While This feature may interfere with other networks and may not support all b and g client cards. In addition, packet bursting techniques are also available in some chipsets and products which will also considerably increase speeds. The first major manufacturer to use Cisco joined by buying up Linksys, an early adopter, and also offers its own wireless mobile adaptors under the name Aironet.
It is projected that There are two competing variants of the The standardization process is expected to be completed by the end of The additional transmitter and receiver antennas allow for increased data throughput through spatial multiplexing and increased range by exploiting the spatial diversity, perhaps through coding schemes like Alamouti coding. Because the IEEE only sets specifications but does not test equipment for compliance with them, a trade group called the Wi-Fi Alliance runs a certification program that members pay to participate in.
Virtually all companies selling The Wi-Fi trademark, owned by the group and usable only on compliant equipment, is intended to guarantee interoperability. Currently, "Wi-Fi" can mean any of Eventually "Wi-Fi" will also mean equipment which implements the Products that say they are Wi-Fi are supposed to also indicate the frequency band in which they operate, 2.
In , a group from the University of California at Berkeley presented a paper describing weaknesses in the 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 titled, These started to appear in products in mid The Wi Fi Alliance definition of interoperability goes well beyond the ability to work in a Wi Fi network.
To gain certification under a specific program, products have to show satisfactory performance levels in typical network configurations and have to support both established and emerging applications. A user that purchases a Wi Fi enabled laptop, for instance, would not be satisfied if the laptop established a connection with the home network, only to get the throughput of a dial-up connection.
Similarly, subscribers using a Wi Fi enabled mobile phone would be disappointed, if a voice call could not go thru or was dropped. The Wi Fi Alliance certification process includes three types of tests to ensure interoperability.
Explore content Browse by Subject. Oral Histories. First Hand Histories. Regular channel measurements are important to monitoring the channel and power settings. Two information elements are defined to allow stations to request measurements and receive reports. Reports are a key component of One of the reasons for the development of dynamic frequency selection was the need to avoid certain military radar technologies.
To find the presence of radar or other interference, an AP can use the Quiet element, shown in Figure , to temporarily shut down the channel to improve the quality of measurements.
Quiet periods are scheduled. The count is the number of Beacon transmission intervals until the quiet period begins. It works in a similar fashion to the Channel Switch Count field. Quiet periods may also be periodically scheduled. If this field is zero, it indicates there are no scheduled quiet periods.
A non-zero value indicates the number of beacon intervals between quiet periods. Quiet periods do not need to last for an entire Beacon interval. This field specifies the number of time units the quiet period lasts.
Quiet periods do not necessarily have to begin with a Beacon interval. The Offset field is the number of time units after a Beacon interval that the next quiet period will begin. Naturally, it must be less than one Beacon interval. In an infrastructure network, the access point is responsible for dynamic frequency selection.
Independent networks must have a designated owner of the dynamic frequency selection DFS algorithm. After the header, it has the MAC address of the station responsible for maintaining DFS information, as well as a measurement interval. The bulk of the frame is a series of channel maps , which report what is detected on each channel. The channel map consists of a channel number, followed by a map byte, which has the following fields:. This bit will be set if frames from another network are detected during a measurement period.
This bit is set if the This bit is set when the received power is high, but the signal cannot be classified as either another The standard does not specify what power level is high enough to trigger this bit being set.
If a radar signal is detected during a measurement period, this bit will be set. Radar systems which must be detected are defined by regulators, not the If the channel was not measured, this bit will be set. Naturally, when there was no measurement taken, nothing can be detected in the band and the previous four bits will be set to zero.
To provide backwards compatibility, the ERP information element , shown in Figure , was defined. In its first iteration, it is three bit flags in a single byte. This bit will be set when an older, non It may also be set when overlapping networks that are not capable of using When stations incapable of operating at This enables backwards compatibility with older stations, as described in Chapter This bit will be set if the stations which have associated to the network are not capable of the short preamble mode described in Chapter With the significant security enhancements in There are several variable components, and in some cases, the RSN information element might run into the limits of the information element size of bytes past the header.
The version field must be present. Zero is reserved, and versions of two or greater are not yet defined. Following the version number is the group cipher suite descriptor. Access points must select a single group cipher compatible with all associated stations to protect broadcast and multicast frames. Only one group cipher is allowed. A cipher suite selector is four bytes long. It starts with an OUI for the vendor, and a number to identify the cipher.
Table shows the standardized cipher suites. Values not shown in the table are reserved. The OUI used by Following the group cipher suite may be several pairwise cipher suites to protect unicast frames. There is a two-byte count, followed by a series of supported cipher descriptors. The suite selector may be set to zero to indicate support for only the group cipher suite.
There is no limit, other than the size of the information element, on the number of supported pairwise ciphers. Like the pairwise cipher suite selector, there may be multiple authentication types defined. Following a count, there is a series of four-byte suite identifiers.
As with the cipher suites, the four-byte identifier consists of an OUI and a suite type number. Table shows the standard authentication types. Key derivation from preshared master key, as described in Chapter 7.
Key derivation from pre-shared key, as described in Chapter 7. This two-byte field consists of four flags used to describe what the transmitter is capable of, followed by reserved bits that must be set to zero. An AP may set this bit to indicate it can perform pre-authentication with other APs on the network to move security sessions around. Otherwise, this bit is set to zero. Preauthentication is discussed in Chapter 8.
This bit is set when a station can support a manual WEP key for broadcast data in conjunction with a stronger unicast key. Although supported by the standard, this configuration should not be used unless absolutely necessary.
Separate replay counters may be maintained for each priority level defined in emerging quality of service extensions. These bits describe the number of replay counters supported by the station. Faster hand-offs between access points are possible when the pairwise master key is cached by the AP. Stations may provide a list of master keys to an AP on association in an attempt to bypass the time-consuming authentication.
PMK caching is discussed in more detail in Chapter 8. The Extended Supported Rates information element acts identically to the Supported Rates element in Figure , but it allows an information element body of up to bytes to be supported.
Wi-Fi Protected Access is a slight modification of a subset of It is identical to the Robust Security Network information element in Figure , but with the following changes:. Only one cipher suite and one authentication suite are supported in the information element. However, many WPA implementations do not follow this restriction. Preauthentication is not supported in WPA, so the preauthentication capabilities bit is always zero. The fixed fields and information elements are used in the body of management frames to convey information.
Several types of management frames exist and are used for various link-layer maintenance functions. Beacon frames announce the existence of a network and are an important part of many network maintenance tasks. They are transmitted at regular intervals to allow mobile stations to find and identify a network, as well as match parameters for joining the network. In an infrastructure network, the access point is responsible for transmitting Beacon frames.
The area in which Beacon frames appear defines the basic service area. All communication in an infrastructure network is done through an access point, so stations on the network must be close enough to hear the Beacons.
Figure shows most the fields that can be used in a Beacon frame in the order in which they appear. Not all of the elements are present in all Beacons. Optional fields are present only when there is a reason for them to be used. The FH and DS Parameter Sets are used only when the underlying physical layer is based on frequency hopping or direct-sequence techniques. The TIM element is used only in Beacons generated by access points, because only access points perform frame buffering.
If the Country-specific frequency hopping extensions were to be present, they would follow the Country information element. Frequency hopping networks are much less common now, though, so I omit the frequency hopping extensions for simplicity.
Mobile stations use Probe Request frames to scan an area for existing The format of the Probe Request frame is shown in Figure All fields are mandatory. Stations that receive Probe Requests use the information to determine whether the mobile station can join the network. To make a happy union, the mobile station must support all the data rates required by the network and must want to join the network identified by the SSID.
This may be set to the SSID of a specific network or set to join any compatible network. If a Probe Request encounters a network with compatible parameters, the network sends a Probe Response frame. The station that sent the last Beacon is responsible for responding to incoming probes. In infrastructure networks, this station is the access point. After a station transmits a Beacon, it assumes responsibility for sending Probe Response frames for the next Beacon interval.
The format of the Probe Response frame is shown in Figure Some of the fields in the frame are mutually exclusive; the same rules apply to Probe Response frames as to Beacon frames. The Probe Response frame carries all the parameters in a Beacon frame, which enables mobile stations to match parameters and join the network. Probe Response frames can safely leave out the TIM element because stations sending probes are not yet associated and thus would not need to know which associations have buffered frames waiting at the access point.
IBSSs have no access points and therefore cannot rely on access points for buffering. When a station in an IBSS has buffered frames for a receiver in low-power mode, it sends an ATIM frame during the delivery period to notify the recipient it has buffered data.
See Figure Disassociation frames are used to end an association relationship, and Deauthentication frames are used to end an authentication relationship. Both frames include a single fixed field, the Reason Code, as shown in Figure Of course, the Frame Control fields differ because the subtype distinguishes between the different types of management frames. Once mobile stations identify a compatible network and authenticate to it, they may attempt to join the network by sending an Association Request frame.
The format of the Association Request frame is shown in Figure The Capability Information field is used to indicate the type of network the mobile station wants to join. Before an access point accepts an association request, it verifies that the Capability Information, SSID, and Extended Supported Rates all match the parameters of the network.
Access points also note the Listen Interval, which describes how often a mobile station listens to Beacon frames to monitor the TIM. Stations that support spectrum management will have the power and channel capability information elements, and stations supporting security will have the RSN information element. Mobile stations moving between basic service areas within the same extended service area need to reassociate with the network before using the distribution system again.
Stations may also need to reassociate if they leave the coverage area of an access point temporarily and rejoin it later. Including this information allows the new access point to contact the old access point and transfer the association data. The transfer may include frames that were buffered at the old access point. When mobile stations attempt to associate with an access point, the access point replies with an Association Response or Reassociation Response frame, shown in Figure The two differ only in the subtype field in the Frame Control field.
As part of the response, the access point assigns an Association ID. How an access point assigns the association ID is implementation-dependent. At the beginning of With If a station uses shared key authentication, it will not be allowed to use the strong security protocols described in Chapter 8. Different authentication algorithms may co-exist. The Authentication Algorithm Number field is used for algorithm selection.
The authentication process may involve a number of steps depending on the algorithm , so there is a sequence number for each frame in the authentication exchange. The Status Code and Challenge Text are used in different ways by different algorithms; details are discussed in Chapter 8.
Allowed frame types vary with the association and authentication states. Stations are either authenticated or unauthenticated and can be associated or unassociated. These two variables can be combined into three allowed states, resulting in the Each state is a successively higher point in the development of an All mobile stations start in State 1, and data can be transmitted through a distribution system only in State 3.
IBSSs do not have access points or associations and thus only reach Stage 2. Figure is the overall state diagram for frame transmission in Frames are also divided into different classes. Class 1 frames can be transmitted in State 1; Class 1 and 2 frames in State 2; and Class 1, 2, and 3 frames in State 3. Class 1 frames may be transmitted in any state and are used to provide the basic operations used by Class 1 frames also allow stations to find an infrastructure network and authenticate to it.
Table shows a list of the frames that belong to the Class 1 group. Class 2 frames can be transmitted only after a station has successfully authenticated to the network, and they can be used only in States 2 and 3. Class 2 frames manage associations. Successful association or reassociation requests move a station to State 3; unsuccessful association attempts cause the station to stay in State 2.
When a station receives a Class 2 frame from a nonauthenticated peer, it responds with a Deauthentication frame, dropping the peer back to State 1. Class 3 frames are used when a station has been successfully authenticated and associated with an access point. Once a station has reached State 3, it is allowed to use distribution system services and reach destinations beyond its access point.
Stations may also use the power-saving services provided by access points in State 3 by using the PS-Poll frame. Table lists the different types of Class 3 frames. If an access point receives frames from a mobile station that is authenticated but not associated, the access point responds with a Disassociation frame to bump the mobile station back to State 2.
If the mobile station is not even authenticated, the access point responds with a Deauthentication frame to force the mobile station back into State 1. Many products will pass all broadcasts up to higher protocol layers without validating the BSSID first.
This convention will be followed throughout the chapter. Presumably, the International Bureau of Weights and Measures would protest the mangling of the traditional form of the prefixes. Filtering prevents frames from a different BSS from triggering a rejection. Skip to main content. Start your free trial. Chapter 4. Data Frames. Table Categorization of data frame types. Frame Control. Addressing and DS Bits. Use of the address fields in data frames. Figure Address field usage in frames to the distribution system.
Address field usage in frames from the distribution system. Variations on the Data Frame Theme. Data Frames of the Data subtype are transmitted only during the contention-based access periods. Null Null frames [ 21 ] are a bit of an oddity. Applied Data Framing. IBSS frames. Frames from the AP. Frames to the AP. Frames in a WDS. Encrypted frames. Control Frames.
Common Frame Control Field. Protocol version The protocol version is shown as 0 in Figure because that is currently the only version. Type Control frames are assigned the Type identifier Subtype This field indicates the subtype of the control frame that is being transmitted.
ToDS and FromDS bits Control frames arbitrate access to the wireless medium and thus can only originate from wireless stations. More Fragments bit Control frames are not fragmented, so this bit is always 0. Retry bit Control frames are not queued for retransmission like management or data frames, so this bit is always 0.
Power Management bit This bit is set to indicate the power management state of the sender after conclusion of the current frame exchange. More Data bit The More Data bit is used only in management and data frames, so this bit is set to 0 in control frames. Protected Frame bit Control frames may not be encrypted.
Order bit Control frames are used as components of atomic frame exchange operations and thus cannot be transmitted out of order. Request to Send RTS. Frame Control There is nothing special about the Frame Control field. Duration An RTS frame attempts to reserve the medium for an entire frame exchange, so the sender of an RTS frame calculates the time needed for the frame exchange sequence after the RTS frame ends.
Address 1: Receiver Address The address of the station that is the intended recipient of the large frame. Clear to Send CTS. Acknowledgment ACK. Duration The duration may be set in one of two ways, depending on the position of the ACK within the frame exchange. Management Frames. The Structure of Management Frames. Address fields. Duration calculations.
Frame body. Fixed-Length Management Frame Components. Authentication Algorithm Number. Values of the Authentication Algorithm Number field. Value Meaning 0 Open System authentication typically used with Authentication Transaction Sequence Number. Beacon interval. Capability Information.
Short Preamble This field was added to PBCC This field was added to Channel Agility This field was added to Short Slot Time Contention-free polling bits Stations and access points use these two bits as a label. Interpretation of polling bits in Capability Information. CF-Pollable CF-Poll Request Interpretation Station usage 0 0 Station does not support polling 0 1 Station supports polling but does not request to be put on the polling list 1 0 Station supports polling and requests a position on the polling list 1 1 Station supports polling and requests that it never be polled results in station treated as if it does not support contention-free operation Access point usage 0 0 Access point does not implement the point coordination function 0 1 Access point uses PCF for delivery but does not support polling 1 0 Access point uses PCF for delivery and polling 1 1 Reserved; unused.
Current AP Address. Listen interval. Association ID. Reason Code. Reason codes. Code Explanation 0 Reserved; unused 1 Unspecified 2 Prior authentication is not valid 3 Station has left the basic service area or extended service area and is deauthenticated 4 Inactivity timer expired and station was disassociated 5 Disassociated due to insufficient resources at the access point 6 Incorrect frame type or subtype received from unauthenticated station 7 Incorrect frame type or subtype received from unassociated station 8 Station has left the basic service area or extended service area and is disassociated 9 Association or reassociation requested before authentication is complete 10 Status Code.
Status codes. Code Explanation 0 Operation completed successfully 1 Unspecified failure Reserved; unused 10 Requested capability set is too broad and cannot be supported 11 Reassociation denied; prior association cannot be identified and transferred 12 Association denied for a reason not specified in the Management Frame Information Elements.
Information elements. Supported Rates. Supported Rate labels. Binary value Corresponding rate Mbps 2 1 4 2 11 Go to the recovery procedure here. I forgot Don't remember your password.
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