ORA-27101: shared memory realm does not exist

I created a 10g database manually on the CentOS4.7 virtual machine. I have no problem to connect as SYS or SYSTEM on the server,
however, when I configured the listner and tried to connect remotely or on the server with the connection string, I got ORA-27101 error.
i.e.

vmlinux1:/app/oracle/product/10.2.0/db_1/network/admin [dbt10g] $ sqlplus system@dbt10g 

SQL*Plus: Release 10.2.0.1.0 - Production on Tue Jan 27 22:04:34 2009

Copyright (c) 1982, 2005, Oracle.  All rights reserved.

Enter password:
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux Error: 2: No such file or directory

It took me a while to figure out the reason. My ORACLE_SID is in upper case, however I used lower case sid name in the listener.ora, i.e.

$ cat listener.ora      
LISTENER =
  (ADDRESS_LIST=
        (ADDRESS=(PROTOCOL=tcp)(HOST=192.168.233.128)(PORT=1521))
        (ADDRESS=(PROTOCOL=ipc)(KEY=PNPKEY)))

SID_LIST_LISTENER=
   (SID_LIST=
        (SID_DESC=
         (GLOBAL_DBNAME=dbt10g)
          (SID_NAME=dbt10g)
        )

   )

Atfer changing them to upper case, I fixed the problem.