Cell Relay Archive

[albert.e.manfredi@boeing.com]

In article <86zp1h52vz.fsf@ironbridgenetworks.com>,
  James Carlson <carlson@ironbridgenetworks.com> wrote:
> "O.B. Kwon" <obkwon@netcom.co.kr> writes:
> > What's POS (Packet over SONET) ?
>
> Also known as PPP over SONET/SDH.  It's a way of transporting
> datagrams over SONET/SDH channels.  See:
>
> 	ftp://ftp.isi.edu/in-notes/rfc2615.txt
>
> > What's the different between POS and ATM ?
>
> POS uses variable-length datagrams as the basic unit of transfer.  ATM
> uses fixed-size cells.  The rest, as they say, is marketing.
>
> > Which technology is more advantage under multimedia environment ?
>
> ... and that's how you start a pointless flame war.

I agree with your last phrase, but not that the only difference beyond
packet size is marketing. The differences are fundamental:

1. Packets over SONET require the addition of layer 3 functionality in
order for the system to achieve global reach. Packets over SONET is
functionally identical to packets over T1, packets over T3, etc. SONET
is a convenient pipe. You still need a network of routers along with
all the trappings of the Internet to make such a  system useful.

2. ATM over SONET, without any additional layer 3 protocol, is a system
with global reach. It can be used as is to carry voice, video, or data
(e.g. data can be sent using ATM's own error-free protocol, SSCOP). In
other words, ATM over SONET was originally meant as an enhancement
beyond ISDN. So it doesn't depend on IP or any other network layer
protocol. Not by _any_ means to say this results in anything simpler
than an Internet-type arrangement. But it is obviously a completely
different beast.

If the only thing you want to do over the SONET is carry data frames,
then POS is clearly a useful scheme. But if you want to use the SONET
infrastructure for non-data services, ATM can give you that.

It's also true that if all of the ongoing efforts in voice over IP, QoS
over IP, etc. etc. pan out as is hoped, then one might question the
need for having that ATM layer over SONET. After all, if IP can carry
all of the data, voice, and video over a global network, no other
scheme would be required.

--
Bert
manfredi@arl.bna.boeing.com


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