Common Features

Although the two proposals are different, there is a great deal of similarity between the two. Practically speaking, some features are required to reach the goal of 100 Mbps throughput.

Multiple-Input/Multiple-Output (MIMO)

Up until 2004, 802.11 interfaces had a single antenna. To be sure, some interfaces had two antennas in a diversity configuration, but the basis of diversity is that the "best" antenna is selected. In diversity configurations, only a single antenna is used at any point. Although there may be two or more antennas, there is only one set of components to process the signal, or RF chain. The receiver has a single input chain, and the transmitter has a single output chain.

The next step beyond diversity is to attach an RF chain to each antenna in the system. This is the basis of Multiple-Input/Multiple-Output (MIMO) operation.[*] Each RF chain is capable of simultaneous reception or transmission, which can dramatically improve throughput. Furthermore, simultaneous receiver processing has benefits in resolving multipath interference, and may improve the quality of the received signal far beyond simple diversity. Each RF chain and its corresponding antenna are responsible for transmitting a spatial stream. A single frame can be broken up and multiplexed across multiple spatial streams, which are reassembled at the receiver. Both the WWiSE and TGnSync proposals employ MIMO technology to boost the data rate, though their applications differ.

[*] MIMO is pronounced "MyMoe." I attended a symposium in which a standards committee attendee described the standardization vote on the acronym's pronunciation.

MIMO antenna configurations are often described with the shorthand "YxZ," where Y and Z are integers, used to refer to the number of transmitter antennas and the number of receiver antennas. For example, both WWiSE and TGnSync require 2x2 operation, which has two transmit chains, two receive chains, and two spatial streams multiplexed across the radio link. Both proposals also have additional required and optional modes. I expect that the common hardware configurations will have two RF chains on the client side to save cost and battery power, while at least three RF chains will be used on most access points. This configuration would use 2x3 MIMO for its uplink, and 3x2 MIMO on the downlink.

Channel Width

802.11a currently uses 20 MHz channels because that is the channel bandwidth allowed by all regulators worldwide. Doubling the channel bandwidth to 40 MHz doubles the theoretical information capacity of the channel. Although promising for the future, some regulators do not currently allow 40 MHz operation. Japan is the most notable exception.

MAC Efficiency Enhancements

As this book has repeatedly pointed out, the efficiency of the 802.11 MAC is often poor. In most usage scenarios, it is very difficult to exceed 50-60% of the nominal bit rate of the underlying physical layer. Every frame to be transmitted requires a physical-layer frame header, as well as the pure overhead of preamble transmission. The 802.11 MAC adds further overhead by requiring that each frame be acknowledged. Overhead is particular bad for small frames, when the overhead takes more time than the frame data itself. Figure 15-1 shows the efficiency, defined as the percentage of the nominal bit rate devoted to MAC payload data, for a variety of frame sizes. The values in the figure are exclusively for MAC payload data. Any network measurement would require additional LLC data, and networks that are encrypted would have additional overhead bytes. Furthermore, most network protocols provide their own acknowledgment facilities, which further reduces real-world efficiency. The point of Figure 15-1 is that small frames have particularly poor efficiency.

Both TGnSync and WWiSE adopt techniques to improve the efficiency of the radio channel. Concepts are similar, but the details differ. Both offer some form of block ACKs (sometimes called frame bursting). By removing the need for one acknowledgment frame for every data frame, the amount of overhead required for the ACK frames, as well as preamble and framing, is reduced. Block acknowledgments are helpful, but only if all the frames in a burst can be delivered without a problem. Missing one frame in the block or losing the acknowledgment itself carries a steep penalty in protocol operations because the entire block must be retransmitted.

Figure 15-1. MAC Efficiency

Frame aggregation is also part of both proposals. Many of the packets carried by 802.11 are small. Interactive network sessions, such as telnet and SSH, make heavy use of rapid-fire small packets. Small packets become small frames, each of which requires physical-layer framing and overhead. Combining several small packets into a single relatively large frame improves the data-to-overhead ratio. Frame aggregation is often used with MAC header compression, since the MAC header on multiple frames to the same destination is quite similar.