You will want to check with the Internet access provider and determine
what sort of access is available, and what sorts of speeds and line
connections are supported, and what sorts of protocols are supported.
If the provider supports it, ISDN and T1 circuits are both likely rather
faster than your current connection. Also possible are connections such
as X.25.
The BNC you refer to is likely a Thinwire Ethernet or IEEE 802.3 network
connection, and while Ethernet is faster than a dial-up, ISDN or T1
connection, one would need to acquire an IP, PPP or SLIP-capable network
router or brouter that can connect to both the local Thinwire Ethernet
and to a point-to-point (synchronous or asynchronous) connection to the
Internet access provider. The local end of the point-to-point link
also requires a provider-specified modem and (potentially) DSU/CSU.
To use IP over the local Ethernet/802.3 network, one needs an IP package
such as DEC TCP/IP Services for OpenVMS (UCX). A license for UCX package
is included with recent NAS releases. X.25 likely requires DECnet OSI.