IEEE 802.11 WLAN Standards
The IEEE 802.11 committee is responsible for ‘Wireless Local Area Network (WLAN)’ standards. WLANs include IEEE 802.11a (WiFi 5), IEEE 802.11b (WiFi), IEEE 802.11g and IEEE 802.11n.
i. The objective of the IEEE 802.11 standard was to define a medium access control (MAC) sublayer, MAC management protocols and services, and three PHYs for wireless connectivity of fixed, portable, and moving devices within a local area.
ii. The IEEE 802.11 WLAN is designed to support a network where most decision making is distributed to mobile stations.
iii. This type of architecture has several advantages. It is tolerant of faults in all of the WLAN equipment and eliminates possible bottlenecks a centralized architecture would introduce.
Fig6. OSI Model for IEEE 802.11 WLAN
1. 802.11 Physical layer
i. The physical layer provides three levels of functionality. These include: (1) frame exchange between the MAC and PHY under the control of the physical layer convergence procedure (PLCP) sublayer; (2) use of signal carrier and spread spectrum (SS) modulation to transmit data frames over the media under the control of the physical medium dependent (PMD) sublayer; and (3) providing a carrier sense indication back to the MAC to verify activity on the media.
ii. The three physical layers are an IR baseband PHY, an FHSS radio in the 2.4 GHz band, and a DSSS radio in the 2.4 GHz. All three physical layers support both 1 and 2 Mbps operations.
iii. Each of the physical layers is unique in terms of the modulation type, designed to coexist with each other and operate with the MAC.
iv. The specifications for IEEE 802.11 meet the RF emissions guidelines of FCC, ETSI, and the Ministry of Telecommunications.
v. In DSSS PHY, two modulation schemes, differential binary phase shift keying (DBPSK) - for 1 Mbps and differential quadrature phase shift keying (DQPSK) - for 2 Mbps are available.
An 11-bit Barker code is used for spreading.
Each DSSS PHY channel occupies 22 MHz of bandwidth and allows for three non-interfering channels spaced 25 MHz apart in the 2.4 GHz frequency band. Fourteen frequency channels are defined for operation across the 2.4 GHz frequency band.
In FHSS PHY, a set of hop sequences is defined for use in the 2.4 GHz frequency band.
The channels are evenly spaced across the band over a span of 83.5 MHz.
In North America, the number of hop channels is 79.
The hop channels occupy a bandwidth of 1MHz.
2. 802.11a
i. It defines ‘Orthogonal Frequency Division Multiplexing (OFDM)’ scheme for modulation at the physical layer.
ii. The idea is to divide a channel into sub-channels, thus using multiple carriers. The modulation on each carrier is independent of each other.
iii. The OFDM PHY provides the capability to transmit PSDU frames at multiple data rates up to 54 Mbps for a WLAN where the transmission of multimedia content is a consideration.
iv. There are 48 data subcarriers and 4 carrier pilot subcarriers for a total of 52 nonzero subcarriers defined in IEEE 802.11a. Each lower data rate bit stream is used to modulate a separate subcarrier from one of the channels in the 5 GHz band.
3. 802.11 Data link layer
i. The data link layer within 802.11 consists of two sublayers: logical link control (LLC) and media access control (MAC).
ii. 802.11 uses the same 802.2 LLC and 48-bit addressing as the other 802 LAN, allowing for simple bridging from wireless to IEEE wired networks, but the MAC is unique to WLAN.
iii. The sublayer above MAC is the LLC, where the framing takes place. The LLC inserts certain fields in the frame such as the source address and destination address at the head end of the frame and error handling bits at the end of the frame.
iv. MAC sublayer defines how a user obtains a channel when he or she needs one.
v. The 802.11 MAC is similar in concept to 802.3, in that it is designed to support multiple users on a shared medium by having the sender sense the medium before accessing it.
vi. The 802.11 MAC scheme includes ‘Carrier sense multiple access with collision avoidance (CSMA/CA)’ to decide which station will access the media. ‘Collision detection’ cannot be used in WLANs.
vii. In IEEE 802.11, the MAC sublayer is responsible for asynchronous data service (e.g., exchange of MAC service data units (MSDUs)), security service (confidentiality, authentication, access control in conjunction with layer management), and MSDU ordering.
viii. The MAC sublayer is also responsible for how a station joins an AP, switch to another AP.
4. 802.11b-High rate DSSS
i. In September 1999 IEEE ratified the 802.11b high rateamendment to the standard, which added two higher speeds (5.5 and 11 Mbps) to 802.11.
ii. To increase the data rate in 802.11b standard, advanced coding techniques are employed. Rather than the two 11-bit Barker sequences, 802.11b specifies complementary code keying (CCK).
iii. The 5.5 Mbps rate uses CCK to encode 4 bits per carrier, while the 11 Mbps rate encodes 8 bits per carrier. Both speeds use QPSK modulation and a signal at 1.375 Msps. This is how the higher data rates are obtained.
iv. The key contribution of the 802.11b addition to the WLAN standard was to standardize the physical layer support to two new speeds, 5.5 and 11 Mbps.
v. To accomplish this, DSSS was selected as the sole physical layer technique for the standard, since frequency hopping cannot support the higher speeds without violating current FCC regulations.
vi. The implication is that the 802.11b system will interoperate with 1 Mbps and 2 Mbps 802.11 DSSS systems, but will not work with 1 Mbps and 2 Mbps FHSS systems.
vii. To support very noisy environments as well as extended ranges, 802.11b WLANs use dynamic rate shifting, allowing data rates to be automatically adjusted to compensate for the changing nature of the radio channel. Ideally, users connect at a full 11 Mbps rate.
viii. However, when devices move beyond the optimal range for 11 Mbps operation, or if substantial interference is present, 802.11b devices will transmit at lower speeds, falling back to 5.5, 2, and 1 Mbps.
ix. Likewise, if a device moves back within the range of a higher-speed transmission, the connection will automatically speed up again.
x. Rate shifting is a physical layer mechanism transparent to the user and upper layers of the protocol stack.
5. 802.11n
i. In response to growing market demand for higher-performance WLANs, the IEEE formed the task group 802.11n.
ii. The scope of this task group is to define modifications to the physical and MAC layer to deliver a minimum of 100 Mbps throughput at the MAC service access point (SAP).
iii. 802.11n uses ‘Multi-input Multi-output (MIMO)’. MIMO divides a bit stream into spatial streams, each directed towards a different antenna. This ‘Space Division Multiplexing (SDM)’ improves OFDM.
iv. The MIMO power saving mode mitigates to multipath only when it improves the overall performance of WLAN.
v. The highest raw data rate will increase to 65Mbps from 54Mbps if the devices show compatibility with 802.11n standard.
vi. ‘Beam forming’ and ‘Diversity’ are two other techniques supported by 802.11n.
vii. Beam forming focuses the beam directly towards the intended antennas at the receiver.
viii. Diversity sums up the response of all antennas, takes the best subset, rejecting weak responses.
ix. 802.11n also supports aggregation. It bundles several frames and sends them together, reducing the total time required. This aggregation enhances the mixed mode operation offered by 802.11g.
x. Doubling the channel width from 20MHz to 40MHz increases data rates. However, it is used by properly managing the needs of clients requiring high speeds and other clients which are connected to the network.
xi. The 802.11n specification was developed with previous standards in mind to ensure compatibility.
Table. Primary IEEE 802.11 specifications and their comparisons
6. WLAN Applications
i. WLANs are designed to operate in industrial, scientific, and medical (ISM) radio bands and unlicensed-national information infrastructure (U-NII) bands. These bands are license free.
ii. Manufacturers have deployed WLANs for process and control applications.
iii. Retail applications have expanded to include wireless point of sale (WPOS).
iv. The health-care and education industry are also fast-growing markets for WLANs.
v. WLANs provide high-speed, reliable data communications in a building or campus environment as well as coverage in rural areas. WLANs are simple to install.
Fig7. WLAN Applications
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