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Site Trunking Site Trunking

Site Trunking is, hands down,  the most misunderstood component of  the statewide 800 MHz system and of any shared communication system.  When the system goes into Site Trunking, many users think that the radio system is down.  This is an inaccurate perception.  When the radio system goes into Site Trunking , it is still working, but at reduced capacity.

Even though the system is working as designed, a system in Site Trunking can frustrate users out in the field and in the dispatch center. A clear understanding of  Site Trunking concepts and practices will alleviate some of that frustration and can help users recognize when their radio goes into that mode.

Shared System 101

Each radio in a shared system, such as Indiana’s 800 MHz SAFE-T system, affiliates with one site (tower) at a time in one Zone Controller and to one talkgroup.    Sites are linked by T-1 lines, phone lines or microwave, which creates a network of  repeated connectivity.  Under normal circumstances, the system operates as a Wide Area network, which allows users to communicate not only with others affiliated with the same site, but also with those affiliated with other sites across the network.  Users can communication across cities, counties and even across the state.

What is Site Trunking?

When  connectivity to the T-1 line, phone line or microwave is severed, the system goes into Site Trunking.   The site will continue to “trunk” on channels at that site and users  affiliated with that site can communicate with other users affiliated with the same site.  They cannot, however, communicate with users affiliated with a different site and vice versa since their connectivity has been temporarily severed.

How to Recognize Site Trunking

  • Field units may received a visual indication on the radio screen (if equipped) and/or may hear a periodic audible indicator (if programmed) to indicate Site Trunking mode. 
  • Dispatchers working on wire line console (hooked directly into the Zone Controller via T-1) will not have a indication from their console that one or more sites in their area are in Site Trunking.  IPSC strongly recommends that Wire Line console users equip their dispatch centers with at least one radio to monitor local site conditions. 
  • Dispatch centers using RF consoles (one or more radios talking directly on one or more sites on one or more talkgroups) may recognize Site Trunking if the dispatch operations radio(s) is programmed to display the  Site Trunking indicator and has Site Trunking alert tones  turned on. 

Site Trunking Plan

Each dispatch center should have a site trunking plan and should test and exercise this plan!  IPSC recommends:

  • Wire Line Console users should have at least one (1) portable or mobile radio as a console back up.  This will provide most users with a site trunking indication, but keep in mind that site distance may prohibit this notification. 
  • RF Console users should  have at least two (2) radios on separate external antennas pointed towards different sites and should create a temporary patch between the Wide Area and Site Trunking sides of the conversation.  Additional resources may allow you to SimulSelect both radios to dispatch to both Wide Area and Site Trunking sides of the conversation. 
  • Agencies should develop policies and procedures to move users to a designated talkgroup(s) for the duration of the Site Trunking event.
  • Consider programming radios with a SITE LOCK button.  When activated, the radio will remain affiliated with the site even if that site is in Site Trunking.  Caution: users must be sure not to lock the radio under normal operations because this will prohibit them from roaming from site to site as normal.

How IPSC Can Help

IPSC is available to help agencies understand site trunking, create site trunking plans, and determine if the system is optimally configured for your agency needs.  IPSC also has the ability to remotely place a site into Site Trunking for testing and exercises!   Use the contact links below to request assistance.

Additional Resources

Download IPSC's Site Trunking White Paper

Site Trunking Myths

MYTH #1: The site is completely down when in Site Trunking. FACT: The site is still operational, and users of that site can communicate normally. Users cannot, however, communicate with those affiliated with other sites since their connection has temporarily been severed.

MYTH #2: It is necessary to report the site to the NOC. FACT: It is not necessary to report Site Trunking issue to the Network Operations Center (NOC). The NOC and Motorola are notified immediately when a site goes into Site Trunking. The only exception to this rule is if the site continuously goes in and out of Site Trunking. Then report the issue.

MYTH #3: IPSC controls all the T-1 lines that connect the towers. FACT: IPSC does not have control over the T-1 lines. The T-1 lines are controlled by the telephone company.