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Q.931: ISDN Network Layer Protocol for Signaling
Network Layer is specified by the ITU Q-series documents Q.930 through Q.939. Q.931, the layer 3 protocol, is used for the ISDN call establishment, maintenance, and termination of logical network connections between two devices.
During the layer 3 call setup, three parties are involved, and are where messages sent and received; 1) the Caller, 2) the ISDN Switch, and 3) the Receiver. Following is an example of call setup steps:
- Caller sends a SETUP to the Switch.
- If the SETUP is OK, the switch sends a CALL PROCeeding to the Caller, and then a SETUP to the Receiver.
- The Receiver gets the SETUP. If it is OK, then it rings the phone and sends an ALERTING message to the Switch.
- The Switch forwards the ALERTING message to the Caller.
- When the receiver answers the call, is sends a CONNECT message to the Switch
- The Switch forwards the CONNECT message to the Caller.
- The Caller sends a CONNECT ACKnowledge message to the Switch
- The Switch forwards the CONNECT ACK message to the Receiver.
- Done. The connection is now up.
What services and features the telco switch provides to the attached ISDN device is specified in the optional filed - Service Profile IDs (SPIDs); when they are used, they are only accessed at device initialization time, before the call is set up. The format of the SPID is usually the 10-digit phone number of the ISDN line, plus a prefix and a suffix that are sometimes used to identify features on the line, but in reality it can be whatever the Telco decides it should be. See ITU spec Q.932 for details.
Protocol Structure - Q.931: ISDN Network Layer Protocol for SignalingInformation Field Structure - the Information Field is a variable length field that contains the Q.931 protocol data:
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1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
|
Protocol Discriminator |
|
0 |
0 |
0 |
0 |
Length of CRV |
|
Call Reference Value (1 or 2 octets)
|
|
0 |
Message Type |
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Mandatory & Optional
Information Elements (variable) |
-
Protocol Discriminator (1 octet) - identifies the Layer 3 protocol. If this is a Q.931 header, this value is always 0816.
- Length(1 octet) - indicates the length of the next field, the CRV.
- Call Reference Value (CRV) (1 or 2 octets) - used to uniquely identify each call on the user-network interface. This value is assigned at the beginning of a call, and this value becomes available for another call when the call is cleared.
- Message Type (1 octet) - identifies the message type (i.e., SETUP, CONNECT, etc.). This determines what additional information is required and allowed.
- Mandatory and Optional Information Elements (variable length) - are options that are set depending on the Message Type.
Related Protocols
LAP-D , BRI, PRI, Q.920-Q.923, LAP-B ,X.25 , ISDN , ATM
Sponsor Source
Q.931 protocol is defined by ITU Q-series documents .
Reference
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