Cell Relay Archive

[fahad@cs.pitt.edu (Fahad A Hoymany)]

JoongSub Lee (kornet) (audience@soback.kornet.nm.kr) wrote:
: Hi,

:  I know this qestion is so silly but I can't proceed to learn
: ATM because of this question.
:  
: With my short knowledge it is impossible to cover WAN network
: with VPI/VCI. If A want to communicate with B, how routing
: is made? In TCP/IP the destionation address never change so they
: can match it eventually but ATM change when transit switches.
: I understand communication is set-up after routing decison making
: via VPI/VCI. But I don't know how routing decision is made when
: virtual circuit is set up.
: Thanks in advance.

 Lee,

    The following is an example I made some time ago... it should answer
your question or part of it.

  Lets say we have user A wishing to connect to user B across an ATM
network:

                  Switch 1           Switch 2
                +--------+         +--------+
                |        |         |        |
   A ___________|        |_________|        |_____________ B
                |        |         |        |
                +--------+         +--------+


  'A' sends a setup message without any VPI/VCI values as we all agreed on,
whether ATM Forum or ITU. Switch 1 selects a VPI = 45, VCI = 27 (as an
example) and sends it back to A in its CALL-PROCEEDING message.  So, we have
this picture:

                  Switch 1               Switch 2
                +--------------+         +--------+
                |  in     out  |         |        |
   A ___________|45 27 a       |_________|        |_____________ B
               a|              |b       a|        |b
                +--------------+         +--------+

  Note that switch 1 does not have a value for the out direction yet. 'a'
and 'b' are the port numbers. Switch 2 then selects a value (37,95) and lets
assume that the NNI protocol requires switch 2 to send a CALL-PROCEEDING
message back to A. Then A updates its table as follows:

                  Switch 1               Switch 2
                +----------------+         +--------------+
                |  in     out    |         |  in      out |
   A ___________|45 27 a 37 95 b |_________|37 95 a       |_____________ B
               a|                |b       a|              |b
                +----------------+         +--------------+


  What this so far says is that any cells coming from A marked 45,27 should
go out on port b as 37,95. The trouble arose on whether switch 2 selects
the remaining entry or user B selects it. The UNI 3.1's method as I learned
today selects some values and sends them to B.  If B cannot accept them the
connection fails (see section 5.5.2.3). The method I suppose suggested by
ITU lets B select its own numbers and return them in its CALL-PROCEEDING
or CONNECT message.  In this case, we would have something like:

                  Switch 1               Switch 2
                +----------------+         +----------------+
                |  in     out    |         |  in      out   |
   A ___________|45 27 a 37 95 b |_________|37 95 a 87 57 b |_____________ B
               a|                |b       a|                |b       87,57
                +----------------+         +----------------+


  So, now any cells out of 'A' marked 45,27 go out of switch 1 as 37,95 on
port 'b'. Cells coming to switch 2 with 37,95 go out as 87,57 on port 'b'.
(Of course I am simplifying the discussion here by showing only one way
of the communication and I am not worried about the exact nature of the
call establishment protocol)

Fahad