ctabka@sri-tsc (08/22/85)
From: David Wolfe <ctabka@sri-tsc>
I currently work at the integration contractor for the DoD
experimental packet radio network and I thought that I might be
able to extend some of the points that Phil Karn mentioned.
Firstly, the IP protocol is used because it provides a bare
bones capability that allows dissimilar networks to intercommunicate
with low overhead. If virtual circuit capability is desired, it
can be added by implementing an appropriate Host-Host protocol on
top of it (TCP is an example) in a modular way. Other protocols
that provide different degrees of service run on top of IP.
Amateur radio would do well to consider IP for any contemplated
national networks for several reasons...
1.) The design has been done
2.) It can run on small, simple gateways (i.e. it`s cheap)
3.) It works (not the least consideration)
4.) It allows the decisions about higher level services to
be delayed until the patterns of network use stabilize.
5.) The ham networks will be able to internet with a wide
variety of other computer networks.
6.) And as Phil mentioned, datagrams are reasonable for
adapting around disasters, topology changes and system
crashes.
Approaches such as using X.75 or other internetwork schemes based on
virtual circuit models should be reconsidered because...
1.) If one guy in the tandem chain crashes, all current calls
going through the switch get lost.
2.) The phone company solves problem 1. by using highly (some
of the most highly reliable ever) reliable switches. This
implies the use of fault tolerant machines for the
communication subnet routing function, an approach that is
useful for large corporations but not hams trying to build
a network on the cheap. Also it is worth remembering that
the phone company is partially locked into this way of doing
things because it is a natural extension of the preexisting
telephone technology (an important consideration with the
size of the existing phone plant).
3.) The virtual circuit way can always be built on top of datagrams
anyway if that`s what people want.
For these reasons, I find the talk about basing Ham networks on X.25/X.75
to be very depressing.
There are currently several DoD experimental networks running around
the country. There is also some military PR work being done over in
the UK. The technology seems to work fairly well,but it is difficult
to tell when it will be deployed operationally. There has been a
packet radio testbed in the Bay area since about 1974.
The commercial situation is almost totally undeveloped and may be
retarded by the advent of Cellular telephone. Probably the most
interesting idea for commercial packet radio is to use it to distribute
computing services in third world countries where they have not been
building telephone services for 100 years. Reliable PR nets can be
set up in the wilds without depending on End or Mobile Telephone Offices.
Finally, one of the reason that spread spectrum and packet radio
go so well together is that the use of SS increases the capture properties
for the packets (similar to FM capture familiar to hams) in the face
of multipath. This is worth keeping in mind as SS seems about to become
legal on some bands. The challenge is to build a low cost approach to
SS as the radios that we use (about the size of an IC290, cost about 12K)
are far too expensive. This advantage pertains to civilian nets as well.
There are some additional advantages that arise out of SS that briefly are.
1.) Using different spreading codes to that different nets
can operate on the same band.
2.) Low probability of intercept for snoopers
3.) If the spreading codes are combined with cryptography, you
get increased communications security.
4.) If number 2. is turned around, communications that is more
jam resistant (The 2 meter repeater guys would love it!)
5.) It provides the basis for precision range measurements.
(The preceeding does not represent the policies of SRI International,
DARPA, or in fact, anyone else but the author)
Dave Wolfe WA4YSS