Differences between SNA and TCP/IP print processing

TCP/IP-attached printers accessed by the Network Print Facility do not behave exactly like their SNA-network counterparts.

The SNA-network user receives verification that the data has been printed:
- In SNA, a positive response from a printer means that the data has actually been printed successfully.
- With the Network Print Facility, a positive response means that the data has been received successfully by the Network Print Facility VTAM® capture point application and queued successfully in a QSAM data set.
If encryption is being used, the Network Print Facility only protects the data's security across the SNA LU-LU session:
- With a real SNA-network printer, the encryption protects the data all the way to the final destination.
- With the Network Print Facility, the data is decrypted as it arrives in the Network Print Facility application's VTAM host; it is unprotected across the TCP/IP portion of the network.
If compression is being used, VTAM-supported data compression is available only between the sender and Network Print Facility's VTAM host.
- With a real SNA-network printer, the compression is available all the way to the final destination.
- With the Network Print Facility, the compressed data is decompressed after it arrives in the Network Print Facility application's VTAM host, but before it arrives at the application itself, not across the TCP/IP portion of the network.
In an SNA network, being in session with a printer gives the application exclusive control of that printer, except for possible interference from local copy operations. The printer's normal session limit of 1 prevents any other application's print output from being interleaved with the owning application's output.
In a TCP/IP network, such exclusive control of the printer is not generally possible because a session limit of 1 applies only to the SNA portion of the flow, and it will not stop other users in the TCP/IP network from sending print jobs to the real printer that the logical printer represents.