70 lines
3.1 KiB
Plaintext
70 lines
3.1 KiB
Plaintext
The X.75 Standard
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CCITT - Consulative Committee on International Telegraphy
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and Telephony
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DCE - Data Circuit-Terminating Equipment
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DTE - Data Terminal Equipment
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The X.75 Standard was developed by CCITT as a supplement to X.25
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It is designed for use between public X.25 networks and is not likely
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to be used or even allowed as an interface between public and private
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networks. However, it could also be used to connect a collection of
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private X.25 networks in an internet that does not include public
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networks
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X.25 specifies an interface between host equiptment (DTE) and user
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equiptment (DCE) that encompasses layers 1 through 3 and permits
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the setup, maintenance, and termination of virtual circuits between
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two DTEs. X.75 specifies signal terminating equipment (STE) that act
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as DCE-level gateways to connect two X.25 networks.
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The interconnection of X.25 networks via X.75 provides a DTE-DTE
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virtual circuit as a connected series of virtual circuits:
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* DCE to STE (intranetwork)
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* STE to STE (internetwork)
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* 0 or more:
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** STE to STE (intranetwork)
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** STE to STE (internetwork)
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* STE to DCE (intranetwork)
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Each section is a distinct entity with a separate virtual circuit, and
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separate flow control and error control.
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From the point of view of the DTE, however, it merely sees an enlarged
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X.25 network; X.75 is invisible. The DTE-DCE interface is still defined
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by X.25. As before, intranetwork protocols are undefined. The internetwork
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STE-STE interface is defined by X.75.
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The transmission of a packet between two hosts works like this:
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Host A sends an X.25 data packet to its DCE with the virtual circuit number
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(group, channel) that it associates with a connection to B. This packet
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is transmitted via network C to an STE. The STE uses the same information
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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
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circuit number that B associates with a connection to A. Three
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important points about this process:
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1. There is no encapsulation by the STEs. The same layer 3 header format
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is reused
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2. There is no end-to-end protocol. As in a single X.25 network, all
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information has local significance only
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3. Because of the 12-bit field, an STE-STE internet link can handle
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a maximum of 4096 connections
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Call Request and Clear Request are handled step by step but must
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propagate end to end. Routing information must exist within DCEs and
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STEs to accomplish this. For example, a CALL REQUEST packet from A
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triggers the set up of a DCE-STE virtual circuit. Using the X.75
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control packet format, wich differs from X.25 only in the addition
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of a network-level utilities field, an STE-STE virtual circuit is set up
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between networks C and D. The CALL REQUEST packet then propagate to B's
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DCE, setting up another virtual circuit. Finally, a CALL
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INDICATION packet is delievered to B. The same procedure is used for
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CALL ACCEPTED and CLEAR REQUEST packets.
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