Hi there, everybody!
This post talks about the dispersion compensation principle. Please see the information displayed below for more details.
BACKGROUND INFORMATION
In general, the transmission fiber features positive dispersion and positive dispersion slope. The DCM provided by Huawei is a fiber with negative dispersion and negative dispersion slope (special fiber: DCF). Currently, Huawei provides three types of DCMs that correspond to G.652 (SMF), G.655 (LEAF) and G.655 (TWRS) fibers, respectively (note: LEAF fiber is a G.655 fiber). It is the most commonly used G.655 fiber. The G.655 (LEAF) DCM is still called the DCM for G.655 fiber.)
If the proper DCM is used for the corresponding fiber, the compensation parameter of the DCM matches the kilometrage of the fiber. For easy calculation, the compensation kilometrage expressed in km is used for calculating the configuration of the DCMs, instead of the dispersion magnitude expressed in ps/nm.
Note: the OTU dispersion tolerance depends on the transmitter (laser).
DISPERSION-RELATED VALUES
The dispersion-related values for engineering calculation are as follows. Please see them below.
The typical dispersion of G.652 single-mode fiber (SMF) is 17 ps/(nm km) (Note 1). The typical dispersion of G.655 SMF is 4.5 ps/(nm km) (Note 2).
Note 1: during the conversion of the OTU dispersion tolerance into dispersion limit, the fiber dispersion value should be taken as 20 ps/(nm km). For example, if the dispersion tolerance is 800 ps/nm, it is converted into the dispersion limit of 40 km.
Note 2: during the conversion of the OTU dispersion tolerance into dispersion limit, the fiber dispersion value should be taken as 6 ps/(nm km).
If a 10 Gbit/s single wavelength is transmitted over G.652 fiber, the dispersion limit is 800/20 =40 (km) instead of 800/17=47 (km) (note: the dispersion limit is 700/20=35 (km) in China.) Because the general hop is longer than 40 km, dispersion compensation is required. If the wavelength is transmitted over G.655 fiber, the dispersion limit is 800/6=130 (km) (note: the dispersion limit is 700/20=35 (km) in China.) Hence, long haul transmission requires dispersion compensation.
During the calculation of the dispersion compensation distance, only the following parameters need to be considered:
1. OTU dispersion tolerance;
2. engineering margin (G.652 fiber: 10–30 km; G.655 fiber: 38–113 km).
The calculation formula is as following:
Dispersion compensation distance (L(DCM)) = transmission distance (L) - OTU dispersion limit + engineering margin.
A 100 km G.652 fiber span is considered as an example:
L(DCM) = transmission distance - OTU dispersion limit + engineering margin = 100 km - 40 km + (10–30 km) = 70–90 km.
Hence, an 80 km DCM need be configured.
LIMITATIONS AND PRECAUTIONS
1. There have been misunderstandings (or misinterpretations) on the external specifications of Huawei OTU dispersion tolerance. For a long time, the internal dispersion tolerance specification of a DCM is defined based on the dispersion limit that is calculated with the dispersion coefficient being 20 ps/km (G.652 fiber). For example, the calculated dispersion tolerance of the 2.5 Gbit/s 640 km DCM is 12800 ps. The actual dispersion tolerance of the DCM, however, cannot reach 12800 ps. If the actual dispersion coefficient of the fiber is considered 17 ps/nm, the calculated dispersion tolerance of the 2.5 Gbit/s 640 km DCM is just about 10880 ps. 10880 ps is closer to the real specification than 12800 ps is. The R&D Department can test the actual transmission performance based on the dispersion limit. The dispersion limit, however, must not be calculated with the dispersion tolerance specification of the DCM being 12800 ps and the dispersion coefficient of the fiber being 17 ps/nm.
2. It is irrational to define the dispersion tolerance specification of a DCM based on the dispersion limit that is calculated with the dispersion coefficient being 20 ps/km for network planning. This definition method, however, is beneficial to Huawei when specified by the customer (there were many such cases in the earlier projects). In that case, the dispersion tolerance specification is surely satisfactory.
