Delay-Based Admission Control to Sustain QoS in a Managed IEEE 802.11 Wireless LANs

Delay-Based Admission Control to Sustain QoS in a Managed IEEE 802.11 Wireless LANs

A. Ksentini, A. Nafaa
DOI: 10.4018/978-1-61520-680-3.ch006
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In this chapter, we present a delay-sensitive MAC adaptation scheme combined with an admission control mechanism. The proposed solution is based on thorough analysis of the trade-off existing between high network utilization and achieving bounded QoS metrics in operated 802.11-based networks. First, we derive an accurate delay estimation model to adjust the contention window size in real-time basis by considering key net-work factors, MAC queue dynamics, and application-level QoS requirements. Second, we use the abovementioned delay-based CW size adaptation scheme to derive a fully distributed admission control model that provides protection for existing flows in terms of QoS guarantees.
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3. Quality Of Service Support And Admission Control In Ieee 802.11

In this section, we provide background material on the 802.11 MAC and QoS enhancements. Related works on AC algorithms in 802.11 networks are also reviewed in this section.

3.1. IEEE 802.11 Basic Access Mechanism: DCF

The IEEE 802.11 MAC defines two transmission modes for data packets: the Distributed Coordination Function (DCF) based on Carrier Sense with Multiple Access (CSMA/CA) and the contention-free Point Coordination Function (PCF), where the Access Point (AP) controls all transmissions based on a polling mechanism. The popularity of IEEE 802.11 wireless LAN (WLAN) is mainly due to DCF, whereas the PCF is barely implemented in today’s products due to its complexity and inefficiency in common network deployment setup, despites its limited QoS support. PCF may cause unpredictable beacon delays and unbounded transmission latencies (Mangold, 2002). On the other hand, DCF is the basic mechanism for IEEE 802.11 that employs a CSMA/CA algorithm (see Figure 1) and allow for a fully distributed wireless medium sharing. Before sending a packet, a wireless station first senses the medium for a duration equivalent to Distributed Inter-Frame Space (DIFS). If the medium is idle for that duration, the wireless station starts sending immediately. Otherwise, if the wireless station senses the medium as busy, the wireless station backs off for a certain number of time slots (see eq. 1).

Backoff = Random (0, CW-1) * SlotTime(1)
Figure 1.

DCF access mechanism


Collisions can only ‎occur in the case where two terminals start transmitting on the same slot. For each unsuccessful transmission the Contention Window (CW) is exponentially increased as follows:

(2) where i is the number of unsuccessful transmission attempts usually referred to as the backoff stage.

Note that, after each successful transmission the CW is initialised with the CWmin.

In order to guarantee undisturbed transmission even in presence of hidden wireless stations, an RTS/CTS (Request to Send/Clear to Send) mechanism is used. When this sender/receiver synchronization mechanism is enforced, the contention winner does not transmit the data immediately. Instead, it sends an RTS frame in to the receiver that replies with a CTS frame. This ensures that all terminals in the range of either the sender or the receiver are not only aware that a transmission will take place, but they are also aware of the duration of the transmission and medium business. In this case, terminals remain silent during the entire transmission, while only the sender is allowed to transmit frames. While the two extra messages present additional overhead, the mechanism is particularly useful in case of large data frames. In fact, only RTS and/or CTS packets can cause collisions, data frames are aware from collisions and no retransmission (data) is required. Thus, in case of large data frames, the channel utilization is considerably increased.

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