ASCII.jp What is the new technology that supports high-speed wireless LAN "IEEE802.11n"?

IEEE802.11n is probably the hottest wireless LAN topic in the next couple of years. It is a new wireless LAN standard whose standardization work was completed in September 2009, two years after the draft 2.0 was approved in 2007. What is the difference from previous wireless LAN standards, and why is it so fast? Let's hold down the technology used in 11n.

Significance of 11n

Since IEEE established 802.11 in 1997, 11b and 11a were established in 1999, and 11g was established in 2003. Along with this, the maximum data transmission speed has also increased from 2 Mbps to 54 Mbps. As complementary standards, 11i with enhanced security functions and 11e with enhanced QoS were also formulated.

The extension from IEEE802.11 to 11g was limited to the physical layer as far as the communication mechanism itself was concerned. In other words, with the exception of 11i and 11e, the MAC (Media Access Control) layer and above have remained unchanged since the formulation of IEEE802.11, and have been used to this day.

The MAC layer is part of the data link layer in the OSI reference model. The role of the MAC layer is to communicate with the physical layer. Ethernet is the LAN standard used in the physical layer and data link layer.

ASCII.jp High-speed wireless LAN

Ethernet was set at 10Mbps in 1983, and since then it has grown from 100Mbps to 1Gbps. Compared to this, wireless LAN is 54 Mbps at 11a/g. Moreover, the effective speed remains at about half of that, 20-30 Mbps, which is a big difference from the speed of Ethernet. I explained in the previous part that the reason for this lies in the mechanism of CSMA/CA.

"IEEE802.11n" breaks through this situation and specifically assumes an execution speed target of 100Mbps or more. In order to achieve the maximum standardized speed of 600 Mbps, it was essential to expand the MAC layer. Understanding why the MAC layer needs to be extended in 11n will deepen your understanding of wireless LAN.

11n high-speed technology

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Key words such as MIMO and channel bonding are attracting attention as technologies to realize 11n. First, let's explain the technology used in the physical layer.

Adoption of MIMO

MIMO (Multiple Input Multiple Output) is a technology that prepares multiple combinations of transmitters (wireless modems) and antennas, parallelizes data and transmits it simultaneously ( Figure 1). With two pairs of transmitters and receivers, two sets of data (data streams) can be sent simultaneously, doubling the data communication speed. This is sometimes called double multiplexing. The 11n standard allows up to 4 pairs of transceivers, that is, 4 multiplexing. Therefore, the maximum data transmission speed is four times higher than before.

Fig.1 Mechanism of MIMO. Multiplex transmission and reception with multiple antennas

Another advantage of MIMO is the ability to use multipath (multiple wave transmission lines). In conventional wireless LANs, reflected waves degrade received signals, leading to a decrease in range and data transfer speed. In MIMO, on the other hand, multiple antennas (receivers) receive multiple data streams simultaneously, so it is advantageous if each data stream has a delay (time difference) due to reflection. Therefore, it can be used even in places where there is no line of sight or where communication was unstable due to reflected waves. In addition, it is said that the transmission distance is about twice that of the conventional 11a/g.

MIMO looks like a good thing, but it has its downsides. Operating multiple transmitters and receivers at the same time consumes a lot of power. In the case of access points, there are cases in which current PoE cannot be used because even with two multiplexing, the power supply capacity of Ethernet cables (PoE) is exceeded. Therefore, products with 3-multiplexing and 4-multiplexing will increase power consumption. It's more serious for battery-powered wireless clients, and we don't expect MIMO to be implemented in all products.

(Continued on next page, "Channel bonding to double speed")

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