Choosing Your Fibre Cable

Key Points

• Explore the factors that need to be considered when buying fibre cable
• Find out how fibre cable is sold
• Read a case study on how Networks Centre assisted a FTTP network provider

With so many applications for fibre optic cables, we take a brief look as some of the considerations when selecting fibre for Datacoms and Telecoms environments: 

When you are looking to buy a length of pre-cut fibre cable from your distributor, you’re probably thinking about the specific requirements

that you may have. The fundamentals, such as transmission performance and compatibility with your optics, will probably be at the top of your list. Depending on the transmission distance and data speeds required, this usually comes down to multimode (OM3, OM4, OM5) or single mode OS2. The only time you may require OM1 is for matching to legacy optics. For Multimode fibre, OM2 has been superseded by OM3 and OM4 which are compatible. OM5 is a wide band multimode fibre for use with WDM (wave division multiplexing) but to date it has not had widespread adoption. 

For more information about Optical Modes (Optical Single mode ‘OS’ and Optical Multimode ‘OM’) and transmission distances see the table below. As can be seen the transmission distance of single mode fibre is vastly superior and consequently is used exclusively in external applications (Telecommunications). The cost of single mode optics has come down in the past decade and with the increasing speeds (40Gbe and 100Gbe) in datacentres, single mode fibre is increasingly being deployed internally as well, particularly in larger, high performance data centres.

Another attribute high up the list to consider is the fibre count, including ‘dark’ fibres for future use. It is always advisable to install more fibres than required on ‘day one’ to allow for expansion. Infrastructure is expensive to install, whereas the cost of fibre cable is relatively cheap.

Fibre cables are supplied in standard core counts and above 12 fibres, will usually be in a doubling sequence, with the fibres arranged in bundles of 12 e.g. 1, 2, 4 ,8 ,12, 24, 48, 96, 144, 288 fibres.  

1 and 2 fibre cables are usually only used for patch cords and will be constructed to be very pliable but also rugged. Twin fibre cables can also be found in use in Telecoms for the last leg link to the customer premises. 

Central Loose Tube Steel Wire Armoured Fibre Cable

A word about diameter. The cable diameter is an important parameter for a few reasons. Where the cable enters an enclosure such as a fibre panel or splicing box, the compression gland that forms a seal will need to be sized to the cable diameter. The diameter will also relate to the bending radius. This should not be exceeded otherwise losses can occur which affect transmission quality. The diameter is also an important consideration for blown fibre cables where diameter (and weight) can relate to blowing distance and the sizing of micro-ducts. The bend radius also needs to be considered as this will have an influence over rack size and other infrastructure. Whether the cable is installed on a cable tray or in an external duct, it must have enough flex to change direction within the confines available.

Cable Environmental factors (e.g., internal or external) will determine the sheath type and cable construction. If the cable is to be direct buried rather than in duct, then crush resistance will be a factor and steel armouring may be required as well as water blocking layers.  External cables with PE (Polyethylene) sheathing or steel armouring will be less flexible and may require duct or draw pits so that they can be installed into a duct, terminated, or for direction changes. If the cable is going to be drawn into ducts using pull ropes, tensile strength will also need to be considered. Tensile strength is provided by elements known as strength members built into the cable.

Blown Cables – In the FTTx arena, blown fibre cables are very popular, where small diameter cables are air blown into micro ducts for last mile installations. This means trenchless techniques can be used to minimise disruption when installing onto private property for example. High fibre count cables can be air blown, typically up to 288 fibres. Networks Centre supply air blown micro fibre cables. For example, a 144-fibre cable from our Micro-lite range has a diameter less than 6mm diameter!

Other considerations such as the flammability rating and internal construction of the cable may be necessary if it is being installed internally. Tight buffered cables are often used inside buildings as they can be used in both horizontal and vertical pathways, and they are easier to prep and terminate than gel-filled loose tube. As they are inside, they do not need the gel to protect the fibres from the moisture ingress. However, the core count is usually limited to twenty-four fibres and generally tight buffered fibre cable is more expensive than loose tube constructions. A dry tube version is also available, which reduces the weight of the cable and does away with the messy gel. When placing loose tube cable types vertically, such as for a building backbone, care must be taken. To eliminate the risk of fibre breakages under tension or the gel shifting due to gravity, loops need to be inserted at regular intervals – usually on every other floor. 

In recent years the CPR (Construction Products Regulations) flammability rating of cables has been introduced and is now a key consideration when specifying data cables. Below, we show a table from one of our premium fibre cable suppliers, Leviton. For commercial applications, only B2ca and Cca provide acceptable protection for most specifiers, particularly in public buildings.

All fire rated cables will have a Declaration of Performance (DoP) which will provide independent verification of fire performance. These will be available from your supplier upon request.

With so many variants available, it’s no surprise that there are quite a few fibre cables to choose from. Distributors typically stock fibre cable in 1km or 2km drums which are bulky and need to be stored. Most LAN customers, require less than 100m length. Customers will not wish to purchase a 2km drum so will usually purchase a cut length.

Here at Networks Centre, we stock one of the best selections of LAN fibre network cable in the UK, so our customers can usually find the exact cable that they need without having to adapt their plans. We also cut cable to length at no cost and re-reel onto smaller drums for ease of handling. For Telecom cables, customers are likely to require much longer lengths. Single mode cables are typically supplied on 2km drums. We can still re-drum cables if needed to make more manageable.

Read one of our case studies where we worked with our FTTx cable manufacturer, STL to assist a FTTP Network Provider.

You may have also seen the following parameters mentioned associated with Single mode fibre: SM G652D or SM G657A2. These refer to universal ITU-T specifications. G652D is the grade of single mode fibre that has been in use for decades. It is still widely used for legacy installations ensuring spicing onto old fibres of the same type gives a better result. G652D fibres provide marginally lower loss and so better for long distance links. However, it is more bend sensitive meaning more susceptible to micro bend losses.

G657 A1 and G657A2, also known as bend insensitive or bend tolerant fibre, have become popular in recent years. They are more likely to be used for high density installations where tighter bend radius cannot be avoided. With increasing data speeds, this fibre is now popular in both data centres and new FTTx last mile installations.

Networks Centre stock a vast range of fibre cables from leading manufacturers. Contact our dedicated sales team by email at enquiries@networkscentre.com or phone us on 01403 754233.

Related Links

• Fibre Cable