To introduce an efficient solution of the problems and challenges facing provisioning QoS for real-time applications in general and for VoIP over IEEE 802.11 in particular, we proposed a new polling scheme called Adaptive Polling Scheme (APS).
In APS we introduced two dynamic polling lists polling, the first list is the Talking Polling List, it contains the stations that are considered to be talking, while the second list is the Silence Polling List, which contains the silent stations. However, to determine the state of the station, we proposed a silence detection mechanism of two phases, the first phase is an Initial Silence Detection phase in which a station sends a request to the HC to access the medium using the HCCA mode, HC receive this request and predict the state of this station according to the traffic direction requested in the TSPEC, if the direction was downlink then HC considers this station as a silent station and insert it into the Silence Polling List according to an algorithm which assures that the order of the newly inserted station satisfies a descending order in the list. Since the list should be sorted according to the number of buffered packets in the transmission queue on the HC, if the traffic direction was upload, the HC will consider this station as a talking station, and insert it into the Talking Polling List according to another algorithm, which do the same ordering as the one for the Silence Polling list. But it sorts the Talking Polling List according to different criteria which is a combination of the number of packets in the transmission queue on the HC, and the number of buffered packets in the station’s buffer with a heavier weight to the number of buffered packets on the station side. The next phase is the Primary Silence Detection phase, in this phase the station already reside in one of the two lists, and after the HC polls any of the stations in the two lists, it receives a reply from the corresponding station, this reply may be a voice packet or a silence packet or a NULL packet, HC decides the state of the station according to its reply. However, if the reply wasn’t a voice packet it will consider this station as a silent station and insert it into the silence polling list according to the algorithm in the Initial Silence Detection, otherwise it will insert it to the Talking Polling List according to the algorithm in Initial Silence Detection.
From the above mentioned updates on the Talking and silence Polling Lists, two dynamic polling lists are generated, both of those lists reflect the state of each station and how much packets it needs to transfer and to receive and within what delay bound. HC then can do a Round-Robin polling by polling all of the stations in the talking polling list from its top to its tail, then polling the silence polling list from its top to its tail. Next round, HC will start polling from the start of the Talking Polling List, moreover, according to the way we sort the two polling lists, where the stations with bidirectional traffic resides at the top of the two lists and by turning the feature of piggybacking on, QoS Data + CF-Poll will be sent from the HC to the station and QoS Data + Ack will be sent from the station to the HC on a negotiated data rate between both of them, which leads to increase the throughput and reduce the polling overhead efficiently.
In APS we took in consideration the two types of codecs, CBR and VBR codecs, because the CBR codes generates packets in a fixed packetization interval, regardless the stations was talking or silent, while the VBR codecs only generates packets when the station is talking.
Moreover, we differentiated between the TXOPs granted to stations in the talking state and the stations in the silence state by calculating the TXOPs for the talking stations depending on the statistics on the station side which is sent to the HC by the TSPEC, while TXOPs for silent stations are calculated according to the statistics on the HC side.
We used ns2 (ns-allinone-2.27) simulator to evaluate the performance of our APS, we compared our scheme with two other schedulers, the reference scheduler mentioned in the standard and the round robin scheme.
APS has shown much better results than the simulation results of two other schemes, in terms of throughput, access delay, packet loss ratio, and polling overhead.
In the future, we will improve our silence detection mechanism to make it independent of the station’s reply, and use a super poll to poll silent stations to reduce the null responses and polling overhead, which leads to the reduction of the access delay and delay jitter.
References:
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