The sync speed of your ADSL modem to the exchange is the absolute limiting factor in your broadband speed. The Sync Speed is what is advertised as your broadband speed, ie 8Mbps broadband and is measured in bps (bits pers second), not to be confused with Bps (bytes per second).
So what factors affect your sync speed? Things like line attenuation and electrical noise reduce the efficiency of your cable and mean the modems have to sync at a slower speed to maintain a decent connection.
Line attenuation, which is the loss of signal as it travels through the cable, is mainly caused by the length of cable from your house to the exchange. Points to note here is that your cable is hardly ever the length quoted by the ADSL checker websites as these always use an approximated "as the crow flies" value between two postcodes. A good rule of thumb is to at least add half again to get your approximate cable length as the cable tends to follow streets rather than go directly to your house. Other things that affect attenuation are any junction boxes in the cables route and any repairs that have been undertaken.
Electrical noise is the quality of the signal on the cable. ADSL pushes the copper cable quite hard compared to voice signals so even though your line may sound fine when you make a voice call its no indication of the amount of electrical noise on the line. One annoying feature is that the ADSL signal itself will effectively introduce noise on the line so sometimes a higher speed signal will generate too much noise to be usable.
"The last mile" is a term you will hear being used with broadband as its where most of the cabling faults occur and it refers to the length of cable from the green junction boxes down your road, off the telegraph pole, into your house to the master socket. Once here the signal may be sent into your houses own internal extension wiring which is the home owners responsibility as once the signal has reached the master socket it ceases to be the responsibility of the telecommunications company.
All this is a little techie but there is a good analogy that can help you understand signal strength, noise and signal to noise ratio. Start off by imagining that your modem and the exchange are two people who want to talk to each other.
Signal strength is how clearly the other person can hear your chat. Obviously the longer the distance and the louder and slower you have to talk to be heard. Try it yourself, you can speak really fast but try and do it while shouting and you can't ... its kind of the same with modems.
Noise is things such as echo from your shouting and other noises from outside sources. As you can see the further you are apart and the more background noise there is the louder you have to speak and therefore the slower the conversation.
Signal to noise ratio margin is how loud you are minus how loud the noise is, simply speaking.
With this in mind . if they stand next to each other in a quiet room they can start talking and they can chat quite quickly and everything is fine. This is your theoretical 8Mbps scenario (fast chat) on a perfect cable (a quiet room).
All cables are not equal however, some are longer some are noisier, some have echo so imagine the same two people at opposite ends of a church. Its still a quiet building but they cannot talk as fast as they have to talk louder and clearer and therefore slower.
Put them at opposite ends of a train station and they would have to scream to be heard over the noise and distance and therefore end up conversing much slower.
One common cause of complaint is people saying that they are a short distance from the exchange so they should get the full speed. Take your two people above and stand then two feet apart and then stick them in a nightclub, the distance is tiny but you have to shout to make yourself heard and therefore, despite being close, you get a slow connection speed.
Its frustrating but its just a part of broadband. Its even been known works the other way sometimes in the same way as it does in real life. In the whispering gallery in St Pauls you can stand one side of the gallery and speak very quietly and hear it the other side incredibly clearly. It happens in real life with people getting exceptionally clear lines despite them being quite long.
In all of thee cases intermittent events can cause the conversation to break down even in the best conditions a loud car passing by could interrupt the conversation on cause the conversation to slow. Temporary road works could turn a very pleasant park bench into an horrible place to sit and chat.
To complete this analogy, the local factors around one person can also limit their ability to converse. local noise being the primary cause. If one party is listening to the radio then although they can talk and be heard the other person has to talk louder to get over the local noise.
As you can see there are a lot of factors there but hopefully that makes it easier to understand why its not a simple science of "You are 1.2 miles from the exchange therefore you will get speed X".
TalkTalk, by default, use an 8Mbps 15db profile. A what ? Well, this is an over simplified explanation that should give you an idea of what it means.
In the early days broadband was simple a fixed speed signal was chosen and sent down the line, if the other party could receive this signal then you were in business and synchronised. The speeds were, typically, 512Kbps, 1Mbps and 2Mbps.
With people wanting faster broadband it was noticed that some people were getting excellent Signal To Noise Ratio even at 2Mbps so in theory they could talk faster and therefore a new generation of ADSL was born.
With the new broadband, instead of sending a fixed speed signal down the line a maximum speed and a target noise ratio are specified. When the modems start to "chat" they start at the maximum speed specified then keep reducing it until they can hear each other and the signal to noise ratio matches the specified ratio.
For example, TalkTalk use a profile they consider to be rock solid which attempts to achieve an 8Mb sync speed with an SNR margin of 15db so when two modems start to chat they will attempt to get the fastest sync speed they can whilst still maintaining a 15db margin.
The minimum SNR Margin is considered to be 6db so they have 4 profiles for 8Mb. 15,12,9 and 6db. The lower the SNR Margin the more sensitive the line is to noise meaning you stand more chance of losing synchronisation. This is another annoying feature of ADSL, a low quality line has a low SNR and requires a profile with a high SNR profile, a high quality line can run with a lower SNR profile. Just the opposite of how you want it sadly.
In most cases there is not a lot you can do short of moving house so you are closer to the exchange. The one area you have control of however is part of "the last mile". You internal wiring.
If you get a problem with your broadband speed you will find most ISPs will ask you to go to the master socket and expose the test socket underneath it and plug in there. The reason for this is it disconnects all the internal wiring which, in one fell swoop, eliminates
In theory plugging a modem into a microfilter and plugging that directly into the master test socket you will achieve the highest sync speed possible for your line and eliminate most local faults.
Decent and correctly fitted microfilters are a must and keeping your ADSL equipment away from noisy electronic equipment can help such as DECT phones. Even flashing Christmas tree lights can generate interference.
Note: One important point thing to note is that if you sync at 4Mbps then you will never be able to download more than 4Mbps. In fact, due to overheads in maintaining the connection, error correction and protocol overheads you should expect about 80% of your sync speed as the download bandwidth available to your client, ie your PC.
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