ADSL'S Technological blur

ADSL is abbreviation of Asymmetric digital Subscriber Line, one technology which memungkinan high speed data is transferred through telephone wire. ADSL enables to accept data until speed 1.5 9Mbps (downstream's speed) and transfers data at a speed 16 640Kbps (upstream's speed).

ADSL divides frequency of thread that utilized by assumption a large part Internet user will more a lot of take (download) data of Internet than transferring (upload) to Internet. Therefore, data speed of ordinary Internet around three until four speed time go to Internet. Since upstream's speed and unegual downstream utilized by Asymmetric's terminology.

There are several reference applicabling to knows deeper about technology ADSL, severally among those is,

http://www. dslforum. org / is index. shtml is ADSL'S Forum

http://electronics. howstuffworks. com / dsl. htm How DSL'S Ways Of Working

http://www. cs. tut. fi / tlt / stuff / adsl / pt_adsl. html Intro goes to ADSL'S technology

http://www. rhapsodyk. net / adsl / HOWTO / ADSL at Linux

http://www. kitz. co. uk / adsl / adsl. htm is ADSL'S Encyclopaedia

[edit] Severally ADSL'S gain

* You can most interlocking go to Internet, and regular gets to utilize telephone to accept / rings.

* Speed much higher of ordinary modem.

* Not necessarily new telephone wire, ADSL enables mengggunakan aught telephone wire.

* Severally ISP ADSL will give ADSL'S modem as part of installation.

[edit] Severally disadvantaged ADSL.

* Thread ADSL will work perfectly if our location last close to sentral rings. At least in distance 2 3 cable disquisition km usually enough safe to be utilized ADSL until 8Mbps's vicinity speed. DSL'S technology that can just send to get at a speed very tall s / d. 100Mbps, obviously to very short distance.

* ADSL'S thread faster to accept data than transfers data via Internet.

* Old copper cable gets to down thread quality and downs speed.

* Upon rainy season, water really bother telephone wire quality. Evenless if flooding and menenggelamkan is Telephone Wire House, at secures will add cable attenuation and will reduce ADSL'S thread quality.

* Service ADSL not is at no region telephone wire.

ADSL

Short for asymmetric digital subscriber line, a new technology that allows more data to be sent over existing copper telephone lines (POTS). ADSL supports data rates of from 1.5 to 9 Mbps when receiving data (known as the downstream rate) and from 16 to 640 Kbps when sending data (known as the upstream rate).

ADSL requires a special ADSL modem.

ADSL is growing in popularity as more areas around the world gain access.

Asymmetric digital subscriber line (ADSL)

Asymmetric digital subscriber line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice telephone call.^[1] A splitter - or microfilter - allows a single telephone connection to be used for both ADSL service and voice calls at the same time. ADSL can generally only be distributed over short distances from the central office, typically less than 4 kilometres (2 mi),^[2] but has been known to exceed 8 kilometres (5 mi) if the originally laid wire gauge allows for farther distribution.

At the telephone exchange the line generally terminates at a DSLAM where another frequency splitter separates the voice band signal for the conventional phone network. Data carried by the ADSL is typically routed over the telephone company's data network and eventually reaches a conventional internet network.

Explanation

The distinguishing characteristic of ADSL over other forms of DSL is that the volume of data flow is greater in one direction than the other, i.e. it is asymmetric. Providers usually market ADSL as a service for consumers to connect to the Internet in a relatively passive mode: able to use the higher speed direction for the "download" from the Internet but not needing to run servers that would require high speed in the other direction.

There are both technical and marketing reasons why ADSL is in many places the most common type offered to home users. On the technical side, there is likely to be more crosstalk from other circuits at the DSLAM end (where the wires from many local loops are close to each other) than at the customer premises. Thus the upload signal is weakest at the noisiest part of the local loop, while the download signal is strongest at the noisiest part of the local loop. It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer end. Since the typical home user in fact does prefer a higher download speed, the telephone companies chose to make a virtue out of necessity, hence ADSL. On the marketing side, limiting upload speeds limits the attractiveness of this service to business customers, often causing them to purchase higher cost Leased line services instead. In this fashion, it segments the digital communications market between business and home users.

On the wire

Currently, most ADSL communication is full-duplex. Full-duplex ADSL communication is usually achieved on a wire pair by either frequency-division duplex (FDD), echo-cancelling duplex (ECD), or time-division duplexing (TDD). FDD uses two separate frequency bands, referred to as the upstream and downstream bands. The upstream band is used for communication from the end user to the telephone central office. The downstream band is used for communicating from the central office to the end user.

Frequency plan for ADSL. The red area is the frequency range used by normal voice telephony (PSTN), the green (upstream) and blue (downstream) areas are used for ADSL.

With standard ADSL (annex A), the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz – 1104 kHz is used for downstream communication. Each of these is further divided into smaller frequency channels of 4.3125 kHz. These frequency channels are sometimes termed bins. During initial training, the ADSL modem tests each of the bins to establish the signal-to-noise ratio at each bin's frequency. The distance from the telephone exchange and the characteristics of the cable mean that some frequencies may not propagate well, and noise on the copper wire, interference from AM radio stations and local interference and electrical noise at the customer end mean that relatively high levels of noise are present at some frequencies, so considering both effects the signal-to-noise ratio in some bins (at some frequencies) may be good or completely inadequate. A bad signal-to-noise ratio measured at certain frequencies will mean that those bins will not be used, resulting in a reduced maximum link capacity but with an otherwise functional ADSL connection.

The DSL modem will make a plan on how to exploit each of the bins sometimes termed "bits per bin" allocation. Those bins that have a good signal-to-noise ratio (SNR) will be chosen to transmit signals chosen from a greater number of possible encoded values (this range of possibilities equating to more bits of data sent) in each main clock cycle. The number of possibilities must not be so large that the receiver might mishear which one was intended in the presence of noise. Noisy bins may only be required to carry as few as two bits, a choice from only one of four possible patterns, or only one bit per bin in the case of ADSL2+, and really noisy bins are not used at all. If the pattern of noise versus frequencies heard in the bins changes, the DSL modem can alter the bits-per-bin allocations, in a process called "bitswap", where bins that have become more noisy are only required to carry fewer bits and other channels will be chosen to be given a higher burden. The data transfer capacity the DSL modem therefore reports is determined by the total of the bits-per-bin allocations of all the bins combined. Higher signal-to-noise ratios and more bins being in use gives a higher total link capacity, while lower signal-to-noise ratios or fewer bins being used gives a low link capacity.

The total maximum capacity derived from summing the bits-per-bins is reported by DSL modems and is sometimes termed sync rate. This will always be rather misleading as the true maximum link capacity for user data transfer rate will be significantly lower because extra data is transmitted that is termed protocol overhead, a reduced figure of around 84-87% at most for PPPoA connections being a common example. In addition some ISPs will have traffic policies that limit maximum transfer rates further in the networks beyond the exchange, and traffic congestion on the Internet, heavy loading on servers and slowness or inefficiency in customers' computers may all contribute to reductions below the maximum attainable.

The choices the DSL modem make can also be either conservative, where the modem chooses to allocate fewer bits per bin than it possibly could, a choice which makes for a slower connection, or less conservative in which more bits per bin are chosen in which case there is a greater risk case of error should future signal-to-noise ratios deteriorate to the point where the bits-per-bin allocations chosen are too high to cope with the greater noise present. This conservatism involving a choice to using fewer bits per bin as a safeguard against future noise increases is reported as the signal-to-noise ratio margin or SNR margin. The telephone exchange can indicate a suggested SNR margin to the customer's DSL modem when it initially connects, and the modem may make its bits-per-bin allocation plan accordingly. A high SNR margin will mean a reduced maximum throughput but greater reliability and stability of the connection. A low SNR margin will mean high speeds provided the noise level does not increase too much, otherwise the connection will have to be dropped and renegotiated (resynced). ADSL2+ can better accommodate such circumstances, offering a feature termed seamless rate adaptation (SRA), which can accommodate changes in total link capacity with less disruption to communications.

Vendors may support usage of higher frequencies as a proprietary extension to the standard. However, this requires matching vendor-supplied equipment on both ends of the line, and will likely result in crosstalk problems that affect other lines in the same bundle.

There is a direct relationship between the number of channels available and the throughput capacity of the ADSL connection. The exact data capacity per channel depends on the modulation method used.

Modulation

ADSL initially existed in two flavours (similar to VDSL), namely CAP and DMT. CAP was the de facto standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL installs at the time. However, DMT was chosen for the first ITU-T ADSL standards, G.992.1 and G.992.2 (also called G.dmt and G.lite respectively). Therefore all modern installations of ADSL are based on the DMT modulation scheme.

ADSL standards

Frequency spectrum of a modem on a ADSL line.

Standard name	Common name	Downstream rate	Upstream rate	Approved in
ANSI T1.413-1998 Issue 2	ADSL	8 Mbit/s	1.0 Mbit/s	1998
ITU G.992.1	ADSL (G.DMT)	12 Mbit/s	1.3 Mbit/s	1999-07
ITU G.992.1 Annex A	ADSL over POTS	12 Mbit/s	1.3 Mbit/s
ITU G.992.1 Annex B	ADSL over ISDN	12 Mbit/s	1.8 Mbit/s
ITU G.992.2	ADSL Lite (G.Lite)	1.5 Mbit/s	0.5 Mbit/s	1999-07
ITU G.992.3	ADSL2	12 Mbit/s	1.0 Mbit/s	2002-07
ITU G.992.3 Annex J	ADSL2	12 Mbit/s	3.5 Mbit/s
ITU G.992.3 Annex L	RE-ADSL2	5 Mbit/s	0.8 Mbit/s
ITU G.992.4	splitterless ADSL2	1.5 Mbit/s	0.5 Mbit/s	2002-07
ITU G.992.5	ADSL2+	24 Mbit/s	1.0 Mbit/s	2003-05
ITU G.992.5 Annex M	ADSL2+M	24 Mbit/s	3.5 Mbit/s

Annexes J and M shift the upstream/downstream frequency split up to 276 kHz (from 138 kHz used in the commonly deployed annex A) in order to boost upstream rates. Additionally, the "all-digital-loop" variants of ADSL2 and ADSL2+ (annexes I and J) support an extra 256 kbit/s of upstream if the bandwidth normally used for POTS voice calls is allocated for ADSL usage.

ADSL1 access utilizes the 1.1 MHz band, and ADSL2+ utilizes the 2.2 MHz band.

The downstream and upstream rates displayed are theoretical maxima. Note also that because digital subscriber line access multiplexers and ADSL modems may have been implemented based on differing or incomplete standards some manufacturers may advertise different speeds. For example, Ericsson has several devices that support non-standard upstream speeds of up to 2 Mbit/s in ADSL2 and ADSL2+.

Installation issues

Due to the way it uses the frequency spectrum, ADSL deployment presents some issues. It is necessary to install appropriate frequency filters at the customer's premises, to avoid interference with the voice service, while at the same time taking care to keep a clean signal level for the ADSL connection.

In the early days of DSL, installation required a technician to visit the premises. A splitter or microfilter was installed near the demarcation point, from which a dedicated data line was installed. This way, the DSL signal is separated earlier and is not attenuated inside the customer premises. However, this procedure is costly, and also caused problems with customers complaining about having to wait for the technician to perform the installation. As a result, many DSL vendors started offering a self-install option, in which they ship equipment and instructions to the customer. Instead of separating the DSL signal at the demarcation point, the opposite is done: the DSL signal is filtered at each phone outlet by use of a low-pass filter for voice and a high-pass filter for data, usually enclosed in what is known as a microfilter. This microfilter can be plugged directly into any phone jack, and does not require any rewiring at the customer's premises.

A side effect of the move to the self-install model is that the DSL signal can be degraded, especially if more than 5 voiceband devices are connected to the line. The DSL signal is now present on all telephone wiring in the building, causing attenuation and echo. A way to circumvent this is to go back to the original model, and install one filter upstream from all telephone jacks in the building, except for the jack to which the DSL modem will be connected. Since this requires wiring changes by the customer and may not work on some household telephone wiring, it is rarely done. It is usually much easier to install filters at each telephone jack that is in use.

DSL signals may be degraded by older telephone lines, surge protectors, poorly designed microfilters, radio frequency interference, electrical noise, and by long telephone extension cords. Telephone extension cords are typically made with small-gauge multi-strand copper conductors, which are more susceptible to electromagnetic interference and have more attenuation than single-strand copper wires typically wired to telephone jacks. These effects are especially significant where the customer's phone line is more than 4 km from the DSLAM in the telephone exchange, which causes the signal levels to be lower relative to any local noise and attenuation. This will have the effect of reducing speeds or causing connection failures.