What is Polarity?
Polarity in its simplest form
To connect two SFP 1Gbps transceivers a duplex (2 fibre cores) cable could be used. For a short distance of say 5m or 10m the ideal cable would be a duplex patch cord. In this exampl the patch cord would have a polarity A-B.
Plugged into two transceivers, the key of the patch cord would be KEY UP at both ends. Both transceivers would transmit into connector B and the signal would be sent out of connector A. Patch cords are sometimes described as either crossed or straight through. This terminology can often cause confusion. An A-B patch cord does not have the fibres physically crossed but they run parallel to each other. But the fibres do connect connector A to connector B.
Transmit (Tx) and Receive (Rx) working in pairs
Maintaining the correct polarity across a fibre network ensures that a transmit signal (Tx) from any type of active equipment will be directed to the receive port (Rx) of a second piece of active equipment and vice versa. Typically, for lower data rates the Tx and Rx signals transmitted in pairs and are therefore networked in pairs. The fibre infrastructure is designed to maintains these as pairs.
For most situations, the connection between two transceivers will require a longer link. It is not considered best practice to directly connect two transceivers but to use an “isolated” trunk / backbone fibre cable. This ensures the trunk cable is unlikely to get damage if further changes occur and also provides greater flexibility in the use of the trunk. A typical set up could look like this:
Transceiver <-> Patch cord <-> Adapter held in a patch panel or box <-> fibre trunk cable <-> Adapter held in a patch panel or box <-> Patch cord <-> Transceiver
The diagram shows that 3 cables used in this way will maintain the polarity across the network. In fact, any number of cables networked in this way will maintain the polarity between two transceivers.
Maintaining polarity with MPO fibre networks
For fibre networks based on MPO type connectors maintaining polarity becomes a little more complicated. Firstly, MPO connectors are used in both duplex type transmission and in multi-fibre parallel type transmission. The four most common types of application are
Duplex type networks based on MPO 12 fibre connectors: up to 10G or 25G Ethernet MPO cassette to cassette using polarity method C
Multi-fibre parallel networking based on MPO 12 fibre connectors: for 40G Ethernet using polarity method B
Multi-fibre parallel networking based on MPO 12 fibre connectors: for 100G SR4 Ethernet using polarity method B
Multi-fibre parallel networking based on MPO 24 fibre connectors: for 100G SR10 Ethernet using polairty X
All these types are defined within international cabling standards. For example TIA/EIA 568-b-1-7 defines 3 standard methods for maintaining polarity within multi-fibre connector fibre networks.
The majority of brands of MPO based systems use these methods for standard ranges. This means that it is possible to mix and match between different cabling brands and still ensure the network polarity is maintained.
A couple of points to note
MPO connectors can be either male (with two guide pins) or female (without pins but with 2 holes). MPO cables can only be connected male to female. QSFP+ and QSFP28 transceivers require MPO cables for connection. The international standard states that transceivers should have male ports and require a female MPO connector
Each 1U panel can also hold M310 24 fibre cassettes and so other options include have a 72 LC fibre presentation with LC cross connects or a mix of LC and MTP connectivity in the HDA.
Each HDA can support any number of equipment racks and is usually located at end of aisle or center aisle and allows for diverse routing of primary and secondary cabling.
The cross connects within the HDA enable inter-aisle connectivity, ease of access and can reduce overall cabling distances. In the figure above the two HDAs are M310 1U MTP through coupler housing panels. Each one of these 1U panels offers up to 288 fibres (144 pairs).
There are two types of MPO adapters; key up to key up (aligned) and key up to key down (opposed). The “key” refers to the MPO connector which is asymmetrical. MPO adapters are used in the rear of MPO cassette modules and in MPO fibre panels for MPO cable extensions and to connect MPO patch cords to MPO trunks.
A Key-Up to Key Up adapter will flip all the fibres when two 12 fibre cables are connected through the adaptor, i.e. fibre no. 1 (of the 12 fibres) in the first cable will connect to fibre no. 12 in the second cable. At every point in a fibre link where a Key-Up to Key-Up adaptor is used the fibres 1 to 12 will be flipped and reverse the polarity of the link.
A Key-Up to Key-Down adapter maintains the fibres so that fibre 1 in cable 1 is joined to fibre 1 in cable 2. This means that no matter how many of these adapters are used in a link the polarity of the link remains.
Key-Up to Key-Down adapters are far easier to use when trying to maintain a consistent polarity across an installation both at the time of installation and for subsequent moves, adds and changes.
Standard Duplex MPO Fibre Networking: Polarity Method C
The building blocks of a bodular MPO network
1. Without a patching field / cross connect
In the simplest form, an MPO network could be used to connect network equipment (server/ switch / storage) at two locations. Two MPO-LC cassette modules (+ 1U 19″ panels, not shown), MPO trunks and LC patch cords would give a 6 channels (using 12 fibre cassettes) or 12 channels (using 12 fibre cassettes).
The key on the MPO connectors means that the MPO trunk cables can only be connected to the MPO cassettes in one orientation. This means that the polarity is fixed when the cables and cassettes are manufactured. All components within the network have a component polarity; patch cords (A-B or A-A), MPO cassettes (A, B or C) and MPO trunks (A,B or C). For the link to work correctly all these component polarities must be correct.
The most common polarity for this type of network is Polarity Method C (see technical drawing). This method uses standard A-B LC patch cords and has the benefit that when extending the network the component polarity of all new components remains the same.
NOTE – the type of MPO adapters used in the rear of the cassettes also are part of the component polarity, not just the internal wiring.
Click Image to Expand
There are two other Network Polarity Methods. These are A and B. Network Polarity Method A uses both types of polarity patch cords (a-A and A-B) and so is not commonly used. Network Method B is most commonly used for SR4 40G and 100G Ethernet networking. See below.
1. With a patching field / cross connects
Taking the above example and introducing a patching field, typically located in a distribution area of a data centre, creates an MPO network with four MPO cassettes.
Complete Connect’s M310 Series
Using Complete Connect gives you the flexibility to chooses products that match the design requirements and so ensure that costs are kept to a minimum without being detrimental to performance. The main aspects taken into consideration when looking at a network are outlined below:
Traffic across various sections of the data centre or network
Quantity and variety of fibre channels, physical space and channel flexibility
Expansion and growth plans for the data centre or network
Expected volume and frequency of MACs
Physical space and outlet density
For Reference: Polarity Method A
For Reference: Polarity Method B
Considerations on Using MTP Based Networks
Some Key Points to Note:
1. M300 products use MTP standard connectors and LC / SC standard connectors and so will be suitable where a combination of either shorter runs or fewer connections are required
2. M310 products use MTP ELITE connectors and LC / SC premium connectors (giving reduced insertion loss at each mating) and so will be suitable when channel lengths are longer or more connections (such as the introduction of Zonal or Horizontal distribution) are required
3. Ruggedised fan-outs allow connectivity without the need for rack space
4. MTP-MTP cross connects can be used as an alternative to LC cassettes in the main distribution area and so reduce both the U space required and the number of connections in the channels
5. M350 HP products give direct cabling to zonal areas or active equipment and can remove U space requirement
6. The MTP 24 fibre connector can be used where rack space is heavily restricted
1. All cassettes have MTP® male internal rear connectors and will require an MTP® female connector for a correct mating
2. All M300 and M310 products are manufactured to be used with only MTP® adaptors that are Key Up – Key Down ensuring continuity of fibre (fibre 1 to fibre 1) at each adaptor
3. Refer to product specifications of trunks and ruggedised fan-outs to ensure the connector to connector distances you require are based around our definitions of trunk and ruggedised fan-out lengths (i.e. when breakouts and stub lengths.
4. Only Complete Connect products can be used across a link.
5. MTP-MTP cross connects can be used in the Main Distribution Area (MDA). To do this the LC cassettes in the MDA are replaced with MTP adaptors housed in adaptor plates. This decreases the insertion loss across the channel and enables greater channel distances to be achieved because 2 LC matings have been removed.
6. The 24 fibre MTP® connector can be used to double the density of fibre within MTP® adaptor plates
7. The 24 fibre MTP® connector can be used to half the number of cables to a cassette.
What Installation Steps are Required?
The same principles apply whether you are installing a 24 fibre link either between 2 buildings, connecting an office floor to a comms room or creating hundreds of channels in a data centre.
There are 5 main steps to adhere to: –
Ensure that the link being created (distance and number of connector matings) meets the required standard.
Ensure that the polarity is managed across the link and in accordance with the network design
Ensure physical forces on the cables and connectors are managed by using strain relief on the cables to limit the tension in the cabling and maintain bend radii in both the cabling and patching according to the allowed specifications.
Follow the fibre connection procedures that you should use when connecting any type of fibre connectors together.
Before connecting a fibre to a cassette or connecting 2 MTP connectors via an adaptor, you should always clean both fibre ferrules with approved cleaning tools, visually inspect both ferrules using an appropriate scope, repeat until the ferrules are clean, and test with a light source and power meter (that is properly referenced).
If the tested link is giving insertion losses greater than expected repeat step 4.
NEVER insert a connector into a cassette without the cleaning and inspection process in step 4 being completed.
Maximum channel lengths are calculated using the Complete Connect channel calculator application tool
Channel lengths can be increased by using M310 products instead of M300 products which reduces the insertion loss at each connector mating
Channel lengths can be increased by using M350 direct cabling products to reduce the total number of connectors in the link
Channel lengths can be increased by using MTP–MTP cross connects instead of LC cassettes
Channel lengths can be increased by using direct cabling such as ruggedised fan outs instead of trunks and cassettes