The next generation of the 802.11 standard--IEEE 802.11ac--promises to finally break the wireless Ethernet gigabit barrier. The new technology will deliver higher bandwidth while delivering a high quality of experience (QoE) for end users, and will be rapidly adopted into all major markets including residential, enterprise, carrier and large venue. However, there are still some significant technical challenges that chip and hardware developers must navigate before these markets can realize promised performance and density benefits.
IEEE 802.11: A brief history
802.11ac is an enhancement of the most recent IEEE wireless networking transmission standard, 802.11n. Image 1 below shows a brief history of the IEEE 802.11 standards.
The 802.11n standard brought with it the advances of MIMO (multiple-in, multiple-out) to deliver traffic over multiple spatial streams. MIMO delivered marked improvements in physical transport rates, enabling more bits/second to be transmitted across Wi-Fi than ever before.
Another advancement of 802.11n was packet aggregation, which delivered equally impressive improvements in transport experience, allowing devices to send more data once they had gained access to the wireless media. Fortunately, these capabilities are also present in 802.11ac because the new standard preserved the aggregation techniques, advanced the physical transport rates yet again, and introduced the concept of parallel transport into Wi-Fi through a technique known as Multi-User MIMO (MU-MIMO), where multiple client devices are receiving packets concurrently.
While this may seem like a minor technical detail, it in fact marks the first time in the Wi-Fi timeline that directed traffic can be delivered to multiple client devices simultaneously. This has a significant impact on the delivery of content to any location with multiple users, especially where that content is revenue generating or critical. In delivering that content, 802.11ac uses 256-QAM and 80 MHz bandwidths, which allows it to achieve the next level of performance. It also makes the signal fidelity, sensitivity and noise demands on the radio systems the most arduous to date. However, 802.11ac must still be compatible with existing legacy and 802.11n client devices.
it will be common for an 802.11ac access point (AP) to carry on conversations with multiple 802.11a phones, 802.11n tablets, and 802.11ac laptops. Each of these different devices supports a variety of features including QoS, power save, and multicast. The new 802.11ac chipsets will have to simultaneously excel at delivering blistering performance to other 802.11ac devices, while gracefully interoperating with several previous devices.
How 802.11ac Achieves Gigabit+ Performance
802.11ac uses a variety of advancements in order to achieve the targeted performance. The new specification addresses the need for performance improvement through three primary initiatives:
- Increasing the raw bandwidth
- Enabling multiple flows to use the medium concurrently
- Optimizing performance to specific clients
Increasing Raw Bandwidth
To increase the physical-layer transport rate, 802.11ac makes use of a higher rate encoding scheme known as 256-QAM. This scheme can transmit 33 percent more data than the 64-QAM used in the 802.11n standard. Thus, signal-to-noise ratios that worked for 802.11n are no longer sufficient for the higher speeds in 802.11ac because the difference in detectable signal level is now significantly smaller.
To further increase the amount of data transported per second, channel bonding approaches, which were made popular in 802.11n, have been expanded to provide 80 MHz-, and ultimately, 160 MHz-wide channels. Increasing channel bandwidth allows for more data to be transmitted simultaneously out of the same antenna. Compare this size to the legacy versions of the 802.11 devices, which commonly used 20 MHz channels.
When using 802.11n, users could only select between a 20 MHz or 40 MHz channel operation. It is important to note that these wider channel bandwidths – and the need for proper channel separation – mean that 802.11ac can only be used in the 5.0 GHz band (where more non-interfering bandwidth is available). While dual-band APs will still be produced, the 2.4 GHz band will be limited to 802.11bgn and will not be able to be configured for 802.11ac.
The use of multiple spatial streams is also a key factor in the 802.11ac equation. The 802.11n standard accommodates for up to four spatial streams to achieve a maximum of 600 Mbps of performance (although most APs today use only three spatial streams for a maximum PHY rate of 450 Mbps). In a significant expansion, the 802.11ac standard allows for up to eight spatial streams.
Lastly, while packet aggregation is present in 802.11n as well, it is worth mentioning because it is often the single biggest performance multiplier on a per-transmission basis. With aggregation, once a high performance device obtains its transmit opportunity, the transmitter strings multiple frames together, and transmits them in succession without having to reacquire the medium.
Enabling Multiple Flows--The Real Information Superhighway
In order to support more users, 802.11ac no longer requires that just one 802.11 device transmit at a time. Fortunately, MU-MIMO allows an access point to transmit data to multiple client devices on the same channel at the same time.
MU-MIMO allows a single AP to simultaneously transmit to multiple client devices by directing some of the spatial streams to one client while directing other spatial streams to a second client. This is much like driving down a four-lane freeway. As traffic is only significantly delayed when all four lanes are blocked, overall throughput can be much higher. There are a number of permutations to this basic principle, but MU-MIMO is critical to performance improvements in environments with high client counts.