Layer 3 - Network Layer

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The ISDN Network Layer is also specified by the ITU Q-series documents Q.930 through Q.939. Layer 3 is used for the establishment, maintenance, and termination of logical network connections between two devices.


Service Profile IDs (SPIDs) are used to identify what services and features the telco switch provides to the attached ISDN device. SPIDs are optional; when they are used, they are only accessed at device initialization time, before the call is set up. The format of the SPID is defined in a recommendation document, but it is only rarely followed. It 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. If an ISDN line requires a SPID, but it is not correctly supplied, then Layer 2 initialization will take place, but Layer 3 will not, and the device will not be able to place or accept calls. See ITU spec Q.932 for details.

Information Field Structure

The Information Field is a variable length field that contains the Q.931 protocol data.

Information Field
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
Mandatory & Optional
Information Elements (variable)

These are the fields in a Q.931 header:
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.

Layer 3 Call Setup

These are the steps that occurs when an ISDN call is established. In the following example, there are three points where messages are sent and received; 1) the Caller, 2) the ISDN Switch, and 3) the Receiver.
  1. Caller sends a SETUP to the Switch.
  2. If the SETUP is OK, the switch sends a CALL PROCeeding to the Caller, and then a SETUP to the Receiver.
  3. The Receiver gets the SETUP. If it is OK, then it rings the phone and sends an ALERTING message to the Switch.
  4. The Switch forwards the ALERTING message to the Caller.
  5. When the receiver answers the call, is sends a CONNECT message to the Switch
  6. The Switch forwards the CONNECT message to the Caller.
  7. The Caller sends a CONNECT ACKnowledge message to the Switch
  8. The Switch forwards the CONNECT ACK message to the Receiver.
  9. Done. The connection is now up.

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Updated December 5, 2006
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