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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:






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.