(SDSL) Is a Digital Subscriber Line Symmetric Digital Subscriber Line (SDSL) Is a Digital

(SDSL) is a Digital Subscriber Line
Symmetric Digital Subscriber Line (SDSL) is a Digital Subscriber Line
(DSL) variant with E1-like data rates (72 to 2320 Kbit/s). It runs over one pair of copper wires, with a maximum range of about 3 kilometers or 1.86 miles. The main difference between ADSL and SDSL is that SDSL has the same upstream data rate as downstream (symmetrical), whereas ADSL always has smaller upstream bandwidth (asymmetrical). However, unlike ADSL, it can't co-exist with a conventional voice service on the same pair as it takes over the entire bandwidth. It typically falls between ADSL and T-1/E-1 in price, and it is mainly targeted at small and medium businesses who may host a server on site, (e.g. a Terminal Server or Virtual Private Network) and want to use DSL, but don't need the higher performance of a leased line.
SDSL was never properly standardized until Recommendation G.991.2 (ex-G.shdsl) was approved by ITU-T. SDSL is often confused with G.SHDSL; in Europe, G.SHDSL was standardized by ETSI using the name 'SDSL'. This ETSI variant is compatible with the ITU-T G.SHDSL standardized regional variant for Europe.
SDSL equipment usually only interoperates with devices from the same vendor, though devices from other vendors using the same DSL chipset may be compatible. Most new installations use G.SHDSL equipment instead of SDSL
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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 voice band modem can provide. It does this by utilizing frequencies that are not used by a voice telephone call. A splitter - or micro filter - allows a single telephone connection to be used for both ADSL service and voice calls at the same time. Because phone lines vary in quality and were not originally engineered with DSL in mind, it can generally only be used over short distances, typically less than 3mi (5.5 km) [William Stallings' book].
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. In the UK under British Telecom the data network in question is its ATM network which in turn sends it to its IP network IP Colossus.
Explanation
A gateway is commonly used to make an ADSL connection. The modem in the picture is also a wireless access point, hence the antenna. 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 Digital Signal 1 services instead. In this fashion, it segments the digital communications market between business and home users
How ADSL works
On the wire

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.
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 canceling duplex (ECD), or time division duplexing (TDD). FDM 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. 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. During initial training, the ADSL modem tests which of the available channels have an acceptable signal-to-noise ratio. The distance from the telephone exchange, noise on the copper wire, or interference from AM radio stations may introduce errors on some frequencies. By keeping the channels small, a high error rate on one frequency thus need not render the line unusable: the channel will not be used, merely resulting in reduced throughput on an otherwise functional ADSL connection.
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 issues 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 flavors (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
الصورة المرفقة هامة وتوضح دون لود والأبلود ستريمالمختلفة
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.
While the ADSL access utilizes the 1.1 MHz band, 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 interferences 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 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 off" at each phone outlet by use of a low pass filter, also known as micro filter. This method does not require any rewiring inside the customer 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 voice band 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.
Footnotes and references
^ a b ADSL2 Annex L is also known as RE-ADSL2, where 'RE' stands for 'Reach Extended.' With this ADSL standard, the power of the lower frequencies used for transmitting data is boosted up to increase the reach of this signal up to 7 kilometers (23,000 ft). The upper frequency limit for RE-ADSL2 is reduced to 552 kHz to keep the total power roughly the same as annex A. Since RE-ADSL2 is intended for use on long loops there isn't much (any) usable bandwidth above 552 kHz anyway. Although this standard has been ratified by the ITU, not all local loop network maintainers allow this protocol to be used on their network, lest the extra power on the lower frequencies cause problems for existing services due to crosstalk.
vs. DSL
Is this better than DSL? What abuot cable? Whats the difference? Is it big or small?
That is not true. The theoretical maximum for G.992.1 for example is 13Mbps (224 downstream carriers * 15bit/carrier * 4kHz symbol frequency). --69.105.224.209
Can someone elucidate DSLAM? I suppose it's DSL access apparatus, but abbreviations should be defined before use.
It's explained here --Tolien 3 July 2005 20:17 (UTC)
I understand that "Annex A" is for ADSL over analog lines while "Annex B" is for ISDN lines. Could someone enter this into the article with some info on how likely you are to need one or the other? Or other compatibilities or non-compatibilities to watch out for? Thanks. ---Ransom/CG
Spectrum allocation not quite right
the spectrum breakdown is not entirely accurate in this article. CAP did allocate spectrum the way that the diagram indicates it, but DMT doesn't - it defines 247 (from memory - this figure might be a bit out) subchannels, each of which can be either upstream or downstream (dynamically). --Dave Symonds 06:11, 21 September 2005 (UTC)
Yep, just looked it up: DMT splits the channel into 247 sub channels, each 4kHz wide. This is the standard way it's done now. --Dave Symonds 06:15, 21 September 2005 (UTC)
Faxes
Out of interest, how does ADSL affect faxes? can you send or receive a fax on an ADSL line? - Ta bu shi da yu 05:34, 14 January 2006 (UTC)
A fax is just a low-speed modem, so it uses the voice band. This band is protected by a splitter from the DSL signal. Biot 09:10, 14 January 2006 (UTC)
That's what I though. Cheers Biot! - Ta bu shi da yu 13:53, 15 January 2006 (UTC)
Asynchronous
ADSL is a synchronous protocol at the lowest data layer. Amusingly enough, it runs ATM (async transfer mode) on top of this layer, but it still used synchronized clocks at the sending and receiving end.
That is, I think, true of most layers atop which ATM runs. Guy Harris 23:22, 25 November 2006 (UTC)
Upload speeds
"Upstream rates start at 64 Kbit/s and typically reach 256 Kbit/s but can go as high as 1024 Kbit/s." Is this also true for ADSL2 and ADSL2+? Or do they allow higher upstream rates? --osmosis 10:26, 26 February 2006 (UTC)
Upload
The site only mentions the word "upload" once, maybe more information should be provided about uploading on ADSL, the uploading speed, and if uploading is affected by downloading. etc.--82.152.138.225 01:29, 8 March 2006 (UTC)
What I think you are referring to is "link saturation", where the maximum upload speed obtainable reduces during a high speed download, or vice versa. But this behavior is not restricted to ADSL. --osmosis 12:24, 8 March 2006 (UTC)
Actually, can ADSL upload and download data at the same time, or is it effectively one or the other? --geoff_o 20:35, 11 April 2006 (UTC)
I thought I should point out that in Japan, download speeds for ADSL go as high as 47Mbps, and upload speeds up to 5Mbps. Without the proper technical background, I wouldn't want to alter this article, but I thought it might be useful info to add. For evidence of this see the NTT page, in English, on this: [فقط الأعضاء المسجلين والمفعلين يمكنهم رؤية الوصلات - إذا كنت مسجل فى المنتدى اتصل بالإدارة لتفعيل عضويتك . إضغط هنا للتسجيل]
I believe that the 47/5 Mbps line is probably a bonded ADSL2+ line, in other words, two ADSL2+ lines used together for the connection. This is covered in the ADSL2/2+ ITU-standard. But I cannot support this claim with any evidence, it's only a guess because the speeds would match pretty nicely.
This is not correct - in Japan "adsl2++" is used , which extends the adsl spectrum to 3.75MHz, thus roughly doubling the downstream rate to 50Mbps. 5 Mbps upstream is with triple upstream - the upstream band is three times the US bandwidth of regular adsl. ~~===
ADSL backbone network
I was wondering, if its appropriate to mention that the shift from ATM backbone networks to Ethernet is also because of the future possibilities of using the same backbone network for other services like POTS or mobile phone networks?
..."or lower signal to noise (SNR)ratios"... Shouldn't this be lower Signal to Noise Ratios(SNR)? Cruxit 15:59, 14 June 2006 (UTC)
Updating ADSL and DSL entries
CarlosRibeiro 17:23, 21 October 2006 (UTC). I've dropped a note at the DSL history. I moved some notes on DSL installation that applied specifically to ADSL here. It refers to some history on the usage of splitters, that is not longer absolutely required but is still of interest, both historically and also to understand some practical aspects of the technology.
Is it an analogue or digital technology?
Is it correct to describe it as a digital technology since it modulates analogue carriers?
I.I.A —The preceding unsigned comment was added by 195.130.120.24 (talk) 09:03, 21 December 2006 (UTC).
Annex M
The page here states the download speed is 28mbit, but the [[ITU G.992.5 Annex M] page says 24mbit... which is it? 125.236.153.184 09:05, 31 January 2007 (UTC)
Capitalization
Reading [1] I think it means this article is incorrectly capitalised, while Digital subscriber line is correctly titled. Does anyone agree that only the first word should be capitalized? Disagree? Think both styles are correct for their respective article? Jim.henderson 20:29, 19 March 2007 (UTC)
I agree, it should be Asymmetric digital subscriber line. But what links here is overwhelmingly in favour of the capitaised version. I will give it a week then move it. -- RHaworth 10:14, 15 October 2007 (UTC)
S=1/2?
My USR router specs says that with S=1/2, it can do 12Mbps, up from 8. What does S=1/2 mean and how does it extend the download speed? Any info appreciated, cheers! 84.9.32.226 14:47, 25 June 2007 (UTC)
OFDM system comparison table
Feel free to add an ADSL column to the OFDM#OFDM system comparison table. Mange01 11:46, 17 July 2007 (UTC)
[
History
I came to this page to find out when ADSL was invented and rolled out. If someone knows, can they add it to the article? It would also be good to know who invented it. exterminator 11:06, 8 August 2007 (UTC)
How Much Faster is ADSL compared to Dialup?
I came here hoping to find a definitive answer, because Verizon's claims make no sense ("768 kbit/s DSL is 21 times faster than 56k dialup"). 768/56 == 13 which is nowhere near Verizon's claim. (It's even less when you consider 56K modems use compression to increase effective thoughput to ~150 kbps).) I'd like to find an actual study that's not biased by salesmanship. - Theaveng 17:37, 14 September 2007 (UTC)
Given that ADSL can run from 768Kb/s (or possibly even lower) to 6Mb/s (or possibly even higher), there isn't a single definitive answer to your question. I've gotten 1.5Mb/s; 1500/56 = 26. Guy Harris (talk) 19:21, 20 November 2007 (UTC)
Typical marketing flim-flam, probably. Remember that the max tech speed of this segment of the SYSTEM that you are using is only one factor in the response you experience. Your computer is a factor. Latencies anywhere along the way are a factor, that remains no matter how fast the peak speeds. As a practical matter, you can hope that DSL will be about ten times faster than dial-up, in real life. -69.87.200.195 (talk) 11:17, 24 March 2008 (UTC)
Does DSL use V.42, V.44, or some other data compression?
My dialup modem uses V.44 to compress **** 6-to-1 (effective throughput of 300 kbit/s) and executables like flash programs 3-to-1 (effective throughput of 150 kbit/s). Does DSL use a similar technology to squash data on the fly? - Theaveng (talk) 18:30, 20 November 2007 (UTC)