Contents

Summary

Background

Explanatory note

Enquiries

Annex A - Letter from NICC DSL Task Group

Annex B - Technical Annex

Summary

S.1 This explanatory note concerns the publication of Issue 2 of the Oftel Technical Requirement: Specification of the Access Network Frequency Plan applicable to transmission systems used on the BT Access Network (OTR004).

S.2 Issue 2 of OTR004 replaces Issue 1.1 in its entirety and extends the capabilities of BT’s access network by permitting the deployment of full rate (2320 kbit/s) G.shdsl.

Background

1.1 An access network frequency plan (ANFP) is a spectrum management plan for controlling interference, caused by cross talk, within a metallic access network. The existence of an ANFP therefore allows operators to deploy services in a predictable and reliable manner.

1.2 In September 1999, following a request from Oftel, the NICC DSL Task Group began the development of an ANFP for BT’s access network. This group agreed the framework and the vast majority of the detail for the ANFP. However, there was one point on which consensus was not reached and this was in relation to the inclusion of a certain HDSL standard – CAP (carrierless amplitude/phase). To facilitate the timely completion of the ANFP Oftel proposed a solution, however at the same time it also encouraged industry to undertake additional theoretical work and to evaluate deployment experience with a view to reformulating the ANFP, if possible. The Oftel publication concerning this issue is available at: www.oftel.gov.uk/publications/broadband/llu/anfp1000.htm The current modification is the result of such additional work and deployment experience.

1.3 The ANFP for BT’s access network has been published as an Oftel Technical Requirement: Specification of the Access Network Frequency Plan applicable to transmission systems used on the BT Access Network (OTR004) and is available at www.oftel.gov.uk/ind_groups/nicc/Public/specs/llu.htm

1.4 The technical interface specifications for BT’s access network make appropriate reference to OTR004. In particular, the attention of users of Baseband Premier and Baseband Standard (EPS8 and EPS9 respectively) is drawn to the restrictions of their use for applications using xDSL technology.

Explanatory Note

2.1 In response to a request from the NICC DSL Task Group the Oftel Technical Requirement: Specification of the Access Network Frequency Plan applicable to transmission systems used on the BT Access Network (OTR004) has been enhanced to accommodate full rate (2320 kbit/s) G.shdsl. The latest version of this specification (Issue 2) is now available on Oftel’s web site at: http://www.oftel.gov.uk/ind_groups/nicc/public.htmwww.oftel.gov.uk/ind_groups/nicc/Public/specs/llu.htm

2.2 Issue 2 of OTR004 replaces Issue 1.1 in its entirety and extends the capabilities of BT’s access network by permitting the deployment of full rate (2320 kbit/s) G.shdsl. This has been achieved by the introduction of a new line category, termed ‘extra short’.

2.3 The rationale for this modification is to facilitate E1 (2048 kbit/s) circuits over the Asynchronous Transfer Mode (ATM) protocol using standardised G.shdsl technology. This modification will therefore bring benefits in the downstream markets, as operators will be able to offer a wider range of products, including leased lines and primary rate ISDN, over BT’s metallic access network.

2.4 Following extensive mathematical modelling, industry experts are satisfied that this modification to BT’s ANFP will not materially affect any existing or planned deployments under the previous plan (Issue 1.1). Accordingly, this modification was unanimously agreed within the NICC DSL Task Group and has subsequently been ratified by the NICC’s Public Network Operators Interest Group and approved by the NICC Board.

A copy of the letter requesting this modification and an accompanying technical annex from the NICC DSL Task Group can be found at Annex A.

Enquires

If you have any enquires about this explanatory note please contact:

David Clarkson
Oftel
50 Ludgate Hill
London
EC4M 7JJ

Tel: 020 7634 8857

Email: david.clarkson@oftel.gov.uk

Annex A

Letter from NICC DSL Task Group

TO: Oftel

FROM: NICC DSL Task Group

Development of BT ANFP Issue 2

The NICC DSL Task Group has been examining developments in xDSL technology standards and the services that they facilitate. As a result of this work it has unanimously agreed to amend the BT ANFP (OTR004:2000, Issue 1.1) that was determined by Oftel in October 2000 to introduce an additional line category termed ES (Extra Short). The resulting BT ANFP Issue 2 was unanimously approved by the DSL Task Group on 27 September 2002. This letter explains the rationale for agreeing to such an amendment and the relationship between this agreement and the previous Oftel determination.

Rationale for Amending the BT ANFP

The majority of DSL deployments use ATM based architectures (as recommended by the DSL Forum) enabling multiple services to be offered across a single network platform supporting quality of service (QoS). This allows legacy voice and leased line circuits together with newer Internet data services to be efficiently transported and managed. Even more forward looking architectures such as those proposed by the FSAN (the Full Service Access Network initiative, comprising over fifteen operators) detail an ATM-centric approach adding video capability and VDSL transmission to the network. The down side of this ATM approach is that in order to deliver a full E1 unstructured leased line, the G.shdsl is required to operate at a line rate of 2.32 Mbit/s to account for the ATM circuit emulation overhead. The original ANFP determination by Oftel did not accommodate this capability. However, since the original determination the ITU G.shdsl standards have been finalized and it has become evident that many DSL operators are deploying ATM-centric architectures and wish to leverage these platforms using G.shdsl together with circuit emulation.

A full E1 service is prohibited by the existing ANFP once ATM overhead is accounted for. The original ANFP was designed so that the maximum permissible speed of G.shdsl transmission was 2 Mbit/s E1 in order to avoid causing interference to ADSL. Consequently the G.shdsl line rate in the ANFP is effectively limited to around 2.056 Mbit/s on the Short or "S" loop category. However this approach assumed that the E1 user data was transported over the copper pair using TDM/SDH techniques. Whilst such equipment does exist, it is not the dominant approach deployed by DSL operators.

It was therefore proposed that an additional line category Extra Short or ES be introduced that would allow an ATM encapsulated E1 service to be provided using G.shdsl whilst having no significant impact on any other access transmission system permitted by the original ANFP (ADSL is the system that is most likely to be affected).

The work of the NICC DSL TG on the feasibility of this ANFP amendment has encompassed simulation modelling and measurement results performed by a number of different operators and vendors. A conclusion has been drawn up proving that it is possible to add to the ANFP the capability for G.shdsl to operate at 2.32 Mbit/s line rate as long as its range (the ES loop length) is restricted to 21 dB at 100 kHz. At this range the impact on ADSL is insignificant.

The introduction of a new line category has impact on the LLU process and commercial agreements. Consequently, the NICC DSL Task Group has been in liaison with the OPF LLU Product & Process/Commercial Group and they have agreed to make the necessary consequential amendments to the process and commercial agreements.

As a result of this feasibility work, the consensus among operators and the concurrence of the LLU commercial group, the NICC DSL TG recommends that Oftel process and undertakes the necessary amendment to the BT ANFP.

Other Changes

Whilst the introduction of the ES line category is the primary reason for amending the ANFP at this time, a number of other changes have also been agreed:

• Voiceband frequency requirements: the ANFP, Issue 1 provided no test specification in Annex 2 for frequencies below 5 kHz. This was a deliberate decision at the time of publication (Nov. 2000) as the Task Group was unable to quickly produce a specification that could readily be applied to both DSL equipment and voiceband equipment (e.g. telephones).The Task Group have now developed an alternative set of requirements and associated test specification that is aimed primarily at enabling voiceband equipment to demonstrate ANFP compliance. This alternative set of requirements is based on voiceband equipment ETSI standards. Care has been taken to ensure that CPE which would have been allowed to be connected to the BT network under the terminal equipment approvals regime that existed prior to the RTTE Directive would meet the ANFP voiceband frequency requirements.
• General updates: the opportunity has been taken to bring references and other general and advisory material up-to-date. In particular, the ANFP User Guide (Annex 4) has been updated, particularly to include the introduction of the ES line category.

Relationship with Oftel Determination

The dispute that resulted in the Oftel determination concerned the proposal to allow 2320 kbit/s HDSL CAP single pair system within the Short line category against the impact that this would have on the performance of ADSL. At the time of the Oftel determination, the ETSI standards for SDSL had not been fully formalised.

The current amendment does not impact the Oftel determination since the amendment does not change the specification of the existing line categories (S, M and L) and modelling results demonstrate that there will be no significant impact on the performance of systems already permitted by the existing ANFP.

Unlike the proposal that resulted in the Oftel determination, the current amendment does not change the specification of any of the existing line categories but introduces an additional line category. The net result is to allow the desired 2320 kbit/s G.shdsl single pair system to be used on lines in the new ES line category without impacting existing systems. It should be noted that the ES category does not permit 2320 kbit/s HDSL CAP single pair system and this exclusion in the specification of the ES line category was made with the unanimous agreement of the Task Group.

Yours sincerely,

David Davies,

Chairman, NICC DSL Task Group

Annex B -

Technical Annex

This annex includes additional background and technical information regarding the need to amend the ANFP in order to support E1 services over G.shdsl using ATM circuit emulation.

The original short and medium (S and M) loop lengths in the ANFP were derived from BT's existing planning rules for 2 and 3-pair HDSL used for Megastream and ISDN 30 services. Concern was raised that the customer transmission from symmetric DSL systems would cause crosstalk back towards ADSL customers on long lines (with weak received ADSL signals) and degrade the ADSL range. Hence network operators deploying ADSL didn't want to make the matter worse by allowing use of symmetric DSL technology with higher bandwidth, range or power than the HDSL that had already been deployed. Any symmetric DSL that fits under the resulting ANFP mask was permissible (including CAP).

Unfortunately, G.shdsl single-pair DSL doesn't quite fit under the S mask (Some vendors have come close via the use of spectral shaping but this could impact interoperability with standardized G.shdsl products if taken to extremes).

when operating at its fastest standardized line-rate, and a maximum line rate of around 2.056 Mbit/s is the existing ANFP limit. This is not sufficient to be able to transport leased line style services when the physical layer and ATM layer framing overheads are accounted for.

G.shdsl potentially allows the replacement of nx64k and E1 services by delivery on unbundled local loops. According to G.shdsl standards, leased line replacement services are supported as follows:

G.shdsl supports user data rates from 192 Kbps to 2304 kbps at the following rates:

192, 256, 384, 512, 768, 1024, 1152, 2048, 1536, 2304 Kbps

At the customer premise, these leased line replacement services can be presented as X.21, V.35 and E1 by CPE. With E1, clarification is needed regarding framing. E1 frames are structured from 32 timeslots (TS) each of 64kbps. E1 framing is provided by TS0. Signalling is contained in TS16. The protocol transported in TS16 can vary depending on application. The remaining channels are user channels. Channels 1-15 are in TS1-15, channels 16-30 are in TS17-31. In a leased line configuration, TS16 can also be used for user data rather than signalling. This becomes channel 31. In this leased line configuration TS16 will be transported as a user channel transparently. These leased line replacement services are often termed fractional E1. The service delivers a local TS0 for E1 framing to the terminal equipment. The subscribed numbers of channels (1-31x64k) are transported from the CPE to the DSLAM and into the core network, but TS0 is NOT transported. Hence the E1 service is fractional. At the remote end, TS0 is recreated locally to provide E1 framing. So, this E1 presentation for leased line services on one pair of copper lines is STRUCTURED, whereby TS0 is created to provide framing for the structured 64k channels.

To provide a leased line replacement service UNSTRUCTURED E1 is required. This provides no framing and the terminal equipment framing is passed end to end. That is to say, that 32TS including TS0 are passed end to end (2.048Mbps).

Leased line services over "next generation" networks are often delivered by a constant bit rate (CBR) circuit emulation service using ATM. For circuit emulation services, non-transparent transport of E1 is possible over AAL1 (Standards are evolving to allow E1 Primary Rate (PRI) services to be deployed over DSL using AAL2 encoding, which is a real time variable bit rate (rt-VBR) class of service. This allows silence suppression and inactive channel suppression for bandwidth optimisation. This contrasts with circuit emulation, which is constant bit rate. Constant bit rate is how service delivery is anticipated in the near term. Further work is required to define a service based on an AAL2 solution and to clarify how TS0 is treated in such a service offering.)

SDT (structured data transport). AAL1 UDT (unstructured data transport) conveyance of E1 may not be supported on G.shdsl over one pair due to the lack of bandwidth remaining for data on a G.shdsl/ATM link (for TS0). Up to 31 DS0 can be transported by the G.shdsl standard, which allows full E1 in non-transparent mode (no end to end TS0). 

There are 3 steps in circuit emulation:

1. Synchronous samples are inserted into ATM payload field (47 bytes wide)

2. Sequence number (SN) and sequence number protection (SNP) fields inserted to verify that the receiving AAL1 protocol has received cells in the right order

3. Remainder of the payload field is filled with enough single bytes to equal 47 bytes

Hence the mapping of a UDT E1 on AAL1 consumes 53 / 47 * 32 * 64 kbps=2310 kbit/s which is more than the supported user bit-rate over G.shdsl (2304 kbps) within the standard. The result is that two copper pairs (with ATM IMA) could be required. 

The work of the NICC DSL TG has shown that it is quite possible that an extra short loop or "injection point" could be added to the existing ANFP permitting 2.32 Mbit/s G.shdsl. The logical ANFP construction confirms that the new extra short loop length is derived as one that ensures the 2.32 Mbit/s G.shdsl transmissions from CPE arrive as crosstalk on long line ADSL CPE at a similar level as existing 2.056 Mbit/s G.shdsl crosstalk or 2-pair HDSL crosstalk from "S" loops. Hence the addition of such a loop will not cause any noticeable additional degradations.

The current Short and Medium ANFP masks are at loop limits of 26 dB and 29 dB at 100 kHz respectively. These are actually very close and only a small percentage of the UK loop length population falls between these limits. The Medium mask at 29 dB allows use of G.shdsl up to a line rate of 1505 kbit/s. Backing off just 3 dB to 26 dB allows use of G.shdsl up to 2056 kbit/s (This is the theoretical limit of G.shdsl capability within an "S" loop ANFP mask. Actual deployed vendor implementations may fall short of this capability, particularly when planning rule margins are accounted for.)

The NICC DSL TG has concluded that a loop length of 21 dB at 100 kHz would be appropriate for locating the new "ES" mask that will accommodate G.shdsl transmit spectrum at a 2.32 Mbit/s line rate.

The ANFP change control procedure requires an impact assessment on at least the systems listed in table 5 of annex 4 in the ANFP. As described above, the mask is to be designed to accommodate 2.32 Mbit/s G.shdsl and the loop length for the injection point of systems conforming to the new mask such that crosstalk impact into long-line ADSL customers is no worse than currently exists using G.shdsl or 2-pair HDSL systems operating on the "S" loops (or 3-pair HDSL systems operating on the "M" loops). Symmetric echo-cancelling systems such as ISDN and HDSL suffer worst-case crosstalk from similar systems with transmitted signals matched to the receiver front-end. This is often termed self-NEXT. Since these systems will also be able to transmit on the new shorter loop length already, their performance will not be further degraded by G.shdsl transmissions with the new mask.