On Sat, 15 May 2010 13:02:58 -0700, Darren New wrote:

> Jim Henderson wrote:
>>> If you have one cable coming into the university campus, and a network
>>> for each building, the router is going to have to send the packets to
>>> each building, duplicating the packets, regardless of how "aware"
>>> anyone is.
>> 
>> Yes, but that's not a 1:1 transmission (compared to the receiving
>> clients).
> 
> I don't know what you mean.

It's not one sender to one receiver.  It's one sender to many receivers.

> For one thing, modern ethernet (the kind that goes over CAT5 instead of
> coax) is indeed 1:1 on every part of the network. Nothing I send from my
> computer to yours does not go through the router.

Not necessarily true if it's a local subnet.  But even if it is across 
routers, so what?  The router is just setting up a multicast domain for 
the downstream network, which may include another router that does the 
same thing.

> You need to duplicate the packets at least once per physical wire. Same
> as any network. IP doesn't force you to duplicate anything less.

True.

> Granted, if you're broadcasting from one point, the broadcaster only has
> to send one packet per physical connection regardless of the number of
> recipients. But that's the same with ISDN and all those technologies
> too. You call into a conference bridge and the bridge duplicates the
> data as necessary. You're not sending out the same data once for each
> receiver there either.

That's also the point; the amount of traffic sent by the sender is 
significantly reduced; the switches also typically have logic so they 
don't have to fill their buffers with multiple copies of the same data 
(for those that have a buffer).

> Firstly, if your subnet isn't broadcast, it doesn't save you any
> bandwidth. The router still needs to duplicate all the packets. If your
> subnet *does* support broadcast (like coax ethernet, or alohanet, or
> something like that) then sure, you can broadcast to everyone, but
> that's because *any* transmission goes to everyone, taking up your
> bandwidth even if it's not addressed to you.

I think there may be a conflation of "broadcast" and "multicast" here.

> Broadcast networks tend to be very small, and they tend to not allow two
> conversations at once. You're not saving bandwidth by broadcasting.
> You're just using it more efficiently. A targeted communication on a
> broadcast network still takes up everyone's bandwidth. They just ignore
> it. Multicast on a broadcast network isn't saving bandwidth as much as
> it's not wasting bandwidth.

Small relative to the size of the Internet, sure.  Small relative to the 
size of a given network, maybe not so much.

That last sentence is just semantics, though.

> Wireless is about the only common broadcast IP-based network still
> around, tho, exactly because it's so bandwidth-inefficient and
> impossible to squelch when something goes wrong.

I assume you're talking about 802.11 networks; some wireless networking 
technologies implement CSMA-CA while others do CSMA-CD; the latter is 
much more efficient but requires hardware that is capable of listening; 
in a spread-spectrum implementation (which most 802.11 networks are), 
CSMA-CA is more common but the DSS implementations mean that there's much 
less chance of a collision (that's part of the point of that type of 
implementation).

Jim

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