The X.75 Standard


CCITT - Consulative Committee on International Telegraphy
        and Telephony

DCE   - Data Circuit-Terminating Equipment

DTE   - Data Terminal Equipment


The X.75 Standard was developed by CCITT as a supplement to X.25
It is designed for use between public X.25 networks and is not likely
to be used or even allowed as an interface between public and private
networks.  However, it could also be used to connect a collection of
private X.25 networks in an internet that does not include public
networks

X.25 specifies an interface between host equiptment (DTE) and user
equiptment (DCE) that encompasses layers 1 through 3 and permits
the setup, maintenance, and termination of virtual circuits between
two DTEs.  X.75 specifies signal terminating equipment (STE) that act
as DCE-level gateways to connect two X.25 networks.

The interconnection of X.25 networks via X.75 provides a DTE-DTE
virtual circuit as a connected series of virtual circuits:

* DCE to STE (intranetwork)
* STE to STE (internetwork)
* 0 or more:
** STE to STE (intranetwork)
** STE to STE (internetwork)
* STE to DCE (intranetwork)

Each section is a distinct entity with a separate virtual circuit, and
separate flow control and error control.

From the point of view of the DTE, however, it merely sees an enlarged
X.25 network; X.75 is invisible.  The DTE-DCE interface is still defined
by X.25.  As before, intranetwork protocols are undefined.  The internetwork
STE-STE interface is defined by X.75.

The transmission of a packet between two hosts works like this:
Host A sends an X.25 data packet to its DCE with the virtual circuit number
(group, channel) that it associates with a connection to B.  This packet
is transmitted via network C to an STE.  The STE uses the same information
for the appropriate STE-STE virtual circuit.  The receiving STE then sends the packet to B's DCE, which presents a a packet to B with the virtual
circuit number that B associates with a connection to A.  Three
important points about this process:

1. There is no encapsulation by the STEs. The same layer 3 header format
is reused

2. There is no end-to-end protocol.  As in a single X.25 network, all
information has local significance only

3. Because of the 12-bit field, an STE-STE internet link can handle
a maximum of 4096 connections

Call Request and Clear Request are handled step by step but must
propagate end to end. Routing information must exist within DCEs and 
STEs to accomplish this.  For example, a CALL REQUEST packet from A
triggers the set up of a DCE-STE virtual circuit.  Using the X.75
control packet format, wich differs from X.25 only in the addition
of a network-level utilities field, an STE-STE virtual circuit is set up 
between networks C and D.  The CALL REQUEST packet then propagate to B's
DCE, setting up another virtual circuit.  Finally, a CALL
INDICATION packet is delievered to B.  The same procedure is used for 
CALL ACCEPTED and CLEAR REQUEST packets.