This document explains the optical connectivity involved in 40G optical QSFP for short reach (40GBASE-SR4), on multimode fibres. The standard specifies MPO12 (or MTP12) as connector to the SR4 QSFP, which employs traditionally 12 fibres, but 40G only need 8 (4 pairs) to carry the 4 parallels bidirectional paths. You might know that QSFPs can be programmed to operate as 4 x 10G.
2) QSFP to QSFP light path on MTP12 cables
Notice in the below QSFP 40G SR4 transceiver that the connector is not LC but a MPO/MTP12 receptacle. You may also notice the 4+4 small orange dots in the center, between the two alignment pins. The dots are the Rx/Tx media entry/exist in the transceiver. The space between the 4 are due to un-used fibre path. The 4 Tx and 4 Rx align with the outer fibre of an MTP cable.
QSFP to QSFP 40G SR4 optical cabling
Rx and Tx light path between SR4 QSFP and MTP cable
- 4 Rx (cable positions 1 to 4)
- 4 Tx (cable positions 9 to 12)
- the 4 positions 5 to 8 are unused by the QSFP
Note: the unused positions do not need to provide light path (it could save some costs for structured cabling), although it would not be standard MTP12 cabling.
40G MTP cable – Fibre position numbering
- key up
- from left to right
- from 1 to 12
This is independent from the fibre colour, which may vary. The fibre position numbering does not vary (E.g.,position 1 might be fibre Blue, or Aqua, etc)
3) What to be careful about
Consider the following when selecting MTP cabling for 40G SR4 QSFPs.
- QSFP Tx/Rx will always line up to the same MTP positions, whatever the cabling behind is
- Rx1 – position 1 – Tx1- position 12
- Rx2 – position 2 – Tx2 – position 11
- Rx3 – position 3 – Tx3 – position 10
- Rx4 – position 4 – Tx4 – position 9
- MTP cable head should be female (no alignment pin, it should have holes)
- alignment pins are inside the QSFP
- pairing : this is particularly important for 10G mode, pairs (Rx and TX) are alway of the same numbering:
- Pair 1 is Rx1 + Tx1 (positions 1 and 12)
- Pair 2 is Rx2 + Tx2 (positions 2 and 11)
- Pair 3 is Rx3 + Tx3 (positions 3 and 10)
- Pair 4 is Rx4 + Tx4 (positions 4 and 9)
- for direct connection (back to back), QSFP to QSFP MTP12 cabling without patch panel or adapter, you would need:
>>> Polarity B <<<
Detailed cable specifications for QSFP to QSFP direct connectivity
- EIA/TIA-604-05 (FOCIS-5) MTP-12 to MTP-12 cable
- polarity B (1 to 12, key up/key up)
4) Mistake examples
4.1) Polarity mistake
Polarity mistake (A instead of B for example), can cause light to come out of an MTP cable in the wrong position. In the below example, light comes wrongly out of the MTP12 cable in positions 9-12, instead of 1-4. Positions 1-4 are for light out of the cable. Positions 9-12 are for light into the cable. The cause of this mistake is usually an MTP cable having a polarity A, while it should be polarity B for direct QSFP to QSFP connectivity. When connecting devices via patch panel, verify the required cable polarity with your patch panel vendor.
Incorrect cable polarity (A instead of B) leading to light being in the wrong fibres
4.1) Wrong connector gender
The MTP cable should have a flat head (no extruding pins). The alignment pins are inside the QSFP, while the MTP connector has for alignment holes The below MTP12 cable has a wrong gender (male instead of female)
Wrong MTP12 cable connector : Male instead of Female
5 ) Physical appearance variant of 100GBASE-SR4 Transceiver
In addition to the above shown appearance of an SR4 transceiver the 100GBASE-SR4 looks like this:
This version appears to have 4 slightly larger holes (two left and two right) instead of 12 tiny fiber links which are arranged in a row.
A real life picture is attached via this link: 100GBASE-SR4
Posted in Tech Tips | Tagged: 40G, 40GBASESR4, cable, MPO, MTP, MTP12, polarity, SR4
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