POL vs FTTO: choosing the best solution for each application

To meet current usage demands, previously disparate platforms and components need to communicate in a unified manner and act as an integrated whole. That requires a common language and an integrated approach to structured cabling and devices. In this respect, POL and FTTO each have their pros and cons. However, both are based on a full fibre based infrastructure with all its advantages. So why not plan a fibre-based infrastructure that can provide both, keeping the flexibility to move between technologies or running both in parallel?

Numerous developments and changing technology drivers affecting the LAN

Around the world, cloud applications, network convergence, (Building Internet of Things) and new ways of working have significantly boosted bandwidth demand and the number of required ports. Cabling needs to be more flexible and easily configurable. Passive Optical LAN (POL) and Fibre To The Office (FTTO) solutions can both offer this – but there are marked differences.

First, a closer look at full fibre based infrastructure

Traditional enterprise network infrastructure (ISO/IEC 11801) is mainly separated in horizontal and vertical cabling and realised in a tree topology. Horizontal copper cabling (primarily Cat 6, Cat 6A or Cat 7) is installed from a floor distributor to offices, where cabling terminates in RJ45 outlets in a cable duct or floor box. Devices such as PCs, IP Phones, or WLAN Access Points connect to this. In the floor distributor, copper cabling is usually terminated in 19” racks and connected to standard 19” access switches to provide Ethernet services. This cabling’s 100m maximum channel length (90m fixed cabling + two 5m patch cords) can result in multiple distributors per floor. Although available bandwidth is up to 10Gbps per port, most connections are based on 1Gbps.

As the floor distributor contains active and passive equipment, it requires a power concept, cooling and  security. Vertical cabling based on fibre connects all access switches to central core switches and forwards aggregated bandwidth to a central distribution room. These connections require more bandwidth and technologies such as 10, 25 or 40Gbps.

Unlike traditional network infrastructure, a ‘full fibre’ approach exclusively uses fibre optic cabling  between a central building or campus distributor and office environment. Depending on the fibre type, distances range from a few hundred metres to several kilometres. This avoids floor distributors and electrical installation work, increases security, and saves space, CAPEX and OPEX. Using fibre offers the option to install a physical tree or ring topology, using extractable cabling. Future-ready single-mode fibre offers practically unlimited bandwidth reserves. It is already possible to transmit hundreds of Gb/s over single-mode. Its lower volume avoids big (copper) cable bundles in the floors. This can be essential in historical buildings with limited options for rolling out network infrastructure. Furthermore, reduced energy consumption over the whole lifetime helps save resources.

Technical basics of two main full fibre enterprise infrastructure technologies

1. Passive Optical Local Area Network (POL or POLAN)

This migrates provider infrastructure and Passive Optical Network (PON) technology to enterprise, campus or hospitality network infrastructures. It is based on a point-to-multipoint topology and connects one central port with multiple clients, mainly using GPON or GEPON technology.

In a typical POL installation the central equipment (OLT), placed in the building or Campus Distributor,  provides signalling that is forwarded via one single-mode fibre to a passive optical splitter that splits/divides the optical signal for the connected ONTs/ONUs. A passive optical splitter is used instead of floor distributors to connect end users. The optical network terminates in an ONU/ONT in the office next to the client. All terminal devices connect to this using RJ45 connectors.

2. Fibre To The Office (FTTO)

This point-to-point topology connects one central port to a single FTTO switch installed in the office environment. using standard Ethernet Point-to-Point.

Small managed Ethernet switches convert optical signals to electrical and vice-versa. FTTO switches in cable ducts or floor boxes usually offer four RJ45 ports to connect terminal devices in the office environment. On the central side, standard Ethernet switches are used to connect FTTO switches to the core network. FTTO uses standard enterprise network equipment and technology, usually Gigabit Ethernet, providing one or two optical uplinks supporting 1Gbps and four user ports that support 1Gbps.

A shared medium is used for communication in a point-to-multipoint POL network, raising several issues:

 

• From a security perspective, sharing multiple connections on a single fibre is less than ideal.
• Available bandwidth has to be split into an existing ratio (1:16, 1:32 or 1:64).
• Limited flexibility during rollouts may result in overspecifying. You might require just one port in a specific location but have to install 16, for example.
• Upgrades affect every connected client.
• PON is based on single-mode fibre optics. Using multi-mode optical fibre is not possible.
• The shared fibre and splitter act as a single point of failure for all connected ONTs and clients. Risk is normally mitigated by redundancy topologies but this is not possible with POL.

Planning a fibre based infrastructure: the best of both worlds

The choice between FTTO and POL depends on numerous variables.

• Type and level of performance users and devices require – now and tomorrow
• Conditions and distances in the building(s)
• Requirements with regard to power, functionality, flexibility and uptime

However, it’s also possible to combine the benefits of POL and FTTO. To do this, fibre infrastructure must be planned with single-mode fibres (POL is based on bidirectional communication via one fibre). Because most PON ONTs are still connected via an APC connection all connectors should be designed as LC or SC/APC. Topology needs to be a logical point-to-point infrastructure between central building or campus distributor and end user. Fibre cabling travels from a central optical distribution frame (ODF) to the floors where ‘Zone Distribution Boxes’ (ZD Boxes) are located. Bundle cabling containing 12 or 24 fibres, for example, acts as vertical cabling between central ODF and the ZD Boxes in the floor. The single fibres are terminated and spliced in the ODF and ZD Boxes. From the ZD Boxes, pre-terminated fibre cabling or fibre patch cords connect ONTs or FTTO switches.

All active equipment is installed in the central distribution room. The passive optical splitter required for a POL installation also needs to be installed in the central building distributor between OLT and the ODF. FTTO switches are usually connected to the ZD Boxes and ONTs via a fibre outlet. ONTs are already equipped with an optical interface (usually SC/APC) whereas FTTO switches need a SFP transceiver and  corresponding optical interface. Finally, a hybrid optical patch cord from SC/APC to LC or SC/PC connection is used to connect the FTTO switches to the APC connector.

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