The Difference between Network Assistance and Network Control

If you have read the sections on cellular handoff, you'll know that there are broadly two different methods for phone handoffs to occur. The first method, network control, is how the network determines when the phone is to hand off and to which base station the phone is to connect. In this method, the mobile phone may participate by assisting in the handoff process, usually by providing information about the radio environment. The second method, network assistance, is where the network has the ability to provide that assistance, but the mobile phone is fundamentally the device that decides.
For transitions across basic service sets (BSSs) in Wi-Fi, the client is in control, and the network can only assist. Why is this? An early design decision in Wi-Fi was made, and the organization broke away from the comparatively long history of cellular networking. In the early days of Wi-Fi, each cell was unmanaged. An access point, compared to a client, was thought of as the dumber of the two devices. Although the access point was charged with operating the power saving features (because it is always plugged in), the client was charged with making sure the connection to the network stayed up. If anything goes wrong and a connection drops, the client is responsible for searching out for one of any number of networks the client might be configured to connect to, and the network needed to learn only about the client at that point. It makes a fair amount of sense. Cellular networks are managed by service providers, and the force of law prevents people from introducing phones or other devices that are not sanctioned and already known about by the service provider. Therefore, a cell phone could be the slave in the master/slave relationship. On the other hand, with Wi-Fi putting the power of the connection directly into the hands of the client, the network never needs to have the client be provisioned beforehand, and any device can connect. In many ways, this fact alone is why Wi-Fi holds its appeal as a networking technology: just connect and go, for guest, employee, or owner.
This initial appeal, and tremendous simplicity which comes with it, has its downsides, and quickly is meeting its limitations. Cellular phones, being managed entities, never require the user to understand the nature of the network. There are no SSIDs, no passphrases to enter. The phone knows what it is doing, because it was built and provisioned by the service provider to do only that. It simply connects, and when it doesn't, the screen shows it and users know to drive around until they find more bars. But in Wi-Fi, as long as the handset owns the process of connecting, these other complexities will always exist.
Now, you might have noticed that SSIDs and passwords have to do only with selecting the "service provider" for Wi-Fi, and once the user has that down (which is hopefully only once, so long as the user is not moving into hotspots or other networks), the real problem is with the BSSID, or the actual, distinct identities of each cell. That way of thinking has a lot to it, but misses the one point. The Wi-Fi client has no way of knowing that two access points—even with the same SSID—belongs to the same "network." In the original Wi-Fi, there is not even a concept of a "network," as the term is never used. Access points exist, and each one is absolutely independent. No two need to know about each other. As long as some Ethernet bridge or switch sits behind a group of them, clients can simply pass from one to the other, with no network coordination. This is what I mean, then, by client control. In this view of the world, there really is no such thing as a handoff. Instead, there is just a disconnection. Perhaps, maybe, the client will decide to reconnect with some access point after it disconnects from the first. Perhaps this connection will even be quick. Or perhaps it will require the user to do something to the phone first. The original standards remain silent—as would have phones, had the process not been improved a bit.
Network assistance can be added into this wild-west mixture, however. This slight shift in paradigm by the creators of the Wi-Fi and IEEE standards is to give the client more information, providing it with ways of knowing that two access points might belong to the same network, share the same backend resources, and even be able to perform some optimizations to reduce the connection overhead. This shift doesn't fundamentally change the nature of the client owning the connection, however. Instead, the client is empowered with increasingly detailed information. Each client, then, is still left to itself to determine what to do and when to do it. It is an article of faith, if you will, that how the client determines what to do is "beyond the scope of the standard," a phrase in the art meaning that client vendors want to do things their own way. The network is just a vessel—a pipe for packets.
You'll find, as you explore voice mobility deployments with Wi-Fi as a leg, that this way of thinking is as much the problem as it is a way to make things simple. Allowing the client to make the choice is putting the steering wheel of the network—or at least, a large portion of the driving task—in the hands of hundreds of different devices, each made by its own manufacturer in its own year, with its own software, and its own applications. The complexity can become overwhelming, and the more successful voice mobility networks find the right combinations of technologies to make that complexity manageable, or perhaps to make it go away entirely.

Inter-Access Point Handoffs

In a voice mobility network with Wi-Fi as a major component, we have to look at more than just the voice quality on a particular access point. The end-user of the network, the person with a phone in hand, has no idea where the access points are. He or she just walks around the building, going in and out of range of various access points in turn, oblivious to the state of the underlying wireless network. All the while, the user demands the same high degree of voice quality as if he or she had never started moving.
So now, we have to turn our focus towards the handoff aspect of Wi-Fi voice networks. Looking back on how Wi-Fi networks are made of multiple cells of overlapping coverage, we can see that the major sources for problems with voice are going to come from four sources:
  1. How well the coverage extends through the building
  2. How well the phone can detect when it is exiting the coverage of one access point
  3. How well the phone can detect what other options (access points) it has available
  4. How quickly the phone can make the transition from the old access point to the new one
Let's try to gain some more appreciation of this problem. Figure 1 shows the wireless environment that a mobile phone is likely to be dwelling within.

Figure 1: The Handoff Environment
As the caller and the mobile phone move around the environment, the phone goes into range and out of range of different access points. At any given time, the number of access points that a client can see, and potentially connect to, can be on the order of a dozen or more in environments with substantial Wi-Fi coverage. The client's task: determine whether it is far enough out of range of one access point that it should start the potentially disruptive process of looking for another access point, and then make the transition to a new access point as quickly as possible. The top part of Figure 1 shows the phone zigzagging its way through a series of cells, each one from an access point on a different channel. Looking at the same process from the point of view of the client (who knows only time), you can see how the client sees the ever-varying hills and valleys of the differing access points' coverage areas. Many are always in range; hopefully, only one is strong at a time.
The phone is a multitasker. It must juggle the processes of searching for new access points and handing off while maintaining a good voice connection. In this section, we'll go into details on the particular processes the phone must go through, and what technologies exist to make the process simpler in the face of Wi-Fi. But first, we will need to get into some general philosophy.

Telecom Made Simple

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