Rethink Research has published an interesting article relating the new Wi-Fi voice certification to the outlook for femtocells.

The idea of the article is that voice over Wi-Fi for cell phones is competing with femtocells, and that femtocells may win out. The article distinguishes between business voice and consumer voice, saying that service providers see femtocells as “an important stalking horse for greater control of corporate customers. ” This gives a hint of why femtocells may be unattractive to businesses: many of them would rather not yield this control.

Consumer voice service is controlled by service providers. They have three options in this space: do nothing, deploy femtocells or deploy Wi-Fi. Do nothing is the obvious best choice, since neither of the other options carries a revenue upside. But poor coverage in a home discourages usage and risks cancellations of subscriptions. So in areas of poor coverage something like femtocells or UMA (voice over Wi-Fi) is attractive to service providers. For both technologies the service provider subsidizes the wireless router, but femtocells will remain more expensive than Wi-Fi routers because of their lower sales volumes, so Wi-Fi is more attractive on this count. But UMA requires phones with Wi-Fi, while femtocells will work with any phone in the service provider’s line-up, including legacy ones. So the customers’ experience of femtocells is better - they can choose or keep the phone they want and still get improved coverage at home. This benefit of femtocells clearly outweighs the marginal price advantage of Wi-Fi routers. Femtocells may help subscriber retention in another way: a Wi-Fi router is not tied to any particular cellular service provider, while a femtocell only works with the carrier that supplied it.

The situation in businesses is different. They generally prefer to control their own voice systems, which is why they have PBXs. But a substantial number of business calls are now made on cell phones, even on company premises. These calls don’t go through the PBX, so they are not least-cost-routed and they are not logged or managed by the IT department. Femtocells don’t fix these problems, but Voice over Wi-Fi does. Not service provider Voice over Wi-Fi, like UMA, but SIP-based Voice over Wi-Fi from companies like DiVitas and Agito. What about phone choice though? Won’t corporate customers be stuck with a limited choice of handsets? The answer is yes, only a limited number of phones have Wi-Fi: less than 10% of those sold in 2008. But in the category of enterprise smart phones, like the Nokia Eseries and Blackberries, the attach rate of Wi-Fi will soon be close to 100%.

So femtocells are a good way for service providers to remedy churn caused by poor residential coverage for consumers, but Wi-Fi may be the better option for businesses that want to regain control over their voice traffic.

In news that is huge for VoWi-Fi, the Wi-Fi Alliance announced on June 30th a new certification program, “Voice-Personal.” Eight devices have already been certified under this program, including enterprise access points from Cisco and Meru, a residential access point from Broadcom, and client adapters from Intel and Redpine Signals.

Why is this huge news? Well, as the press release points out, by 2011 annual shipments of cell phones with Wi-Fi will be running at roughly 300 million units. The Wi-Fi in these phones will be used for Internet browsing, for syncing photos and music with PCs, and for cheap or free voice calls.

The certification requirements for Voice-Personal are not aggressive: only four simultaneous voice calls in the presence of data traffic, with a latency of less than 50 milliseconds and a maximum jitter of less than 50 milliseconds. These numbers will produce an acceptable call under most conditions, but a network round-trip delay of 300 ms is generally considered to approach the limit of acceptability, and with a Wi-Fi hop at each end running at the limit of these specifications there would be no room in the latency budget for any additional delays in the voice path. The packet loss requirement, 1% with no burst losses, is a very good number considering that modern voice codecs from companies like GIPS can yield excellent sound quality in the presence of much higher packet loss. This number is hard to achieve in the real world, as phones encounter microwave ovens, move through spots of poor coverage and transition between access points.

Since this certification is termed “Voice-Personal,” four active calls per access point is acceptable; a residence is unlikely to need more than that. Three of the four access points submitted for this certification are enterprise access points. They should be able to handle many more calls, and probably can. The Wi-Fi Alliance is planning a “Voice-Enterprise” certification for 2009.

There are several things that are good about this certification. First, the WFA has seen fit to highlight voice as a primary use for Wi-Fi, and has set a performance baseline. Second, this certification requires some other certifications as well, like WMM power save and WMM QoS. So far in 2008, of 99 residential access points certified only 6 support WMM power save, and of 52 enterprise access points only 13 support WMM power save. One of the biggest criticisms of Wi-Fi in handsets is that it draws too much power. WMM power save yields radical improvements in battery life - better than doubling talk time and increasing standby time by over 30%, according to numbers in the WFA promotional materials.

Phybridge is a Canadian startup (founded May 2007) aiming to solve some of the problems of VoIP implementation. Its premise is that in many cases, an organization seeking to move from a traditional TDM phone network to a VoIP network does not have an Ethernet LAN capable of supporting VoIP. This inadequacy may result from insufficient capacity, QoS or reliability.

The conventional solution in these cases is to upgrade the Ethernet network while junking the old phone wiring.

Phybridge proposes to leave the Ethernet network alone, and to reuse the old phone wiring to implement a parallel data network, using Ethernet over a flavor of DSL. This is similar to HomePNA, but aimed at business use rather than consumer, and done point-to-point rather than into a shared medium.

The solution consists of two parts: a central box called “Uniphyer” has 24 ports connected to the legacy phone wiring. At the other end of each cable run is a “phy adapter” the size of a pack of cigarettes that you plug into the legacy phone jack, and into which you plug your Ethernet VoIP phone.

The Uniphyer provides power over the same copper pair as the data, so you can plug power-over-Ethernet phones into the client adapters.

The data rate is 3 megabits per second upstream, 30 down. This is slow for a data network, but certainly adequate for VoIP, so an organization that is replacing a conventional PBX phone system with a VoIP one may find Phybridge a cost effective solution if their existing data network isn’t up to VoIP, and the required improvements are extensive.

The Uniphyer is scheduled to launch at the end of September.

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 56 Mbps, but the higher layer throughput is only 26 Mbps; the MAC overhead is 54%! 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.

STBC
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.

David Pogue, the gadget-maven at the New York Times, went to a cell phone conference in Italy last week, and learned a few home truths.

On Independence Day he wrote a column that lambasted the US cellular carriers for their conservatism, and the following day he eulogized T-Mobile for deploying UMA. The UMA writeup is a PR flack’s dream. All true, too.

In the column on the calcification of the US cellular carriers, he indulged in a bit of wishful thinking:

If the iPhone becomes a hit, then, it could wind up loosening the carriers’ stranglehold on innovation.

Seasoned denizens of this industry may scoff, but it must be possible. And while UMA strives to exploit the VoIP genie while still keeping it in the bottle, at least its another step in the right direction. In the column on UMA, Pogue made a prediction that I happen to agree with:

But hard to believe though it may be, T-Mobile did make an announcement last week. And even harder to believe, its new product may be as game-changing as Apple’s.

The Wall Street Journal has already made the observation that the network operators don’t necessarily have their subscribers best interests at heart. But these two events in the same week may mark some kind of a turning point. I hope they do.

***Update: I went to the T-Mobile store this morning and signed up. The service here in Dallas is $10 per month, not $20 as reported by Reuters. The store manager also told me that people with poor cellular reception at home can use the UMA service at no additional monthly charge, but that this usage is treated the same way as cellular usage - in other words, it counts against your cellular minutes.***

***Update 2: Here are some details on the T-Mobile launch campaign. ***

Reuters reported this morning that T-Mobile is rolling out FMC service nationally.

Subscribers would pay an extra fee of up to $19.99 per line or $29.99 for five lines on top of regular monthly cellular bills for unlimited calls in a subscriber’s home or the nearly 8,500 places T-Mobile runs Wi-Fi, like Starbucks coffee shops.

This pricing model seems ambitious, compared to what it is competing with. T-Mobile’s MyFaves 300 plan gives you unlimited minutes nights and weekends and unlimited minutes to a list of five people that you choose. So the 300 minutes are consumed during the day, calling to people whom you call infrequently. For $20 more you can bump this to 1,000 minutes. Alternatively, you can spend that $20 on the FMC service. It seems like the FMC service would only be a better deal for people who are home all day (or at Starbucks), who want to talk a lot to people beyond their five most frequently called. MyFaves 1000 would be a better deal for people who want to talk to a large variety of people during the day when they are not at home, for example in the car or out of range of a Starbucks - like at work, for example.

So who are these people that this “HotSpot@Home” service is aimed at? Surely there can’t be many. Why doesn’t T-Mobile use this technology to gain more customers, by giving it away free to subscribers? This would appeal to all the people who have poor reception at home, who would feel bilked by having to pay extra just for acceptable quality of service there (Hey! They do! See the update above). Another way to increase customer appeal would be to go with a wideband codec for Wi-Fi calls, guaranteeing CD-quality sound to Wi-Fi on-network calls. Or why not do both? This would provide a viral motivation to complement MyFaves, it would be unique among US carriers, it would improve retention, and it would bring new subscribers to start exploiting all that spectrum that T-Mobile picked up in the AWS auction in September 2006.

Reuters ran a story today from the FMC World Congress in Amsterdam.
The article cites very weak consumer uptake leading to the cancellation of T-Mobile’s T-One service in Germany, and weak uptake also at Neuf Cegetel. It seems strangely unbalanced, since it doesn’t mention T-Mobile’s imminent national rollout of FMC in the USA, the BT/Vodafone Fusion service and the FT/Orange Unik service. There are several other UMA deployments that would have made the outlook seem less gloomy.
The T-Mobile service was survived in Germany by T-Com’s similar service, Telekom-Vorteil, a “fixed/Wi-Fi” service that routes wireline calls over Wi-Fi, so you can use a Wi-Fi phone or the Wi-Fi of your dual mode phone to pick up calls on your home number when you are at home. This is not UMA based, and drops the call when you move out of Wi-Fi range. People like it.

Today’s Wall Street Journal has a good article about T-Mobile’s UMA trial in Seattle. It says that T-Mobile may be rolling it out nationally as early as next month, despite some trial particpants’ complaints about handoff and battery life issues. T-Mobile will be offering a home router to help with QoS and battery life. I presume that for the battery life this is just WMM Power Save (802.11e APSD) since that is what the phones in the trial (Samsung T709 and Nokia 6136) support. For QoS side I expect these APs will support WMM (802.11e EDCF), but they could also support some proprietary QoS on the WAN access link, the way that the AT&T CallVantage routers do, which would be interesting.

There is some background on the trial here.

The article goes on to put the trial into the context of other FMC deployments, from BT Fusion, Telecom Italia and Orange. The article quotes a Verizon Wireless spokesman saying that they aren’t convinced that Wi-Fi can deliver high enough voice quality to carry Verizon branded calls. This is amusing bearing in mind the usual quality of a cellular call in a residence.

The article also quotes Frank Hanzlik, the head of the Wi-Fi Alliance as saying that business FMC may have more potential than consumer. I agree.

In-Stat has just published a report called “Wi-Fi for Voice: Consumer Research Around Wi-Fi Phones.”

The report finds that consumers are unenthused about Wi-Fi-only phones, which is not exactly news. This is of course not a strike against VoWLAN, since everybody will be using VoWLAN on their cell phones in just a few years.

Reuters picked up on one little sentence buried in a Nokia press release entitled “Nokia Demonstrates Leadership in Technologies for Internet on Mobile Devices at Web 2.0 Expo.” The relevant paragraph, in its entirety, reads:

“Nokia Shows Commitment to WiMAX as Web 2.0 Enabler

“Nokia is dedicating significant research, development and intellectual property to WiMAX and supports efforts in making it a global broadband standard. The combination of WiMAX broadband technology and Web 2.0 services offers people an enriched high-speed Internet experience free from the desktop PC. Nokia plans to bring its first WiMAX enabled mobile device to market in early 2008.”

With no apparent evidence, the headline of the Reuter’s story mentioned the word “phone,” and the Internet echo chamber commenced to spawn dozens of stories saying that Nokia is going to release a WiMAX phone in 2008. Actually it looks more as though they are talking about an Internet Tablet like their N800, which is much less exciting.

A Nokia phone based on WiMAX would either have to have a regular cellular radio for the voice channel, or it would use WiMAX for voice. A phone that uses WiMAX for voice would most likely be aimed at a wireless Internet provider that doesn’t have a cellular network, for example ClearWire in the USA. This would put a date on their anticipated entry into mobile voice over WiMAX to compete with the incumbent cellular operators.

But that’s not what the press release says.