When you transfer data over a slow link ( RAS, RRAS, ...), the Windows NT redirector normally uses core mode, because one of the requirements for the much faster raw mode is not satisfied, namely that the I/O must complete within 5 seconds.
Before the redirector can use raw mode, the following conditions must be met (and commonly are):
The Server Message Block (SMB) protocol negotiated between the client and server must support raw mode.
The server must be configured to support raw mode.
The client redirector must be configured to allow raw mode.
No other SMB requests are pending on the same Virtual Circuit.
The I/O is not on a blocking named pipe.
For a READ request: the requested data size is equal to or greater than 2 times
the currently configured request buffer size, and the read offset is not
past the redirector's currently configured end-of-file.
For a WRITE request: the requested data size is equal to or greater than 1.5 times
the currently configured request buffer size, and the write offset is not more
than approximately 1 MB past the current end-of-file.
The I/O is to take no longer than 5 seconds to complete.
Edit or Add Value name RawIoTimeLimit as a type REG_DWORD. The valid range is 0 - 65535 seconds (using the decimal Radix).
You must stop and restart the redirector for a change to take effect. At a CMD prompt, type:
net stop rdr
net start rdr