Products built to the new IEEE 802.11ac amendment feature "wired-like" speeds, data rates beyond 1 Gig, unprecedented user density, scalability in large venues, and better client battery life. All this new capability is provided by larger swaths of bandwidth available in the 5 GHz band and sophisticated modulation and coding schemes enabled by advanced signal processing techniques. Among the technologies offered by the latest 802.11ac products is Multi-User Multiple Input Multiple Output or MU-MIMO. MU-MIMO is the ability of the wireless access point (AP) to engage multiple clients simultaneously. Of course, prior generations of APs could engage multiple clients "simultaneously", but only in the sense that multiple clients could be associated with a AP and be communicating with it, but only one at a time. The new MU-MIMO will permit multiple client devices to be communicating, truly, at the same time.
Perhaps an analogy to the wired network will help demonstrate the difference between the 802.11ac MU-MIMO and prior wireless protocols. Years ago, Ethernet was a "shared medium" technology. Some readers might remember thick net and thin-net coaxial cabling and Ethernet hubs used as the transmission medium. Multiple Stations (PCs or other clients) would be connected to the same cable through taps, and to avoid collision, the station would "listen" first to see if the line was clear before transmitting. If two stations transmitted simultaneously causing a collision, there is a Collision Detection (CD) mechanism built in. The station detects the collision then retransmits after a random interval. This is called Carrier Sense Multiple Access with Carrier Detection (CSMA-CD) and is contention protocol built into the IEEE 802.3 standards. With CSMA-CD, only one station on the network can be transmitting at a time, and thus, by its very nature, the system is half duplex. This was an effective technology at the time, but data throughput could really be impacted by multiple connected stations, and if enough stations were connected to the medium, throughput would drop to zero.
The solution to this wired contention problem is what we use now - Switched Ethernet. With switched Ethernet, every client is connected to a switch port with its own dedicated, full duplex, communications cable. There is no contention or collision on the dedicated line. So throughput is not limited by the number of stations connected, it is only limited by the throughput of the transmission medium and the switch fabric.
This is a highly simplified description, but the point is that we have much higher throughput capability when we use a dedicated (not shared) medium as with switched Ethernet. However, for IEEE 802.11 Wi-Fi wireless client devices, the contention problem has remained in place. Wireless devices still use a shared medium. IEEE 802.11 based products use Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA) protocol. Again, the transmitter "listens" to know if the channel is clear, then transmits. The problem in the wireless domain is that the transmitter may sense that the channel is clear but the receiver may not have a clear channel at its remote location. So while the transmitter clearly transmits, the receiver is unable to receive due to interference from a local hidden transmitter. This is called the "hidden node" problem, and is one reason why wireless devices do not, perhaps cannot, provide the same high throughput that a dedicated communications line (copper or fiber) can provide.
\Another problem is that the Wi-Fi client or wireless access point antenna is generally omni-directional. It transmits, and receives, waveforms from all directions. So, unlike a wired connection, the Wi-Fi receiver is receiving not only the intended Wi-Fi waveform, but also unintended Wi-Fi waveforms, microwave ovens, cosmic background radiation and everything else that happens to permeate the electromagnetic space.
The wireless transmission medium is truly a challenging medium. A properly cabled transmission medium is like a drive on a new 4 lane interstate highway. A typical wireless medium is like a bushwack up a jungle mountainside, with numerous machete wielding bushwackers moving against you.
Single User MIMO (SU-MIMO) and Multi-User MIMO (MU-MIMO)
SU-MIMO was introduced with the 802.11n generation products. With SU-MIMO, a device can transmit multiple spatial streams from multiple antennas at once, but only directed to a single device or address. The multiple spatial streams can improve the reception and throughput to a device, but only one at a time (see figure 1).
802.11ac products are being introduced in two waves. Wave 1 products still engage SU-MIMO, but Wave 2 introduces MU-MIMO. MU-MIMO provides the ability for the Access Point (AP) transmit to several clients simultaneously. Using advanced signal processing and beam-forming, the AP creates a "beam" to each individual client. In fact, the AP can create a beam to several clients, at the same time, on the same frequency channel. Think of it like this: until now, Wi-Fi operated like shared medium Ethernet and a hub. Now, with Wave 2 802.11ac and MU-MIMO, Wi-Fi has become more like switched Ethernet!
The transition from the old shared medium, hub based Ethernet to Switched Ethernet resulted in dramatic improvements in wired network capacity. This transition to MU-MIMO will yield a comparable improvement for wireless capability. Although Wave 2 802.11ac product will probably not be available until 2015, some manufacturers' are presently providing a MU-MIMO mode. With 802.11ac products, expect the following:
- Wired-like experience at higher speed, noticeably faster connectivity for the end user
- Higher density deployments enabled through clients getting on and off the network faster
- Significantly better client battery life
- Wide selection of client devices now available with integrated 802.11ac
FIGURE 1 SU-MIMO versus MU-MIMO. The number of client spatial stream cannot exceed the number of spatial streams offered by the AP.
How Many Clients can a MU-MIMO AP Communicate with Simultaneously?
802.11ac Wave 2 permits up to 8 spatial streams. However, in practice, wave 2 products will support 4 spatial streams, while expanding the channel width from 80 MHz to 160 MHz. The total number of clients an AP can support simultaneously is dependent on the number of spatial streams demanded by the client devices. For example, a 4SS (spatial stream) AP can support four 1SS clients (such as smart phones, figure 2) or one 2SS client (tablets) plus two 1SS clients (figure 3). The total number of client spatial streams must not exceed the maximum number of spatial streams supported on the AP.
FIGURE 2 -A 4SS MU-MIMO AP and four 1SS clients. MU-MIMO uses beam-forming and multiple spatial streams to simultaneously engage multiple clients.
Credit for figures: Cisco will ride the 802.11ac Wave2, Bill Rubino, Cisco Mobility blog
Installation Considerations for 802.11ac
Given that new Wi-Fi deployments with 802.11ac will be mission critical, it is all the more important to plan for effective location and positioning of access points. The antennas will be required to effectively create spatial streams and beam-form to multiple clients. WAPS should be positioned to achieve optimum antenna coverage and performance, and may be located densely to serve high densities of client devices. Some important considerations are as follows:
- Mount access points in the preferred horizontal orientation, whether in the ceiling or on the wall. All leading AP manufacturers recommend this.
- Mount APs in the ceiling if possible, in a high location above obstructions where possible.
- Do not mount APs above ceilings. The ceiling tiles and gridwork attenuate and disrupt the signal.
- In auditoriums, stadiums, and large classrooms, it may not be possible to mount APS in the ceiling. An alternative is to mount APs underneath seating to achieve the density required. APs should be properly protected in this environment.
- Physically protect the AP and antenna from the environment
- Installations should provide the physical security, code compliance, and aesthetics mandated by the installation environment.
Oberon’s wireless AP mounting and enclosure solutions provide a means to securely, conveniently, and aesthetically mount the AP while optimizing antenna performance.
APs are ideally secured in the ceiling or other high location with antennas unobstructed by ceiling tiles or ceiling grid. - Oberon Model 1064 locking ceiling mount for Cisco APs
APs should be mounted in the preferred horizontal orientation to achieve best antenna pattern coverage. - Oberon Model 1012 locking right angle surface mount bracket for access points
Access points with detachable or external antennas can be secured in ceiling enclosures with plenum rated non-metallic domes. - Oberon Model 1059 locking ceiling enclosure
Access points may be mounted beneath seats to improve AP density, but must be protected from weather, spilling liquids, and impacts. - Oberon Model 1020 compact NEMA 4 non-metallic enclosure for access points
Physically secured installation can be achieved with non-metallic surface mount lock box. - Oberon Model 1015
External antennas can provide optimal coverage if deployed and protected correctly. - Oberon Model 1016
Indoor/Outdoor installation in public venues may require weather protection with a water tight, non-metallic NEMA4 enclosure designed for wireless APs. - Oberon Model 1024