IEEE 802.11e-2005

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IEEE 802.11e-2005 or 802.11e is an approved amendment to the IEEE 802.11 standard that defines a set of quality of service (QoS) enhancements for wireless LAN applications through modifications to the media access control (MAC) layer. [1] The standard is considered of critical importance for delay-sensitive applications, such as voice over wireless LAN and streaming multimedia. The amendment has been incorporated into the published IEEE 802.11-2007 standard.

Contents

Original 802.11 MAC

Distributed coordination function (DCF)

The basic 802.11 MAC layer uses the distributed coordination function (DCF) to share the medium between multiple stations. (DCF) relies on CSMA/CA and optional 802.11 RTS/CTS to share the medium between stations. This has several limitations:

Point coordination function (PCF)

The original 802.11 MAC defines another coordination function called the point coordination function (PCF). This is available only in "infrastructure" mode, where stations are connected to the network through an Access Point (AP). This mode is optional, and only very few APs or Wi-Fi adapters actually implement it.[ citation needed ] APs send beacon frames at regular intervals (usually every 100 TU or 0.1024 second). Between these beacon frames, PCF defines two periods: the Contention Free Period (CFP) and the Contention Period (CP). In the CP, DCF is used. In the CFP, the AP sends Contention-Free-Poll (CF-Poll) packets to each station, one at a time, to give them the right to send a packet. The AP is the coordinator. Although this allows for a better management of QoS, PCF does not define classes of traffic as is common with other QoS systems (e.g. 802.1p and DiffServ).

802.11e MAC protocol operation

A diagram of the seven-layer OSI model with the modifications made by the 802.11 standard and the 802.11e amendment OSI-80211e.png
A diagram of the seven-layer OSI model with the modifications made by the 802.11 standard and the 802.11e amendment

The 802.11e enhances the DCF and the PCF, through a new coordination function: the hybrid coordination function (HCF). Within the HCF, there are two methods of channel accessed, similar to those defined in the legacy 802.11 MAC: HCF Controlled Channel Access (HCCA) and Enhanced Distributed Channel Access (EDCA). Both EDCA and HCCA define Traffic Categories (TC). For example, emails could be assigned to a low priority class, and voice over wireless LAN (VoWLAN) could be assigned to a high priority class.

Enhanced distributed channel access (EDCA)

EDCA is a supported QoS mechanism in 802.11e. With EDCA, high-priority traffic has a higher chance of being sent than low-priority traffic: a station with high priority traffic waits a little less before it sends its packet, on average, than a station with low priority traffic. This is accomplished through the TCMA protocol, which is a variation of CSMA/CA using a shorter arbitration inter-frame space (AIFS) for higher priority packets. [3] The exact values depend on the physical layer that is used to transmit the data. In addition, EDCA provides contention-free access to the channel for a period called a Transmit Opportunity (TXOP). A TXOP is a bounded time interval during which a station can send as many frames as possible (as long as the duration of the transmissions does not extend beyond the maximum duration of the TXOP). If a frame is too large to be transmitted in a single TXOP, it should be fragmented into smaller frames. The use of TXOPs reduces the problem of low rate stations gaining an inordinate amount of channel time in the legacy 802.11 DCF MAC. A TXOP time interval of 0 means it is limited to a single MAC service data unit (MSDU) or MAC management protocol data unit (MMPDU).

The levels of priority in EDCA are called access categories (ACs). The contention window (CW) can be set according to the traffic expected in each access category, with a wider window needed for categories with heavier traffic. The CWmin and CWmax values are calculated from aCWmin and aCWmax values, respectively, that are defined for each physical layer supported by 802.11e.

Calculation of contention window boundaries
ACCWminCWmax
Background (AC_BK)aCWminaCWmax
Best Effort (AC_BE)aCWminaCWmax
Video (AC_VI)(aCWmin+1)/2-1aCWmin
Voice (AC_VO)(aCWmin+1)/4-1(aCWmin+1)/2-1

For a typical of aCWmin=15 and aCWmax=1023, as used, for example, by OFDM (802.11a) and MIMO (802.11n), the resulting values are as following:

Default EDCA parameters for each AC
ACCWminCWmaxAIFSNMax TXOP
Background (AC_BK)15102370
Best Effort (AC_BE)15102330
Video (AC_VI)71523.008ms
Voice (AC_VO)3721.504ms
Legacy DCF15102320

ACs map directly from Ethernet-level class of service (CoS) priority levels:

– 802.1p 802.11e
PriorityPriority code point (PCP)AbbreviationTraffic typeAccess category (AC)Designation
Lowest1BKBackgroundAC_BKBackground
2–SpareAC_BKBackground
0BEBest effortAC_BEBest effort
3EEExcellent effortAC_BEBest effort
4CLControlled loadAC_VIVideo
5VIVideoAC_VIVideo
6VOVoiceAC_VOVoice
Highest7NCNetwork controlAC_VOVoice

The primary purpose of QoS is to protect high priority data from low priority data. There are also scenarios in which the data needs to be protected from other data of the same class. Admission Control in EDCA address these type of problems. The AP publishes the available bandwidth in beacons. Clients can check the available bandwidth before adding more traffic.

Wi-Fi Multimedia (WMM) is the Wi-Fi Alliance specification which is a subset of IEEE 802.11e. Certified APs must be enabled for EDCA and TXOP. All other enhancements of 802.11e are optional.

HCF controlled channel access (HCCA)

The HCF (hybrid coordination function) controlled channel access (HCCA) works a lot like PCF. However, in contrast to PCF, in which the interval between two beacon frames is divided into two periods of CFP and CP, the HCCA allows for CFPs being initiated at almost anytime during a CP. This kind of CFP is called a Controlled Access Phase (CAP) in 802.11e. A CAP is initiated by the AP whenever it wants to send a frame to a station or receive a frame from a station in a contention-free manner. In fact, the CFP is a CAP too. During a CAP, the Hybrid Coordinator (HC)—which is also the AP—controls the access to the medium. During the CP, all stations function in EDCA. The other difference with the PCF is that Traffic Class (TC) and Traffic Streams (TS) are defined. This means that the HC is not limited to per-station queuing and can provide a kind of per-session service. Also, the HC can coordinate these streams or sessions in any fashion it chooses (not just round-robin). Moreover, the stations give info about the lengths of their queues for each Traffic Class (TC). The HC can use this info to give priority to one station over another, or better adjust its scheduling mechanism. Another difference is that stations are given a TXOP: they may send multiple packets in a row, for a given time period selected by the HC. During the CFP, the HC allows stations to send data by sending CF-Poll frames.

HCCA is generally considered the most advanced (and complex) coordination function. With the HCCA, QoS can be configured with great precision. QoS-enabled stations have the ability to request specific transmission parameters (data rate, jitter, etc.) which should allow advanced applications like VoIP and video streaming to work more effectively on a Wi-Fi network.

HCCA support is not mandatory for 802.11e APs. In fact, few (if any) APs currently available are enabled for HCCA.[ citation needed ] Implementing the HCCA on end stations uses the existing DCF mechanism for channel access (no change to DCF or EDCA operation is needed). Stations only need to be able to respond to poll messages. On the AP side, a scheduler and queuing mechanism is needed.

Other 802.11e specifications

In addition to HCCA, EDCA and TXOP, 802.11e specifies additional optional protocols for enhanced 802.11 MAC layer QoS:

Automatic power save delivery

In addition to the Power Save Polling mechanism, which was available pre-802.11e, new power save delivery and notification mechanisms have been introduced in 802.11e. APSD (automatic power save delivery) provides two ways to start delivery: ‘scheduled APSD’ (S-APSD) and ‘unscheduled APSD’ (U-APSD). With APSD, multiple frames may be transmitted together by the access point to a power-saving device during a service period. After the end of a service period, the device enters a doze state until next service period. With S-APSD, service periods start according to a predetermined schedule known to the power-saving device, thus allowing the Access Point to transmit its buffered traffic without the need for any signaling. With U-APSD, whenever a frame is sent to the Access Point, a service period is triggered, which allows the access point to send buffered frames in the other direction. U-APSD can take a ‘full’ U-APSD or ‘hybrid’ U-APSD form. With Full U-APSD, all types of frames use U-APSD independently of their priority. With Hybrid U-APSD, either U-APSD or the legacy Power Save Polling mechanism is used, depending on the access category. S-APSD is available for both channel access mechanisms, EDCA and HCCA, while U-APSD is available only for EDCA. [1] [4]

APSD is a more efficient power management method than legacy 802.11 Power Save Polling, leading to lower power consumption, as it reduces both the signaling traffic that would otherwise be needed for delivery of buffered frames to power-saving devices by an AP and the collision rate among power-save polls, typically transmitted immediately after the beacon TIM. S-APSD is more efficient than U-APSD because scheduled service periods reduce contention and because transmission between the access point and a power-saving device starts without the need for any signaling. A power-saving device using U-APSD must generate signaling frames to retrieve buffered traffic in the absence of uplink traffic, as for instance in the case of audio, video, or best effort traffic applications found in today's smartphones. U-APSD is attractive for VoIP phones, as data rates are roughly the same in both directions, thus requiring no extra signaling—an uplink voice frame can trigger a service period for the transmission of a downlink voice frame. [5] Hybrid U-APSD is less efficient than Full U-APSD because the Power Save Polling mechanism it employs for some access categories is less efficient than APSD, as explained above. The relative advantages of the various power-save mechanisms have been confirmed independently by simulations. [6] [7]

Block acknowledgments

Block acknowledgments allow an entire TXOP to be acknowledged in a single frame. This will provide less protocol overhead when longer TXOPs are specified.

NoAck

In QoS mode, service class for frames to send can have two values: QosAck and QosNoAck. Frames with QosNoAck are not acknowledged. This avoids retransmission of highly time-critical data.

Direct Link Setup allows direct station-to-station frame transfer within a basic service set. This is designed primarily for consumer use, where station-to-station transfer is more commonly used. For example, when streaming video to a television across the living room, or printing to a wireless printer in the same room, it can be more efficient to send Wi-Fi frames directly between the two communicating devices, instead of using the standard technique of always sending everything via the AP, which involves two radio hops instead of one. Also, If the AP is far away in some distant part of the home, sending all the frames to the AP and back may require them to be sent at a lower transmission rate. However, DLS requires participation from the AP to facilitate the more efficient direct communication, and few, if any, APs have the necessary support for this. Tunnelled Direct Link Setup was published as 802.11z (TDLS), allowing devices to perform more efficient direct station-to-station frame transfers without support from the AP. Both DLS and TDLS require that stations be associated with the same AP. Both DLS and TDLS improve the speed and efficiency of communications between members of a basic service set, but they do not facilitate communication between devices that are near to each other but not associated with the same AP.

Nearby communication between devices not associated with the same AP can be performed using technologies like Wi-Fi Direct, but so far Wi-Fi Direct has not seen widespread adoption.

Microsoft's Virtual Wi-Fi initiative was designed to accomplish the same goal as DLS. Virtual Wi-Fi allows gamers to connect wireless while accessing the Internet through an AP by allowing station adapters to have multiple MAC addresses. [8]

Related Research Articles

<span class="mw-page-title-main">IEEE 802.11</span> Wireless network standard

IEEE 802.11 is part of the IEEE 802 set of local area network (LAN) technical standards, and specifies the set of medium access control (MAC) and physical layer (PHY) protocols for implementing wireless local area network (WLAN) computer communication. The standard and amendments provide the basis for wireless network products using the Wi-Fi brand and are the world's most widely used wireless computer networking standards. IEEE 802.11 is used in most home and office networks to allow laptops, printers, smartphones, and other devices to communicate with each other and access the Internet without connecting wires. IEEE 802.11 is also a basis for vehicle-based communication networks with IEEE 802.11p.

<span class="mw-page-title-main">Wireless LAN</span> Computer network that links devices using wireless communication within a limited area

A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication to form a local area network (LAN) within a limited area such as a home, school, computer laboratory, campus, or office building. This gives users the ability to move around within the area and remain connected to the network. Through a gateway, a WLAN can also provide a connection to the wider Internet.

<span class="mw-page-title-main">Carrier-sense multiple access with collision avoidance</span> Computer network multiple access method

Carrier-sense multiple access with collision avoidance (CSMA/CA) in computer networking, is a network multiple access method in which carrier sensing is used, but nodes attempt to avoid collisions by beginning transmission only after the channel is sensed to be "idle". When they do transmit, nodes transmit their packet data in its entirety.

<span class="mw-page-title-main">Wireless access point</span> Device that allows wireless devices to connect to a wired network

In computer networking, a wireless access point, or more generally just access point (AP), is a networking hardware device that allows other Wi-Fi devices to connect to a wired network or wireless network. As a standalone device, the AP may have a wired or wireless connection to a router or router, but, in a wireless router, it can also be an integral component of the networking devices itself. An WAP & AP is differentiated from a hotspot, which can be a physical location or digital location where Wi-Fi or WAP access is available.

<span class="mw-page-title-main">Medium access control</span> Service layer in IEEE 802 network standards

In IEEE 802 LAN/MAN standards, the medium access control (MAC), also called media access control, is the layer that controls the hardware responsible for interaction with the wired or wireless transmission medium. The MAC sublayer and the logical link control (LLC) sublayer together make up the data link layer. The LLC provides flow control and multiplexing for the logical link, while the MAC provides flow control and multiplexing for the transmission medium.

Distributed coordination function (DCF) is the fundamental medium access control (MAC) technique of the IEEE 802.11-based WLAN standard. DCF employs a carrier-sense multiple access with collision avoidance (CSMA/CA) with the binary exponential backoff algorithm.

<span class="mw-page-title-main">Hidden node problem</span> Problem in wireless networking

In wireless networking, the hidden node problem or hidden terminal problem occurs when a node can communicate with a wireless access point (AP), but cannot directly communicate with other nodes that are communicating with that AP. This leads to difficulties in medium access control sublayer since multiple nodes can send data packets to the AP simultaneously, which creates interference at the AP resulting in no packet getting through.

<span class="mw-page-title-main">WiMAX</span> Wireless broadband standard

Worldwide Interoperability for Microwave Access (WiMAX) is a family of wireless broadband communication standards based on the IEEE 802.16 set of standards, which provide physical layer (PHY) and media access control (MAC) options.

IEEE 802.11i-2004, or 802.11i for short, is an amendment to the original IEEE 802.11, implemented as Wi-Fi Protected Access II (WPA2). The draft standard was ratified on 24 June 2004. This standard specifies security mechanisms for wireless networks, replacing the short Authentication and privacy clause of the original standard with a detailed Security clause. In the process, the amendment deprecated broken Wired Equivalent Privacy (WEP), while it was later incorporated into the published IEEE 802.11-2007 standard.

Wireless Multimedia Extensions (WME), also known as Wi-Fi Multimedia (WMM), is a Wi-Fi Alliance interoperability certification, based on the IEEE 802.11e standard. It provides basic Quality of service (QoS) features to IEEE 802.11 networks. WMM prioritizes traffic according to four Access Categories (AC): voice (AC_VO), video (AC_VI), best effort (AC_BE), and background (AC_BK). However, it does not provide guaranteed throughput. It is suitable for well-defined applications that require QoS, such as Voice over IP (VoIP) on Wi-Fi phones (VoWLAN).

IEEE 802.11r-2008 or fast BSS transition (FT), is an amendment to the IEEE 802.11 standard to permit continuous connectivity aboard wireless devices in motion, with fast and secure client transitions from one Basic Service Set to another performed in a nearly seamless manner. It was published on July 15, 2008. IEEE 802.11r-2008 was rolled up into 802.11-2012. The terms handoff and roaming are often used, although 802.11 transition is not a true handoff/roaming process in the cellular sense, where the process is coordinated by the base station and is generally uninterrupted.

<span class="mw-page-title-main">Beacon frame</span> Type of management frame

A beacon frame is a type of management frame in IEEE 802.11 WLANs. It contains information about the network. Beacon frames are transmitted periodically; they serve to announce the presence of a wireless LAN and to provide a timing signal to synchronise communications with the devices using the network. In an infrastructurebasic service set (BSS), beacon frames are transmitted by the access point (AP). In ad hoc (IBSS) networks, beacon generation is distributed among the stations. For the 2.4 GHz spectrum, when having more than 15 SSIDs on non-overlapping channels, beacon frames start to consume significant amount of air time and degrade performance even when most of the networks are idle.

Point Coordination Function (PCF) is a media access control (MAC) technique used in IEEE 802.11 based WLANs, including Wi-Fi. It resides in a point coordinator also known as access point (AP), to coordinate the communication within the network. The AP waits for PIFS duration rather than DIFS duration to grasp the channel. PIFS is less than DIFS duration and hence the point coordinator always has the priority to access the channel.

IEEE 802.11w-2009 is an approved amendment to the IEEE 802.11 standard to increase the security of its management frames.

The network allocation vector (NAV) is a virtual carrier-sensing mechanism used with wireless network protocols such as IEEE 802.11 (Wi-Fi) and IEEE 802.16 (WiMax). The virtual carrier-sensing is a logical abstraction which limits the need for physical carrier-sensing at the air interface in order to save power. The MAC layer frame headers contain a duration field that specifies the transmission time required for the frame, in which time the medium will be busy. The stations listening on the wireless medium read the Duration field and set their NAV, which is an indicator for a station on how long it must defer from accessing the medium.

Block acknowledgement (BA) was initially defined in IEEE 802.11e as an optional scheme to improve the MAC efficiency. 802.11n amendment ratified in 2009 enhances this BA mechanism then made it as mandatory to support by all 802.11n-capable devices.

Arbitration inter-frame spacing (AIFS), in wireless LAN communications, is a method of prioritizing one Access Category (AC) over the other, such as giving voice or video priority over email. AIFS functions by shortening or expanding the period a wireless node has to wait before it is allowed to transmit its next frame. A shorter AIFS period means a message has a higher probability of being transmitted with low latency, which is particularly important for delay-critical data such as voice or streaming video.

Traffic indication map (TIM) is a structure used in 802.11 wireless network management frames.

IEEE 802.11ah is a wireless networking protocol published in 2017 called Wi-Fi HaLow as an amendment of the IEEE 802.11-2007 wireless networking standard. It uses 900 MHz license-exempt bands to provide extended-range Wi-Fi networks, compared to conventional Wi-Fi networks operating in the 2.4 GHz, 5 GHz and 6 GHz bands. It also benefits from lower energy consumption, allowing the creation of large groups of stations or sensors that cooperate to share signals, supporting the concept of the Internet of things (IoT). The protocol's low power consumption competes with Bluetooth, LoRa, and Zigbee, and has the added benefit of higher data rates and wider coverage range.

In a WLAN, packets can be a stream of video, voice, or data, which each have different priorities to be served by an access point. The Traffic Identifier (TID) is an identifier used to classify a packet in Wireless LANs. When a base station receives an 802.11 frame with the TID set for audio, for example, the priority given is higher than a data frame.

References

  1. 1 2 M. Benveniste, "WLAN QoS", Chapter 3 in Emerging Technologies in Wireless LANs: Theory, Design, and Deployment, (B. Bing, ed.), Cambridge University Press, 2008, ISBN   978-0-521-89584-2.
  2. ↑ "802.11n: Next-Generation Wireless LAN Technology" (PDF). Broadcom Corporation. 21 April 2006.
  3. ↑ M. Benveniste, "Tiered Contention Multiple Access' (TCMA), a QoS-Based Distributed MAC Protocol", Proceedings PIMRC 2002, Lisboa, Portugal, September 2002
  4. ↑ Perez-Costa, X.; Camps-Mur, D.; Sashihara, T. (2005). "Analysis of the integration of IEEE 802.11E capabilities in battery limited mobile devices". IEEE Wireless Communications. 12 (6): 26–32. doi:10.1109/MWC.2005.1561942.
  5. ↑ M. Benveniste, "Guidelines for Power Management", Doc IEEE 802.11-04/073, January 2004
  6. ↑ Pérez-Costa, X.; Camps-Mur, D. (August 2010). "IEEE 802.11e QoS and Power Saving feature: Overview and Analysis of Combined Performance". IEEE Wireless Communications Magazine (WirComMag). Vol. 17, no. 4.
  7. ↑ Pérez-Costa, Xavier; Camps-Mur, Daniel; Vidal, Albert (2007). "On distributed power saving mechanisms of wireless LANs 802.11e U-APSD vs 802.11 power save mode". Computer Networks. 51 (9): 2326–2344. doi:10.1016/j.comnet.2007.01.026.
  8. ↑ "Windows 7 adds native Virtual WiFi technology from Microsoft Research". 16 May 2009. Retrieved 7 July 2010.
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