# Description on the GCC byte mode of NG WDM products

Question:
In the OTN architecture, the ESC mode is adopted. Sometimes, the GCC byte mode needs to be set using the :cm-set-fiberport command. The last parameter of this command indicates the channel rate. 13 denotes three-byte GCC0, 14 denotes the GCC12_9 byte, and 15 denotes the GCC12_18 byte. How are three-byte GCC0, GCC12_9, and GCC_18 calculated?
Root cause:
None
Answer:
1. The channel rate is an equivalent rate. Three-byte GCC0 indicates that the bandwidth provided by GCC0 is 3 x 64K, and GCC12_9 indicates that the total bandwidth provided by GCC1 and GCC2 is 9 x 64K. In the OTN architecture, GCC0 is of two bytes, and the frame structure is always 4 x 4080 bytes.
2. At the 2.5G rate, the bandwidth of GCC0 is calculated using the following formulas:
Frame frequency = 2.5G/(4 x 4080 x 8)
Bandwidth of GCC0 = 2x frame frequency x 8
Therefore, the approximate bandwidth is as follows: 320K = 5 x 64K
Likewise, at the 5G and 10G rates, the approximate bandwidths are 10 x 64K and 20 x 64K respectively. (The length of an OTN frame is fixed, whereas the frame frequency varies.) Due to the limit on CPU channels, the bandwidth provided by GCC0 must be reduced so that the equipment can be used like the previous equipment. That is, GCC0 is used as three bytes only. To be specific, the supervisory bandwidth provided by GCC0 is 3 x 64K.
3. According to the preceding method, at the 2.5G rate, GCC1 and GCC2 together provide 10 x 64K bandwidth. That is, GCC1 and GCC2 are used as nine bytes. GCC1 and GCC2, which are always bound together when used, provide 20 x 64K bandwidth at the 5G rate and 40 x 64K bandwidth at the 10G rate.
Suggestion and conclusion:
The OTN-framed OTU board supports DCC types GCC0, GCC12_9, and GCC12_18. The OTN 2.5G board supports GCC0 and GCC12_9, and the OTN 5G and OTN 10G boards support GCC0 and GCC12_18.