Skip to main content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.


The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

P25 ISSI Simulation Tool

P25 ISSI Simulation Tool Software

The P25 ISSI Simulation Tool is designed to allow Public Safety users to analyze and to understand the behavior of the network when deploying the Inter-RF Subsystem Interface (ISSI) that connects multiple Radio Frequency Subsystems (RFSSs). The tool combines an easy to use Graphical User Interface (GUI) and a network simulator that models the ISSI protocol. By using the P25 ISSI Simulation Tool, the Public Safety users can evaluate different scenarios without the need for purchasing hardware and/or without affecting their current deployment. The tool can be used to simulate networks containing many RFSSs and several hundred Subscriber Units (SUs) generating both talk group calls and unit to unit calls. Each network element and SU can be configured separately allowing the user to tweak the parameters and see the impact on the user application. The tool also supports generic IP traffic (e.g. FTP) in order to simulate both dedicated and shared networks. The traffic generated by the SUs during the call can derive from a statistical function (e.g. exponential, uniform), or can be specified by the use of an audio file. When audio files are used, the tool records the traffic received by the SUs during the calls and the result can be heard during the simulation playback allowing the user to assess the quality of the call.

Graphical User Interface
The GUI allows the user to create the network topology, configure the applications, run the simulations, and view the results of the simulations, using both quantitative and qualitative data. The main features of the GUI are:

  • Creation of the network topology using drag and drop.
  • Computation and display of network and application statistics.
  • Simulation playback with visual markers to help understand the results (e.g. change in link load, traffic flows).
  • Support of audio files to assign traffic to the SUs and listen to the audio (as it would be perceived by a user) during simulation playback.
  • Automatic analysis of the simulation results (e.g. link capacity, call failures, packet loss).
  • Generation of pdf file for simulation results.

Simulation Model
The ISSI network model is an extension to the NS-2 simulation platform.  NS-2 is a discrete event simulator for network research.  NS-2 contains models for many of the existing network technologies and protocols upon which new protocols can be added or existing ones extended or modified.  Some example protocols that already exist for NS-2 that can be used to support the P25 ISSI model are Ethernet, IPv4, User Data Protocol (UDP), Session Initiation Protocol (SIP), and the Real-time transport protocol (RTP).

Our implementation of the ISSI model includes the Mobility Manager, Call Control Manager, and Transmission Control Manager as specified in TIA-102.BACA-A.

  • The Mobility Manager handles the registration and roaming of the SUs from RFSS to RFSS. It is also responsible for managing the registration of the talk groups for which the SUs are members.
  • The Call Control Manager enables the establishment and termination of talk group calls and subscriber unit to subscriber unit (SU-to-SU) calls, as well as the various configuration parameters (e.g., priority, availability check, emergency call/ non-emergency call).
  • The Transmission Control Manager manages the talk spurts and handles the transmission of the RTP (e.g., voice) packets.

For more information on the various functional services, visit our ISSI protocol overview page.

The network model also includes an implementation of the Session Initiation Protocol (SIP), which is used by the ISSI protocol to carry signaling messages.

To ensure that our simulation model is accurate as possible, we simulated and validated the scenarios defined in the standard TIA-102.CACD-A (Inter-RF Subsystem Interface –Interoperability Test Procedures for Trunked Systems Involving the ISSI), for all the features that are available in the model.


The P25 ISSI Simulation Tool provides a wide range of information making it a versatile tool to analyze the behavior of the ISSI protocol. The following data is provided:

  • Simulation-wide analysis such as link capacity planning, call success/failure statistics, or call setup delay distributions.
  • Time based statistics are available for RFSSs (e.g. active SUs/Talk Groups), SUs (e.g. traffic sent/received), applications/calls, routers, and links.
  • Log file can be configured to trace the events that occurred during the simulation and view the decisions made by the RFSSs regarding call admission and spurt control.

As a result, the user is able to:

  • Perform network planning. By testing a multitude of configuration and parameters, the user can configure the network to satisfy the application requirements.
  • Troubleshoot problems occurring in a real deployment. The tool can be used to replicate an existing network and give insights into the application performance.
  • Use the tool to learn about the ISSI protocol. The user can create simple scenarios and use the log file to follow a call progress step by step.
Created December 15, 2009, Updated October 25, 2016