Colorado Springs, Colo. -- Development engineers at VeriWave Inc. have discovered an unexpected problem with the physical-layer convergence procedure (PLCP) in existing 802.11 Wi-Fi networks--one that could have implications for voice-over-Wi-Fi and Radius authentication of wireless networks.
VeriWave chief technology officer Tom Alexander discovered the problem in work for the 802.11 Task Group on test generation and wireless performance (TGt), which later became 802.11t.
PLCP headers are common convergence protocols used in Internet Protocol, asynchronous transfer mode and wireless networks. In the case of 802.11, a single parity bit in the header can allow header corruption to take place as the rate and length fields of a packet are computed. Typical frame errors lead to packets' being retransmitted, but in a PLCP corruption, the network may remain blind to the problem, and the packet is simply dropped.
"You could maybe say [the problem] should have been foreseen with only a single bit of parity in the PLCP header," said Eran Karoly, vice president of marketing at VeriWave. "At first, it might seem of interest only to geeks. But this is a problem that could affect the voice quality of Wi-Fi more than delay or jitter and could also create some end-user problems in network authentication."
Karoly gave the example of a 100-byte OFDM packet being sent at 54 Mbits/second in an 802.11g network, with bit errors introduced as a result of channel noise. Those bit errors would cause the packet to be interpreted as a 4,096-byte packet traveling at 6 Mbits/s. The receiver would not detect the error, because the PLCP corruption would blind the receiver for 5.5 ms. All of the packet retransmission attempts would occur during the receiver's "blind" period.
In a worst-case scenario, 0.1 percent of packets could be lost through PLCP corruption. That rate might not sound high; but during a handshake using the four-way Extensible Authentication Protocol (EAP), a handshake packet might be lost completely, causing a Radius authentication server to stall. That could impact roaming in larger Wi-Fi networks and compromise voice quality more significantly than jitter would.
VeriWave executives said they doubt that any amendments will be made to existing 802.11a/b/g standards, but they are pushing for slight changes to be made to 802.11n. Acceptable loss tolerances for throughput tests should be set at higher than zero, for example--perhaps at 0.1 percent, according to VeriWave. And frame error ratios could be minimized for roaming and voice-over-IP throughput tests.
Members of the TGt group from VeriWave and Aruba Networks are approaching some vendors of Radius servers and physical-layer Wi-Fi chips about minor changes that could be made in legacy 802.11a/b/g networks. Handshake time-outs from EAP protocols should be filtered in authentication servers, Karoly suggested, and physical-layer chips could add a cyclic redundancy check on top of the single-bit parity check.
"We want to be pragmatic here. Few people are going to want to change the existing Wi-Fi standards," Karoly said. "But it's a good time to address this during the 802.11n balloting process, and we may see some chip and server vendors make some changes for existing Wi-Fi networks to address this problem."
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