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2001 Arizona APCO/NENA State Training Conference

2001 Arizona APCO/NENA State Training Conference. Traffic Loading Considerations in a Trunked Two-Way Radio System Ken Ballard, Ph.D. Communications System Design. How will the system be used? Group Calls, Dispatch Operations Security Needs Interoperability with Outside Agencies

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2001 Arizona APCO/NENA State Training Conference

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  1. 2001 Arizona APCO/NENA State Training Conference Traffic Loading Considerations in a Trunked Two-Way Radio System Ken Ballard, Ph.D. Traffic Loading in Trunked Radio

  2. Communications System Design • How will the system be used? • Group Calls, Dispatch Operations • Security Needs • Interoperability with Outside Agencies • One-to-one Calls (e.g., telephone interconnect, paging, etc.) • Radio Coverage & Sites • Availability of Frequencies • Capacity (# of Users, Expected Growth & Expansion) • Reliability / Grade of Service • Cost • Acquisition • Maintenance (Software & Hardware) Traffic Loading in Trunked Radio

  3. Conventional or Trunked? • Number of Users • Wide Area System • Users per “Cell” (> 200 indicated Trunking) • Number of “Talk Groups” • Departments or Organizations of People • Overlapping air time requirements (> 5 per cell) indicate Trunking • Required System Functions • Priority Calls, including “Emergency” • Internal Interoperability (especially situation dependent) • “Storm Plans” or Re-grouping • Mobile Data, Automatic Vehicle Location, etc. • System Cost Traffic Loading in Trunked Radio

  4. Traffic Testing & Analysis • Testing of Conventional System • Call Statistics can be collected (over-the-air or at base/ voters) • Extrapolate Measured Data to Trunked System Performance? • This may be a waste of time & money! • Call Patterns Change on a Trunked System • Users Sense that the new System is Never Busy • Only 1 Person can Talk at a time (forced discipline) • Talk Groups are now Separate (less chatter from other departments) • Recommend that Statistics from Trunked Systems be Applied • Upgrades / Expansion for Trunked Systems • Do use Historical Reports from the System Manager • Model Upgrade Configuration with known Call Patterns Traffic Loading in Trunked Radio

  5. Trunking and Traffic Loading • Some Selected “Rules of Thumb” • 50 Radios per Channel (conventional) • 100 Radios per Channel (trunked) • FCC, APCO and NPSPAC use these as “Rules” for Licensing • Reality • Trunking can be Modeled via “Queuing Theory” • Erlang C Model is most commonly used • Loading for Trunked Systems is Not Linear • Examples: • 146 Radios can be Added when going from 3 to 4 Channels • 371 Radios can be Added when going from 27 to 28 Channels Traffic Loading in Trunked Radio

  6. Typical Erlang C Model Results Traffic Loading in Trunked Radio

  7. Transmission vs. Message Trunking • Transmission Trunking • Each Push-to-Talk (PTT) results in a new channel assignment • Channel Access Time varies from 0.25 sec to 0.600 sec • Channel Release Time (on the trailing edge) is minimal • Generally the Most Efficient Use of Spectrum • Message Trunking • Channel Assignment made to Talk Group by first PTT • Channel Resource is held for 1 – 3 sec when PTT is released (programmable) • Next Caller in Same Talk Group can Access Channel w/o Wait • Efficient for crisp back-and-forth Conversations • Standard Message Trunking - No Caller ID after 1st PTT • PTT-ID is a hybrid where each caller has ID attached • Must be used for some kinds of data calls Traffic Loading in Trunked Radio

  8. Public Safety Statistics Used • Use Erlang C Model for Trunked System • Assumed Call Statistics for Transmission Trunking • 3.8 sec per Voice Transmission • 0.5 sec Control Channel Overhead • 2 calls/hour per radio • 1 sec Maximum Acceptable Call Delay • 1% Delayed Call Grade of Service • Assumed Call Statistics for Message Trunking • 9 sec of Audio Transmission (3 back-and-forth calls) • 3 sec for Hang Time & Control Channel Overhead (for 1st PTT) • 1 message/hour per radio • 1 sec Maximum Acceptable Call Delay • 1% Delayed Call Grade of Service Traffic Loading in Trunked Radio

  9. Impact of Digital Voice on Traffic Caller 2 Hears Audio Channel Access (0.5 sec) Caller 2 Voice Caller 1 Voice Caller 1 Hears Audio Digital Latency (1 sec) Digital Latency (1 sec) PTT Traffic Loading in Trunked Radio

  10. Impact of Digital Voice on Traffic • System Access Time • Time between the PTT and the Channel Grant • Typically ranges between 0.25 and 0.600 sec • Voice during this period of time is lost • Digital Latency • Time Delay for Audio to pass through system (caller to receiver) • Typically is about 1 sec in duration • Latency is caused by Analog-to-Digital (A/D) and D/A Conversions • Voice during this period is not lost • Most noticeable when the users can see each other • Digital Calls Add 1 Sec for each call made • Significant accumulation of time (about 20% additional overhead) • Noticeable to the users, Slows back-and-forth messages Traffic Loading in Trunked Radio

  11. Advanced Features (Bells & Whistles) • Private (or Individual) Call • Two Radios involved in a one-to-one call • Ties up a channel for the duration of the call • Some radio models will “forget” to hang up! • Telephone Interconnect • Radio Unit receives or initiates a telephone call on a landline via switch • Ties up a channel for the duration of the call • Can program a time limit • Automatic Roaming • In a multi-site (or zone) system, Radio Unit affiliates with optimum site • Software in subscriber equipment searches for & selects site • Ties up additional channels on multiple tower sites • Recommend that these features be restricted in use Traffic Loading in Trunked Radio

  12. Capacity and System Configuration • Wide Area Considerations • More than 1 Site Required to Obtain Desired Coverage Levels • Talk Group Structure and Regional Operations • User Density in Each “Region” • Requirement for frequent wide area calls • Multi-Site or Zone Configuration • Each Site (or Cell) has a unique frequency set • Each Site/ Cell has a separate Control Channel • Wide Area Calls involve at least 2 control channels & 2 voice channels • Simulcast Configuration • Optimum Solution for Dense Operations involving limited spectrum • Single Control Channel, 1 set of frequencies Traffic Loading in Trunked Radio

  13. Planning Efficient Operations • Talk Group Structure / Fleetmap Planning • Keep Talkgroups large enough to involve some traffic (20-50 units) • Don’t Assign Separate Talkgroups for every conceivable occasion • Do Create “Wild Card” Talkgroups for Tactical Situations or Joint Operations • Plan for emergencies or disasters – e.g., assign priority levels • Limit (or eliminate) Scanning (especially in Zoned Systems) • Limit ad hoc use of special talkgroups for private conversations • Interoperability with Outside Agencies • Avoid Patching Trunked Systems • Develop Agreements for cross-programming units • Prepare plans for handling joint operations (which system will host) • Trunked/ Conventional Patching – Avoid permanent or routine patches Traffic Loading in Trunked Radio

  14. System Management Issues • Trunking is Based on the Sharing of a Limited Resource, but • It Forces Disparate Organizations to Cooperate! • Effective System Management must be Authoritative • Real-time Decisions on Limiting Resources & Features • Revocation of “privileges” to individuals & organizations • Allocation of Maximum Quantities to Each Organization • System Management must limit # of radios by organization • Must Study the Impact of Adding New Groups or Radios to System • Having the Funds is no longer the only consideration • Must Study, or pay for study of, Impact on Capacity of: • Adding new features or functions to system • Converting to all digital operation • Expanding Coverage Area by Adding New Sites Traffic Loading in Trunked Radio

  15. Mobile Data Traffic Considerations • Mobile Data Traffic Loading is Fundamentally Different • Short Data Bursts lasting ~ 1 sec • All data must be received without residual errors • Immediate transmission & reception is not critical • Rarely (if ever) used for “life-and-death” calls • Channel Bandwidth is Critical to Throughput & Functions • Narrowband Channels (12.5 kHz) has a slow data rate • NPSPAC Channels (821-824 MHz) have a max. Baud rate of 9600 • 806-821 MHz Channels have a max Baud rate of 19,200 • Sharing Voice & Data on Same System is a Bad Idea (usually) • Data Channels use different “Digital Signal Processing” schemes • Exception: System has > 10 channels, excess capacity, few MDT’s • Do not plan on using a single radio for simultaneous voice & data Traffic Loading in Trunked Radio

  16. New Technologies & Capacity • TETRA • New Standard in Europe and around the World (except North America) • Uses 25 kHz channels, 4-slot TDMA with low power transmitters • 40 MHz of spectrum in Europe for Public Safety/Government (380-420) • Each Slot is a “Talk Path” • First Slot is the equivalent of a “control channel” • 4 Channels have 1 control and 19 voice/data talk-paths • 4 TETRA Channels are thus equivalent to 20 Trunked Channels today • OpenSky • Also uses 25 kHz channels, but with a 3-slot TDMA scheme • Each slot is a “Talk Path” • 5 Channel site would have traffic handling capacity of a 15 channel site Traffic Loading in Trunked Radio

  17. New Technologies & Capacity (cont.) • APCO Project 25 Phase I • FDMA (12.5 kHz Channels) • FCC has no plans to re-farm 800 MHz band (ever?) • Will not improve throughput or capacity with current FCC rules • Can be applied to other bands (VHF, UHF & 700 MHz) • Voice talk paths will be doubled initially in the 700 MHz band • APCO Project 25 Phase II • FDMA (6.25 kHz Channels) • Probably not feasible as initially envisioned • May solve with 12.5 kHz channels and 2-slot TDMA • Initial channel allocations in 700 MHz band to be in 25 kHz blocks • Voice could use 4-slot TDMA • Data may want entire 25 kHz channel (or even larger) for NCIC 2000 Traffic Loading in Trunked Radio

  18. Summary • Traffic Capacity is a Major Consideration in System Design • Extrapolating Conventional system operations to trunking is unwise • Multi-Site vs. Simulcast Configurations • System Functions & Features Can Change Capacity • Digital vs. Analog, Message Trunking vs. Transmission Trunking • Limit use of features like Private Call, Telephone, automatic “roaming” • Fleetmapping & Planning for Incidents & Emergencies • Build your Talkgroup Structure around group calls & dispatch operation • Have agreements for cross-programming with outside organizations • Management must Control Growth of System within Capacity • Mobile Data should be on separate wideband channels • New Technologies will have even more spectral efficiency Traffic Loading in Trunked Radio

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