The General Communication Channel (GCC) was defined by G.709

G.709

ITU-T Recommendation G.709 "Interfaces for the Optical Transport Network " describes a means of communicating data over an optical network...

 is an in-band side channel used to carry transmission management and signaling information within Optical Transport Network

Optical Transport Network

ITU-T defines an Optical Transport Network as a set of Optical Network Elements connected by optical fibre links, able to provide functionality of transport, multiplexing, switching, management, supervision and survivability of optical channels carrying client signals...

 elements.

Two types of GCC are available:

• GCC0 - two bytes within OTUk overhead. GCC0 is terminated at every 3R (Re-shaping, Re-timing, Re-amplification) point and used to carry GMPLS signaling protocol and/or management information.

• GCC1/2 - four bytes (each of two bytes) within ODUk overhead. These bytes are used for client end-to-end information and shouldn't be touched by the OTN equipment.

In contrast to SONET/SDH where Data Communication Channel (DCC) has a constant data rate, GCC data rate depends on the OTN line rate. For example, GCC0 data rate in the case of OTU1 is ~333kbit/s, and for OTU2 its data rate is ~1.3 Mbit/s.