Preterminated Solutions, Past Present and Future

Geoff Day, Technical Manager at CMS plc, examines the use of preterminated cabling solutions in the Data Centre and their ability to support 40 and 100 Gigabit Ethernet speeds.

Preterminated cabling solutions have been around for a while, but have seen dramatic growth in recent years, which can be primarily attributed to their use in Data Centre environments, which seem to have a never ending demand for bandwidth. The demand for fast and reliable delivery of critical applications is driving Data Centre technology at an ever increasing pace.

Pre terminated solutions have been available for both copper and fibre cabling with fibre coming to the fore in recent years by virtue of its bandwidth capabilities. These solutions have been available in two forms: factory terminated solutions supplied by OEM’s and products terminated off-site by System Integrators and installers. They offer a number of advantages over traditional structured cabling solutions including:-

  • Minimum on-site disruption
  • Minimum waste and disposal costs
  • Reduced security risks
  • Fully tested product delivered to site
  • Consistent product quality
  • Reduced installation time
  • Improved return on investment.

These attributes made them particularly attractive for Data Centre Applications. Another primary driver which has contributed to the accelerating adoption of preterminated solutions is the increasing need for higher density cabling in the Data Centre. Cabling blocks airflow so the higher the density of the cable, the better the thermal management – an important consideration for Data Centres applications. The development of MPO (Multi-push-on) fibre trunk cables significantly increases cable density and addresses the 40 and 100Gbps bandwidth requirements of the latest Ethernet protocols.

Architecture and Planning

When planning Data Centres, designers need to provision both for flexibility and scalability for future growth. Data Centre architecture has altered significantly in recent years leading to the modular “pod” design which can be a top-of-rack (TOR), middle-of-row (MOR) or end-of-row (EOR) configuration. This pod design allows the minimum investment to meet the current requirements, but permits the flexibility and scalability to build additional pods when required. This approach spreads the financial investment on a “pay as you grow” basis and improves payback. It is possible for entire pods to be built off-site and delivered as and when required.

The advent of MPO solutions years ago provided a level of modularity and ease of configuration of cables within the data centre environment. Most good pre-terminated solutions comprise a range of trunk cables, array cables, plug-and-play cassettes ,which, when combined with high density frames and modular panels, give network managers a choice of cable configurations to suit their specific needs.

Just as Data Centre architecture and design has evolved over the years MPO solutions have also undergone a number of changes. Some of the first generation MPO solutions were regarded with caution by some network managers as they were not considered completely reliable. The products were not seen as sufficiently robust and durable and they also seemed vulnerable in low temperature environments and so their use was perceived as limited. In order to overcome these vulnerabilities most manufacturers of pre-terminated solutions have revamped their products to ensure they are fit for purpose in the data centre environment, which is one of the most demanding. Connectors and cable assemblies have been refined to improve performance and durability in the field. It is easy to see how this technology will be adopted in more traditional networking environments. Network Managers are replacing their existing infrastructure with MPO cassettes and running a 12 fibre MPO cable from a cassette from one side of the building to a cassette on the opposite side, which is a really simple way to supply data for 12 connections in an instant. The high fibre count of MPO solutions creates numerous possibilities.

Faster Data Centres

Most forward-thinking network managers will want to future-proof their installations such that they will accommodate the next generation of networking equipment. Already we are seeing 40 & 100 Gigabit Ethernet equipment available on the marketplace, and current predictions forecast shipments of 40 Gigabit Ethernet servers to grow by more than 40% in the short term. Network managers will want to provide for this technology with their new data centre builds. Fibre is the medium that supports 40 and 100 Gigabit Ethernet and logically it follows that MPO solutions are well suited to support the next generation requirements. However these new Ethernet speeds place considerably higher demands on the cabling technology required to support them.

Let’s look at this in more detail: If we take a 10 Gigabit Ethernet system installed today using trunk cables and breakout cassettes with MPO to LC connectors, the upgrade path would seem simple – just swap the cassette for an MPO coupler. But in practice it isn’t that straightforward. As an example, a 12-fibre MPO trunk cable can be broken out at the cassette to provide six 10 Gigabit Ethernet channels with LC connectivity, providing an aggregate of 60 Gigabits. But if you replace the cassette with an MPO coupler you would have a single 40 Gigabit Ethernet channel (four transmit and four receive fibres), with four dark fibres within the trunk cable and no capability to provide a 100 Gigabit Ethernet channel. It is apparent, therefore, that network managers who intend to use MPO solutions for their data centre cabling infrastructure need to plan very carefully to ensure that they have sufficient high speed channels to meet their future requirements and to understand how the architecture may impact on the number of cassettes in the channel. It is important to note that when redeploying your cabling to support the higher speeds of the next generation Ethernet protocols it is necessary to re-test the channels.

Impact of return loss on high speed solutions

Many systems on the market today struggle to deliver 10 Gigabit performance when the channel comprises four or six MPO cassettes. The predominant reason for this is poor optical performance which is primarily due to return loss (RL). The maximum return loss for a 10 Gigabit Ethernet channel is 12dB at the optical receiver. Each cassette that is added to the channel generates more reflection and consequently reduces the RL budget. Increasing the fibre bandwidth has no effect on the RL budget as it is directly related to the number of connections in the channel.

So how can this constraint be overcome so that 40 and 100 Gigabit performance levels can be achieved? The key to overcoming these issues is to improve the performance of the connectors and developing products with extremely tight tolerances. What does this mean in reality? It means controlled end-face geometry in the connector and the precise location of fibres within the connector interface and very tight tolerances. These tight tolerance requirements are recognised in the new Ethernet standards: to achieve 150m with an OM4 cable the maximum return loss is 1dB.

Clearly the manufacturers of MPO solutions need to ensure that their connectors are up to the task. TE Connectivity have invested significantly in the design and performance of their MPO products, resulting in a solution that will support 10Gbit/s for any architecture design using between two and six cassettes for distances up to 300m. Their solution provides an upgrade path to 40 and 100Gbit/s for OM3 (100m) and OM4 (150m) cable, thus “ticking all the boxes” in terms of flexibility of design and future-proofing.

Checklist

So for the discerning Network Manager, needing to design their next Data Centre I would offer the following advice:-

  1. Remember you still need to fully test the solution, as once it has been shipped from the manufacturer’s factory, it is transported, stored, bent and pulled during installation. Testing on-site, post-installation is the only way of guaranteeing performance.
  2. Decide how much you want to future-proof the system and design your cabling system to support this.
  3. Determine the architecture you plan for the Data Centre as this will impact on the channel and return loss budgets for high speed solutions.
  4. If provisioning for 40 and 100Gbit performance levels check the product performance level of the manufacturer’s to ensure that their products can support this.
  5. If you reconfigure your fibre cabling to support 40 and 100 Gigabit Ethernet, you will need to re-test the channel.