CWDM relieves wireless Backhaul Bottlenecks

Certainly one of the most urgent issues presently facing Network Operators concerns the dramatic and sustained acceleration of mobile data traffic growth. Traditional mobile telephone, but more significantly, smart phones, mobile personal computing, mobile commerce and new 4G, Wi-Max and LTE services demand bandwidth and contribute to overwhelm the capacity of existing wireless backhaul links everywhere. Thus, ground-based infrastructure weakness previously evident primarily in links from distribution / aggregation nodes to cell sites in cities and urban zones continues to intensify and is spreading to industrial sites and rural areas.

The problem boils down to the classic mobile backhaul bottleneck problem: not enough fibers available to handle the bandwidth. Moreover, the difficulty is compounded by the fact that many wireless carriers either desire or require dedicated links through network operator's connectivity and traditionally dictating the provisioning of individual strands in the first mile backhaul.

Cube Optics (CUBO) has tackled this fundamental bandwidth exhaust problem applied to telecom, HFC, datacenter and wireless backhaul and access infrastructure. CUBO has assisted many network operators to deploy Coarse Wave Division Multiplexing (CWDM) solutions quickly and efficiently as a proven low cost, quick-to-install, future-compatible and a minimal latency fully protocol interoperable approach for addressing fiber exhaust in the mobile backhaul without the substantial space, power needs of maintenance encumbering active systems.


Simply said, CWDM in the vast majority of situations relieves backhaul bandwidth exhaust cost effectively and elegantly in essentially perfect agreement with Occam's age old Principle: "the simplest solution, other things being equal, is the better among more complex solutions". Indeed, sophisticated, costly and maintenance intensive active optical gear could comfortably address the wireless backhaul problem leading in the majority of cases to gross overkill in terms of the technical requirement, CAPEX and OPEX. CWDM equipment expands capacity of existing fiber infrastructure to essentially make individual fibers function as multiple optical links effortlessly accommodating 10Gb/s plus speeds at spans of up to 80km. CUBO's CWDM approach exploits passive optical technologies to satisfy Telcordia "Outside Plant" operating conditions (-40C to +85C) without need of electrical power while permitting CWDM capacity to reap the benefits of the entire 18-channel ITU CWDM grid in extremely compact form factors.

Custom solutions are routinely tailored to accommodate options for supplementary or overlay optical links including RFOG, fiber channel, up to 80 channels of DWDM, WDM-LAN channels or even fully customized features including monitor ports, secure channels or non-standard wavelengths. Service provider tower link requirements for dedicated channel wavelengths, and in some cases even dedicated fiber links, become much more manageable when integrating WDN methods into the solution.

When planning and designing capacity for cell sites, it is important to carefully consider architectural alternatives and how the architecture selected supports capacity growth in the long-term. Also important in terms of security is the used of wavelength segregation for banks and mission critical data links. CWDM renders a multiplicity of possibilities to upgrade, overlay, partition, segregate and / or otherwise expand and craft backhaul links. Legacy network constraints, options for modifying network architecture in anticipation of new subscription and capacity growth may be effectively addressed while avoiding changes to existing infrastructure and minimizing investment and operating cost.

Upgrading Backhaul Architectures

The depiction below represents a typical outside plant (OSP) backhaul network. The feeder cable extends often several km from the central office (CO) to a remote terminal (RT) in the vicinity of the wireless tower or cell cite. One is often confronted with an existing link often comprising only a limited number of 6, 8 of 12 fiber strands.

Electrical supply cables tend to accommodate the optical cable along the same trench. The link of Figure 1 transitions to copper (twisted pair / coax) beyond the RC carrying mobile telephone, community (police, ambulance, fire department) microwave relay services. WIMAX and other private dedicated or industrial and military antennas may be located at the tower. The infrastructure and / or fiber may be owned by an operator and the tower infrastructure either leased from the operator from yet another third-party enterprise.

Figure 1. Legacy low-bandwidth wireless backhaul example

Legacy installed fiber adequately supplying wireless relatively low bandwidth 2G and some 3G services are relatively easily expanded via upgrading the speeds of CO and RT transceivers or by adding blocks of 4 wavelengths of CWDM channels. However, bandwidth hungry 4G and LTE services in most cases will require expansion of optical bandwidth of the CO / RT link and very possibly converting the RT / DT links and the coax tower drops to fiber links.

The majority of costs associated with laying/upgrading additional physical cables comprises trenching and duct engineering regardless of installing twisted pair, coax, electrical power grid or fiber. Thus, when the opportunity arises, laying flexible and future-proof optical cable yields a very high ROI (Return-on-Investment) proposition whenever trench or conduit infrastructure work becomes a consideration as in Figure 2 below.Figure 2. Upgrading to tower fiber links for 4G and LTE

Where fiber cable does become deployed, larger cables (48+ strands) are typically selected since the overall project cost is not strongly impacted by the number of the optical strands in the cable. Future capacity restrictions may be virtually eliminated beyond the DT with the bottleneck now becoming the backhaul feed link. By the same token, the business case for upgrading the link CO / RT CWDM part of the OSP using CWDM wins handsomely over any option involving retrenching. Obviously, the longer the trench span, the greater the comparative cost advantage of deploying CWDM.

Let CWDM Shine

   Increase the capacity from the CO straight to the DT using WDM and increases the capacity on the existing DT / WSPx fibers by increasing the bit rate and requires only 2 fibers out of the feeder cable (maybe even eliminating the RT in the process).

Increase the capacity of the CO to the DT but also extend WDM channels all the way to the WSPs. There are ample fibers in the new fiber cable to dedicate fibers from the CWDM enclosure right through to the multiplexers belonging to each WSP.

Another architecture utilizing CWDM consists of a stitching a series of cell sites along a fiber (4 in this case) using the add/drop capabilities of CWDM. One such example is shown in Figure 3. Here a CO serves 4 cell sites with 4 pairs of wavelengths. A wavelength pair is add/dropped at each cell site. The cell sites could tens of kilometers from the CO, so that 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 suspended or buried pods.

The ultra-compact form factor of Cube Optics' CWDM Add/Drop Multiplexers (ADMs) has been deployed in large numbers via various enclosures and splice cassettes including those offered by Tyco-FIST, ADC, Corning, 3M, Multilink and PFP (to name a few vendors) in remote nodes situated in uncontrolled environments. Fusion splicing is often the preferred means of connecting the gear although connector-fitted solutions perform equally well - link loss margins permitting.

Figure 3 Linear architecture with intermediate add/drop nodes

Depending on the particular regulatory situation or jurisdiction, not all imaginable WDM upgrade configurations may be practicable. First, telecommunications regulations may, for example, prohibit certain digital multiplexing of data or channels from particular subscribers (state institutions, security and emergency services, financial networks etc.).

Other subscribers traditionally prefer dedicated fibers strands to guarantee privacy and internal network integrity. Circumstances stipulating dedicated fiber(s) may be simply accommodated through consigning WDM capacity upgrade or other fiber strands.

Further network reliability and incorruptibility considerations typically arise in respect of latency. Networks transporting SONET overhead or frame relay protocols strive to eliminate, by all means possible, delays resulting from provisioning, queuing, buffering, switching or other electronic processing. Of course the WDM technologies offer certainly one of the most effective approaches to minimizing latency since end-to-end delays are essentially reduced to propagation speed of the optical signal through the optical link. In a vast majority of deployments channels allocated via wavelengths easily satisfies customer needs.

CWDM - The Broader View

An alternative architecture is to use CWDM multiplexers to partition a single fiber strand (or pair) to in effect create virtual fibers. CWDM multiplexers are placed at the CO and in a remote enclosure as depicted in Figure 4.

Figure 4. Using CWDM for the critical sections 

A CWDM system uses 1 to 16 wavelengths based on the ITU-T standard grid (in fact 18 if low "water peak" fiber has been deployed). The transmission equipment at the cell site (DS-1, SONET, Ethernet) can utilize CWDM small form-factor pluggable (SFP) transceivers. If not, a separate CWDM transponder can be used to convert a low power 1310 nm signal to the desired CWDM wavelength.

CWDM SFPs and transponders can support a loss budget of up to 28+ dB which would, in turn, support transmission over fiber spans of in the range of at least 60 km (again depending also on the legacy fiber deployed). A CWDM system can characteristically scale as demand from the wireless subscribers grows. Additional wavelengths can be made available to particular wireless sites in anticipation of future well in advance of lighting up addition wavelengths. Typically a range of flexibility in terms of transmission speed per wavelength permits the wireless providers to increase bandwidth to particular cell sites independent of the WDM equipment. Alternatively, link capacity may be increased simply by add/dropping more wavelengths.

In a vast majority of cases, boosting capacity of the Outside Plant (OSP) infrastructure using CWDM technology adequately relieves wireless bandwidth bottlenecks. For example, overlaying DWDM wavelengths onto the CWDM grid permits dramatic expansion of capacity. The option to adopt DWDM connectivity however carries with it the controlled environment enclosures and deployment of appropriate transceivers including the electrical power and possibly space requirements may arise in remote terminals DWDM add/drop locations.

Finally, existing access networks may become recruited to add wireless capacity where the network subscription areas overlap with cell phone, WiMax and private wireless footprints in these cases, typically the operational continuity and integrity of legacy subscription base must be maintained while augmenting bandwidth to wireless sites. Figure 5 demonstrates such a situation. The network segment of Figure 5 typically forms part of ring in urban areas but often a linear topology in rural or remotely populated areas. Both configurations are possible. In this case new wireless capacity supplements the existing 10Gb/s connectivity linking subdivisions, enterprises and institutions to the Co-locations / Distribution Hub / Headend.