Category: Wifi

Gone are the days when standards were pure and simple (well, probably never simple, but at least pure…). Today, it seems they are cluttered with options of which most are probably never going to be implemented. The Wireless LAN 802.11 standard seems to be no exception. Let me make two examples:

Packet Transmission:

• Default: This is the good old "backoff period – send – ack" mechanism. Easy, works well but performance is not that great.

• Frame Bursting: Packets are sent in the following manner: "packet – ack – packet – ack – packet – ack". Still easy, was  implemented as a proprietary enhancement in many 802.11g products and has been sort of legalized with 802.11e (WMM).

• Block Acknowledgments: An addition to frame bursting which allows transmissions without ack’s. A whole set of frames are then acknowledged once they are all sent. To make things just a bit more complicated there’s immediate ACK and delayed ACK (which seems to have been defined for devices which can’t tell right away if all went fine).

• Aggregation: And on top, 802.11n has now specified that several MAC frames can be put into a physical frame which can now have a size of up to 64kByte. Looks like this is mandatory so all 802.11 devices should support this.

The statistics on this one are not so bad. Even low end 802.11n devices should support the default method, frame bursting and aggregation. Haven’t seen block ack’s implemented in the devices that have come by me, however.

Power Saving:

I can see at least four possibilities here:

• Standard Power Save (PS): This has been in the standards since the beginning. Devices tell the AP that they are going to sleep and the access point buffers incoming packets. When devices wake up and see that the access point has packets waiting for them they poll for each buffered frame.

• U-APSD: Unscheduled Automated Power-Save Delivery: Introduced by 802.11e, optional in the WMM (Wireless Multimedia) specification. Similar to PS above but once a device sends a trigger frame, the access point forwards all frames of in the buffer that fit into the service period during which the device is active. Once the service period is over, the device automatically goes back to sleep.

• S-APSD: Scheduled Automated Power-Save Delivery: No trigger frames. Instead, a schedule is agreed between the access point and wireless devices. The devices then wake up at predefined instants and packets are delivered automatically. This one is not included in the WMM specification, so this one probably has no chance of seeing the light of day.

• PSMP: Power Save Multi Poll. Yet another power save scheme which was lately introduced with the 802.11n High Throughput specification. This one schedules uplink and downlink transmissions of end user devices. Outside the scheduled times, devices can enter sleep mode. It looks like this power save mode has been designed for devices and applications that have constant data streams with a static bandwidth requirement (e.g. VoIP, video streaming etc.). Nice but also optional.

Statistics on this one are bad. I haven’t seen an access point yet that supports more than the classic PS mode. Has anyone seen more than this implemented yet?

In the past, things were pretty much clear when it came to Wireless LAN performance. If the box in the shop said it’s an 802.11g device, users could pretty much assume the device would do 54 MBit/s on the physical layer and application layer speed would be around 20 MBit/s. Things are much less clear with the new Draft 802.11n standard, which contains a myriad of options a device may or may not implement.

The standard for example contains three different flavors of MIMO. The most popular one, MIMO spatial multiplexing will be implemented in many devices. But the standard gives devices the option to use 2, 3 or 4 transceivers/antennas. The more receivers, the higher the speed, if of course the receiver has at least as many. O.k. one might be able to sell this story in a fashion like "We do 4×4 MIMO compared to the competition which only does 2×2, so we are twice as fast".

The story doesn’t end there, however. There are two other MIMO modes, namely MIMO beamforming and MIMO STBC (Space Time Block Code) which can significantly enhance range and link stability. It just might turn out that these MIMO modes are just as important for applications such as video streaming to devices that are not close to the Wifi Access Point. Ruckus wireless for example is doing interesting things in this area.

And it certainly doesn’t end here. Draft 802.11n contains further options like multimedia power save (PSMP), shorter OFDM guard intervals, Antenna selection, Maximum Ratio combining, Modulation and Coding Scheme (MCS) feedback, etc. etc. In the end, marketing words on boxes in the electronics store are cheap. Let’s see, how about the Wifi Alliance coming up with something standardized about "enhanced 11n options"? Time will tell.

Standards are well and good but usually contain a zillion options nobody ever implements. The 802.11 ‘draft-n’ standard is no exception. To find out which options different vendors have actually implemented, the best thing to do is to trace the beacon frames of ‘draft-n’ access points. A couple of months ago I described how to trace WLAN frames here.

If you are looking for beacon frames traced by other people to compare functionalities with your own ‘draft-n’ access point at home, take a look here. The beacon frame traced by ‘swordfish’ seems to be from a Linksys access point, judging from the first three bytes of the MAC address which identifies the manufacturer of the device. So here are the main ‘draft-n’ features this access point supports taken from the ‘HT Capability‘ (HT = high throughput) parameter (element ID 45):

• 20 MHz and 40 MHz channel operation support
• Greenfield mode support, i.e. protection mechanisms used to allow 802.11b and 11g devices to be part of the network can be switched off.
• the access point has two independent transmitters and supports 2 spatial MIMO streams.
• All of the other gazillion options such as short guard interval, STBC diversity, beamforming, MIMO power save, advanced coding, MCS feedback, antenna selection, etc. etc. are not supported.

The current mode of operation according to the ‘HT Information‘ parameter (element ID 61) erroneously called ‘Additional HT Capability’ in the trace):

• The access point currently operates in 20 MHz mode only (either set by user or due to other networks using the same channel)
• The access point runs in greenfield mode, i.e. only 11n devices have joined the network

Also interesting to see that the size of beacon frames has dramatically increased. Current 11g access points send beacon frames with a length of around 110 bytes. This ‘draft-n’ beacon frame has a length of 228 bytes!

Just saw an article in a German computer magazine testing a number of different pre-802.11n access points and related client adapters. Performance was around 50 MBit/s which is not a lot considering the test here and here showed performance in the range of about 100 MBit/s on the application layer.

What also struck me was that all the top 3 access points were 2.4 GHz only products. So looks like except for the Apple Airport Extreme we have to wait for a while before other 5 GHz pre-802.11n products make it to the market!?

The top three candidates where:

• the TEW-631BRP
• the SMCWBR14-N2
• the Netgear WNR834B

If anyone knows a pre-802.11n Access Point for 5 GHz except for Apple’s access point, please let me know!

I am having a good time these days browsing through the current draft D2.00 of the 802.11n standard to find out about the details of the compromise reached in the IEEE working group for the new 100+ MBit/s Wifi standard. Besides MIMO, one of the corner stones of reaching speeds beyond 100 MBit/s on the application layer is to combine two standard 20 MHz channels and transmit on them simultaneously.

This is pretty difficult to impossible in the 2.4 GHz band which only has space for 3 independent 20 MHz networks or a single 802.11n 40 MHz network together with one 20 MHz legacy network (for details see here). In my Paris flat, for example, there are already 13 networks operating in this band, many using the same channels.

In such an environment, a ‘draft n’ compliant access point has no chance to use a 40 MHz channel as according to chapter 9.20.4 of the draft standard, an access point detecting frames of another network on it’s primary or secondary 20 MHz channel has to immediately deactivate the 40 MHz channel mode. Further, it has to remain in 20 MHz channel mode for at least 30 minutes after the last frame from a different network has been detected.

I guess the standard allows the access point to switch to another channel to avoid the detected network but in the 2.4 GHz ISM band there is only one alternative. So I wonder if some vendors have put an option into their settings that allows locking the access point to a 40 MHz channel!? Not that this would be very polite, or not cause any problems to other networks and one’s own if traffic of other networks is higher than an occasional traffic burst.

So if you have a ‘draft n’ network at home, what kind of access point do you have and does it allow locking operation to 40 MHz?

As a follow up to my recent entry on the growing number of Wifi networks in the apartment building I live in Paris I did a bit of research of how much bandwidth there is available in the 5 GHz range compared to the 2.4 GHz ISM band 99% of today’s Wifi networks use.

Before taking a closer look it should be noted that both the 2.4 GHz and the 5 GHz band for Wifi are controlled by national regulators. Thus values such as bandwidth and transmission power are country dependent. For this blog entry I’ve chosen to work with the values applicable for Germany.

Let’s look at the 2.4 GHz ISM band first: It ranges from roughly 2.4 GHz to 2.483 GHz. An 802.11b or 802.11g channel requires around 22 MHz of bandwidth which means that there can be at most 3 non overlapping Wifi networks in the 2.4 GHz range. In my Paris example, however, 13 networks share this frequency range. Things work o.k. as long as the individual networks don’t carry a lot of traffic as packets are always marked for which access point or client device they are intended. Packets sent on one network are received on devices of other networks using the same channel, too, but are simply ignored.

Nevertheless, it’s obvious that the performance of individual networks on the same channel won’t be great once more than one carries video streaming and other bandwidth hungry applications. With the new 802.11n standard, things become even worse. To reach ever higher transmission speeds it’s possible to double the channel bandwidth compared to current 11b or 11g networks. In the 2.4 GHz ISM band this means that it’s not even possible to squeeze in two such networks in a non overlapping fashion.

The only way out of this is to put some of the traffic into the 5 GHz band. Compared to the 70-80 MHz available in the 2.4 GHz band, there’s 455 MHz available for unlicensed wireless networks in Germany (see German RegTP info PDF here). The band spans the frequency ranges of 5,15 to 5,35 GHz (200 MHz) and from 5,47 to 5,725 GHz (255 MHz). Consequently, around 18 single channel 802.11n Wifi networks can co-exist in this space or around 9 that use a double channel. The standard and regulatory requirements also foresee that the networks dynamically select an appropriate channel based on interference encountered. This is required to prevent Wifi networks from interfering with other applications such as military radar. It also has the nice benefit of removing the necessity for users to select a channel.

Disadvantages of Using the 5 GHz band

Unfortunately, there are also two disadvantages in using the 5 GHz band. The higher the frequency the shorter the range with a certain power level. In the 5 GHz band a Wifi device must therefore transmit with a higher power level to cover the same distance compared to the 2.4 GHz band. The power level, however, is restricted: In the 5250 to 5350 MHz band (4 channels) power output is limited to 10 mW / MHz, which translates into around 200 mW per Wifi channel. Between 5.470 – 5.725 GHz (about 11 channels) power output is limited to 800 mW. Since most 2.4 GHz Wifi equipment today transmits at less than 50 mW, this is probably not going to be a big problem.

The second disadvantage of using the 5 GHz band is price. Devices supporting the band must also support the 2.4 GHz band for backwards compatability. Access points should even support both bands simultaneously to serve both legacy and new high speed devices. So the question is how much more 5 GHz power amplifiers will cost compared to 2.4 GHz amplifiers and if combined 2.4/5 GHz chips become available soon. Apple’s airport is one of the first mass market access points that makes use of both the 2.4  and 5 GHz bands. Current retail price is $179.- I’d say that looks pretty promising already as prices will surely go down over time.