Hinting at Apple\'s next-generation AirPort products

The emerging 802.11n specification — the successor to 802.11g — will differ from its predecessors in that it will provide for a variety of "optional modes" and configurations that dictate different maximum raw data rates, according to a white paper released this week by Apple wireless chip supplier Broadcom.

These optional modes will enable the emerging standard to provide baseline performance parameters for all 802.11n devices, while allowing hardware manufacturers to enhance or tune capabilities to accommodate different applications and price points.

With every possible option enabled, 802.11n could offer raw data rates up to 600 Mbps, the company said. However, most WLAN hardware will not need to support every option, and therefore most 802.11n hardware is expected to advocate support for raw data rates of up to 300 Mbps.

Demand for Wi-Fi technologies has experienced phenomenal growth during the past several years, evolving quickly from novelty into necessity. As a measure of this expansion, WLAN chipset shipments in 2005 surpassed the 100-million-unit mark, a more than tenfold increase from 2001 shipments of less than 10 million units.

So far, demand for the technology has been driven primarily by users connecting notebook computers to networks at work and to the Internet at home as well as at coffee shops, airports, hotels, and other mobile gathering places. As a result, Wi-Fi technology is most commonly found in notebook computers and Internet access devices such as routers and DSL or cable modems. In fact, more than 90 percent of all notebook computers now ship with built-in WLAN, according to Broadcom.

The growing pervasiveness of Wi-Fi is helping to extend the technology beyond the PC and into consumer electronics applications like Internet telephony, music streaming, gaming, and even photo viewing and in-home video transmission. Personal video recorders and other A/V storage appliances that collect content in one spot for enjoyment around the home are accelerating this trend.

With all its new users, as well as a growing number of conventional users, Wi-Fi networks are often becoming saturated. The emerging 802.11n specification promises to alleviate most of this strain once when it is finalized early next year, offering not only higher data rates but increased reliability.

Apple waiting on ratification

Although hardware that conforms to an 802.11n draft specification is already hitting the market in small doses, Apple is reported to be waiting until the specification is ratified by the IEEE standards-setting body before including it in shipping products.

The first requirement of the 802.11n draft specification, which is currently under review, is to support an OFDM (Orthogonal Frequency Division Multiplexing) implementation that improves upon the one employed by the 802.11a/g Wi-Fi products, using a higher maximum code rate and slightly wider bandwidth. Broadcom says this change will improve the highest attainable raw data rate to 65 Mbps from 54 Mbps in the existing standards.

MIMO to improve performance

One of the most widely known components of the 802.11n draft specification is called Multiple Input Multiple Output, or MIMO. This technology exploits a radio-wave phenomenon called multipath, where transmitted information bounces off walls, doors, and other objects, reaching the receiving antenna multiple times via different routes and at slightly different times.

An uncontrolled, multipath would distort the original wireless signal, making it more difficult to decipher and degrading Wi-Fi performance. The goal of MIMO is to harnesses multipath with a technique known as space-division multiplexing. Using this technique, the transmitting WLAN device actually splits a data stream into multiple parts, called "spatial streams," and transmits each spatial stream through separate antennas to corresponding antennas on the receiving end.

The current 802.11n draft provides for up to four spatial streams, even though compliant hardware is not required to support that many. Previous Wi-Fi specifications, including 802.11a/b/g, each allowed for only a single spatial stream. Doubling or quadrupling the number of spatial streams will have the effect of doubling or quadrupling the raw data rate.

Some of the trade-offs to this technique include increase power consumption and slightly higher costs. Therefore, the 802.11 draft specification calls for a MIMO power-save mode, which mitigates power consumption by using multiple paths only when communication would benefit from the additional performance.

The two features in the draft specification that focus on improving MIMO performance are called beam-forming and diversity. Beam-forming is a technique that focuses radio signals directly on the target antenna, thereby improving range and performance by limiting interference. Meanwhile, Diversity exploits multiple antennas by combining the outputs of, or selecting the best subset of, a larger number of antennas.

Although the draft specification supports up to four antennas, a notebook computer with two antennas may still connect to an access point with three antennas. In this case, only two spatial streams will be used. If the notebook were to include four antennas, and the access point also had that many, four spatial streams would be utilized to increase performance, according to the specification.

Improved throughput and higher data rates

Another optional mode of the 802.11n draft specification will reportedly double data rates by doubling the width of a WLAN communications channel from 20 MHz to 40 MHz. The primary trade-off to this technique is fewer channels available for other devices.

In the case of the 2.4-GHz RF band (802.11n calls for support of both 2.4GHz and 5GHz bands), there is only enough room for three non-overlapping 20-MHz channels, so a 40-MHz channel would not leave much room for other devices to join the network or transmit in the same airspace.

Therefore, the draft specification employs intelligent, dynamic management to ensure that the 40-MHz channel option improves overall WLAN performance by balancing the high-bandwidth demands of some clients with the needs of other clients to remain connected to the network.

Other possible enhancements

Other optional features of draft specification include high-throughput duplicate mode, which helps extend the network's range, and short guard interval, which improves efficiency by further limiting overhead.

With all the optional modes and back-off alternatives of the 802.11n draft specification, the array of possible combinations of features and corresponding data rates amounts to a whopping 576 possible data rate configurations. By comparison, 802.11g standard used in today's Apple AirPort products provides for 12 possible data rates, while 802.11a and 802.11b specify eight and four, respectively.

Coexisting with today’s Wi-Fi networks

Owners of 802.11a/b or g hardware need not fret — the draft 802.11n specification has been crafted with the previous standards in mind to ensure compatibility with more than 200 million Wi-Fi devices currently in use. It calls for 802.11n Wi-Fi products to communicate with 802.11a devices on the 5-GHz band as well as 802.11b and 802.11g hardware on the 2.4-GHz frequencies. In addition to basic interoperability between devices, 802.11n will provide for greater network efficiency in mixed mode over what 802.11g offers.

Because wireless environments are much more challenging to orchestrate than wired networks, there is generally more overhead to ensure that data sent is actually received, and that other clients leave the channel open during transmission. Broadcom says the presence of 802.11b nodes will make communications difficult on the 2.4-GHz band because the older standard does not recognize OFDM, which is employed by 802.11g and the draft specification for 802.11n.

This means that if OFDM clients want to communicate in the presence of 802.11b clients, they need to use the older standard’s communication protocol at least to protect their higher-rate OFDM transmissions. This drops network efficiency considerably because data packets take far less time to transmit with 802.11g and 802.11n than they do under the old 802.11b standard, according to Broadcom. Some WLAN chipset suppliers, including Broadcom, devised innovative schemes to improve the efficiency of mixed 802.11b/g networks. However, this issue is addressed directly in the draft 802.11n specification.

Broadcom believes one of the most important features in the draft-n specification aimed at improving mixed-mode performance is aggregation. Rather than sending a single data frame, the transmitting client bundles several frames together. Thus, aggregation improves efficiency by restoring the percentage of time that data is being transmitted over the network (see figure below).

It will be much easier for 802.11n devices to coexist with 802.11g and 802.11a hardware because they all use OFDM. Even so, Broadcom says there are features in the specification that increase efficiency in OFDM-only networks.

One such feature is Reduced Inter- Frame Spacing, or RIFS, which shortens the delay between transmissions. For the best possible performance, the the 802.11n draft specification provides for what is called "greenfield" mode, in which the network can be set to ignore all earlier standards.

It is not clear at this stage whether greenfield mode will be a mandatory or an optional feature in the final 802.11n draft, but it is likely to be an option, Broadcom said.

The company also believes that, realistically, battery-powered WLAN hardware will continue to be built around 802.11g and even 802.11b for some time. Despite the improved efficiency built into the draft 802.11n specification, it is difficult to eliminate all of the obstacles of 802.11b. This means that consumers looking for the best possible network performance may want to consider replacing 802.11b WLAN hardware on their networks.

What's driving demand for 802.11n?

According to Broadcom, some of the current and emerging applications that are driving the need for 802.11n are Voice over IP (VoIP), streaming video and music, gaming, and network attached storage.

The company says VoIP phones and online gaming can benefit from the increased range and reliability of an 802.11n access point. Similarly, it says hardware based on the emerging standard "may be better suited to streaming music" than older-generation WLAN hardware.

Another emerging application that demands all that 802.11n has to offer is Network-Attached Storage, or NAS. This technology has become popular in the enterprise as an inexpensive, easy-to-install alternative for data backup.

802.11n to aid Apple's digital media hub

In its white paper on 802.11n, Broadcom acknowledges NAS is taking hold in small offices and even some homes, with users wanting to safeguard their growing digital photo albums and other important data from hard-drive failure. And new, more exciting applications for NAS are emerging, such as video storage centers that demand reliable, high-bandwidth connections to stream prerecorded TV shows, music videos and full-length feature films to televisions and computers throughout the house.

With existing WLAN technologies, transferring large files such as prerecorded TV shows from a personal video recorder onto a notebook computer or portable media player takes planning and patience. The 802.11n standard promises to make these tasks simple and snappy.

At the best data transfer rate, copying a 30-minute video file on a 802.11b wireless network would take 42 minutes, while copying the same file using a dual antenna 802.11n network would take about 44 seconds.