Explicit in the concept of load balancing is that it is actually possible to balance load—that is, to transfer load from one access point to another in a predictable, meaningful fashion. To understand this, we need to look at how the load of a call behaves from one access point to another, assuming that neither the phone nor the access points have moved.
Picture the environment in Figure 1. There are two access points, the first one on channel 1 and the second on channel 11. A mobile phone is between the two access points, but physically closer to access point 1. The network has two choices to distribute, or balance, the load. The network can try to guide the phone into access point 1, as shown in the top of the figure, or it can try to guide the phone into access point 2, as shown in the bottom of the figure.
This is the heart of load balancing. The network might choose to have the phone associate to access point 2. We can imagine that access point 1 is more congested—that is, it has more phone calls currently on it. In the extreme case, access point 1 can be completely full, and might be unable to accept new calls. The advantage of load balancing is that the network can use whatever information it sees fit—usually, loads—to guide clients to the right access points.
There are a few wrinkles, however. It is extremely unlikely that, in a non-channel-layered environment, the phone is at the same distance from each of the two access points. It is more likely that the phone is closer to one access point than another. The consequence of the phone being closer to an access point is that it can get higher data rates and SNR, which then allows it to take less airtime and less resources. It may turn out that, if the network chooses to move the phone from access point 2 to access point 1, that the increase in data rate because the phone is closer to access point 1 allows possibly two calls in for access point 2. In this case, the same call produces unequal load when it is applied to different access points, all else being equal.
For this reason, within-band load balancing has serious drawbacks for networks that do not use channel layering. Load balancing should be thought of as a way to distribute load across equal resources, but within-band load balancing tends to work rather differently and can lead to performance problems. If the voice side of the network is lightly used—such as having a small CAC limit—and if the impact of voice on data is not terribly important, then this sort of load balancing can work to ease the rough edges on networks that were not provisioned properly. However, for more dense voice mobility networks, we need to look further. The concept of load balancing among near equals does exist, however, with band load balancing. Band load balancing can be done when the phones support both the 2.4 GHz and 5 GHz bands (some newer ones do) and the access points are dual-radio, having one 2.4 GHz radio and another 5 GHz radio in the same access point. In this case, the two choices are collocated: the client can get similar SNR and data rates from either radio, and the choice is much closer to one-to-one. Figure 2 illustrates the point.
A variant of band load balancing is band steering. With band steering, the access point is not trying to achieve load balancing across the two bands, but rather is prioritizing access to one band over the other—usually prioritizing access to the 5GHz band for some devices. The notion is to help clear out traffic from certain devices, such as trying to dedicate one band for voice and another for data. Using differing SSIDs to accomplish the same task is also possible, and works across a broader range of infrastructures.
There are differences between the two bands, of course, most notably that the 5 GHz band does not propagate quite as far as the 2.4GHz band. The 5GHz band also tends to be unevenly accessed by multiband phones: sometimes, the phones will avoid the 5GHz band unless absolutely forced to go there, leading to longer connection times. On the other hand, the 2.4GHz band is subject to more microwave interference. And, finally, this mechanism will not work for single-band phones. (The merits of each band for voice mobility are summarized later in this chapter.) Nevertheless, band load balancing is an option for providing a more even, one-to-one balance.
For environments with even higher densities, where two channels per square foot are not enough, or where the phones support only one band or where environmental factors (such as heavy microwave use) preclude using other bands, channel layering can be employed to provide three, four, or many more choices per square foot. Channel layering exists as a benefit of the channel layering wireless architecture, for obvious reasons, and builds upon the concept of band load balancing to createcollocated channel load balancing. The key to collocated channel load balancing is that the access points that are on different channels are placed in roughly the same areas, so that they provide similar coverage patterns. Because channels are being taken from use as preventatives for co-channel interference and are instead being deployed for coverage, channel layering architectures are best suited for this. In this case, the phone now has a choice of multiple channels per square foot, of roughly similar, one-to-one coverage. Figure 3 illustrates this.
Bear in mind that the load balancing mechanisms are in general conflict with the client's inherent desire to gain access to whatever access point it chooses and to do so as quickly as possible (see Section 6.2.2). The network is required to choose an access point and then must ignore the client, if it should come in and attempt to learn about the nonchosen access points. This works reasonably well when the client is first powered up, as the scanning table may be empty and the client will blindly obey the hiding of access points as a part of steering the load. On the other hand, should the client already have a well-populated scanning table—as voice clients are far more likely to do—load balancing can become a time-consuming proposition, causing handoff delays and possible call loss. Specifically, what can happen is that the client determines to initiate a handoff and consults the information in its scanning table, gathered from a time when all of its entries were options, based on load. The client can then directly attempt to initiate a connection with an access point, sending an Authentication or Reassociation frame (depending on whether the client has visited the access point before) to an access point that may no longer wish to serve the client. The access point can ignore or reject the client at that point, but usually clients are far less likely to abandon an access point once they choose to associate than when they are scanning. Thus, the client can remain outside the access point, persistently knocking on the door, if you will, unwilling to take the rejection or the ignoring as an answer for possibly long periods of time. This provides an additional reason why load balancing in an environment where multiple handoffs are likely can have consequences for the quality of voice calls.
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