[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

RE: real-time digital protocols



Hello Maggie

> Point well taken

Yes I was slightly concerned that I might be seen as being pedantic on that
point.

> Multicast *is* IP, isn't it? I mean, it's the IP archetecture for
addressing a
> packet to more than one destination address, if I understand it properly.
Or is
> it further down the stack than that?

Multicasting is an IP term, and has been bolted onto IP in a way which could
be considered an afterthought. The physical way it works depends on how
smart your network is. If you use only layer 2 switches or hubs, it's merely
broadcast traffic.

If you use either these smart layer three switches, or routers, the traffic
can be managed slightly more sensibly - but not by much.

Add to that the fact that older OS's, NIC's and drivers are unable to run
multicast on the card and so have to be placed into promiscuous mode by the
OS - essentially you're still dealing with broadcast traffic at the host.

When Joe Ham sees multicast coming through his radio it's really a broadcast
anyway - in every sense except in nomenclature. The only reason to make it
multicast rather than a broadcast is to enable the packets to legitimately
leave the local amprnet subnet. There's the difference. Whereas broadcasts
generally don't leave the local subnet, multicasts can if they're so routed.

So, let's get that 44.193.0.0/16 subnet activated with some 224.0.0.0/8
addresses!

The new VoIP configurations are very smart routers which understand even
further up the protocol stack even into the H.323 sub layers. Big problem
for routing H.323 is that both dynamnically allocated port numbers and IP
addresses appear in the payload, and there's QoS (Quality of Service)
datagrams, as well as some appallingly slow conversation controls floating
about too. Here's where the dividing line between a gateway and a router
becomes rather jaded.

BTW 1: H.323 is really a collection of several underlying protocols and
formats - call setup, control channel protocol, specific audio & video
codecs, security, gateways etc etc - some of these are lots of reasons _not_
to use H.323. It's also not only IP - although IP is the most popular
transport. There's also other less established VoIP protocols than H.323
such as SIP. H.323 is an ITU standard, SIP is IETF.

BTW 2: the best book on VoIP IMHO is IP Telephony by Hersent, Gurle & Petit
published by Addison Wesley. Covers pretty much all you need to know.

73 Howard G6LVB

----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home