r together than channels 1, 6, and 11 (for example, 1, 4, 7, and 10), overlap between the channels may cause unacceptable degradation of signal quality and throughput [9]. However, overlapping channels may be used under certain circumstances. This way, more channels are available [10].
[edit] Frames
Current 802.11x standards define "frame" types for use in transmission of data as well as management and control of wireless links. Frames are divided into very specific and standardized sections. Each frame has a 2-byte frame control field that provides detailed information on the wireless link. This field is segmented 11 ways and will be presented in order, with the first two bits reserved for identification of the protocol being used (e.g. 802.11g, 802.11b, etc.). These respectively two and four bit fields are used for identification of which frame type is used. The next two segment are reserved for type and subtype. The next two bits are the To DS and From DS fields. They indicate whether a frame is headed for a distributed system. All frames will have one of these bits set. The More Fragmentation bit is set most notable when higher level packets have been partitioned and will be set for all non-final sections. Some management frames may required partitioning as well. Sometimes frames require retransmission, and for this there is a Retry bit which is set to one when a frame is resent. This aids in the elimination of duplicate frames station side. The Power Management bit indicates the power management state of the sender after the completion of a frame exchange. Access points are required to manage the connection and will never set the power saver bit. The More Data bit is used to buffer frames received in a distributed system. The access point uses this bit to facilitate stations in power saver mode. It indicates that at least one frame is available and addresses all stations connected. The WEP bit is modified after processing a frame. It is toggled to one after a frame has been decrypted or if no encryption is set it will have already been one. The last bit is the Order bit and is only set when the "strict ordering" delivery method is employed. Frames and fragments are not always sent in order as it causes a transmission performance penalty.
The next two bytes are reserved for the Duration ID field. This field take on one of three forms, Duration, contention-free period (CFP), and PS-Poll.
An 802.11 frame can contain up to four address fields. Six bytes are reserved for each address field. Each field is numbered are is used for different purposes. Address 1 is the receiver, Address 2 is the transmitter, Address 3 is used for filtering purposes by the receiver. As addresses are only 46 bits long and there are 48 bits reserved for each address, the first bit has a special function. A 0 indicates a single stations address (unicast), while a 1 represent a group of stations (multicast). If all the bits are 1's then the frame is broadcast to all station connected to an access point. The Sequence Control field is a two byte section used for identifying message order as well as eliminating duplicate frames. The first 4 bits are used for the fragmentation number and the last 12 bits are the sequence number. The Frame Body field is variably size, from 0 – 2132 bytes, an contains information from higher layers. The Frame Check Sequence (FCS) is the last four bytes in the standard 802.11 frame. Often referred to as the Cyclic Redundancy Check (CRC), it allows for integrity check of retrieved frames. As frames are about to be sent the FCS is calculated and appending. When a station receives a frame is can calculate the FCS of the frame and compare it to the one received. If they match it is assumed that the frame was not distorted during transmission.[11]
Management Frames allow for the maintenance of communication. Some common 802.11 subtypes include:
- Authentication frame: 802.11 authentication begins with the WNIC sending an authentication frame to the access point containing its identity. With an open system authentication the WNIC only sends a single authentication frame and the access point responds with an authentication frame of its own indicating acceptance or rejection. With shared key authentication, after the WNIC sends its initial authentication request it will receive an authentication frame from the access point containing challenge text. The WNIC sends an authentication frame containing the encrypted version of the challenge text to the access point. The access point ensures the text was encrypted with the correct key by decrypting it with its own key. The result of this process determines the WNIC's authentication status.
- Association request frame: sent from a station it enables the access point to allocate resources and synchronize. The frame carries information about the WNIC including supported data rates and the SSID of the network the station wishes to associate with. If the request is accepted the access point reserve memory and establishes and association ID for the WNIC.
- Association response frame: sent from an access point to a station containing the acceptance or rejection to an association request. If it is an acceptance the frame will contain information such an association ID and supported data rates.
- Beacon frame: Sent periodically from an access point to announce its presence and provide the SSID, and other parameters for WNICs within range.
- Deauthentication frame: Sent from a station wishing to terminate connection from another station.
- Disassociation frame: Sent from a station wishing to terminate connection. It's an elegant way to allow the access point to relinquish memory allocation and remove the WNIC from the association table.
- Probe request frame: Sent from a station when it requires information from another station.
- Probe response frame: Sent from a station containing capability information, supported data rates, etc., after receiving a probe request frame.
- Reassociation request frame: A WNIC sends a reassociation request when it drops from range of the currently associated access point and finds another access point with a stronger signal. The new access point coordinates the forwarding of any information that may still be contained in the buffer of the previous access point.
- Reassociation response frame: Sent from an access point containing the acceptance or rejection to a WNIC reassociation request frame. The frame includes information required for association such as the association ID and supported data rates.
Control frames facility in the exchange of data frames between station. Some common 802.11 control frames include:
- Acknowledgement (ACK) frame: After receiving a data frame the receiving station will send an ACK frame to the sending station if no errors are found. If the sending station doesn't receive an ACK frame within a predetermined period of time the sending station will resend the frame.
- Request to Send (RTS) frame: The RTS and CTS frames provide an optional collision reduction scheme for access point with hidden stations. A station sends a RTS frame to as the first step in a two-way handshake required before sending data frames.
- Clear to Send (CTS) frame: A station responds to an RTS frame with a CTS frame. It provides clearance for the requesting station to send a data frame. The CTS provides collision control management by including a time value for which all other stations are to hold off transmission while the requesting stations transmits.
Data frames carry packets from web pages, files, etc. within the body.
[12]
[edit] Standard and amendments
Within the IEEE 802.11 Working Group,[4] the following IEEE Standards Association Standard and Amendments exist:
- IEEE 802.11 - THE WLAN STANDARD was original 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and IR standard (1997), all the others listed below are Amendments to this standard, except for Recommended Practices 802.11F and 802.11T.
- IEEE 802.11a - 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001)
- IEEE 802.11b - Enhancements to 802.11 to support 5.5 and 11 Mbit/s (1999)
- IEEE 802.11c - Bridge operation procedures; included in the IEEE 802.1D standard (2001)
- IEEE 802.11d - International (country-to-country) roaming extensions (2001)
- IEEE 802.11e - Enhancements: QoS, including packet bursting (2005)
- IEEE 802.11F - Inter-Access Point Protocol (2003) Withdrawn February 2006
- IEEE 802.11g - 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003)
- IEEE 802.11h - Spectrum Managed 802.11a (5 GHz) for European compatibility (2004)
- IEEE 802.11i - Enhanced security (2004)
- IEEE 802.11j - Extensions for Japan (2004)
- IEEE 802.11-2007 - A new release of the standard that includes amendments a, b, d, e, g, h, i & j. (July 2007)
- IEEE 802.11k - Radio resource measurement enhancements (2008)
- IEEE 802.11l - (reserved and will not be used)
- IEEE 802.11m - Maintenance of the standard. Recent edits became 802.11-2007. (ongoing)
- IEEE 802.11n - Higher throughput improvements using MIMO (multiple input, multiple output antennas) (November 2009)
- IEEE 802.11o - (reserved and will not be used)
- IEEE 802.11p - WAVE - Wireless Access for the Vehicular Environment (such as ambulances and passenger cars) (working - 2009?)
- IEEE 802.11q - (reserved and will not be used, can be confused with 802.1Q VLAN tagging)
- IEEE 802.11r - Fast roaming Working "Task Group r" - (2008)
- IEEE 802.11s - Mesh Networking, Extended Service Set (ESS) (working - Jul. 2010?)
- IEEE 802.11T - Wireless Performance Prediction (WPP) - test methods and metrics Recommendation (2008)
- IEEE 802.11u - Interworking with non-802 networks (for example, cellular) (proposal evaluation - Mar 2010?)
- IEEE 802.11v - Wireless network management (early proposal stages - Sept 2010?)
- IEEE 802.11w - Protected Management Frames (early proposal stages - 2009?)
- IEEE 802.11x - (reserved and will not be used, can be confused with 802.1x Network Access Control)
- IEEE 802.11y - 3650-3700 MHz Operation in the U.S. (2008)
- IEEE 802.11z - Extensions to Direct Link Setup (DLS) (Aug. 2007 - Dec. 2011)
- IEEE 802.11aa - Robust streaming of Audio Video Transport Streams (Mar. 2008 - May. 2011)
There is no standard or task group named "802.11x". Rather, this term is used informally to denote any current or future 802.11 amendment, in cases where further precision is not necessary. (The IEEE 802.1x standard for port-based network access control is often mistakenly called "802.11x" when used in the context of wireless networks.)
802.11F and 802.11T are recommended practices rather than standards, and are capitalized as such.
[edit] Standard or amendment?
Both the terms "standard" and "amendment" are used when referring to the different variants of IEEE 802.11.
As far as the IEEE Standards Association is concerned, there is only one current standard; it is denoted by IEEE 802.11 followed by the date that it was published. IEEE 802.11-2007 is the only version currently in publication. The standard is updated by means of amendments. Amendments are created by task groups (TG). Both the task group and their finished document are denoted by 802.11 followed by a non-capitalized letter. For example IEEE 802.11a and IEEE 802.11b. Updating 802.11 is the responsibility of task group m. In order to create a new version, TGm combines the previous version of the standard and all published amendments. TGm also provides clarification and interpretation to industry on published documents. New versions of the IEEE 802.11 were published in 1999 and 2007.
The working title of 802.11-2007 was 802.11-REVma. This denotes a third type of document, a "revision". The complexity of combining 802.11-1999 with 8 amendments made it necessary to revise already agreed upon text. As a result, additional guidelines associated with a revision had to be followed.
[edit] Nomenclature
Various terms in 802.11 are used to specify aspects of wireless local-area networking operation, and may be unfamiliar to some readers.
For example, Time Unit (usually abbreviated TU) is used to indicate a unit of time equal to 1024 microseconds. Numerous time constants are defined in terms of TU (rather than the nearly-equal millisecond).
Also the term "Portal" is used to describe an entity that is similar to an IEEE 802.1D bridge. A Portal provides access to the WLAN by non-802.11 LAN STAs.
[edit] Community networks
With the proliferation of cable modems and DSL, there is an ever-increasing market of people who wish to establish small networks in their homes to share their broadband Internet connection.
Many hotspot or free networks frequently allow anyone within range, including passersby outside, to connect to the Internet. There are also efforts by volunteer groups to establish wireless community networks to provide free wireless connectivity to the public.
[edit] Security
In 2001, a group from the University of California, Berkeley presented a paper describing weaknesses in the 802.11 Wired Equivalent Privacy (WEP) security mechanism defined in the original standard; they were followed by Fluhrer, Mantin, and Shamir's paper entitled "Weaknesses in the Key Scheduling Algorithm of RC4". Not long after, Adam Stubblefield and AT&T publicly announced the first verification of the attack. In the attack they were able to intercept transmissions and gain unauthorized access to wireless networks.
The IEEE set up a dedicated task group to create a replacement security solution, 802.11i (previously this work was handled as part of a broader 802.11e effort to enhance the MAC layer). The Wi-Fi Alliance announced an interim specification called Wi-Fi Protected Access (WPA) based on a subset of the then current IEEE 802.11i draft. These started to appear in products in mid-2003. IEEE 802.11i (also known as WPA2) itself was ratified in June 2004, and uses government strength encryption in the Advanced Encryption Standard AES, instead of RC4, which was used in WEP. The modern recommended encryption for the home/consumer space is WPA2 (AES PreShared Key) and for the Enterprise space is WPA2 along with a radius server the strongest is EAP-TLS.
In January 2005, IEEE set up yet another task group, TGw, to protect management and broadcast frames, which previously were sent unsecured. See IEEE 802.11w
[edit] Non-standard 802.11 extensions and equipment
Many companies implement wireless networking equipment with non-IEEE standard 802.11 extensions either by implementing proprietary or draft features. These changes may lead to incompatibilities between these extensions.[citation needed]
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[edit] See also
[edit] External links
[edit] References
