IEEE 802.11e-2005
IEEE 802.11e is an industry standard of the Institute of Electrical and Electronics Engineers ( IEEE) and an extension of the wireless LAN standards IEEE 802.11 to support the quality-of- service concept.
The busy with the preparation of the IEEE working group has completed its work in July 2005. The final publication was completed in late 2005.
802.11e operates on a similar principle as the QoS DiffServ method. The data packets in WLAN are marked by the sender with a priority. The access point should treat packets with higher priority are preferred. " True" QoS supports only the IEEE 802.16 protocol.
With the help of 802.11e real-time applications such as voice over IP better supported. The protocol guarantees a certain bandwidth in the network so that it is ensured that data packets arrive within a certain time at the receiver. This is to ensure that there is no eg Internet telephony due to other network traffic dropouts in the connection.
- 2.1 EDCA
- 2.2 HCCA
- 3.1 APSD
- 3.2 BA
- 3.4 DLS
Originally 802.11 MAC format
DCF
The original 802.11 Medium Access Layer ( MAC) using the distributed coordination function (DCF, Distributed Coordination Function ), to share the medium with a plurality of stations. The DCF is based on CSMA / CA ( carrier sense multiple access with collision avoidance ) and optional 802.11 RTS / CTS (Request to Send / Clear to Send ). This has several disadvantages:
- If many stations want to communicate at the same time, which caused thereby, frequently occurring collisions in reduced net data rate lead ( such as Ethernet, which CSMA / CD used).
- There is no prioritization for traffic.
- When a station gains access to the medium, they can keep this access as long as necessary. A low data rate (for example, 1 Mbit / s) of a station causes it prevents the access of other stations. Those suffering.
- Generally speaking: There are no QoS guarantees.
PCF
The Point Coordination Function ( PCF point coordination function) is true in the standard, but it is rarely implemented. In PCF mode, the access point takes full control over the access to the medium. The normal Access Control CSMA / CA is thereby turned off repeatedly according to certain rules for a short time. The Access Point calls this all mobile stations (polling), which have previously registered as a PCF participants at him, and allow them to send their data. Then is switched back to the CSMA / CA operation. The start of a PCF period ( Beacon frame ) signals in a signal frame with the "Duration " field.
802.11e MAC protocol
With 802.11e a new coordination function was introduced: the Hybrid Coordination Function (HCF, hybrid coordination function). Within this HCF, there are two ways of channel access ( which are similar to the original ): HCF Controlled Channel Access ( HCCA ) and Enhanced Distributed Channel Access ( EDCA ). Both Define Traffic Classes (TC, Traffic Classes). Through these different data protocols can be given different priorities.
EDCA
With EDCA important data packets are sent to get a higher chance because höherprioritisierter traffic must wait an average of less long before the packet can be sent. In addition, each priority level 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, in order to transmit a frame within a TXOP, the frame must be divided. Thus the problem is reduced to slow stations slow down faster stations.
Access points that are certified for Wi- Fi Multimedia ( WMM) support, EDCA TXOP and need to support. All other improvements are optional for WMM.
HCCA
HCCA works similarly to PCF: The interval between two signal frames ( beacon frames) is divided into two periods, called the Contention Free Period ( CFP) and Contention Period ( CP). During the CFP, the Hybrid Coordinator (HC) called Access Point controls access to the medium. During the CP, the stations operate in EDCA mode. The main difference with PCF is that even here Traffic Classes ( TC) are defined. Also, decide how he wants to direct the traffic of the HC. In addition, the stations provide information on their queue length of each TC. The HC can then decide whether to grant a station priority.
HCCA is commonly seen at the moment the most advanced ( and complex ) coordination function. With HCCA, QoS can be configured with great precision. QoS stations have the ability to transfer certain parameters (data rate, jitter, etc. ) query. This should help to make applications like VoIP and video transmission work more effectively.
HCCA is not mandatory for 802.11e access points. Currently, only few available APs have this functionality. The Wi- Fi Alliance has therefore a postgraduate certificate (WMM Scheduled Access) introduced in order to detect APs easier with HCCA can.
More 802.11e specifications
In addition to HCCA, EDCA TXOP and there are other optional protocols for 802.11e:
APSD
Automatic Power Save Delivery (automatic power-saving delivery ) is a more efficient power management method. Most newer devices already support a service that works similar to APSD. APSD is very useful for VoIP devices. Whenever voice data is received from the AP, the AP sends the buffered speech data also same in the other direction. After the VoIP phones go into twilight until the next data to be sent.
BA
Block Acknowledgements allow it to confirm an entire TXOP with only one frame. This produces less protocol superstructure for longer TXOPs.
In QoS mode, the service class for outgoing frames can have two values: QosAck and QosNoAck. Frames with QosNoAck not be confirmed. This avoids retransmissions of time-critical data.
DLS
Direct Link Setup allows direct station - to-station frame transfer despite base station. This way you can use a higher bandwidth with less delay multimedia components, because the traffic is not through the access point, but will be handled directly. This is particularly desirable when two components to one another at a smaller distance than to the access point.