ADSL diagnostic

[The Natural Philosopher]

There was a major outage last week in BT's ATM network that caused I, and others, to lose ADSL connectivity..

..The support I got when I reported it is not good, but in following up the issues to do with the (lack of) support this website came up.

it allows you (or someone else, if you are current;ly ****ed) to check to see if there is a fault within BT's major ATM backbone that affects one or more exchanges.

Out of interest, I also discovered that my router (D-link D504, older than Methuselah) had ADSL diagnistics (which passed, showing I could talk to the DSLAM at the local exchange) and indeed ATM diagnostics, which showed that that was as far as I could get.

(Essentially ADSL is what you use to talk to the exhange - like a super modem - ATM is the next protocol level which BT uses to pass your signal off to the ISP, and the PPP layer protocol is what you use to talk over that to your ISP, log in, and start up IP services over the rest)

In this case the ATM link was clearly broken... I pass this on as potentially useful info to people who have broadband problems...at least you can 'phone a friend' and get them to use this to see if YOU are currently suffering a known outage, or whether as the helpful BT man suggested ' you router,she is obviously broken'

:-)

It also allows you to see if your exchange is subject to a high contention ratio, and is therefore slower than it should be.

Enjoy, you sad nerds like me :-)

[Jim Gregory]

Brilliant info for u nerds on BT. Is there an equivalent diagnostic thingy for ntl cabled broadband which is known as DSL? Jim

[Alex]

Cable broadband is not DSL. ADSL is DSL.

Alex.

[Jim Gregory]

Oops, I had thought it was! Jim

[Andy Hall]

Neither of them are broadband, but that's another story.

[Dave]

Agreed!

Quite why the term 'broad band' came into use in computers, I have no idea.

The use of two wires to connect to the internet is called a serial connection. As such, it can never be specced as broad band. Broad band is the ability of a signal to occupy a wider spectrum. As such, only a modulated signal can be broad band. i.e. the data is not in a serial stream, but is spread across a wide frequency.

What is called broad band, these days, is just a very fast serial connection.

To clarify this...

Broadcast FM transmissions use a broad band system. About 25 KiloHertz. Don't quote me, it is late at night and the whisky bottle is low in content.

Medium wave transmissions uses a narrower band.

Morse code is a true narrow band system, signal on, signal off, just like the way we connect to the internet.

HTH

Dave

[tony sayer]

In

|Thats OK:))

9 kHz

Lo lo hz

[Gary Cavie]

My memory of these things is very rusty, as I haven't worked in the area for some time, but isn't ADSL transmitted (on the local loop at least) using DMT (Discrete Multi-Tone)? From what I remember, it uses many channels, spread over the 1MHz or so of bandwidth on a local pair (effectively Frequency Division Multiplexing). If correct, is this not broadband as you defined it above?

I agree once it gets into the DSLAM, it is converted to true serial bits on the STM-1 (4? / 16?) link into the ATM network. Even serial data uses bandwidth though, proportional to the rate of change of bits, or something like that!

Probably all changed significantly in the last 5 years though!

[John Rumm]

I expect you are agreeing at crossed purposes! ;-)

The connection between my TV and its aerial is with two wires... does that make it serial also?

Which is exactly what ADSL does - transmits a modulated waveform down a copper pair with a wide bandwidth (relatively at least compared to the normal voice channel bandwidth of 3.4kHz).

depends on what level you look at it... you could argue that ethernet is a serial link with differential manchester encoded signalling on the wire - but it is not what most people would call a serial link.

ADSL uses just under 1MHz of bandwidth on the downlink side. The modulation scheme used is form of Orthogonal frequency-division multiplexing (OFDM) although many DSL modems routers refer to it as discrete multitone modulation due to the way the bandwidth is broken up into a multiple of parallel tone bands to overcome any dropout frequency regions in your phone line's response.

(the ADSL signal also starts at over 25kHz - way above the audio signal on the phone line)

[Andy Hall]

I can give an insight here..

When I first got into data networking in the early 80s, ethernet was pretty new and IP was just coming into being through the works of Vint Cerf and Jon Postel among others; the term 'broadband' was used to describe multiservice networks based around bandwidth division of cable TV infrastructure.

In fact there were systems for delivering 10Mbit ethernet presentation using three channels of such a system provided that it was engineered correctly. There were data networks using this and more channels to obtain what were, for the time, quite high bandwidths over areas much in excess of what could be done using ethernet with repeaters.

Thus broadband came to mean an infrastructure over which one could deliver data, voice and broadcast quality TV services.

More recently, the notion of being able to deliver data/voice/video over phone lines to the home had great appeal to the marketeers at the telcos. So much so, that they thought that calling this 'broadband' would be a great way to describe it. They even believed their own bullshit to the extent of trying to sell it in this way full of the promises of multimedia.

The reality has fallen far short of that. None of them ever described their services as slow. The 110 and 300 baud modems of the 70s gave way to 1200, 2400 and so on up - each having "blazing" speeds. At one point, even 128k ISDN was described as "broadband". Even in comparison with 56k modems that's a stretch.

To describe standard ADSL as "broadband" is a downright lie. The higher speeds are only available very close to the exchange. 512k and

1Mb services are much more typical for most users.

Broadcast quality TV (meaning full size screen on a TV set) isn't possible at these bandwidths (at least with today's codecs) The 256k return bandwidth makes the service unusable for any form of quality video conferencing other than fairly crude desktop stuff.

So not only are these incompetent telco marketeers not delivering a broadband service, they are not even delivering on what they tried to convince us that the technology could do.

Arguably, the closest technologies to broadband are cable modem and satellite. Both can deliver data services, but neither are very good at it. At least they can deliver broadcast quality TV.

[Douglas de Lacey]

Ignoring your terminology,

is a good starting-point for all things cable modem.

Douglas de Lacey

[Paul-S8]

I thought everyone was on BT (the infrastructure at least) - just like gas and leccky you don't have another line installed.

Good info anyhow!

Off topic - Many years ago when privatisation was happening I was talking to some guys and the mention of one of the electric companies suppling gas, one guy said "how do they get it to you"? which followed the comment of "They fit a special adapter on your electric cable and push the gas down the small gap between the wires"

Paul

[usenet]

Strictly "neither of them is broadband" if you're going to be pedantic.

But the system used to provide ADSL *is* a high frequency signal divided into pieces with each 'channel' used to carry data. Exactly what you have described as 'broadband'.

It's most certainly not serial where all the 'bits' of data are sent one after the other.

[Ben Willcox]

Not sure about that. My understanding is that the definition of broadband is (something like) multiple different signals modulated onto a single transmission medium. Therefore ADSL and Cable Internet are both broadband.

Broadband is the opposite of baseband (as in 100BaseT Ethernet for example).

Ben.

[manatbandq]

Except those who have "cable" installed. It's just that, another cable, not BT.

MBQ

[manatbandq]

Not in any normal sense of the term "serial" when applied to data comms.

I suggest you get a "broadband" connection and use it to do some reading on how ADSL actually works. Google will be your friend here.

MBQ

[Andy Hall]

That is true as well. However, at the time that it was defined, the technology having multiple signals modulated onto one medium was RF cable, and "broad" was also used in the sense of having lots of bandwidth. It was sold and used on the basis of being able to deliver data, voice and proper video services.

While ADSL (together with POTS) can do voice and some degree of data, it doesn't have the bandwidth to deliver proper broadcast TV quality video.

Generally people don't derive their main video entertainment from sitting in front of business card sized videos on a PC screen.

In that sense, the marketeers have massively overhyped ADSL.

[douglas.farnan]

I'm not.

I've got Telewest, and (ICBW) they have all their own cables and things in the street, and it comes into it's own box inside the house. I also have a BT box in the hall however.

I remember when Telewest launched around here, I was doing GIS work for Scottish Water and the amount of PU enquiries they generated (since they laid cables in every street) was enormous.

-- Doug "Doug's cool. He's metal ;)" - Fnook Ignore the old spamtrap work address; mail me on: doug at fruitloaf dot net

[Jim Gregory]

Thanx for that url, Doug. BTW, a test on this reference calls a cabled service DSL, and ntl use fibre-optical links to their distribution centres. Jim

[Capitol]

2Mbit modems were around in the early 70's for telecoms purposes. The range on a pair however was only around 1.5KM ( and at that point, the signal was apparently lost in the noise and took a lot of effort to recover it). I doubt that today is much different as the cables have often not been changed. However, there's no reason not to use a fibre cable to extend the range from the exchange to almost any distance you wish. This leaves only the black box to customer range at 1.5KM for 2Mbits. I am intrigued by the thought of 8Mbits. I'd expect the range from the fibre interface to be down in the 400M range, unless the modems have increased tremendously in power.

Regards Capitol