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| Access to Bandwidth: Determination on the Access Network Frequency Plan (ANFP) for BTs Metallic Access Network | |
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October 2000 Determined Access Network Frequency Plan This document may also be downloaded in .pdf format. Please click here to download. 1.1 In September 1999 Oftel requested that the Network Interoperability Consultative Committee (NICC) DSL Task Group reconvene to address certain technical issues related to local loop unbundling (LLU). 1.2 One of the tasks presented to this group was to develop a process to control interference in the access network, later to become known as the access network frequency plan (ANFP). The ANFP will essentially determine which digital subscriber line (DSL) systems/technologies can be deployed in BT’s access network. 1.3 Four proposals were submitted to the group in late 1999 and early 2000. The essential difference between the proposals concerned the extent to which it would be possible to deploy a particular technology for offering symmetric services (so called "CAP" - based HDSL) systems for offering symmetric services. It was considered that the maximum deployment of CAP systems could limit the deployment of ADSL to some extent because of the risk of interference between loops which had been upgraded to ADSL and loops which are still being used only for voice telephony. 1.4 At the meeting on 5th May 2000 the NICC DSL Task Group were unable to reach agreement and therefore it was necessary for Oftel to intervene and propose a way forward. 1.5 Since the group was unable to reach a consensus, on 7th June Oftel published a consultation document - Access to Bandwidth: Proposed Solution for the Access Network Frequency Plan (ANFP) for BT’s Metallic Access Network and invited comments by 5th July. 1.6 Oftel received nine responses to this consultation and these are summarised in this document. 1.7 Given the continued lack of industry consensus in this area, the urgent need for a stable plan and the responses to the consultation, Oftel believes that the ANFP proposed in the consultation document strikes the right balance of public policy. While this ANFP does limit the deployment of CAP - based technology, it does not limit the offering of symmetric DSL services to the same extent. The plan allows deployment of equipment conforming to the recently developed European standard ("SHDSL") for symmetric DSL technology. Oftel therefore confirms this proposal for Issue 1 of the ANFP.
2.1 At the request of Oftel, the Network Interoperability Consultative Committee (NICC) DSL Task Group was reconvened to address various technical aspects of local loop unbundling (LLU). An important element of their work was the development of an access network frequency plan (ANFP) to control interference in the metallic access network. 2.2 After nine months of analysis and debate it became apparent that this group would not be in a position to agree a suitable ANFP by the specified delivery date. At this time Oftel intervened and proposed a solution to ensure that the LLU times scales were not jeopardised. Oftel’s proposed solution: Access to Bandwidth: Proposed Solution for the Access Network Frequency Plan (ANFP) for BT’s Metallic Access Network was published on 7th June 2000 with responses being invited from interested parties by 5th July 2000. 2.3 This document summarises the responses in the light of the consultation responses and spells out Oftel’s final determination for issue 1 of the ANFP. The NICC DSL Task Group was informed of the result of this determination in Aug 2000 so that they could continue producing the ANFP specification.
3.1 When deploying systems in a metallic access network, operators will want 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 a metallic access network as deployed systems couple with one another within the shared access cables (binder groups); this process is generally referred to as cross talk. 3.2 In order to control interference within an access network, and thus produce a predictable environment on which deployment decisions consistent with an appropriate service quality can be based, it is necessary to have some form of frequency plan to which all deployed systems need to conform. 3.3 In April 2000, a new condition [83] was inserted into BT’s licence, which requires it to offer unbundled local loops to its competitors. This condition came into force in August 2000. To avoid undue interference in the BT metallic access network, as noted above, it will be necessary for all operators seeking unbundled local loops to adhere to an ANFP published or approved by the Director General. BT has also made a commitment to adopt this ANFP.
4.1 Most respondents agreed that Oftel's approach was a reasonable one while noting that the ANFP should be reviewed in the light of experience. Some respondents continued to adhere to the lines they had taken in the preceding discussions. ASCOM, a manufacturer, raised a new issue discussed in section 6 4.2 Further details are in Annex B.
5.1 The issue that prevented the NICC DSL Task Group from reaching agreement on the ANFP was the inclusion of CAP based HDSL systems at various data rates. When BT first started deploying HDSL systems in their network it elected to use 2B1Q based systems and has continued to do so ever since. The NICC DSL Task Group agreed, in January 2000, that the ANFP should accommodate these existing deployments. This meant that if CAP based systems were permitted into the network then it must be assumed that both CAP and 2B1Q systems could be present simultaneously. 5.2 The spectral profile of CAP technology is different to that of 2B1Q and therefore the Task Group were investigating the effect that introducing CAP systems would have on the resultant interference environment and consequently what effect this would have on the deployment of ADSL. 5.3 Typically, for a given technology, as the data rate is increased the amount of power and/or bandwidth that it uses also increases. Thus a 2Mbit/s system will create more interference than a 1Mbit/s system. Because of this feature the debate surrounding the inclusion of CAP based HDSL systems was about the data rate that the systems could support. 5.4 CAP based HDSL systems can be configured to operate at various data rates, however the debate was mainly about 1168kbit/s and 2320kbit/s. Clearly the advantage of the 2320kbit/s data rate is that a standard E1 circuit could be supported on a single pair whereas the 1168kbit/s rate would require two pairs. 5.5 CAP is not the only technology that is capable of supporting an E1 circuit on a single pair, for instance 2Mbit/s 2B1Q HDSL systems are available. Although this particular system has been standardised for several years it has not been deployed in BT’s network as it has been considered to cause excessive interference. Additionally a new symmetric technology is currently in an advanced stage of standardisation within ETSI and the standard is expected to be complete before the end of 2000. This technology can also support 2Mbit/s on a single pair and is compatible with the ANFP proposed by Oftel. 5.6 Although Oftel considers it desirable that high-speed symmetrical services can be offered competitively, it is apparent from the NICC DSL Task Group debate and from the responses that simply altering the PSD mask to accommodate 2320kbit/s CAP systems would cause significant concern. 5.7 The responses were clear that additional work by the industry should be conducted to address the current uncertainties and in light of this work the ANFP should be reviewed. Oftel endorses this position. Previous Standards 5.8 The response from Ascom raised a separate issue. Its response highlighted the differences between the proposed ANFP and a previously used standard, NTR14, that describes the spectral characteristics of equipment that can be attached to analogue leased circuits. The introduction of the Revised Telecommunications Terminal Equipment (RTTE) directive in April 2000 rendered the NTR14 standard obsolete and this is an additional reason why it is important to produce an ANFP that is adhered to by all operators. 5.9 The main driver for the development of NTR14 was BT’s Analogue Baseband Leased Circuit product, which is essentially two local loops connected together, either at the MDF or within a flexibility cabinet. As the end user, rather than the operator, is responsible for the equipment attached to the baseband circuits it was felt necessary to produce a standard to control interference although at this time the mass deployment of DSL technology was not considered. To avoid interference problems, all equipment attached to the network was required to comply with the NTR14 standard, however, with the exception of baseband circuits, BT was responsible for all deployments. 5.10 NTR14 describes a single PSD mask that applies to all loops in the access network whereas the proposed ANFP divides the loops into three categories, short, medium and long. A comparison between the proposed ANFP and NTR14 for each loop category is made below and illustrated in Figure 1:
5.11 So potentially, equipment currently deployed on medium and long loops which conforms to NTR14 may not be compliant with the ANFP. However, as stated above, any concern would arise only in respect of baseband circuits where the operator has no control over the deployment. 5.12 A survey has revealed that there are approximately 2200 baseband circuits in use by BT customers. Given that it would be expected that about 65% of loops would be categorised as short, this means that potentially around 800 baseband circuits throughout the UK could be non-compliant with the ANFP. However, it is unclear whether equipment used on such circuits actually is non - compliant. Correction to SDSL Data Rate 5.13 In the consultation document it was stated that the long mask would support SDSL at 294 kbaud, however it has been brought to Oftel’s attention that this is inconsistent with the PSD masks shown in Annex A. The correct value for SDSL on long loops is 261.33 kbaud (784kbit/s). This error stemmed from some earlier NICC DSL Task Group documents but has now been corrected.
Determined Access Network Frequency Plan 6.1 Given the continued lack of industry consensus in this area, the urgent need for a stable plan and the responses to the consultation, Oftel believe that the ANFP proposed in the consultation document strikes the right balance of public policy. Oftel therefore confirms this proposal for Issue 1 of the ANFP and outlines it in Annex A of this document. 6.2 On the NTR14 issue, Oftel does not feel it appropriate to try to identify non-compliant equipment currently deployed on baseband circuits. Procedures will be put in place to identify and deal with real interference problems as they arise. Accordingly, it is unnecessary to invest effort in identifying equipment which may not be causing a problem in practice. Operators should however make it a condition of provision of baseband circuits after the publication of the ANFP that all equipment deployed must conform to the ANFP. 6.3 In common with several of the responses Oftel would support a reformulation of the ANFP if additional theoretical work and deployment experience supported this action. For the moment it will leave open the date of any such review. 6.4 Oftel has informed the NICC DSL Task Group of this decision and they have now finalised issue 1 of the ANFP document, which can be found on the public NICC area within Oftel’s web site. References Access to Bandwidth: Proposed Solution for the Access Network Frequency Plan (ANFP) for BT’s Metallic Access Network - Oftel consultation document, June 2000. Further developments Oftel would welcome further information which might be useful in reviewing the ANFP in due course. The contact is: Dave
Clarkson Tel:
020 7634 8857 Issue 1 of the Access Network Frequency Plan (ANFP) will consist of three upstream PSD masks and one downstream PSD mask. The precise values describing these masks will form part of the ANFP specification document produced and the responsibility of, the NICC DSL Task Group. The PSD mask approach to the ANFP is intended to provide technical neutrality as any system with a PSD output at or below the levels described by the ANFP will be deemed as compliant. However an indication of typical systems that conform to the ANFP, as identified by the NICC DSL Task Group, is given in Table 1. Table 1: Example of Technologies that Comply with BT’s ANFP
Note: This table gives examples of compliant systems. It is not intended to be a complete list.
The following nine companies responded to Oftel’s consultation on the proposed solution for the ANFP.
A brief summary of each response is given below, however the complete responses are available from Oftel’s Research and Intelligence Unit at 50 Ludgate Hill, London. Alcatel Cross talk and potential interference problems recognised and in 1992 BT raised issue within ETSI, as a result BT decided to adopted 2 and 3 pair HDSL systems to protect network for future use. Given the continuing disagreement between technical experts on this matter Alcatel supports Oftel’s proposal and encourages continuation of the work. Ascom The proposed plan does not accommodate NTR14, which has previously been used for controlling the spectral characteristics of equipment approved for attachment to BT’s network. Ascom suggest that the medium and long mask be amended to follow to contours of NTR14 above 220 kHz. British Telecom BT supports Oftel’s proposed solution and believes it to be a reasonable compromise as CAP and 2B1Q systems can be deployed on equal terms whilst retaining the maximum headroom for ADSL services. BT also point out that the proposed ANFP will permit 2Mbit/s SDSL on a single pair that has been categorised as ‘short‘. BT will continue to work with various technical groups to try and reach a common understanding. Energis Energis welcome Oftel’s intervention and support the thrust of the proposal of initially adopting a cautious approach. The paper does not address policing and enforcement and these must be addressed urgently. The ANFP should be reviewed at an early stage to ensure that operators are not unnecessarily constrained. Ericsson Until a test method is agreed it will be difficult to verify the suitability of a particular system. The ANFP is defined in a technology neutral manner and this should be emphasised. In summary Ericsson support Oftel’s proposal. Kingston The introduction of CAP based systems constitutes a risk to Kingston, however in the spirit of compromise Kingston supports Oftel’s proposal but reserve the right to change their position if CAP based systems are subsequently found to be detrimental to their planned service deployments. Paraydne Paradyne state that CAP based systems are more spectrally compatible with ADSL than 2B1Q based systems. The use of a single pair to support 2Mbit/s (E1) has two advantages; firstly there will only be one transmitter instead of two or three thus less noise, secondly it uses existing infrastructure more efficiently. Paradyne supports proposal 2 as it believes this will open competition and reduce operational costs. QSC QSC believe that Oftel’s proposed solution will have a detrimental effect on competition in broadband and state that symmetrical technology like HDSL are effectively ruled out as their bandwidth is restricted. Also competitors will not bring much competition to the leased lines market. QSC do not see any danger that accepting proposal 2 will have significant negative impact on ISDN or HDSL and recommend that Oftel support proposal 2. Siemens Siemens believe that Oftel’s proposed solution is a good compromise and that it will adequately protect the various DSL systems from each other. Siemens conclude that its ADSL, SDSL and HDSL products will operate satisfactorily within the resulting noise environment.
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