GR-303

Introduction

The GR-303 interface, originally defined by Telcordia, provides a digital interface between a Remote Digital Terminal (RDT) and a Central Office switch using T1-based bearer and signaling channels. In TelcoBridges implementations, the device operates as an Integrated Digital Terminal (IDT), providing the switch-side termination of GR-303.

GR-303 enables:

• Concentration of analog subscriber loops
• Digital transport over T1
• Logical grouping of subscriber channels into IDT groups
• Separation of signaling and bearer channels
• Scalability across thousands of lines

Since Toolpack/Tmedia v3.3, TelcoBridges platforms support full GR-303 IDT functionality on Tmedia appliances.

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Overview

GR-303 separates the interface into three logical components:

• TMC (Time Slot Management Channel) — bearer channel mapping
• EOC (Embedded Operations Channel) — OAM & control
• Channel Associated Signaling (CAS) — subscriber line state signaling

In a TelcoBridges solution:

• The RDT provides the analog subscriber access.
• The IDT (the TelcoBridges device) terminates the RDT’s GR-303 CAS/T1 channels.
• The IDT maps those channels to either:

 * H.248 terminations
 * SIP subscriber endpoints

== Key Objectives ==

• Provide a scalable GR-303 digital switch interface
• Support a wide range of legacy RDT vendors
• Allow migration paths to SIP and VoIP architectures
• Ensure consistent signaling between CAS, H.248, and SIP

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Features and Capabilities

System Capabilities

• Up to 42 IDT groups per system
• Up to 28 T1s per group
• Primary/Backup TMC & EOC redundancy
• 1+1 High Availability
• CAS loopstart signaling on GR-303 T1s
• Support for industry-standard RDT vendors
  • Adtran TA5000
  • Calix C7
  • AFC
  • Tellabs UMC1000
  • Motorola/Arris AXS2200

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GR-303 Call Flow (Engineering Detail)

Call Routing

Routing differs:

SIP Mode

• Routing is determined by the TMG routing engine
• The subscriber is defined in a csv file (subscriber to GR303 crv value)

H.248 Mode

• Softswitch determines call flow
• IDT simply reports events and manages media terminations
• Full feature set is supported

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GR-303 Overview in SIP Architecture

SIP Architecture Diagram

GR303 RDT -- GR303 CAS/T1 -- TMG800 / TMG3200 IDT -- SIP + RTP -- Softswitch

SIP Integration Details

• Each subscriber registers individually to the SIP registrar
• Each subscriber must have an entry in the CRV mapping CSV file
• CAS → SIP signaling translation is handled by the IDT

Supported SIP features

• Basic call
• SIP registration
• Single dial plan

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GR-303 Overview in H.248 Architecture

H.248 Architecture Diagram

GR303 RDT -- GR303 CAS/T1 -- TMG800 / TMG3200 IDT -- H.248 + CAS -- Softswitch (MGC)

H.248 Integration Details

• IDT appears as a standard H.248 Media Gateway
• Subscriber loops appear as Terminations
• Softswitch has full control over:
  • digit collection
  • feature activation
  • ring cadence
  • call routing
  • media negotiation

Supported features

• Caller ID
• Call waiting
• Call transfer
• 3-way conference
• ESA (Emergency Standalone), switch dependent

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Conclusion

The TelcoBridges GR-303 implementation provides a robust, scalable, and flexible IDT-side interface suitable for modern softswitch architectures, enabling a seamless migration path from legacy TDM access lines to VoIP core networks via SIP or H.248.