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X.25 User-Facility Support and Code References

page 62

* The Fast Select facility code is present in the incoming call packet if reverse charging and/or

fast select is indicated.

Facility Types

Facilities fall into two groups: those specified at subscription time, and those specified on a

per-call basis.

Subscription Facilities

The first group includes CCITT facilities such as nonstandard default window and packet sizes,

Closed User Group definitions, barring of incoming or outgoing calls, and reverse charging.

Per-Call Facilities

The second group of facilities are agreed upon at the time of subscription but can be used on

a per-call basis. These facilities include: reverse charging, indexing of a Closed User Group

(CUG), or specification of a Network User Identification (NUI).

Facility Syntax

Facility codes are entered as a string of numbers and/or ASCII characters. The first parameter

of a facility code identifies the facility. The subsequent parameters supply information about

the facility.

A request for CCITT facilities must always precede all requests for non-CCITT facilities. The

National Facility Marker (0,0) need only be included when at least one request for a non-CCITT

facility is present.

The syntax for Network User Identification and Flow Control Negotiation is explained below.

Network User Identification (NUI): C6,NUIlength,NUI

The first parameter after the facility code specifies the length, in bytes, of the NUI. This is

followed by the NUI itself, in a format determined by the network administration. The following

shows how to encode the seven-character NUI pass.id.

C6,07,”pass.id”

Flow Control Negotiation (packet size):42,insize,outsize

Insize and outsize specify, respectively, the maximum length of incoming and outgoing

packets. They are coded as the logarithm base 2 of the packet size, and may be offered by

networks in the range of 4 through 12, that is, packet sizes from 16 to 4096 bytes. All networks

must offer packet size 7 (128 bytes).

Examples of User Facilities

The following table provides examples of user facilities and their corresponding codes:

User Facility Code

National Facility Marker 0,0

Reverse Charging 1,1

Throughput Class Negotiation 2,table

1 2 ... 59 60 61 62 63 64 65 66 67 68 69 ... 90 91

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