Network Working Group                             L. Peter Deutsch  (PARC-MAXC)
Request for Comments: 567			              September 6, 1973
NIC #18970
 
 
 
            CROSS-COUNTRY NETWORK BANDWIDTH
 
 
 
The following computation of cross-country network bandwidth was
contributed by Butler Lampson of PARC.
 
Consider what happens when a TIP user on the West Coast, connected to a
full-duplex Host on the East Coast, strikes a key on his terminal.
 
The TIP sends a one-character message (1 packet).
 
The destination IMP sends a RFNM (1 packet).
 
The destination Host sends an ALLocate - this seems to be the strategy
used by TENEX Hosts, at least (1 packet).
 
Thc TIP sends a RFNM for the ALLocate (1 packet).
 
The same sequence repeats itself, with roles interchanged, for the echo
character (4 packets).
 
This constitutes 4 packets or 4OOO bits in each direction. The current
cross-country transmission capability of the ARPANET is 3 5OKb phone
lines; ergo, it can only support 3*50000/4000=37.5 such characters per
second!
 
It may be that RFNMs are transmitted between IMPs more efficiently; at
best this can only double the network capacity.
 
This computation may help explain why cross-country TIP users (e.g. the
substantial West Coast community of BBN-TENEX users) experience such
bad echo response, at least in bursts: the network itself may be
experiencing momentary peak loads.
 
If this argument is correct, the proposed remote echoing facilities of
the new TELNET protocol could have a major effect on network operation.