How Optical Multiplexing Eliminates Data Bottlenecks

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  •   The deluge of mobile data traffic circling the world is exposing an important vulnerability inherent in the physical network with which infrastructure operators connect transmissions to end users. Bottlenecks are choking the access links, including those linking mobile radio towers to the wider network.

      It is a classic communication link problem: insufficient bandwidth to accommodate the increasing number of Gbps (gigabits/second) connecting the wireless tower with the metro fiber rings. Compounding the problem is the practice that many wireless carriers of maintaining dedicated links through the network optical fiber connectivity.

    Depending on the mandate of the wireless service (state security for example), the provider may even demand dedicated backhaul fiber strands linking access and core meshes with mobile tower sites depleting further the number of fiber strands available and depriving new service providers use of mobile towers.

    This article examines advantages of using passive Coarse Wavelength Division Multiplexing (CWDM) and shows how CWDM can expand fiber capacity without the need to install addition fiber of new fiber links.


    More wavelengths, more capacity
      A CWDM optical multiplexer either introduces or extracts several signals propagating via fiber of different wavelengths (or a pair of fibers as the case may be when separate fibers for the send and return paths are used) to effectively create more separate channels. The diagram of Figure 1 shows a schematic of discrete optical elements arranged to construct a multiplexer (MUX). At the sending end of the link, a MUX aggregates the individual light channels onto the fiber.

    Upon arrival at the destination, a demultiplexer (DEMUX) employs a similar optical configuration and a reverse direction of propagation through the device. The optical filters of the DEMUX discriminate the incoming wavelengths and couples each channel into a separate fiber and off to another destination. Thus the MUX / DEMUX pair effectively expands the number of signal channels transmitted over the optical fiber.

      A CWDM equipped fiber link based on the ITU-T standards functions using a grid spacing of 20nm extending from 1271nm to 1611nm for the full ITU complement of 18 channels. Although low "water peak" fiber is now widely available, most legacy fiber deployed decades ago and typically exhibit H2 absorption losses at the wavelengths 1371nm and 1391nm. Typically only 16 channels available in older fiber link deplyoments.

    The equipment at the CO-location (CO) or Remote Terminal (RT) typically utilizes CWDM small form-factor pluggable (SFP) transceivers to convert the optical signal to its electrical counterpart for the incoming channels and from electrical to optical for outbound transmissions. Modern CWDM transponders reliably span fiber transmission ranges of up to 80 km or more.

    Besides boosting capacity of optical links, CWDM multiplexing easily scales to supply the additional bandwidth as capacity demand from subscribers grows. Thus, bandwidth to particular locations to relieve bandwidth bottlenecks without substantial modification of the equipment, enclosures and cabling already installed in the field. The CWDM package is exceptionally compact and withstands environmental conditions designated Outside Plant (OSP) according to IEEE standards enabling deployment in unheated and uncooled cabinets, pedestals or equipment racks.

      Existing access networks operators seldom have the luxury of needing to connect a sole tower or single type of customer over a given geographical area or "footprint". As a rule, optical networks carry data for many different subscribers requiring various bandwidths over extended network locations including residences, businesses and mobile phone and data users. In such circumstances, the operational continuity and integrity of the existing subscription base must be maintained while augmenting bandwidth growth to individual sites or nodes.

    In this case, the network segment comprises a segment of a ring connecting urban and commercial areas. Whenever possible, ring topologies are preferred since the ability to exploit counter-travelling signals can provide protection against cable breaks or the failure of a node. Never-the-less rings often branch to linear or tree architectures in rural or less densely populated areas.

    Both configurations, ring or linear, are possible where existing 10Gbps connectivity to subdivisions, enterprises and institutions to provide additional bandwidth for existing wireless service providers or even to connect new service providers to the CO facility or distribution hub. Judicious addition of wavelengths also permits to some extent selective capacity allotment for individual user types and locations. Consideration of the operating environment as well as space and physical limitations determines the type of packaging, the necessary equired on-site housing and the means of connection required for each optical MUX / DEMUX "location". Without optical multiplexing, serving so many varied users with high speed internet, on-demand video and telephony would be impossible.

    The Value of CWDM
      Before assessing the technology options, let's list the requirements that operators of access networks must satisfy: 

    • Segregation of bandwidth per-wireless carrier
    • Guaranteed bandwidth of up to 10Gbps per each first mile backhaul link
    • Uncomplicated and reliable operation (truck roll avoidance)
    • Provisioning simplicity
    • no substantial changes required to installed equipment
    • Typical spans of a few kilometers in some cases may need to reach 80km
    • Durable and yet packaged to suit installation environmental constraints
    • Facility to preserve undisturbed legacy 1550nm or 1310nm fiber connections

    The technology options for this portion of the network include active optical networking, passive Dense Wavelength Division Multiplexing (DWDM) and passive CWDM. Let's look at each in turn.

    Active optical. One approach to mitigating wireless backhaul congestion, active optical networking equipment more often amounts to overkill. The complexity of active solutions presents an abundant superset of features and functionality that the operator pays for in hardware costs, software licensing, ongoing maintenance, electrical power and upgrade costs. Segregating or partitioning bandwidth is done at a logical level within the realm of the active electronics. These higher-level logical approaches, however, yield only best-effort bandwidth performance when emulating individual physical connections and unavoidably increase latency.

    Next are costs related to training personnel to maintain and manage proprietary network gear. Associated operating, spare-stocking and repair costs further diminish Return on Investment (ROI). Active optical equipment is best placed in close proximity to the network core; unsophisticated and low-cost passive gear belongs in the access and middle mile networks.

    Passive DWDM. This represents a more practical option over active equipment in a vast majority of cases. When the total number of connections or channels required exceeds sixteen (or 10Gbps x 16 which is 160Gbps), DWDM technology may offer viable alternatives. But operators considering DWDM should be aware that it is inherently a more expensive technology and simply will not accommodate the plethora of form factors, from pedestal to line card to CO rack etc., that characterize this "first-mile" of an access network. In that light, it compares less favorably with highly adaptable and universally retrofit-able and environmentally friendly CWDM schemes.

    Passive CWDM. An optimal balance between right-sized functionality and right-sized cost, CWDM satisfies the best-fit rule of Occam's razor. (The simplest solution, all things being equal, is the best among more complex solutions.) CWDM expands capacity of existing fiber infrastructure by making individual fibers function as multiple optical links, each countenancing at least 10Gbps over spans of up to 80km. CWDM is unique in its capability to support legacy 1310nm and 1550nm single fiber connections while permitting additional CWDM links via the same fiber pair.

    Both DWDM and CWDM physically partition connections at the physical layer. In other words, a partitioned 10G connection in passive CWDM and DWDM in fact provides a unique and exclusive optical connection for each individual 10Gbps traffic channel. The flexibility of CWDM also factors into planning and designing cell site capacity. Existing legacy fiber network does not constrain CWDM. Rather, previously deployed and newly introduced channels are handled similarly; they are relayed and routed undisturbed. CWDM provides the simplest, most robust and yet most multifaceted option for future expansion. Electrical power for climate control or active equipment beyond a handful of additional pluggable transceivers is unnecessary as is changes to the physical housing

    Access Network Upgrades
      Several connectivity issues impact network operator architectures extending to wireless towers located sometimes far from urban centers such as along highways, secluded subdivisions or rural residences. In the typical case of a network linking wireless tower sites, the feeder cable often stretches several km from the CO or aggregation point to a Remote Terminal (RT) in the vicinity of the wireless tower or cell site.

    In such situations it is common to be confronted with the existing legacy link comprising only a limited number of 6, 8 of 12 fiber strands. Electrical supply lines accompany the optical cable along a common trench to the distribution terminal (DT). WiMax and other private, dedicated or industrial and security antennas may also be situated at the tower.

    Expanding digital capacity for installed legacy fiber supplying wireless bandwidth is often accomplished by adding blocks of four wavelengths of CWDM channels. Satisfying bandwidth-hungry 4G and LTE services is easily addressed by lighting up new wavelengths over the CO-to-RT link.

      Normally the trenching and laying of conduit which brings the electrical power grid and fiber to a remote locations comprises the vast majority of costs associated with connecting remote wireless towers and antennas. Thus, the business case for upgrading the CO-to-RT link using CWDM wins handsomely over any option involving drawing new fiber or further trenching.

    Another architecture taking advantage of CWDM consists of stitching a series of cell sites along a fiber (four in this case) using the add/drop capabilities of CWDM. One such example is shown in Figure 4.

      Here a CO serves four cell sites with four pairs of wavelengths. A fiber pair carrying a single wavelength is added or dropped at each cell site. Because cell sites may reside tens of kilometers from the CO, minimizing insertion loss and selecting the appropriate optical power of transceivers becomes an essential priority. Individual cell sites may be housed in pedestals, small cabinets or even within suspended or buried pods.

    Furthermore, network reliability and incorruptibility considerations typically arise with respect to latency. Networks transporting ATM / SONET overhead (protected serial synchronous relay approaches) or frame relay or pseudo wire protocols (STM / OC synchronous transmission) in Figure 4 strive to eliminate delays resulting from provisioning, queuing, buffering, switching or other electronic processing. Wavelength division multiplexing technologies offer one of the most effective approaches to minimizing latency since end-to-end delays essentially reduce to the speed of propagation of the optical signal through the optical link. Conclusion The rising tide of wireless backhaul traffic is creating bottlenecks in the access networks. CWDM relieves bandwidth exhaustion in these networks in harmony with the dictum that the simplest choice, all things being equal, tends to be the best.

    Applicable in a range of scenarios, CWDM demonstrates admirable flexibility - especially when combined with highly reliable, customizable and compact, low-cost components. It solves the problem of future expansion with a minimalist approach: only as much as you need, when you need it, without expensive or unnecessary extras. CWDM is thus able to remedy the "tower to the router" backhaul link challenges of today and tomorrow.

      Usually a metro network already is in place. You can't avoid that! They are often relying on multiple 1Gbps or 10Gbps services which are multiplexed over the dark fiber network. But we can avoid, that you have to take the network down to replace it by a new infrastructure.

    Simply add the new services without effecting the legacy installation. At lowest possible CAPEX and OPEX, while still being simple to install and maintain.

      This can easily be achieved by adding the 40Gbps / 100Gbps service via a passive transport approach. CUBO has ready-to-implement strategies fulfilling the 40GBase-LR4 / 100GBase-LR4 (or future -ER4) transceiver (as e.g. CFP, QSFP, CFP2 etc MSA format) which will be directly plugged into the terminal equipment or the Ethernet switch or router etc.

    New services may be added by simply overlaying them onto the existing DWDM (or CWDWM) network via a 1310nm band pass port on the (eventually existing) DWDM (or CWDM) passive multiplexer:


    Integrating the 1310 band port into the 40 channel 100GHz passive multiplexer permits transport of up to 40 additional services at 10 Gbps and at 100 GBbps bandwidths over a single fiber pair. Now it adds up to a total of 500Gbps, implemented into 1HU/19" rack space without consuming any electric power.

    The effective reach sometimes is inadequate given the power budgets of available LR4 transceivers. In such a case the 100GBase and 40GBase transceiver reach can be extended, by a matching CUBO SOA (Semiconductor Optical Amplifier) to distances of around 40 / 60km.


    Passive, multiple 100Gbps WDM Networks

      If an additional 1 X 100 Gbps is not compatible with the architecture at hand, but 100 Gbps is what customers demand, by passive WDM transport means can deliver the results you need. In the same way that passive DWDM 10Gbps networks have been used to augment capacity in metro environments.

    Similar to the LR4 versions featuring 4 lasers and 4 detectors (each at 25Gbps) each at specific lambdas, the heart of overlay solutions are 100Gbps DWDM CFP MSA compliant transceivers only without the optical WDM multiplexer integrated into a single package. This requires an external (e.g. 19" based, passive DWDM) multiplexer.

    In contrast to CFP LR4 types, the CFP DWDM versions are based on tunable lasers (50GHz DWDM grid) in the 1550nm spectrum.

    A set of up to 24 "differently colored" 100Gbps DWDM can be transported via a 96 channel passive DWDM mux (each CFP uses 4 lambdas) in parallel over a single-mode fiber pair.

    Similar to 10Gbps DWDM networks, those signals can be reach-extended by the use of standard EDFA (Erbium Doped Fiber Amplifier) amplifiers. Allowing the transport of up to 2.4 Tbps in a passive WDM manner without additional and costly amplifiers over distances of hundred and more km's.