802.11n MAC MSDU integrity

ovides a superior level of service. In addition to the general efficiencies that the IEEE 802.11 working group builds into new specifications, products often add clever features to further extract capacity increases from the new physical layer. One common method of doing so is to bias transmissions toward frames that require shorter times to transmit. Even though 802.11ac can transmit large numbers of bits, the extremely high data rates mean that even very large amounts of data are transmitted faster than small packets were in 802.11b.

Latency

Some applications benefit primarily from lower latency, especially real-time streaming applications such as voice, videoconferencing, or even video chat. Improving latency can be done by building a more efficient network, but often the best way to improve latency is to reduce the load on the network. 802.11 measures load by airtime utilization, so moving to faster physical layer standards improves latency by reducing the airtime load. Multi-user MIMO also has the potential to decrease network load by enabling parallel transmissions. Reducing latency means that even a few 802.11ac devices may benefit the entire network by decreasing airtime demand.

As part of the IEEE project authorization process, a task group in the formation process needs to discuss compatibility with previous technology standards. Early adopters of wireless LANs made significant investments in the technology, and the IEEE process is designed to protect that investment. Backward compatibility with prior 802.11 standards was a key consideration in the 802.11ac standardization process, and there was extensive work done in the protocol to ensure that 802.11ac would work with the many existing wireless LAN devices. In addition to physical-layer compatibility, 802.11ac has extensive MAC-layer compatibility, which enables newer 802.11ac devices to perform at their best even when surrounded by older devices. In fact, these functions were designed to enable a little bit of 802.11ac to speed up any network.

802.11ac was designed from the beginning to be compatible with prior standards (802.11n, 802.11a/g, and 802.11b). Don’t let compatibility worries slow you down—adding 802.11ac speeds up any network, even if it has only a few 802.11ac client devices.

Even though 802.11ac is the future physical layer in wireless LANs, it will not be the only physical layer. APs that are sold as “802.11ac APs” will have one 5 GHz radio running 802.11ac, and they will also have a second 2.4 GHz radio running 802.11n. Even as 802.11ac becomes established, the 2.4 GHz band will continue to depend on the same 802.11n technology that has been used for the past several years.

Catching the 802.11ac Technology Wave

Early in the development of wireless LAN technology, a new PHY was brought to market all at once. With 802.11n, however, the standards started to become much more complex, and different levels of capability came to the market in distinct “waves” or “phases.” Once the basic technical details are worked out, it can often be much easier to write a standard than to build a product. For example, the work required to add four-spatial-stream support into the 802.11n standard was relatively minimal after the basic ground rules were complete, but as of the 2013 publication date of this book, four-stream 802.11n devices have yet to be brought to market because of the engineering challenges involved in building the powerful DSP required to perform the spatial mapping while staying within the 15-watt 802.3af power limit.
802.11n came to the market in waves due to the overall complexity of the standard. 802.11ac will follow this well-worn path, with a rough estimate of the contents of the first two waves in Table 5-1. The first generation of 802.11ac delivers another jump in channel bandwidth, along with a new modulation. Taken together, these two features are enough to nearly double the speed of a typical three-stream client device. The second wave of 802.11ac will add even wider channels, four-stream support, and beamforming. Although there is a temptation to focus on the headline rates only, beamforming has the potential to deliver significant gains in network capacity by improving the data rates at which most clients transmit. Not all transmissions occur at the fastest rate, so the beamforming boost can be substantial if it increases the data rates used by clients.