Virtual Cell/Virtual Port - Part 1: Virtual Port Migration
Wireless LAN Virtual Cell/Virtual Port - Part 1 of 2
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In this video, I'd like to talk about the design of Virtual Cell, migrating to Virtual Port. Now, the traditional way in which wireless LANs were designed -- what we think of as the legacy design of wireless LANs -- is really based on micro cell architecture. Now, the way micro cell works is the following. You have multiple access points, each access point has a certain coverage region, and it advertises its presence. The way it advertises its presence is by means of a wireless MAC address, or what is known in the trade as BSSID. This is just an index. It looks like a MAC address. And what it does is it advertises the presence of this access point. So when a client comes into a network, or in other words when a client is trying to establish connectivity, it does a probe. It looks to see what are all the BSSIDs or access points that are in the network, and the client sends out a probe request.
All the access points that hear this probe request and want to serve the client will send a probe response, then the client decides which access point to use. So in this instance, let's assume there is, first off -- there are two access points, call them AP1 and AP2. AP1 advertises for a certain SSID, which is a wireless service, let's say BSSID1. And AP2 has its own unique wireless MAC address, BSSID2. So there's a client that started out here. It comes into the network, it probes and it says I need connectivity. Only access point 1 hears it, it responds with BSSID1, and therefore the client is connected to AP1. As the client moves, it starts to slowly see AP2. And at some point, the client is going to come out here, and the signal strength with respect to BSSID1 is going to get weaker, and it's going to try to figure out -- it has to make its own decision on which other access point to connect to. So at this point it'll start to do another probe. AP1 and AP2 will both hear it, and they will both respond. It's up to the client to choose BSSID2.
So notice a couple of things that are implicit in what I just said. First off, access points only advertise their presence and their willingness to take in clients is the client's decision. And secondly, as the client moves, when it decides to hand off is the client's choice. There are, as of today, about 5,000 different options in terms of drivers, chipsets and settings, and in many of these it's the client decision that is highly variable. So what is the right algorithm or how does a client decide when to make this tradeoff? Nobody knows because it's a function of the client.
The key point is you don't want a client-initiated network because one client can start affecting the behavior of another. So the question here is how do you take the decision away from the client? Well there is one way of doing it. Let's take both these cells and let's make them advertise the same BSSID or wireless MAC address. Now whether the client is here or here or here, when it seeks the presence of access points, what does it hear? It hears only BSSID1. Whether there is a single AP responding to it or multiple APs responding to it, they all respond with the same wireless MAC address. The same BSSID. Notice you took the decision making transparently away from the client.
As the client moves, it is up to the infrastructure to figure out whether access point 2 is better served to serve the client than access point 1, and transparent change the association or transparently change this assignment. From a client perspective, there is no hand off. So how does this happen? Access points are connected through a back end fabric to a controller. And periodically, access points compose a message digest of all the devices that they see with the corresponding RSSIs, frame rates, number of transmissions, etc. up to a controller. So the controller basically has a global view.
How do you differentiate between this client and this client? Well, one way of doing that is by giving them different BSSIDs. So notice that rather than making these BSSIDs a function of the access point or unique to an access point, when a client comes in, let's make sure we assign a unique BSSID per client. And let's make sure when a client moves, the BSSID basically moves along with it. So one way of thinking about it is rather than virtualizing a cell, you really personalize an access point. You create a personal access point for each client, and as that client moves from location to location you figure out which is the best access point to serve it, and then instantiate the BSSID or the virtual access point on that corresponding physical AP.
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