Context :

Aims : We present an overview of a high-mass star formation region through the major ( sub- ) mm , and far-infrared cooling lines to gain insight into the physical conditions and the energy budget of the molecular cloud .

Methods : We used the KOSMA 3m telescope to map the core ( 10 ^ { \prime } \times 14 ^ { \prime } ) of the Galactic star forming region DR 21/DR 21 ( OH ) in the Cygnus X region in the two fine structure lines of atomic carbon ( C i ^ { 3 } \mathrm { P } _ { 1 } \mbox { $ - $ } ^ { 3 } \mathrm { P } _ { 0 } and ^ { 3 } \mathrm { P } _ { 2 } \mbox { $ - $ } ^ { 3 } \mathrm { P } _ { 1 } ) and four mid- J transitions of CO and ^ { 13 } CO , and CS J = 7 \mbox { $ - $ } 6 .
These observations have been combined with FCRAO J = 1 \mbox { $ - $ } 0 observations of ^ { 13 } CO and C ^ { 18 } O .
Five positions , including DR21 , DR21 ( OH ) , and DR21 FIR1 , were observed with the ISO/LWS grating spectrometer in the [ O i ] Â 63 and 145 \mu m lines , the [ C ii ] Â 158 \mu m line , and four high- J CO lines .
We discuss the intensities and line ratios at these positions and apply Local Thermal Equilibrium ( LTE ) and non-LTE analysis methods in order to derive physical parameters such as masses , densities and temperatures .
The CO line emission has been modeled up to J = 20 .

Results : From non-LTE modeling of the low- to high- J CO lines we identify two gas components , a cold one at temperatures of T _ { \mathrm { kin } } \sim 30 - 40 K , and one with T _ { \mathrm { kin } } \sim 80 - 150 K at a local clump density of about n ( H _ { 2 } ) \sim 10 ^ { 4 } -10 ^ { 6 } cm ^ { -3 } .
While the cold quiescent component is massive containing typically more than 94 % of the mass , the warm , dense , and turbulent gas is dominated by mid- and high- J CO line emission and its large line widths .
The medium must be clumpy with a volume-filling of a few percent .
The CO lines are found to be important for the cooling of the cold molecular gas , e.g .
at DR21 ( OH ) .
Near the outflow of the UV-heated source DR21 , the gas cooling is dominated by line emission of atomic oxygen and of CO. Atomic and ionised carbon play a minor role .

Conclusions :