Browse Prior Art Database

Elimination of Dialog-2 State from a Dynamic Switch

IP.com Disclosure Number: IPCOM000115031D
Original Publication Date: 1995-Mar-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 2 page(s) / 78K

Publishing Venue

IBM

Related People

Franaszek, PA: AUTHOR [+2]

Abstract

When two units which are interconnected by a dynamic circuit switch, such as the IBM* ESCON* Director, send connection requests to each other at the same time, the IBM ESCON I/O Interface architecture requires that the switch make the connection and the interconnected units determine, between themselves, which of the conflicting requests be honored. In order to make the connection, the switch recognizes that two units are attempting to connect to each other and causes the connection to be created in a special state, called dialog-2, which records the fact that two connection requests were received. To break a dialog-2 connection, the switch must receive two disconnection requests. This ensures that one of the units does not break the connection while the other unit is still processing the request it received.

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This is the abbreviated version, containing approximately 52% of the total text.

Elimination of Dialog-2 State from a Dynamic Switch

      When two units which are interconnected by a dynamic circuit
switch, such as the IBM* ESCON* Director, send connection requests to
each other at the same time, the IBM ESCON I/O Interface architecture
requires that the switch make the connection and the interconnected
units determine, between themselves, which of the conflicting
requests be honored.  In order to make the connection, the switch
recognizes that two units are attempting to connect to each other and
causes the connection to be created in a special state, called
dialog-2, which records the fact that two connection requests were
received.  To break a dialog-2 connection, the switch must receive
two disconnection requests.  This ensures that one of the units does
not break the connection while the other unit is still processing the
request it received.

      The dialog-2 state adds complexity to the switch.  It is
possible to eliminate the dialog-2 state by enabling the switch to
determine which request to honor.

      Following is a summary of the conflicting-request rules of the
ESCON I/O Interface.  In these rules, the term "accedes" means that
the unit which accedes accepts the other unit's request.  Assume that
unit A and unit B sent each other requests and unit B accedes.  Unit
B processes unit A's request and must later re-send its own request.
Unit A sends a response to unit B's request which breaks the pending
switch connection for unit B's request (the switch connection reverts
from the dialog-2 state to a normal connection initiated by unit A).
  1.  If simultaneously a channel and a control unit send each other
       device-level initiation frames, the control unit accedes.
  2.  If simultaneously one unit sends a link-control frame and the
       other sends a device frame, the sender of the device frame
       accedes.
  3.  If simultaneously two units send link-control frames to each
       other, the unit with the lower link address accedes.

      To enable the switch to enforce the rules, a priority indicator
in the link header of the connection request frame is introduced.
When two requests conflict, the switch gives priority the request
with the higher-valued p...