We describe the function of X.25 but ignore interface details here. The designer can learn from this section the function available from an X.25 circuit. We believe that the X25 circuit keys provide the same function. In this regard we believe that this section is complete and precise. Other sections are required, however, to find out how to code these functions. Section (x25map) relates this terminology to that of the official X.25 definition.

The role of X.25 is to transport data both ways between a pair of "DTE"s (data terminal equipment) by X.25. One DTE may belong to many pairs at once.

A DTE may specify the other member of a pair when communication is desired. After agreement by the other DTE communications can proceed until one of the DTEs terminates the relation. This is called a "virtual call".

Alternatively these pairs may be permanently established administratively thru the network operators in which case the DTE need not specify a remote DTE in order to communicate. These connections are called "Permanent Virtual Circuits" (PVC).

The Data Stream (Version One)
The data transmitted in each direction may be viewed conceptually as a symbol stream transmitted in its natural order. Each symbol is a bit, a mark or a qualifier. The stream is composed by the sender and sensed by the receiver. There are two bits {0 and 1} and three marks, the "bare record mark", the "bare delivery mark" and the "combo mark". By record mark we mean the bare record mark or the combo mark. By "delivery mark" we mean the bare delivery mark or the combo mark. There is one qualifier, written here as "Q". The only syntax rule for a symbols stream is that every qualifier is either the first symbol of the stream or immediately preceded by a mark.
Synopsis: The symbols are bits, marks or qualifiers. The bits are 0 or 1. The marks are (1) bare delivery marks (D) or (2) bare record marks (R) or (3) combo marks (C). The qualifier is Q. The delivery marks are (1) the bare delivery mark or (2) the combo mark. The record marks are (1) the bare record mark or (2) the combo mark.

{/{Q^}{/{0^1}}{C^B^R}}

In Gnosis, (and in most X25 networks) the spans of bits between other symbols must be a multiple of eight bits in length.

The sender may wait for an indication that the symbols before all transmitted delivery marks have been acknowledged by the remote DTE. The receiver gets the stream in its natural order. Bits following the most recently transmitted mark may not arrive at the receiver until more bits or marks are transmitted. Transmitted marks and bits preceding them will eventually arrive. Any other symbol significance is between by the sender and the receiver.

If the recipient is slow to receive and acknowledge the symbols, the sender will eventually be blocked from sending more.

The Data Stream (Version Two)
The data stream between DTEs in each direction may be conceptually described as a stream of bytes punctuated by marks. These marks divide the stream into units meaningful to the application, and arrange for delivery confirmation.

The punctuation marks are BRM (bare record mark), BDM (bare delivery mark) and CM (combination mark). By "record mark" we mean BRM or CM and by "delivery mark" we mean BDM or CM. CM is for efficiency, two for the price of one.

After a mark there may be a "Q" symbol that designates as "qualified" the following stream of bytes up to the next mark.

{Purpose of delivery marks:} The sender may wait for an indication that all data before a given delivery mark have been delivered to the remote DTE.

Data following the most recently transmitted mark may not arrive at the receiver until more data or marks are transmitted. A transmitted mark and data preceding it will eventually arrive even without further transmission by the sender.

If the recipient is slow to receive and acknowledge the data, the sender will eventually be blocked from sending more.

Interrupts
An additional function is to carry "interrupts" between the pair of DTEs. An interrupt delivery is not delayed by undelivered portions of the data stream but is made available to the remote DTE even while the DTE is not accepting data from the network. An interrupt carries from 1 to 32 bytes of data from the sender to the receiver and is explicitly designated as an interrupt to the receiver. Data from the stream never overtake interrupts. The sender may request confirmation of the delivery of an interrupt to the remote DTE. The confirmation is not delayed by queued data in the other direction and such confirmations are not overtaken by data.

An additional function is to carry "interrupts" between the pair of DTEs. An interrupt delivery is not delayed by undelivered portions of the symbol stream but is made available to the remote DTE even while the DTE is not accepting symbols from the stream. An interrupt holds from 1 to 32 bytes of data from the sender to the receiver and is explicitly designated as an interrupt to the receiver. Symbols from the stream never overtake interrupts. The sender may request confirmation of the delivery of an interrupt to the remote DTE.

Resets
A DTE may request a reset. This will eventually result in a reset indication to the remote DTE. The remote DTE should confirm the reset. The confirmation will be delivered to the requesting DTE.

Reset signals and their confirmation may overtake data and interrupts and are thus not blocked by a DTE that is not receiving data. Overtaken data and interrupts are discarded by the network and never delivered. The requestor of the reset may continue to receive data and interrupts until the confirmation arrives. {All data and interrupts arriving after a reset indication, were sent after the corresponding reset request and all data and interrupts delivered after the arrival of the reset confirmation was sent after the confirmation were sent.}