By using the different MAC addresses to connect to the different APs, multiple access between the infrastructure BSSs is possible for a STA. However, there should be other mechanisms provided to prevent the packet loss during the network switching. Furthermore, we should model the real-time communication behaviors to determine the switch timing. The following depicts the buffering protocol and the switching algorithm for a DualMAC-capable STA that performs real-time communications:
1. Buffering protocol: Since adjacent WLAN APs are always on the independent channels, STA should perform channel switching periodically to listen to arrival packets from both the serving AP and the target AP. A good switching method always relies on the accurate prediction for the arrival time of the incoming packets. However, it is hard to achieve due to the fluctuation of network traffics or the channel condition. Instead of providing such a prediction method, we take a buffering approach. The IEEE 802.11 provides
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packet buffering for incoming traffics while a STA enters the power saving mode (PSM).
Figure 5. The scenario of the buffering protocol for both the infrastructure BSSs
During the power saving interval, the serving AP buffers any packet destined for the STA.
STA can retrieve the buffered packets later by sending a PS-Poll to the AP. The buffering protocol is used in the following two cases:
z The real-time communications: STA enters the PSM of the serving AP to prevent the packet loss of the incoming real-time packets.
z The handoff procedures after the association: STA enters the PSM of the target AP to prevent the packet loss of the handoff traffics that includes the security context exchange and the four-way handshake.
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Figure 5 shows the scenario of the PSM buffering.
Since the power saving mode can be activated only after a successful association, there should be packet loss problem for the handoff procedures before the association.
Here we describe the method to prevent such the condition. The first step of the handoff is to probe all available APs. STA uses active scan in this case. STA that sends the probe request should wait for minChannelTime period if there is no available AP on this channel or maxChannelTime period for all probe responses. During the wait interval, the STA should stay on the target channel to listen for any response, or the responses may be lost. After probing a channel, STA switches back to the serving channel to retrieve the buffered packets, and then probes the next channel. Until the scan procedure is finished, the authentication and association is then performed. The authentication and association should be performed atomically without switching to prevent the packet loss of the response packets.
2. Switching protocol: In order to ensure the service quality of real-time communications, the delay of the arrival packet should be bounded to an acceptable range. Therefore, we should design a scheduling algorithm to access both the channels under the QoS constraint. Real-time communications are always modeled as a periodic traffic with a small inter-arrival time. Because the real-time packets arrive periodically in the ideal case, there must be free time between the arrival intervals. We define this interval as the idle interval. During the idle interval, the DualMAC-capable STA can switch to another
infrastructure BSS to perform the handoff procedures. After the end of each idle interval, the STA switches back to the serving AP, transmits uplink packet, and then polls for the buffered packet. However, the STA cannot stay on the target channel too long, or the reception of the real-time packets may be delayed. The time quota to enforce the handoff procedures for each idle interval is depicted as the following equation.
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T = (packet inter arrival time) – 2*(channel switching time)
Since the STA should switch to the target channel and then switch back to the serving channel, the channel switching time is multiplied by two. Notice the real-time packets may not arrives periodically in the real environment, we should analysis the introduced delay by the above switching algorithm in the best case and the worst case.
z The packet arrives just before the predicated arrival time: In this case, the packet can be directly retrieved while the STA switches back to the serving channel. The introduced packet delay is close to zero.
z The packet arrives just after the predicated arrival time: In this case, the packet can not be retrieved since it has not arrived at the serving AP. The retrieval of the packet is postponed until the end of the next idle interval. As a result, the worst case delay is close to the ideal inter-arrival time.
Figure 6. Packet p1 arrives just before the predicated arrival time, so it can be retrieved directly without any delay. However, packet p2 arrives just after the predicated arrival time.
The retrieval of the packet is delayed for a period
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According to the analysis above, the additional delay introduced by the network multiplexing is bounded to a small range. Therefore, real-time packets that arrive at any time may be acceptable for most of the real-time applications by the proposed switching algorithm. Figure 6 shows both the cases.
DualMAC provides an opportunity to access multiple infrastructure BSSs with a single WLAN card. Based on the multiplexing scheme over different networks, the handoff can be performed in the background while STA keeps real-time communication. Therefore, we can facilitate a make-before-break handoff approach which minimizes the link creation latency.
IV. Simulations and Results
In this section, we present the simulation model and results of the DualMAC handoff.
Furthermore, we will consider the same case under an inter-subnet handoff. We focus on the service quality of the real-time communications while creating a new association in a secured WLAN, and compare its connectivity under different approaches. The WLAN environment is based on 802.11b PHY, where only 11 channels are available. The real-time traffics are simulated using IxChariot [18], a well-known network tester which supports most types of network flow, including the voice call. We use G.711 as the voice sampling codec which generates voice packets every 20 milliseconds.
In order to simulate our approach in a real environment and, we have setup the IEEE 802.1x framework and activate the dynamic key management. We experiment the handoff in such a secured WLAN for several times in order to log the traffics and its relative arrival time, which will be taken as our simulation input. The experiment environment is constructed by a supplicant that runs wi-fi protected access (WPA) client provided by Windows XP service
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pack 1, an Planex GW-AP54SGX AP that acts as an authenticator, and an authentication server that runs freeRADIUS [16]. We use EAP-TLS as the default authentication method which is based on public key certificates. Wireless packets are captured using Airopeek developed by WildPackets [17]. Our simulation takes the log files of both voice traffic and handoff traffic as
the input. We also implement the functions of the buffering protocol and the switching algorithm in our simulation. The objective of the simulation is to observe:
1. Packet delay of the real-time communications caused by the buffering protocol and the switching protocol.
2. The service disruption time of the real-time communication..
3. The duration of each handoff procedure (the IEEE 802.11 scan, authentication, and association, the IEEE 802.1x EAP authentication, the IEEE 802.11i four-way handshake, the DHCP handshaking) and the overall cost.