The elements of fibre cable management

August 26th, 2015, Published in Articles: EngineerIT

As service providers upgrade their networks to transport high-bandwidth broadband services, an increase in fibre usage is essential to meet both bandwidth and cost requirements. But just deploying this additional fibre is not enough – a successful, well- built network must also be based on a strong fibre cable management system.

Proper fibre management has a direct impact on the network’s performance, stability, reliability and cost. Additionally, it affects network maintenance, operations, expansion, restoration, and the rapid implementation of new services, ideally without disturbing the transmission in other active circuits. The four primary elements of a strong fibre cable management system provides:

  • Storage and protection of fibres, splices, connections, passive optical components, cables.
  • Fibre and cable routing paths with bend radius control.
  • Modular circuit separation (to reduce transient losses in adjacent groups of optical circuits).
  • Fibre and cable identification and accessibility.

Executing these concepts correctly will enable the network to realise its full competitive potential.

With strong demand steadily increasing for broadband services including several bandwidth-hungry technologies such as high-definition television (HDTV) and higher internet speeds for larger file sharing requirements, fibre is pushing closer and closer to the customer premises.

This, in turn, creates a need for additional fibre in the central office (CO)/data centre and for active equipment that must be managed to accommodate future network growth.

Any new broadband network infrastructure must have the inherent capability to easily migrate to the next generation of technologies and services. This is a key consideration for any provider of triple-play broadband services. This is a key consideration for any provider of triple-play broadband services – whether it’s from the multiple service operator (MSO) headend, a CO, or a wireless mobile switching center (MSC). As the amount of fiber dramatically increases, the importance of properly managing fibrer cables becomes a more crucial issue.

The manner in which fibre cables are connected, terminated, routed, spliced, stored, and handled will directly and substantially impact the network’s performance, stability and, more importantly, its profitability.

The four fundamental elements of fibre cable management – physical and environmental protection, circuit separation, cable routing paths with bend radius control, accessibility and identification – will be discussed in this article, as well as new technologies and products developed in the last few years to improve these elements.

Storage and protection

A fibre management system provides storage and protection (both physical and environmental) of the installed fibres, splices, connectors and passive optical components. Every fibre throughout the network must be protected against accidental damage by technicians or by equipment handling. Fibres traversing from one piece of equipment to another must be routed with physical protection in mind, such as using raceway systems that protect from outside disturbances.

Without proper physical protection, fibres are susceptible to uncontrolled bends which cause transient optical losses and damage that can critically affect network performance and reliability.

Fibre and cable routing paths with bend radius control

Uncontrolled bends in fibres or cables can affect the fibre network’s long-term reliability and performance.

Uncontrolled bending of fibres is often caused during handling of the fibres. As the bend occurs, the radius can become too small and allow light to escape the core and enter the cladding. The result is an increase of attenuation at best and, at worst, the signal is decreased or completely lost due to a mechanical fibre fracture. These macrobends can, however, be reduced and even prevented through proper fibre handling and routing in a fibre management system.

Fig. 1: Wavelength dependence of macrobending loss.

Fig. 1: Wavelength dependence of macrobending loss.

The attenuation caused by macrobending is wavelength dependent (see Fig. 1). For the same bend, the increase in attenuation will be higher for the longer wavelengths. As specified in the standards IEC 61756-1 [1] and ITU-T L.13 [2] the recommended minimum permanent storage radius for the conventional single-mode fibres (ITU-T G652D) is 30 mm, however, in local cases a radius of 20 mm is allowed (for example a bend at a connector boot).

The minimum bend radius of a cabled fibre will vary depending on the cable construction. In general, the minimum bend radius of a cable should not be less than ten times its outer diameter. Thus, a 5 mm cable should not have any bends less than 50 mm in radius. Also, if a tensile load is applied to a fibre cable, such as the weight of a cable in a long vertical run or a cable pulled tightly between two points, the minimum bend radius is increased due to the added stress on the fibre.

Fig. 2: Expected mechanical reliability in various optical networks.

Fig. 2: Expected mechanical reliability in various optical networks.

The advent of bend-insensitive fibre according to ITU-T G.657 [3] is an example of how technology has addressed the bend radius issue and associated attenuation. Whereas the minimum bend radius should not be less than ten times the outer diameter of the fibre cable in typical fibre, bend-insensitive fibre provides more leeway. A bend-insensitive fibre can be bent with a 30% smaller radius compared to the conventional single-mode fibre without additional attenuation penalty. This allows the reduction of the size of fibre management systems for FTTH applications where a recommended minimum bend radius of 20 mm is used and in some limited cases a 15 mm radius is allowed (for example in wall outlets). Some bend-insensitive fibres (ITU-T G.657B3) are specified with a bend radius of 5 mm, which makes these fibres ideal for customer premises cabling. However, care must be taken; for such small bends the mechanical failure probability will increase dramatically. For FTTH customer premises cabling, a mechanical failure probability of 10- to 5 is considered acceptable, but for long haul networks a failure probability of minimum 10 to 7 is required since one failure could affect the transmission of several thousand customers (see Fig. 2).

However, service providers must understand that these new ITU-T G.657 fibres do not diminish the need for solid fibre cable management. On the contrary, the increase in the sheer number of fibres being added to the system to accommodate broadband upgrades makes bend radius control as important as ever. Additionally, the future NG-PON2 systems will use transmission wavelengths up to 1625 nm, which makes the fibres very sensitive to macrobending at these wavelengths.

As fibres are added on top of installed fibres, macrobends can be induced on the installed fibres if they are routed over an unprotected bend. A fibre that had been working fine for many years can suddenly have an increased level of attenuation, as well as a potentially shorter service life. Although bend-insensitive fibre is an innovative breakthrough addressing the issue of bend radius protection, it may be some time before service providers replace existing fibes with a bend-insensitive variety of fibre. Meanwhile, the importance of bend radius protection is critical to avoid operational problems in the network. Improper routing of fibres and cables by technicians is one of the major causes of bend radius violations. Wherever fibre is used, routing paths must be clearly defined and easy to follow – to the point where the technician has no other option than to route the cables properly. Leaving cable routing to the technician’s imagination leads to an inconsistently routed, difficult-to-manage fibre network.

The quality of the cable routing paths, particularly within a fibre distribution frame system, can be the difference between congested chaos and neatly-placed, easily accessible, patch cords. It’s often said that the best teacher in fibre routing techniques is the first technician to route it properly. Conversely, the worst teacher is the first to use improper techniques since subsequent technicians are likely to follow his lead. Well-defined routing paths, therefore, reduce the proficiency training time required for technicians and increase the uniformity of the work done by ensuring and maintaining bend radius requirements at all points to improve overall network reliability.

It is important to note that, again, the use of bend-insensitive fibre does not diminish the need for clear cable routing paths – there are benefits that go beyond bend radius protection.

Defined routing paths make accessing individual fibres easier, quicker and safer – reducing the time required for reconfigurations. Fibre twists are reduced so tracing a particular fibre for rerouting is much easier. Even with new technologies, such as the use of connection point identification (CPID) at both ends of patch cords for easy identification, well-defined cable routing paths still greatly reduce the time required to route and reroute patch cords. All of this directly affects network operating costs and the time required to turn up or restore service.

Modular circuit separation system

The easiest way to increase fibre capacity is to add as many fibres and cables as possible in the equipment. This results in splice trays with more than 96 splices per tray. The drawback of such mass storage systems is that during an intervention by a maintenance crew, many fibres or cables will be touched, either intentionally or by accident. The probability of an uncontrolled fibre bend becomes high, resulting in rapid changes in attenuation, also called “transient losses”.

Fig. 3: Transient optical loss.

Fig. 3: Transient optical loss.

Fig. 3 shows a typical transient loss recorded when an installer handles fibres. The effects of transient optical losses on the transmission quality has been described in many papers [4], [5]. Transient optical losses are fast changes in attenuation up to 10 dB with a duration from 1 ms to several seconds. The transient loss should be reduced to a level below 0,5 dB to avoid transmission errors (or bit errors) in an active optical circuit.

In network locations that require frequent interventions, such as the access network, it makes sense to separate the optical fibre circuits by storing the fibres on individual splice trays. This reduces the occurrence of transient optical losses in adjacent circuits. Circuit separation is defined and described by the international standards IEC 61756-1 [1] and ITU-T L51 [6]. The following separation levels are defined:

  • Single circuit (SC): only the fibres of one customer per splice tray.
  • Single ribbon (SR): only one ribbon per splice tray.
  • Single element (SE): all fibres from a cable element (e.g. a loose tube) per splice tray.
  • Multi-element (ME): fibres from multiple cable elements on one splice tray (also called mass storage trays).

A fibre management system should allow modular combination of splice trays with the above mentioned circuit separation levels. Depending on the application, the type of splice tray can be changed. For example, in a long distance network where cable segments are spliced together (in track joints) and where closure re-entry is not expected, the SE and ME splice trays are typically used. In the distribution points of an access network, where frequent re-entry is expected over the lifetime of the product, the SC and SE splice trays are employed.

Fibre identification and cable access

Cable access and identification is another important element to good fibre cable management and refers to the accessibility of the installed fibres. As the number of fibres grows dramatically in both the distribution frame and the active equipment, cable access becomes an increasingly important issue for broadband service providers. In the past, an active equipment rack might have had about 50 fibres exiting, and managing those fibres was much less of an issue. But as that same ack is fitted for next generation broadband services, there may be as many as 500 fibres involved, making proper management, identification and accessibility a vitally important matter. Using SC and SE circuit separation levels will help identify the correct fibre circuits.

With huge amounts of data – as well as revenue – moving across those fibres, the ability for technicians to have quick, correct and easy access is critical. When there are service level agreements in place, particularly for customers with high priority traffic, the last thing any service provider wants is service interruptions caused by mishandling one fibre to gain access to another.

As previously mentioned, there are patch cords designed today with connection point identification at both ends to help technicians identify particular cable runs with no chance of error. These innovations can be implemented into a good cable management system to minimise problems caused by disconnecting the wrong patch cord. There are many other tools and techniques for ensuring that every fibre can be installed or removed without bending or disturbing an adjacent fibre.

The accessibility of the fibres in the fibre cable management system can mean the difference between a network reconfiguration time of 20 minutes per fibre and one of over 90 minutes per fibre. Since accessibility is most critical during network reconfiguration operations, proper cable access directly impacts operational costs and network reliability.

A final word – planning

Finally, since many service providers are in the process – or soon will be – of upgrading networks for delivery of high-bandwidth broadband services, it is important to stress the need for planning in terms of fibre and cable management. Today’s network is a living and growing entity – and what is enough today will almost certainly be too little for tomorrow. With that in mind, future- proofing the network wherever possible should be a major consideration – and fibre cable management is no different.

For example, the current upgrades to broadband service delivery taking place in COs, MSOs, or MSCs requires more fibre deployment. Four- and six-inch (102 mm and 132 mm) fibre raceway systems are quickly becoming inadequate to properly manage larger amounts of fibre. Service providers must plan ahead for a centralised, high-density fibre distribution frame lineup using 610 mm raceways that not only accommodate today’s fibre requirements, but also those expected in the future. Although installing a 610 mm raceway system is more expensive today, the cost of going back in and retrofitting the system in a few years represents a much higher cost and significant risk to the fibre. Ignoring future growth, particularly in terms of fibre, will result in higher long-term operational costs resulting from poor network performance or a requirement to retrofit products that can no longer accommodate network demand.

Another consideration in planning for good fibre cable management concerns the active equipment rack. Most manufacturers have traditionally overlooked the need to provide cable management within their active equipment. Before purchasing, service providers should insist that cable management is included within every piece of active equipment to ensure their investment will operate at peak efficiently over time.

References

[1] IEC 61756-1 Fiber optic interconnecting devices and passive components – Interface standard for fiber management systems – Part 1: General and guidance

[2] I TU-T L.13 Performance requirements for passive optical nodes: Sealed closures for outdoor environments

[3] ITU-T G.657 Transmission media and optical systems characteristics –Optical fiber cables. Characteristics of a bending-loss insensitive single-mode optical fiber and cable for the access network

[4] D. Daems, “Effects of Transient Optical Loss in STM-64 Transmission Systems,” NFOEC Proceedings, Vol. 1, 290 (2002).

[5] John W. Peters and Osman S. Gebizlioglu, “FDF operations testing: transient optical loss and BER (Bit Error Ratio) measurements. NFOEC Procee

Contact Stefaan Fagot, TE Connectivity, stefaan.fagot@te.com

 

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