Public Safety Communications Network Dramatically Increases Network Availability

A major networking company owns and operates a public safety trunked radio system serving statewide public safety agencies. The P25 radio system utilizes T1 trunks for connection between approximately 180 tower sites and three regional centers. State Police, Highway Authority, county and municipal agencies all rely on the public safety radio system for critical public safety communications.

To ensure a high availability of service, the networking company’s engineers focused on minimizing service interruptions caused by T1 outages. In the event of a T1 failure, they wanted the circuit to automatically switch to an alternate path on a different access network. Cellular, with its increasing capacity and reliability, was determined to be a viable solution for this backup path. However, next generation 3G/4G cellular networks are packet based and don’t support native T1 circuit transport. The engineers needed equipment that would convert a T1 signal to IP, and automatically switch to the cellular path when the primary T1 failed.

The Engage Communication IPTube circuit emulation product line was just the solution the engineers were looking for. They had successfully used the IPTube for automatic protection switching of T1 over satellite links, and decided a similar solution would be effective for cellular backup of the critical P25 network. Engage Communication IPTube T1-over-IP solutions encapsulate TDM data into IP packets for transmission over IP & Ethernet connections. The IPTube uses circuit emulation to transparently transport T1 voice and data, enabling interconnection of T1-based PBX/Voice, public safety radio (LMR) and WAN router equipment over packetbased IP links.

The Link Protector option expands IPTube T1-over-IP functionality, adding auto protection switching and eliminating manual intervention in the case of a T1 outage.

Under normal operation, IPTube with Link Protector functions in a PassThru (transparent) mode, continuously monitoring the quality of the primary leased T1 connection. In the event of a T1 outage, the IPTube immediately detects the failed connection, switches from PassThru mode to T1-over-IP mode, and routes customer T1 traffic over an IP link.

In the public safety radio application, IPTube equipment is installed at the radio tower sites and at the regional centers. The primary T1 trunks, linking tower sites back to the regional centers, are monitored for quality on an ongoing basis. If a T1 link drops below customer configured performance levels, the IPTube automatically switches tower traffic over the cellular network, restoring T1 connectivity between the tower and the regional center. See Figure 1.



Fig 1 – Engage IP-Tube provides Auto Backup of T1s over Cellular Network 

  •  Under normal conditions, IPTube provides PassThru of Primary T1, monitoring the primary T1 path
  •  Upon T1 outage, IPTube routes T1 traffic over Cellular network, immediately restoring T1 connection  


    The IPTube automatically switches back to the primary T1 trunk when service is restored. With the IPTube again in PassThru mode, the public safety radio traffic is returned to the primary T1 trunk. The primary T1 is monitored for future outages, and the cellular connection can be terminated.

    IPTube T1 units are available in standalone configurations for deployment at remote tower sites, and in chassis versions for aggregation at regional centers. IPTube circuit emulation solutions from Engage Communication offer a number of specialized features which help to ensure a high quality T1 connection over the cellular network:

    • Lossless Data Compression reduces Ethernet bandwidth by eliminating idle and redundant data, minimizing cellular bandwidth required;
    • Far-End Echo Cancellation suppresses echo in PBX and other Voice applications;
    • Assured Delivery Protocol greatly improves on UDP best-effort transport used in TDM-over-Ethernet products by enabling retransmission of dropped packets as well as packet out-of-sequence correction
    • Transparent TDM over IP mode ensures all customer traffic types can be carried over the emulated T1 circuit;
    • Alternatively, the HDLC-over-IP protocol makes efficient use of cellular bandwidth for WAN router traffic.

    The availability of higher capacity/higher reliability cellular networks and IPTube T1 Circuit Emulation makes it possible to significantly improve availability for critical public safety communications networks.