Wireless Local Area Networks
`and the 802.11 Standard
`
`March 31, 2001
`
`Plamen Nedeltchev, PhD
`
`Edited by Felicia Brych
`
`IPR2020-00202
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`Table of Contents1
`
`Introduction................................................................................................................................................. 3
`Upper Layer Protocols of OSI..................................................................................................................... 3
`WLAN Architecture.................................................................................................................................... 4
`WLAN topologies.................................................................................................................................... 4
`WLAN Components ................................................................................................................................ 5
`IEEE 802.11, 802.11b and 802.11a Physical Layer.................................................................................... 5
`802.11 Physical Layer.............................................................................................................................. 5
`802.11b – The Next Step ......................................................................................................................... 7
`Sub-layers in the PHY layer .................................................................................................................... 8
`The last step – 802.11a ............................................................................................................................ 9
`IEEE 802.11, 802.11b and 802.11a MAC Layer ...................................................................................... 10
`802.11 MAC Layer Services ................................................................................................................. 10
`Collision Sense Multiple Access with Collision Detection................................................................... 11
`Collision Sense Multiple Access with Collision Avoidance ................................................................. 12
`The “Hidden Station” challenge ............................................................................................................ 13
`MAC Level Acknowledgements ........................................................................................................... 15
`Extended Backoff Algorithm................................................................................................................. 16
`Frame Types .......................................................................................................................................... 16
`MAC Frame Formats............................................................................................................................. 16
`MAC Layer for 802.11a......................................................................................................................... 17
`802.11 Security ......................................................................................................................................... 17
`Roaming Approach, Association and Mobility......................................................................................... 19
`Power Management................................................................................................................................... 20
`Known Issues and Development Directions ............................................................................................. 20
`Wireless Device Interoperability in 802.11 ........................................................................................... 21
`Safety ..................................................................................................................................................... 21
`Conclusion ................................................................................................................................................ 21
`Glossary .................................................................................................................................................... 23
`References................................................................................................................................................. 24
`
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`Introduction
`
`Support for wireless local area networks (WLANs) in corporate offices and employee’s homes is
`becoming a necessary activity for networking professionals, requiring new knowledge and training. The
`purpose of the article is to provide readers with a basic understanding of the 802.11 techniques,
`concepts, architecture and principles of operations. The standard was designed as a transmission system
`between devices by using radio frequency (RF) waves rather than cable infrastructure, and it provides
`mobile, cost-effective solutions, significantly reducing the network installation cost per user.
`Architecturally, WLANs usually act as a final link between end user equipment and the wired structure
`of corporate computers, servers and routers.
`
`The standard not only defines the specifications, but also includes a wide range of services including:
`•
`support of asynchronous and time-bounded (time-critical) delivery services;
`• continuity of service within extended areas via a Distributed System, such as Ethernet;
`• accommodation of transmission rates;
`•
`support of most market applications;
`• multicast (including broadcast) services;
`• network management services; and,
`•
`registration and authentication services.
`
`The target environment of the standard includes:
`•
`inside buildings such as offices, convention centers, airport gates and lounges, hospitals, plants
`and residences; and
`• outdoor areas, such as parking lots, campuses, building complexes, and outdoor plants.
`In 1997, the IEEE released 802.11 as the first internationally sanctioned standard for wireless LANs,
`defining 1 and 2 Mbps speeds. In September 1999, they ratified the 802.11b “High Rate” amendment to
`the standard, which added two higher speeds (5.5 and 11 Mbps) to 802.11[1]. The basic architecture,
`features and services of 802.11b are defined by the original 802.11 standard, with changes made only to
`the physical layer. These changes result in higher data rates and more robust connectivity.
`
`Data Link Layer
`
`Physical layer
`
`Upper Layer Protocols of OSI
` ---------------------------------------------------------
`
`Logical Link Control (LLC) - 802.2
`
`Media Access Control (MAC)
`
`Physical Layer
`FH, DS, IR
`
`Figure 1. 802.11 standard focuses on the bottom two levels of the ISO model: PHY and MAC
`
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`WLAN Architecture
`
`WLAN topologies
`
`IEEE 802.11 supports three basic topologies for WLANs: the Independent Basic Service Set (IBSS), the
`Basic Service Set (BSS), and the Extended Service Set (ESS). All three configurations are supported by
`the MAC layer implementation.
`
`The 802.11 standard defines two modes: ad hoc/IBSS and infrastructure mode. Logically, an ad-hoc
`configuration is analogous to a peer-to-peer office network in which no single node is required to
`function as a server. IBSS WLANs include a number of nodes or wireless stations that communicate
`directly with one another on an ad-hoc, peer-to-peer basis, building a full-mesh or partial-mesh
`topology. Generally, ad-hoc implementations cover a limited area and aren’t connected to any larger
`network.
`
`Using infrastructure mode, the wireless network consists of at least one access point connected to the
`wired network infrastructure and a set of wireless end stations. This configuration is called a Basic
`Service Set (BSS). Since most corporate WLANs require access to the wired LAN for services (file
`servers, printers, Internet links), they will operate in infrastructure mode and rely on an Access Point
`(AP) that acts as the logical server for a single WLAN cell or channel. Communications between two
`nodes, A and B, actually flow from node A to the AP and then from the AP to node B. The AP is
`necessary to perform a bridging function and connect multiple WLAN cells or channels, and to connect
`WLAN cells to a wired enterprise LAN.
`An Extended Service Set (ESS) is a set of two or more BSSs forming a single subnetwork. ESS
`configurations consist of multiple BSS cells that can be linked by either wired or wireless backbones.
`IEEE 802.11 supports ESS configurations in which multiple cells use the same channel, and use
`different channels to boost aggregate throughput.
`
`Distribution System (DS)
`
`Access Point (AP)
`
`Access Point (AP)
`
`Basic Service Set (BSS)
`Wireless station single cell
`
` Basic Service Set (BSS)
` single cell
`
`Wireless station
`
`Wireless station Wireless station
`
`Wireless station
`
`Extended Service Set (ESS) – multiple cells
`
`Figure 2. IEEE 802.11 BSS and ESS topologies
`
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`WLAN Components
`
`802.11 defines two pieces of equipment, a wireless station, which is usually a PC equipped with a
`wireless network interface card (NIC), and an access point (AP), which acts as a bridge between the
`wireless and wired networks. An access point usually consists of a radio, a wired network interface
`(e.g., 802.3), and bridging software conforming to the 802.11d bridging standard. The access point acts
`as the base station for the wireless network, aggregating access for multiple wireless stations onto the
`wired network. Wireless end stations can be 802.11 PC Card, PCI, or ISA NICs, or embedded solutions
`in non-PC clients (such as an 802.11-based telephone handset).
`
`An 802.11 WLAN is based on a cellular architecture. Each cell (BSS) is connected to the base station or
`AP. All APs are connected to a Distribution System (DS) which is similar to a backbone, usually
`Ethernet or wireless. All mentioned components appear as an 802 system for the upper layers of OSI
`and are known as the ESS.
`
`The 802.11 standard does not constrain the composition of the distribution system; therefore, it may be
`802 compliant or non-standard. If data frames need transmission to and from a non-IEEE 802.11
`LAN, then these frames, as defined by the 802.11 standard, enter and exit through a logical point
`called a Portal. The portal provides logical integration between existing wired LANs and 802.11 LANs.
`When the distribution system is constructed with 802-type components, such as 802.3 (Ethernet) or
`802.5 (Token Ring), then the portal and the access point are the same, acting as a translation bridge.
`
`The 802.11 standard defines the distribution system as an element that interconnects BSSs within the
`ESS via access points. The distribution system supports the 802.11 mobility types by providing logical
`services necessary to handle address-to-destination mapping and seamless integration of multiple BSSs.
`An access point is an addressable station, providing an interface to the distribution system for stations
`located within various BSSs. The independent BSS and ESS networks are transparent to the LLC Layer.
`http://wwwin.cisco.com/cct/data/itm/wan/sdlc/wtsdllca.htm.
`
`IEEE 802.11, 802.11b and 802.11a Physical Layer
`
`802.11 Physical Layer
`
`At the Physical (PHY) layer, IEEE 802.11 defines three physical techniques for wireless local area
`networks: diffused infrared (IR), frequency hopping spread spectrum (FH or FHSS) and direct sequence
`spread spectrum (DS or DSSS). While the infrared technique operates at the baseband, the other two
`radio-based techniques operate at the 2.4 GHz band. They can be used for operating wireless LAN
`devices without the need for end-user licenses. In order for wireless devices to be interoperable, they
`have to conform to the same PHY standard. All three techniques specify support for 1 Mbps and 2
`Mbps data rates.
`
`Photonic Wireless Transmission - Diffused Infrared (IR). The only implementation of these types of
`LANs use infra-red light transmission. Photonic wireless LANs use the 850 to 950 Nm band of infra-
`red light with a peak power of 2 Watts. The physical layer supports 1 and 2 Mbps data rates. Although
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`photonic wireless systems potentially offer higher transmission rates than RF based systems, they also
`have some distinct limitations.
`• First, infra-red light like visible light, is restricted to line of sight operations. However, the use
`of diffuse propagation can reduce this restriction by allowing the beam to bounce off passive
`reflective surfaces.
`• Second, the power output (2 Watts) is so low to reduce damage to the human eye, that
`transmissions are limited to about 25 metres.
`• Finally, sensors (receivers) need to be laid out accurately, otherwise the signal may not be picked
`up.
`
`
`Photonic-based wireless LANs are inherently secure and are immune (as are optical fiber networks)
`from electromagnetic radiation which can interfere with cable and RF based systems.
`
`Diffused Infrared (IR). IR communications are described as both indirect and non-line-of sight. The
`diffused infrared signal, which is emitted from the transmitter, fills an enclosed area like light and does
`not require line-of-sight transmission. You can point the infrared adapters at the ceiling or at an angle,
`and the signal will bounce off your walls and ceiling. Changing the location of the receiver does not
`disrupt the signal. Many diffused infrared products also offer roaming capabilities, which enables you
`to connect several access points to the network, then connect your mobile computer to any of these
`access points or move between them without losing your network connection. Usually IR provides a
`radius of 25 to 35 feet and a speed of 1 to 2 Mbps.
`
`Spread Spectrum (RF) Transmissions. Spread Spectrum (SS) RF systems are true wireless LANs
`which use radio frequency (RF wireless) transmission as the physical layer medium. Two major sub-
`systems exist: Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum
`(DSSS). DSSS is primarily an inter-building technology, while FHSS is primarily an intra-building
`technology. The actual technique of spread spectrum transmission was developed by the military in an
`attempt to reduce jamming and eavesdropping. SS transmission takes a digital signal and expands or
`spreads it so as to make it appear more like random background noise rather than a data signal
`transmission. Coding takes place either by using frequency shift keying (FSK) or phase shift keying
`(PSK). Both methods increase the size of the data signal as well as the bandwidth. Although the signal
`appears louder (more bandwidth) and easier to detect, the signal is unintelligible and appears as
`background noise unless the receiver is tuned to the correct parameters.
`
`Frequency Hopping Spread Spectrum Technology (FHSS). Frequency Hopping Spread Spectrum
`(FHSS) is analogous to FM radio transmission as the data signal is superimposed on, or carried by, a
`narrow band carrier that can change frequency. The IEEE 802.11 standard provides 22 hop patterns or
`frequency shifts to choose from in the 2.4GHz ISM band. Each channel is 1MHz and the signal must
`shift frequency or hop at a fixed hop rate (U.S. minimum is 2.5 hops/sec). This technology modulates a
`radio signal by shifting it from frequency to frequency at near-random intervals. This modulation
`protects the signal from interference that concentrates around one frequency. To decode the signal, the
`receiver must know the rate and the sequence of the frequency shifts, thereby providing added security
`and encryption.
`
`FHSS products can send signals as quickly as 1.2 to 2 Mbps and as far as 620 miles. Increasing the
`bandwidth (up to 24 Mbps) can be achieved by installing multiple access points on the network. In FS,
`the 2.4 GHz band is divided into 75 one-MHz sub-channels. In order to minimize the probability that
`two senders are going to use the same sub-channel simultaneously, frequency-hopping is used to provide
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`a different hopping pattern for every data exchange. The sender and receiver agree on a hopping
`pattern, and data is sent over a sequence of sub-channels according to the pattern. FCC regulations
`require bandwidth up to 1 MHz for every sub-channel which forces the FHSS technique to spread the
`patterns across the entire 2.4 GHz, resulting in more hops and a high amount of overhead.
`
`
`Direct Sequence Spread Spectrum (DSSS). Spread spectrum was first developed by the military as a
`secure wireless technology. It modulates (changes) a radio signal pseudo-randomly so it is difficult to
`decode. This modulation provides some security, however, because the signal can be sent great
`distances, you do risk interception. To provide complete security, most spread spectrum products
`include encryption.
`
`DSSS works by taking a data stream of zeros and ones and modulating it with a second pattern, the
`chipping sequence. The sequence is also known as the Barker code which is an 11-bit sequence
`(10110111000). The chipping or spreading code is used to generate a redundant bit pattern to be
`transmitted, and the resulting signal appears as wide band noise to the unintended receiver. One of the
`advantages of using spreading codes is even if one or more of the bits in the chip are lost during
`transmission, statistical techniques embedded in the radio can recover the original data without the need
`for retransmission. The ratio between the data and width of spreading code is called processing gain. It
`is 16 times the width of the spreading code and increases the number of possible patterns to 216 (64k),
`reducing the chance of cracking the transmission.
`
`The DS signaling technique divides the 2.4 GHz band into 14 twenty-two MHz channels, of which 11
`adjacent channels overlap partially and the remaining three do not overlap. Data is sent across one of
`these 22 MHz channels without hopping to other channels, causing noise on the given channel. To
`reduce the number of re-transmissions and noise, chipping is used to convert each bit of user data into a
`series of redundant bit patterns called “chips.” The inherent redundancy of each chip, combined with
`spreading the signal across the 22 MHz channel, provides the error checking and correction functionality
`to recover the data.
`
`Spread spectrum products are often interoperable because many are based on the IEEE 802.11 standard
`for wireless networks. DSSS is primarily an inter-building technology, while FHSS, is primarily an
`intra-building technology. DSSS products can be fast and far reaching.
`
`
`802.11b – The Next Step
`
`All previously mentioned coding techniques for 802.11 provide a speed of 1 to 2 Mbps, lower than the
`wide spread IEEE 802.3 standard speed of 10 Mbps. The only technique (with regards to FCC rules)
`capable of providing higher speed is DSSS which was selected as a standard physical layer technique,
`supporting 1 to 2 Mbps and two new speeds of 5.5 and 11 Mbps.
`
`The original 802.11 DSSS standard specifies the 11-bit chipping, or Barker sequence, to encode all data
`sent over the air. Each 11-chip sequence represents a single data bit (1 or 0), and is converted to a
`waveform, called a symbol, that can be sent over the air. These symbols are transmitted at a 1 MSps (1
`million symbols per second), using a sophisticated technique called Binary Phase Shift Keying (BPSK)
`(see http://www.physics.udel.edu/wwwusers/watson/student_projects/scen167/thosguys/psk.html). In the
`case of 2 Mbps, a more sophisticated implementation called Quadrature Phase Shift Keying (QPSK) is
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`used (see http://www.ee.byu.edu/ee/class/ee444/simulink/oqpsk/oqpsk.html). It doubles the data rate
`available in BPSK, via improved efficiency in the use of the radio bandwidth.
`
`To increase the data rate in the 802.11b standard, in 1998, Lucent Technologies and Harris
`Semiconductor proposed to IEEE a standard called CCK (Complementary Code Keying). Rather than
`the two 11-bit Barker code, CCK uses a set of 64 eight-bit unique code words, thus up to 6 bits can be
`represented by any code word (instead of the 1 bit represented by a Barker symbol). As a set, these code
`words have unique mathematical properties that allow them to be correctly distinguished from one
`another by a receiver, even in the presence of substantial noise and multi-path interference (e.g.,
`interference caused by receiving multiple radio reflections within a building).
`
`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 as the modulation technique and signal at 1.375 MSps. QPSK uses four
`rotations (0, 90, 180 and 270 degrees) to encode 2 bits of information in the same space as BPSK
`encodes 1. The trade-off is that you must increase power or decrease range to maintain signal quality.
`Due to the fact the FCC regulates output power of portable radios to 1 watt EIRP (equivalent
`isotropically radiated power), range is the only remaining factor that can change. Thus, for 802.11
`devices, as you move away from the radio, the radio adapts and uses a less complex (and slower)
`encoding mechanism to send data, resulting in the higher data rates. Table 1 identifies the differences.
`
`
`
`
`Data Rate
`
`1 Mbps
`2 Mbps
`5.5 Mbps
`11 Mbps
`
`
`
`
`
`
`
`
`
`
`
`Code Length
`
`Modulation
`
`Symbol Rate
`
`Bits/Symbol
`
`11 (Barker Sequence)
`11 (Barker Sequence)
`8 (CCK)
`8 (CCK)
`
`BPSK
`QPSK
`QPSK
`QPSK
`
`1 MSps
`1 MSps
`1.375 MSps
`1.375 MSps
`
`1
`2
`4
`8
`
`Table 1. 802.11b Data Rate Specifications
`
`
`Sub-layers in the PHY layer
`
`The PHY layer is divided into two sub-layers, called the PLCP (Physical Layer Convergence Protocol)
`sub-layer and the PMD (Physical Medium Dependent) sub-layer. The PMD is responsible for the
`encoding. The PLCP presents a common interface for higher-level drivers to write to, and it provides
`carrier sense and CCA (Clear Channel Assessment), which is the signal the MAC (Media Access
`Control) layer needs to determine whether the medium is currently in use.
`
`
`
`
`
`
`Figure 3. IEEE 802.11b DSSS PHY frame format
`
`PLCP header
`Service
`Length HEC
`
`Payload
`(variable)
`
`PLCP preamble
`Synchronization
`
`SFD
`
`Signal
`Header
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`
`PLCP Preamble. The PLCP consists of a 144-bit preamble that is used for synchronization to
`determine radio gain and to establish CCA. This is PHY dependent, and includes:
`• Synch: A 128-bit sequence of alternating zeros and ones, which is used by the PHY circuitry to
`select the appropriate antenna (if diversity is used), and to reach steady-state frequency offset
`correction and synchronization with the received packet timing.
`• SFD: A Start Frame delimiter which consists of the 16-bit binary pattern 1111001110100000,
`which is used to define frame timing and mark the start of every frame and is called the SFD
`(Start Frame Delimiter)..
`
`
`PLCP Header. The header consist of 48 bits, it is always transmitted at 1 Mbps and contains logical
`information used by the PHY Layer to decode the frame. It consists of:
`• Signal: 8 bits which contains only the rate information, encoded in 0.5 Mbps increments from 1
`Mbit/s to 4.5 Mbit/s;
`• Service: 8 bits reserved;
`• Length: 16 bits and represents the number of bytes contained in the packet (useful for the PHY
`to correctly detect the end of packet);
`• Header Error Check Field: 16 Bit CRC of the 48 bit header.
`
`
`The PLCP introduces 24 bytes of overhead into each wireless Ethernet. Because the 192-bit header
`payload is transmitted at 1 Mbps, 802.11b reduces the efficiency on the PHY layer by 15%.
`
`
`The last step – 802.11a
`
`As we have mentioned earlier 802.11b pick for a coding technique is based on DSSS, a technology,
`developed by the military as a secure wireless technology. This technology works by modulating
`(changing) a radio signal pseudo-randomly so that it is difficult to decode. This modulation provides
`some security; however, because the signal can be sent great distances, you do risk interception. To
`provide complete security, most spread spectrum products include encryption. Spread spectrum products
`are often interoperable because many are based on the proposed IEEE 802.11 standard for wireless
`networks. Direct sequence spread spectrum is primarily an inter-building technology, while frequency
`hopping spread spectrum, on the other hand, is primarily an intra-building technology.
`Unlike 802.11b, 802.11a was designed to operate in the more recently allocated 5-GHz UNII
`(Unlicensed National Information Infrastructure) band. Unlike ISM band, which offers about 83 MHz in
`the 2.4 GHz spectrum, IEEE 802.11a utilizes almost four times that of the ISM band, because UNII
`band offers 300 MHz of relatively free of interference spectrum. And unlike 802.11b, the 802.11a
`standard is using a frequency division multiplexing technique, which is expected to be more efficient
`in inter-building environments. As previously mentioned, the FCC has allocated 300 MHz of spectrum
`for UNII in the 5-GHz block, 200 MHz of which is at 5,150 MHz to 5,350 MHz, with the other 100
`MHz at 5,725 MHz to 5,825 MHz. The first advantage of the 802.11a before 802.11b is that the
`standard operates in 5.4 GHz spectrum, which gives it the performance advantage of the high
`frequencies. But frequency, radiated power and distance together are in an inverse relationship, so
`moving up to the 5-GHz spectrum from 2.4 GHz leads to shorter distances and/or requirements for more
`power. That is why the 802.11a Standard increases the EIRP to the maximum 50 mW. The 5.4 GHz,
`spectrum is split into three working "domains" and every domain has restrictions for maximum power.
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`
`The second advantage lies on the coding technique, 802.11a is using. The 802.11a uses an encoding
`scheme, called COFDM or OFDM
`(coded orthogonal
`frequency division multiplexing)
`http://www.cclinf.polito.it/~s83797/cofdm.htm. Each sub-channel in the COFDM implementation is
`about 300 KHz wide. COFDM works by breaking one high-speed data carrier into several lower-speed
`sub-carriers, which are then transmitted in parallel. Each high-speed carrier is 20 MHz wide and is
`broken up into 52 sub-channels, each approximately 300 KHz wide. COFDM uses 48 of these sub-
`channels for data, while the remaining four are used for error correction. COFDM delivers higher data
`rates and a high degree of signal recovery, thanks to its encoding scheme and error correction. Each sub-
`channel in the COFDM implementation is about 300 KHz wide. To encode 125 Kbps, well-known
`BPSK is used, yielding a 6,000-Kbps, or 6 Mbps, data rate. Using QPSK, it is possible to encode up to
`250 Kbps per channel, which combined achieves 12-Mbps data rate. And by using 16-level quadrature
`amplitude modulation encoding 4 bits per hertz, and achieving data rate of 24 Mbps, the Standard
`defines basic speeds of 6,12 and 24 Mbps, which every 802.11a-compliant products must support. Data
`rates of 54 Mbps are achieved by using 64QAM (64-level quadrature amplitude modulation), which
`yields 8 bits/10 bits per cycle, and a total of up to 1.125 Mbps per 300-KHz channel. With 48 channels,
`this results in a 54-Mbps data rate. On February 15, 2001 Cisco Systems completed its acquisition of
`Radiata Incorporated, a company, supporting the standard speeds and 36Mbps, 48Mbps and 54 Mbps as
`well. The maximum theoretical data rate of COFDM is considered 108 Mbps.
`
`
`
`IEEE 802.11, 802.11b and 802.11a MAC Layer
`
`802.11 MAC Layer Services
`
`The MAC layer provides various services to manage authentication, de-authentication, privacy and data
`transfer.
`
`Authentication. The authentication service is the process of proving client identity which takes place
`prior to a wireless client associating with an AP. By default, IEEE 802.11 devices operate in an Open
`System, where essentially any wireless client can associate with an AP without checking credentials.
`True authentication is possible with the use of the 802.11 option known as Wired Equivalent Privacy or
`WEP, where a shared key is configured into the AP and its wireless clients. Only those devices with a
`valid shared key will be allowed to be associated to the AP.
`
`De-authentication. The de-authentication function is performed by the base station. It is a process of
`denying client credentials, based on incorrect authentication settings, or applied IP or MAC filters.
`
`Association. The association service enables the establishment of wireless links between wireless
`clients and APs in infrastructure networks.
`
`Disassociation. The service which cancels the wireless links between wireless clients and APs in
`infrastructure networks.
`
`Re-association. The re-association service occurs in addition to association when a wireless client
`moves from one BSS to another. Two adjoining BSSs form an ESS if they are defined by a common
`ESSID, providing a wireless client with the capability to roam from one area to another. Although re-
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`association is specified in 802.11, the mechanism that allows AP-to-AP coordination to handle roaming
`is not specified.
`
`Privacy. By default, data is transferred in the clear allowing any 802.11-compliant device to potentially
`eavesdrop on similar PHY 802.11 traffic within range. The WEP option encrypts data before it is sent
`wirelessly, using a 40-bit encryption algorithm known as RC4. The same shared key used in
`authentication is used to encrypt or decrypt the data, allowing only wireless clients with the exact shared
`key to correctly decipher the data.
`
`Data transfer. The primary service of MAC layer is to provide frame exchange between MAC
`layers. Wireless clients use a Collision Sense Multiple Access with Collision Avoidance (CSMA/CA)
`algorithm as the media access scheme.
`
`Distribution. The distribution function is performed by DS and it is used in special cases in frame
`transmission between APs.
`
`
`Integration. This is a function performed by the portal, where essentially the portal is design to provide
`logical integration between existing wired LANs and 802.11 LANs.
`
`Power management. IEEE 802.11 defines two power modes: an active mode, where a wireless client is
`powered to transmit and receive; and, a power save mode, where a client is not able to transmit or
`receive, consuming less power. Actual power consumption is not defined and is dependent upon the
`implementation.
`
`
`Collision Sense Multiple Access with Collision Detection
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`The classic (CSMA/CD) method is a very effective mechanism in a wired environment, enabling speeds
`of 10 (T-base), 100 (Fast-Ethernet), or 1000 (Gigabit-Ethernet). However, this mechanism immanently
`allows conflicts (collisions) and supports exponential backoff mechanism, reducing the throughput in a
`very competitive environment with a high number of active users. Collision levels of 30-40 %, even
`less, could cause a very significant degradation of the overall performance of the active users [2], [3 see
`http://eman.cisco.com/NETWORKING/tech_ref/access_capacity_planning.pdf]. On the other hand, the
`backoff algorithm could defer the transition of the data for up to 367 ms in the 10Mbps networks.
`Therefore, the CSMA/CD mechanism creates an opportunistic discipline to access the common media
`and makes the response time a predictable value for at least a “not worst than” scenario.
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`Creating a mechanism to prevent the potential conflicts in the shared med