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VDSL - A light touch
With ADSL2+ technology now being pushed to its absolute limits, carriers are talking about the next generation of broadband, VDSL and VDSL2, with speeds of up to 200 Mbits/s on relatively short line lengths from the DSLAM. Jorg Franzke explains how it is possible to roll out the speed benefits of VDSL and VDSL2 to most urban and city customers, without breaking the telco bank
It’s been an eventful decade across Europe as former state incumbents, cable companies and virtual telcos have all, seemingly en-masse, jumped on the broadband telecoms wagon and rolled out an increasing range of high-speed broadband services to their customers.
Most experts agree that, even with ADSL2+ offering customers access to up to 24 Mbits/s downstream data speeds, customers’ appetites for even faster speed services are still increasing, with some cable companies already talking about offering 100 Mbits/s as standard.
The only problem with this new generation of very high-speed broadband services is that they rely on VDSL and VDSL2 technology. Whilst ADSL2+ can happily support copper line lengths of two or more kilometres, the maximum available rates are achieved with VDSL at a maximum range of just 300 meters from the DSLAM (digital subscriber line access module), which gives around 52 Mbits/s. When we move on up to VDSL2 (ITU G.993.2) technology, carriers are even talking about rates of up to 200 Mbits/s.
But VDSL2 deteriorates quickly from a theoretical maximum of 250 Mbit/s at zero metres from the DSLAM to 100 Mbits/s at 500 metres, and 50 Mbits/s at 1.0 kilometre. As a result of these line length limitations, very few customers will be within the coverage range of VDSL2 DSLAMs installed at the central exchange. So, most local loop carriers are discussing moving the active electronics, including the DSLAMs, out of their central offices and into larger versions of the roadside cabinets that form an integral part of the street furniture we see every day.
A major issue is that, with the move out of the central office comes the de-centralisation of the main distribution frame where connections have to be moved to initiate new services such as ADSL and VDSL. Each time a customer requests a change of service, jumper wires have to be moved - fairly easy and efficient in a warm, dry clean centralised environment – but once the connections have to be made in the cold and rain it becomes an operational issue. To give the reader an idea of the massive scale involved, however, a network the size of BT in the UK would require around 65,000 of these externally deployed active electronics cabinets. A network the size of Germany’s T-Com would require around 100,000 such cabinets.
In theory, the incumbent telcos could employ teams of roving engineers to maintain and provision the cabinets in much the same way as central offices are serviced at the moment, but the costs associated with the necessary engineering `truck rolls’ is anathema on the financial and ecological fronts. Even one visit per fortnight, at say ?50 per technician visit, would clock up annual costs of ?130 million per annum on a 100,000 cabinet network.
Consequently, any carrier electing to stay with the status quo and implement manual re-jumpering at the thousands upon thousands of active equipment roadside cabinets, will be forced to reduce their costs by making only scheduled visits. The corollary of this is that each cabinet may only figure in the schedule once every fortnight, meaning that the time-to-provision each customer will become much longer than currently is the case.
New approach
A markedly different technique is needed and newly developed automatic cross-connects (ACX) can now be used to replace manual distribution/jumpering frames in the remote cabinets and so save carriers significant sums of money on the operational expenditure (OpEx) front.
With an automated ACX solution, not only are there no delays in waiting for an appropriate technician truck roll, but also the control of the ACX can be integrated directly into the carrier’s operations support system. Using this approach allows for the service connection to follow on automatically from the customer's order, within an hour or two, rather than the customer facing a wait of several days, as is currently normally the case with a central office, or several weeks in the case of above manual re-jumpering scenario.
Many carriers and manufacturers alike are chasing a holy-grail of Zero Touch for their networks. We have been pioneering a slightly less pipedream approach based on practicality and best Return on Investment.
The aim of a Zero Touch network would, of course, be that the field technician never needs to visit the remote site. Which is all very well until you take into account that active equipment, and its power supplies and air-conditioning go wrong from time to time. So occasional technician visits are inevitable.
Zero touch systems have other drawbacks, not the least of which is the fact that the purchase costs can be substantial, so reducing the installation's return on investment. In the case of automated cross connects, Zero-touch would need a non-blocking switching matrix which is very expensive, and current non-blocking technology simply isn’t up to the job of transmitting 100Mbit/s signals.
The third issue with zero touch systems is the fact that the cabinet needs to be equipped with a large degree of reserve DSLAM and splitter capacity, ditto power supplies and air-conditioning, so seriously increasing the levels of capital expenditure required.
Our theory is simple. What happens if we introduce a minimum number of technician truck tolls to the mix, creating a `minimum touch’ not zero touch active electronics-based local loop?
This is where the financials begin to get interesting, as a minimum touch network is far more financially viable. It requires significantly lower levels of capital expenditure with very similar levels of operating costs. Less spare capacity is needed as this can be added when demand dictates. Likewise a much less expensive semi-blocking ACX can be used – with the full frequency range for 100Mbit/s service delivery
A good minimum touch system has the advantage of automating the provisioning and re-provisioning of lines without incurring the high capital costs of a zero touch system, or attenuating the signal levels required for effective VDSL and VDSL2 transmissions.
Well before the ACX system reaches saturation levels, it can signal its status to the central exchange, allowing engineers to make a planned site visit, install additional capacity if needed and hardwire connections already switched through to VDSL freeing up the switch ports to be used again for the next six or twelve months.
The result of this approach is good scalability, lower cost per line and a reduced space requirement. And all without affecting those all-important customer satisfaction levels.
Using a minimum touch ACX approach means that only one or two maintenance visits each year are required for each cabinet, with remote monitoring shouldering the responsibility of maximising network up-time.
In the event that something like a DSLAM card fails, the ability of ACX to connect ‘any-to-any’ can be employed to ensure that customers are only minimally affected by any technical problems. Depending on the severity of the failure, the cabinet's active technology can be remotely reconfigured to maintain service for the customers affected and the network operations centre can schedule a truck roll when it suits the operator. This makes for a more cost-effective maintenance strategy.
ACX technologies
In a survey of switching technology for remote automated cross connect devices used in next generation carrier networks, research company Venture Development Corporation (VDC) considered a number of technologies, but rejected robotic and solid-state/electronic switching, the former being error prone, expensive and with poor life expectancy, whilst solid-state/electronic switches have electrical parameters that make them unsuitable for the high bandwidth requirements of xDSL services like VDSL2. VDC also noted that a very specific `electromagnetic’ variation of the MEMS relay may become a suitable technology, but this is currently only in testing as regards ACX applications and, as yet, has no field application track record.
VDC concluded: “We believe the electromagnetic relay is acceptable technology because of its proven reliability, ruggedness and minimal transmission impairment.” It did not judge any other technology to be currently acceptable. This, and the fact that the power requirements are so minimal, are the reasons that we had chosen to develop our own ACX product range around the tried and tested electromagnetic relay.
Obviously whether or not to implement ACX or to manage the service provision process manually is a matter for individual carriers. The choice of technology is critical from the perspective of reliability, minimal power consumption and the ability to handle the very high frequencies needed for VDSL2, but far more important in this rapidly changing telecoms word is the need for rapid return on investment.
It is our contention that Zero Touch is a step too far and that in the world of every day engineering issues, Minimum Touch networks and minimum touch ACX are the way to minimised costs.
Jorg Franzke is ACX product manager for ADC KRONE, and can be contacted via tel: +49 308453-2498; e-mail: jörg.franzke@adckrone.com
www.adckrone.com
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