January 18, 2001Traffic Loading in Trunked Radio Arizona APCO/NENA State Training Conference Traffic Loading Considerations in a Trunked Two-Way Radio System Ken Ballard, Ph.D.

January 18, 2001Traffic Loading in Trunked Radio2 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)

January 18, 2001Traffic Loading in Trunked Radio3 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

January 18, 2001Traffic Loading in Trunked Radio4 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

January 18, 2001Traffic Loading in Trunked Radio5 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

January 18, 2001Traffic Loading in Trunked Radio6 Typical Erlang C Model Results

January 18, 2001Traffic Loading in Trunked Radio7 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 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 1 st PTT –PTT-ID is a hybrid where each caller has ID attached –Must be used for some kinds of data calls

January 18, 2001Traffic Loading in Trunked Radio8 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 1 st PTT) –1 message/hour per radio –1 sec Maximum Acceptable Call Delay –1% Delayed Call Grade of Service

January 18, 2001Traffic Loading in Trunked Radio9 Impact of Digital Voice on Traffic Channel Access (0.5 sec) PTT Digital Latency (1 sec) Caller 1 Voice Caller 2 Hears Audio Caller 2 Voice Digital Latency (1 sec) Caller 1 Hears Audio

January 18, 2001Traffic Loading in Trunked Radio10 Impact of Digital Voice on Traffic System Access Time –Time between the PTT and the Channel Grant –Typically ranges between 0.25 and 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

January 18, 2001Traffic Loading in Trunked Radio11 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

January 18, 2001Traffic Loading in Trunked Radio12 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

January 18, 2001Traffic Loading in Trunked Radio13 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

January 18, 2001Traffic Loading in Trunked Radio14 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

January 18, 2001Traffic Loading in Trunked Radio15 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 ( MHz) have a max. Baud rate of 9600 – 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

January 18, 2001Traffic Loading in Trunked Radio16 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 ( ) –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

January 18, 2001Traffic Loading in Trunked Radio17 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

January 18, 2001Traffic Loading in Trunked Radio18 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