Cisco 1231G - Aironet - Wireless Access Point Manual de usuario descargar pdf (Pagina 21)

Q. I have a problem with my AP. It continues to send too many RTS

messages in bursts which cause the unexpected disassociation of

associated clients. These clients were associated with this AP at a signal

level between −91 and −95 dBm. What is the reason for this unexpected

disassociation? Is this an expected behavior of the AP?

A. Yes, this is an expected behavior. Your client is at the very edge of the 1 Mbps cell. Since

you see it at −91 to −95 dBm, the erratic behavior is expected.

Install more APs in order to address this issue. Or, if your desired coverage is in a focused

area rather than omni−directional, use directional antennas.

RTS is caused by the retry mechanisms kicking in. The client should respond to an RTS with

a CTS, but if the client sees them in a sniffer as a group of around eight RTS frames with no

corresponding CTS, then the client does not hear the AP, or the client is so far away that the

AP cannot hear it. Both devices have to hear each other, not just your AP hearing the client.

So, if the antenna on the client is not of great design (probable), or their transmitter does not

transmit at 100 mW (very probable), or their receiver is nowhere near −90 to −95 dBm

sensitivity (almost guaranteed if it is not a Cisco client), then you get the operation that you

describe.

Q. We use Cisco LWAPP Wireless APs. Although I have seen many TCP

retransmissions and duplicate ACKs at clients, I do not see those in our

wired enviroment. Is that normal for wireless?

A. Corrupted packets and retransmitted packets are two of the fundamental metrics of an

802.11 WLAN. Analysis of corrupted and retransmitted packets in 802.11 differs from

analysis in a wired LAN for three reasons:

First, 802.11 WLANs typically have many more corrupted packets than do wired

LANs, so the importance of corrupted frames in an 802.11 WLAN is enhanced.

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Second, 802.11 defines a reliable data−link layer, which means that every corrupted

packet must result in a retransmission. Wired LANs typically do not define a reliable

data−link layer, so a retransmission only occurs if a reliable upper−layer protocol is

in use.

♦

Finally, upper−layer reliability is typically end−to−end, which means that a corrupted

packet anywhere between the source and destination causes a retransmission. An

802.11 retransmission, since it occurs at layer 2, is implemented between wireless

interfaces, so an 802.11 retransmission can only be caused by corruption on the local

"segment." This makes it much easier to identify the location of corruption in an

802.11 WLAN than in a traditional wired LAN. Let us explore the implications of

these differences.

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One of the challenges of a wireless environment is that it is difficult to determine whether the

analyzer sees the same things as do the clients. Differences between the analyzer and the

clientdifferent radios, antennae, or physical locationscan cause the analyzer to see different

things than does the client. For example, if the analyzer is far from the AP, but the wireless

client is close to the AP, the analyzer can see a corrupted frame, while the station sees an

uncorrupted frame. Since we know that every corrupted frame results in a retransmission, we

can use the relative numbers of retransmissions and corrupted frames to evaluate the degree to

which the analyzer sees what the station(s) on the network see.

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