USB is a simple standard for the end-user. But for the System Integrator, it is often a pain to work with. How many support cases have you had where a customer calls in about a device which isn't working on one PC but works flawlessly on another?
One of the things you need to know about USB is that it has several limitations. One of which is the number of tiers one can use in a setup. Tiers are sometimes also referred to as levels or hubs. The USB standard defines a maximum of 7 tiers. So what are these tiers? Tiers are the number of USB hub-chips that can be linked together to connect to the Host controller.
Usually, a PC only has a single USB Host controller, this is Tier 1 and usually only contains ports for 2 USB connections. Your average PC these days, has many more ports than just two. To achieve this, they incorporate one or more hub chips These hub chips account for Tier number 2. With this setup, your PC can have about 6 USB ports, which isn't uncommon these days. Keep in mind, that we are still talking about internal hubs, we haven't gone outside the PC yet.
Once you run out of available USB ports on your PC, you turn to USB hubs or port-replicators. These usually perform the same way as it works within your PC. They have a receiving USB hub, which splits into another two hubs, which in turn provide you with an extra 4-7 USB ports. These are tier 3 & 4
Extenders usually do not account for an extra tier as they simply mimic a USB cable, but provide you with much longer cable length capabilities, however, some extenders do actually incorporate USB hub chips. You can usually simply tell by looking at the number of USB ports provided to you on the far end of the extender. This could be tier 5
As you can see, we're rapidly approaching our 7th tier. And we haven't done anything crazy here. We simply have a PC, hooked up to a dock with multiple USB ports and are using a USB extender. This is an everyday life experience for many. So far we haven't run into trouble as we're still only at tier 5. However, keep in mind that a USB HID device, like any mouse or keyboard, accounts for a tier in itself! So even with this simple setup, we're reaching the limits of USB capabilities. We've reached tier 6.
There have even been reports of USB devices that incorporate an additional USB hub internally to distribute a signal within the device itself (7 port USB HUB incorporate 2 tiers). This would add an additional tier. In this case, we would have maxed out our tiers by now. There is really no straightforward way to tell if this is the case, other than studying the PCB itself, looking for USB hub chips.
Now, this is where things get interesting. In the previous examples, we'd be talking about your own personal workspace or home office. But now we're taking it to the meeting rooms in your offices, where everybody has to be able to work with their own laptops and have to make use of the devices installed in the room. You can probably name them yourself, but common devices include the big touch screen, the video conference camera, the speakerphone on the table, all of which use the USB standard to connect to your PC. In this example, usually, there is a dedicated device in the room that connects to all of these devices, so you only need to hook up a single USB cable to your laptop. This typical setup can easily incorporate 2 or 3 tiers extra depending on the BYOD solution you choose.
When designing these installations, it is very much necessary that you take into account how many USB tiers you are creating and whether that might pose a problem for some users. Especially in a BYOD environment, not all users have the same hardware, they might bring additional docks, simply because their high-end ultraportable laptop only has a single USB port, which is also used for charging. Since you do not have control over what the end-user brings into the room, ensuring enough margin is key for a successful installation.