New 802.11ac technology – what it means for WLANs

802.11ac is the latest IEEE standard for wireless LAN (WLAN) network technology, and is expected to be ratified by end of 2013 or early in 2014.

The Wi-Fi Alliance has already begun certification programs. It is expected that certifications and equipment will be rolled out in at least two phases, with the first phase happening now, and another phase to realise all of the IEEE specification’s enhancements coming at least a year from now. Several access points (APs) and client radios supporting the 802.11ac draft specification have been released in 2013 and are shipping with this first phase of enhancements.

802.11ac is essentially about higher bit rates over a WLAN connection. It builds on many of the techniques introduced in 802.11n to deliver higher rates. These include:

Wider channels: 802.11n introduced 40 MHz channels, which improved rates over previous 20 MHz channels. 802.11ac introduces 80 MHz channelsnow, and 160 MHZ channels in a next wave. Higher modulation and coding schema (MCS). 802.11ac introduces 256 QAM, which allows more bits to be encoded in a single symbol. This can provide up to a 33% improvement in bit rates.

More spatial streams via more antennae: 802.11n introduced multiple input /multiple output (MIMO) transmission systems, which uses up to four antennae for transmitting and four antennae for receiving (4×4). This allows a single bit stream to be divided into four different streams to be transmitted simultaneously, and then aggregated back to the original bit stream at the receiving end. 802.11ac increases this to up to eight streams via eight antennae (8×8).

Multi-user MIMO: 802.11ac introduces the technique of an AP using multiple antennae to transmit simultaneously to multiple clients. For example, a 4×4 AP can transmit simultaneously to four 1×1 clients.

The first wave of this technology includes 80 MHz channels and 3 x 3 APs. Wider 160 MHz channels, MIMO configurations greater than 3 x 3, and multi-user MIMO is expected to come in the next wave.

Physical layer connection rates will eventually be 6,9 Gbps. But most implementations in the first wave of deployments, using 3 x 3 MIMO, will support 1,3 Gbps.

What all of this really means is user throughput (as measured in bits per second) will increase from what they are now in 802.11n networks. A higher user throughput will, in turn, increase the capacity of 802.11ac APs. Because a user can download a file and upload an email attachment at faster transmission rates, they can use less time on the shared RF media. Therefore, more users transmitting at these higher rates can access the shared RF media of an AP.

However, the higher rates are neither certain nor guaranteed. They depend on a number of factors such as signal levels and signal/noise ratios (which determine which MCS can be used), co-channel interference, MIMO and spatial streams, beam steering and RRM techniques of an AP, and the hardware and firmware of radio adapters and the APs themselves. Higher rates also depend on the number of users on the shared RF media, and especially on the presence and activity of 802.11a and 802.11n clients.

802.11ac has been designed to be fully compatible with 802.11a and 11n client radios. If an 802.11ac AP operates on a wide 80MHz channel, RF media controls (e.g., beacons, RTS, CTS messages) are transmitted on the “primary” 20 MHz channel in 11a format. This allows an 802.11a/n client to operate on an 802.11ac AP. However, 802.11a clients can only transmit on 20 MHz channels, and 802.11n on 20 or 40 MHz channels. Therefore, when an 11a client transmits to an 11ac AP, the 80 MHz channel must fallback to 20 MHz. It must fallback to a 40 MHz channel to accommodate an 11n client. This means that an 11ac client must wait for the slower transmissions of an 11a or 11n client before it can transmit, thus slowing down overall performance.

Even with these conditional impacts on performance from legacy clients, an 802.11ac AP network will provide overall better performance in terms of throughput and user capacity. This improvement will only increase as the client base transitions more to 802.11ac clients from legacy 802.11a and 11n clients.

Another key difference from 802.11n is that 802.11ac operates only in the 5 GHz band. This will drive the population of WLAN client radios more to the 5 GHz band in order to take advantage of the performance improvements. Nonetheless, most 802.11ac APs now shipping support dual-band operation in both the 2.4 GHz and the 5 GHz band, and support 802.11a/b/g/n along with 11ac.

802.11ac requires replacement of current legacy APs. However, the rest of the LAN/WAN infrastructure does not require replacement. The only consideration that needs to be made is whether an increase in Ethernet capacity is required from the AP to the network aggregation and WAN connections.

Since 802.11ac APs can provide higher overall throughput, one can expect to have more user traffic hit the Ethernet backhaul network. If a single Gigabit Ethernet link is currently installed to an 802.11a/n AP, then it is likely that link will be sufficient for an 802.11ac AP. Since the current maximum physical data rate of 802.11ac is 1300 Mbps, and the user throughput will not likely exceed 900 Mbps, a 1 Gbps link will handle this. However, consider first that in a 1-2 year timeframe 802.11ac APs may handle more than 2,6 Gbps physical data rates and more than 1 Gbps user rates, thereby driving the need for two Gigabit Ethernet links. Consider also that the aggregate network uplinks from multiple 802.11ac APs may require an upgrade in Ethernet capacity, especially if that uplink is currently only 1 Gbps.

Top five tips for deploying 802.11ac

Know your user base

There is no single set of rules for designing all WLANs; it all depends on the user base. Understand your user density requirements. Know if the majority of your user devices are 11a/n and if you can rely on just 20 or 40 MHz channels (which will reduce channel overlap complexities). Know if application needs are mostly around high bit rate apps like video, or low bit rate apps like web browsing.

Design for capacity

Although 802.11ac provides an increase in WLAN capacity of an AP by increasing throughput rates, the proliferation of WLAN devices and users means that most likely your network will require more APs to handle the increased user density.

Survey your site prior to deployment

Determine the coverage of your current legacy network, to help determine if more APs will be needed. See how the physical environment (e.g., walls and other obstacles) affect the RF propagation of your current APs. Determine if any interference sources are present that will impact performance, and if you can mitigate or remove them. Also determine availability of dynamic frequency selection (DFS) channels. DFS channels in the 5 GHz band are used by radars, and APs are required to abandon a DFS channel if the AP detects radar on it. If DFS channels are available, they may be used for more channels if your client base supports them.

Plan your network carefully, using pre-deployment survey as input

Based on your user base and pre-deployment survey, carefully determine the number of APs needed and their placement. Also determine if wider channels are needed, and how they will be assigned on each AP to minimise co-channel interference of nearby APs and maximise coverage.

Survey your network after deployment to validate

Perform a site survey of underlying performance factors including signal levels, channel widths, interference patterns, and MCS indices. Determine if the coverage provide meets your design requirements. If not, adjust AP placements, channel assignments, transmission levels, etc., while still on site and avoid having to come back. Then validate your final deployment with a survey of the ultimate performance objective: user throughput. This can be done with a survey of iperf throughput.

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New 802.11ac technology – what it means for WLANs