Cell Relay Archive

[albert.e.manfredi@boeing.com]

In article <864pkl$bc7$1@nnrp1.deja.com>,
  mredondo@my-deja.com wrote:

> > With LANE, you need a router to go to other Layer 3 networks (e.g.
to
> > reach another IP subnet). Or a bridge to go to other segments of
that
> > same Layer 2 network (e.g. LANE and an Ethernet segment are both in
> the
> > same IP subnet).
>
> What do you mean with Layer 2 network?
> Maybe an Ethernet MAC (LEC client) member of the same ELAN?

No, I mean that your local IP subnet _could_, in principle, consist of
several Ethernet hosts connected together via Ethernet hubs _and_
several LANE hosts using the Ethernet framing option of LANE
(preferably), connected together by an ATM mesh. And the ATM and
Ethernet LANs, which form a single Layer 2 network, would be tied
together with a bridge (or two or three or more).

As you can see, this would be a single Layer 2 network, because two
stations communicating to each other in such a network would never need
to cross a router. The IP net ID is the same for these two stations.
Yet, one might be Ethernet and the other ATM LANE.

> > > How many bytes do you have add (AAL5,LANE,...) to the ethernet
frame
> > to
> > > have a packet ready to be cut in cells of 53 bytes?
> >
> > Don't worry about it. SAR (segmentation and reassembly) worries
about
> > packing cells correctly. It happens in the ATM NIC, all by itself.
>
> Maybe I didn´t explain well. I just wanted to know how many bytes are
> added by LANE and AAL5 to the Ethernet frame (Overhead). Just to know
> the different bandwidth used in the Ethernet and ATM link.

The difference in overhead caused by packet framing, for full-speed
transfers like FTP, will be small. You can count up the bytes if you
want, by downloading the LANE Spec at www.atmforum.com, in the approved
specs folder. The bigger cause for overhead will be the cells
themselves, though, which basically amounts to 48 payload bytes for 53
cell bytes.

BUT, don't forget that you're comparing apples and oranges.

When you say "100 Mb/s Ethernet" you are really saying 125 Mbaud
Ethernet.

When you say 155 Mb/s ATM, that refers to essentially a baud rate, not
bit rate. So a 155 ATM net is actually 136 Mb/s system, if you want the
actual data rate to compare with Ethernet.

Different conventions were used when identifying Ethernet and ATM nets,
so this whole overhead thing got blown out of proportion. In ATM, it's
the actual symbol rate over the physical (SONET) layer.

--
Bert
albert.e.manfredi@boeing.com


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