New study shows how broccoli helps reduce cancer risk

WASHINGTON (AFP) - A scientific study published here Wednesday sheds light on why men who eat a lot of broccoli are less likely to develop prostate cancer.

Scientists have already observed that diets rich in cruciferous vegetables -- including broccoli, cauliflower, cabbage, Brussels sprouts, bok choy and kale -- may reduce the risk of prostate cancer and other chronic disease.

But the new research, by scientists at Britain's Institute of Food Research, is the first attempt to show how that works in a clinical study involving people, as opposed to animals or cell models.

For the new study, men at risk of developing prostate cancer ate 400 grams of broccoli or 400 grams of peas a week for 12 months, in addition to their normal diet.

The researchers took samples of prostate gland tissue from the men at the start of the study, at six months and at 12 months, to monitor changes in genes linked to cancer.

"There were more changes in gene expression in men who were on the broccoli-rich diet than on the pea diet, and these changes may be associated with the reduction in the risk of developing cancer," said a statement by the Public Library of Science, which published the work Wednesday in its online journal PLoS ONE.

"The results of the study suggested that relatively low amounts of cruciferous vegetables in the diet -- a few portions per week -- can have large effects on gene expression by changing cell signalling pathways," the statement said.

The signalling pathways are the routes by which information is transmitted to the nucleus of the cell where gene expression occurs.

"Other fruits and vegetables have been shown to also reduce the risk of prostate cancer and are likely to act through other mechanisms," said lead researcher Richard Mithen.

"Once we understand these, we can provide much better dietary advice in which specific combinations of fruit and vegetable are likely to be particularly beneficial.

"Until then, eating two or three portions of cruciferous vegetable per week ... should be encouraged."

Prostate cancer is the most common form of non-skin cancer affecting men in western countries.

Fossil of most primitive 4-legged creature found

WASHINGTON - Scientists unearthed a skull of the most primitive four-legged creature in Earth's history, which should help them better understand the evolution of fish to advanced animals that walk on land.

The 365 million-year-old fossil skull, shoulders and part of the pelvis of the water-dweller, Ventastega curonica, were found in Latvia, researchers report in a study published in Thursday's issue of the journal Nature. Even though Ventastega is likely an evolutionary dead-end, the finding sheds new details on the evolutionary transition from fish to tetrapods. Tetrapods are animals with four limbs and include such descendants as amphibians, birds and mammals.

While an earlier discovery found a slightly older animal that was more fish than tetrapod, Ventastega is more tetrapod than fish. The fierce-looking creature probably swam through shallow brackish waters, measured about three or four feet long and ate other fish. It likely had stubby limbs with an unknown number of digits, scientists said.

"If you saw it from a distance, it would look like a small alligator, but if you look closer you would find a fin in the back," said lead author Per Ahlberg, a professor of evolutionary biology at Uppsala University in Sweden. "I imagine this is an animal that could haul itself over sand banks without any difficulty. Maybe it's poking around in semi-tidal creeks picking up fish that got stranded."

This all happened more than 100 million years before the first dinosaurs roamed Earth.

Scientists don't think four-legged creatures are directly evolved from Ventastega. It's more likely that in the family tree of tetrapods, Ventastega is an offshoot branch that eventually died off, not leading to the animals we now know, Ahlberg said.

"At the time there were a lot of creatures around of varying degrees of advancement," Ahlberg said. They all seem to have similar characteristics, so Ventastega's find is helpful for evolutionary biologists.

Ventastega is the most primitive of these transition animals, but there are older ones that are oddly more advanced, said Neil Shubin, professor of biology and anatomy at the University of Chicago, who was not part of the discovery team but helped find Tiktaalik, the fish that was one step earlier in evolution.

"It's sort of out of sequence in timing," Shubin said of Ventastega.

Ahlberg didn't find the legs or toes of Ventastega, but was able to deduce that it was four-limbed because key parts of its pelvis and its shoulders were found. From the shape of those structures, scientists were able to conclude that limbs, not fins were attached to Ventastega.

One question that scientists are trying to figure out is why fish started to develop what would later become legs.

Edward Daeschler, associate curator of vertebrate zoology at the Academy of Natural Sciences in Philadelphia, theorizes that the water was so shallow that critters like Ventastega had an evolutionary advantage by walking instead of swimming

Nature: http://www.nature.com/nature

Salmonella probe adds foods served with tomatoes

WASHINGTON - Adding to tomato confusion, the government is about to start testing numerous other types of fresh produce in the hunt for the source of the nation's record salmonella outbreak — even as it insists tomatoes remain the leading suspect.

Investigators are mum on exactly what other vegetables are getting tracked.

Items commonly served with fresh tomatoes is the only hint Food and Drug Administration food safety chief Dr. David Acheson would give, calling it "irresponsible" to point a finger until he has more evidence that some other food really deserves the extra scrutiny.

"Tomatoes aren't off the hook," he stressed. "It's just that there is clearly a need to think beyond tomatoes."

Still, Acheson widened FDA's probe on Tuesday, activating an emergency network of food laboratories around the country in anticipation of lots of additional samples to test.

The reason is that the outbreak continues, with 869 people now confirmed having taken ill. Most troublesome, at least 179 of them fell ill in June, the latest on June 20. That is more than two months after the first salmonella illnesses appeared, meaning the outbreak is continuing weeks longer than food-poisoning specialists had expected — and suggesting the culprit is still on the market.

Over the weekend, disease detectives with the Centers for Disease Control and Prevention began interviewing people sickened in June to find out what they ate and to compare their diets with those of healthy relatives and neighbors. Officials wouldn't reveal early findings, except to say they supported the investigation's new move.

Among the possibilities FDA is exploring is whether tomatoes and other produce are sharing a common packing or shipping site where both might become contaminated, or whether multiple foods might be tainted while being grown on adjoining farms or with common water sources.

Pressure is increasing on the FDA to solve the case, with the tomato industry suffering millions of dollars in losses and pushing for Congress to investigate how the agency handled the outbreak.

But Acheson said Tuesday that there's a growing misconception in the public that if tomatoes really were to blame, the outbreak would only have lasted six weeks.

That's just not true, he said, pointing to farms that rotate harvests so as to keep producing tomatoes for months.

Tomatoes first became a suspect because of what are called "case-control" studies rapidly conducted in New Mexico and Texas, the outbreak's center, CDC food-poisoning specialist Dr. Robert Tauxe said.

Those kinds of studies compare the sick to people who are otherwise similar — in income, lifestyle, where they live — but healthy. In those initial studies, about 80 percent of the ill reported eating certain types of fresh tomatoes, far more than the healthy group did, Tauxe said. Statistically, the association was too strong to think it a coincidence.

Some food-poisoning experts say the CDC missed a key step in not taking those studies a step further and trying to trace why some of the healthy ate tomatoes without harm.

For now, the FDA continues to urge consumers nationwide to avoid raw red plum, red Roma or red round tomatoes unless they were grown in specific states or countries that the agency has cleared of suspicion. Check the FDA's Web site — http://www.fda.gov — for an updated list. Also safe are grape tomatoes, cherry tomatoes and tomatoes sold with the vine still attached.

That advice is coming under fire too because tomatoes are sent through multiple repacking and distribution sites around the country, even to Mexico and back, regardless of where they're grown. But Acheson said the advice would be fine-tuned only if new science emerges.

Even Health and Human Services Secretary Michael Leavitt expressed frustration Tuesday that the case isn't solved.

"Nothing happens fast enough when you have a problem like this," Leavitt said as he asked Congress for more funds and stronger legal powers for food and consumer safety agencies. Still, "I feel confident we will find the solution to this problem."

Including VoIP over WLAN in a Seamless Next-Generation Wireless Environment Part 2

2. Network Interfaces, Architectures, and Timing Issues

This section reviews the requirements of the PSTN with regards to a VoIP application as well as the timing issues that are critical for toll quality voice deployment.

How VoIP over WLAN applications will be deployed will have an effect on the design and integration of the equipment. The following issues have a bearing on equipment design:

• VoIP voice compression algorithm(s)
• Voice packet size, packet rate and delay
• Timing requirements for signaling and call set up
• Call control protocol
• Capacity and range of QoS capabilities that will be supported beyond voice

The market can be roughly divided between residential/SOHO and enterprise deployments.

The following figure illustrates enterprise deployments:

Figure 8

VoIP integration in the enterprise will be evolutionary, not revolutionary. Greater than 90 percent of enterprises use analog and ISDN (P-Phone)-based PBX equipment. In some cases the PBX equipment is modular and supports VoIP interface LRUs (line replaceable units). For older legacy PBX equipment, a VoIP gateway can introduce VoIP into the enterprise.

Given the expense of analog PBXs and phones, enterprises typically decide to gradually transition the deployment of VoIP. The enterprise deployment diagram above illustrates a phased deployment such as the following:

1. The analog PBX/voice mail system connects to the PSTN. A small number of users have VoIP phones connected via a VoIP gateway. (See Hybrid Network-1.)
2. A VoIP PBX/gateway connects the enterprise to the PSTN. A majority of users have VoIP phones. Legacy phones are supported by an IP/PBX converter. This network will be typical in larger enterprises with a large number of PSTN connections. (See Hybrid Network-2.)
3. The enterprise has an IP connection to a remote gateway/PBX that may serve one or more business customers. Legacy analog phones, if any, are supported by an IP/PBX converter. This network will be typical in small and medium size enterprises and in the remote facilities of large enterprises. (See IP Network.)

The enterprise solution is highly dependent on the data network topology. A large number of routers and the type of hierarchy among the routers could delay the network's throughput. The transition through each router will add to the delay budget for IP packets. Additional delay in the network could be caused by centralized security and authentication servers. Depending on the network topology, 802.11 clients who are roaming between APs may experience extended delays in accessing centralized authentication servers as well as longer latencies in completing= handoffs between APs.

A residential network topology, as shown in the following diagram, will have its own set of challenges.

Figure 9

Residential/SOHO customers typically have one or more analog voice circuits which are connected directly to the PSTN through a CLASS 5 switch or through a DLC remote terminal. In many cases, subscribers are served by advanced DLC/DSLAM (DSL access multiplexer) remote terminals that provide a combination of DSL and POTS service on a single line as well as standard POTS voice circuits.

As the PSTN network has been upgraded to support DSL, integrated voice/data service (VoDSL) using ATM AAL1/2 packet voice has become available. In general, most residential and SOHO users will continue to have POTS as the primary voice interface. The following are several of the interface configurations that will emerge for home users:

1. POTS voice interfaces to the PSTN in a way that is identical to a cordless phone. VoIP conversion to/from the analog wire pair would be integrated into the VoIP-enabled access point.
2. VoDSL interfaces to the PSTN and VoATM is converted to VoIP for use with a WLAN AP.

Integrated DSL/WLAN gateways will become a trend because these gateways will be able to serve both of these residential/SOHO configurations.

If the residential/SOHO user is interfacing to a broadband cable modem, the topology is nearly identical to that of DSL. Where voice over cable is available, it is typically an independent system from the DOCSIS® cable modem interface. An example of this independent interface is the ARRIS system, which results in termination of two-wire analog POTS interfaces at the customer premise. As DOCSIS 1.1 and 2.0 become readily available, integrated VoIP over cable modem will become prevalent.

The time delay of the communications path for 802.11 VoIP in a cordless residential application will typically be much shorter and have a limited number of sources of delay. The voice interface to the home will typically be a POTS interface. Specific signaling requirements must be supported with analog line pairs to DLC and/or CLASS 5 switching equipment. (See note below.)

WLAN Lower Sleep Modes and POTS Call Processing

The design of DLC and Class 5 switch equipment is based on the premise that the wire connection to the phone is in place and operational. Typically, establishing a call with Q.391 protocol is completed in less than 200 msec. The caller ID modulation receiver (FSK Modem) must be ready after the first ring, which is less than or equal to two seconds. Worst case signaling delay should be less than 100 msec.

This creates a problem.

Any WLAN implementation will include VoIP handsets that are often in power-saving sleep modes where much of the device is not operating. In this mode, the WLAN will "wake up" and establish communication in intervals of 200 msec to one sec, well beyond the telephony system delay specification.

Clearly, the 802.11 AP will have to maintain an attached state for a given handset and provide a call proxy during call set up until the call can be handed off to the handset after it has awakened from sleep mode.

Including VoIP over WLAN in a Seamless Next-Generation Wireless Environment

1. WLAN Network Capacity Analysis

For network planners who are deploying a VoIP over WLAN application, one of the first issues to be addressed should be network capacity. To ensure the network is able to deliver the required QoS capabilities for a voice application, designers must anticipate and analyze how the WLAN will be used. Several questions, such as the following, must be answered:

• What types of QoS capabilities will be deployed?
• How much network capacity must be set aside for these QoS capabilities?
• What is the projected growth rate for QoS capabilities on the WLAN?

The questions above are network design considerations for a variety of contemporary applications, including VoIP, video and other services requiring QoS capabilities.

The purpose of this discussion is to explore the various facets of network capacity planning for the future deployment of WLANs. While the intent here is to analyze VoIP-enabled systems, network designers should also expect a significant amount of multimedia traffic over home/SOHO WLANs as well as video conferencing traffic over enterprise WLANs.

The remainder of this section describes the following:

• Over-subscription of voice networks (voice concentration)
• Throughput requirements for typical voice, video and media applications using IP packet technology
• WLAN network capacity for enterprise applications
  • RF frequency planning and reuse for large network deployments
• WLAN network capacity for home applications
  • Consideration of wireless repeaters (mesh) to extend home coverage

Over-subscription of Voice Services (Concentration)
It is important for designers of VoIP over WLAN applications to understand some of the basic concepts that have been applied for years in the PSTN. A basic understanding of oversubscription, for example, can assist network planners who are evaluating network capacity for enterprise VoIP over WLAN applications.

Telephone systems have been very closely monitored for over 100 years. The public telephone system has always incorporated "statistical over-subscription" of phone lines. In the United States, there are typically between four and eight phones per active (served) phone line in the network. POTS (plain old telephone system) networks are designed to have a specific probability that a call can be blocked from time to time. In the United States, call blocking is typically limited to 1% or 0.5% of total calls.

Phone lines typically are terminated at a Class 5 switch or a Digital Loop Carrier (DLC) connected to a Class 5 switch. The Class 5 switch manages call connections and rejects calls when the system capacity has been reached. (A caller is aware of this when receiving a fast busy tone or the "all circuits busy" message.) In cellular networks, some consideration is given to reserve a fraction of the active phone line capacity for handoff purposes between one cell and the next.

A measure of phone usage capacity is the ERLANG function, which equates to one active call hour (or 3,600 call seconds) of voice line use. The amount of phone concentration (oversubscription) can be determined with the ERLANG-B function, (the ERLANG blocked call function). Because the telephone network must be designed for the worst-case load, phone usage is defined as that level that is achieved during the busiest hour of the day. Accurate average measurements for peak busy hour phone usage in the United States are as follows:

• 0.15 ERLANG (15 me) for a business phone
• 0.1 ERLANG (10 me) for a residential phone

Based on the ERLANG B function and an acceptable percentage of blocked calls, the following diagram illustrates the number of active phone lines needed to support a set of phone users attached to a given switch or bandwidth resource.

Figure 2

As the pool of attached phone lines increases, efficiency, in terms of fewer blocked calls, and over-subscription also will increase. This should not be surprising because the efficiency of all systems that use statistical multiplexing improves as the number of channel resources increases at the multiplexer. The concentration level moves from around 2:1 at 10 subscriber phone lines to more than 3:1 at 60 user phone lines.

Voice, Video and Media Throughput over IP
The following sections discuss several WLAN network capacity issues as they relate to the transmission of voice, video and multimedia data using IP.

Voice Compression and VoIP
Voice compression algorithms help network designers derive as much capacity from an infrastructure as possible, but compression algorithms involve tradeoffs between efficiency and overhead that planners should consider.

In wireless networks, voice is digitized with the G.711 coding standard and transported at 64 Kbps. While G.711 is the mainstream digital codec for toll-quality voice services, a number of more efficient codes are used for both cellular and voice "pair gain applications." In an IP network, voice codecs are placed into packets with durations of 5, 10 or 20 msec of sampled voice, and these samples are encapsulated in a VoIP packet.

The following figure illustrates the encapsulation for various protocols, including IPv4, UDP and RTP (Real Time Protocol). For IPv4, the packet overhead is 40 Bytes. As the industry transitions to IPv6, this overhead will grow to 60 bytes.

Figure 3

Clearly VoIP has an overhead issue that is compounded when high levels of voice compression are deployed in conjunction with voice packets of short duration. The tradeoff between overhead and packet duration is shown in the following table. Other issues affecting VoIP network capacity planning, such as delay, jitter and packet duration, are discussed later in this tutorial.

	Voice Packet Frame duration (msec) efficiency
CODEC	5	10	20	40
IPv4 G.711	47.6%	64.5%	78.4%	87.9%
IPv6 G.711	38.5%	55.6%	71.4%	83.3%
IPv4 G.726	31.3%	47.6%	64.5%	78.4%
IPv6 G.726	23.8%	38.5%	55.6%	71.4%
IPv4 G.729	10.2%	18.5%	31.3%	47.6%
IPv6 G.729	7.2%	13.5%	23.8%	38.5%

Table 1

The following table lists the one-way throughput requirements for typical voice codecs using VoIP. For the purposes of capacity analysis, a typical throughput of 64 Kbps per direction was used, assuming a combination of G.726 and G.711 codecs.

Coding algorithm		Bandwidth	Sample	Typical IP bandwidth (one way)
G.711	PCM	64kbps	0.125ms	80kbps
G.723.1	ACELP	5.6kbps	30ms	16.27kbps
G.723.1	ACELP	6.4kbps	30ms	17.07kbps
G.726	ADPCM	32kbps	0.125ms	48kbps
G.728	LD-CELP	16kbps	0.625ms	32kbps
G.729(A)	CS-ACELP	8kbps	10ms	24kbps

Table 2

VoIP Complexity Options To deploy VoIP WLANs, two tiers of voice-capable access points (APs) probably will be needed: Low-end consumer VoIP APs will use G.711 and/or G736 APs for the enterprise and wide-area applications will support a full suite of possible cellular and standard codecs for a wide variety of user devices such as PDAs and others. To achieve completely seamless ubiquity of IP services even low-APs must support handoffs from cell phone traffic as well as a full set of codecs.

Video Media over IP
Although voice will be the first application requiring QoS capabilities over WLANs, several other multimedia applications will soon follow, including the distribution of audio (net radio, MP3 music, etc.) and video (streaming video, DVD, HDTV, etc.) over WLANs. Fortunately for network planners, media compression codecs will ease the bandwidth requirements for these multimedia applications. Specifically, improvements in the quality of video codecs like MPEG4 will allow DVD-quality compression at throughput rates of approximately one Mbps. For HDTV, the standard MPEG2 video stream can be reduced from 19.2 Mbps to around eight Mbps.

The following table illustrates the one-way throughput for various consumer video codec devices using maximum IP packet lengths.

Video Media	Bandwidth (MBPS)	Packet Size	Packets/sec	Delivered Bandwidth (MBPS)	Overhead (%)
MPEG-1 Basic Media	2.5	1,500	2083	2.5663	2.58%
MPEG-2 SDTV format (DVD)	8	1,500	6666	8.2125	2.59%
HDTV MPEG 2 committee 18	19.2	1,500	16000	19.7120	2.60%

Table 3

It should be noted that the FCC has mandated that by 2006 all televisions sold in the U.S. must include digital receivers. At that point, the integration of wireless interfaces into television electronics could be widespread.

Video Conferencing and IP Streaming Media
WLAN planners and designers should realize that video conferencing is an application that will have an impact on WLAN network capacity even though video conferencing has not yet become as pervasive in the enterprise or home markets as had been expected. This will change over the next few years as broadband connections become pervasive in households, the number of telecommuting workers increases, and enterprises improve their IT resources to allow greater use of video. As a result, WLAN designers should consider the requirements of video conferencing as they deploy infrastructure.

The following table provides a summary of the throughput requirements for several typical video conferencing and streaming media applications. Similarly to DVD and HDTV, the same types of improvements in lower resolution video and conferencing compression are expected in the years ahead. Network capacity models, especially in home networks, should anticipate increasing use of these applications.

Video Product	Bandwidth (MBPS)	Packet Size	Packets/sec	Delivered Bandwidth (MBPS)	Overhead (%)
Business-Quality Conference	915	781,693	107	735,466	6.3%
NetMeeting Video LAN	779	478,312	77	445,156	7.4%
NetMeeting Video DSL	363	187,726	64	159,800	17.4%
NetMeeting Video 28K	288	10,497	5	8,529	23.1%
Read Audio Radio	681	165,118	30	152,025	8.6%
Media Player 80K Stream	687	81,171	15	74,882	8.4%
Media Player 20K Stream	476	27,600	7	24,469	12.8%
Real video 28K Stream	384	25,173	8	21,633	16.4%

Table 4

Throughput of WLAN Access Points (AP)
To optimize the network capacity of a WLAN with a voice or multimedia application, network planners must give special attention to the throughput of the APs which govern how quickly data of any sort can be placed on the network.

The following two basic functions affect the throughput of an AP:

1. Area and modulation density supported by the cell
   1. Small cells can support high data rate modulations (peak rates)
   2. Larger cells will use lower rate 802.11 modulations and are an aggregate sum of areas covered and the modulation rate
2. The WLAN MAC protocols have the following effects:
   1. The Ethernet (CSMA/CA) protocols, DCF and EDCF, limit capacity at approximately 37% of the peak data rate
   2. Scheduled TDMA protocols such as HCF can theoretically reach around 90% capacity of the network, but under full load they will typically carry only approximately 75% of capacity
   3. DCF/EDCF MAC protocols do not effectively manage network latencies as the capacity limit is approached
   4. HCF protocols control latencies by providing fair weighted queuing so that all users will receive service even under full load conditions

The following table shows the throughput rates for HCF and DCF/EDCF for various modulations. These values can be de-rated when applied to larger cells that operate with lower capacity modulations.

	Throughput (MBPS)
Modulation	HCF (75%)	DCF/EDCF (37%)
54 MBPS OFDM	40.5	19.98
22 MBPS PBCC	16.5	8.14
11 MBPS CCK	8.25	4.07
5.5 MBPS CCK	4.125	2.035

Table 5

By and large, network designers do not use theoretical peak performance rates when planning a WLAN. As a rule of thumb, most network planners de-rate the theoretical performance figures to approximately 70% to 80% of the peak capacity.

Note: With packet aggregation and proper use of 802.11 protection mechanisms, DCF/EDCF can achieve higher levels of throughput (approximately 50% to 55% higher) with a limited number of users and limited number of connections requiring QoS capabilities. This does not address the concern many enterprise WLAN designers have for the stability of DCF/EDCF under a high user load.

Enterprise Capacity Analysis
Because an enterprise 802.11 WLAN deployment will involve covering a workplace with a series of APs, the network planner must analyze the bandwidth capacity of each cell and the bandwidth demands that users will make on each cell in the network. In an enterprise deployment, the APs will be connected to a router either directly or through an Ethernet switch. In larger enterprises, multiple sub-nets may be connected hierarchically so that a wireless subscriber actually passes through several routers before reaching the IP network.

This type of WLAN essentially represents a micro-cellular architecture using 802.11 Aps interconnected via broadband IP links over Ethernet. APs have a certain coverage range which provides network access to users in a circular area around the location of the AP.

The analysis of enterprise network capacity that follows was based on the following assumptions:

• The average density of enterprise users is one per 200 square feet of floor space.
• The work day is eight hours long.
• 150 Mbytes of data as file downloads, e-mails and web accesses are transferred per user over the WLAN. No streaming media is supported.
• A sustained peak-to-average data throughput rate of three was used, essentially making the average data load three x 150 Mbytes or 450 Mbytes.
• Users require 0.15 ERLANG (15 me) of voice load. (This is based on current Bellcore and SBC business user peak busy hour loads.)
• A VoIP connection places a load on the WLAN of 64 Kbps in each direction (a combination G.726 and G.711).

Based on this profile, the following table illustrates the peak busy hour load on a WLAN cell as a function of the radius of the cell.

Cell Radius (feet)	50	75	100	125
Users	39	88	157	245
Active Phone Lines	12	22	34	49
Concentration X:1	3.25	4.00	4.62	5.00
Bandwidth (MBPS)
Voice Uplink	0.77	1.41	2.18	2.18
Voice Downlink	0.77	1.41	2.18	2.18
Data Downlink	3.25	7.33	13.08	20.42
Data Uplink	1.63	3.67	6.54	10.21
Total Throughput	6.41	13.82	23.98	34.98

Table 6

This network capacity analysis shows that even for a small cell with a radius of just 50 feet, a typical 802.11b network would not have the capacity for applications like VoIP or the "completely unwired workplace." However, if an 802.11a/802.11g WLAN with 54 Mbps modulation were combined with an HCF MAC in a cell with a 100-ft. radius, the cell would have nearly 40 percent reserve (excess) bandwidth. Alternately, if the inefficient EDCF MAC were used, a dual-mode 802.11a/g solution would be required to cover the same cell. Two RF channels would be required if the EDCF MAC were used.

"Wired When Docked" Workplace The analysis presented above is abased on the unrealistic assumption that users of a WLAN would always be completely wireless. In reality, a typical workplace will consist of wired and wirelss users, and most wireless users will be "docked," or connected to a wired network, when they are at their desks. Windows XP supports intelligent docking. Users are automatically switched from the WLAN network to a wired IP backbone when a device is docked. WLAN planners should take into consideration the effects that "wired when docked" will have on wireless networks' capacity requirements. For example, what if fewer than 20 percent of a workforce are un-tethered wireless workers. This has a profound impact on WLAN capacity needs, as shown in the following table. Cell Radius (feet) 50 75 100 125 20% wireless 1.28 2.76 4.80 7.00 30% wireless 1.92 4.14 7.19 10.49 40% wireless 2.56 5.53 9.59 13.99 Based on this analysis, planners can conclude that an enterprise WLAN with a "wired when docked" strategy can be supported by 802.11a/b/g dual-frequency access points using either HCF or EDCF. In other words, a deployment of a "sea of simple Ethernet-powered access points" would be sufficient.

Table 7

In order to fully utilize the bandwidth of an access point, co-channel and adjacent channel interference must be addressed. The following section will briefly address RF planning.

RF Frequency Planning for Enterprise Deployment

To analyze properly the overall capacity of a WLAN deployment, planners must consider the effects co-channel and adjacent channel interference will have on the throughput and bandwidth of the APs in the infrastructure. As WLAN APs are deployed for wide area coverage, WLAN RF interference issues take on characteristics similar to those that are faced in the planning of micro-cellular RF networks.

RF network planning begins with a consideration of the frequencies that are available. 802.11 a/b/g radios have the following independent frequencies:

• 5.1 to 5.3 GHz with eight frequencies
• 2.4 GHz with three frequencies (There is some discussion in the industry that four frequencies actually could be used.)

For access points that are based on simple omni-directional antenna configurations, the following diagram illustrates both the seven-frequency and the three-frequency repeat patterns with frequency reuse of one. The seven-frequency plan can be used for 5.x GHz 802.11a, and the three-frequency plan can be used for 802.11b/g systems.

Figure 4

For these types of deployments, the cell reuse distance, Ru, can be defined as follows:

• C = 7 (7 frequency): Ru = Rcell*sqrt (3C) = 4.48* Rcell
• C = 3 (3 frequency): Ru = Rcell *sqrt (3C) = 3.00* Rcell

Where:

C is the cluster size, which is the number of frequencies used in the reuse pattern
Ru is the reuse radius of the cell cluster
Rcell is the radius of coverage of a single cell

For distances greater than the AP's cell radius, it is assumed that RF propagation loss will not be free space (R²) but will be R³ to R⁴. This would result in interference reductions between cells of at least the following:

• C = 7: 19.5 dB to 26.1 dB (allows 36 to 54 Mbps OFDM)
• C = 3: 14.3 dB to 19.1 dB (allows 22 Mbps PBCC to 36 Mbps OFDM)

Based on larger deployments, it would be possible to implement 802.11 a/b/g WLANs with omni-directional antenna coverage and allow automatic frequency selection at the access point so that the AP is able to establish the most effective frequency plan.

It is possible to use sectored access points and improve frequency reuse. However, in an enterprise environment this would require very careful placement of the APs and alignment of cell sectors. Where frequencies are at a premium, deployments based on four-frequency sectors per AP can provide optimal reuse. The interference reduction is equivalent to the omnidirectional seven-frequency plan previously discussed. The following diagram illustrates the optimal four-frequency reuse plan.

Figure 5

Home Capacity Analysis

Unlike the enterprise market where some assumptions can be made about typical usage patterns, network capacity analysis for WLANs in the residential market will be greatly influenced by the rate of market penetration and the implementation of multimedia applications.

The following are some probable multimedia applications for the home:

• 802.11 VoIP cordless phones and home PBX/voice mail integrated into an 802.11 access point
• Streaming audio distribution to 802.11 speaker systems
  • Home PC as an MP3 audio service
• Streaming video from a cable television network, DVD system, etc.
• Telemetry applications, such as:
  • 802.11-enabled cameras/video for security
  • Meter reading for utilities
  • Smart appliances
• Wireless print server connections

If none of these applications are in demand by residential consumers in the near term, 802.11b with security features and QoS enhancements (802.11e/i) will meet the needs of most consumers. (Note: For consumers, speed will always sell. The concept that "faster is better" is compelling. For this reason, dual-mode 802.11b/g devices will have strong market acceptance as long as devices are backwards compatible with the nearly 20 million 802.11b subscriber base.)

Considering that the FCC has mandated that all TVs sold in the US must have a digital tuner, there is a very strong possibility of some level of wireless video distribution in the home. Video applications certainly will have the largest effect on the throughput and capacity requirements of home WLANs. The following table lists the bandwidth requirements for a number of current video codecs:

Video Media	Bandwidth (MBPS)	Packet Size	Packets/sec	Delivered Bandwidth (MBPS)	Overhead (%)
MPEG-1 Basic Media	2.5	1,500	2083	2.5663	2.58%
MPEG-2 SDTV format (DVD)	8	1,500	6666	8.2125	2.59%
HDTV MPEG 2 committee 18	19.2	1,500	16000	19.7120	2.60%

Table 8

This data indicates that a single MPEG2 SDTV/DVD quality channel requiring eight Mbps of bandwidth cannot be supported by current 802.11b MAC/PHY components. Fortunately, advances in video compression (MPEG-4) should reduce the bandwidth requirements for video applications to approximately one Mbps for DVD-quality video and about eight Mbps for HDTVquality applications.

Over the next two to three years, many in the industry expect that a typical broadband-enabled household could have WLAN peak capacity needs as indicated in the table below:

Service	Rate Upstream MBPS	Rate Downstream	Number of Channels	Total Rate Upstream	Total Rate Downstream
MPEG DVD-TV	0.5	8	2	1	16
Toll Quality Voice	0.064	0.064	2	0.128	0.128
Streaming Media	0.01875	0.3	2	0.0375	0.6
ABR Web Service	0.0965	0.386	1	0.0965	0.386
TOTAL				1.262	17.114

Table 9

As shown in the following table, the market acceptance of HDTV and the absence of MPEG-4 compression could increase a home's WLAN throughput needs by a factor of four over the next four to five years.

Service	Rate Upstream MBPS	Rate Downstream	Number of Channels	Total Rate Upstream	Total Rate Downstream
HD-TV	1.5625	25	2	3.125	50
Toll Quality Voice	0.064	0.064	4	0.256	0.256
Streaming Media	0.01875	0.3	1	0.01875	0.3
ABR Web Service	0.0965	0.386	2	0.193	0.772
TOTAL				3.59275	51.328

Table 10

These numbers indicate that high-throughput 802.11g/a PHY technology will be needed as a minimum in order to support these applications. Further, an efficient MAC (HCF) will be needed to optimize throughput.

VoIP applications do not require a significant amount of bandwidth in any of these capacity scenarios. Given the small number of phones in a typical home, the system must be designed for 1:1 concentration (that is, there would be no over-subscription of phone lines in the home). The more important benefits of WLAN-enabled cordless phones are twofold:

1. Removing cordless phones as a source of RF interference in the 2.4 GHz and 5.2-5.8 GHz frequencies could accelerate the acceptance of video applications over WLANs.
2. A new market for 802.11 cordless phones would be created with a sales potential of approximately 100 million units a year.

Residential 802.11 Link Asymmetry Usage models of residential applications show that the typical data transfer load is very asymmetric. That is, the downlink from the AP to the subscriber usually requires 10 times more throughput than the up-link from the subscriber to the AP. Radio archetectures for 802.11 APs can be differentiated to improve coverage and throughput by simply "bolting on" a booster LNA and PA capability (much like an "afterburner"). This capability is most appropriate in North America where spectrum rules allow 10 dB greater EIRP (power) than in Europe. The range of an AP can be nearly doubled at the highest modulation rate with simple link budget improvemetns in the AP.

Application of Repeaters/Small-mesh Access Points for Residential/SOHO Coverage
For developers of WLAN access points for the residential/SOHO marketplace, cell coverage and throughput are the most crucial issues facing WLAN implementations in this market. Wireless repeaters, which can be used to implement small mesh residential networks, are a low-cost method of improving coverage and throughput.

One possible technique for extending coverage and improving residential service is the use of multiple APs in a mesh/repeater architecture. A simple example featuring two access points is illustrated in the diagram below:

Figure 6

Access point B is a repeater (mesh element) to access point A, which connects to the Internet. Access point A functions as a router to access point B. Access point A must maintain a routing list for all clients in the home network while access point B only must maintain a routing list of attached clients. For example, B may be a simple bridge or a more intelligent router. Clearly, the mobility/roaming between the cells in this sort of arrangement will generate overhead messaging to update and maintain the routing information.

A real-world example of mesh WLAN architecture was the Aironet system, which was one of the first large-scale deployment platforms for WLANs. In this system, a client would probe for APs that could provide coverage, and the APs would reply with information on signal quality and on how much of their resources were currently in use. The subscriber would then associate and authenticate with the AP with the best signal quality and lowest usage. Once this was completed, re-routing updates would be completed.

Mesh networks can be nested deeper than a single connection. This is known as multi-hop. However, this creates even greater delays because of the cumulative time needed to route and retransmit from one AP to another. For voice, video phone and video conferencing, the round trip delay would be excessive for any architecture with more than one hop.

There are two possibilities for operating a residential mesh network. They are the following:

• Single-Frequency Mode: Access points are not dual-mode and can only support a single frequency of operation from an AP to another AP and from an AP to a client/subscriber.
• Dual-Frequency Mode: Access points are dual frequency, supporting two separate links on two separate frequencies simultaneously.

The single frequency mode of operation is backwards compatible to older single-frequency APs, but it is highly inefficient because the coverage provided by all APs in a WLAN is overlapping. Any communication initiated by an AP or a subscriber can interfere with any other communication. Under worst case conditions, the throughput is reduced by 1/(N+1) where N is the number of repeater/mesh APs attached to an AP.

The dual-frequency mode requires that all access points support two frequencies simultaneously. Typically, 802.11a (5.x GHz) would be used for AP-to-AP backbone communications while AP-to-subscriber communication would be provided by 802.11b/g (2.4 GHz). Using the 2.4 GHz frequency for subscriber coverage ensures support for low-cost and legacy 802.11b clients/subscribers. Because three independent 802.11b/g frequencies are available in the 2.4 GHz band, WLANs designed with a primary AP and one or two repeater Aps actually improve the coverage of the home. Stated another way, as long as three APs are implemented, the coverage area is greater and throughput will be consistently high without RF interference between the APs.

The dual frequency configuration is shown in the following diagram:

Figure 7 Mesh/Micro-Cell and the Interface Environment

The IEEE community is debating whether to use MIMO and/or beam steering techniques for 802.11 standards as a way to improve throughput and coverage.

A simople mesh extension for the 802.11g standard combined with improved video compression could be available to consumers immediately, and this would provide a "virtual" performance improvement. Final approval of IEEE 802.11 HTSG is at least three years away.

The mesh repeater architechture has another benefit in that it improves signal-to-interference (S/I) performanace because the architecture ensures subscribers are consistently closer to access points. This, in turn, ensures better link margins.

Network Evolution toward IP Multimedia Subsystem

Abstract
As opposed to mobile next generation network (NGN) approach aiming at just carrying circuit services on top of the Internet protocol (IP), the IP multimedia subsystem (IMS) offers operators the opportunity to build an open IP based service infrastructure that will enable an easy deployment of new rich multimedia communication services mixing telecom and data services. Before trying to emulate circuit switched domain, IMS will first provide new services (such as instant messaging, presence, push-to-talk over cellular [PoC]) that are not too demanding for the underlying radio access network. These services will attract customers and create new telecommunication needs without inducing the technical risks and costs of speech with stringent quality over packet radio.

Introduction
IMS is a new framework, basically specified for mobile networks, for providing IP telecommunication services. It has been introduced by the third generation partnership project (3GPP) in two phases (Release 5 and Release 6) for universal mobile telecommunications system (UMTS) networks. An IP multimedia framework was later introduced by 3GPP2 as the multimedia domain (MMD) for third generation code division multiple access 2000 (CDMA2000) networks, and finally harmonized with IMS. Real-time services can only be properly supported using the Release 6 IMS specifications. Currently, a Release 7 is being worked out by 3GPP taking into account the requirements from Telecoms and Internet-Converged Services and Protocols for Advanced Networking (TISPAN) and European Telecommunications Standard Institute (ETSI) on fixed IMS and NGN requirements.

The IMS concept was introduced to address the following network and user requirements:

• Deliver person-to-person real-time IP-based multimedia communications (e.g. voice or videotelephony) as well as person-to-machine communications (e.g. gaming service).
• Fully integrate real-time with non-real-time multimedia communications (e.g. live streaming and chat).
• Enable different services and applications to interact (e.g. combined use of presence and instant messaging).
• Easy user setup of multiple services in a single session or multiple simultaneous synchronized sessions.

3GPP(2) defines a reference architecture, functional splits, protocol specifications, use cases, and so on, for the IMS phases in the context of packet-based UMTS (CDMA2000) access. Many of the features, such as quality of service (QoS) control, full security and full roaming, need specific features to be available in the radio access and/or core network. Other features, such as the session initiation protocol (SIP) session control, integrated access and charging for combined applications, open up the integration of third-party applications. They can be provided today on pre-IMS packet access networks, such as the general packet radio service (GPRS), enhanced data rates for Global System of Mobile Communications (GSM) evolution (EDGE), and CDMA 1x as well as fixed NGN based packet access.

IMS Market Trends and Business Challenge
Global carriers are facing decreasing average revenues per user (ARPU). Due to strong competition, the prices of voice calls and Internet access are declining and the carriers' margins are being reduced. Carriers can no longer afford to offer only voice and ever faster Internet access because these items are becoming mere commodities. They need to identify attractive services that can be bundled with their basic access offer in order to reduce churn.

Most carriers have started looking for such services. Mobile networks operators have introduced portals (CDMA, i-modeTM, and GPRS) with the aim of offering content on mobile phones. Initial attempts were met with varying degrees of success: i-mode was successful in Japan, while the wireless application protocol (WAP) proved to be unsuccessful in Western Europe.

Besides e-mail, most of these portal services are person-to-server services in which a user interacts with a content server. One of the drawbacks of such services is that carriers have to obtain content, which means entering a new business field and negotiating with content providers. This process is costly and undermines the anticipated revenue opportunities from portal services.

Today, however, mobile network operators derive the bulk of their revenues from person-to-person services, mainly voice and short message service (SMS). The core of their business is to enable two users to communicate. Content is created by the users, so wouldn't it make sense to look for new person-to-person services?

E-mail and instant messaging are proving to be successful mobile services, as they already are on the wireline Internet. The multimedia messaging service (MMS), which enables mobile users to exchange pictures, is an example of carriers launching a new person-to-person service.

One area that mobile carriers have not yet been able to address is in real-time rich multimedia person-to-person communication. Video telephony over a UMTS network is a first step in this direction; the communication is enriched with a real-time video component. A UMTS videophone enables you to see the person to whom you are talking, and vice versa. However, because it is based on circuit switching, UMTS video telephony suffers some limitations; for example, a chat component cannot be added to a video telephone call.

Mobile carriers cannot offer their customers the freedom to mix multimedia components (text, pictures, audio, voice, video) within one call. Nowadays, a two party voice call cannot be extended to a multi-party audio and video conference. IMS overcomes such limitations and makes the previous scenario possible.

Moreover, in the future, it will make sense for operators to bridge the gap between traditional telecom services (voice call, SMS) and data services (e-mail, browsing, and instant messaging). Why would not it be possible to start a video call from an Internet directory, extend a text chat to a voice call, show a web and WAP page while making a video call, or show a photograph (provided by the caller) instead of a numerical caller identifier? Mobile carriers will make their offer more attractive if they can provide their customers with the right blend of telecommunication and data services seamlessly interwoven. Because of its IP foundations and the extensibility of SIP, along with the IMS signaling protocol, IMS enables telecommunication and data services to be mixed.

Real-time rich multimedia communication mixing telecom and data services has started to happen in wireline broadband networks. The major instant messaging providers are offering a mix of e-mail, text chat, multimedia file exchange, voice and video calls, and video chat. They have been successful in attracting millions of young people to this service mix, who are in turn tomorrow's adult consumers. Mobile carriers will have to offer them the same multimedia rich environment if they want to prevail over the pure information technology (IT) players, like Microsoft MSN, Yahoo, and America Online (AOL).

However, mobile carriers will have to choose whether they want to become IP pipe providers or IP telco providers. On the one hand, if they become "IP pipe providers", they will focus on transporting IP packets while the value-added services will be offered by others. This is similar to the situation in fixed networks in which broadband access is offered by the carriers, while the most successful IP services (e-mail, instant messaging, peer-to-peer file exchange) are offered by application service providers (ASP). The risk is that network access might become a commodity, thus leaving only small margins for the carrier. On the other hand, if the mobile carriers can become "IP telco providers" they will retain a greater part from the value chain, not only network access but also applications and services, payment, service brokering and even content. The portals and associated payment facilities (money charged for the application service providers and sharing the revenue with them) are a first step in this direction. IMS is a further step because it is carrier-centric, thus enabling them to retain control over rich multimedia sessions.

Evolution toward Work-Style Services
The integration of different media opens up new possibilities for far richer services than those available today and for which users will probably be willing to pay.

IMS is concerned with the creation and deployment of multimedia telecommunication services over any IP network. Most importantly, this includes person-to-person real-time services (such as voice) over packet switched (PS) networks, removing the need for a circuit switched (CS) domain. However, emulation of mobile CS services is not the primary goal of IMS, although their replacement by IMS services is unavoidable in the long run. Furthermore, it will be necessary for IMS to interwork with external CS networks, such as the public switched telephone network (PSTN) or public land mobile network (PLMN), even if there is no longer a circuit switched core network (CN) domain.

The main argument for introducing IMS is to support creative services, for example, from IT-oriented third parties.

IMS brings the power and wealth of Internet services to the mobile environment to a far greater extent than is possible today. In effect, IMS takes the best from the switched and packet worlds. In this context, a more controlled value chain is possible which is of greater benefit to the operators than are current Internet business models. For mobile operators, such models should continue to persist since users will certainly be willing to pay for the combination of rich Internet services and mobility.

IMS enables sessions to be established between multiple users and devices, and allows multiple services to be carried on a single bearer channel. Examples are the ability to set up:

• Separate services within a single session; these may be synchronized (e.g. voice and video for video telephony) or not (e.g. video and chat).
• Multiple simultaneous single sessions of unrelated services (e.g. voice call in parallel with a presence session).
• Easy switching between point-to-point and point-to-multipoint person to person sessions, without the burden of pre-programmed conference services on the PSTN. Combined with the flexibility of packet networks, this will allow walkie-talkie like services, such as PoC, in which a user simply has to push a button to send voice messages to a group of peers.
• Ease of simultaneously ringing a called users different terminals.

The service combination also guarantees session completion, which does not depend on the called party having a compatible terminal. For instance, a session can be accepted without the video component or the video component can be routed to another destination. Together all these features enable IMS to provide a significantly enhanced user experience and promises to be the basis for radically new services.

IMS is designed to be agnostic to the underlying access network, although initially it has been designed to run over mobile packet networks (e.g. GPRS, EDGE, UMTS, CDMA-based). In a second phase, it will also run on fixed (wireless) broadband networks (e.g. x digital subscriber line (xDSL), wireless local area network [WLAN], and cable). Consequently, IMS holds out the promise of seamlessly supporting converged services on fixed and mobile networks.

Safe Deployment of IMS
Although IMS is access agnostic, initially it will be deployed in mobile networks. Consequently, taking an end-to-end service perspective, mobility and particularly radio constraints must be taken into account. However, some other network-related constraints are also relevant, although not necessarily linked to mobile networks.

The use of IPv6 for IMS — as required by 3GPP — makes it necessary for an operator to deploy almost entirely new network layers at the same time as rolling out IMS:

• New service layer elements to provide new IMS services
• Totally new session layer, that is, a new networking layer based on SIP
• New transport layer based on IPv6
• Associated back-office features, such as security, routing and charging. Since some IMS services are totally new they might require new charging concepts and new charging information to be stored in the relevant network nodes.

Providing initial support for IPv4 over IMS overcomes the challenge of deploying IMS services over a new transport layer. Furthermore, deploying IMS solely on IPv6 may be an issue for communication services provided by the mobile operator to enterprise networks, which might be deploying IPv6 at their own pace.

IMS is indeed a good tool to deliver the same set of services to both wire-line and wireless users. For this reason, it might prove beneficial to deploy IMS over IPv4, at least during the roll-out phase. Nevertheless, the operator should not wait too long to carry out the necessary migration to IPv6 in order to avoid high migration costs when IMS has become established. The efficient transport of voice over a radio interface is another challenge that must be met when delivering real-time conversational services (such as VoIP over IMS). As the radio is the most expensive part of a PLMN, it must be optimized, otherwise the reduction capital expenditures (CAPEX) and operational expenditures (OPEX) potentially brought by IMS in the CN would be ruined.

A first step in providing efficient transmission of conversational voice over radio has been provided by robust header compression (RoHC), but further improvements are still needed. Answers to various questions, including the following, have to be provided before deploying IMS services that supersede the CS domain services:

• How is it possible to cope with variations in the length of voice frames over radio as a result of RoHC compression?
• How can one deal with real-time control protocol (RTCP) considering that its traffic is very sporadic but may correspond to frames with very different lengths which might delay real-time protocol (RTP) traffic?
• How to optimize the transfer of adaptive multi-rate (AMR) speech over the packet domain using techniques such as AMR codec mode control and unequal error protection (UEP)?

It is important to point out that the level of end-to-end QoS supported by the infrastructure determines the nature of the services that can be supported (e.g. real-time or non-real-time). In the case of real-time mobile services over IMS, the GPRS and UMTS infrastructure has to meet hard QoS constraints, for instance, full support for conversational quality. Such constraints cannot be met by today's network, for example, because of the lack of handover support for 2.5G packet radio.

Conclusion
IMS can provide new person-to-person multimedia communication services going beyond those available on today's 3G networks. Because IMS is IP based, it will blend telecommunication and data services, keeping the best of both the circuit-switched and packet worlds. As IMS was designed for network operators, it keeps multimedia call control with the mobile carriers, who can thus win a bigger part of the value chain (access, application and services, brokering, payment, etc.) and can become IP telco providers instead of "IP pipe providers".

Educational Content Provided by:

Sajid Soormally, IMS Solution Manager, Alcatel-Lucent
Michael Tadault, Technical Director, Alcatel-Lucent
Laurent Thiebaut, Senior End-to-End Solution Architect, Alcatel-Lucent

"Good" cholesterol may protect memory, study finds

WASHINGTON (Reuters) - Middle-aged people with low levels of so-called good cholesterol may be at higher risk for memory decline that could foreshadow Alzheimer's disease or other forms of dementia, European researchers said on Monday.

The study, involving about 3,700 British men and women, found that falling levels of high-density lipoprotein, or HDL, cholesterol were linked to declining memory by age 60. Such memory declines often precede the development of dementia such as Alzheimer's in the elderly.

Experts predict increasing numbers of people worldwide will develop Alzheimer's in the coming decades as populations in many countries grow older. Scientists are trying to identify risk factors that may appear years before the onset of dementia to help find ways to prevent or postpone it.

"Considering the way the population is aging -- the 65-plus age group being the fastest-growing age group -- we are facing a dementia time bomb," said Archana Singh-Manoux of the French National Institute for Health and Medical Research and the University College London, who led the study.

Singh-Manoux said she hoped the findings will focus attention on the possible role of higher levels of HDL cholesterol in protecting against memory loss.

The researchers looked at blood cholesterol readings and the results of a simple memory test collected when the people in the study were on average 55 years old and then again when they were on average 60.

WORD QUIZ

In the memory test, the participants had 20 words read to them, and then were asked to write down in 2 minutes all the words they could remember.

At age 55, those with low HDL cholesterol had a 27-percent higher risk of memory loss when compared to those with high HDL. At age 60, those with low HDL had a 53-percent higher risk of memory loss compared to those with high HDL levels.

The study did not track whether or not the people went on to develop dementia.

Low HDL was defined as less than 40 milligrams per deciliter (mg/dL) and high HDL was defined as at least 60 mg/dL in the study, published in the American Heart Association's journal Arteriosclerosis, Thrombosis and Vascular Biology.

Cholesterol is a fatty substance made naturally by the body and also found in many foods. High HDL levels can cut heart attack risk.

As opposed to low-density lipoprotein (LDL) cholesterol that can build up in artery walls, making them hard and narrow, HDL cholesterol takes excess cholesterol back to the liver.

LDL cholesterol is dubbed the "bad" cholesterol.

In the study, total levels of cholesterol and triglycerides

-- another type of fat found in the blood -- had no association with memory decline.

Singh-Manoux said the study did not look at the reasons that HDL cholesterol may protect memory. She said one possible explanation is that it wards off formation of beta-amyloid plaques in the brain that are a hallmark of Alzheimer's.

Previous studies have identified other early risk factors for Alzheimer's disease. In March, U.S. researchers found that having a big belly in middle age appears to greatly increase one's risk of developing Alzheimer's or another form of dementia decades later.

Mental Test Spots Alzheimer's Risk

TUESDAY, July 1 (HealthDay News)

A new questionnaire may help in both diagnosing older adults facing dementia and also in identifying individuals who need help with daily living.

The Everyday Cognition instrument consists of 39 questions to be answered by people who know the patient well.

"There have been a number of studies that show that people with mild cognitive impairment who have functional problems in addition to performing poorly on neuropsychological testing are more likely to progress in the near future," said study author Sarah Tomaszewski Farias, an assistant professor of neurology at the University of California, Davis, Medical Center in Sacramento. "One of our hopes is that this instrument will be able to help identify very early on those people at increased risk for developing Alzheimer's disease."
That would help both patients and family members prepare for what lies ahead and identify patients who need to be more closely monitored.

In addition, Farias said, the test would also help identify "people who are having [functional] problems so that we know who needs help and who doesn't."

"What's nice about this is that it is designed to pick up very early memory problems, and it's an entirely caregiver-based survey," said Dr. Scott Turner, incoming director of the Memory Disorders Program at Georgetown University Medical Center, in Washington, D.C. "This is something the caregiver can fill out, while the practitioner is looking at the patient. It could be used for screening, for diagnosis and for drug development, if you want to look for some proof that your drug is having some effect, so it has a lot of potential uses."

"They want something that they could use to ask a family member about the potential patient's everyday functioning to see if that's sensitive to picking up the likelihood of dementia early on," added Dr. Gary J. Kennedy, director of geriatric psychiatry at Montefiore Medical Center in New York City.

The findings were published in the July issue of Neuropsychology.

Existing neuropsychological tests tend to be very abstract. For the last 40 years, these tests have looked at two categories: so-called "basic" activities (such as grooming, feeding, dressing), which are affected in later stages of dementia, and "instrumental" activities of daily living (such as managing medication, finances, cooking, driving).

"I was interested in understanding how our neuropsychology tests translated into everyday problems, how our cognitive tests . . . translate into everyday problems that a person is experiencing and that a caregiver is concerned about," Farias explained.

Farias and her colleagues divided everyday functioning into seven cognitive "domains:" memory, language, semantic or factual knowledge, visual and spatial abilities, planning, organization and divided attention.

An original list of 138 items was eventually culled to 39, which was then tested in 576 older adults: 174 of whom were cognitively normal, 126 who had mild cognitive impairment (MCI), and 276 who had been diagnosed with dementia.

"Informants" (people who had known the patient for an average of almost 45 years) provided details on whether the patient could remember shopping items without a list, reading a map, balancing the checkbook, and cooking or working and talking at the same time.

Not only did the instrument confirm established diagnoses, it was also able to distinguish people with MCI from those with full-blown dementia, meaning it was able to pick up on subtle differences in function.

The results also weren't highly influenced by occupation and education levels, as are existing tests.

"This is really the first step in development the instrument," Farias said. "What we're really interested in doing is to track people over time to get a better understanding of the early signs of functional impairment."

More information

Visit the Alzheimer's Association for more on this condition.