PS Consultants - ideas & solutions

Microwaves and DSL – going digital and locally loopy
October 1998

Last month’s news of the plan to prise the local subscriber loop from the clammy clasp of BT was quickly followed by tales from HMG of the 20-40GHz spectrum being made available for “radio tail” deliveries. Let me explain this jargon, and why you will be interested. Indeed, why it may also help change the face of Two Jags Preston and his pathetic transport strategies, if we can at last provide a viable means for teleworking and conferencing.

The path from the nearest phone exchange to your front door is either along telegraph poles that get struck by lightening, or in trenches that get dug up by any one of the 5 or so utility operations that like to plant their services in the ground. That route is traditionally carried with a pair of copper wires, many locations may still have copper that is 30 or more years old – and there is nothing particularly wrong with that. The problems in old cable systems arise at the connection nodes where corrosion can set in, and produce effects ranging from an occasional crackle to a completely open circuit.

In analogue circuits, a bit of a crackle is not a show stopper – indeed, analogue circuits of all sorts can handle intermittent connections better than digital circuits, and we should not get carried away with the notion that digital is indubitably superior to analogue in all circumstances.

Certain HiFi buffs who cling to vinyl (an analogue medium) seem willing to kill in order to preserve their way of life in the face of not only digital CDs, but digital media that increasingly apply lossy compression techniques (the Sony Minidisk, MP3). For the purist, audio recording with lossy compression techniques is tantamount to painting the Mona Lisa in watercolours or the same sort of sacrilege as rebuilding a cherished Bentley S2 Continental Flying Spur with the engine borrowed from a 1984 Ford Sierra 1.6L.

Lobbing gigabits worth of bandwidth between exchanges (also these days referred to as “points of presence”) has become something of a no-brainer as a lot of US high tech money has been poured into the process of producing ever faster switching technologies. There is very little of the finesse required in these technologies that analogue design demands in this business – it’s all down to brute force and speed, where the art of the science is pretty much reduced to producing switching devices that switch quickly enough between the power supply rails without every quite getting so far ahead of itself as to short circuit them – because that’s where power dissipation and heat arises, and dissipation is traditionally the limiting factor in this type of speed evolution.

Much recent effort has been devoted to find ways to divide and haul this monster bandwidth from the exchange to the individual subscribers. Operations with fibre to the door are laughing – there are lots of choices, but digging in and managing fibre is costly from all perspectives, so since 100% of locations are served by copper pairs, guess what..? There is a huge effort to develop (and contrive) ways and means of using the copper to haul the higher data rates.

ISDN is pretty much a “base band” switching scheme – which means that the switching is “simple” transitions of ones and zeros on the end of the wire at both ends. The copper delivered ISDN 2 service operates as two 64kBit duplex (two-way) channels. But as any mathematicians and physicists amongst you will realise, the square waves of digital switching waveforms are made up by the combination of a sine wave and many higher frequency harmonics. So although (simplistically) the clock rate may mean poking 64kHz square waves up and down, there is probably significant energy within the waveforms at anything up to 30MHz.

Long twisted pair transmission circuits have finite characteristics on capacitance and resistance that mean that the speed of delivery is a compromise with length of circuit – and I know that many of you who have asked BT to supply its Highway ISDN services have been fazed to learn that you can’t have it because the nearest exchange is too far away. 4-8000 feet is not an untypical range at which Home Highway cops out, although the industrial strength ISDN service should be good for 5-6km. The cables used form most UK telephone pairs are approx 0.5mm diameter of copper, and this sets the basic rules (fatter cables are better because they have lower resistance).

DSL (digital subscriber line) technology is a hybrid that deploys variations on the theme of carrier modem technology, it doesn’t attempt to poke megabits of baseband along the wires, but operates by modulating a higher frequency carrier signal that is generally spread through traditional radio broadcast frequencies.

Yes, this radio signal radiates from the unshielded copper pair conductors. Yes, this means adjacent circuits can suffer from “cross talk” effects, and with 50 pair cables being used for many exchange-to-home deliveries, this is not an inconsiderable issue. And yes, despite assurances to the contrary, this all creates a lot of radio interference – but just as cellular radio operations have been inclined to overlook that problems their services are creating in terms of potential health hazard, until the market has reached a point where the idea that cellular radio should be in any way curtailed is almost economically unthinkable for most countries, so DSL is likely to be allowed to become incumbent, and turf out other radio spectrum uses in the interests of enabling the cheap digital future. I know I may one day come to regret this assertion, but there really is no health hazard in DSL at the power and frequencies concerned, just considerable inconvenience to other spectrum users. But since radio will be delivered via IP connections anyway soon enough – why worry..?


DSL comes in many variants, with two basic propositions: ADSL where A stands for asymmetric with different “suck and blow” rates – just like in the good old days when Prestel modems were asymmetric at 1200 baud in and 75 baud out.

It’s possible to transmit a signal a greater distance from the exchange to a remote home or office than in the opposite direction. This is due to crosstalk effects which are bigger exchange end of the copper wire than on the remote subscriber side. This is due to the fact that more copper wires, each of which introduces a crosstalk component, are combined in large bundles as they get closer to an exchange where the “listening devices” determine the return speed of the remote connection. But at the user end, the loops branch apart for connection, resulting in fewer copper wire loops. Therefore, less aggregated crosstalk is introduced at the subscriber end.

And the other variant is – you guessed – symmetric where the upstream and downstream rates are the same.

BT has been using a variant of SDSL for some time to deliver primary rate ISDN services in areas where fibre isn’t yet an option. This is HDSL (H for high speed) and it uses 2-3 pairs of copper to do its stuff up to 3-4km from the exchange node. VDSL is the seriously fast version for so-called Video on Demand services which has an overall bandwidth of circa 60Mbit, which can be divided symmetrically or asymmetrically.

DSL also features that holy grail of internet users – an “always on” services which is packet switched not circuit switched (timed calls). General ADSL services are delivered as flat rate subscriptions in the US (and probably in the UK when the telco act eventually comes together).

If you want some more on DSL, then look here – it’s possibly the best short form guide I have found on the web (unless you know better..?)..

And alternative to DSL to the home is starting to emerge amongst microwaves services as frequencies in the 20-40GHz range are being offered for land deliverables. This are seriously short wavelengths – much shorter than a microwave oven. In fact, 40Ghz might even escape from the holes in the mesh on the glass door, and although that isn’t necessarily an indication of the potential health hazard, the frequencies are definitely more likely to fry your brain than Radio one FM. (errr, perhaps that was a bad example…)

20-40GHz is barely short of Infra Red in the scheme of things, and so line of site is the rule. Atmospheric interruption needs to be taken into account, The “dishes” for the links are very small and tidy affairs – no more than about 5-9 inches diameter.

The bandwidth deliverable along these beams is considerable 10Mbit ought to be simple – and the rang from 30m to 15kM means that this is easily the lowest cost of the broadband delivery technologies, at the presently suggested £600.

The question of managing contention – the situation where many users are scrambling for a share of the same overall bandwidth – remains to be tackled in many technologies, notably cable modem services where a notional 10Mbit can very quickly become a real 60-100kBit. And all bandwidth ultimately relies on reaching the “far end” of the chain at the same bandwidth. But the reality remains that as you traverse the internet from your 2Mbit SDSL connection and arrive some 15 routers later at the server offering the content, you probably turn up with as many as 1000 other users trying to get a share of the server’s 10Mbit (if you are lucky) bandwidth. That’s right, you will get a (10Mbit/1000) 10kBit share of the available bandwidth.

However, there are some creative ideas emerging on managing bandwidth through improved caching techniques where “popular content” only has to make the trek from a cache/proxy located at your nearest exchange. But once again, 1000 users all wanting to connect to a video server cerates a monster bandwidth management issue for content that is delivered on demand. this isn’t going to be solved overnight, so take all the tales of ADSL and cable modems with a pinch of salt. Just as the result of building more lanes on motorways results in more congestion on the feeder roads, the same applies to trunk bandwidth.

However, some variants of satellite delivery are providing some interesting alternatives along lines of guaranteed bandwidth and capacity to up to an unlimited number of users, and with the prospect of some really serious bandwidth becoming available to the users in the foreseeable future, this month’s R&R focus is on web tools – page development and content manipulation for video.