Tuesday, 18 November 2014

I passed CWDP exam

I passed CWDP exam, time to prepare for CWAP now.

Improving network performance by disabling lower speed rates

One of the ways to improve overall network performance is to disable support for lower speed rates. Management frames are transmitted on the lowest basic speed rate, by default this rate is 1 Mbps.

One of the implications of sending frames on low speed rates is the fact that in free space, 1 Mbps signal can be received 14 times farther than 54 Mbps signal.
By changing the lowest basic rate from 1 Mbps to 5.5 or higher we can decrease the effective BSS (Basic Service Set) area that can be serving clients. It doesn't mean the coverage has been decreased, this is achieved by manipulating the Tx power of the access point (this technique will be discussed in a separate post).

By disabling lower basic speeds we effectively improve network airtime utilization, because sending a frame on 1 Mbps speed rate takes twice as long than sending the same frame on 2 Mbps, and eleven times longer than sending the same frame on 11 Mbps. This leaves more airtime to be available for other traffic.

The important fact is that by disabling the lower speeds on the AP (or wireless lan controller) we only prevent the AP/WLC from sending the frames on these speeds. What that means is that when a client sends a broadcast probe request (always on 1 Mbps) the AP will hear it, but it will respond back to the client with a probe response on it's lowest basic rate. The implication of it, is if the client can't operate on this speed it won't be able to connect to the network, for example if we disable speeds 1, 2, 5.5 and 11 Mbps, effectively we will prevent 802.11b clients from connecting to the network.

Let's examine another scenario, let's take a client that has already associated to the network, with a 5.5 Mbps set as a basic speed rate, it currently operates at 24 Mbps and is moving away from the AP. At some point, along with signal quality degrading, it will start switching down to a rate with less demanding modulation, to a point when it will have to switch to 2 Mbps rate (operating with QPSK modulation). At this stage, since the AP is not supporting this rate, the client will be kicked of the network.

Here's an example of how to disable speeds 1 and 2 Mbps on Cisco controller:

Another important fact is that broadcast and multicast frames are also send on the lowest mandatory (basic rates are sometimes called mandatory) rates. Some vendors, Cisco for example, send multicast frames on the highest mandatory rate. This is why on the above screenshot you can see two mandatory rates selected, 5.5 and 11 Mbps, broadcast and management frames will be sent with 5.5 Mbps  speed, and multicast frames will be sent with 11 Mbps speed.

By changing the mandatory speed rate we also decreasing the overall overhead of the wireless network. Every AP sends a beacon frame, advertising it's presence and capabilities, this frame is a broadcast frame, sent by default at about 100 ms interval.
Because this frame is a broadcast frame, it is sent at the lowest basic rate (1 Mbps by default). By disabling lower speeds, we are forcing the beacon frames to be sent at higher data rates, and at the same time decreasing protocol overhead and improving the overall performance of the wireless network.

Here's an example of how disabling 1 and 2 Mbps speed rates decreases the protocol overhead. For a network with 4 SSIDs per AP (where each AP is within a range of three other APs on the same channel) with default basic rate of 1 Mbps, the overhead is 38.70%. When speeds of 1 and 2 Mbps are disabled, leaving 5.5 Mbps as the lowest basic rate, the overhead is reduced to 8.42%.


For overhead calculations I used SSID overhead calculator, an incredibly useful tool created by Andrew von Nagy, that can be found on his website www.revolutionwifi.net