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CWDM - A low risk tactic
CWDM technology provides the answer to bridging demand for bandwidth, both fast and cost 
effectively says Francis Nedvidek
FTTx (Fibre To The Home, Business, etc.) is gradually gaining momentum in Europe. Projects realised to date have tended to be modest but all opinions concede that FTTx has entered the mainstream. Network operators have plans on drawing boards in the Netherlands, Norway, Sweden, Denmark, France, Italy, Slovenia, the UK and Germany among others. Certainly, regulatory and legal frameworks concerning the use of legacy infrastructure and of newly installed fibre; the jurisdiction of regional and city carriers vs telecom vs CATV / HFC (Cable Television / Hybrid Fibre Cable) operators; and, the access to multiple-dwelling buildings, still need resolution. In addition, the technical debates concerning PON vs Point-to-point architectures evolve as broadband demand, telecommunications legislation and network technology advance.
As networks expand in terms of the number of subscribers, the offer and take up of services and the expansion of geographic footprints, Coarse Wave Division Multiplexing (CWDM) has emerged as the preferred method for increasing link capacities of these optical access networks quickly, simply and at a low cost. Passive CWDM requires absolutely no electrical power and the technology has proven itself to be sufficiently robust and reliable for installation in the most demanding environmental conditions.
Modern CWDM technology enables network capacity upgrades in the form of install-and-forget hardware allowing network operators to multiply the bandwidth of their presently overloaded fibre spans. A CWDM technology platform permits enhanced flexibility in terms of network planning and installation without sacrificing scalability to far higher transmission volumes as bandwidth needs inevitably grow. CWDM is inherently transparent to protocol, coding and bit rate and therefore ideally suited for aggregating fibre bandwidth. Capacity increases of factors from 4X to 8X or even up to 18X at a fraction of the cost of laying new cable in trenches or drawing additional fibre strands through conduits are routinely achieved. Operators implement network functionality upgrades literally within hours, while continuing to operate legacy ATM, TDM/TDMA, SDH/SONET or whatever topology their legacy or new architectures embrace. Furthermore, CWDM bandwidth augmentations are network transparent and fully operable with BPON, (Broadband PON (BPON ITU-TG.983.x), GPON or Gigabit capable PON (GPON ITU-TG.984.x), Ethernet or EPON (EPON IEEE 802.3ah) or various versions of DOCSIS. Even 1310 nm and 1550/1490 nm analogue modulation combining full digital overlays may be accommodated.
The goal of the network operator is to provide ever more subscribers with service while containing the cost to reach each additional customer. Reaching more subscribers with higher bandwidths attains higher penetration densities and consequently greater revenue generation potential. Increasing the bandwidth of existing fibre lines promotes higher degrees of network utilisation by permitting the price of each router port and laser transceiver to be shared across many connection drop points. Increasingly, attracting new subscribers also means providing the bandwidth that customers need for the services and the programming that they are signing up to enjoy. In all, CWDM is a very attractive means for network operators to achieve their objectives.
At the very edge of the network, FTTx architectures traditionally exploit an optical platform to carry downstream traffic to approximately 16 to 32 residential drop points or subscribers and upstream traffic back in the opposite direction. FTTx deployments, whether telecom-centric or HFC-centric, ultimately require extending sufficient optical bandwidth from the central offices and headends all of the way to these subscribers.
In its simplest form, CWDM multiplexers aggregate additional wavelengths, or in other words, additional data channels, onto an optical fibre where previously only one wavelength or channel had been transmitted. Upon arrival at the opposite end of the fibre, a CWDM demultiplexer discriminates and physically separates the different wavelengths so that each wavelength is rendered once again as an individual communications channel. In practice passive CWDM may be deployed in simple ring or protected ring distribution, point-to-point setups, PON configurations, via bidirectional or unidirectional arrangements or utilised to carry analogue signals simultaneously with bidirectional digital overlays. CWDM equipment may be packaged to fit 19-inch telecom central office installations, splice cassettes for mounting in street cabinets, hand-holes or CATV-pedestal closures. The most advanced CWDM components work over temperatures spanning the Telcordia GR-standards for outside plant operating conditions and are small enough for convenient insertion or retrofit into existing fibre splice cassettes. In fact, upgrading network capacity, in practical terms, becomes a task of modifying outside plant fibre connectivity rather than procurement and installation of new inside plant equipment.
Network operators are increasingly taking advantage of CWDM-enhanced architectures and their accompanying low CAPEX, minimal OPEX, and simple and straightforward planning and implementation. Decisions to adopt CWDM typically revolve around the following priorities:
Low and predictable equipment and operating cost - CWDM network upgrade approaches require significantly lower CAPEX and offer much more economical OPEX scenarios compared with any active equipment deployment. Especially attractive are the quicker return-on-investments. We often encounter network operators who redeem the cash flows generated via newly CWDM-acquired subscribers and enterprise service contracts for financing their next access network expansion.
Ability to upgrade portions or the entire network quickly and efficiently - Agility has a major impact on launch strategy and timing. Rapid response is key to pre-emptively or defensively capturing and holding market share. Our experience over the past three years with numerous European network operators exploiting CWDM building blocks involving many thousands of nodes clearly confirms that four or eight channel upgrades may be installed and fully operational within days or less.
Simplicity of specification, simplicity of deployment and simplicity of upgrade / reconfiguration - An inherent attraction of passive CWDM-based solutions is that the technical expertise required to design, manage and upgrade or otherwise adapt the existing or new network are well within the capabilities of virtually any network operator. Risks and burdens of complex network design and planning may be minimized without sacrificing options to further scale the bandwidth or network configuration. Deployment means plug-and-play installation with no need for additional power supplies or software updates.
Solutions that facilitate rather than constrain future expansions - Network operators strive to add subscribers, extend geographical reach and transport ever more data traffic. CWDM is a low cost and low risk tactic that complements other capacity enhancements whether future expansion strategies incorporate further passive or active equipment or even a complete change of operating philosophy. Roll out may be planned to ensure that technical improvements and the financial resources associated with upgrade scenarios remain decomposable into predictable and non-cost-prohibitive phases. Network Operators preserve the freedom to roll out capacity, coverage and services as the changing demand and competitive landscape require and as cash flows dictate.
Freedom from becoming locked into proprietary schemes - A CWDM approach tied to the established open standards typically operates unconstrained with any of the routers, switches, DSLAMs and even the WDM systems offered by major Telecom / CATV / HFC / Datacomm vendors. As a passive element, CWDM modules are functionally agnostic to all data transmission protocols and are equally immune to the incompatibility problems often encountered when connecting disparate equipment or accessories supplied by different vendors. Risks of becoming captive to any particular proprietary approach or attendant service agreement are eliminated.
Dr. Francis Nedvidek is CEO, Cube Optics AG, and can be contact via tel: +49 (0) 162 263 8032; e-mail: nedvidek@cubeoptics.com
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