switching versus routing ATM redux
William Allen Simpson
wsimpson at greendragon.com
Sat Sep 13 03:21:40 UTC 1997
Wading thru the flood in the past two days, I was wondering why Carmen
Hillenburg <carmenh at onecall.net> is answering Richard Irving
<rirving at onecall.net> email?
And why, with all the terminology being flung around, they don't realize
that the term "Broadband ISDN" came years before "ATM"? The former is
Engineering Speak, the latter Marketing Speak.
FYI, they are confused. BRI ISDN is "baseband", not "narrowband".
10-base-2, 100-base-T, and gigabit ethernet are also "baseband".
Cable TV channels (6 Mbps) are "broadband". Old 2400 bps satellite
links are also "broadband".
And why cannot either of them tell the difference between measured RTT
of a ping and measured RTT using a real IP hop measurement tool, such as
is available from LBL?
And why there is any argument about forwarding latency, since by
definition the last bit of the last ATM cell is not going to arrive any
sooner than the last bit of a packet containing any IP datagram?
Measuring the first bit arrival of cut-thru-switching is a waste of
time. Processing of the IP datagram at the destination will not occur
any sooner than the last bit.... And we all know that the task switch
in the source and destination dominates the end-to-end timing
(especially of non-fast-path ICMP messages like ping).
A good ATM switch will buffer the entire packet anyway, to avoid wasting
downstream bandwidth when one of the cells is dropped. No different
than a router.
By definition, there is much more link overhead (at least 2 ATM headers)
and forwarding overhead (at least 2 ATM cell switches per SDH hop) for
an IP/ATM/SDH datagram than an IP/PPP/SDH datagram (1 small header and 1
packet switch per SDH hop).
Thus, ATM takes more bandwidth (is slower) and more cell switches (is
slower), cumulatively at every hop. Slower is slower is slower....
And, on another thread, I was pretty sure that NetStar cum Ascend had a
622 Mbps OC-12c (STM-4) IP/PPP/SDH card long before Cisco. They were
engineering it nearly 2 years ago. Did they not sell any?
At the time, both Cisco and NetStar had very talented engineers on the
OC-3 and OC-12 projects (and I'm not easily impressed), but the Cisco
folks were hampered by the speed of their backplane and buffering on
their card. NetStar had the backplane bandwidth and plenty of
buffering, but had routing software instability. Reports on this list
were that they were fixed.
WSimpson at UMich.edu
Key fingerprint = 17 40 5E 67 15 6F 31 26 DD 0D B9 9B 6A 15 2C 32
BSimpson at MorningStar.com
Key fingerprint = 2E 07 23 03 C5 62 70 D3 59 B1 4F 5E 1D C2 C1 A2
More information about the NANOG