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Wireless a shared medium with a high throughput?



It is not something new, wireless is using a shared medium. Having a shared medium is a problem, as you share it with others. Every new 802.11 standard comes with a faster network. I saw the news release of 802.11ac with up to 6.93 Gbit/s. When you walk into the retail stores you see the boxes with 6.93 Gbit/s, so influencing the customers with technical marketing messages, who expects that a speed test will show the same speed as the box tells us.

A quick review about the 802.11 history.

802.11n
In 2007 the 802.11n standard came on the market with multiple-input and multiple-output (MIMO). We are here talking about MIMO, so this means not yet the Multi User MIMO (MU-MIMO). MU-MIMO will be discussed later. MIMO is sometimes called Single User MIMO (SU-MIMO) and it gives the possibility to transmit with multiple antennas or receive with multiple antennas. Re-read the last sentence it is “or” and not “and”. A client cannot transmit and receive at the same time (as radio communication is half-duplex communication), but concurrent communication it is possible in the wired network (also known as full-duplex). Conclusion on 802.11n, it is still for only one user at a time (SU-MIMO). When a user wants to send 2 MB all the other devices must wait till this device is done transmitting.

802.11ac (wave 2)
With 802.11ac wave 2 MU-MIMO is a new feature. It sounds more interested than it is. Let’s look deeper in this technology. Multi User is only in a downstream situation (from the access point to the client). With MU-MIMO it is possible that the access point can transmit to multiple clients (max. 4 concurrent down streams). It is not possible that multiple clients can transmit to the same access point at the time. Besides that, there is a limitation in the number of clients. An 8-spatial stream access point can, in theory, transmit to eight 1-spatial stream clients or transmit to four 2-spatial stream clients. I said in theory, because an access point can that can support 8-spatial streams, can maybe not support 8 clients at one time. For example, the maximum clients are 4 for this access point. It is possible that this will change and that there will come access point that can support more clients than 4.

Concluded, in 802.11ac, it is possible to transmit with multiple antennas to multiple devices, but it is not possible to receive from multiple devices with multiple antennas. Secondly clients can still transmit or receive at the same time and not transmitting and receiving at the same time. So yes, in the latest version of the 802.11 family it is still a shared medium and still half duplex.

As written in the introduction, 802.11ac is still faster than 802.11n. True, but still there is still a catch in this marketing slogan. The 6.93 Gbit/s is the total speed that 802.11ac can deliver. If we have an 8-spatial stream access point, using 160 MHz bandwidth, and Short Guard Interval (SGI) enabled. I tried to verify if there are any clients that supports 160 MHz channels, I couldn’t find anything on that for now, but still 160 MHz is not smart, there are multiple disadvantages of using 160 MHz channels, so the top speed of 6.93 Gbit/s is as told in the introduction an illusion.

Since Wi-Fi is still a shared medium, you share the total speed, so let’s say theoretically we have 4 2-spatial stream devices associated to an 8-spatial stream access point with a 160 MHz channel and SGI we share the 6.93 Gbps among the four clients, this will be 1.7 Gbps per client. To pick a more real environment; a 4-spatial stream access point with two 2-spatial stream clients with a 20 MHz channel and SGI we share the maximum of 346.7 Mbit/s (at the Access Point) so in this case we only talk about a maximum speed of 173.3 Mbit/s per device (as the devices can only do 2 spatial streams). At this point, only some mac books support 3-spatial stream

With MU-MIMO (downstream) we are a step closer to a medium where you don’t need to wait on each other. It is still sharing a medium and still transmitting or receiving. Sadly, at this moment there are a handful devices that support MU-MIMO, and the future is not standing still. Since the wireless industry is working on a new standard that enhanced the MU-MIMO.

802.11ax
Let’s look in to one of the newest family member of the 802.11 family. It is not ratified yet, but announced. With this announcement, I kind of see 802.11ac wave-2 as the Windows Vista of the wireless. Not that 802.11ac is bad, but at the time that there are clients that support all the features 802.11ax is ready to take place. 802.11ax is called High Efficiency Wireless and have some nice features.

I pick three enhancements that are valuable for this blog. The first one is an enhancement for MU-MIMO. Where 802.11ac was only downlink, 802.ax is downlink and uplink MU-MIMO. Where the maximum number of clients in 802.ac was four with MU-MIMO 802.11ax goes up to eight and the last one that I want to address is Orthogonal Frequency Division Multiple Access (OFDMA) an enhancement for OFDM.

OFDMA
With OFDM, if five clients want to transmit each a small package, they need to wait on each other. So, you have five data packages and five times a preamble and the whole CMSA/CA process with contention windows and so on. This is very inefficient. OFDMA uses resource units and have the possibility to assign the subcarriers to multiple clients. There is still a preamble, but it is possible that five clients claim all a resource within a package that uses the OFDMA modulation. They share the medium together, but don’t need to wait on each other, so they can transmit together. Instead of using the whole channel to transmit a small package, now five clients can transmit a small package in the channel.

MU-MIMO DL/UL
In 802.11ac it was new that the access point was able to transmit to multiple clients, 802.11ax takes it a step further. Clients can transmit their data in the same way (but don’t need to wait for other clients), the access point is now able to accept multiple data packages from different clients at the same time. Another limitation for 802.11ac was the maximum number of clients. In the situation of an access point with 8-spatial streams, it was not able to transmit to eight different clients, the maximum number of clients in a MU-group was four. With 802.11ax the maximum number of clients in a MU-group is eight. In this situation, an access point has the possibility to transmit or receive to eight different clients.

The focus with 802.11ax was not about fast, faster and fastest, but more about capacity and efficiency. How to make a shared medium more efficient. Of course, 802.11ax will be faster than 802.11ac, but that is not the focus this time. As long as Wi-Fi is a shared medium we need to find out how-to use it in the most efficient way. We started with MU-MIMO DL, now we have MU-MIMO DL/UL in combination with OFDMA. For now, the conclusion is still shared medium and still transmitting or receiving, but at least in a more efficient way. Don’t want to say 802.11ax will solve all our problems, but it looks like a good step in the right direction. The big question is, is what is next.

Future
March 2017, I read a Dutch news article with 100 times faster Wi-Fi. Are we talking here about another marketing slogan that we must not believe, or is there something true about this article? Reading this news article, I start believing in a great future for wireless (starting with 802.11ax).

The Eindhoven University of Technology(TU/e) in The Netherlands did research with Wi-Fi in combination of infrared. Infrared as a non-shared medium with a speed around 40 Gbit/s sounds like all problems are solved. Since it is not shared the maximum speed of 40 Gbit/s is the maximum speed for one device. Infrared operate in 200 terahertz, so no interferences from microwaves or radars. I want to read, at some point, the whole thesis from C.W. Oh for a deep dive in this ‘new’ technique. The question is, like with Li-Fi, how is this working in buildings, with walls, and windows.

The article, for who are interested, News article.