Ieee 802.11 mac frame format

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Writing code in comment? Please use ide. It only offers asynchronous service.

IEEE 802.11

It uses a centralized, contention-free polling access method. It offers both asynchronous and time-bounded service. Frame Control FC — It is 2 bytes long field which defines type of frame and some control information. Various fields present in FC are: It is a 2 bit long field which indicates the current protocol version which is fixed to be 0 for now.


It is a 2 bit long field which determines the function of frame i. The value 11 is reserved. It is a 4 bit long field which indicates sub-type of the frame like for association request, for beacon. To DS: It is a 1 bit long field which when set indicates that destination frame is for DS distribution system.

From DS: It is a 1 bit long field which when set indicates frame coming from DS. More frag More fragments: It is 1 bit long field which when set to 1 means frame is followed by other fragments. It is 1 bit long field, if the current frame is a retransmission of an earlier frame, this bit is set to 1.

Power Mgmt Power management: In the case of an IBSS, no access points are used, and no distribution system is present. The transmitter is the source, and the receiver is the destination. All frames carry the BSSID so that stations may check broadcasts and multicasts; only stations that belong to the same BSS will process broadcasts and multicasts.

The transmitter sends a frame on to the wireless medium but does not necessarily create the frame. A similar distinction holds for destination addresses and receiver addresses. A receiver may be an intermediate destination, but frames are processed by higher protocol levels only when they reach the destination. When mobile stations associate with an access point, the access point assigns a value called the Association ID AID from the range , The AID is used for a variety of purposes that appear throughout this book.

Several distinct To maximize the probability of creating a unique address, 46 random bits are generated for the BSSID. BSSID broadcasts are used only when mobile stations try to locate a network by sending probe requests. In order for probe frames to detect the existence of a network, they must not be filtered by the BSSID filter. To expand on these distinctions, consider the use of the address fields in infrastructure networks.

In the case of frames bound for a destination on the distribution system, the client is both source and transmitter. The receiver of the wireless frame is the access point, but the access point is only an intermediate destination. When the frame reaches the access point, it is relayed to the distribution system to reach the server. Thus, the access point is the receiver, and the ultimate destination is the server. In infrastructure networks, access points create associated BSSs with the address of their wireless interfaces, which is why the receiver address Address 1 is set to the BSSID.

When frames are relayed through the access point, the access point uses its wireless interface as the transmitter address. Frames are ultimately sent to the client, which is both the destination and receiver. Frames bound from the client to the server traverse the The source and destination addresses of the wireless frames remain the client and server addresses. These frames, however, also identify the transmitter and receiver of the frame on the wireless medium. For frames bound from the client to the server, the transmitter is the client-side access point, and the receiver is the server-side access point.

Separating the source from the transmitter allows the server-side access point to send required Variations depend on whether the service is contention-based or contention-free. Contention-free frames can incorporate several functions for the sake of efficiency. Data may be transmitted, but by changing the frame subtype, data frames in the contention-free period may be used to acknowledge other frames, saving the overhead of interframe spaces and separate acknowledgments. Here are the different data frame types that are commonly used:. Frames of the Data subtype are transmitted only during the contention-based access periods.

They are simple frames with the sole purpose of moving the frame body from one station to another. Null frames [ 21 ] are a bit of an oddity. In a traditional Ethernet, empty frames would be extraneous overhead; in When stations sleep, the access point must begin buffering frames for the sleeping station. If the mobile station has no data to send through the distribution system, it can use a Null frame with the Power Management bit in the Frame Control field set.

Access points never enter power-saving mode and do not transmit Null frames. Several other frame types exist for use within the contention-free period. The form of a data frame can depend on the type of network. The actual subtype of the frame is determined solely by the subtype field, not by the presence or absence of other fields in the frame.

The first address identifies the receiver, which is also the destination address in an IBSS. The second address is the source address. IBSS data frames have the subtype data or Null; the latter is used only to communicate power management state. The second address holds the transmitter address. On infrastructure networks, the transmitter address is the address of the station in the access point, which is also the BSSID. Finally, the frame indicates the source MAC address of the frame. The split between source and transmitter is necessary because the Nothing in the Access points are forbidden from using the power-saving routines, and they can acknowledge Null frames from stations without using Null frames in response.

In practice, access points send Data frames during the contention-based access period, and they send frames incorporating the CF-Poll feature during the contention-free period. Access points do not perform filtering, but instead use the third address to forward data to the appropriate location in the distribution system. Mobile stations in an infrastructure network cannot become the point coordinator, and thus never send frames that incorporate the contention-free polling CF-Poll functions. Like all other data frames, WDS frames use the first address for the receiver of the frame and the second address for the transmitter.

Two more address fields are necessary to indicate the source and destination of the frame and distinguish them from the addresses used on the wireless link. On a wireless bridging link, there are no mobile stations, and the contention-free period is not used. Access points are forbidden to enter power-saving modes, so the power management bit is always set to 0. Frames protected by link layer security protocols are not new frame types.

Control frames assist in the delivery of data frames. They administer access to the wireless medium but not the medium itself and provide MAC-layer reliability functions. Other versions may exist in the future. Control frames are assigned the Type identifier By definition, all control frames use this identifier. This field indicates the subtype of the control frame that is being transmitted. Control frames arbitrate access to the wireless medium and thus can only originate from wireless stations.

The distribution system does not send or receive control frames, so these bits are always 0. Control frames are not queued for retransmission like management or data frames, so this bit is always 0. This bit is set to indicate the power management state of the sender after conclusion of the current frame exchange. The More Data bit is used only in management and data frames, so this bit is set to 0 in control frames. Control frames may not be encrypted. Thus, for control frames, the Protected Frame bit is always 0. Control frames are used as components of atomic frame exchange operations and thus cannot be transmitted out of order.

Therefore, this bit is set to 0. Access to the medium can be reserved only for unicast frames; broadcast and multicast frames are simply transmitted. Like all control frames, the RTS frame is all header. No data is transmitted in the body, and the FCS immediately follows the header. There is nothing special about the Frame Control field. The frame subtype is set to to indicate an RTS frame, but otherwise, it has all the same fields as other control frames. The most significant bits in the 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.

Fragmentation bursts use subsequent fragments to update the Duration field. The number of microseconds required for the transmission is calculated and placed in the Duration field. If the result is fractional, it is rounded up to the next microsecond. CTS frames were later adopted for use by the The protection mechanism is described with the rest of By the time the CTS frame is transmitted, though, only the pending frame or fragment and its acknowledgment remain.

Rules for CTS frames used in protection exchanges are described with the protection mechanism. CTS frames used in Quality-of-service enhancements relax the requirement for a single acknowledgment per Data frame. The duration may be set in one of two ways, depending on the position of the ACK within the frame exchange. ACKs for complete data frames and final fragments in a fragment burst set the duration to 0.

The data sender indicates the end of a data transmission by setting the More Fragments bit in the Frame Control header to 0. If the More Fragments bit is 0, the transmission is complete, and there is no need to extend control over the radio channel for additional transmissions. Thus, the duration is set to 0. If the More Fragments bit is 1, a fragment burst is in progress. In fact, the The receiver address is copied from the transmitter of the frame being acknowledged.

Technically, it is copied from the Address 2 field of the frame being acknowledged. Acknowledgments are transmitted in response to directed data frames, management frames, and PS-Poll frames. When a mobile station wakes from a power-saving mode, it transmits a PS-Poll frame to the access point to retrieve any frames buffered while it was in power-saving mode. This is a numeric value assigned by the access point to identify the association.

Including this ID in the frame allows the access point to find any frames buffered for the now-awakened mobile station. The automatic NAV update allows the access point to transmit an ACK with a small probability of collision with a mobile station. Management is a large component of the Several different types of management frames are used to provide services that are simple on a wired network. Establishing the identity of a network station is easy on a wired network because network connections require dragging wires from a central location to the new workstation.

In many cases, patch panels in the wiring closet are used to speed up installation, but the essential point remains: Wireless networks must create management features to provide similar functionality. Mobile stations in search of connectivity must first locate a compatible wireless network to use for access. With wired networks, this step typically involves finding the appropriate data jack on the wall. Next, the network must authenticate mobile stations to establish that the authenticated identity is allowed to connect to the network.

The wired-network equivalent is provided by the network itself. If signals cannot leave the wire, obtaining physical access is at least something of an authentication process. Finally, mobile stations must associate with an access point to gain access to the wired backbone, a step equivalent to plugging the cable into a wired network.

The MAC header is the same in all management frames; it does not depend on the frame subtype. Management frames use information elements , little chunks of data with a numerical label, to communicate information to other systems. Some management frames are used to maintain properties within a single BSS. To limit the effect of broadcast and multicast management frames, stations are required to inspect the BSSID after receiving a management frame, though not all implementations perform BSSID filtering.

The one exception to this rule is Beacon frames, which are used to announce the existence of an BSSIDs are assigned in the familiar manner. One exception to the rule: Any frames transmitted in the contention-free period set the duration to 32, Frames transmitted during the contention-based access periods using only the DCF use the Duration field to block access to the medium to allow any atomic frame exchanges to complete.

If the frame is a broadcast or multicast the destination address is a group address , the duration is 0. Broadcast and multicast frames are not acknowledged, so the NAV is not needed to block access to the medium. If a nonfinal fragment is part of a multiframe exchange, the duration is set to the number of microseconds taken up by three SIFS intervals plus the next fragment and its acknowledgment. Final fragments use a duration that is the time required for one acknowledgment plus one SIFS.

Management frames are quite flexible. Most of the data contained in the frame body uses fixed-length fields called fixed fields and variable-length fields called information elements. Information elements are blobs of data of varying size. Each data blob is tagged with a type number and a size, and it is understood that an information element of a certain type has its data field interpreted in a certain way.

New information elements can be defined by newer revisions to the Fortunately, new options usually can be easily turned off for compatibility. This section presents the fixed fields and information elements as building blocks and shows how the building blocks are assembled into management frames.

This book shows all the frame building blocks in the specified order, and the discussion of each subtype notes which elements are rare and which are mutually exclusive. Fixed-length fields are often referred to simply as fields to distinguish them from the variable-length information elements. Fields do not have a header to distinguish them from other parts of the frame body. Because they have a fixed length and appear in a known order, fields can be delimited without using a field header. This field identifies the type of authentication used in the initial Only two values are currently defined.

Other values are reserved for future standardization work. Authentication is a multistep process that consists of a challenge from the access point and a response from the mobile station attempting to associate. It takes values from 1 to 65,; it is never set to 0. Beacon transmissions announce the existence of an Beacon frames carry information about the BSS parameters and the frames buffered by access points, so mobile stations must listen to Beacons.

One time unit, which is often abbreviated TU, is 1, microseconds ms , which is about 1 millisecond. It is common for the Beacon interval to be set to time units, which corresponds to an interval between Beacon transmissions of approximately milliseconds or 0. In this field, each bit is used as a flag to advertise a particular function of the network. Stations use the capability advertisement to determine whether they can support all the features in the BSS. Stations that do not implement all the features in the capability advertisement are not allowed to join. These two bits are mutually exclusive.

Setting the Privacy bit to 1 requires the use of WEP for confidentiality. In infrastructure networks, the transmitter is an access point. This field was added to Zero means the option is not in use and is forbidden in the BSS. Zero means that the option is not in use and is forbidden in the BSS. This bit is set to one to indicate the use of the shorter slot time supported by Stations and access points use these two bits as a label.

Station supports polling and requests that it never be polled results in station treated as if it does not support contention-free operation. This field is used to ease associations and reassociations. Stations transmit the address of the access point that handled the last association with the network. When an association is established with a different access point, this field can be used to transfer the association and retrieve any buffered frames. To save battery power, stations may shut off the antenna units in While stations are sleeping, access points must buffer frames for them.

Dozing stations periodically wake up to listen to traffic announcements to determine whether the access point has any buffered frames. When stations associate with an access point, part of the saved data is the Listen Interval , which is the number of Beacon intervals that stations wait between listening for Beacon frames.

Higher listen intervals require more access point memory for frame buffering. Access points may use this feature to estimate the resources that will be required and may refuse resource-intensive associations.

Wireless Frames Format And Types

When stations associate with an access point, they are assigned an Association ID to assist with control and management functions. Even though 14 bits are available for use in creating Association IDs, they range only from , The master timekeeper for a BSS periodically transmits the number of microseconds it has been active. When the counter reaches its maximum value, it wraps around. Counter wraps are unlikely given the length of time it takes to wrap a bit counter. At over , years, I would bet on a required patch or two before the counter wrap. Stations may send Disassociation or Deauthentication frames in response to traffic when the sender has not properly joined the network.

Fully understanding the use of reason codes requires an understanding of the different classes of frames and states of the Station has left the basic service area or extended service area and is deauthenticated. Station has left the basic service area or extended service area and is disassociated. Disassociated because of unacceptable values in Power Capability element. Disassociated because of unacceptable values in Supported Channels element. Invalid information element added with Status codes indicate the success or failure of an operation.

Reassociation denied; prior association cannot be identified and transferred. Authentication rejected; the next frame in the sequence did not arrive in the expected window. Association denied; the mobile station does not support all of the data rates required by the BSS. Association denied; the mobile station does not support the Short Preamble option. Association denied; the mobile station does not support the PBCC modulation option. Association denied; the mobile station does not support the Channel Agility option. Information elements are variable-length components of management frames.

Reserved [ a ] formerly for challenge text extension, before However, it is widely implemented, so I include it in the table. Network managers are only human, and they usually prefer to work with letters, numbers, and names rather than bit identifiers. Stations attempting to join a network may scan an area for available networks and join the network with a specified SSID. The SSID is the same for all the basic service areas composing an extended service area.

Some documentation refers to the SSID as the network name because network administrators frequently assign a character string to it. Some products require that the string be a garden variety ASCII string, though the standard has no requirement on the content of the string. In all cases, the length of the SSID ranges between 0 and 32 bytes. The zero-byte case is a special case called the broadcast SSID ; it is used only in Probe Request frames when a station attempts to discover all the Several data rates have been standardized for wireless LANs.

The Supported Rates information element allows an When mobile stations attempt to join the network, they check the data rates used in the network. Some rates are mandatory and must be supported by the mobile station, while others are optional. It consists of a string of bytes. Each byte uses the seven low-order bits for the data rate; the most significant bit indicates whether the data rate is mandatory.

Mandatory rates are encoded with the most significant bit set to 1 and optional rates have a 0. Up to eight rates may be encoded in the information element. As the number of data rates has proliferated, the Extended Supported Rates element was standardized to handle more than eight data rates. In the initial revision of the When 7 bits are used to have a multiple of kbps, the maximum data rate that can be encoded is Research and development on wireless LAN technology has made this rate achievable in the near future.

As a result, the IEEE changed the interpretation from a multiple of kbps to a simple label in Previously standardized rates were given labels corresponding to the multiple of kbps, but future standards may use any value. This is encoded as a mandatory 2-Mbps rate and an optional Mbps rate. The FH Parameter Set has four fields that uniquely specify an The amount of time spent on each channel in the hopping sequence is called the dwell time.

It is expressed in time units TUs. Several hopping patterns are defined by the This field, a single byte, identifies the set of hop patterns in use. Stations select one of the hopping patterns from the set. This field, also a single byte, identifies the hopping pattern in use.

Each pattern consists of a long sequence of channel hops. This field, a single byte, identifies the current point in the hop sequence. Direct-sequence High-rate direct sequence networks use the same channels and thus can use the same parameter set. Access points buffer frames for mobile stations sleeping in low-power mode.

Periodically, the access point attempts to deliver buffered frames to sleeping stations. A practical reason for this arrangement is that much more power is required to power up a transmitter than to simply turn on a receiver. The designers of The meat of the traffic indication map is the virtual bitmap , a logical structure composed of 2, bits. Each bit is tied to the Association ID. When traffic is buffered for that Association ID, the bit is 1.

Explain IEEE MAC frame format in detail

If no traffic is buffered, the bit tied to the Association ID is 0. DTIM frames indicate that buffered broadcast and multicast frames will be delivered shortly.

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Zero is reserved and is not used. The DTIM count cycles through from the period down to 0. The Bitmap Control field is divided into two subfields. Bit 0 is used for the traffic indication status of Association ID 0, which is reserved for multicast traffic. The remaining seven bits of the Bitmap Control field are used for the Bitmap Offset field.

To save transmission capacity, the Bitmap Offset field can be used to transmit a portion of the virtual bitmap. The Bitmap Offset is related to the start of the virtual bitmap. By using the Bitmap Offset and the Length, The CF Parameter Set information element is transmitted in Beacons by access points that support contention-free operation. The initial Rather than continue to revise the specification each time a new country was added, a new specification was added that provides a way for networks to describe regulatory constraints to new stations. Each constraint descriptor specifies a unique band, and they may not overlap, since a given frequency has only one maximum allowed power.

The first two letters are the ISO country code e. Many countries have different indoor and outdoor regulations, and the third character distinguishes between the two. When a single set of omnibus regulations covers all environments, the third character is a space. The first channel number is the lowest channel subject to the power constraint.

Channel number assignment for each PHY is discussed in the appropriate chapter. The size of the band subject to the power constraint is indicated by the number of channels. The size of a channel is PHY-dependent. The size of the information element must be an even number of bytes. If the length of the information element is an odd number of bytes, a single byte of zeroes is appended as a pad. These two elements can be used to build a hopping pattern that complies with regulatory constraints in additional countries, which allows further adoption of the frequency-hopping PHY without requiring additional revision to the specification.

In Probe Request frames, the Request information element is used to ask the network for certain information elements. The shared-key authentication system defined by The Power Constraint information element is used to allow a network to describe the maximum transmit power to stations. In addition to a regulatory maximum, there may be another maximum in effect. The only field, a one-byte integer, is the number of decibels by which any local constraint reduces the regulatory maximum. For stations to know how to tune transmission power, it helps to know the attenuation on the link.

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  5. The first, the transmit power, is the transmit power of the frame containing the information element, in units of dBm. The second, the link margin , represents the number of decibels of safety that the station requires. The Supported Channels information element is similar to the Country information element, in that it describes sub-bands that are supported. After the header, there is a series of sub-band descriptors.

    For example, a device that only supported channels 40 through 52 would set the first channel number to 40, and the number of channels to When the operating channel is changed, it disrupts communication. If this field is set to 1, associated stations should stop transmitting frames until the channel switch has occurred.

    If it is set to zero, there is no restriction on frame transmission. The new channel number after the switch. At present, there is no need for this field to exceed a value of Channel switching can be scheduled. This field is the number of Beacon frame transmission intervals that it will take to change the channel. Channel switch occurs just before the Beacon transmission is to begin. A non-zero value indicates the number of Beacon intervals to wait; a zero indicates that the channel switch may occur without any further warning. 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. 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.