Intel’s Primary Wireless Campus

Intel published a white paper last year about a trial deployment of 802.11a as a replacement for wired Ethernet at a 5,000 person campus. The results were lower costs and happier workers. This was just for PC connectivity. The dual-mode phone phase of the deployment is still to come.

There are several interesting findings in the white paper. First, while the latency of the network increased somewhat, the difference was imperceptible to the users. Second, Intel chose to abandon the VPN, relying on 802.11i for security. This made joining the network faster and easier.

The decision to use 802.11a was presumably for the greater capacity (more non-interfering channels than 11g), and for the cleaner spectrum. 802.11n is superior to 802.11a in capacity and rate at range. This means that what was doable with 11a will be even easier with 11n.

Reliable VoIP

QoS metrics are important, and several companies have products that measure packet loss, jitter, latency and so on. But you can have perfect QoS, and your VoIP system can still be defective for all sorts of reasons.

I spoke with Gurmeet Lamba, VP of Engineering, at Clarus Systems at the Internet Telephony Expo this week. He said that even if a VoIP system is perfectly configured on installation, it can decay over time to the point of unusability. Routers go down and are brought up again with minor misconfigurations; moves, adds and changes accumulate bad settings and policy violations.

VoIP systems are rarely configured perfectly even on installation. For example, IP phones have built-in switches so you can plug your PC into your desk phone. Those ports are unlocked by default. But some phones are installed in public areas like lobbies. It’s easy for installers to forget to lock those ports, so anybody sitting in the lobby can plug their laptop into the LAN. There are numerous common errors of this kind. Clarus has an interesting product that actively and passively tests for them; it monitors policy compliance and triggers alarms on policy violations.

Clarus uses CTI to do active testing of your VoIP system, looking for badly configured devices and network bottlenecks. Currently it works only on Cisco voice networks, but Clarus plans to support other manufacturers.

Clarus started out focusing on automated testing of latency, jitter and packet loss for IP phone systems. It went on to add help desk support with remote control of handsets, and the ability to roll back phone settings to known good configurations.

The next step was to add “Business Information,” certifying deployment configurations, and helping to manage ongoing operations with change management and vulnerability reports. Clarus’ most recent announcement added passive monitoring based on a policy-based rules engine.

Clarus claims to have tested over 350 thousand endpoints to date. It has partners that offer network monitoring services.

T-Mobile UMA service details

I called the T-Mobile customer service line and received a clarification about the HotSpot@Home service. The charge for this service is just for unlimited calling at home and at T-Mobile hotspots. If you don’t mind using your regular service minutes in these situations, there is no need to subscribe to the @Home service – you can still use the Wi-Fi connection for better reception. So since I never seem to use more than half my minutes, I cancelled the @Home service. The phone still uses Wi-Fi when it can, so the customer service agent appears to be correct.

This makes me a lot happier with T-Mobile. When the Wi-Fi is being used to offload their network and provide better coverage, they don’t charge for it. This is as it should be. If the offload/coverage effect turns out to be a significant benefit for T-Mobile, and as the price of Wi-Fi in handsets comes down, it is conceivable that T-Mobile will find it worthwhile to add Wi-Fi to all their phones.

I remain curious about why my Nokia 6086 can’t use the Wi-Fi for web browsing.

How does 802.11n get to 600Mbps?

802.11n incorporates all earlier amendments to 802.11, including the MAC enhancements in 802.11e for QoS and power savings.

The design goal of the 802.11n amendment is “HT” for High Throughput. The throughput it claims is high indeed: up to 600 Mbps in raw bit-rate. Let’s start with the maximum throughput of 802.11g (54 Mbps), and see what techniques 802.11n applies to boost it to 600 Mbps:

1. More subcarriers: 802.11g has 48 OFDM data subcarriers. 802.11n increases this number to 52, thereby boosting throughput from 54Mbps to 58.5 Mbps.

2. FEC: 802.11g has a maximum FEC (Forward Error Correction) coding rate of 3/4. 802.11n squeezes some redundancy out of this with a 5/6 coding rate, boosting the link rate from 58.5 Mbps to 65 Mbps.

3. Guard Interval: 802.11a has Guard Interval between transmissions of 800ns. 802.11n has an option to reduce this to 400ns, which boosts the throughput from 65 Mbps to 72.2 Mbps.

4. MIMO: thanks to the magical effect of spatial multiplexing, provided there are sufficient multi-path reflections, the throughput of a system goes up linearly with each extra antenna at both ends. Two antennas at each end double the throughput, three antennas at each end triple it, and four quadruple it. The maximum number of antennas in the receive and transmit arrays specified by 802.11n is four. This allows four simultaneous 72.2 Mbps streams, yielding a total throughput of 288.9 Mbps.

5. 40 MHz channels: all previous versions of 802.11 have a channel bandwidth of 20MHz. 802.11n has an optional mode (controversial and not usable in many circumstances) where the channel bandwidth is 40 MHz. While the channel bandwidth is doubled, the number of data subcarriers is slightly more than doubled, going from 52 to 108. This yields a total channel throughput of 150 Mbps. So again combining four channels with MIMO, we get 600 Mbps.

Lower MAC overhead
But raw throughput is not a very informative number.

The 11a/g link rate is 54 Mbps, but the higher layer throughput is only 26 Mbps; the MAC overhead is over 50%! In 11n when the link rate is 65 Mbps, the higher layer throughput is about 50 Mbps; the MAC overhead is down to 25%.

Bear mind that these numbers are the absolute top speed you can get out of the system. 802.11n has numerous modulation schemes to fall back to when the conditions are less than perfect, which is most of the time.

But to minimize these fall-backs, 11n contains additional improvements to make the effective throughput as high as possible under all circumstances. These improvements are described in the following paragraphs.

Fast MCS feedback – rate selection.
Existing equipment finds it hard to track rapid changes in the channel. Say you walk through the shadow of a pole in the building. The rate may go from 50 to 6 to 50 mbps in one step. It’s hard for conventional systems to track this, because they adapt based on transmit errors. With delay sensitive data like voice you have to be very conservative, so adapting up is much slower than down. 11n adds explicit per-packet feedback, recommending the transmission speed for the next packet. This is called Fast MCS (Modulation and Coding Scheme) Feedback.

LDPC (Low Density Partity Check) coding
LDPC is a super duper Forward Error Correction mechanism. Although it is almost 50 years old, it is the most effective error correcting code developed to date; it nears the theoretical limit of efficiency. It was little used until recently because of its high compute requirement. An interesting by-product of its antiquity is that it is relatively free of patent issues.

Transmit beam-forming
The term beam-forming conjures up images of a laser-like beam of radio waves pointing exactly at the client device, but it doesn’t really work like that. If you look at a fine-resolution map of signal intensity in a room covered by a Wi-Fi access point, it looks like the surface of a pond disturbed by a gust of wind – it is a patchwork of bumps and dips in signal intensity, some as small as a few cubic inches in volume. Transmit beam-forming adjusts the phase and transmit power at the various antennas to move one of the maxima of signal intensity to where the client device is.

In a phone the chances are that there will only be one Wi-Fi antenna, so there will be only one spatial channel. Even so, the MIMO technique of STBC (Space-Time Block Coding) enables the handset to take advantage of the multiple antennas on the Access Point to improve range, both rate-at-range and limiting range.

Incidentally, to receive 802.11n certification by the Wi-Fi Alliance, all devices must have two or more antennas except handsets which can optionally have a single antenna. Several considerations went into allowing this concession to handsets, mainly size and power constraints. STBC is particularly useful to handsets. It yields the robustness of MIMO without a second radio, which saves all the power the second radio would burn. This power saving is compounded with another: because of the greater rate-at-range the radio is on for less time while transmitting a given quantity of data. STBC is optional in 802.11n, though it should always be implemented for systems that support 802.11n handsets.

Hardware assistance
Many of these features impose a considerable compute load. LDPC and STBC fall into this category. This is an issue for handsets, since computation costs battery life. Fortunately these features are amenable to hardware implementation. With dedicated hardware the computation happens rapidly and with little cost in power.

CSR pitches better sound quality, battery life in Bluetooth headsets

CSR announced their Bluecore 6 chip today. It will ship in production volumes in January 2008. CSR claims a more robust connection – with increased transmit power and receive sensitivity. CSR also claims a breakthrough in sound quality, achieved by going from a Continuous Variable Slope Delta (CVSD) codec to Adaptive Differential Pulse Code Modulation (ADPCM). This enables packet retransmission and a halving of transmission bandwidth. The reduced bandwidth requirement results in a reduction in power consumption, and the ADPCM codec yields a MOS of 4.14 compared with a maximum of 2.41 for CVSD.

This is a welcome change, but doesn’t really go far enough. What’s needed is a wideband codec like AMR-WB to yield better-than-toll quality sound. While this would be redundant in a regular cell phone – ADPCM is more than adequate to carry a signal that has been encoded in GSM – it would make a huge difference in dual-mode phones carrying Voice over Wi-Fi.

Muni Wi-Fi on the ropes?

Wired has a story about the struggles of several municipal Wi-Fi deployments. Turns out that good coverage requires double the number of access points than planned, and that consumer subscriptons were an order of magnitude less than anticipated (1-2% instead of 10-25%).

The article holds out a few rays of hope: 802.11s for mesh deployments in residential areas, concentrate on high-traffic areas rather than trying to replace fixed broadband access, get the cities to become “anchor tenants.”

On the other hand, there still seem to be plenty of lower profile metro Wi-Fi deployments that are doing OK.

Bluetooth headset for the iPhone

I went to an Apple store today, to buy an iPhone Bluetooth headset. I asked the clerk how the iPhone is selling, and he said “Steady, to both business users and consumers.” I came back to my desk to find a press release from iSuppli saying that the iPhone was the best selling smartphone in the US in July, with 1.8% of the overall cell phone market.

The iPhone Bluetooth headset comes in a box about the same size as the iPhone’s box. As usual with Apple, the box and all its contents are seductively designed, a pleasure to unpack and examine. The headset itself is tiny, but it comes with two USB connectors, one a nice docking station for the phone plus the headset, and one a traveling cable for both the phone and the headset.

One benefit of these dual connectors is pairing. To pair the devices simply plug them both in at the same time. That’s all. It worked for me. Another nice touch is the charging progress indicator that appears on the screen of the iPhone. It shows the battery status of both the headset and the phone.

The headset comes pre-charged; it was only plugged in to the dock for a couple of minutes before the light went green. Even plugging it in brought a little lift of the spirit, as I discovered that it uses the same magnetic engagement technology as the MacBook power connector.

What a disappointment when I made a call, though! There was a lot of static and a sound like running water at both ends of the connection. This is par for the course in my experience of Bluetooth headsets (there are half a dozen discards in my desk drawer), but still not acceptable. The headset was about 3 feet from the phone. There are several Wi-Fi transmitters in my office, but Bluetooth is supposed to be immune from this kind of interference due to its adaptive frequency hopping, which is supposed to learn which frequencies are conflicting and avoid them. ** Update: on subsequent calls I didn’t experience the same degree of impairment, so this initial experience may have been anomalous. The call quality on most calls appears to be acceptable. Even better, this is the first in-ear headset I have used that is so comfortable that I forget I am wearing it. This is a breakthrough. But now that it’s in my ear all the time I am beginning to be concerned about battery life. **

About a billion cell phones were sold in 2006, of which about 50% had Bluetooth capability. About 100 million bluetooth headsets were sold in 2006. Although 100 million of anything is a lot, it is only a 10% attach rate for headsets to phones. I believe the attach rate would be higher if the comfort, sound quality and ease of use were improved.

A strange omission in the iPhone Bluetooth headset is the apparent lack of support for playing iPod content through it. While it may make sense to think that music listeners must have stereo, not every MP3 is music. I play a lot of saved NPR clips through my iPhone, and the headset wires are constantly getting tangled up. Balanced against this inconvenience I would be quite satisfied with monaural playback of this content through a Bluetooth headset. It seems high-handed to deny this option to those who might find it useful. Perhaps this design decision has something to do with battery life.