Q.850

REMORA Category: Q

Q.850 is an ITU-T recommendation that defines the usage of cause and location codes in telecommunications signaling. It is specifically applied in:

  1. Digital Subscriber Signalling System No. 1 (DSS1): Used in Integrated Services Digital Network (ISDN) environments to signal between user equipment and the network.
  2. Signaling System No. 7 (SS7): Used in core telecommunications networks to facilitate signaling for ISDN and other services.

The primary role of Q.850 is to standardize the interpretation and application of cause and location codes that indicate why and where a call or signaling event was released or failed.

Benefits of Q.850
Efficient Troubleshooting: Helps operators resolve problems faster by pinpointing issues.
Interoperability: Standardizes signaling across different networks and equipment vendors.
Diagnostic Precision: Provides detailed insights into network and user issues.

Q.850 Concepts

1. Location Codes

Cause codes are standardized numeric values that describe the reason for a specific signaling event, such as the release of a call or the failure to establish a connection. These codes ensure interoperability and provide diagnostic information for troubleshooting.

Common Cause Codes:
  • 16: Normal call clearing – The call ended normally.
  • 17: User busy – The called party is busy.
  • 18: No user responding – The called party did not respond.
  • 21: Call rejected – The call was rejected by the user or network.
  • 34: No circuit/channel available – The network lacks resources to complete the call.
  • 41: Temporary failure – A transient issue occurred in the network.
  • 47: Resource unavailable – Required resources (e.g., bandwidth) are not available.

Each cause code is defined with a description and contextual usage. For example, “No circuit available” applies when a network link is overloaded or not operational.

2. Location Codes

Location codes indicate where in the network the cause event occurred. They provide critical context for identifying the source of an issue.

Common Location Codes:
  • 0: User (U): Indicates the issue is at the user equipment or endpoint.
  • 1: Private Network Serving Local User (PNL): Suggests a problem in a private network close to the user.
  • 2: Public Network Serving Local User (PNP): Points to an issue in the public network near the caller.
  • 3: Transit Network (TN): Indicates an issue in the transit portion of the network.
  • 4: Public Network Serving Remote User (PNR): Denotes a problem near the recipient’s side.
  • 5: Remote User (RU): Suggests the issue is at the far-end user’s equipment.
3. Cause-Location Pairing

Together, the cause and location codes provide a full picture of why and where the event occurred. For instance:

  • Cause Code: 34 (No circuit available)
  • Location Code: 2 (Public Network Serving Local User)
    This pairing indicates the call failed due to a lack of network resources near the caller’s side.

Usage in DSS1 (ISDN) and SS7

In DSS1:
  • Purpose: To signal between the user’s equipment and the ISDN network.
  • Scope: Deals with issues like call setup failures, busy conditions, or resource unavailability.
  • Diagnostic Role: Helps end-users or network administrators understand issues affecting ISDN connections.
In SS7:
  • Purpose: To facilitate signaling between network nodes (e.g., exchanges, routers).
  • Scope: Provides deeper network-level diagnostics, useful for carrier-grade telecommunication services.
  • Application in ISUP (ISDN User Part): Q.850 cause and location codes are used during call setup, management, and teardown to identify and resolve issues efficiently.

Real-World Applications of Q.850

  1. Call Failure Diagnostics:
    • When a call drops or fails to connect, the cause and location codes provide information about the root cause.
    • Example: Cause code 34 (No circuit available) with location 3 (Transit Network) indicates an issue in the backbone transit network.
  2. Troubleshooting Tools:
    • Telecom operators use Q.850 codes to monitor network health and pinpoint issues affecting service quality.
    • Network logs, combined with Q.850 codes, streamline fault diagnosis.
  3. Interoperability Between Networks:
    • In scenarios where multiple networks interconnect (e.g., roaming or inter-carrier calls), Q.850 ensures consistent communication about signaling events.

Examples

  1. Busy Signal (Cause 17, Location 4):
    • A user attempts to call another party, but the call fails with Cause Code 17 (User Busy) and Location Code 4 (Public Network Serving Remote User).
    • This suggests the problem lies at the recipient’s network, and the called party is actively on another call.
  2. Call Rejection (Cause 21, Location 0):
    • A call fails with Cause Code 21 (Call Rejected) and Location Code 0 (User).
    • This indicates the user equipment deliberately refused the incoming call.

Q.850 remains an essential standard in telecommunication systems, ensuring clarity and consistency in signaling event reporting across networks.