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20.0 General

Below is a collection of terms and their definitions as provided by the forum members. Feel free to suggest additions or clarifications and don't forget about online resources such as »www.mpirical.com/companion.html or »www.mpirical.com/companion.html

802.11: A term often used for 802.11a, 802.11b, and 802.11g or for wireless in general. But it properly refers to a 2Mbps wireless protocol in the 2.4Ghz band which is no longer in wide use.

802.11b: A wireless protocol in the 2.4Ghz band which achieves 11Mbps raw throughput. 802.11b is the first technology to be widely implemented by home users and is still one of the most popular.

802.11g: A wireless protocol in the 2.4Ghz band which can provide up to 54Mbps raw throughput. 802.11g is designed and marketed as a faster direct replacement for 802.11b, and is backward compatible with it, though at some cost in performance.

802.11a: A wireless protocol in the 5.4Ghz and 5.8Ghz band which achieves 54Mbps raw throughput. 802.11a has less range than 802.11b or 802.11g, and is marketed to the office/commercial market. It is incompatible with 802.11b and 802.11g.

AP: An Access Point in its simplest form is essentially a wireless hub. It serves to allow wireless clients to connect to a wired LAN as well as to each other. An AP in Access Point Mode cannot wirelessly negotiate its own connection to the LAN; it must be hardwired to a switch or other node. Many access points can be configured for use as an Access Point, Bridge, Multi-point Bridge, Repeater, or Wireless Client.

Bridge: An AP mode in which two access points form a dedicated wireless link between two separate LAN segments. Bridging is commonly used to join two spatially separated networks within a limited proximity. When set to bridge mode, the AP cannot serve any wireless clients; its sole function becomes to complete the link with the other AP in bridge mode. Each AP in bridge mode must be wired to a switch or other network node.

Multi-point Bridge: A scenario in which three or more access points form a dedicated wireless link between themselves. Serves to wirelessly join separate LAN segments. Each access point must be set to multi-point bridge mode.

Repeater: An AP set to repeater mode serves to extend the range of a wireless network. The repeater receives a wireless signal from an access point and in turn broadcasts it out to wireless clients. Throughput for wireless clients of the repeater is cut in half.

Wireless Router: A wireless router combines the functions of an access point with those of a router. Thus it will have both a WAN and LAN interface. The WAN interface is most often used for connection to a broadband modem. The router uses NAT to allow the computers on the LAN to share the one WAN connection.

Client (Station): An AP in client mode serves to wirelessly connect a separate LAN segment to the rest of the LAN. Its function is similar to that of a bridge, however in this scenario, one AP is set to access point mode, and the other AP is set to client mode. This allows other wireless clients to also connect to the first AP.

Infrastructure Mode: A situation in which wireless adapters associate with an access point rather than directly with each other.

Ad-Hoc Mode: A situation in which two wireless adapters form a peer-to-peer network with each other. Useful for situations in which a network infrastructure is not available, or is not needed.

IBSS: Independent Basic Service Set. Same as an Ad-Hoc network. A direct connection between two wireless adapters.

dB: decibels. A measurement of power difference, defined so that increasing the power by a factor of 10 gives an increase of 10 dB.

dBm: decibels relative to one milliwatt. An absolute measurement of power, where 0dBm = 1 milliwatt. SO 10dBm = 10mw, 20dBm = 100mW, 30dBm = 1W.

Receive sensitivity: The power level below which a connection cannot be maintained. Typically expressed in dBm. This will often be qualified by a data rate -- to say that a wireless card has a receive sensitivity of -80dBm at 11Mbps means that if there is less signal available than -80dBm, an 11Mbps connection cannot be maintained.

Transmit power: The power level a wireless card emits. Usually expressed in either dBm or milliwatts. Typical values are 15dBm (~33mW), 100mW (20dBm), and 200mW(~23dBm). Note that transmit power is always in _positive_ dBm (greater than 1mW) whereas receive sensitivity is in _negative_ dBm (less than 1 mW)

SNR (Signal to Noise ratio: The ratio between the power level of the desired signal at the receiver, and the power level of noise (any undesired RF energy) at the reciever. This is usually expressed in decibels, and when expressed in decibels is the _difference_ between the signal and the noise.

AES: Advanced Encryption Standard. The United States Department of Commerce (DOC) Federal Information Processing Standard (FIPS) 197 standard to replace Data Encryption Standard (DES). AES is currently considered the most robust encryption algorithm available. 802.11 devices that support AES require faster and more current hardware technology (manufactured in or since 2003). When choosing to use AES, you must also choose a key exchange technology, such as WPA with a Radius Server, WPA-PSK, or TKIP.

TKIP: Temporal Key Integrity Protocol. A key exchange authentication technology drafted by the WiFi Alliance prior to ratifying the more complete 802.11i security standard. Implementing TKIP enhances the original Wired Equivalent Privacy (WEP) technology by by adding per-packet keying on all WEP encrypted data frames. Prior to TKIP, it was possible to break the WEP technology after collecting a large number of packets.

WPA: WiFi Protected Access. A key exchange authentication technology drafted by the WiFi Alliance prior to ratifying the more complete 802.11i security standard. Implementing WPA requires a RADIUS server to manage keys, which can then easily scale upwards to manage a large number of users.

WPA-PSK: WiFi Protected Access Pre-Shard Key. A key exchange authentication technology allowing the use of the AES encryption technology without requiring a RADIUS server. WPA-PSK is a viable solution to permit home users or small businesses the ability to implement AES or TKIP encryption technology.

Antenna: a device to transmit and/or receive electromagnetic wave (often referred to as radio waves). Antennas are resonant devices, which must be tuned to operate most efficiently at a given frequency. A single antenna can be constructed to operate on more than one frequency if required.

Omnidirectional Antenna: A omnidirectional antenna is designed to radiate and receive radio waves equally in all horizontal directions. Because of this, omnidirectional antennas typically have the lowest gain (or operational distance) when compared with other antenna types. An omnidirectional antenna with an increased gain is achieved by focusing the antenna's energy in a more narrow, donut-shaped pattern, decreasing it's effectiveness above and below the antenna. Most SOHO wireless networking products come standard with a low-gain omnidirectional antenna which will operate with a minimal amount of consideration given to the environment. Switching to a high-gain omnidirectional antenna may increase the effective gain or distance the product operate in a horizontal plane, while reducing it's effectiveness above and below this plane. For example, a 3 dB gain omnidirectional antenna may work better than a 5 dB gain omnidirectional antenna in a multi-story environment because it operates better above and below the horizontal plane. A 5 dB gain omnidirectional antenna may be best in a single-story environment because it will focus it's energy across the horizontal plane better than a 3dB gain omnidirectional antenna.

Yagi Antenna: A directional antenna, designed to radiate and receive radio waves in one focused direction, increasing the antenna's effectiveness in that direction. Different yagi antenna designs will determine how focused the beam width, or operational area, will be. Yagi antennas typically have a boom supporting a series of elements, which are spaced a precise distance apart, and precise lengths to cause the antenna to operate most efficiently for a given radio frequency. Wireless networking yagi antennas may have this appearance or may be contained in a long cylinder-like tube or pipe to protect it and make it easier to install. For wireless networking, yagi antennas are typically most effective for fixed, point-to-point installations, often between wireless access points. This antenna type gets it name from one of the Japanese inventors, Shuji Yagi.

Panel Antenna: A directional antenna, designed to radiate and receive radio waves in one general direction, increasing the antenna's effectiveness in that direction. Different panel antenna designs will determine how focused the beam width, or operational area, will be. Panel antennas, like their name implies, are often flat panel-like devices. Panel antennas are most useful when it is desired to have the operational area in one general direction as opposed to all directions (omnidirectional), but not to the degree that a yagi antenna would focus it. For wireless networking, a panel antenna may be useful when attaching it to a wireless router or access point that is placed off to one side of an operational area (e.g., one side of a home or office as opposed to centrally located). The beam width will be relatively wide (when compared with a yagi antenna beam width), so precise antenna aiming and device placement isn't necessary.

Antenna Polarization: The orientation of the electromagnetic waves radiating or being received by a given antenna. Antennas constructed with it's elements primarily in a vertical orientation will radiate and receive vertically-polarized electromagnetic waves most effectively, horizontally-polarized antennas will operate most effectively with horizontally-polarized electromagnetic waves. Environmental factors, such as metallic objects, can affect polarization of electromagnetic waves. Two similarly polarized antennas will inter-operate most effectively. For wireless networking products, it may be possible to increase effectiveness between devices by simply reorienting an antenna.

Antenna Gain: A term used to compare the effective range or distance of a given antenna. Gain is described in decibels (dB), with a higher dB gain antenna being more effective in it's designed radiation pattern.

by No_Strings See Profile edited by DrTCP See Profile
last modified: 2005-01-14 18:10:39

IEEE 802.11 is a standard for wireless network connectivity using either Frequency Hopping Spread Spectrum (FHSS) or Direct Sequence Spread Spectrum.

This term originally referred to communication speeds up to 2Mbps in 2.4Ghz band, but it is now often used it identify all 802.11 family of protocols such as 802.11b, 802.11a, 802.11g, etc. The MAC level specification that covers these procols can be downloaded here.

Related References:

by DrTCP See Profile
last modified: 2004-12-07 15:38:33

IEEE 802.11a is an extension of 802.11 using Direct Sequence Spread Spectrum (DSSS) in 5Ghz band. It supports multiple data rates up to 54Mbps.

The text of specification can be found here.

by DrTCP See Profile
last modified: 2004-05-10 20:00:56

IEEE 802.11b is a popular extension of 802.11 using Direct Sequence Spread Spectrum (DSSS) in 2.4Ghz band.

It supports multiple data rates: 11Mbps, 5.5Mbps, 2Mbps, 1Mbps

The text of specification can be found here.

Related References:

by DrTCP See Profile
last modified: 2004-05-10 20:21:33

IEEE 802.11g is a variant of IEEE 802.11 that is backwards compatible with 802.11b but implementing 802.11a like OFDM modulation in 2.4Ghz ISM band to achive raw speeds of 54Mbps.

The IEEE 802.11g protocol specification is available here.

Related References:

by DrTCP See Profile
last modified: 2004-05-10 23:25:51

By definition, if things conform to a standard designed for interoperability they will interoperate. However, since these speed "improvements" rely on proprietary extensions that are not defined by the standard, you should not expect interoperability between different proprietary implementations. Vendors hope to lock you into their brand with their proprietary improvements. In reality, you have to question whether you really need such proprietary features. For example, if you are mostly using your wireless connection to connect to the internet using normal broadband, the extra speed will not buy you anything. On the other hand, if you are moving files on your local network, it can make a significant difference. If you are a mobile user, the proprietary features are typically of no value since you have to be able to connect to access points from many different manufacturers.

Thanks to claudeo See Profile for the excellent answer to xxxin See Profile's question.

by No_Strings See Profile
last modified: 2004-10-21 01:43:40

dBi is "decibels relative to isotropic radiator" which means that the radiator (an antenna, usually) sends the signal equally strong in all directions, including above and below. Such a radiator would have 0 dBi.

(I have yet to see an antenna like that -- it may exist only in math theory.)

A higher dBi is obtained by designing the antenna so that the strength of the signal is more focused.

The little vertical 'rubber duck' antenna that you see on most wireless routers or Access Points (AP's) focusses the signal such that it is stronger from the sides of the antenna and weaker above and below. It is omni-directional. If the signals were visible, it would resemble a fat donut shape with the radiator in the middle. This focusing gives a relative strength of 2.15 dBi.

After-market vertical antennas get stronger dBi by narrowing the beam. The signals can be "heard" more from farther away, but less from above and below. If the signals were visible, it would look like a bigger donut, but with skinnier dough.

Directional antennas are designed to focus the beam such that it prefers one direction from the antenna. If the signals were visible, it would look like a bulb syringe with the radiator at the narrow-skinny end.

Choosing between directional or omni-directional usually has a lot to do with where the device is going to be in your home or office. If the wireless router or AP is in a corner near an outside wall, a directional antenna pointed in toward the bulk of your space is usually a better choice. But if it is near the center of the space, then omni-directional is probably the right answer.

editorial comment: This link adds some pictures and colors to funchords See Profile' very nice FAQ. »antennas.ee.duth.gr/TheLaborator···hort.htm

Feedback received on this FAQ entry:
  • So, does that mean I want a small 8 dbi instead of a large 12 dbi? You're not very clear on that.

    2012-11-12 10:50:36



by funchords See Profile edited by No_Strings See Profile
last modified: 2005-08-17 16:47:02

No.

There are several legal and ethical considerations, as well as security and performance reasons, all leading to the conclusion that one should not connect to a neighbor's network without permission.

In most jurisdictions, when you access a network or use a service (bandwidth, for example) belonging to another, you are committing a criminal act. In addition to laws governing computer-related crimes, you may also run afoul of laws concerning theft of utility services. While one can reasonably argue that these laws were often written to prevent corporate espionage and vandalism, that argument is one that would be made in court -- after an arrest and/or confiscation of your equipment.

Although there are some areas where the laws have not caught up to the technology, it is difficult to morally justify consuming someone else's internet service without permission. When criminal laws do not apply, it does not mean that civil laws or claims cannot result. And if the legal system can be avoided, it hardly keeps the peace of the neighborhood when one is found intruding on the property, services, and privacy of another.

An unsecure network probably means that other security precautions are lax. You can safely assume that most wireless networks are unencrypted due to the owner's unintentional failure to secure the network. When you access a home network, all of the machines (yours and theirs) are on the LAN side -- the more trusted side -- of the owner's firewall. As you may know, there is less protection against virus infection or unauthorized access between machines on the same network segment. You may find your computer under attack from a bug they caught earlier!

The performance of wireless networks depends on the ability to avoid collisions -- the simultaneous transmission of two or more network devices. Connecting to a wireless network usually means directly accessing only the Access Point or wireless router. When you use a neighbor's connection, you will likely be the most distant station on the wireless network. Your station may not be "visible" to the other stations on the network; as a result, your hidden station may transmit simultaneously with another station on the network. This interference will reduce the performance of the overall network.

Because of all of these reasons -- legal, moral, security, and performance -- it is generally inadvisable to use a neighbor's network without permission.

by funchords See Profile edited by adsldude See Profile
last modified: 2005-08-28 09:10:41

Wi-Fi is designed to share the bandwidth of the other networks around yours (within 20 MHz, or 5 channels width).

Some users expect that by putting a wi-fi client in the same room as the Access Point (AP), that it will override the other nearby signals and communicate faster. This is not the case.

Generally, Wi-Fi equipment will not transmit over other wi-fi equipment. Instead, the protocols are designed so that all of the users on the frequency have an opportunity to transmit.

This does suggest a problem that often occurs. If two Wi-Fi stations are transmitting a lot of data (such as transferring a large file), and one cannot hear the signals of the other, then interference occurs when both stations try to transmit simultaneously. You can mitigate this problem by selecting different channels, moving the physical location of one of the stations, or upgrading the antenna.

Related article: /faq/12308



by funchords See Profile edited by mjf See Profile
last modified: 2005-11-13 19:22:54

It depends. When communicating from a wired device to a wireless device, a good rule of thumb is that you can expect to average roughly a third of the datarate. That may surprise you, or disappoint you, but it is true of most two-way wireless networks including Wi-Fi networks. The difference in throughput is because of the nature of the technology, network protocols, and radio transmissions.

For example, suppose you have a desktop computer that is connected by Ethernet cable to a 802.11g "54 Mbps" wireless router. When sending a large file to a wireless laptop, you can expect roughly 18 Mbps of actual throughput.

Similarly, suppose you have a 1XRTT network-capable wireless telephone and are attempting to connect to a video stream. Although it reports that you are connected at 230.4 Kbps, you can expect throughput of roughly 76.8 Kbps.

If your transfer is taking multiple wireless "hops," such as from a wireless computer to an AP, and then again from the AP to your computer, expect to cut the throughput by roughly half again.

I emphasize roughly because it can be considerably higher or lower depending upon...
    •The strength and clarity of the wireless signals
    •The interference caused by non-WiFi devices that share the same radio frequencies as the wireless network
    •The activity of neighboring WiFi devices that are sharing the radio frequency
    •The number and activity of clients in your own network
    •The efficiency of the transfer protocol that you are using
    •Any proprietary enhancements you might be using in your network (such as those that purport to go up to 108 Mbps or 125 Mbps on an 802.11g device)
    •The output speed of the device from which you are downloading the file
    •Other activity on your local computer or the sending computer
    •Other factors

This is not an evil plot by the networking manufacturers (although a little marketing is certainly involved). Network devices are always described by their maximum data rate (signalling) rate -- the speed at which a network device can send or detect the electrical pulses (1's and 0's). But because of all the factors above, the maximum throughput of any network device is always going to be less.

If you're still awake at this point, here are some terms that will make you sound smart:
    •The port speed is what the hardware and driver reports to the Operating System (OS). You can think of this as the maximum data rate (sometimes spelled datarate). The port speed is also the speed that the OS will talk to the driver. While it usually stays at the maximum, the driver may adjust the port speed lower if it detects errors when communicating to the OS. An example of this is when a USB 2.0 network device is inserted into a USB 1.1 connection. In several versions of Windows, when you hover over the icon in the system tray for your wireless network card (the Windows icon, not the manufacturer's icon), you will see the port rate in a pop-up window.

    The Windows icon looks like this:

    •The data rate is the current signalling rate of the wireless transmitters and receivers. In 802.11, and other protocols, this rate often varies up and down along a set of agreed-upon supported rates. This increase and decrease is based on the error rate of the wireless link. If the errors are increasing, then the link is slowed down (because slower packets are easier to decipher). Many of the Wi-Fi utilities provided by the manufacturers will report both the TX and RX rate in their monitoring utilities, You often see that they are somewhere below the port speed.

    •Finally, the throughput is the rate of the delivery of the actual cargo of the network -- your desired data. This is what really matters to you! As a rule, the faster the data rate, the faster the throughput unless there are too many errors. Errors cause retransmissions, which takes bandwidth. It is more efficient to slow the datarate to the point where fewer errors occur. (This is why it's a bad idea to lock the Transmit rate or "TX Rate" instead of setting it to automatically adjust.)


Feedback received on this FAQ entry:
  • Another note, the advertised data rate (example: 802.11g maximum 54mbps)is going to concur some lose due to management and control frames from the AP and wireless client (vice/versus). In a clean environment/best case scenario, you can expect 20mbps-30mbps, using the example of 802.11g.

    2008-11-23 01:48:06



by funchords See Profile edited by No_Strings See Profile
last modified: 2005-12-26 19:04:48

An excellent answer to this question is found here:

»Networking Forum FAQ »What is the relationship between dBm and milliwatts

by funchords See Profile edited by mjf See Profile
last modified: 2006-01-11 15:00:35

Multiple-input and multiple-output is a method for increasing bandwidth and link reliability in WiFi systems. As is the case with many technologies, there are also multiple implementations. Marketing terms can sometimes confuse: N300, N450, two antennas, three?

This paper attempts to break down the concepts and provide the reader with a better understanding of MIMO.

»djw.cs.washington.edu/papers/mim···mies.pdf

by No_Strings See Profile