For GPON: This attribute indicates service ability of ONU type,
there are three service mode :N:1,1:M,1:P.
If support the service mode,set the corresponding bit to 1,
if not support, set to 0.   
Default value is 7.               
     
| MSB                                  LSB| 
|-----|-----------|-----------|-----------|
| ... |    1:P    |    1:M    |    N:1    |
| ... | (1bits)   | (1bits)   | (1bits)   |
+-----+-----------+-----------+-----------+
This attribute is not used for EPON.
This variable must comply with the multi-variables binding 
rule described in 'zxAnPonOnuTypeEntry'.