Fibre optic cable continues to light the way to higher bandwidth and data speeds. The ratification of two new classes of fibre, the looming introduction of 40 & 100 Gigabit Ethernet (GbE), and the revisiting of low-bend sensitivity technology, means network owners and installers face a dizzying array of considerations when specifying fibre-optic physical infrastructure in enterprises or data centres. Ken Hodge considers the growing complexity of choice:

The insatiable appetite for bandwidth and higher-speed data has ensured a thriving fibre-optic cable industry driven by technology innovation, standards development and ever-widening scope of application. New applications have caused significant diversity in cabling options, with more than 15 types of single mode fibre, two types of single-mode and four types of multi-mode fibre cable now available.

New cable standards

ISO/IEC and TIA industry standards have been updated to include two new classes of fibre: OS2 and OM4. OS2 is a single mode fibre cable introduced in the industrial premises standards as a solution for long links in outdoor environments, whereas OM4 develops proprietary, higher-bandwidth OM3 solutions (OM3+/extended etc.) This new OM4 solution is ideal for multiple applications such as backbone connections in enterprise LANs and data centres, and is now referenced in the new IEEE 802.3ba designation for 40/100GbE.

Deployments of OM1 (62.5/125) and OM2 (50/125) are in steady decline, as adoption of OM3 (laser optimised 50/125) has accelerated. Latest to the market OM4 (50/125), is the premium-grade multi-mode fibre with the highest quality core, making it the best platform available for the transmission of higher-speed protocols (such as 10, 40 & 100 GbE). Signals transmitted on OM4 suffer the least geometric dispersion, which enables longer reach (up to 550m for 10GbE) or the accommodation of more connectivity on shorter links. The real benefits are seen with applications such as 10GBASE-SX, which employ vertical-cavity surface-emitting lasers (VCSELs) operating in first window. However, despite VCSELs providing the lowest-cost option in terms of optical transceiver equipment, the perennial problem of network lifecycle still applies.

Issues to consider when choosing cable

Before any decision can be taken regarding the type of fibre cabling to specify, there are a number of factors to consider regarding the objectives of the planned system:

• Should it support the applications that are running today, or those that are expected to be running in the future – e.g. Ethernet, Fibre Channel, Fibre Channel over Ethernet (FCoE), or Infiniband?
• Is it be designed to last a few years or for the lifetime of the building? Are the necessary funds available to allow a modular and scalable system to be deployed, or do you install dark fibre from the outset?
• What is the type of environment – is it a data centre, general office, or industrial site used for retail/manufacturing; or a metro or campus-wide installation?

Standards dictating choice

There are many optical fibre types specified by international industry bodies, but only four are commonly used for in-building cabling, while three more are being introduced. The most important specifications relating to enterprise and data centre fibre deployments are:

• ISO/IEC 50173-1, which is almost identical to the international cabling standard ISO/IEC 11801 provides general advice for the design of generic cabling
• ISO/IEC 51073-2, 50173-3, 50173-4, and 50173-5 specify generic cabling in general offices, industrial premises, the home and data centres respectively
• ISO/IEC 24764 and TIA-942 data centre infrastructure standards

EN 50173-1 is an important standard because it defines system performance parameters and recommends cabling dimensioning and configurations. The other parts of this series are more prescriptive to the environment – e.g. EN 50173-5 recommends OM3 as the minimum performance requirement, effectively rendering OM1 and OM2 redundant within the data centre. It also advises the use of LC or MPO connectors. The LC connector is a small form factor connector with particularly good return loss performance, whilst the MPO is suited to quick, multi-fibre installation of backbones, and for short links that use multi-lane optical technologies (Infiniband & 40/100GbE). These standards-driven infrastructure decisions simplify choice, protect against the requirements of newer technology applications, and ensure that components are interoperable and backwards compatible.

Another area of interest and ongoing standardisation is low-bend sensitivity fibre (both singe and multi mode), which is an old technology being revisited to aid FTTH (fibre-to-the-home) rollouts and high connectivity areas such as data centres (patch cords and cables see tight bend radii in data centres). The aim is to protect the power of the signal and provide more security in maintaining the optical link – allowable optical margins per application have dropped from >10dB at lower speeds to <2dB. This places more importance on the quality of the connectors and the performance of the fibre-optic cable.

Cost remains king…

Ultimately, the choice of multi-mode or single mode cable within the LAN and/or data centre is a commercial decision governed by the total cost of the system, distance traversed and transmission bit-rate. Single mode duplex fibre links may be less complex than multi-lane multi-mode links (e.g. for 40GbE that uses 8 multi-mode fibres), yet the active equipment required for single mode can be more than three times as expensive – and 20 times more expensive at higher speeds.

As the number of single mode deployments increases (driven largely by FTTx applications) the cost of active equipment may fall, but given that most enterprise networks comprise short-distance, connector and transceiver dense links, multi-mode is by far the dominant fibre type employed today. Backbone applications typically account for more than 60 percent of deployed multi mode fibre, with data centres and campus networks each accounting for approximately 13 percent, and horizontal applications 10 percent.

…and so does design

Both single mode and multi-mode solutions will be used in the network as the LAN today supports a growing number of new applications – such as surveillance (CCTV) and video telephony. Reliability is a given. In the case of a large campus, or a data centre, a dual redundant ring topology is also vital. The most common cause of network down time is accidental damage and if there is only one fibre backbone supporting the network, downtime can be extremely costly and disabling to the business. Indeed, such is the critical nature of today’s fibre systems that higher reliability and availability is a must when compared with other parts of the network.

Copper isn’t dead yet

A thorough understanding of current and future LAN technology has become essential to finding the best structured cabling solution for the design’s lifetime. This is not a simple equation, and this is why in backbone and campus installations, blown fibre remains a favoured option, offering the flexibility and modularity to perform future upgrades and changes with a minimum of disruption. In the data centre, the introduction of 40/100 GbE speeds with their low optical budget means that OM4 must realistically be considered from the outset to provide a robust system. Fibre is essential in many parts of the infrastructure, particularly in outside plant, and is viewed by some as the future of the data centre core. However, in the latter, the commercial imperative may determine that copper has some life left yet.