Articles: Networking

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The performance growth provided by the 40MHz channel is illustrated by the diagram that shows performance depending on the signal-to-noise ratio.

The resulting picture of performance – a wide channel plus MIMO – is shown in the next diagram:

It shows the bandwidth growth dynamics for different “Number of Channels x Number of Data Streams – Channel Frequency Bandwidth” configurations. The 2x2-40 configuration is superior to the others. Comparing the 2x2-40 and 4x4-20 configurations, you can see that the use of a 40MHz channel is optimal not only in performance but also in the cost of implementation because a system with two antennas is going to be much cheaper than a system with four antennas. The 2x2-40 configuration is taken as the basis for building wireless communication systems in the draft version of the standard while the maximum number of streams is limited to four as yet. However, the final version will describe some means to increase performance of an 802.11n device in case of emergency. Currently, most manufacturers that produce draft 802.11n-compliant devices install three antennas on them.

Besides the changes on the physical level, the MAC level was revised in the 802.11n standard as well. This level is important for reliability and performance of network equipment. As we wrote above, the declared bandwidth used to be measured for the physical level, but the effective speed of data transfers between network devices was much lower than specified due to auxiliary information, like the headers of physical-level packets, which was useless for the information user. The speed was reduced even more by various latencies that resulted from signal reflections, etc. The most annoying thing was that the speed reduction didn’t depend much on the overall connection speed. The most obvious way to increase the effective bandwidth is to minimize the overhead on the MAC level. The main approach to increasing the efficiency of communication in the future standard is the use of so-called aggregate exchange sequences . It means combining several MAC Protocol Data Units (MPDUs) into a single PHY Protocol Data Unit (PPDU). Aggregate exchange sequences allow to increase the effective bandwidth to the level of the theoretical one. Without them, a physical speed of 500Mbps would be required to reach a speed of 100Mbps on the MAC level.

Another method to make communication more effective is the new MAC-level data transfer mechanisms that allow transferring data in both directions without the need to initiate transmission. They minimize the time it takes to reverse transmission between the initiator and the responder. Besides that, the new standard is expected to introduce a new MPDU format that will allow sending physical-level packets to several recipients at once. Finally, error-correction algorithms have been improved to increase the coverage and Quality-of-Service mechanisms – this should have a positive effect on such services as high-quality streaming video and Voice over IP.

The problem of backward compatibility with the older Wi-Fi standards has been taken care of, too. First, the 5GHz range, necessary for 802.11a, has become a standard feature, instead of optional, in the new draft version of the standard. Second, the mechanism of operation of devices with 40MHz and 20MHz channels has been clearly defined. If a 20MHz device connects to an 802.11n access point, data transfers between them will utilize one of the two channels that make up a wide 40MHz channel. Third, the new standard will surely inherit all the modulation methods employed earlier in 802.11a/b/g.

Compatibility of different 802.11n devices between each other is important, too, but it’s all right here. Chipsets from Broadcom and Atheros are quite friendly to each other. Other manufacturers’ chipsets have a high degree of compatibility, too, which is determined by the connection speed only. Serious problems can only occur on the level of drivers and firmware, which is not critical.

Now that we’ve started talking about available chipsets, we should name a couple of them here.

As soon as the next day after the ratification of the first draft version of the 802.11n standard Broadcom announced that it was ready to begin shipments of its new Intensi-fi chipset. This chipset combines three pieces: a BCM2055 radio module, a BCM4321 MAC-controller, and a BCM4705 processor. The processor is not directly related to 802.11n. For the other two chips we can give you a general specification taken from the manufacturer’s product brief:

Click to enlarge

The specification suggests that the chips are universal in terms of connection interface and antenna configuration.

Atheros, another world leader in manufacturing wireless communication chipsets, was catching up with Broadcom by releasing the xspaN AR5008. Unlike Broadcom’s, this chipset may contain a lot of different chips because each interface and antenna configuration is supported by a separate chip. Thus, the following versions of the chipset are available: AR5008-2NG, AR5008-2NX, AR5008-3NG, AR5008-3NX, AR5008E-2NG, AR5008E-2NX, AR5008E-3NG, and AR5008E-3NX. These versions are combinations of a total of six chips. The letter E in the first part of the marking denotes the AR5418 MAC-controller that supports a miniPCI Express interface. Otherwise, the AR5416 with a miniPCI/CardBus interface is employed. The digits 2 and 3 in the second part of the marking stand for an RF module with two (AR2122 and AR5122) or three (AR2133 and AR5133) antennas, respectively. The letter G at the end means that the RF module works in the 2.4GHz range only (AR2122 and AR2133) whereas the letter X means both 2.4GHz and 5GHz ranges (AR5122 and AR5133). We won’t publish the specification of the xspaN chipset because its versions are very similar and the parameters of one of them will be reflected in the specification of the WRT300N router.

Marvell has released its TopDog chipset, too, but there’s little information about it yet. It consists of an 88W8360 MAC-controller and an 88W8060 RF module that supports both 2.5GHz and 5GHz ranges.

The AGN400 chipset that is expected to arrive in the first quarter of 2007 has been announced by Qualcomm that has recently acquired the assets of Airgo Networks Inc. This chipset consists of an AGN403BB MAC-controller and an AGN403RF RF module and features True MIMO Gen-N technology. Here’s its specification taken from the manufacturer’s product brief:

PCIe 1.1, CardBus (PC Card 7.1), MiniPCI 1.0 , PCI 2.2, USB 2.0,

Frequency Band 2.400 - 2.485 GHz
4.920 - 5.825 GHz
Network Standard IEEE 802.11n, IEEE 802.11a, IEEE 802.11b, IEEE 802.11g
Modulation Techniques
Orthogonal Frequency Division Multiplexing: BPSK, QPSK, 16 and 64 QAM
Direct Sequence Spread Spectrum: DBPSK, DQPSK, CCK
Data Rates
802.11b: 1 - 11 Mbps
802.11a: 6 - 54 Mbps
802.11g: 1 - 54 Mbps
802.11n: 24 -144 Mbps (20 MHz channel)
48 - 315 Mbps (40 MHz channel)

AGN403BB ™ Single-chip CMOS integrated MAC and baseband
AGN403RF ™ Single-chip SiGe 2.4/5 GHz transceiver

Encryption / Decryption
• 64 bit WEP
• 128 bit WEP

802.11e, WMM & WMM-SA

And finally we can’t but acknowledge Intel’s having played an active part in promoting the new standard, too. The company has released a miniPCI Express adapter Next-Gen 4965AGN which will be installed into Intel Centrino Duo notebooks.

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