The Scanning Process | Inter-Access Point Handoffs

The scanning table's contents come from beacons and probe requests. Scanning is a process that can be requested explicitly the user—often by performing an operation that is labeled "Reconnect." "Update," or "Scan." But far more often, scanning is a process that happens in the background or when the client decides that it is needed. To understand why the client makes those choices, we will need to look at the mechanisms of scanning itself.

There are two ways that the scanning table can be updated. When a client is associated to an access point, it has the ability to gather information about other access points on that channel. Especially when the client is not in power save mode, the client will usually ask its hardware to let it receive all beacon frames from any access point. Each beacon frame is then used to update the scanning table entry for that access point.

On the other hand, the client may want to survey other channels to find out what other access point options are out there. To do this, the client clearly needs to leave the channel of its access point for at least a small amount of time. Therefore, before engaging in this process, the client will usually tell the access point that it is going into power save mode, even though it is doing no such thing. That way, the access point will buffer traffic for the client, who can then look around the network with impunity.

When the client changes channels, it has two methods it can use to find out about the access points. The quickest method is to send out the probe request mentioned earlier. This probe request contains the SSID the client desires (with the option of a null SSID, an empty string, if the client wants to learn about all SSIDs), and is picked up by all access points in range that support the SSID and wish to make themselves known to the client.  Each access point that wishes to answer and that supports the SSID in question will respond with the probe response, a frame that is nearly identical to a beacon but is sent, unicast, directly to the client who asked for it. This procedure is called active scanning, though it can also be called probing, given the name of the frames that carry out the procedure. The other option is called passive scanning, and, as the name suggests, involves sending no frames out by the client. Instead, the client waits around for a beacon. Keep in mind that passive scanning clients do not know, ahead of time, how many access points are on a channel or when these access points may transmit the beacons. Therefore, a client may need to wait for at least one beacon period to maximize its chances of seeing beacons from every access point of possible interest.

In these two ways, the client goes from channel to channel, collecting as much information as possible about the available networks.

Clients may choose between active or passive scanning for a number of reasons. The advantage of active scanning is that the client will get definitive answers about the access points that are on that channel and in range in short order. Sometimes the client needs to send more than one probe request, just to make sure that none of those broadcast frames were lost because of transient RF effects or collisions. But the process itself concludes rather quickly. Furthermore, active scanning with probe requests is the only way to learn about which access points serve SSIDs that are hidden, where hidden SSIDs are not put in beacons and require the user to enter the SSID by hand. On the other hand, active scanning comes with two major penalties. The first one is for sheer network overhead. A probe request can trigger a storm of probe responses to the client, all of which take up valuable airtime. Especially when there is a network fluctuation (access point reboots, power outages, or RF interference), all of the probes pile onto an already fragile network, making traffic significantly worse. The second penalty is that active scanning is simply not allowed on the majority of the 5GHz channels. Any channel that is in a DFS band cannot be used with active scanning. Instead, the client is always required to wait for a beacon (an enabling signal), to know that the channel is allowed for operation, does not have a radar, and thus can be used. (Note that, once a client has an enabling signal, it is allowed to proceed with a probe request to discover hidden SSIDs. However, the time hit has been taken, and the process is no faster than a normal passive scan.)

Therefore, to better understand scanning, we need to look at the timing of scanning. Active scanning, of course, is the quicker process, but it too has a delay. Active scanning is limited by a probe delay, required by the standard to prevent clients from tuning into a channel in the middle of an existing transmission. The potential problem is that a client abruptly tuning into a channel might not be able to detect that a transmission is under way—carrier sense mechanisms that are based on detecting the preamble will miss out, and thus produce a false reading of a clear channel. Thus, if the client were then to send a probe request, the client could very well destroy the ongoing transmission and lose out on the access points' seeing the probe request, because of a collision. As it turns out, many voice clients set that probe delay to a trivial value, in order to not have to wait. But the common value for that delay is 12ms, which is a long time in the world of voice. Passive scanning is worse. Most access points send their beacons every 102.4ms, or as close as they can get. This means that a client who tunes to a channel has a good chance of having to wait 50ms just to get a beacon, and may have to wait the entire 100ms in the worst case, for just that one access point.

The timescale that dominates, for voice mobility, is the voice packet arrival interval. Normally, that value is 20ms (though it can be 30ms in some cases). A client will usually want to get all of the scanning it can get done in those 20ms, so that it can return to its original channel and not miss the next voice packet. Certainly, the client will not want to take 100ms unless it has to, because 100ms is a long enough jitter that it can be quite noticeable. Again, this tends to make active scanning the choice for voice clients, who are always in a hurry to learn about new access points.

If the client is going to scan between the voice packets, then the client's ability to scan will probably be limited to one channel at a time. When limited this way, the client may take up to a second, easily, to scan every possible channel. There are 11 channels in 2.4GHz, 9 non-DFS channels in 5GHz, and 11 more in the DFS bands, for a total of 31 channels to scan (or 23 channels if clients make the assumption that service is provided only on channels 1, 6, and 11 in the 2.4GHz band). Of course, scanning is also a battery-intensive process, and so a client may choose to spread out the scanning activity over time.

Furthermore, the process of changing channels is not always instantaneous. Depending on the radio chip vendor, some clients will have to wait through a multimillisecond radio settling and configuration time, reprogramming the various aspects of the radio in order to ensure proper transmission on the new channel. This adds additional padding time to the individual scanning channel transitions.

Overall, this scanning delay is a major source of handoff delays, and some methods for reducing the scanning time have been created, which we will examine shortly.