Hybrid Change Makes WLAN QoS Come to Life

Hybrid Change Makes WLAN QoS Come to Life

Andrew Shepard, Bermai
Apr 28, 2004 (6:00 AM)
URL: http://www.commsdesign.com/showArticle.jhtml?articleID=19200133

Wireless LAN (WLAN)-enabled consumer electronics (CE) products are an emerging and rapidly growing market. But, for these devices to gain hold in the communication sector, the quality of service capabilities provided by a WLAN system must dramatically improve.

To improve QoS, the IEEE 802.11 committee formed the .11e task group to put together a specification that improves the QoS capabilities of a WLAN system. With ratification expected at the end of 2004, the 802.11e specification adds a host of elements designed to improve QoS in a WLAN system. The most significant addition, however, is the hybrid coordination function (HCF), which improves upon (and coexists with) the distributed coordination (DCF) in existing WLAN products.

In this article, we'll take a look at the current 802.11 medium access methods. We'll then take a detailed look at the HCF functions and the QoS improvements these functions provide.

Access Medium
The existing 802.11 protocol¹ has two wireless medium (WM) access methods: the distributed coordination function (DCF) and the point coordination function (PCF). The DCF and PCF allow devices to share the WM and may co-exist. The PCF currently has little commercial acceptance and will not be discussed in this article.

The DCF provides "fairness" to devices trying to access the WM. When a station (STA) needs to access the WM it listens to see if the medium is idle. If it is not idle, the STA waits for the idle state and then starts a backoff timer with a random backoff interval chosen from a uniform distribution—with the minimum and maximum backoff intervals being defined as network-wide constants.

The backoff timer is decremented until either the WM becomes busy again, at which time it stops decrementing until the WM is again idle or until the timer expires. When the timer expires, if the medium is idle, the STA transmits.

With the DCF mechanism, over a long period of time all devices on a given channel will get equal access to the WM. This works well for traditional data applications like ftp transfers, web-browsing, and other non-multimedia based applications. A user of these applications will, in general, not notice that the application is sharing the WM with other users.

However, fairness is not good when dealing with CE oriented applications such as video, gaming, and voice-over-IP (VoIP). For example, if a VoIP application is not given expeditious access to the WM because it is being preempted by other STAs desiring access to the WM, there will be unacceptable latencies. This will result in a negative user-experience due to the difficulty in carrying on a conversation.

In a gaming environment, high latencies will result in important game events like trigger pulls or control stick movements not being synchronized to the game. In an HDTV application, there needs to be an assurance to the application that the high bandwidth it requires is available and will remain available and will not be preempted by applications that don't require high bandwidth such as traditional data applications.

Figure 1 is a plot of actual data showing the effect the DCF channel mechanism has on a network with high bandwidth traffic like HDTV data in concert with best effort traffic. Because there is no QoS, the HDTV data rate falls when the best effort data transfer begins. Thus, as shown in this figure, the standard 802.11 DCF simply does not provide the QoS that these and other CE applications require.

Figure 1: DCF channel access: HDTV Data throughput in the presence of best-effort traffic.
Enter 802.11e
The 802.11e effort was created to design a standardized method for providing QoS to wireless networked applications. The 802.11e standard² builds upon the existing 802.11 suite of protocols. It provides QoS functionality while retaining full backward compatibility with the large installed base of 802.11 products. Legacy 802.11 products will continue to work in the presence of 802.11e devices and vice versa.
In 802.11e, ahybrid coordination function (HCF) is added which provides QoS capabilities not available from the DCF while still coexisting with the DCF (in fact the DCF provides the foundation for the HCF). The HCF provides two WM access methods: enhanced distributed channel access (EDCA) and HCF controlled channel access (HCCA). Let's look at both of these methods in more detail.
EDCA
EDCA is a superset of the 802.11 DCF. In the DCF, all STAs compete for the WM with the same priority. In EDCA, on the other hand, this mechanism is extended to four levels of priorities or access categories (AC).
Similar to the DCF, EDCA QoS stations (QSTAs) listen for the WM to be idle and use a backoff mechanism to know when they can transmit. However, unlike DCF, where the same network-wide maximum backoff time is used by all STAs, the maximum backoff times are different for the different ACs under the EDCA approach. Higher-priority ACs have a shorter maximum backoff time than lower priority ACs.
With a shorter maximum backoff time, the higher priority AC wins access to the WM more frequently than the lower priority AC. Therefore, statistically, the packets with the highest AC are given access to the medium more frequently than those packets with a lower AC.
It's important to note that packets within the same AC have the same maximum backoff time and hence compete fairly with each other within that AC for access to the WM. Another enhancement EDCA makes to the DCF is that once a device has accessed the WM, it has the opportunity to continue transmitting for a specified transmission opportunity (TXOP).
Due to the non-deterministic nature of EDCA, it is not possible—except in a very lightly loaded network—to guarantee parameters such as bandwidth, jitter, and latency. In addition, due to the backoff mechanism, the medium usage is much less efficient than HCCA (more to follow below).
It is likely that EDCA will be most successful in traditional data-centric enterprise environments where admission control and tight QoS constraints are not required. Here the EDCA mechanism requires little administration or infrastructure change. With simple translation of the eight priorities defined in 802.1d⁴ into the four 802.11e ACs of EDCA, prioritization of data-centric traffic can be done.
HCCA
If an application requires a fine-grained level of QoS with tight, guaranteed constraints for parameters such as bandwidth, jitter, and latency, HCCA is a better option for designers.
HCCA provides deterministic behavior and a high level of control and fidelity to multimedia applications that require parameterized QoS. Unlike the non-deterministic behavior of EDCA where QSTAs must listen for the WM to be free and go through a backoff mechanism before transmitting, HCCA-equipped QSTAs are specifically granted TXOPs as necessary by the QoS access point (QAP) without the overhead and statistical uncertainty of random backoffs.
In HCCA, admission control is required. Thus, all QSTAs must request permission from the QAP to join the network. This request includes a traffic specification (TSPEC) that specifies in detail the QoS required by the QSTA. The QAP determines if it can support the requested TSPEC and admits or denies the QSTA.
The TSPEC² includes the following parameters: nominal MSDU size, maximum MSDU size, minimum service interval, maximum service interval, minimum data rate, mean data rate, peak data rate, and delay bound. This richness of parameters provides considerable flexibility for defining the QoS required for different types of multimedia data including gaming, video, audio or voice.
The QAP maintains a centralized schedule based on all the QSTAs that have been admitted to the network. At the appropriate time, the scheduler in the QAP sends out a QoS poll message to each QSTA in the network enabling that QSTA access to the network. The QoS poll message includes the time that the QSTA may have access to the WM.
Upon receipt of a QoS poll message, a QSTA can begin transmitting and has contention-free access to the WM for the period specified in the QoS poll message. Once the QSTA has completed its TXOP, the QAP can send out another QoS poll message to the next QSTA desiring service.
The timing between QoS polls messages and the TXOP durations is deterministic without the randomness created by the backoff mechanism of EDCA. Hence, with HCCA QoS can be guaranteed with tighter constraints and much higher medium efficiency.
Figure 2 is a plot of actual data showing the effect the HCCA channel mechanism has on a network with high bandwidth traffic like HDTV data in concert with best effort traffic. Unlike the DCF example of Figure 1, with HCCA the HDTV data is maintained at its requested rate when best effort data begins.

Figure 2: HCCA channel access: HDTV data throughput in the presence of best-effort traffic.
The admission control requirements imposed by HCCA add additional infrastructure requirements that are not optimal for the data-centric enterprise environment where the simpler prioritization scheme of EDCA is adequate. However, for a home CE environment (or for an enterprise environment where multimedia data is networked) where the non-deterministic nature of EDCA is not good enough and maximum utilization of the available bandwidth is desired, the deterministic behavior of HCCA makes it the protocol of choice.
Interop Tests
The Wi-Fi Alliance is currently defining interoperability test suites for 802.11e-enabled products. There are two different test suites being defined. The WME test suite is targeted to ensure products employing EDCA are interoperable and the WSM test suite is targeted to ensure products employing HCCA are interoperable. Products that pass these interoperability tests will be certified as being WME and/or WSM interoperable. Note: WSM certification is a superset of WME certification.
As part of the certification suite, the Wi-Fi Alliance has defined a set of TSPEC templates that address QoS parameters required for different categories of multimedia data. These templates define TSPEC parameters for voice, CD audio, interactive gaming, video conferencing, standard definition TV, and HDTV. For example, an HDTV product that passes the certification suite with the HDTV TSPEC will be interoperable at the wireless network protocol layer with other vendor's HDTV products that have passed the certification suite.
References

IEEE Std 802.11 First edition 1999-00-00 ANSI/Information technology —Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications.

[2] IEEE P802.11e/D8.0, February 2004 (Draft Amendment to IEEE Std 802.11, 1999 Edition [Reaff 2003]).

ANSI/IEEE Std 802.1D, 1998 Edition Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Common specifications—Part 3: Media Access Control (MAC) Bridges.

Dongyan Chen, Daqing Gu and Jinyun Zhang. Supporting Real-time Traffic with QoS in IEEE 802.11e Based Home Networks, TR-2004-006, February 2004.

About the Author
Andrew Shepard is Site Manager for the Wireless Multimedia Products Group managing software development for Bermai's 802.11e wireless products. Andy holds a BSEE from California State University at Sacramento and can be reached at ashepard@bermai.com.

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