JOOST GRILLEART LC STRIKES BACK
LC STRIKES BACK the future of LC connectivity in the data centre
With IEEE protocols once again changing cabling in data centres, the expected rise of parallel optics and MPO/MTP connectivity seems less certain. What does the future hold for LC connectivity, asks Joost Grillaert , Product Manager at Nexans Cabling Solutions .
working group for 25G single mode. We expect standards to be published by the end of 2018 or early 2019. This will allow the use of 50GBASE-SR on duplex LC and 200GBASE-SR4 on MTP 8 connectors. By the end of 2018/2019 we will have the option of using transceivers for 200G that require only eight fibres. In access networks, there will be the choice of 10G, 25G and 50G with LC connectivity, the life of LC has been extended to 50G. In the core network there will be migration from 40G to 100G moving on to 200G. In most data centres, OM4 is currently preferred over OM3 because of its extended reach.
JOOST GRILLAERT I n 2005, access networks were generally based on 1G copper or 10G fibre LC connectivity (Lucent Connector). However, as core network speeds increased, standards body the IEEE released a new protocol in 2010: 40G based on parallel optics. These used four fibres to send and four fibres to receive data and gave rise to a new type of connector: MTP, a brand name for a version of the MPO (“multi-fiber push on”) connector. This drastically changed cabling infrastructure. In the access network – the largest part of the cabling infrastructure – operators could migrate from 1G to 10G with LC connectivity. In 2010, forward-looking DCs knew they would probably require a 100G core network in five years’ time (achieving that using existing protocols would require 20 fibres.) They also knew they would want 40G on the access network by 2015 which would require MTP connectivity. So MTP trunks were installed between racks and panels. Conversion modules would be installed and traditional LC duplex patch cords would be used at the front. Moving to 40G involved swapping out conversion modules and replacing them with MTP adapter modules. LC patchcords would be exchanged for MTP patch cords. This allowed operators to migrate from LC connectivity to MTP connectivity. DIFFICULT DECISIONS This migration path had one major drawback, however. One MTP connector could support six 10G ports and connect six devices. Switching out the conversion module would leave only one 40G MTP port, supporting just one device in the
rack. The IT manager had to decide whether to keep port count high with lots of extra cable, or install additional cabling when migration was necessary. Around 2013, the industry saw non- IEEE transceivers that used different wavelengths on a single fibre – an attempt to avoid migration to parallel optics for 40G (such as BiDi from Cisco and Univ 40G from Arista). These set- ups did not accept full duplex or LC patchcords, but did accept MTP. Yet it turned out that customers wanted to retain LC connectivity allowing them to expand on existing infrastructure. The first generation of 100G, developed in 2010, required 20 fibres. In 2015-2016, IEEE developed a second generation of 100G on multimode fibre. The number of fibres was reduced to 8, simplifying fibre cabling. Technological evolutions had made it possible to increase the speed of transceivers from 10G to 25G. As IEEE were developing this, they realised it would be interesting to develop a 25G multimode standard for the downlinks, based on – remarkably - LC connectivity. In 2010, forward-looking data managers saw only MTP connectivity in the access network and core network in the future. But new IEEE protocols have changed this. LC connectivity still has a place in the access network. The LC connector is being extended in the access network up to speeds of 25G. Speed may be reduced from 40G to 25G but previously installed cable infrastructure can stay in place. In the core network 8 fibre 100G can be used. In 2016, a new working group was formed to work on multiples of 50G. Multimode and single mode 50G, 100G and 200G are expected, and there’s a
IMPLICATIONS FOR DC NETWORKS
The core network between core switches is a relatively small part of the network. The much larger access network from the core switches to the access switches or severs can use copper, but in many DCs fibre is the preferred solution. The approaches we propose should make it possible to use the same cabling infrastructure for the next three generations. No migration of cabling infrastructure in the access or core network will be required. In the core network, Nexans suggests installing parallel optics and MTP trunks. That approach should make it easy to migrate from 40G to 100G and 200G. In the access network it proposes LC connectivity, allowing operators to migrate from 1G or 10G to 25G or 50G. There is no real need to migrate from LC to MTP connectivity moving forward. There are two possible approaches in the access network: (i) install MTP trunks between panels and conversion modules from MTP to LC inside the panels, which allows very fast installation; and (ii) there is an alternative for access networks that allows operators to migrate from 10G to 50G, based on LC preterms. The typical migration path used around 2010, where you install MTP trunks and swap out MTP conversion modules is no longer viable. In low speed networks Nexans now sees new life for duplex LC, moving from 10G to 25G to 50G. Parallel optics will remain in core networks migrating from 40G to 200G to 400G. We see no need to migrate from LC to MTP in cabling infrastructure.
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ISSUE 9 | Q2 2017
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