WLAN performance, and aims to deliver a minimum effective data rate of 100 Mbps through modifications to the 802.11 PHY and MAC layers.

This target data rate, at the MAC service entry point (MAC SAP), will require a PHY layer data rate in excess of 200 Mbps, representing a fourfold increase in throughput compared to 802.11a and 11g networks. Backward compatibility with 11a/b/g networks will ensure a smooth transition from legacy systems, without imposing excessive performance penalties on the high rate capable parts of a network.

Although there is still considerable debate among the supporters of alternative proposals, the main industry group working to accelerate the development of the 802.11n standard is the enhanced wireless consortium (EWC) which published Rev 1 of its MAC and PHY proposals in September 2005. The following description is based on the EWC proposals.

The two key technologies that are expected to be required to deliver the aspired 802.11n data rate are multi-input multi-output (MIMO) radio and OFDM with extended channel bandwidths.

MIMO radio, discussed in previous chapters, is able to resolve information transmitted over several signal paths using multiple spatially separated transmitter and receiver antennas. The use of multiple antennas provides an additional gain (the diversity gain) that increases the receiver’s ability to decode the data stream.

The extension of channel bandwidths, most likely by the combination of two 20 MHz channels in either the 2.4 GHz or 5 GHz bands, will further increase capacity since the number of available OFDM data tones can be doubled.

To achieve a 100 Mbps effective data rate at the MAC SAP it is expected to require either a 2 transmitter×2 receiver antenna system operating over a 40 MHz bandwidth or a 4×4 antenna system operating over 20 MHz, with respectively 2 or 4 spatially separated data streams being processed. In view of the significant increase in hardware and signal processing complexity in going from 2 to 4 data streams, the 40 MHz bandwidth solution is likely to be preferred where permitted by local spectrum regulations. To ensure backward compatibility, a PHY operating mode will be specified in which 802.11a/g OFDM is used in either the upper or lower 20 MHz of a 40 MHz channel.

Maximizing data throughput in 802.11n networks will require intelligent mechanisms to continuously adapt parameters such as channel bandwidth and selection, antenna configuration, modulation scheme and coding rate, to varying wireless channel conditions.

A total of 32 modulation and coding schemes are initially specified, in four groups of eight, depending on whether one to four spatial streams are used. Table 6.14 shows the modulation and coding schemes for the highest rate case—four spatial streams operating over 40 MHz bandwidth providing 108 OFDM data tones. For fewer spatial streams, the data rates are simply proportional to the number of streams.

Table 6.14. 802.11n OFDM modulation methods, coding and data rate

Modulation	Code bits per subcarrier (per stream)	Code bits per symbol (all streams)	Coding rate	Data bits per symbol (all streams)	Data rate (Mbps)
BPSK	1	432	1/2	216	54
QPSK	2	864	1/2	432	108
QPSK	2	864	3/4	648	162
16-QAM	4	1728	1/2	864	216
16-QAM	4	1728	3/4	1296	324
64-QAM	4	2592	2/3	1728	432
64-QAM	6	2592	3/4	1944	486
64-QAM	6	2592	5/6	2160	540

As for 802.11a/g, these data rates are achieved with a symbol period of 4.0 μs. A further data rate increase of 10/9 (e.g., from 540 to 600 Mbps) is achieved in an optional short guard interval mode, which reduces the symbol period to 3.6 μs by halving the inter-symbol guard interval from 800 ns to 400 ns.

MAC framing and acknowledgement overheads will also need to be reduced in order to increase MAC efficiency (defined as the effective data rate at the MAC SAP as a fraction of the PHY layer data rate). With the current MAC overhead, a PHY layer data rate approaching 500 Mbps would be required to deliver the target 100 Mbps data rate at the MAC SAP.

6.4.5. Mesh Networking (802.11s)

As described in Section 6.1, the 802.11 topology relies on a distribution system (DS) to link BSSs together to form an ESS. The DS is commonly a wired Ethernet linking access points (Figure 6.3), but the 802.11 standard also provides for a wireless distribution system between separated Ethernet segments by defining a four-address frame format that contains source and destination station addresses as well as the addresses of the two access points that these stations are connected to, as shown in Figure 6.9.

Figure 6.9. Wireless distribution system based on four-address format MAC frame

The objective of the 802.11s Task Group, which began working in 2004, is to extend the 802.11 MAC as the basis of a protocol to establish a wireless distribution system (WDS) that will operate over self-configuring multi-hop wireless topologies, in other words an ESS mesh.

An ESS mesh is a collection of access points, connected by a WDS, that automatically learns about the changing topology and dynamically re-configures routing paths as stations and access points join, leave and move within the mesh. From the point of view of an individual station and its relationship with a BSS and ESS, an ESS mesh is functionally equivalent to a wired ESS.

Two industry alliances emerged during 2005 to promote alternative technical proposals for consideration by TGs; the Wi-Mesh Alliance and SEEMesh (for Simple, Efficient and Extensible Mesh).

The main elements of the Wi-Mesh proposal are a mesh coordination function (MCF) and a distributed reservation channel access protocol (DRCA) to operate alongside the HCCA and EDCA protocols (Figure 6.10). Some of the key features of the proposed Wi-Mesh MCF are summarized in Table 6.15.

Figure 6.10. Wi-mesh logical architecture

Table 6.15. Wi-Mesh mesh coordination function (MCF) features

Wi-Mesh MCF feature	Description
Media access coordination across multiple nodes	Media access coordination in a multi-hop network to avoid performance degradation and meet QoS guarantees.
Support for QoS	Traffic prioritization within the mesh; flow control over multi-hop paths; load control and contention resolution mechanisms.
Efficient RF frequency and spatial reuse	To mitigate performance loss resulting from hidden and exposed stations, and allow for concurrent transmissions to enhance capacity.
Scalability	Enabling different network sizes, topologies and usage models.
PHY independent	Independent of the number of radios, channel quality, propagation environment and antenna arrangement (including smart antennas).

The final ESS mesh specification is likely to include prioritized traffic handling based on 802.11e QoS mechanisms as well as security features and enhancements to the 802.11i standard.

The evolution of 802.11 security will be fully described in later chapters, but mesh networking introduces some security considerations in addition to those that have been progressively solved for non-mesh WLANs by WEP, WPA, WPA2 and 802.11i. In a mesh network additional security methods are needed to identify nodes that are authorized to perform routing functions, in order to ensure a secure link for routing information messages. This will be more complicated to achieve in a mesh, where there will commonly be no centralized authentication server.

The work of the 802.11s TG is still in progress, and ratification of the final accepted proposal is not expected before 2008.

6.5. Other WLAN Standards

Although the wireless LAN landscape is now comprehensively dominated by the 802.11 family of standards, there was a brief period in the evolution of WLAN standards when that dominance was far from assured. From 1998 to 2000, equipment based on alternative standards briefly held sway. This short reign was brought to an end by the rapid market penetration of 802.11b products, with 10 million 802.11b based chipsets being shipped between 1999 and end-2001. The HomeRF and HiperLAN standards, which are now of mainly historical interest, are briefly described in the following sections.

6.5.1. HomeRF

The Home Radio Frequency (HomeRF) Working Group was formed in 1998 by a group of PC, consumer electronics and software companies, including Compaq, HP, IBM, Intel, Microsoft and Motorola, with the aim of developing a wireless network for the home networking market. The Working Group developed the specification for SWAP—Shared Wireless Access Protocol—which provided wireless voice and data networking.

SWAP was derived from the IEEE’s 802.11 and ETSI’s DECT (digitally enhanced cordless telephony) standards and includes MAC and PHY layer specifications with the main characteristics summarized in Table 6.16.

Table 6.16. Main characteristics of the HomeRF SWAP

SWAP specification	Main characteristics
MAC	TDMA for synchronous data traffic—up to 6 TDD voice conversations.
	CSMA/CA for asynchronous data traffic, with prioritization for streaming data.
	CSMA/CA and TDMA periods in a single SWAP frame.
PHY	FHSS radio in the 2.4 GHz ISM band. 50–100 hops per second. 2- and 4-FSK modulation deliver PHY layer data rates of 0.8 and 1.6 Mbps.

Although the HomeRF Working Group claimed some early market penetration of SWAP based products, by 2001, as SWAP 2.0 was being introduced with a 10 Mbps PHY layer data rate, the home networking market had been virtually monopolized by 802.11b products. The Working Group was finally disbanded in January 2003.

6.5.2. HiperLAN/2

HiperLAN stands for high performance radio local area network and is a wireless LAN standard that was developed by the European Telecommunications Standards Institute’s Broadband Radio Access Networks (BRAN) project. The HiperLAN/2 Global Forum was formed in September 1999 by Bosch, Ericsson, Nokia and others, as an open industry forum to promote HiperLAN/2 and ensure completion of the standard.

The HiperLAN/2 PHY layer is very similar to the 802.11a PHY, using OFDM in the 5 GHz band to deliver a PHY layer data rate of up to 54 Mbps. The key difference between 802.11a and HiperLAN/2 is at the MAC layer where, instead of using CSMA/CA to control media access, HiperLAN/2 uses time division multiple access (TDMA). Aspects of these two access methods are compared in Table 6.17.

Table 6.17. CSMA/CA and TDMA media access compared

Media access method	Characteristics
CSMA/CA	Contention based access, collisions or interference result in indefinite back-off.
	QoS to support synchronous (voice and video) traffic only introduced with 802.11e.
	MAC efficiency reduced (54 Mbps at PHY=ca. 25 Mbps at MAC SAP).
TDMA	Dynamically assigned time slot based on a station’s throughput.
	Support for synchronous traffic.
	Higher MAC efficiency (54 Mbps at PHY=ca. 40 Mbps at MAC SAP).
	Ability to interface with 3G as well as IP networks.

The technical advantages of HiperLAN/2, namely QoS, European compatibility and higher MAC SAP data rate, have now to a large extent been superseded by 802.11 updates, such as the QoS enhancements introduced in 802.11e (see Section 6.4.1) and the 802.11h PHY layer enhancements specifically introduced to cater for European regulatory requirements (see Section 6.4.2). As a result, the previous support for HiperLAN/2 in the European industry has virtually disappeared.

Given the overwhelming industry focus on products based on the 802.11 suite of standards, it seems unlikely that HiperLAN/2 will ever establish a foothold in the wireless LAN market, the clearest indication of this being perhaps that Google News returns zero hits for HiperLAN/2!

6.6. Summary

Since the ratification of 802.11b in July 1999, the 802.11 standard has established a dominant and seemingly unassailable position as the basis of WLAN technology. The various 802.11 specifications draw on a wide range of applicable techniques, such as the modulation and coding schemes shown in Table 6.18, and continue to motivate the further development and deployment of new technologies, such as MIMO radio and the coordination and control functions required for mesh networking.

Table 6.18. 802.11a, b and g mandatory and optional modulation and coding schemes

Rate (Mbps)	802.11a		802.11b		802.11g
	Mandatory	Optional	Mandatory	Optional	Mandatory	Optional
1 & 2			Barker		Barker
5.5 & 11			CCK	PBCC	CCK	PBCC
6, 12 & 24	OFDM				OFDM	CCK-OFDM
9 & 18		OFDM				OFDM, CCK-OFDM
22 & 33						PBCC
36, 48 & 54		OFDM				OFDM, CCK-OFDM

As future 802.11 Task Groups make a second pass through the alphabet, the further enhancement of WLAN capabilities will no doubt continue to present a rich and fascinating tapestry of technical developments.

This chapter has provided a grounding in the technical aspects and capabilities of current wireless LAN technologies. We will build on that foundation in future chapters.