Operators are facing the challenge of how to increase capacity and therefore revenue. On the other hand there is only so much fibre available and it is a relatively expensive undertaking to install more fibre for a traditional duplex or dual fibre configuration.
One increasingly popular way to double the amount of links between devices without deploying more fibre is to use Bi-Directional transceivers. This type of transceiver is denoted by “D” or “U” or “Single Fibre” or “BiDi” in the description on the label and electronically.
The way in which these transceivers work is to run the signal in both directions on the same fibre. This means that we use WDM technology and an optical filter / reflecting mirror to separate the two wavelengths, or colours, into transmitting and receiving paths. Thus we will have two wavelengths denoted by a U and a D to denote upstream and downstream over a single strand of fibre.
The other way to identify the BiDi transceiver is by noticing that it only has one single connector, usually LC type but SC is also available if required.
It is standard practice to use the following wavelength pairings for the 1G transceivers: 1490 and 1310, whereas in 10G transceivers 1550 and 1310 is more common. For the 1G transceivers a reach of 120km is possible whereas for 10G the limitation is around 80km.
A common use case is where the telco is connecting business customers and we see both 1G and 10G BiDi transceivers are used frequently.
The form factors that are applicable to the BiDi are SFP, SFP+, XFP, QSFP and CSFP.
The CSFP form factor is a special case because it is in fact a double BiDi in a single form factor and is used in OLT scenarios.
OEMs supported for this type of bidirectional transceiver include Juniper, Huawei, Alcatel Lucent, HP, Transmode (now Infinera) and so on.