Bits

8

7

6

5

4

3

2

1

Octets

Calling party subaddress information element identifier

0

1

1

0

1

1

0

1

1

Length of calling party subaddress contents

2

1 ext.

Type of subaddress

Odd/even
indicator

Spare

3

0

0

0

Subaddress information

4
etc.

Type of subaddress (octet 3)

Bits

7 6 5

0 0 0

NSAP (CCITT Rec. X.213 [23]/ISO 8348 AD2 [24])

0 1 0

User specified

All other values are reserved.

Odd/even indicator (octet 3)

Bit

4

0

Even number of address signals

1

Odd number of address signals

Note 1 - The odd/even indicator is used when the type of subaddress is "user specified" and the coding is BCD.

Subaddress information (octets 4, etc.)

The NSAP CCITT Rec. X.213/ISO 8348 AD2 address, shall be formatted as specified by octet 4 which contains the Authority and Format Identifier (AFI). The encoding is made according to the "preferred binary encoding" as defined in CCITT Rec. X.213/ISO 8348 AD2 except when used for Terminal selection at the S interface (see Note 3). For the definition of this type of subaddress, see Recommendation I.334 [25].

For user specified subaddress, this field is encoded according to the user specification, subject to a maximum length of 20 octets. When interworking with X.25 [5] networks BCD coding should be applied.

Note 2 - It is recommended that users apply the NSAP subaddress type since this subaddress type allows the use of decimal, binary and IA5 syntaxes in a standardized manner.

Note 3 - It is recommended that users apply the Local IDI format (the AFI field coded 50 in BCD) when the subaddress is used for terminal selection purposes at the S interface. In this case the IA5 character syntax using only digits 0 to 9 shall be used for the DSP. Each character is then encoded in one octet according to Recommendation T.50 [49] and ISO/IEC 646, with zero parity in the most significant position.