Cell Relay Archive

["M.S.Srikrishna" <vagdevi@bellsouth.net>]

Eike Kock wrote:
> 
> Martin Winter wrote:
> >
> > Eike Kock wrote:
> > >
> > > At which state of the connection setup process are the routing tables of
> > > the switches set up and how?
> > >
> > > Is the DTL IE the Transit Network Selection IE or is it a different
> > > information element?
> > >
> > > Thanx in advance,
> > > Eike
> >
> > For a detailled description of this take a look at the ATM Forum PNNI
> > spec at
> >
> > ftp://ftp.atmforum.com/pub/approved-specs/af-pnni-0055.000.pdf
> >
> > To give a preliminary, short and for sure not complete answer: PNNI
> > consists of two protocol parts: a PNNI routing protocol and a PNNI
> > signaling protocol.
> >
> > The routing protocol operates over a reserved VC (0/18). It is a dynamic
> > routing protocol. Informations about nodes and links are flooded within
> > the peer groups and enable the switches to know the topology of the
> > network and to find routes through the network. The routing protocol is
> > only responsible for this information exchange and to maintain the
> > necessary databases in the node, not for call setup.
> >
> > That's the signaling protocols task. It receives SETUP messages from end
> > systems, asks the routing database for a proper route to the
> > destination, builds a DTL containing this route and forwards the SETUP
> > message to the next node, which forwards the SETUP further along the
> > path given in the DTL.
> >
> > Signaling messages are transfered over another VC (0/5). A permanent
> > SAAL connection is maintained over this VC to assure the delivery of the
> > signaling messages.
> >
> Thanx for your answer. With "routing tables" I ment the forwarding
> tables in switches where VPI, VCI and incoming port of a cell is
> determined, looked up in the table and where the VPI (and VCI) is
> overwritten and the cell is put on some outgoing port. When are they set
> up when a switched virtual connection is set up?

When a signalling SETUP message is received by the switch, there are two 
aspects of call processing that happens in the switch. One is the call 
control and the other is the connnection control. Note that this happens 
for each half of the call i.e. the originating call half which received 
the SETUP and the terminating call half which (receives an internal 
setup indication) is going to send the SETUP.

The call control processing takes care of the signalling aspect of the 
call i.e. it represents the CR state machine. The connection control 
processing takes care of the connection aspect of the call i.e. 
selecting a VPCI and allocating bandwidth resources (i.e. go through 
CAC) for each half of the call including inside the switching fabric. It 
is at this point, a self routing TAG has been generated for both the 
originating port and terminating port. However this TAG is not sent to 
the line card (hosting this port) until a CONNECT message is received 
from the terminating user and a CONNECT has been sent to the originating 
user. In fact the self routing TAG is sent to the terminating port if an 
ALERTing message is received prior to a CONNECT so that a backward 
cut-through connection from the terminating to the originating is done 
but not from the originating to the terminating.

After the TAG has been sent to the respective line cards, the user data 
cells for this VPCI (VPI/VCI) are self routed via the hardware using the 
TAG information, to the other port. Possibly, the incoming VPI and VCI 
in the data cell is used to access this TAG information from a table in 
the line card to and put it on top of every data cell which is then used 
by the hardware to route it to the other port. As the cell progresses 
through the hardware, the various fields of the TAG data is removed so 
eventually at the time the cell is ready to depart from the terminating 
port, it is back to a 53 byte cell.

Depending upon whether it is a point to point VCC or point to multipoint 
VCC, the outgoing VPI/VCI ma actually be determined at the incoming port 
and overwritten in the VPI/VCI field of the user data cell header so 
that nothing is required to be done athe outgoing port. This mos 
probably happens for a point to point SVC. However for a point to 
multipoint VCC, depending upon where the data cell is replicated, it may 
have to be written at the outgoing port. Frankly, I do not see any other 
way for the point to multipoint case. In fact in order to reduce the 
traffic through the swiych, the P-MP cell should be replicate as close 
as possible to the terminating port (leaves) I think.

I apologize for this long message, but I sincerely hope it is relevant 
to your question.

You can send me email if you like..

Thanks...