Hi Carl, X.21 is really the protocol, with X.24 covering the cabling.
http://www.itu.int/rec/T-REC-X.21/en
http://www.itu.int/rec/T-REC-X.24/en
X.24 is really helpful and says that cable length is limited by the spec of the interchange circuits.
So, to the BT site as a best fuess to who the provider may be - there is no technical info I can find for Megastream, only kilostream (includes Kilostream N), where a max length of 10M is specified.
http://www2.bt.com/static/i/media/pdf/kilostream_network_terminating.pdf
Paul.
