AT&T 7200 series, 7500 series, 7100 series, 2500 series manual Communication Pathways and Endpoints

COMMUNICATIONS PROTOCOLS D-19

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Physical Layer. This layer for BX.25 is compatible with the RS-232C and RS-449 standards.

Data-Link Layer. At this layer, BX.25 uses a procedure is compatible with the HDLC (High-Level Data Link Control) procedure and is performed by DCIU firmware and by an HDLC chip on each of the DCIUs eight links.

Packet Layer. At this layer, BX.25 multiplexes different logical streams of data through layer 2 and regulates the flow of data on each of these logical streams so as to prevent the overrunning of receiver buffers. Packet layer responsibilities are performed by firmware in the DCIU.

Session Layer. This layer binds two applications into a session and shields the applications from the details of controlling the layer 3 interface. For BX.25, session layer services are provided by DIP on the 501CC.

Application Layer. This layer is named but not regulated in BX.25.

Communication Pathways and Endpoints

BX.25 terms describing communication pathways and their endpoints are link, logical channel, port, virtual circuit, and permanent virtual circuit.

Link. Link is a layer 2 term describing a segment of a communication path (e.g., between two DCIU’s or between a DCIU and an AP line controller) on which there is established a layer 2 protocol. Link includes whatever entities are performing layer 2 services at each end of the segment. Note that since link is a layer 2 term, it does not imply what kind of hardware is used at layer 1.

Logical Channel. Logical channel is a layer 3 term describing one of many logically independent streams of data which are multiplexed on a single link. The definition of a logical channel begins and ends with the layer 3 entities at each end of a given link. This means logical channel numbers are defined on a given segment of a communication path and that a single logical stream of data may be assigned different logical channel numbers on different segments of a communication path.

Port. Port is a session layer term describing a gateway between session layer and a single application. To the application, a port looks like an I/O device. Once a session has been established, it is safe to think of ports as the endpoints of a virtual circuit between two applications. Although the port numbers at opposite ends are generally not equal, the session layer entity at each end knows the port numbers at both ends.

Virtual Circuit. Virtual circuit is a layer 3 term referring to the entire communication path between two layer 3 entities. Since the communication path may consist of several links, with a different logical channel number used on each link, the logical channel numbers at opposite ends of a virtual circuit can differ.

Permanent Virtual Circuit. A permanent virtual circuit is a virtual circuit that is permanently established, as opposed to a virtual call which is a virtual circuit that is dynamically established as the need arises and then released when no longer needed.