If you think about it, it makes a lot of sense. Using VoIP over wireless can reduce the usage of cell phones in the work environment (where the company pays an airtime fee). This reduced use of cell phones gives network administrators a tangible dollar value to develop a return on investment (ROI) for a WLAN deployment.
QoS is a relatively mature technology for wired networks and is generally available on routers, switches, and end devices such as wired IP phones. For 802.11 WLANs, the contrary is true. It is an emerging technology that is hotly debated with the IEEE and the WLAN industry as a whole. The key challenges for a QoS mechanism in 802.11 networks include the following:
Cochannel Overlap
Cochannel overlap is a common occurrence in 2.4 GHz WLAN deployments with more than three APs. Because of the restriction of three nonoverlapping channels, some APs end up adjacent to APs on the same channel. What does this mean for the clients in those BSSs? Figure 6-1 shows a client in a cochannel overlap area. If both APs begin to transmit at the same time, the frames collide and both stations must back off and retransmit.
QoS Mechanism Overview
The 802.11e task group has debated many issues, including those discussed in the previous section. It has devised two proposed solutions for the future of 802.11 MAC. Bear in mind that the proposed specifications are not yet ratified, and changes might occur after this book is printed. The current two proposed solutions are
- Hybrid coordination function (HCF) with contention operation— More commonly known as Enhanced DCF (EDCF)
- HCF with polled access operation
HCF in Contention Mode—The EDCF Access Mechanism
The draft 802.11e specification attempts to provide classification for up to eight classes of data. EDCF and HCF polled access leverages these eight classes, known as traffic classes (TC), which map to the eight classes defined in the 802.1D standard, as shown in Table 6-1. Traffic from QoS-enabled clients is categorized into four broader categories known as access categories (AC). ACs 0 to 3 map to the 802.1D priority classes.
Summary: The Challenges Facing EDCF and HCF
At the time of this writing, there are two major obstacles perplexing the IEEE with respect to 802.11e: an effective yet simple admission control for EDCF and the operation performance of HCF. These issues are hotly debated among the various vendors in the working group who endeavor to solve application issues.
DAC still presents performance problems because it does not strictly enforce admissions control. Stations may potentially transmit and negatively impact existing traffic streams. Resolution seems to surround the adoption of parameterized admissions control for EDCF as well (the use of TSPECs to admit or deny EDCF traffic). HCF has its share of issues. Proponents extol the virtues of polled access as the panacea for effectively using the medium and also providing the ability to nearly guarantee service. Detractors believe that practical implementations of HCF will fail, as did early PCF implementations, because of the cochannel overlap issues that plague the 2.4 GHz band. The effectiveness of HCF diminishes quickly with cochannel overlap.
Although the working group has not finalized the 802.11e standard, it continues to strive toward a practical and effective set of tools to extend and expand the implementations of 802.11 WLANs.
