The Challenge

An aging ATM/TDM backbone that could not scale

A state government agency responsible for public safety and utility communications needed to replace its aging ATM/TDM backbone built on legacy ATM switches interconnected by OC3 microwave radios. The existing infrastructure could not scale to support growing bandwidth demands from IP-based applications while continuing to carry critical TDM services including land mobile radio trunking, voice circuits, and SCADA telemetry.

The agency required a migration path that would modernize the transport backbone to IP/MPLS while preserving all existing TDM services through circuit emulation.

The Solution

An IP/MPLS backbone tuned for microwave transport

The solution replaced legacy ATM switches with compact MPLS service routers at hub sites and service aggregation routers at remote mountaintop and tower locations, interconnected by new 150 Mbps Ethernet microwave radios in ring topologies.

Design Elements

  • 01

    Single-Area OSPF-TE

    Single-area OSPF with Traffic Engineering extensions across all ~24 nodes, appropriate for the network's moderate scale while enabling RSVP-TE for traffic-engineered LSP tunnels.

  • 02

    Differentiated BFD Detection

    BFD-based fault detection with differentiated timer profiles: 200 ms intervals over microwave links to avoid false triggers from radio fading, and 100 ms intervals over fiber interconnects for faster failure detection.

  • 03

    Extended OSPF Backoff

    Carefully tuned OSPF SPF and LSA timers with extended backoff values (120s max SPF wait, 180s max LSA generate) to absorb microwave link flapping caused by weather-related signal degradation.

  • 04

    RSVP-TE with Standby Paths

    RSVP-TE signaled LSPs with secondary standby paths for transport redundancy, leveraging the ring topology to provide diverse backup paths around any single link or node failure.

  • 05

    Nine Service Types

    Nine distinct service types implemented using a mix of Cpipe (TDM circuit emulation for LMR and voice), Epipe (point-to-point Ethernet), Ipipe (IP interworking), and VPLS (multipoint LAN extension) services.

  • 06

    Network Synchronization

    Network synchronization architecture supporting multiple clock sources including GPS, SyncE, and adaptive clock recovery to maintain TDM service quality across the packet backbone.

Architecture Highlights

Design approach by area

  • Routing

    Single-area OSPF-TE; RSVP-TE LSPs with secondary standby; port egress rate-limiting matched to microwave link capacities (150/50/24 Mbps).

  • Convergence

    Dual BFD profiles for microwave vs. fiber; extended OSPF backoff timers absorb radio fade events without unnecessary SPF recalculations.

  • TDM Services

    Cpipe circuit emulation for LMR radio trunking and legacy voice; T1/E1 ASAP adapters at remote sites preserving existing TDM device connections.

  • IP Services

    VPLS for routed data services; Epipe for point-to-point Ethernet; Ipipe for IP interworking with legacy serial devices.

  • Synchronization

    GPS primary timing at hub sites; SyncE and adaptive clock recovery distribution to remote sites for TDM service quality.

  • Security

    CPM filters with default-deny policy; MD5 protocol authentication; RADIUS/TACACS+ AAA; role-based access profiles.

Service Portfolio

Nine service types carried across the new transport

The migration preserved all nine existing service types while transitioning them from ATM to MPLS transport. Land Mobile Radio trunking, the most critical public safety service, was carried as Cpipe circuit emulation providing transparent TDM connectivity between radio sites and dispatch centers.

Voice circuits were similarly carried as Cpipes until VoIP migration could be completed. SCADA telemetry and video monitoring services were implemented as Epipe and VPLS services respectively, while the telemetry and network management services used routed VPRN or Ipipe configurations depending on the endpoint device capabilities.

Each service type was assigned a structured numbering scheme with distinct customer IDs, service ID ranges, and SDP numbering conventions to ensure consistent operations across the network as services were provisioned and maintained.

Results & Benefits

Outcomes of the migration

  • Bandwidth Upgrade

    Transition from OC3 (155 Mbps) to 150 Mbps Ethernet microwave per link, with capacity for future radio upgrades.

  • Service Continuity

    All nine legacy service types preserved through circuit emulation and pseudowire technologies during migration.

  • Radio-Aware Design

    Differentiated BFD timers and OSPF backoff values prevent false convergence events from microwave signal fading.

  • Operational Simplicity

    Single-area OSPF and centralized NMS with redundant management servers simplify operations for a lean government IT team.

  • Future Readiness

    IP/MPLS backbone positions the agency for VoIP migration, IP-based SCADA, and broadband public safety applications.

Conclusion

A legacy ATM backbone replaced by a microwave-tuned IP/MPLS network

This migration replaced a legacy ATM backbone with a modern IP/MPLS network purpose-tuned for microwave transport, preserving all critical public safety TDM services while delivering a foundation for IP-based service evolution.

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