The reader can refer the Part 1 of the article here — <EDCA (WMM) Information Elements – part 1> Continuing from the previous article — the QoS information element is shown below The U-APSD bit indicates if Unscheduled Power Save Delivery is supported for the Access category The Parameter set count is a counter value […]
Point Coordination Function (PCF)
The Point Coordination Function was another media access mechanism that is described in the 802.11 WLAN standard. In the PCF mechanism, the Access Point would function as a Point Coordinator. The time for which the Access Point would control the medium was termed as the Contention Free Period (CFP). The Access Point can place the […]
Drawback of the DCF Mechanism and development of EDCF and HCF
The DCF Physical carrier sense/Virtual carrier sense/ Contention window back-off allow different stations to synchronize usage of the air medium. However, the one drawback of the DCF mechanism was that it was a fair mechanism for all Application level user priorities. It treated all user application priorities at the same priority. This caused packet loss […]
802.1p Priority in Ethernet Frame
As was seen in the previous article <Understanding Packet User Priority>, the IP header incorporates a Differentiated Services Code Point (DSCP) field which provides the packet priority. Another level of priority was introduced in the 802.1p standard which featured a 3 bit priority field in the Ethernet header. The 802.1p standard was merged into the […]
Understanding Packet User Priority
Packet user priority was earlier defined by IP Precedence (IPP) and Type of Service (TOS) field in IPv4 header. The IP Precedence/TOS fields could specify the route delay, priority and reliable service for a packet. Later on, the TOS field was extended to 6 bits(RFC 2474) and was called Differentiated Services Code Point (DSCP). The […]
Enhanced Distributed Co-ordination Function (WMM) – Part 1
The Enhanced Distribution Co-ordination Function (EDCF) or also called as Enhanced Distribution Co-ordination Access (EDCA) is the modification of the basic Distributed Co-ordination Function (DCF) that was developed by the IEEE 802.11 standard’s body initially. It was developed to provide priority queuing to different traffic classes. To understand the need for EDCA – refer here […]
802.1p mapping to 802.11 (WMM) Quality of Service
The 802.1p priority mapping was discussed here — <802.1p Priority in Ethernet Frame>. When an 802.11 WLAN device sends out a packet, the upper layers should provide an 802.1p priority mapping. The access points that receive the frame should map the 802.1p priority to the appropriate DSCP value and forward the same. For 802.11 QoS, […]
Contention Window and NAV Timer
The contention window provides a buffer to prevent multiple 802.11 stations from transmitting at the same time which may lead to collisions. To prevent collisions on air – the Contention window for back-off is selected as an exponentially growing back-off counter. The contention window has a minimum and a maximum value given by CWmin and […]
Enhanced Distributed Co-ordination Function/WMM – Part 2
The EDCA (Enhanced Distributed Channel Access) mechanism proposed different contention and back-off times for the different access categories. Voice access category is configured to wait the least before transmission, followed by Video, Best Effort and Back-ground traffic class. Hence a degree of un-fairness was introduced giving the higher traffic class more priority for transmission. The […]
Legacy Power Save in WLAN
Wi-Fi Station devices are usually Battery operated devices. WLAN radios consume a lot of battery power and hence efficient power saving schemes should be introduced which conserve battery power. The earliest 802.11 standard introduced a power save mechanism which is now termed as Legacy Power save in 802.11. It is based on a polling mechanism […]