Re: [Autoconf] aspects of multi-hop wireless communication

Thanks for your feedback Alex,

On Fri, Dec 19, 2008 at 1:07 PM, Alexandru Petrescu <
alexandru.petrescu@gmail.com> wrote:

> Thanks for the description.  I think it's good to lay out our
> understandings of terminology to clarify and eventually converge.
>
> It's a short comprehensive draft covering a few essential points recently
> discussed on the list about particularities of wireless communications.
>
>  In this document, we consider a multi-hop ad hoc wireless network to
>>  be a collection of devices that all have radio transceivers using
>> the same physical and medium access protocols.  All are configured to
>>  provide store-and-forward functionality on top of these protocols,
>> as needed to enable communications; consequently, they can be classified
>> as routers in the resulting wireless network.
>>
>
> It would be good to clarify that the "top" mentioned above is actually
> the networking IP layer, decrementing TTL/HopLimit.  Otherwise one may
> read the "routers" mentioned above as "MAC bridges".
>
> Or you didn't mean that "top" to be the IP layer?
>

I guess that in the context of the IETF, yes. We should spell this out in
-01.

>
> I continue assuming you meant the IP layer.
>
>  First, there is no guarantee that a router C within S can, symmetrically,
>> send IP packets directly to router A. In other words, even though C can
>> "hear" packets from node A (since it is a member of
>>  set S), there is no guarantee that A can "hear" packets from node C.
>>  Thus, multi-hop ad hoc wireless communications may be "asymmetric". Such
>> asymmetry is often experienced on multi-hop ad hoc wireless networks, due to
>> well-known properties of wireless communication.
>>
>
> Sorry, could one mention why?  What's the example of this?  I haven't
> ever experienced this problem with wifi.  I may have noticed a slight
> difference in bandwidth upstream vs downstream but not a complete cut of
> communication in one direction while the other direction was ok.
>

I have nothing to add to what Chris replied to that ;)

>
> What are the well-known properties of wireless communication making this
> asymmetric behaviour (communication works in one direction but not in
> the other).
>
>  Second, there is no guarantee that two given routers within S can directly
>> communicate with one another.  In other words, even though two routers R1
>> and R2 can both "hear" packets from router A, there is
>>  no guarantee that R1 can hear packets from R2, and there is likewise
>>  no guarantee that R2 can hear packets from R1.  Thus, multi-hop ad hoc
>> wireless communications may be "non-transitive".
>>
>
> It is of paramount important for me to understand what is meant by
> "hearing".  Is it MAC level or IP level.
>
> It is very possible for R1 to not receive from R2 (although the
> intermediary A receives from both) and this does not represent a problem
> at all, even less a particular problem of wireless communications.  When
> R1 doesn't hear from R2 it's because it's too "far" from it; the
> solution could be A to bridge R1 to R2.  It is the same problem in wired
> communication.

 Actually this is out of the question because, as stated in the previous
paragraph in the draft, there is no guarantee that A can hear either R1, or
R2... (all we know is that R1 and R2 can hear A).

>
>  Lastly, there is no guarantee that, as a set, S is at all stable. The
>> membership of set S may in fact change at any rate, any time.
>>
>
> One would however differentiate this dynamic nature from a completely
> disconnected on-off behaviour.  One would set the limits of
> connectivity.  If we don't have limits for connectivity then we can't
> even talk PHY (let alone MAC, Networking, Transport and Application).
>
> If it's needed, I could help contribute text defining what movement may
> mean in terms of fixed points, subnets, TTL/HoLimit and of IP address
> change.  I could also contribute text about radio communication being
> the same through the air as through copper or fiber, as seen from the
> MAC layer.
>

Thanks. But I think this is not in scope for this document. The goal of this
document is simply to describe what was experienced over the years on
multi-hop wireless networks in terms of communication characteristics. Such
"conclusions", based on the characteristics described in this draft, should
go in a separate document, in my mind.

Emmanuel