Q.931, as a network layer protocol of the telecom system, mainly for ISDN, providing call establishment, maintenance, and termination of the logical network connection between the ISDN. Q.931 is one of the telecommunications system network layers (third layers) protocol, which is described in detail by the ITU Q series Q.930 file.
During the third layer call installation, third part sends and receive information:
Call party conversion receiver.
Below is an example of the call installation step:
The caller sends a installation information to the conversion mechanism; if the setup displays OK, the conversion mechanism sends a call to the caller, returns a setup to the recipient; the recipient receives the setup. If OK is displayed, it is dialing the phone and sends a Alerting information to the conversion mechanism; the conversion mechanism forwards the alerting information to the caller; when the recipient answers the call, send a Connect information to the conversion mechanism; the conversion mechanism forwards the Connect information Give the call party; the call party sends a Connect response information to the conversion mechanism; the conversion mechanism forwards the Connect response information to the recipient; The connection is established.
Telephone company conversion mechanism provides services and features to assist ISDN devices in the optional field-service profile IDS (SPIDS), but they can only be accessed during the device initialization period before the call creation. The general format of the SPID is 10 digital phone numbers on the ISDN line, and has a prefix and suffix to identify online features, but the specific content is determined by the telecommunications company.
Protocol structure
Information Field Structure - Information Field is a variable length field, including Q.931 protocol data:
12345678Protocol Discriminator0000Length of crvcall Reference Value (1 Or 2 Octets) 0Message TypeMandatory & Optional Information Elements (Variable)
Protocol Discriminator (1 OCTET) - Identify Level 3 Protocol. If it is q.931, the value is generated to 0816. Length (1 OCTET) - Indicates the length of the CRV in the next field. Call Reference Value (1 or 2 OCTS) - Uniquely identifies each call on the user network interface. Assign this field value at the start of the call. When the call is cleared, the field value can be used for other calling procedures. Message Type (1 OCTET) - Identify information type (that is, setup, connect, etc.). This field decides which other information is permitted. MANDATORY AND OPTIONAL Information Elements (Variable length) - Options, depending on the Message Type.
