The X.25 program supports both standard X.25 facilities and CCITT-defined facilities.
Several types of facilities may be requested in a call packet. Standard X.25 facilities are the most usual but you may find also non-X.25 facilities specific to your network or CCITT-defined facilities to be used with the OSI network services. Ask your network provider which facilities are available.
Nonstandard facilities are preceded by a facility marker: 0x0000, 0x00FF or 0x000F. The Facilities Format figure shows the format of these facility markers:
|non-X.25 facilities provided by the local network|
|non-X.25 facilities provided by the remote network|
|CCITT-specified DTE facilities|
If any section is not required, both it and the preceding facility marker can be left out.
Within each section, the facilities format is defined as a series of facility codes, followed by a number of bytes of arguments. The number of bytes of arguments is defined by the first two bits of the facility code as shown in the Facility Code Format figure.
|0||Class||| | | | | | | | | ||
The class can have one of the following values:
|00||Class A. This has a single-byte parameter field.|
|01||Class B. This has two bytes as a parameter.|
|10||Class C. This has three bytes as a parameter.|
|11||Class D. The next byte defines the length of the parameter.|
One special facility code, 0xFF, is reserved for extension of the facility codes. The octet following this code indicates an extended facility code having the format A, B, C or D class. Repetition of the facility code 0xFF is permitted, resulting in additional extensions.