IEEE 802.11e

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IEEE 802.11e is an industry standard of the Institute of Electrical and Electronics Engineers (IEEE) and an extension of the wireless LAN standard IEEE 802.11 to support the quality of service concept .

The IEEE working group involved in developing the standard completed its work in July 2005 . The final publication took place at the end of 2005.

802.11e works on a similar principle as the QoS method DiffServ . The sender marks the data packets in the WLAN with a priority. The access point should give priority to packets with a higher priority. “Real” QoS only supports the IEEE 802.16 protocol.

With the help of 802.11e, real-time applications such as B. Voice over IP better supported. The protocol guarantees a certain bandwidth in the network so that it is ensured that data packets arrive at the recipient within a certain time. This is to ensure that there are e.g. B. in Internet telephony there is no dropout in the connection due to other network traffic.

Original 802.11 MAC format

DCF

The original 802.11 Medium Access Layer (MAC) uses the Distributed Coordination Function (DCF) to share the medium with several stations. The DCF is based on CSMA / CA ( Carrier sense multiple access with collision avoidance ) and optionally on 802.11 RTS / CTS (Request to Send / Clear to Send). This has several disadvantages:

  • If many stations want to communicate at the same time, the more frequent collisions they cause lead to a reduced net data rate (as with Ethernet, which uses CSMA / CD).
  • There is no prioritization for traffic.
  • Once a station gains access to the media, it can keep that access as long as necessary. A low data rate (e.g. 1 Mbit / s) of a station means that it prevents access from other stations. These suffer .
  • Generally speaking, there are no QoS guarantees.

PCF

The Point Coordination Function (PCF) is included in the standard, but is rarely implemented. In PCF mode, the access point takes full control of access to the medium. The normal access control CSMA / CA is switched off repeatedly for a short time according to certain rules. The access point calls up all mobile stations ( polling ) that have previously registered with it as PCF participants and allows them to send their data. It then switches back to CSMA / CA mode. The start of a PCF period is signaled in a signal frame ( beacon frame ) with the "Duration" field.

802.11e MAC protocol

A new coordination function was introduced with 802.11e: the Hybrid Coordination Function (HCF, hybrid coordination function). Within this HCF there are two options for channel access (the purpose of which is similar to the original): HCF Controlled Channel Access (HCCA) and Enhanced Distributed Channel Access (EDCA). Both define data traffic classes (TC, Traffic Classes ). These protocols allow different data to be given different priorities.

EDCA

With EDCA, important data packets have a higher chance of being sent, since higher-priority traffic has to wait less long on average before the packet can be sent. In addition, each priority rank is marked with a Transmit Opportunity (TXOP). The TXOP is a fixed time interval in which a station is allowed to send as many frames as possible. If the time is too short to send a frame within a TXOP, the frame must be split. This reduces the problem of slow stations slowing down faster stations.

Access points that are certified for Wi-Fi Multimedia (WMM) must support EDCA and TXOP. All other enhancements are optional for WMM.

Access delay on the radio channel with 4 different traffic classes

HCCA

HCCA works in a similar way to PCF: The interval between two signal frames ( beacon frames ) is divided into two periods, called the Contention Free Period (CFP) and the Contention Period (CP). During the CFP, the Access Point called Hybrid Coordinator (HC) controls access to the medium. During the CP, the stations work in EDCA mode. The main difference to PCF is that Traffic Classes (TC) are also defined here. The HC can also decide how to direct the traffic. The stations also provide information about their queue length for each TC. The HC can then decide whether to give a station priority.

Generally speaking, HCCA is currently the most developed (and most complex) coordination function. With HCCA, QoS can be configured with great accuracy. QoS stations have the ability to query certain transmission parameters (data rate, jitter, etc.). This should help make applications such as VoIP or video transmission work more effectively.

HCCA is not mandatory for 802.11e access points. Few APs currently available have this functionality. The Wi-Fi Alliance has therefore introduced a further certificate ( WMM Scheduled Access ) in order to be able to identify APs with HCCA more easily.

Other 802.11e specifications

In addition to HCCA, EDCA and TXOP, there are other optional protocols for 802.11e:

APSD

Automatic Power Save Delivery is a more efficient power management method. Most newer devices already support a service that works in a similar way to Power Safe Mode. APSD is very useful for VoIP devices. Whenever voice data is received by the AP, the AP sends the buffered voice data in the other direction as well. The VoIP telephones then go into twilight mode until the next data has to be sent.

BA

Block acknowledgments allow an entire TXOP to be confirmed with just one frame. This creates less protocol overburden for longer TXOPs.

NoAck

In QoS mode, the service class for frames to be sent can have two values: QosAck and QosNoAck. Frames with QosNoAck are not confirmed. This avoids retransmissions of time-critical data.

DLS

Direct Link Setup allows direct station-to-station frame transfer despite the base station. In this way, multimedia components can use a higher bandwidth with less delay, since the traffic is not handled via the access point but directly. This is particularly desirable when two components are closer to each other than to the access point.

See also

WMM is the Wi-Fi multimedia specification of the Wi-Fi Alliance , which is a subset of IEEE 802.11e.

literature

  • Hermann Pommer: Roaming between Wireless Local Area Networks: A technical system must be viewed in an interdisciplinary manner in order to meet the requirements of the market . Vdm Verlag Dr. Müller, 2008, ISBN 978-3-8364-8708-5 .

Web links

supporting documents

  1. ↑ Pommer, p. 144