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What you ought to know before deploying an 802.11n Wireless Network

admin — Tue, 28 Dec 2010 08:37:43 +0000

We all know that the IEEE 802.11n standard for Wi-Fi supports up to 600 Mbps for creating a wireless network. But are all 802.11n compliant devices same? Is the 600 Mbps throughput feasible as of today? What other network/ device considerations do we need to account for, to achieve maximum 802.11n throughput levels? These are some of the questions that will be answered in this article.

Gone are the good old days of the 802.11a/b/g wireless networks where the throughput levels were simple and consistent for the entire standard. All the 802.11b compliant devices (clients and access points) could achieve 11 Mbps throughput (theoretically). All the 802.11a/g compliant devices could achieve 54 Mbps throughput (theoretically). This can be compared to the black and white era, where there were only two shades – black or white!

But in the coloured world of 802.11n, there are various options and configurations possible. For example, each 802.11n compliant device (access point or client adapter) could have unique T x R : S configurations. In addition to the 1×1, 2×2, 3×3, 4×4 configurations and throughput levels shown in the above diagram, there could even be configurations like 1×2, 2×3, 3×2 etc. Each transmit and receive antenna comes with its own radio. Each 802.11n compliant device can support a certain number of spatial streams (transmitted data streams, that have different amplitude and phase, each of which can be independently received at the receiver end). 802.11n compliant devices can work at 20 Mhz or 40 Mhz (bonded channels) and each gives a different throughput. Some of the 802.11n compliant devices work in both 2.4Ghz band as well as 5 Ghz band, while some of them work in either of the two.

So, next time you want to buy a laptop or access point that supports the latest wireless standard, don’t just ask for 802.11n compliance! Below are some of the common configurations of laptops/ home access points/ business access points that support 802.11n.

Common configurations for 802.11n compliant Laptop / Desktop wireless clients (adapters):

1×2 MIMO – Works in 2.4 or 5 Ghz (single band) – Supports up to 150/300 Mbps.

2×2 MIMO – Works in 2.4 and 5 Ghz (dual band) – Supports up to 300 Mbps.

3×3 MIMO – Works in 2.4 and 5 Ghz (dual band) – Supports up to 450 Mbps.

Common 802.11n compliant Access Point configurations:

Business: 2×2 MIMO (single band) – Supports 2.4 or 5 Ghz; 2×2 MIMO (dual band) – Supports 2.4 Ghz and 5 Ghz; 3×3 MIMO (dual band) – Supports 2.4 Ghz and 5 Ghz.

Home: There are no common configurations in this segment and the MIMO diversity could range from 1×1 to 3×3 configurations.

Note: 4×4 configurations (that support 600 Mbps, in theory) are not yet available, meaning they may not provide a significant performance improvement over 3×3 (if deployed currently, at the time of writing this article) and hence are not very popular. But very soon, they are expected to become available; 1×1 is not actually a MIMO but SISO (Single Input / Single Output) configuration and some of them support bandwidths in the range of 65 Mbps, which is only a slight improvement over the current 802.11a/g standards that support 54 Mbps.

Still, all the above devices are referred to as Wireless ‘n’ compliant devices! But cost varies with each product, and perhaps its good to have options!

Some suggestions for achieving highest possible throughput/bandwidth with 802.11n wireless implementations:

The wired (backbone) network to which each of these 802.11n access points are connected should be end to end Gigabit (1GE) network (Switch ports, cables, patch panels, patch cords, fiber transceivers, etc).
Access Points and client wireless adapters should support at least two spatial streams (three might be even better).
If access points are going to be used in 2.4 Ghz band, and channel bonding is applied (recommended for 802.11n performance), out of the three non overlapping channels (20 Mhz each), two would be bonded (to achieve a 40 Mhz channel for use in 802.11n networks) and only one would be left out for legacy 802.11b/g clients. Any network today would definitely have some legacy wireless b/g devices and hence those devices would be prone to higher interference – hence poor connectivity.
Hence it is suggested that, for effective 802.11n performance, it better for access points and clients to operate in 5 Ghz spectrum which has many more non over lapping channels. In fact using 40 Mhz channels in 5 Ghz would provide the best performance for 802.11n wireless networks.
Its even better if both wireless clients (laptops, etc) and wireless access points support both 2.4 Ghz and 5 Ghz spectrum. The legacy clients could operate in the 2.4 Ghz spectrum (most of todays legacy clients are 802.11g and hence operate in 2.4 Ghz) and the newer 802.11n clients could operate in the 5 Ghz spectrum (preferably).
By the way, 5 Ghz spectrum is cleaner and does not suffer from interference with blue-tooth devices, Microwave Owens, etc.
In dense wireless access point deployments in the 2.4 Ghz spectrum (for the legacy clients), channel bonding could be disabled as otherwise there would be a lot of interference from neighboring access points operating in the same channel.
The POE (Power Over Ethernet) standard – 802.3af supplies around 12.95 Watts or slightly higher per port (POE Switches or Power over Ethernet Injectors). This may not be sufficient for 802.11n access points. If the 802.11n access points are still used with them, some of the additional radios might be turned off and hence there would be lower throughputs. Either the POE Plus (802.3at) standard switches/ POE injectors could be used (or) multiple cables/shorter cables could be run to the 802.11n compliant access point from existing POE switches. But some vendors support 802.11n operation with 802.3af or equivalent power supplies. Hence its better to check the vendor specifications on this.
The RF Survey tool (used to predict the number of access points required in a wireless network, before implementation) should be specifically designed for 802.11n and should take in to consideration all the interfering objects on the way (as the reflections from these objects are critical for multiple path propagation) in order to come up with the right network sizing.
Though a network can be designed exclusively for 802.11n, there will be a number of legacy devices that operate in 802.11a/b/g, which needs to be taken in to account during the design stage itself. Probably, the 802.11n access points could co-exist with legacy access points, in certain cases.
In the 5 Ghz spectrum, wireless signals travel lesser distances than the 2.4 Ghz spectrum. Also, the signal strength for any client near the access point is always high and it decreases with client’s increasing distance from the access point.
It may be better to dis-allow the legacy 802.11b clients in the 802.11n network as they might slow down the performance of all the clients considerably. Generally, the more the number of legacy devices(802.11b,g) lesser is the performance of 802.11n network.
Integrated client adapters for 802.11n based clients (laptops, etc) perform better than the external adapters (USB based, etc).
Since encryption is a tedious process that can slow down the performance, its better if a 802.11n client can perform encryption at chip level than at the software level.
Similar type of network adapters for all the clients (as far as possible), increases the performance of 802.11n networks.
Some client adapters (and even access points) prefer to connect in the 2.4 Ghz spectrum by default, even though 5 Ghz spectrum might be available. In such cases, it might be better to disable the 2.4 Ghz spectrum operations in the client adapters so that they operate in 5 Ghz spectrum only.

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What is IEEE 802.11n, what are the advantages and challenges for 802.11n in Wi-Fi networks

admin — Thu, 11 Jun 2009 09:11:04 +0000

This article explains the IEEE 802.11n standard for wireless (Wi-Fi) networks, what is the bandwidth supported by this standard, what is the distance covered by access points employing 802.11n, the advantages of 802.11n, what are the challenges faced by this standard, what steps must someone ensure before upgrading to 802.11n and if it is backward compatible with other earlier standards.

What is IEEE 802.11n in Wi-Fi networks?

This is the new standard (protocol) which would be supported fully by IEEE very soon (Meaning currently there is only a ‘Draft n’ standard which can be upgraded to full n, once it is ratified) to give a higher bandwidth per access point (About 300 Mbps currently), to support a greater distance than the earlier standards like 802.11 a,b,g and hence allow us to get an enhanced network performance in wireless.

This is possible by a technology employed by 802.11n access points called MIMO. MIMO stands for Multiple In and Multiple Out which means that there are multiple radios inside the AP, all of which work together to maximise the throughput. The main features of MIMO in 802.11n are:
¤ There are three antennas and two spatial streams.
¤ Provides optional 40 Mhz channel which provides twice the data rates of a 20 Mhz channel.
¤ Uses Orthogonal Frequency Division Multiplexing (OFDM).
¤ It supports MAC aggregation: Packs small packets in to a larger frame. This reduces the number of frames and hence the time lost to contention for the medium. So, the throughput is increased.
¤ It supports Block acknowledgement: One acknowledgement signal for many frames (as opposed to each frame) and hence making it better for real time applications like video, voice etc.

Update: Click here to read about the different versions of MIMO (Like 1×1,2×2,3×3 etc) and also the steps you need to take before upgrading to IEEE 802.11n wireless network.

Advantages of IEEE 802.11n in Wi-Fi networks:

¤ It provides increased bandwidth per access point : Currently, the draft-n specification supports up to 300 Mbps per access point (This is in ideal conditions, and the actual bandwidth may vary between 100 to 200 Mbps). Still, it is a huge jump from the 54 Mbps (26 Mbps, practically) supported by the nearest 802.11 a, g standards. This feature alone might become the game changer in the industry – especially given the fact that they would be supporting 600 Mbps in the very near future when the n standard is ratified fully. Wireless networks could become the primary networks for the edge – then. Of course, the bandwidth per access point is shared by approximately 15-20 computers, but still there is enough for everyone, considering the current requirements/applications of networks.

¤ The Access Points that support 802.11n standard covers roughly 1.5 times more distance than the earlier standard – 802.11g , per Access Point (802.11g gave about 30 meters indoor coverage and 100 meters outdoor coverage in the open. But you don’t necessarily keep one access point every 30 meters as the positioning depends on the number of concurrent connections to the access point, assured bandwidth per user, site conditions etc.).

¤ This standard works better with NLOS circumstances as multiple antennas can collect information from reflections from surfaces better. So, not only they support more distances, they also support a more uniform bandwidth over such distances.

¤ 802.11n is totally backward compatible. It can support 802.11a, b and g standards for connecting the legacy clients too. And it can work both on 2.4 Ghz and 5 Ghz radios. The specifications for this goes something like: 802.11 a/n and 802.11b/g/n. Dual radio is supported.

¤ Many new laptops come with built in adaptors for 802.11n (draft n) and it can also be added via an interface like USB etc.

¤ The draft-n specifications is expected to be fully compatible with the final n and most devices might just need a software upgrade to support the final version, with some more advancements. The draft-n is being promoted by the Wi-Fi alliance, to ensure interoperability for different vendors.

¤ Most of the features of the earlier standards like Radio Management, Wireless Multi Media, WPA2 encryption, Intrusion Detection etc. would work with 802.11n too. Some are supposed to be better.

Challenges facing 802.11n Wi-Fi deployments:

¤ The cost of each 802.11n access point is two to three times that of b/g access points. This is however expected to come down soon.

¤ While the operation of 802.11n in 5 Ghz is good due to the availability of more non-overlapping channels, it remains a challenge on 2.4 Ghz as this has only three non-overlapping channels and would be highly used by the other radios and neighboring access points. If the signature feature of 40 Mhz spectrum is used here, there would be overlapping.

¤ Power for the 802.11n AP’s don’t follow the earlier 802.3af, which is the standard for POE. While newer POE injectors are available to support the minimum 17 Watts (for three radios and two spatial streams), all the switches (with POE) and power injectors may not support this. Some vendors also offer to operate their access points in the earlier standard by switching off one of the radios, for a slightly degraded performance instead of totally powering off the AP’s in that case.

¤ If the wired back-end networking doesn’t support 10/100/1000 Mbps (perhaps only supporting fast ethernet – 10/100), then all the components, including cables (cat 6 cables are required for 1GE), patch panels, fiber interfaces, edge/core switches need to be 1GE enabled to realize the enhanced performance of 802.11n as every access point can easily utilize more than 100 Mbps.

¤ The 802.11n adaptors are available only in the clients shipped recently. Most of the earlier ones don’t support. That means they should also be upgraded. It is better to upgrade the earlier ones as the presence of more 802.11 b/g/a adaptors in the network brings down the performance of 802.11 n clients, as they reduce the effective capacity of the cell.

¤ The network planning for 802.11n access points is different from the network planning for earlier standards. The biggest challenge is to accommodate the earlier access points in a few ‘non-critical’ places in the network or discard them completely. A lot of companies have made huge investments in the older access points, which most of the time is sufficient for their applications.

¤ Controller should be able to support the higher bandwidth provided by 802.11n access points. But how ever it is vendor and model specific. So, for some installations, new controllers might be required too.

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