« Previous | Contents | Next »
Listen
3. The Local Loop
This section provides a necessary technical overview of the local loop, which is used to support the delivery of ADSL services. This then places in context the later discussion on scale and possible technical options.
3.1 Overview
The local loop comprises the network portion between the local BT exchange and the customer. Starting from the customer premises, a copper wire pair runs to a junction point called a Distribution Point ( DP). At a typical DP, several customers' wires are grouped into a single cable, and this cable then usually runs underground to a cabinet on the pavement (the 'green cabinet') or a manhole - technically known as a Primary Cross-connection Point ( PCP). At the cabinet the cables are bunched into main cables, which then connect into the local exchange. There are approximately 1200 of these local exchanges in Scotland.
At the exchange, the copper pairs are connected into a Main Distribution Frame ( MDF) which links the customer copper pair to their assigned number. From the MDF, a splitter is used to separate voice and data with data passed to the DSL Access Multiplexer ( DSLAM) for onward transmission and voice on to a Concentrator and the PSTN. The Concentrator converts the analogue voice signals into digital, and also multiplexes the individual customer lines together. Multiplexing interleaves many calls together on a single wire or fibre. For the majority of lines this unit is a Remote Concentrator Unit ( RCU), while for the remainder it is the Digital Local Exchange ( DLE).
Figure 3.1: Local Access Network (source: BT, Mason)
3.2 Technical Description of ADSL Provision
The ADSL service is provided through a DSLAM unit located in the local exchange, or at the RCU. The DSLAM incorporates a number of DSL line cards, which interface to the copper wire pair feeding to the customer. These line cards generate the DSL signals for the subscriber lines. Each line card may support multiple subscriber lines, and a single DSLAM cabinet may support up to 480 lines.
ADSL is specifically designed to operate on the telephone line at the same time as the PSTN voice service, by operating at frequencies above the voice band.
The human range of audible frequencies is roughly 0-20kHz, although everyday speech can be adequately transmitted using only 300Hz - 3.3kHz. PSTN is, therefore, designed to support only 0 - 4kHz.
The frequency map for a phone line supporting both voice on the PSTN and ADSL is shown below in Figure 3.2.
Figure 3.2: ADSL Frequency Plan
As can be seen, ADSL uses two separate frequency bands, one for upstream and one for downstream. With standard ADSL, the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz - 1104 kHz is used for downstream communication.
Each of these two bands is further divided into smaller elements of 4.3125 kHz. During initial service set-up, the ADSL modem at either end of the line tests which of the available chunks have an acceptable level of signal, referred to as the signal-to-noise ratio. The distance from the telephone exchange reduces the signal level, and noise on the copper wire, may introduce errors on some of the small frequency elements. By keeping these frequency elements small, an error on one frequency need not render the line unusable: that particular element will not be used, merely resulting in reduced throughput on an otherwise functional ADSL connection. There is a direct relationship between the number of frequency elements available and the throughput capacity of the ADSL connection.
BT's fixed speed wholesale products have a performance range defined in terms of signal strength at the end of the line (determined by a factor known as insertion loss) and a low failure rate. In supplying fixed speed products, BT aims to strike a balance between service availability and the operational costs of service failure. As we are dealing with a huge population of lines - about 32 million in the UK- taking engineering measurements at the far end of each individual line would be completely uneconomic. So, the Broadband Availability Checker provided by BT is based on estimates of insertion loss derived from various data sources, including line test systems, capacitance measurements, physical plant records and geographical line length estimates based on postcode data. The Checker uses these records to determine the likelihood of obtaining a particular service on that particular customer line. This is described further in Section 3.8.
3.3 Regulatory Position
Providers, such as BT, have to abide by the regulatory regime governing spectrum usage and transmission power (known as the ANFP). Access networks and regulatory frameworks vary from country to country, so the experience in France or Germany, for example, will not be the same as in the UK.
When deploying systems in a copper access network, operators need to have a high level of confidence that their services will operate as expected, both at the time of the deployment and in the future. A major factor that determines the successful operation of a system is the level of interference presented to it. Interference is inherent in such an access network, as signals couple with one another within the shared access cables in a process, is generally referred to as crosstalk.
In order to control this interference, and thus produce a predictable environment on which deployment decisions based on appropriate service quality can be based, it is necessary to have some form of frequency plan to which all deployed systems need to conform.
The ANFP is applicable to the whole of the BT copper pair access network, and is in place to ensure the prevention of undue interference between transmission systems, by placing limits on the power and range of frequencies that can be used on the network.
3.4 Attenuation and Noise on the Access Network
The most important measurement in determining the performance of the local loop is the Signal-to-Noise Ratio (S/N or SNR) measured in dBs. The higher the ratio, the better the performance. The signal on a line is reduced by attenuation, which increases with distance, and will make the ratio worse. The noise is introduced by a number of factors, the greater the noise, then the worse the ratio.
Attenuation is the amount of signal that is lost over a length of a cable: the longer the distance, the more the attenuation. Copper cables will always have some attenuation at normal temperature ranges, and is the big limiting factor for providing ADSL. Attenuation on a perfectly made piece of copper cable, with no joints, is predictable, hence why distance is often quoted as the limit. BT Wholesale has a set of attenuation figures that are based on their own line qualification tests; from September 2004 these are 43dB for 2Mbps, 60dB for 1Mbps and now with no limit set for 512kbps.
The 2Mbps ADSL limit, in particular, is set so that there is a reasonable safety margin of provision, i.e.BT can be confident that if they estimate the line to be a 42dB loss line, then a 2Mbps service will work. On lines with low levels of noise, ( i.e. a high signal to noise ratio) 2Mbps may work, even with attenuation as high as 55dB. Again, it is all down to an acceptable ratio.
The speed and stability of DSL on any given line in the access network is a function of the signal to noise ratio at the end of that line, which, in turn, depends on:
- the length, quality and dimensions of the copper cable
- the amount of crosstalk (which is directly related to 'cable fill', the proportion of pairs in the cables carrying DSL)
- noise from sources in the home or premises (including home wiring)
- noise picked up from the environment, e.g. radio frequency interference
- any faults that might be present on the line.
Accurate detection of the broadband signal by the customer's ADSL modem depends on this S/N ratio. If the signal strength reduces in relation to the noise, the receiver has more difficulty in distinguishing the signal from the noise that will affect the performance.
ADSL modems measure the loss of the line over their bandwidth and compute S/N figures at these frequencies. Based on the loop attenuation and noise values, the modem assigns data bits to individual frequencies and this then determines the overall ability of a line to support a certain data rate.
The key factor on getting higher speeds to work above these limits is the amount of S/N Margin.
One example to reduce noise is to install a new ADSL faceplate ( NTE5) on the master phone socket of the house, which can be done by the BT engineer. This can help to keep the ADSL signal away from the noisy bell wire in the telephone wiring, and is an example of improving the S/N Margin by reducing the noise.
Some other examples of the causes of attenuation and noise are as follows:
3.4.1 Cable Fill
Crosstalk simply explained is interference between copper pairs within the local loop cable. This interference causes reduction in the signal strength, which reduces the ADSL performance. The effect on the ADSL service is seen most when other ADSL customers are fed from within the same copper bundle of pairs. As the number of ADSL customers' increases within the copper bundle, so this effect increases. This is, therefore, an effect that is constantly changing, and cannot be predicted to any great accuracy.
3.4.2 Wire Gauge
The gauge or thickness of the copper wire in the local loop affects the attenuation of signal. The narrower the gauge, the more the signal is attenuated. Within the UK, the majority of local loops are 5mm, although there are some 4mm and 9mm used, although this is not a standard build, and is only used in special conditions. In addition, loops are not always of uniform gauge throughout the route between customer and exchange, and signal reflections can result from the impedance change caused by splicing wires of two different gauges. This can increase attenuation on the line. Even with uniform wire gauge, wire splices can potentially cause problems as, if they are not sealed properly, oxidation can take place which causes a high resistance and, hence, signal loss.
3.4.3 Aluminium
Aluminium has been used in some local loops instead of copper. The resistivity of aluminium is worse than copper, by about 50%, which means it will attenuate the signal more, and thus reduce the coverage of DSL on that line. DSL can still be provided over aluminium, it is the distance that will be limited.
3.4.4 Load Coils
Long loops sometimes have load coils on them to flatten out the voiceband frequency response. This enhances the quality of the voice call on the line. Unfortunately, these coils act as filters, which prevent the use of the higher frequencies required for ADSL. As a general rule, they most commonly exist on loops longer than 5.5km. However, as loops have been shortened, in many cases the loading coils weren't removed, so can still be present on even shorter lengths.
3.5 DACS - Digital Access Carrier System
In normal circumstances, a customer's phone line is provided by one copper pair, all the way back to the exchange. However, where there is a shortage of copper pairs in the local access network, DACS is used to provide two phone lines over one copper pair. Using this method, the analogue signals in each phone line are digitised in the DACS unit located on a nearby pole for example, and multiplexed ( i.e. combined with the other phone line) across a single copper pair back to the exchange. At the exchange another unit de-multiplexes the signal and converts it back to two analogue lines for connection to a standard line card on the switching equipment.
DACS has been a problem with modems previously - the 56 kbps speed of recent modems can only be achieved if there is a single digital to analogue conversion in the route from the ISP to the end user. Since DACS involves an additional conversion to digital, and then back to analogue, this means that the maximum possible bitrate over a DACS line is 33.6 kbps.
A DACS line will not support ADSL, due to the line-sharing feature, which does not support the bandwidth required. The BT position with DACS and ADSL is that the line will have the DACS equipment removed if there is a spare copper pair available, or if the cost to BT is less than £1,000. It is useful to note that some service providers will refuse orders from lines with a DACS unit, since they do not want the additional hassle these orders can generate.
3.6 Line Concentrators
The access network for PSTN consists of a variety of differing transmission methods and equipment in addition to the standard copper twisted pair cable.
Local line concentrators are remotely sited concentrators for PSTN lines and as ADSL requires a direct connection between the customers' premises and the serving exchange, customers connected through one of these devices cannot receive ADSL.
A CMUX is a remotely sited multi-service multiplexor used to combine lines at a suitable intermediary point on the local access network. As ADSL requires a direct copper connection between its ends, delivery through a CMUX can only be provided if fibre is used to connect the CMUX to the exchange, and ADSL equipment is deployed within the CMUX.
3.7 TPON
TPON (Telephony Over Passive optical Network) was a method used by BT mainly in the 1980's and early 1990's to deliver lots of phone lines easily (and cheaply) to new build areas where there wasn't enough existing copper capacity available. This decision was made before ADSL was envisaged, and as ADSL requires a copper pair route between customer and exchange, this means customers in TPON areas are currently unable to receive ADSL. BT no longer implements TPON to new geographic sites, but will do to extend existing housing estates, for example. BT are currently installing copper overlay into TPON areas to allow delivery of ADSL and this programme will proceed as part of BT's overall infrastructure investment programme.
3.8 Testing Process
BT's Broadband Availability Checker is based on estimates of insertion loss derived from various data sources including, in order of accuracy:
- Celerity testing system
- Capacitance measurements on the line
- Physical plant records
- Geographical line length measurements.
Loop pre-qualification has become more accurate with performance improvements aimed at minimising false positives - where a line is given the OK, but fails to work - and false negatives - where a useable line is wrongly judged to be unsuitable. These systems are generally based on GIS systems, network records and line testing. The use of indirect measures leads to the small amount of error, which is managed by the Red/Amber/Green process to provide a 98% confidence level.
Lines are flagged as red for the following conditions:
- Line records show provision through a line concentrator
- Engineer visit has tried and failed to provide
- ADSL is already provided on the line.
Celerity is an automated testing system that uses insertion loss (rather than length) to predict DSL speed. It measures the loss of the line with a single-ended test to enable accurate pre-qualification and to identify network problems such as load coils, which can impede DSL performance.
According to BT, the system covers about 90% of BT lines, and with a programme in place to roll it out to the remaining exchanges. The system comprises a Loop Diagnostic Unit located at the exchange, which feeds back to a centralised Test System Controller, which is integrated into the BT Operational Support Systems ( OSS). A single TSC platform typically caters for up to 2,000,000 lines of network coverage, and multiple TSCs can be transparently networked together enabling the system to test millions of lines.
Celerity data is the most accurate way of assessing availability of fixed speed broadband, and where it is not available, BT falls back to other sources of line data (capacitance records etc) as described above. BT also uses Celerity to help predict DSL Max performance on lines that do not have broadband currently. Lines on Celerity are tested pretty much every weekend to keep the data fresh and is necessary because line routings can change, bits of network can be upgraded, faults can occur that may not impact telephony etc. From August 2005, Local Loop Unbundling ( LLU) operators have had access to Celerity data on their unbundled lines, which should have helped the accuracy of their own end-to-end test and diagnostic processes.
In the event a requested service is not supported by the customer's line, an alternative (lower speed) service may be offered as an alternative via the Celerity speed binning capability. Speed binning identifies what speeds a line is capable of supporting based on the particular telco service offerings (512k, 2M, etc.) so that at least a minimum speed may be identified as supported.
If the BT checker, which takes input from this pre-qualification process, shows the line is unlikely to support 512kbps, there is the option to proceed with an engineer visit that will, where possible, supply the broadband service. If after this visit the service still cannot be provided, the line will be flagged as Red, i.e. 'unsuitable for ADSL'.
If ordering a Max based product, the only check done is whether the service is available on the exchange, and that the line has not been marked as Red, due to a previous failed ADSL activation.
It is worth noting that these testing predictions can never be 100% accurate and, for example, it is possible for ADSL to perform on a few long lines at near 8Mbps and run at less than 1Mbps on some short lines. BT cannot predict these performance 'quirks' in advance, which is why estimating line speed and availability is difficult at times.
« Previous | Contents | Next »