IBM 3705 Communications Controller - Maximum Communication Line Connection Estimates

Maximum Communication Line Connection Estimates

With modem supplied clocking, a single line was limited to 56 kbit/s; with internal clocking the limit was 2400 bit/s.

Aggregate line connection ability was limited by processing speed.

For a Type II Communications Scanner, processing of a received or transmitted character might take fifty storage cycles (forty instructions). In a 3705-II this gives an aggregate capacity of 20,000 byte/s. This would allow about 160 half-duplex lines running at a mean speed of 120 cps. Alternatively sixteen half-duplex lines running at 1200 cps (9600 bit/s) would be the theoretical limit. IPSANET experience was that six full-duplex 9600 bit/s lines carrying a heavy load was the limit. IBM software may have had superior performance.

For a Type III Communications Scanner cycle steal processing of a single character was fairly inexpensive—a single storage cycle was required. This would give a theoretical limit of a million cps (eighty 9600 bit/s half-duplex lines). In practice the limit was probably lower as some processing would be required for end of frame. Also if the frame contents were moved about in storage this would require 3.5 storage cycles per byte.