[Asus  Eee Pad Transformer Prime] demandarin:
Some myths on Wifi interference: Cisco provided

Myth #11: "Wi-Fi interference doesn't happen very often."

There is a growing body of evidence that points to the fact that Wi-Fi interference is an extremely common and troublesome issue. Here are a few recent examples:

• The technical support engineers at a major Wi-Fi infrastructure vendor reported to Cisco that in a recent service call to a major customer they found almost 20 sources of interference, contributing to over 50 percent of the problems on the customer's Wi-Fi network.

• The manager of a large group of outsourced wireless service representatives stated to Cisco that "one out of every three Wi-Fi problems our service technicians get called out for is related to interference."

• In a recent survey of 300 of their customers, a major Wi-Fi tools provider reported that "troubleshooting interference won `top honors' as the biggest challenge in managing a Wi-Fi network."

• Jupiter Research reports 67 percent of all residential Wi-Fi problems are linked to interfering devices, such as cordless phones, baby monitors, and microwave ovens.

Summary: There's no point burying your head in the sand: Wi-Fi interference happens.

Myth #12: "I should look for interference only after ruling out other problem sources."

In any networking system, it's critical that the physical layer is solid. When the physical layer is not operating properly, the higher protocol layers tend to operate in inefficient and sometimes confusing ways. For this reason, it always makes sense to verify your physical layer first before going on a wild-goose chase looking higher layer problems.

As an analogy, when you hook your computer up to an Ethernet cable and the network does not appear to be working, your first diagnostic step is to look at the lights on the side of your Ethernet adapter. If the lights are not on, there is no point looking for a subtle network configuration problem: you simply don't have physical layer connectivity.

The potential for physical layer problems with Wi-Fi is much worse than with Ethernet. With an Ethernet cable, you worry about the physical-layer connectivity issue only the first time you plug in the cable. If the connection was working that first day, it's reasonable to expect it will keep working day after day. But in the RF world, the quality of the physical connection can change hour by hour, as people introduce other devices or obstructions into the environment.

Summary: Avoid wasting your time. Fix your RF physical layer first.

Myth #13: "There's nothing I can do about interference if I find it."

The most common cure for interference is simply to replace or remove the offending interference device. For instance, you might replace an old leaky microwave oven or a 2.4-GHz cordless headset used by the receptionist with a different model that operates in a non-Wi-Fi frequency band. Many times interference is caused unwittingly by well-intentioned employees. One Wi-Fi administrator found an employee who sat with his back to his door, and had brought in a wireless camera so he could see behind him. Unfortunately, it operated at 2.4GHz. In this case, a policy was created to ban these types of devices on the campus.

Another solution is to work around the interference device by moving the affected access point, or changing its operating channel to a frequency that is not impacted by the interfering device. This is simple once you understand the location and frequency parameters of the interfering device. Note that because some devices frequency-hop (for example, Bluetooth devices) it's not always possible to change channels and eliminate the interference.

A final cure is to move or shield the offending device. For example, in a hospital, a piece of equipment that causes RF interference might be isolated to a particular room where Wi-Fi network access is not critical. If that's not possible, adding electromagnetic interference (EMI) shielding can limit propagation of the interference to a small area. You can implement shielding with grounded mesh or foils in the walls (essentially Faraday cages) or with insulating foams or paints.

Summary: There's always a cure for interference, but you need to know what's ailing you.

Myth #14: "There are just a few easy-to-find devices that can interfere with my Wi-Fi."

With the huge proliferation of wireless devices in the unlicensed band, it is no longer obvious what might be a source of interference-wireless links are now embedded in watches, shoes, MP3 players, and many other tiny consumer devices.

In some cases, previously benign devices have been updated with RF technology. Motion detectors, which appear in many offices for lighting control, are a good example. A new breed of hybrid motion detectors uses a combination of passive infrared sensor (PIR) and 2.4-GHz radar to detect motion. These devices, which look identical to their benign predecessors, generate significant interference that can disrupt your Wi-Fi network.

Unintentional emitters are also hard to find. A defective ballast on a fluorescent light fixture can generate broadband RF interference that can impact Wi-Fi. This is impossible to identify by simply looking at the device. "Hidden devices" are becoming more common as well. We have seen numerous instances where a security group has hidden wireless cameras-unbeknownst to the networking group-not realizing that they are jamming the Wi-Fi network

---------- Post added at 01:24 PM ---------- Previous post was at 01:19 PM ----------

For Wifi/BT:

Will Bluetooth and Wi-Fi signals interfere, since they share the 2.4 GHz spectrum?

Interference can occur when radio signals occupy similar frequencies at the same time and place.

Therefore, it seems logical to expect interference when Bluetooth and Wi-Fi are used together.

However, such interference is not absolute and can be virtually eliminated with proper preventive measures. For example, in the unlicensed 900 MHz band, a wide variety of devices share similar frequencies without interference problems, including cordless phones, garage door openers, and baby monitors.

---------- Post added at 01:28 PM ---------- Previous post was at 01:24 PM ----------

Even more detailed here: A scientific research study and look at Wifi/BT interference

http://www.ce-mag.com/archive/01/05/lansford.html

Wi-Fi and Bluetooth: Enabling Coexistence

Jim Lansford, Ron Nevo, and Brett Monello

The coexistence of Bluetooth and Wi-Fi (IEEE 802.11b) requires particular attention to simultaneous operation of both systems in very close proximity.

The 2.4 GHz Industrial, Scientific, and Medical (ISM) band is poised for strong growth. Fueling this growth are two emerging wireless technologies: wireless personal-area networking (WPAN) and wireless local-area networking (WLAN). The dominant WPAN technology is a short-range wireless technology called Bluetooth. Cahners In-Stat (Scottsdale, AZ) predicts the market for Bluetooth devices will reach 800 million units by 2004.1

Designed principally for cable-replacement applications, most Bluetooth implementations support a range of up to ~10 m and speeds of up to 700 Kb/sec for data or isochronous voice transmission. Bluetooth is ideal for applications such as wireless headsets, wireless synchronization of personal digital assistants (PDAs) with computers, and wireless peripherals such as printers or keyboards.

WLAN has several technologies competing for dominance; however, based on current market momentum, it appears that Wi-Fi (IEEE 802.11b) will prevail. Wi-Fi offers speeds of 11 Mb/sec and covers a range of up to 100 m. With WLANs, applications such as Internet access, e-mail, and file sharing can now be implemented with new levels of freedom and flexibility. The market for WLAN shipments is predicted to reach more than 5 million units in 2004, implying an installed base of nearly 20 million systems.2

WPAN and WLAN are complementary rather than competing technologies. Moreover, with both expecting rapid growth, collocation of Bluetooth and Wi-Fi devices will become increasingly likely. Because both technologies occupy the 2.4 GHz frequency band, there is potential for interference between the two technologies. Coexistence of the two technologies has become a key topic for analysis and discussion throughout the industry. In this article, the term coexistence means that the wireless systems can be collocated without significantly affecting the performance of any device.3

Previous Work

Analysis of interference between Wi-Fi and Bluetooth devices is not new, as indicated by the literature such as Kamerman.4 Early attempts to quantify the mutual interference effects have been based on simple geometric models of Bluetooth deployment rather than actual usage models.5,6 In Ennis, the investigation focused on the problem of calculating the probability of an overlap, in both time and frequency, of a continuous sequence of Bluetooth packets and an IEEE 802.11b direct sequence 11-Mb/sec packet.7 Relative power levels between the Bluetooth and IEEE 802.11b packets were not considered. Zyren and others made several refinements on previous assumptions.8–10 These efforts, however, did not examine in detail the ramifications of the physical (PHY) layer such as hopping, spectral masks, and filter selectivity, nor did they discuss implementation. In addition, the geometries studied did not necessarily correspond to practical usage models.

Results presented by Golmie modeled both PHY and medium access controller (MAC) behaviors. Such modeling is necessary to predict performance accurately. This article uses some of the same approaches to modeling, providing a firm foundation from which to evaluate the effectiveness of interference-reduction techniques.

Interference Basics

With characteristics such as low cost, modest speed, and short range (<10 m), Bluetooth was designed as a cable-replacement radio-frequency (RF) technology. Bluetooth can support piconets of up to eight active devices, with a maximum of three synchronous-connection-oriented (SCO) links. SCO links are designed to support real-time, isochronous applications such as cordless telephony or headsets. Bluetooth also supports asynchronous connection links (ACLs) that are used to exchange data in non-time-critical applications. The Bluetooth PHY layer uses frequency-hopping spread spectrum (FHSS) at a rate of 1600 hops/sec and Gaussian frequency shift keying (GFSK) modulation. The majority of Bluetooth devices transmit at a power level of about 1 mW (0 dBm) with a raw data rate of 1 Mb/sec.

Like Ethernet, Wi-Fi supports true multipoint networking with such data types as broadcast, multicast, and unicast packets. The MAC address built into every device allows a virtually unlimited number of devices to be active in a given network. These devices contend for access to the airwaves using a scheme called carrier sense multiple access with collision avoidance (CSMA/CA). The Wi-Fi physical layer uses direct-sequence spread spectrum (DSSS) at four different data rates using a combination of differential binary phase-shift keying (DBPSK) for 1 Mb/sec, differential quaternary phase-shift keying (DQPSK) for 2 Mb/sec, and QPSK/complementary code keying (CCK) for the higher speeds: 5.5 and 11 Mb/sec. The RF power level can vary, but is typically between 30 and 100 mW (up to 20 dBm) in most commercial WLAN systems.

Sharing the Same Frequency Band

Wireless communication systems use one or more carrier frequencies (frequency bands) to communicate. Bluetooth and Wi-Fi share the same 2.4 GHz band, which under Federal Communications Commission (FCC) regulations, extends from 2.4 to 2.4835 GHz. Under the ISM band rules defined in FCC Part 15.247, this frequency band is free of tariffs.11 It is license exempt in Europe. However, systems must operate under certain constraints that are supposed to enable multiple systems to coexist in time and place.

A system can use one of two methods to transmit in this band; both are spread-spectrum techniques. Frequency-hopping spread spectrum (FHSS) enables a device to transmit high energy in a relatively narrow band, but for a limited time. Direct-sequence spread spectrum (DSSS) allows a device to occupy a wider bandwidth with relatively low energy in a given segment of the band, and it does not hop.

As discussed earlier, Bluetooth selected FHSS, using 1-MHz-wide channels and a hop rate of 1600 hops/sec (625 microseconds in every frequency channel). Bluetooth uses 79 different channels in the United States and most of the rest of the world. IEEE 802.11b (Wi-Fi) opted for DSSS, using 22 MHz of bandwidth (passband) to transmit data with speeds of up to 11 Mb/sec. A Wi-Fi system can use any of 11 22-MHz-wide subchannels across the allocated 83.5 MHz of the 2.4 GHz frequency band. A maximum of three Wi-Fi networks can coexist without interfering with one another. Geographies outside of the United States may support more or fewer than 11 selectable subchannels. However, regardless of the portion of the band in which Wi-Fi operates, sharing with Bluetooth is inevitable. Two wireless systems using the same frequency band would have a high propensity to interfere with each other.

Bluetooth and Wi-Fi Interference Cases

If Bluetooth and Wi-Fi operate at the same time in the same place, they will interfere (collide) with each other. Specifically, these systems transmit on overlapping frequencies, creating in-band colored noise for one another. The sidebands of each transmission must also be accounted for. Interference between Bluetooth and Wi-Fi occurs when either of the following is true:

An Wi-Fi receiver senses a Bluetooth signal at the same time a Wi-Fi signal is being sent to it. The effect is most pronounced when the Bluetooth signal is within the 22-MHz-wide passband of the Wi-Fi receiver.

A Bluetooth receiver senses a Wi-Fi signal at the same time a Bluetooth signal is being sent to it; the effect is most pronounced when the Wi-Fi signal is within the passband of the Bluetooth receiver.

It is worthwhile to note that neither Bluetooth nor Wi-Fi was designed with specific mechanisms to combat the interference that each creates for the other. As a fast frequency-hopping system, Bluetooth assumes that it will hop away from bad channels, minimizing its exposure to interference. The Wi-Fi MAC layer, which is based on the Ethernet protocol, assumes that many stations share the same medium, and therefore, if a transmission fails, it is because two Wi-Fi stations tried to transmit at the same time. Later, this article examines how this assumption drives system behavior that actually worsens the impact of Bluetooth interference.

Interference Measurements

It is important to understand a few simple measurements that demonstrate the potential magnitude of the interference

Figure 1. Geometry of measurement environment.

issue in scenarios that involve simultaneous operation.

Figure 1. Geometry of measurement environment.

Measurement Environment. A geometric arrangement of Bluetooth and Wi-Fi devices (shown in Figure 1) was tested. The configuration was intended to be representative of a laptop (a device which needs simultaneous operation and collocation) equipped with collocated Wi-Fi and Bluetooth interacting simultaneously with a Wi-Fi access point and another Bluetooth node. Ganymede's Chariot, a LAN traffic excitation program used by the Wireless Ethernet Compatibility Alliance (WECA) for certification of IEEE 802.11b interoperability, was used to measure Wi-Fi throughput.

In this test, Chariot drove IEEE 802.11b RoamAbout PC cards (Cabletron Systems) with Ethernet packet sizes of 1500 bytes (typical for such data-transfer applications). Data transfer was from LT1 to LT2, and RF output power was 30 mW. Measurements were captured at varying distances from LT1 to LT2. The distance between the collocated Wi-Fi and Bluetooth (LT2 to BT1) was fixed at 10 cm, and the second Bluetooth node (BT2) was located 1 m from LT2. The 10-cm spacing is less than the maximum achievable inside a laptop, but greater than what could be achieved in a single PC card. The two Bluetooth nodes were laptops with Digianswer PC cards that ran DH5 data transfers from BT1 to BT2 at an RF power output of 1 mW.

Measurement Results. Figure 2 presents two sets of measurement results that capture Wi-Fi throughput as a function of received signal strength at the Wi-Fi station (LT2). The diamond line shows Wi-Fi performance with all Bluetooth devices

Figure 2. Measured throughput of Wi-Fi in the presence of Bluetooth (IEEE 802.11b Mb/sec versus received signal).

turned off (baseline). The square line shows Wi-Fi throughput with Bluetooth active.

Figure 2. Measured throughput of Wi-Fi in the presence of Bluetooth (IEEE 802.11b Mb/sec versus received signal).

Ultimately, it is important to understand the effect on Wi-Fi performance over distance. However, given that the effect of distance is highly dependent upon the actual physical environment, received signal strength indicator (RSSI) was selected. In this way, results could be extended across environments by making simpler physical energy-level measurements rather than having to reproduce and execute full test suites. Table I shows the received signal strengths and distances for a particular office environment with cubicles. Using RSSI enables interpretation of data without having to account for the effects of selective fading.

In the scenarios measured, even Wi-Fi stations with <5–7 m of free space from their access point suffer >25% degradation in throughput. This degradation exceeds 50% by the 30-m mark. In an office environment with cubicles, the range associated with each throughput level would be reduced significantly. When cubicles must be penetrated, Wi-Fi loses nearly one-third of its expected throughput within the first couple of meters. Erosion of performance exceeds 50% with stations <8 m from their access point. The focus of this particular experiment was on Bluetooth's effect on Wi-Fi. However, subsequent simulations demonstrate that although the effect of Wi-Fi on Bluetooth is not as deleterious as that of Bluetooth on Wi-Fi, coexistence issues can substantially degrade Bluetooth voice performance