Fibre in the Access Network

Transcription

1 Fibre in the Access Network The ACTS projects PLET, TOBASCO and WOT have tested systems for upgrading the access network to carry broadband services. Their experiences have been brought together in a Guideline entitled Advanced and Evolution Friendly Deployment of Fibre in the Access Network. This describes three approaches to upgrading the network: Extending reach and coverage Extending wavelength usage Extending flexibility Extending Reach and Coverage This approach is suitable for the gradual upgrade of a telephony network to handle demand for broadband services. The first step is the replacement of most of the existing copper access network with APON (ATM over Passive Optical Network) systems. Core network SDH transport FTTC/B APON 1: / 155 Mbit/s FTTC/B (Fibre to the Curb/Building) topologies will be used and the last copper drop will be reused. Digital Subscriber Loop (xdsl) techniques will provide broadband access where this is required. The access node will be installed at the conventional local exchanges. In the, the narrowband () data are switched to the network, while the broadband () data are transported via an SDH transport network to a more distant broadband switch in the core network. The APON provides the operator with future-proof fibre cable plant in the access network. The reuse of the last copper drop and the sharing of electro-optical components among several subscribers results in a lower first installed cost than for a FTTH (Fibre to the Home) scenario. This is particularly attractive when the number of broadband users is low. Standard APONs have splitting ratios up to 32 and a reach of about 20 km. This is about twice the reach of conventional copper pairs so some smaller local exchanges could be eliminated using this technology. However by adding optical amplifiers the so-called SUPER-PON - the splitting ratios can be increased to 2048 and the range to around 100km. Page 1 of 5

2 Core network Feeder repeater Amplified splitter SuperPON 1: / 311 Mbit/s FTTC/B The traditional local exchanges can be eliminated and the access switches concentrated at the current trunk exchanges. This removes a level from the switching hierarchy and reduces the number of switching nodes by a factor between 40 and 100, offering network operators considerably lower operating costs. When broadband services become commonplace, the final copper drop can be replaced by fibre to realise a full FTTH network. Core network Feeder repeater Amplified splitter FTTH SuperPON 1: / 311 Mbit/s The removal of the curb unit reduces the complexity of the network. It also allows it to be managed remotely up to the network termination in the customer s home. A further benefit is that upgrades such as wavelength division multiplexing can be introduced without civil works. This offers further savings in operations and maintenance costs. Extending Wavelength Usage This approach is suitable for the upgrade of a tree and branch coaxial cable CATV network to handle interactive services. The starting point is an analogue CATV network with point-to-point fibre connections from CATV Headend () (or Local Centre) to layer A (up to the Optical Network Unit ()). The fibre-node at this point serves at most ~2400 subscribers through a tree-and branch coaxial cable system having uni-directional passband amplifiers at layers B and C. These serve typically ~300 and subscribers respectively. Page 2 of 5

3 A B C 1:8 (~2400) 1:8 (~300) The introduction of broadband (10 Mbit/s downstream and 2 Mbit/s upstream) interactive services is hampered by the small bandwidth (typical 20 MHz) available and a low Carrier-to-Noise Ratio (CNR) on the upstream channel. However cable modems offer a solution for low take-up rates and moderate capacity demand. A B C 1.5 DS Modems 1.3 IS Mb/s (ATM) 1:8 (~2400) 1:8 (~300) The upgrade involves adding upstream amplifiers in the band from MHz and between one and four ATM cable modem system(s) at point A. These cable modems are connected to the head-end by a 1.3 µm ATM/STM1 (155 Mb/s) link. A single cable-modem has a maximum upstream capacity of 8 Mbit/s and a downstream capacity of 32 Mbit/s. The interactive services capacity can be increased by Frequency Division Multiplexing (FDM) the cable modems. However, given the small upstream bandwidth (~ 20 MHz) and the severe ingress noise, broadband switching or concentration functions will have to be introduced closer to the customer when demand begins to grow. A B C 1.5 DS OLT 1.3+/- IS 1..2 Modems 622/155 APON 1:8 (~300) An APON system, with simple low cost 1.3/1.5 µm WDM components, in regarded as the most cost effective solution at this stage. A 622/155 Mbit/s APON system, operating at 1.3 µm with a 1:8 split, is introduced between the and layer B. One APON, one AM-CATV optical receiver and 2 cable modem systems are placed at each B location. The analogue CATV signal is carried over the same PON at 1.5µm. An optical amplifier is needed to boost the CATV signal. Page 3 of 5

4 As demand grows further, the network capacity can be increased by extending fibre to level C, where 40 subscribers are connected to one by coax. A B C 1.5+ DS TWOBA Modem OLT 1.5 IS * 622/155 APON on 1.5- (HDWDM) The number of APON systems is increased to four using high-density wavelength division multiplexing (HDWDM) in the 1.5 µm window. A second 1:8 splitter is introduced and the fibre is extended to level C. The additional amplification required is provided by a TWOBA (Two-Window-Optical-Branching Amplifier). A WDMPON (optically modified APON ) an AM-CATV optical receiver and a cable modem system are placed at each C location. Extending Flexibility Passive Optical Networks offer a limited number of basic services and the traffic capacity per customer is limited to a few tens of Mbit/s. Businesses are already demanding 155 Mbit/s links and are expected to want Gbit/s in the not too distant future. It is predicted that, within 10 years, electronic switches will no longer be able to handle the traffic presented. The use of WDM and wavelength agility allows PONs to be incrementally upgraded to introduce new, higher data rate, services. This type of upgrade is called high-capacity bypass. The APON or SPON operates on one accurately defined and stabilised wavelength. Other wavelengths, originating from Wavelength Agile Modules (), are WDM multiplexed onto the PON fibre and carried through the amplified PON. At the customer premises the signal is optically split to: a fixed optical filter, on the SPON s wavelength, and terminating in its ; a for the bypass traffic. The bypass channel from the customer can be connected to high capacity services at the access node via another or can be connected directly to the serevices through the core optical transport network. The ACTS project WOT is testing these options with data rates of up to 2.5 Gbits/s. As can be seen from the figure below, the principle of WDM high-capacity bypass can applied to: FTTC/B systems APON or SPON FTTH systems Hybrid fibre Coax systems provided a fibre is installed from the to the premises where the is to be located. Page 4 of 5

5 Broadband & High-Capacity Networks APON APON/ SPON APON ADSL/ VDSL NT NT FTTC/FTTB FTTH HFC The number of simultaneous bypass connections available depends on the width of the optical amplifier and the wavelength spacing used. As the number of channels is increased, the optical link budget is reduced because the optical power available must be divided among the channels. As the demand for bypass channels increases, the point at which the signals need to be injected into and extracted from the PON has to move closer to the customer. Conclusions The ACTS projects PLET, TOBASCO and WOT have explored how to upgrade the access network by: Extending reach and coverage Extending wavelength usage Extending flexibility The approaches proposed allow broadband services to be launched with relatively low up-front costs but, at the same time, are future-proof against a number of demand evolution scenarios. Key components for each of the approaches have been identified and prototypes or early production samples are being tested under operational conditions in the projects trials. The technologies are already sufficiently mature for network operators to upgrade their networks for low penetration broadband services and there is good evidence that components with suitable performance and quality will be avialable when demand reaches a level that justifies the later upgrade steps. Full texts of the ACTS guidelines can be found at Hill Stewart ACTSLINE Page 5 of 5