A fibre-to-the-node (FTTN) architecture comprises a fibre-optic link between the central office and an intermediate distribution point. From this distribution point, the home and/or building is served with voice and data through either a wireline or wireless connection.
If the existing copper plant is used for the ‘final drop’, which is usually the case with a FTTN architecture, then a remote DSLAM (Digital Subscriber Line Access Multiplexer) forms the distribution point. It cross-connects to the street cabinet, which already serves a set of local homes and/or buildings with copper connections. In a hybrid fibre/copper FTTN architecture, the twisted copper pair connection between the end-user and the remote DSLAM uses VDSL (Very high bit-rate Digital Subscriber Line) technology. The ONU, co-located with the DSLAM, feeds the signals back to the central office via the fibre-optic link. It is possible to use BPON+VDSL in a FTTN architecture; Verizon says it is working on how to implement GPON+VDSL2.
If a wireless connection is used for the final drop, an antenna forms the distribution point. WiMax (Worldwide Interoperability for Microwave Access) is the most promising candidate for FTTN in a hybrid fibre/wireless architecture. The 802.16d WiMax standard, designed for fixed access, provides theoretical maximum speeds of 70Mbps and an average coverage range of 5-8 kilometres. However, the longer the distance between the antenna and the CPE, the lower the transmission speed. WiMax is also a shared medium. The more active users, the less bandwidth is available.
Like all DSL variants, the data throughput performance of VDSL and VDSL2 is a function of the distance between the DSLAM and the end-user: the further the distance, the greater the performance deterioration. Asymmetrical VDSL, which was standardised by the ITU in June 2004 (G.993.1), offers a theoretical maximum downstream speed of 52Mbps (2Mbps upstream), but attaining that performance level would only be possible at distances of less than 300 metres between the remote DSLAM and the end-user. For distances in excess of 300 metres, VDSL performance rapidly deteriorates. Between 1km and 1.5km, VDSL offers downstream speeds of around 15Mbps, which is similar to the performance level of ADSL2+ at that distance.
Symmetrical VDSL2, whose standard specifications were recommended by the ITU in May 2005 (ITU G.993.2), offers a better throughput performance. Using a greater amount of frequency spectrum over the copper – 30MHz as opposed to 12MHz for VDSL – VDSL2 can offer a maximum theoretical symmetrical speed of 100Mbps. But again, this would only be possible if the DSLAM was located very near to the enduser (under 300 metres). At 1km, VDSL2 throughput drops to around 25Mbps.
The topology of the existing copper network is therefore a crucial factor in determining how suitable FTTN+VDSL/VDSL2 deployment is. In Germany, the average distance between the end-user and the street cabinet in urban areas is 300 metres; in France, the average distance is 800 metres. Deutsche Telekom is pursuing a FTTN+VDSL2 strategy and France Telecom intends to roll out FTTH. However, there is a ‘long reach’ version of VDSL, which uses the 12MHz frequency band. Its proponents say that this can deliver around 35Mbps downstream at distances of 1km, which would allow operators to provide faster speeds to more customers. The flexibility of VDSL2 to function in different bandwidths is specified in the G.993.2 recommendation. In fact, to cater for different band plan requirements in different regional markets, as well as different applications (such as central office, remote DSLAM, digital loop carrier and MDU connections), the VDSL2 standard specifies eight different configuration profiles based on the parameters of power and frequency range. VDSL2 is designed to be a truly universal standard.
It is also backwards compatible with ADSL2+. VDSL2 line cards in the DSLAM can be inserted and still serve ADSL2+ modems. The ADSL2+ modems can then be upgraded to VDSL2 when necessary – if the customer is eligible for the faster service – which makes rollout faster and easier for the operator. But with this amount of flexibility comes complexity. The ITU G.993.2 recommendation is 221 pages long and vendor interoperability will probably remain a VDSL2 issue over the next 12-24 months. Even so, this hasn’t stopped major network operators, such as AT&T, Deutsche Telekom, Belgacom and KPN, pushing ahead with their own FTTN+VDSL2 implementations.
VDSL, due to its less attractive performance characteristics than VDSL2, is unlikely to gain momentum among operators pursing FTTN strategies.
FTTN+VDSL/VDSL2 versus FTTH/FTTB
■ Faster time-to-market than FTTH/FTTB to deliver higher-speed services than
ADSL2+. No need for operators to negotiate rights of way to lay fibre
between intermediate distribution point and the home/building; no need to
negotiate with landlords for fibre access into the building.
■ Less upfront capex required than FTTH/FTTB through the re-use of existing
copper resources; no need for costly civil engineering works between the
home/building and the intermediate distribution point.
■ FTTN+VDSL/VDSL2’s higher bandwidth speeds are only available to a small
percentage of operators’ customers compared to FTTH/FTTB architectures.
According to IDATE, if all remote DSLAMs in France were equipped with
VDSL2, less than 10% of the population would be eligible for 50Mbps.
Similar coverage assumptions can be made about other markets, says IDATE.
■ FTTN operators have higher operational expenditure (opex) than PON operators
through the need to manage multiple small nodes within the network11.
■ The GPON standard has a 20km reach between the ONU and the OLT, which
offers the operator the prospect of central office consolidation and a
further reduction in opex. This option is not available to FTTN operators.
3.5 Enhanced xDSL cables
The performance of both ADSL2+ and VDSL2, as outlined above, is not necessarily
set in stone. Through ‘enhanced’ xDSL copper cabling, which has been available
in the market since early 2006, dramatic improvements in either distance or
bandwidth can be achieved with a minimal capex outlay. (If an operator needed to
repair degraded copper plant, anyway, the extra cost of replacement with
enhanced cabling would be negligible.)
According to Nexans, an ADSL2+ operator could provide 50% more services to
customers located within a 2.5km copper loop radius from the central office using
enhanced DSL copper cable. Or, if the operator preferred, the bandwidth reach of
ADSL2+ could be extended by some 40%.