To be hybrid or not to be

[Emmanuel Duros <Emmanuel.Duros@sophia.inria.fr>]

From: Harri Hakulinen NRC/Tre <Harri.Hakulinen@research.nokia.com>

> Emmanuel Duros wrote:
> > The sat dish of the VSAT can be used for transmission/reception.
> > Why wouldn't we plug a ROI in a feed ?
> > 
> >    PC
> >     +                                                              1.8 m
> >    SOI ------------> Modulator ------------> Amplifier ---------> sat dish
> >     +                                                                |
> >    ROI <-------------------------------------------------------------/
> > 
> > A driver could support both SOI and ROI, and we then get a bidirectional
> > communication. Feeds can communicate with each other without the need of
> > tunnels or AFM group.
> 
> Somehow it seems, that this doesn't have much to do with UDLR
> (I recall that U stands for Unidirectional ?).

Right but couldn't we have some hybrid systems that would interoperate
with 'pure' Unidirectional systems ?

I attach a mail sent by Steve Deering a while ago about this issue.

Maybe we could include a mechanism in DTCP (or whatever tunneling
establishment protocol) that would also take care of hybrid
systems. Just an idea...

Emmanuel

------
From: Steve Deering <deering@cisco.com>
Subject: Re: Purpose of terminology I-D
Cc: Scott Michel <scottm@rush.aero.org>,
        udlr@sophia.inria.fr (Unidirectional Link Routing Group),
        dabbous@milou.inria.fr

At 3:22 AM -0700 7/25/97, Walid Dabbous wrote:
> do you think that two way nodes attached to a non-trans
> link require specific treatment? or is it just for
> complete terminology purpose/

There exist satellite communication systems in which every groundstation
has both send and receive capabilities, and which therefore act like a
normal multi-access, bidirectional links.  There also exist satellite
communication systems in which there are one or more send-only ground
stations and many receive-only ground stations, e.g., DBS TV systems.
I imagine that there are, or will be, hybrid systems, in which there
are multiple groundstations capable of both send and receive, plus
other groundstations that are receive-only (and perhaps also some send-
only stations).  For communication between those nodes with send-and-rcv
capability, the satellite channel is bidirectional, whereas for
communication between other pairs of nodes, it is unidirectional.

For designing IP routing schemes for satellite networks, I thought it
might be helpful to keep the more general non-transitive case in mind.
In particular, a subset of nodes with both send-and-rcv capability can
use the satellite channel to exchange routing information, rather than
resorting to a terrestrial backchannel, and there might be good reasons
to exploit that capability.

Alternatively, one could split such a hybrid network into two virtual
networks, one unidirectional and one bidirectional, and ignore the more
general non-transitive case.  But I suggest that it would be worth at
least thinking about the pros and cons of those two different approaches.

Steve