Q931 Call Control Messages

The message structure and signaling elements of Q.931 are used in ISDN networks to provide call control capabilities. Q.931 messages are sent from the network to the user and from the user to the network. They are referred to as user-network and network-user messages, as illustrated in Tables 3-11 and 3-12.

Some of the most important Q.931 messages are listed in Table 3-11. The message type field in the general format of the Q.931 message is used to determine the type of message being sent.

Table 3-11. Q.931 Messages and Type Codes

Q.931 Message Type

Message Type Value

Setup message (SETUP)

00000101

Setup acknowledgment message (SETACK)

00001101

Call proceeding message (CALPRC)

00000010

Progress message (PROG)

00001111

Alerting message (ALERT)

00000011

Connect message (CONN)

00000101

Connect acknowledgment message (CONACK)

00000111

Disconnect message (DISC)

01000101

Release message (RLSE)

01001101

Release complete message (RLCOM)

01011010

Information message (INFO)

01111011

Source: ITU-T Q.931 3/93

The information or signaling elements of each message type are listed in Table 3-12. Table 3-12 also indicates the mandatory (M) and optional (O) fields for each network-to-user message.

Table 3-12. User to Network—Information Elements in Q.931 Messages

Information Elements

SETUP

CALPC

ALERT

CONN

CONAK

DISC

RLSE

RLCOM

INFO

Bearer Capability

M

Called party number

M

Calling party number

O

Called party subaddress

O

Calling party subaddress

O

Cause

M

O

O

Channel identification

O

M

O

O

High-layer compatibility

O

Keypad

O

M

Low-layer compatibility

O

Transit network selection

O

User-to-user information

O

O

O

O

O

O

Table 3-13 indicates the mandatory (M) and optional (O) fields for each network-to-user message.

Table 3-13. Network to User—Information Elements in Q.931 Messages

Signaling Elements

SETUP

SETACK

CALPC

PROG

ALET

CONN

CONAK

DISC

RLSE

RLCOM

INFO

Bearer Capability

M

Called party number

M

Calling party number

O

Called party subaddress

O

Calling party subaddress

O

Cause

O

M

O

O

Channel identification

M

M

O

High-layer compatibility

OO

Low-layer compatibility

O

Progress indicator

O

O

M

O

O

Signal

M

O

O

M

O

O

O

O

O

O

User-to-user information

O

O

O

O

O

O

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