Ref: 13930002
Title: How to Use Rotaries on the GS/X.25-XNS
date: 09-27-89

Copyright 3Com Corporation, 1991.  All rights reserved.

This article explains the versatile function of the rotary.  Several
examples demonstrate its capabilities when used in combination with the
other features of the X.25 gateway.

Normally each port can be addressed individually for X.25 calls.  For
outgoing calls, the port number is specified explicitly in the connect
syntax:

C %080002001234!25#123456789

where:  080002001234 is the Ethernet address of the gateway
        25 is the port number
        123456789 is the X.25 address of the remote host

When receiving an X.25 incoming call, the gateway compares the called
address (inside the incoming call indication packet) to the X.25 local
address as configured in the GS/X.25-XNS.  The subaddress digits, if any,
indicate which gateway port to use.

A specific set of ports can be assigned to a rotary.  When the rotary is
specified as subaddress digits, the gateway will use the next available
port number in the set assigned to the rotary.

A port-to-line mapping table allows the network manager to distribute
the ports among the various X.25 lines.


Notes:

a) Ports must be mapped to a line to allow connections to be made on
this line.

b) For outgoing calls, the gateway uses the port-to-line mapping table
to determine on which X.25 line to place the call.  On a rotary, ports are
allocated on a round robin basis.

c) For incoming calls, the gateway uses the lowest numbered available port
assigned to the X.25 line in the port-to-line mapping table.


Examples:

I.  Configuration with one X.25 line

All ports must be mapped to this line.  Rotaries are used to pick up ports
with different configurations and to differentiate between departments for
security or billing purposes.


II.  Configuration with two X.25 lines

If the lines will be used for different purposes, ports must be shared among
the lines.  This case is similar to the configuration with one X.25 line.

If the lines are going to the same destination, follow one of the two
procedures below as appropriate.

1.  If line costs are equivalent--pseudo "load balancing" can be achieved by
the following:

a.  Map all even ports to line 0.

b.  Map all odd ports to line 1.

c.  Create a rotary which contains all ports.

To place an outgoing call, use the rotary previously defined; the first call
will use line 0, the second will use line 1 and so on.

NOTE:  Load balancing works for outgoing calls only since it is the active
end's responsibility to place the call to the appropriate line.  This is
pseudo load balancing since routing is done at call setup.  Even if the call
does not last, the port will be available when the rotary has made a full
round.

2.  If line cost is different--for example, a private line is used to connect
two remote locations and the PDN is used as a backup:

a.  Choose the maximum number of connections that can use the private line.
Additional connections will be routed through the PDN.

b.  Assign this number of ports to the private line, by defining a rotary
which contains all these ports.

c.  Name this rotary "private_line".

d.  Map the remaining ports to the line connected to the PDN; define a second
rotary and name it "pdn_line".

e.  Define a global rotary and name it "x25."  This name will include both
the private-line and pdn_line names as: "NAme x25=private_line,pdn_line."

f.  Use the "x25" name to place the calls.

The calls will be routed through the private line first and when saturation
is reached, will be routed through the PDN.  If the private line is down,
calls will be automatically routed through the PDN.


III.  Configuration with several gateways

This case is an extension of the configuration with two X.25 lines, above,
since the global rotary concept can span across the LAN.


Careful use of the features described above--direct port addressing,
rotary, global rotary and port-to-line mapping--allows the network designer
to build a cost effective, secure, and redundant network when using a X.25
public network.

