SID和SERVICE_NAME的疑问

[latahu]

我的tnsnames.ora

ORCL =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.57.125)(PORT = 1521))
)
(CONNECT_DATA =
(SERVICE_NAME = orcl)
)
)
在net configuration assistant里测试，提示无法解析SERVICE_NAME，
改成
ORCL =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.57.125)(PORT = 1521))
)
(CONNECT_DATA =
(SID= orcl)
)
)
测试就通过了。我装的是Oracle9i.
请高手明示，本人才疏，不知道为什么？什么时候用SID，什么时候用SERVICE_NAME呢？