Black Box Explains...Wireless Ethernet standards.
The precursor to 802.11b, IEEE 802.11 was introduced in 1997. It was a beginning, but 802.11 only supported speeds up to 2 Mbps. And it supported two entirely different... more/see it nowmethods of encodingFrequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). This led to confusion and incompatibility between different vendors equipment.
802.11b is comfortably established as the most popular wireless standard. With the IEEE 802.11b Ethernet standard, wireless is fast, easy, and affordable. Wireless devices from all vendors work together seamlessly. 802.11b is a perfect example of a technology that has become both sophisticated and standardized enough to really make life simpler for its users.
The 802.11b extension of the original 802.11 standard boosts wireless throughput from 2 Mbps all the way up to 11 Mbps. 802.11b can transmit up to 200 feet under good conditions, although this distance may be reduced considerably by the presence of obstacles such as walls.
This standard uses DSSS. With DSSS, each bit transmitted is encoded and the encoded bits are sent in parallel across an entire range of frequencies. The code used in a transmission is known only to the sending and receiving stations. By transmitting identical signals across the entire range of frequencies, DSSS helps to reduce interference and makes it possible to recover lost data without retransmission.
The 802.11a wireless Ethernet standard is new on the scene. It uses a different band than 802.11b—the 5.8-GHz band called U-NII (Unlicensed National Information Infrastructure) in the United States. Because the U-NII band has a higher frequency and a larger bandwidth allotment than the 2.4-GHz band, the 802.11a standard achieves speeds of up to 54 Mbps. However, its more limited in range than 802.11b. It uses an orthogonal frequency-division multiplexing (OFDM) encoding scheme rather than FHSS or DSSS.
802.11g is an extension of 802.11b and operates in the same 2.4-GHz band as 802.11b. It brings data rates up to 54 Mbps using OFDM technology.
Because it's actually an extension of 802.11b, 802.11g is backward-compatible with 802.11b—an 802.11b device can interface directly with an 802.11g access point. However, because 802.11g also runs on the same three channels as 802.11b, it can crowd already busy frequencies.
Super G® is a subset of 802.11g and is a proprietary extension of the 802.11g standard that doubles throughput to 108 Mbps. Super G is not an IEEE approved standard. If you use it, you should use devices from one vendor to ensure compatibility. Super G is generally backwards compatible with 802.11g.
80211n improves upon 802.11g significantly with an increase in the data rate to 600 Mbps. Channels operate at 40 MHz doubling the channel width from 20 MHz. 802.11n operates on both the 2.4 GHz and the 5 GHz bands. 802.11n also added multiple-input multiple-output antennas (MIMO).
Multiple-Input/Multiple-Output (MIMO) is a part of the new IEEE 802.11n wireless standard. It’s a technique that uses multiple signals to increase the speed, reliability, and coverage of wireless networks. It transmits multiple datastreams simultaneously, increasing wireless capacity to up to 100 or even 250 Mbps.
This wireless transmission method takes advantage of a radio transmission characteristic called multipath, which means that radio waves bouncing off surfaces such as walls and ceilings will arrive at the antenna at fractionally different times. This characteristic has long been considered to be a nuisance that impairs wireless transmission, but MIMO technology actually exploits it to enhance wireless performance.
MIMO sends a high-speed data stream across multiple antennas by breaking it into several lower-speed streams and sending them simultaneously. Each signal travels multiple routes for redundancy.
To pick up these multipath signals, MIMO uses multiple antennas and compares signals many times a second to select the best one. A MIMO receiver makes sense of these signals by using a mathematical algorithm to reconstruct the signals. Because it has multiple signals to choose from, MIMO achieves higher speeds at greater ranges than conventional wireless hardware does. collapse
SHDSL, VDSL, VDSL2, ADSL, and SDSL.
xDSL, a term that encompasses the broad range of digital subscriber line (DSL) services, offers a low-cost, high-speed data transport option for both individuals and businesses, particularly in areas without... more/see it nowaccess to cable Internet.
xDSL provides data transmission over copper lines, using the local loop, the existing outside-plant telephone cable network that runs right to your home or office. DSL technology is relatively cheap and reliable.
SHDSL can be used effectively in enterprise LAN applications. When interconnecting sites on a corporate campus, buildings and network devices often lie beyond the reach of a standard Ethernet segment. Now you can use existing copper network infrastructure to connect remote LANS across longer distances and at higher speeds than previously thought possible.
There are various forms of DSL technologies, all of which face distance issues. The quality of the signals goes down with increasing distance. The most common will be examined here, including SHDSL, ADSL, and SDSL.
SHDSL (also known as G.SHDSL) (Single-Pair, High-Speed Digital Subscriber Line) transmits data at much higher speeds than older versions of DSL. It enables faster transmission and connections to the Internet over regular copper telephone lines than traditional voice modems can provide. Support of symmetrical data rates makes SHDSL a popular choice for businesses for PBXs, private networks, web hosting, and other services.
Ratified as a standard in 2001, SHDSL combines ADSL and SDSL features for communications over two or four (multiplexed) copper wires. SHDSL provides symmetrical upstream and downstream transmission with rates ranging from 192 kbps to 2.3 Mbps. As a departure from older DSL services designed to provide higher downstream speeds, SHDSL specified higher upstream rates, too. Higher transmission rates of 384 kbps to 4.6 Mbps can be achieved using two to four copper pairs. The distance varies according to the loop rate and noise conditions.
For higher-bandwidth symmetric links, newer G.SHDSL devices for 4-wire applications support 10-Mbps rates at distances up to 1.3 miles (2 km). Equipment for 2-wire deployments can transmit up to 5.7 Mbps at the same distance.
SHDSL (G.SHDSL) is the first DSL standard to be developed from the ground up and to be approved by the International Telecommunication Union (ITU) as a standard for symmetrical digital subscriber lines. It incorporates features of other DSL technologies, such as ADSL and SDS, and is specified in the ITU recommendation G.991.2.
Also approved in 2001, VDSL (Very High Bitrate DSL) as a DSL service allows for downstream/upstream rates up to 52 Mbps/16 Mbps. Extenders for local networks boast 100-Mbps/60-Mbps speeds when communicating at distances up to 500 feet (152.4 m) over a single voice-grade twisted pair. As a broadband solution, VDSL enables the simultaneous transmission of voice, data, and video, including HDTV, video on demand, and high-quality videoconferencing. Depending on the application, you can set VDSL to run symmetrically or asymmetrically.
VDSL2 (Very High Bitrate DSL 2), standardized in 2006, provides a higher bandwidth (up to 30 MHz) and higher symmetrical speeds than VDSL, enabling its use for Triple Play services (data, video, voice) at longer distances. While VDSL2 supports upstream/downstream rates similar to VDSL, at longer distances, the speeds don’t fall off as much as those transmitted with ordinary VDSL equipment.
ADSL (Asymmetric DSL) provides transmission speeds ranging from downstream/upstream rates of 9 Mbps/640 kbps over a relatively short distance to 1.544 Mbps/16 kbps as far away as 18,000 feet. The former speeds are more suited to a business, the latter more to the computing needs of a residential customer.
More bandwidth is usually required for downstream transmissions, such as receiving data from a host computer or downloading multimedia files. ADSLs asymmetrical nature provides more than sufficient bandwidth for these applications.
The lopsided nature of ADSL is what makes it most likely to be used for high-speed Internet access. And the various speed/distance options available within this range are one more point in ADSLs favor. Like most DSL services standardized by ANSI as T1.413, ADSL enables you to lease and pay for only the bandwidth you need.
SDSL (Symmetric DSL) represents the two-wire version of HDSL—which is actually symmetric DSL, albeit a four-wire version. SDSL is also known within ANSI as HDSL2.
Essentially offering the same capabilities as HDSL, SDSL offers T1 rates (1.544 Mbps) at ranges up to 10,000 feet and is primarily designed for business applications.
Black Box Explains...vDSL.
VDSL (Very High Bit-Rate Digital Subscriber Line or Very High-Speed Digital Subscriber Line) is a “last-mile” broadband solution for both businesses and homes, providing economical, high-speed connections to fiber optic... more/see it nowbackbones.
VDSL enables the simultaneous transmission of voice, data, and video on existing voice-grade copper wires. Depending on the intended applications, you can set VDSL to run symmetrically or asymmetrically. VDSL’s high bandwidth allows for applications such as high-definition television, video-on-demand (VOD), high-quality videoconferencing, medical imaging, fast Internet access, and regular voice telephone services—all over a single voice-grade twisted pair. The actual VDSL distances you achieve vary based on line rate, gauge and type of wire, and noise/crosstalk environment.
Black Box Explains...Power over Ethernet (PoE).
What is PoE?
The seemingly universal network connection, twisted-pair Ethernet cable, has another role to play, providing electrical power to low-wattage electrical devices. Power over Ethernet (PoE) was ratified by the... more/see it nowInstitute of Electrical and Electronic Engineers (IEEE) in June 2000 as the 802.3af-2003 standard. It defines the specifications for low-level power delivery—roughly 13 watts at 48 VDC—over twisted-pair Ethernet cable to PoE-enabled devices such as IP telephones, wireless access points, Web cameras, and audio speakers.
Recently, the basic 802.3af standard was joined by the IEEE 802.3at PoE standard (also called PoE+ or PoE plus), ratified on September 11, 2009, which supplies up to 25 watts to larger, more power-hungry devices. 802.3at is backwards compatible with 802.3af.
How does PoE work?
The way it works is simple. Ethernet cable that meets CAT5 (or better) standards consists of four twisted pairs of cable, and PoE sends power over these pairs to PoE-enabled devices. In one method, two wire pairs are used to transmit data, and the remaining two pairs are used for power. In the other method, power and data are sent over the same pair.
When the same pair is used for both power and data, the power and data transmissions don’t interfere with each other. Because electricity and data function at opposite ends of the frequency spectrum, they can travel over the same cable. Electricity has a low frequency of 60 Hz or less, and data transmissions have frequencies that can range from 10 million to 100 million Hz.
There are two types of devices involved in PoE configurations: Power Sourcing Equipment (PSE) and Powered Devices (PD).
PSEs, which include end-span and mid-span devices, provide power to PDs over the Ethernet cable. An end-span device is often a PoE-enabled network switch that’s designed to supply power directly to the cable from each port. The setup would look something like this:
End-span device → Ethernet with power
A mid-span device is inserted between a non-PoE device and the network, and it supplies power from that juncture. Here is a rough schematic of that setup:
Non-PoE switch → Ethernet without PoE → Mid-span device → Ethernet with power
Power injectors, a third type of PSE, supply power to a specific point on the network while the other network segments remain without power.
PDs are pieces of equipment like surveillance cameras, sensors, wireless access points, and any other devices that operate on PoE.
PoE applications and benefits.
Use one set of twisted-pair wires for both data and low-wattage appliances.
In addition to the applications noted above, PoE also works well for video surveillance, building management, retail video kiosks, smart signs, vending machines, and retail point-of-information systems.
Save money by eliminating the need to run electrical wiring.
Easily move an appliance with minimal disruption.
If your LAN is protected from power failure by a UPS, the PoE devices connected to your LAN are also protected from power failure.
IEEE 802.3 af
|PoE IEEE 802.3 at
|Power available at powered device
|Maximum power delivered
|Voltage range at powred source
|Voltage range at powred device
|Maximum cable resistance