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A Little Background

Back in the days of the modem, you had the option of either using the telephone or connecting to the Internet. This was OK for companies that could afford to have dedicated phone lines for their modems or people that only had to briefly connect to download mail etc... but for the home user, who wanted to download files and browse the net for any period of time, this caused a big inconvenience, as they did not have access to a spare phone line so could not use the phone as well.

This may sound insignificant but how would you feel if you didn't receive that emergency family phone call because one of the kids was on the computer all afternoon? The ideal scenario here would be to be able to use the phone line while being connected at the internet at the same time. Although the technology was originally thought about for providing TV signals down a telephone line, DSL has since been developed for transmitting IP data traffic along the line with the telephone signal.

In this guide I refer to it as ADSL (Asynchronous - Digital Subscriber Line) because this is the most commonly used DSL service at present. Basically the average user will download quite a lot more than they will upload. i.e. they receive more data than they send. ADSL places a large bias on downloading, rather than uploading and downloading data at the same rate. There are also other variants of ADSL like ADSL 2+ and VDSL but the majority of the larger ISP's are currently using ADSL.

Splitting It Up

ADSL works by splitting up the available frequency in the phone line into bands. The first band is used for the voice part of your service. In standard ADSL the frequency range of 0 - 4 kHz is used for this.

The band between 4 - 25.875 kHz is not used as the voice signal is filtered off using microfilters and a slight variance in the range of the microfilter could cause some of the ADSL signal being filtered off.

The next band is the Upstream band which is the part used for sending data from your computer to the Internet. This is in the range 25.875 - 138 kHz.

The final band is the Downstream band which is the part used for receiving data from the Internet and uses the range 138 - 1104 kHz.

ADSL uses Microfilters and ADSL routers or modems to place the correct signals on the line at the correct frequency to allow them all to travel through the same cable at the same time.

Understanding The Terms

The biggest difficulty with ADSL is that the terms used are not used in everyday life by most people. So before moving on it will be a good idea to get an understanding of some of the common terms as some are not generally understood. Listed below is a selection of terms, with explanations, used when dealing with ADSL.

1. Sync Speed

The Sync Speed is the speed at which your router is connected to the equipment at the telephone exchange and is negotiated when your router starts or is connected to the line. The speed at which your router Syncs with the exchange depends on a number of factors including your Line Attenuation, Signal to Noise Ratio and Target Margin. This is how the ADSL Max service works, however, your wholesale provider can also choose to manually set what rate your router Syncs at, this is known as a fixed speed service.

2. Attenuation (ATT)

Attenuation means Loop Loss or Resistance. As your ADSL service travels through cables there is a certain amount of resistance as the Signal travels through the cabling between your house and the telephone exchange, which is called the Local Loop. The longer the length of cable between your house and the telephone exchange, and it can go via a long detour, the longer distance the Signal has to push through. The degree of resistance can depend on the quality of the metal used in the cable, thickness of the metal and type of metal used in the cable. If you have Aluminium in your line, rather than all copper, you will probably have a high Attenuation. Little will be done to lower your Attenuation if it is high.

3. Signal To Noise Ratio (SNR)

The Signal to Noise Ratio is exactly how it sounds, it's the amount of Signal compared to the amount of Noise being received. The Signal is directly affected by the Attenuation, the higher resistance of the cable and the longer the cable between you and the exchange the less of the Signal is going to reach you. The Noise is indirectly affected by Attenuation, the longer the cable the greater distance it has to pick up Noise as it will be running past more sources of interference. Generally your lines SNR will be lower at night as am radio waves, which use the same frequencies as ADSL, are stronger at night so the line will pick up more 'noise'.

4. Signal to Noise Ratio Margin

The Signal to Noise Ratio Margin is basically what is left over when Sync 'd with the exchange at a certain speed. i.e. If you are Sync 'd with the exchange at a speed of 2272kbps (A fixed 2M service) and you had a SNR Margin of 10dB, this would basically say that the line can cope with a further 10dB of noise before it will loose Sync. In a fixed Sync speed environment the Sync is set at a specific speed. If the SNR Margin is over the 2dB mark (depending on modem type) you should still be able to hold Sync with the exchange but if it falls below this level you are simply disconnected until such a time that you regain enough SNR Margin to connect at the set Sync speed.

5. Target Margin

The Target Margin is used in the BT Max setup of ADSL, known as Rate Adaptive ADSL. Unlike a fixed speed service, a rate adaptive service will say I want to connect at the highest possible speed with a SNR Target Margin of 6dB. If the SNR Margin falls to a point where the line is unstable, the router will re-Sync, again with a SNR Margin of 6dB. This extra stability will come from the Sync speed being lower. Usually a wholesale provider will set the Target Margin, an ISP can ask for a particular Target Margin or, if your ISP is receptive you can request it be changed. Different wholesale providers have different systems to raise your Target Margin if your line is unstable.

6. Download Rate

Your Download Rate is the total speed at which you can download files from the Internet and it is based on your IP Profile if on Max and your Sync Speed if on a fixed speed connection. You can test your Download (and Upload) Rate with numerous speed testers available on the Internet but you should take a collection of figures from different sites as the way that they are connected to your ISP and the way they conduct there test can cause the odd anomaly. In an ideal world it should be around 5 - 10 % below your IP Profile if on Max or Sync Speed if on a fixed speed service, however exchange or ISP congestion and ISP Port throttling can cause it to be somewhere between a little and quite significantly lower.

7. IP Profile or BRAS Data Rate

The IP Profile, also known as the BRAS Data Rate, is the Limit set to your available download speeds. This is based on the lowest Sync over the last 3 days. This is the problem with BT's implementation of the Rate Adaptive system. If you Sync at 6M all of the time but every 3-4 days something produces a massive amount of Noise on your line for a few seconds, your router could Sync at 1/4M causing a constantly low IP Profile, even though 99.99% of the time you Sync fine at 6M speeds.

The IP Profile system is needed due to the ATM nature of the backbone of the BT Wholesale network. This is being upgraded, but it may take years. IP Profiles have been known to get stuck. This is usually at the 2M speed, so a download rate of around 1.8Mbps. If this happens you need to contact your ISP. This process is used by BT wholesale so is used by all ISP's that use the BT wholesale network.

If your Broadband line is unbundled (LLU) then the IP Profile is not used, your download rate will usually be just lower than your current sync speed.

The following table explains what Sync Speed you need to achieve a particular IP Profile, remember, the lowest Sync over the last 3 days is the one that counts.

Sync Rate (Kbps)	288	576	1152	1728	2272	2848	3424	4000	4544	5120	5696	6240	6816	7392	7968	8128
IP Profile (Mbps)	0.25	0.5	1	1.5	2	2.5	3	3.5	4	4.5	5	5.5	6	6.5	7	7.15

Note : The Upto 8Mbps service never reaches an 8M Profile.

So if your lowest Sync in the last 3 days was 2688Kbps you should be on a 2Mbps IP Profile, which would give you a Download Rate of around 1.8Mbps.

8. Interleaving

Interleaving is a process which is used as a way to decrease the amount of packets which need to be re-transmitted on unstable lines and is usually employed with error correction algorithms. When Interleaving is set on a line it will split data up and send it non sequentially. This way if a segment of data is corrupted due to noise on the line only a fragment of the packet is damaged not the entire packet. This makes the error correction algorithms more likely to be able to correct the entire packet using extra error correction bits added before transmission.

Interleaving is usually a good thing, but it can be bad if you are using applications which want low latency connections like games, for example. Interleaving can be set at different levels called the Interleave Depth. This represents the number of segments sent before returning to the first packet for the next segment.

How Do I Find My Stats?

The above mentioned statistics can tell a great deal about what is happening with your telephone line and Broadband connection when having problems. The main statistics of concern are the Sync Rate, Signal To Noise Ratio Margin, Attenuation and, if your ISP is using a BT Wholesale provided product, the IP Profile.

The main point to note when you have a problem and want to check the stats for your line is that you need to get the stats from your modem or router when it is plugged into the test socket of your NTE5 as this eliminates any problems that may be caused by your internal telephone wiring. If your problem does not appear when plugged into the test socket the problem is not with your Broadband connection but with your internal telephone wiring. If this is the case you need to read the page on optimizing our wiring.

Finding these stats is usually quite straight forward as most are listed on your routers web admin pages or can be pulled off of your modem using command line tools like Telnet. However, due to the fact that there are many different modem / router manufacturers, makes and models finding the correct place to look for these stats can sometimes cause there own problems.

When trying to find where to locate the stats on modems / routers that I have not used before and are not immediately obvious, generally the first place I start is looking at KITZ web site as she has spent a great deal of time and effort to provide details for finding the statistics from various routers and modems. Even if you can't find your own specific make and model of modem / router you should be able to find at least a similar model by the same manufacturer which will usually use a similar process.

Your modem / router will usually give the stats for Sync Speed, Attenuation and SNR Margin but it will not give you your IP Profile. To find this you will need to connect to the BT Speedtester and run the speed test. After running the BT speed test, and it may run a couple of times, you will be provided with Current Up and Down Sync Speed, the current download rate and the IP Profile currently assigned to your line.

Be Warned that the BT Speedtester usually gets very busy in the afternoon and evenings and on certain days you will find that the only time it will run will be in the early hours of the morning.