Cell Relay Archive

[albert.e.manfredi@boeing.com]

In article <8dlj3m$68f$1@nntp.itservices.ubc.ca>,
  "Patrick Lam" <patrickl@ece.ubc.ca> wrote:
> Hi all:
>
> I took a project that has to design an ATM switch.  I understand the
> buffering, scheduling and traffic control stuff.  What's bothering me
is how
> the routing table work!
>
> Does the routing table contain only incoming VPI/VCI, outgoing
VPI/VCI and
> output port?

The cell routing table contains this info.

> When a signaling cell comes in with a particular VPI/VCI (or should
we use
> VPCI/VCI in this case?), how does the switch calculate the outgoing
VPI/VCI
> for that cell for the routing table?

The signaling cells are always sent to VPI/VCI 0/5 during setup.

> What is the calculation based on?

In the NNI, during setup, the switches have to determine to which
switch port an incoming setup message has to be routed, based on the
AESA of the destination (contained in the setup message). The correct
port to use is determined by the routing protocol, typically PNNI these
days. This is a protocol much like OSPF.

The difference between ATM and normal packet switching (by "normal" I
mean that I'm not including schemes like MPLS or tunnels) is that in
ATM, this destination address and routing calculation are only
accomplished once, during setup. Thereafter, as long as the SVC remains
alive, each switch just relays cells the same way every time, using the
VPI/VCI table. With packet switched systems, the route determination is
done individually for every packet. And could potentially change from
one packet to the next.

> Would that be based on the ATM address contained in the CPS layer?
>
> As far as I know, after a VC has been setup, all cells containning
the same
> VCI will be switched according to this VC.  So are these cells (with
the
> same VCI) going to be switched to the same VCs (in other words, will
they
> carry the same VCI) along the whole connection (with same VPCI)?

The VPI/VCI label attached to a VC only applies to that local interface
between the ports of two adjacent ATM switches, or between the ATM
switch and the directly-connected end system host. During the setup
process, the VPI/VCI values are assigned independently at each hop, and
are then retained that way for the duration of that SVC. PVCs are much
the same, except the VPI/VCI value can be specified ahead of time. (I
think that SPVCs are like SVCs in this regard, though. You wouldn't
constrain the network to use any particular VPI/VCI values. The network
chooses its own when it sets up the SPVC.)

> Also, how often does the routing table update itself?  Does it have to
> update itself whenever a VC is setup or torn down?

Yup. The VPI/VCI values are reassigned every time an SVC is torn down.

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


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