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Getting the most from ADSL It’s a fair bet that most readers of SILICON CHIP enjoy their daily fix of internet access by courtesy of ADSL. While some readers are luxuriating with optical fibre – and an unhappy minority are still using dial-up – most of us owe our ongoing communication to the distinctly freakish technology of ADSL. But what exactly is ADSL and what came before it? How does it work and why is it often called a “freak” technology? Are your internet speeds painfully slow? Can anything be done to speed them up? Do you curse your ISP? Read on! By ALAN FORD M any of us remember the early days of text-only bulletin boards (which could be regarded as the fore-runners to today’s internet), to which we connected via an acoustic modem. Bulletin boards were set up in the early 1980s by special interest groups, some businesses and even altruistic individuals. Most specialised in a particular subject or brand and we connected to them by dialling a number specific to that bulletin board. We then carefully inserted the telephone handset into a contraption of cups and flexible joints. We laid it on its side to prevent the carbon granules in the microphone from A Radio Shack (Tandy in Australia) acoustic modem from the 1980s. These did not work well with the carbon microphone used in Australian telephones at the time. 22  Silicon Chip coalescing, moved the cat out of the room to prevent the heavy tread of its paws from interrupting data flow and settled down to enjoy the lightning fast data transfer speed of approximately 300 bits per second (bps). Without delving into the esoteric realms of parity bits, overhead or consideration of baud versus bits (don’t ask!), you can take that as about 35 characters/bytes per second (8 bits = 1 byte). In practice, various factors contributed to delays (as they do today) and we would usually see text characters emerging on our computer screens one by one or in groups of a few at a time. How did that old acoustic modem work? Computer binary data streams (well, trickles) would be converted in the modem (or “modulated”) and transmitted over the telephone line as frequency shifted audio tones. At the ISP’s end another modem would convert the sounds to data (or “demodulated”), or vice versa. In fact, that’s where the word “modem” comes from: it’s a MODulator/DEModulator. That was fine for plain text but then along came graphics, with Microsoft Windows a pioneer (but certainly not the only one), as well as the World Wide Web. Now we needed to access the Web with its rich images and sounds as well. The direct modem Enter the direct-wired modem, connecting the computer electrically to the PSTN (Public Switched Telephone Netsiliconchip.com.au Aaahhh – the way we were! This photo, taken in 1981, shows a youthful Dick Smith talking bits and bytes with an equally youthful and then-hirsute Leo Simpson. But the main point about this picture is not so much the all-new System 80 computer and its external floppy disk drive, it’s that whizz-bang acoustic modem in which the telephone handset resides. The problem with this (which obviously Dick and Leo didn’t understand) was that the modem needed to be turned on its side to work properly, otherwise the carbon granules in the microphone would tend to coalesce – and cause data loss. work), with much anguish on the part of the telcos (well, Telecom Australia!). They (Telecom) even took to placing adverts in the media warning of the dangers of using unapproved (ie, not supplied by them!), mainly imported modems and the heavy fines for doing so. It wasn’t too long before they realised the horse had well and truly bolted so instead started issuing approvals for imported equipment. Wired modems used more complicated methods of coding and offered much faster communication than the simple two-tone system of the acoustic device. Speeds increased as modulation methods became more clever, until in theory 56kb/s could be reached. In case you haven’t done the sum, that’s about 187 times faster than the acoustic modem! Of course, we still were using the PSTN speech path and bandwidth, so for a time it was thought that 56kb/s was the limit. But we could now get our images — even some (jerky) moving ones! But as well as new Web applications needing even more speed, there was another big disadvantage to the technology — the engaged line syndrome. While we were using the ‘net’, the Mother-inlaw received the engaged signal and could not impart any telephonic wisdom to the family. Neither could the kids call their friends. Today both would use email or even the dreaded text messaging but we are getting ahead of ourselves. Or if by chance we had the call-waiting facility activated, the internet experience would be rudely but soundlessly interrupted; neither the Mother-in-law nor our Net aspirations would be fulfilled. A fortunate few might have had a separate line installed for Net use but it was not a general rule for households. POTS and carriers Then along came ADSL, really a “freak” technology and After acoustic modems came direct-connect modems such as this D-Link DFM-526E 56K. At the time, everyone thought they were unbelievably fast compared to acoustic models. But ADSL has consigned them to the rubbish-heap of history! siliconchip.com.au May ay 2012  23 in some ways it’s surprising it works at all. But it does work if all or most of a large number of aspects are at or near optimum, as I shall explain. Hopefully my words will reduce the total of frustrated users and prevent many of those newsgroup or forum posts that sometimes use violent language to blame the ISP for shortcomings that are entirely outside its control! To appreciate how ADSL works it is helpful to go right back to the basic telephone network — the Plain Old Telephone System (POTS) in the mid part of last century. At first, most lines were strung overhead, including long distance lines, before the much later advent of coaxial and tower-to-tower microwave links. The stringing of a dozen or so wires between say Sydney and Melbourne was expensive and there were obvious limitations of space as the bare wires could not be allowed to touch and short in any foreseeable winds. To have just a dozen connections between Sydney and Melbourne seems ludicrous now and in fact it was ludicrous then. So it was necessary to somehow concentrate several speech channels down one pair of lines in order that the best use be made of that expensive (and expensively erected) copper — even galvanised iron in some places! The solution was carrier telephony. A number of telephone channels were modulated onto several different radio frequency (RF) carriers, sent down the overhead wires and separated and demodulated at the other end. Normal speech occupies a relatively narrow bandwidth; typically in those days the speech path was designed for a bandwidth of 200Hz-3kHz. But the lines were capable of carrying frequencies of several hundred kilohertz — radio frequencies but still carried by line. A typical carrier system in use in the 1950s was capable of concentrating 17 RF channels down one pair of wires, spaced by 4kHz, with the highest being 68kHz. Later systems used even higher frequencies. ADSL and more carriers Years later, internet engineers reckoned (correctly) that they should be able to do the same sort of thing with internet signals. The ADSL method consists of modulating a large group of separate RF channels, often called bins or buckets, and sharing the data to be transmitted digitally amongst them. That’s the Digital Subscriber Link (DSL) but what about the A for Asymmetric? Think about how we typically use the internet. We type a few characters of a website address and in return we get pages of visual information and plenty of text too. So most of the data traffic is downloaded and therefore most of the bins are allocated to it. An ADSL2+ capable line carries the normal speech and telephony (POTS) signals in the first 4kHz of bandwidth, followed by a guard (unused) band from 4 to 25kHz, and then a large block of separate frequencies spaced 4.3125kHz apart (up to 512 of them) above that for the internet data. These separate channels are the bins and about 5% of them are used for upload with the rest for download. The number of bins and the allocation between upstream and downstream varies according to which version (‘Annex’) of the standard is in use. (And before you pundits TYPICAL ADSL2+ FREQUENCY ALLOCATIONS (not to scale) ADSL BINS AT 4.3125kHz SPACING POTS 0-4kHz GUARD UPSTREAM 25kHz DOWNSTREAM 138kHz 142kHz 26 BINS (25 available) 2.2MHz 479 BINS (446 available) NOTE: some bins are used for pilots or other special purposes Here’s how ADSL is arranged on a standard PSTN telephone line. The bottom 4kHz is reserved for your phone calls, followed by a number of channels (‘bins’) for uploaded data and a much larger number for downloaded data. 24  Silicon Chip siliconchip.com.au reach for your keyboards, I am simplifying the position for the benefit of newbies). Separating the information How is all the information kept separate? First of all let’s deal with telephony, because this touches on a great advantage of ADSL — the end of the ‘engaged line’ syndrome! When ADSL is in use, each telephone should be provided with a low pass filter that allows the DC signalling (such as on-hook condition), AC ring current and audio frequencies (such as speech and DTMF dialling) to pass normally. The filter passes the lowest part of the total passband (up to 4kHz), to the telephone and keeps it separate from the RF of the internet connection. The internet is always connected but the telephone, duly filtered, operates normally. Whether or not we are using the net, the phone will still ring if someone is calling and neither telephone party will hear the internet signals. Now to the Net connection. All the bins, whether allocated to upload or download, are kept separate by the special ADSL modem, a complex piece of technology now relatively cheap. At the telephone exchange end the allocation of the bins is controlled by the Digital Subscriber Line Access Multiplexer (DSLAM). The Modem-DSLAM combination does more than keep all those bins separate. It is also a smart self-training combination, passing information on a per bin basis according to how free of interference each bin is. Later we will see why this is one possible reason for a slow internet connection. Meanwhile you can see that the provision of a large number of separate but simultaneous bins (= channels) offers a vastly improved speed. Are your expectations too high? It is not good for the blood pressure to pursue the unattainable. Because it involves transmitting RF down a copper pair, with corresponding attenuation and other effects, your speeds will depend on your cable distance from the nearest exchange and of course, the cable distance will be more than the ‘crow flying’ distance. Because ADSL gets progressively slower as the cable distance rises, it becomes marginal at 4km and will probably not work at all at 5km, although there can be exceptions. I am lucky enough to be 167 metres cable length from my exchange and on ADSL2+ I enjoy at least 10Mbit/s and sometimes nearly 16Mbit/s download speeds, at the same time as 0.8Mbit/s upload speed, although the copper cabling here is not very good. To put that in perspective, the download speed is up to over 283 times faster than a dial-up modem and 53,000 times faster than the old acoustic modems! Because speed varies so much with cable distance and quality of connection, it is not possible to lay down hard and fast rules but there are many ISP and other sources on the net where you can compare your speeds to others in your area. There’s a possible fly in the ADSL ointment. You may be on a telephone line concentrator system, such as a RIM (Remote Integrated Multiplexer), where many lines are multiplexed and share a fibre (or even coax) link to the exchange. Since ADSL is itself multiplexed there can be clashes and speed penalties. Your telco will tell you if you are on a RIM or similar concentrator and whether you can expect a good ADSL experience. In fact, when you enquire about ADSL, one of the first things that happens is that you are asked for your phone number to check whether you are on a multiplexed system. Unfortunately, that’s all that is checked – the line is not physically checked to see if ADSL is possible until you actually apply for the service. Low speed, dropouts & throttling Basically there are two types of trouble you can face as an ADSL subscriber: low speed and drop-outs. What about throttling? No, not the person at the ISP help desk, the speed. Because this check is so simple it ought to be the first that you do. Many ISP plans have a data limit, after which data speeds are deliberately restricted or “throttled”. This is irritating but you will probably agree that the alternative of receiving an unexpected bill for excess data usage would be a tad more annoying! Check via your ISP’s website to see if you are being throttled. Telephone line problems Bearing in mind that ADSL is really a freak technology where we stuff RF signals down a copper pair that it was not designed for, for optimum speed and especially for freedom from drop-outs the line needs to be in good electrical condition. There are many joints in a phone line between you and the exchange as separate ‘pairs’ of wire are connected at street cabinets or pits. If any of those joints are faulty you will have problems. Your Reliable Partner in the Electronics Lab ab LPKF ProtoMat E33 – small, accurate, affordable Hardly larger than a DIN A3 sheet: The budget choice for milling, drilling and depaneling of PCBs or engraving of front panels – in LPKF quality. www.lpkf.com/prototyping Embedded Logic Solutions Pty. Ltd. Ph. +61 (2) 9687 1880 siliconchip.com.au Email. sales<at>emlogic.com.au May 2012  25 DSL Glossary ADSL Asymmetric Digital Subscriber Line. ATM Asynchronous Transfer Mode. Authentication Auto-negotiation Bandwidth Cross-talk A digital subscriber line (DSL) technology in which the transmission of data from server to client is much faster than the transmission from the client to the server. A cell-based data transfer technique in which channel demand determines packet allocation. ATM offers fast packet technology, real time, demand led switching for efficient use of network resources. A security feature that allows access to information to be granted on an individual basis. Procedure for adjusting line speeds and other communication parameters automatically between two computers during data transfer. The range of frequencies a transmission line or channel can carry: the greater the bandwidth, the greater the informationcarrying capacity of a channel. Signal currents being induced into neighbouring wires and causing errors. bit (“BINary digiT”) A single unit of data, where there are only two possible states. The smallest amount which can be carried/transmitted. bps bits per second A standard measurement of digital transmission speeds. Bridge A device that connects two or more physical networks and forwards packets between them. Broadband Characteristic of any network that multiplexes independent network carriers onto a single cable. This is usually done using frequency division multiplexing (FDM). byte DMT Discrete Multitone 26  Silicon Chip (usually!) 8 bits = 1 byte; origin is the number of bits needed to define one text character. The leading method of signal modulation for DSL service. The usable frequency range is separated into 512 frequency bands (or channels) spaced 4.3125kHz apart. DMT uses the FFT (fast Fourier transform) algorithm as its modulator and demodulator. Downstream rate The line rate for return messages or data transfers from the network to the customer. DSL Digital Subscriber Line A technology for bringing highbandwidth information to homes and small businesses over ordinary copper telephone lines. DSLAM Digital Subscriber Line Access Multiplexer A device at the telephone exchange which enables connection to multiple customers simultaneously. Encapsulation The technique used by layered protocols in which a layer adds header information to the protocol data unit (PDU) from the layer above. FTP File Transfer Protocol The Internet protocol (and program) used to transfer files between hosts. HTML Hypertext Markup Language The most common page-coding language for the World Wide Web. HTML browser (or web browser) A browser used to traverse the world wide web. http Hypertext Transfer Protocol. The protocol used to carry world-wide web (www) traffic between a www browser computer and the www server being accessed. Internet address An IP address assigned in blocks of numbers to user organizations accessing the Internet. Internet A collection of networks interconnected by a set of routers which allow them to function as a single, large virtual network. IP Internet Protocol The network layer protocol for the Internet protocol suite. IP address The 32-bit address assigned to hosts that want to participate in a TCP/IP Internet. Written as four numbers separated by dots. ISP Internet Service Provider A company that allows home and corporate users to connect to the Internet. LAN Local Area Network A data communications network restricted to a small area (often within one building or office) siliconchip.com.au Last mile The final connection between the nearest exchange and the subscriber (for most people at the moment a copper pair). Line rate The speed by which data can be transferred over a particular line type, express in bits per second (bps). Loopback A diagnostic test that returns the transmitted signal back to the sending device after it has passed through a network or across a particular link. The returned signal can then be compared to the transmitted one. MAC Media Access Control Layer. A computer’s interface to a physical network. Multiplexer A device that can send several signals over a single line. They are then separated by a similar device at the other end of the link. Router A system responsible for making decisions about which of several paths network (or Internet) traffic will follow. SNMP Simple Network Management Protocol The network management protocol of choice for TCP/IP-based internets. Split pair Where the earth leg of one twisted pair is inadvertently swapped with the earth leg of another during jointing, leading to the noise cancelling effect of the twist being defeated. Spoofing A method of fooling network end stations into believing that keepalive signals have come from and return to the host. Polls are received and returned locally at either end of the network and are transmitted only over the open network if there is a condition change. Synchronous connection During synchronous communications, data is not sent in individual bytes, but as frames of large data blocks. TCP Transmission Control Protocol The major transport protocol in the Internet suite of protocols providing reliable, connectionoriented full-duplex streams. NAT Network Address Translation. Allows multiple computers to share one IP address. Packet The unit of data sent across a packet switching network. PAP Password Authentication Protocol. Pair The pair of copper wires making up an individual telephone circuit. UTP Unshielded Twisted pair Port The abstraction used by Internet transport protocols to distinguish among multiple simultaneous connections to a single destination host. Two insulated copper wires twisted together to reduce potential signal interference between the pairs. Upstream rate The line rate for message or data transfer from the source machine to a destination machine on the network. Also see downstream rate. VC Virtual Connection A link that seems and behaves like a dedicated point-to-point line or a system that delivers packets in sequence, as happens on an actual point to point network. In reality, the data is delivered across a network via the most appropriate route. WAN Wide Area Network A data communications network that spans any distance and is usually provided by a public carrier (such as a telephone company or service provider). POTS Plain Old Telephone Service Also known as PSTN – the public switched telephone network. PPP Point-To-Point-Protocol. Provides router-to-router and hostto-network connections over both synchronous and asynchronous circuits. Protocol A formal description of messages to be exchanged and rules to be followed for two or more systems to exchange information. RIM Remote Integrated Multiplexer Where many lines are locally multiplexed and share a link to the exchange. Often precludes ADSL. Route The path that network traffic takes from its source to its destination. siliconchip.com.au May 2012  27 vals, it turned out to be a paging alarm system on another subscriber’s line, with, you guessed it, a split pair. Local RF Interference Flat patch lead at top and standard UTP (unshielded twisted pair) below. The twisted pair lead is preferable for minimising noise and interference. Pick up your phone and dial 1. In the few seconds before the “number unobtainable” tone kicks in you should have absolute or very near silence (possibly a very faint steady hiss with some types of phone). There should definitely be no intermittent crackles (caused by bad joints) and no cross talk from other telephone users. Cross talk or hum may indicate a split pair somewhere on the route between you and the exchange. Is a split pair the same as split end? A split pair is where the technician has in error used someone else’s telephone earth line instead of yours. The phone still works because all the earth wires are connected together at the exchange but it is noisy, and ADSL does not like noise. The pair of telephone wires that make up your normal connection are twisted together in the multi-way cable that usually runs underground in ducts. The twists in the pair go a long way towards minimising noise because the noise currents in the adjacent wires balance out. But if by error your earth wire is really someone else’s, it is no longer twisted with your other wire. The pairs are split and noise will result. Returning to crackly joints, these are often far worse in extreme weather, by which I mean excessive heat or cold, or heavy rain. If you hear line noise, even when all other equipment is removed from your phone connection, there is a problem either inside or outside your premises. If inside you will need the services of a competent licenced cabler. If outside, your approach needs to be to Telstra or any other line retailer involved. Be careful here. You may be slugged with a charge if no faults are found, so do the other checks first. Also, you may be unlucky with your technician. Many a case has been marked as ‘no fault found’ when there was a glaring one. I have no magic solution to this human problem. Patch leads & RJ plugs How long is the patch lead joining your modem/router to the telephone point in your house? Such leads are usually not twisted, probably because it is cheaper to make flat ones. Although it’s a comparatively short run, the interferencecancelling effect of a flat pair is much less than a twisted pair. I would recommend that patch leads be no longer than two metres. While we are considering the patch lead, internet problems may also be due to a loose RJ plug on the patch lead, where it has not been fully pressed home in the modem or telephone point. Problems at given times Internet users often experience drop-outs at specific intervals (every four hours is common) and these have turned out to be Securitel or similar auto-paging alarm units on the same line. For optimum internet speed and freedom from drop-outs, it is best to have only telephones (and possibly faxes), all with filters, on a line that is to carry ADSL traffic. In one recent case of drop-outs exactly at 4-hourly inter28  Silicon Chip In one case I assisted with, the user experienced a dramatic reduction of internet speed at 6PM every evening. He was convinced that his ISP was deliberately throttling his speed at peak times, that there were insufficient ISP servers etc. Fortunately, he was not one to rush into un-researched blame and in due course he found by experiment that the problem was his new plasma TV which was switched on at 6PM for the news each evening. The switch-mode power supply was poorly screened and filtered (as is often the case) and it radiated pulses throughout the house and into his untwisted patch lead. RF interference can even be caused by a faulty lamp, including an incandescent one that is ‘singing’ just before failing. To test for this, appliances should be completely disconnected one by one (not just switched to standby). (But do not emulate a certain friend who noted that if he switched off at his main fuse panel, the interference certainly stopped — but along with everything else…). The sheer field strength of a nearby radio or TV broadcaster will result in some induced signals in nearby telephone lines. Your telephones are fitted with capacitors to reduce or eliminate rectified audio currents from interfering with normal telephony but the RF itself will often interfere with ADSL. Why does RF interference matter? We’ve looked at the modem/DSLAM combination and how it dynamically negotiates the best use of the big block of bins available to it. If there is interference on one or more of these bins, caused by harmonics or fundamental frequencies, the modem/DSLAM will agree together not to use it/them and so the total number in use reduces, leading to reduced overall speed. Power interruptions Some time ago, I was puzzled about drop-outs that I was suffering after upgrading from ADSL1 to ADSL2+ (a more ‘fussy’ technology, because the frequencies are higher). Previously, if power visibly failed for a few seconds (as it often did in thunderstorms), the modem would reset itself and consequently there was delay. This I understood but since the upgrade to ADSL2+, the drop-outs also happened at times when there was no flicker of the lights. A small Uninterruptible Power Supply (UPS) for the modem and router solved this problem. Indeed, almost immediately after fitting the UPS it went into alarm mode several times, although the lights did not blink. Here was the solution: power occasionally dipped for a few milliseconds, not enough for an incandescent lamp to flicker (because of thermal inertia) but certainly enough to cause the modem and router to reset. Modems and routers Although I have been lucky myself, there are many reported cases of these units becoming flaky. There is a tendency to leave them on continuously and thus heat fatigue may occur, particularly when there are underrated electrolytic capacitors fitted, which is the case with much overseas siliconchip.com.au equipment or when the mains voltage is abnormally high. Modem and router firmware may need upgrading as manufacturers discover shortcomings in current versions. Sometimes settings may be lost for one reason or another and the modem or router may have to be reset to the factory configuration. Although it is not always practical, the best way of eliminating the modem or router from your list of suspects is to borrow another; as long as you are quite sure you have set it up properly. Sometimes when the Net connection fails it is necessary only to perform a simple power reset of the modem/router, where you disconnect the low voltage power for 20 seconds or so (to let the electrolytics discharge) and then power it up again. After a few minutes to let the ADSL connection become established, you may find that you have normal service. Computer My experience was that by far the biggest contributor to low internet speeds was the state of my own computer. Running XP it had become somewhat cluttered, with much software installed or de-installed over the years. Consequently the machine often ‘froze’. By pressing CTRLALT-DEL on an XP machine you can see its operating state, via Windows Task Manager. The CPU usage is the critical figure. If this sits at 100% for more than a few seconds then the machine is in a virtual ‘locked up’ state and all your computing will experience considerable delays. In my case, I had sufficient backups and original software CDs/DVDs to be able to clean off my hard disk and do a complete reload of everything. And I had another machine to work on while this was in progress. I admit that it took many hours and I know it’s not a luxury open to all but in my case when it was finished my internet speeds were revolutionised! When working at the computer I now make a point of calling up Windows Task Manager and then minimising it. On the task bar I can then see the loading of the CPU as a partially green square all the time. Possibly, you are not able to perform (or risk) a complete reload from the start but at least, after taking good backups and verifying them, check your machine for viruses or other malware. Also, avoid having other applications running when on the Net and defrag your disk from time to time. General infrastructure failure The telephone network that we rely on so heavily can experience local or even national outages for a number of reasons. You can be sure that every effort is made to minimise the effects of this, especially if over a wide area, because the telco concerned is losing valuable revenue. A reputable ISP will have a page on its website that will list outages and general locations (although admittedly this is of no value when you can’t access it!). Phone help lines may also be a source of information. (I hear you groan — but remember that the help-desk jockey is a human being reading from a script). We now come to less likely reasons for poor speeds or for drop-outs. Exchange DSLAMs are very reliable, since they feed many subscribers, perhaps up to 1,000 or more. If one is faulty it will give rise to many simultaneous complaints and is likely to be swapped out quickly. Exchange congestion is a fairly rare occurrence, though some remote exchanges are notorious. 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Alternatively, contact us on 1300 811 355 or email enquiries<at>tekmarkgroup.com age of potential revenue for the telco and the likelihood is that it will be fixed fairly quickly. Unfortunately, in these cases ‘fairly quickly’ for a telco may mean a number of months. Incompetent or poorly resourced ISPs I have placed this last because although it is possible I have never experienced it. I can only think that in this case subscribers would leave in droves and the ISP concerned would fold. How can you tell if your ISP is the guilty party? I would like to say be guided by forum and newsgroup postings. Unfortunately this is not completely reliable. In the first place, among the millions of users there will be many who are experiencing your problem, even in your own immediate area, and are convinced they know the causes (invariably they cite the ISP). Some posters pop up under cover of a different user name to their normal one, make a disparaging post and then disappear, possibly returning under another user name and agreeing with their own post. Are they paid stooges from another ISP? Or are they genuinely frustrated and distressed? There is no way of checking. You could go by praise but to be fair, praise posts could also be made by stooges! I can only be certain about my own experience. To paraphrase a certain person recently in the news, I am a happy little Vegemite now that I’ve cleaned up my computer and fixed my power outage problem. And all without any helpdesk jockeys being harmed… SC May 2012  29