X.25 User-Facility Support and Code References

---

This section describes X.25 user-facility support features and includes details on DNICs, X.25 diagnostic and cause codes, and ASCII control codes specific to the X.25 protocol.

 
User-Facility Support

---

User facilities are optional network services that let you perform tasks such as reversing charges on your calls, accessing a Closed User Group, or specifying a Network User Identification (NUI).

Two of Eicon Networks' additions to the X.28 Command set--call and conn-- support full CCITT Facility request capability.

Facilities are encoded into the facility field of the call request packet without any interpretation on the part of the X.PAD program. The facility field contains both facility codes and their associated parameters. Code format varies, since facilities may have one or more parameters. You can enter the contents of the user-facilities field in three different formats:

	Hexadecimal numbers separated by commas
	ASCII characters within either single or double quotation marks
	Combination of hexadecimal numbers and ASCII characters

The table below summarizes the facility codes for these facilities, including the applicable packet types. The facility code is given in hexadecimal.

* 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:

Packet size

The packet size for transmissions from the remote DTE is shown in the low nibble of the first octet in the parameter field. The packet size for transmissions from the local DTE is indicated in the low nibble of the second octet. The high nibble of each octet must be zero.

The four bits indicating each packet size are binary coded and indicate the packet size as follows:

Note: Some networks may offer a subset of these values. Default packet size is 128 bytes.

Window Size

The window size for transmissions from the remote DTE is represented in bits 6 to 0 of the first octet in the parameter field. The window size for transmissions from the local DTE is represented in bits 6 to 0 of the second octet. Bit 7 of each octet must be 0.

The bits referring to window size are binary coded and directly indicate the size of the window.

Window sizes of 1 to 7 are standard. Window sizes of 8 to 127 are valid only if extended sequence numbering is used. The default window size is 2. A value of 0 is not allowed.

Fast Select

The fast select facility is controlled by bits 6 and 7 of the octet in the parameter field. If fast select is not requested then bits 6 and 7 are set to 0. If fast select is requested with no restriction on response then bit 6 is set to 0 and bit 7 is set to 1. Otherwise, if fast select is requested with a restriction on response then bits 6, 7 are set to 1.

*See Reverse Charging.

Bits 1, 2, 3, 4, 5 may be used for other facilities or set to 0

Note: The reverse charging and Fast Select facilities share the same facility code and may be used concurrently.

Throughput Class Negotiation

The throughput class for transmissions from the remote DTE is represented in the high nibble of the octet in the parameter field. The throughput class for transmissions from the local DTE is indicated in the low nibble.

The two groups of four bits referring to throughput class are binary coded and indicate the throughput classes as follows:

Closed User Group (CUG) Selection

The index number to the closed user group selected for the virtual call is in the form of two decimal digits. Each digit is coded in BCD, in a nibble of the parameter field. The high nibble represents the first digit and the low nibble represents the second digit.

Note: Indices to the same Closed User Group at different DTE/DCE interfaces may be different.

CUG with Outgoing Access Selection

The index number to the closed user group selected for the virtual call is in the form of two decimal digits. Each digit is coded in BCD, in a nibble of the parameter field. The high nibble represents the first digit and the low nibble represents the second digit.

Note: Indices to the same Closed User Group at different DTE/DCE interfaces may be different.

Bilateral CUG Selection

The index number to the bilateral closed user group selected for the virtual call is in the form of four decimal digits. Each digit is coded in a semi-octet, in BCD. The high and low nibbles of the first octet of the parameter field represent the first and second digits. The high and low nibbles of the second octet represent the third and fourth digits, respectively.

Note: Indices to the same Bilateral Closed User group at different DTE/DCE interfaces may be different.

Reverse Charging

The reverse charging facility is controlled by bit 0 of the octet in the parameter field. If reverse charging is not requested then bit 0 is set to 0. Otherwise if reverse charging is requested then bit 0 is set to 1.

*See Fast Select.

Bits 1, 2, 3, 4, 5 may be used for other facilities or set to 0

Note: The reverse charging and Fast Select facilities share the same facility code and may be used concurrently.

Network User Identification (NUI)

The octet following the facility code field indicates the length, in octets, of the Password and NUI fields. The following octets contain the user password and network user identification.

The Length field is set to the combined number of octets in the password and the NUI fields.

Each octet in the Password and NUI fields contain a single ASCII character. The maximum Password length is 6 characters. The maximum NUI length is 8 characters.

If both the NUI Charging facility and the reverse charging facility are specified then the reverse charging facility will apply to the call.

Example of a Non-CCITT Facility

The Datapac Traffic Class Facility is an example of a non-CCITT facility. One of its significant aspects is that it is compulsory when making an international call. Its coding is 1,1 and it must be preceded by the National Facility Marker (0,0). For example:

Other networks also define their own facilities. If you receive repeated "Invalid Facility Request" messages or if you wish to find out more about the facilities applicable to you, contact your network representative.

Further Information on User Facilities

The previous sections are not intended as an exhaustive description of the user facilities supported either by the CCITT or by the network you may be using. For information on CCITT user facilities, consult the CCITT Recommendation International User Services and Facilities in Public Data Networks, Vol. VIII, Fascicle VIII.2, Rec. X.2.

 
Networks and DNICs

---

CCITT Recommendation X.121 defines a general address format containing 12, 13, or 14 digits. The first four digits of a general address format constitute the Data Network Identification Code (DNIC). The first three digits of the DNIC generally identify the country--much like telephone area codes--with the exception of large countries such as the United States. The fourth digit identifies a particular network within the country.

The following table shows the DNICs of various public data networks around the world and information/test numbers, as applicable. You can call these numbers for further information about a given network, or to test your X.3 PAD parameter settings. The numbers in this table are in effect at the time of publication.

 
X.25 Diagnostic Codes

---

The following table describes X.25 diagnostic codes. These codes can help you in tracing the source of problems with an X.25 connection

 
X.25 Cause Codes

---

The table below describes X.25 generated cause codes. This includes all clearing, resetting, and restarting causes. These codes can help you to trace the source of problems with an X.25 connection.

 
ASCII Control Codes

---

The following table lists ASCII control codes.