Motorola's SmartNet Systems

Trunking is a method of sharing a small number of communication paths among a large number of users. Trunking is an old telephone company term, which was originally referred to the sharing of a bundle of copper telephone lines (perhaps looking like a tree trunk!) between telephone switching centers.

Imagine 4000 telephone customers in each of two cities that share these trunked lines. It would require 4000 copper lines between the citys if they do not want to experiance a busy condition. This is an inefficient way of connecting the cities because:

• Most calls are relatively short.
• It is statistically improbable that all telephone users will be engaged in a non-local call at the same time.

That is why it is possible to statistically determine the minimum number of lines, which would result in an acceptable "busy condition" probability.

The advantages that trunking offers for the telephone systems also apply to radio systems. In two way radio communications, trunking refers to the automatic, dynamic sharing of a small number of radio channels between a large number of users. Trunking system efficiency distributes message traffic among the available channels and reduces channel-waiting time.

Conventional vs. Trunking Radio Systems

Drawbacks of Conventional Radios
A conventional radio system does not offer the most efficient use of the limited available frequencies.

Typical problems of conventional radio systems are:

• Uneven traffic loading on radio channels.
• Some channels are over utilized and blocked.
• Lack of privacy (more than one user group can listen to a conversation).
• No redundancy in case of failure.
• All users must monitor the channel for traffic before transmitting.

Trunked Radio Systems

Advantages of Trunking
Trunked radio offers many advantages over conventional radio and other forms of wireless communication, such as cellular telephones. These advantages include:

• Faster system access: No need to monitor the channel.
• Better channel efficiency: All users share all channels, which decreases channel congestion.
• User privacy: Users in the same talkgroup are assigned exclusive use of a voice channel for the duration of a conversation.
• Types of communications: Users can initiate a wide variety of calls including one-to-one, one-to-talkgroup, and one-to-all.
• Queuing: Provides a more orderly approach to system utilitization.
• Priority levels: Ensure that critical users have more immediate access to the system.
• Flexible expansion: Talkgroups can be added without necessarily adding channels or modifying existing radios.

Loading Criteria: The FCC considers 100 subscribers (users) per channel as a fully loaded channel. A trunked system licensed by the FCC must be constructed within the first year. The system must be 70% loaded with in the first five years of the license. A five-channel system will have a minimum of 350 subscribers to show the 70% minimum loading. A 10-channel system will have 700 units (portable/mobile/control stations) within 5 years. The FCC part 90 states the balance of the licensed frequencies will automatically be relinquished if the loading quantity and times are not met.

Trunking Protocols

• An Addressing Scheme: The simplified three step call flow demonstrates how in every step of the call flow, the mobiles and the Central Controller need to use IDs to identify appropriate radios or user groups. The process of assigning IDs is referred to as "Fleet Mapping".
• A Signaling Scheme: The process of sending messages to convey actions to be taken by either the radio or the Controller is called "Signaling".

With the development of Motorola Trunking two methods, or protocols, have evolved: Type-I (SMARTNET) and Type-II (SMARTNET-II). In 1977 Motorola developed a protocol for their first trunking system that assigned digital addresses to specific radios. In 1987, to add more features such as capacity for more radios and group IDs, Motorola developed a new trunking protocol. It was called Type-II, and the original protocol became known as Type-I.

Each method uses different addressing (Fleet Mapping) and control (Signaling) schemes. Since Type-II offers more features, it is now the standard protocol on new trunking systems. For example, 900 Mhz and UHF systems use only Type-II. The MTC 3600 Central Controller is only available for Type-II.

Only a few strictly Type-I systems are still in existence. To allow customers to maintain the original Type-I subscribers, Motorola developed their system to work with both Type-I and Type-II. These combined systems are called Hybrid Systems. This allows customers with Type-I radios to add Type-II radios and take advantage of Type-II features.

Type-I and Type-II Fleet Mapping Differences

• Type-I Systems:

  The original protocol, which became known as Type-I, divides the system into groups called Fleets. Each Fleet is further divided into smaller groups known as SubFleets. SubFleet calls are more common than Fleet-wide calls. A SubFleet call is heard only by radios in the same SubFleet. A Fleet-wide call is heard by all radios in the Fleet.

  Each radio is assigned a unique digital address that identifies it as a member of a particular Fleet. Using this address, the radio can make group calls in any of the SubFleets affiliated with its assigned Fleet.

  The key to understanding Type-I is to remember that every radio is assigned a Fleet and a unique ID within the Fleet. To address a particular radio, the Central Controller must indicate the radio's Fleet number and unique ID within the Fleet. Together, a Fleet number and ID give a radio a unique digital address within the system.

  The maximum number of SubFleets and radio IDs a Fleet can contain is fixed, though the maximum varies between trunking systems based on each system's "Size Code" mapping (Size Codes are described in Type-I and Type-II Fleet Mapping modules).

• In Type-II Systems:

  The two main levels of groups are Multigroups and Talkgroups. Multigroups are comparable to Fleets, and Talkgroups are similar to SubFleets. Multigroups are groupings of Talkgroups, just as Fleets may be considered groupings of SubFleets.

  Type-I and Type-II have one major difference. A Type-I radio ID includes the ID of the Fleet with which the radio is affiliated. But a Type-II radio ID does not identify any group, so radio and group IDS can be assigned independently.

  In Type-II, any number of radios can be assigned to a group, whereas Type-I limits the number of radios in a Fleet. However, in a Hybrid System, any number of Type-II (interoperable) radios can be added to a Type-I Fleet.

  Similarly, for Type-II radios, talkgroup calls are more common than Multigroup calls. A talkgroup call is heard only by radios in the Talkgroup. A Multigroup call is heard by all radios selected to Talkgroups within the Multigroup.

Subscriber/Central Controller Affiliation Tables

The following is applicable to Type-II only. The relationship between individual ID and Fleet ID is inherent in Type I Fleet-Mapping/Signaling. With Type I, the Individual ID, SubFleet and Fleet information is predetermined and is contained in one ISW.

Central Controller: The Central Controller is programmed with a static list of individual ID's. The Central Controller has no pre-programmed listing of relationships between talk groups and announcement groups. A dynamic list of talk group Affiliations is initially empty until the Subscriber Radios inform the Central Controller of the talkgroup that they want to operate in.

Subscriber: The radio informs the Central Controller of the current talk group it is operating in by Affiliation. The Affiliation update may be performed by two methods, depending on how the radio was programmed to Affiliate:

• Push-to-talk: The operator changes the talk group it is working in. When the PTT is depressed the radio will send in a dual Inbound Signaling Word (ISW). The first word contains the Individual ID and call type; the second word contains the updated talk group ID. ISW's will be described within the Trunked System Signaling pages.
• Auto: The operator changes the talk group it is currently operating in. A few seconds after the change, the radio will automatically send in an ISW to update the Central of the new talk group ID. When the operator depresses it's PTT it will now only need to send in a single word ISW with it's Individual ID and call type. This will be a faster access time. If the operator depresses it's PTT prior to time set to send in the new Affiliation, the radio will use PTT Affiliation to update the Central Controller.

Type-I and Type-II Signaling Differences

The major differences between Type-I and Type-II signaling schemes are:

• For Type-I operation, following the channel assignment, the Central Controller sends a High Speed Handshake signal on the newly assigned voice channel. The Central Controller sends this signal to make sure that the originating subscriber has correctly switched to the assigned channel. If the subscriber switches to the correct channel, it will send an Acknowledge tone signal to let the Controller know that it has switched correctly. In Type-II operation these steps are deleted and the Controller assumes that the subscriber has switched to the correct voice channel.

• In Type-II systems, unlike the Type-I systems, the Talkgroup ID is not always sent to the Controller. The radio may send its selected Talkgroup to the Central Controller when it is powered up. The Central Controller upon receiving a call request refers to its Talkgroup Affiliation Table and looks for the affiliated Talkgroup ID. If for some reason, the Central Controller does not find the Talkgroup ID, it sends an Affiliation request to the subscriber. The subscriber, in return, sends in its Talkgroup Affiliation. The call request in Type-I systems always includes the SubFleet ID information.

Basic Trunked System Elements

A basic 5-channel (up to 28 channels are possible) system configuration is centrally located in the area requiring radio coverage. For simplification only 2 Multigroups (or Fleets) and 3 Talkgroups (or SubFleets) are shown.

Each Multigroup may employ a remote or local control station (Dispatcher). A remote dispatcher can operate via an RF channel. A local dispatcher (local to the Controller) using CENTRACOM for example, is directly connected to the Central Controller.

The system may be equipped with local or remote network management tools.

System users can be connected to the Public Switched Telephone Network (PSTN) for telephone access using a Central Interconnect Terminal (CIT) connected to the Controller.

SMARTNET trunked systems may include ASTRO digital equipment.

Central Controller Description

• Control Subsystem:
  • Monitoring and controlling each call sequence:
    • Has a call been received?
    • Is a voice channel available?
    • Has a channel assignment been made?
  • Monitoring voice channel activity:
    • Is a particular channel available for reassignment?
  • Selecting and assigning vacant channels as required.
  • Controlling base repeater stations:
    • a) Keying the repeaters when assigned.
    • b) Rotating the control channel frequency on a daily basis so as to equalize equipment aging.
  • Provide a system manager interface through an optional keyboard terminal.
  • Monitoring and reporting of alarm conditions.
• Receive Subsystem:
  • Recovery and decoding of inbound signaling requests (subscriber unit originated service requests).
  • Monitoring of the sub-audible tones used in voice channel communications.
  • Monitoring of alarm conditions and reporting through the control subsystem.
• Transmit Subsystem:
  • Generating and encoding outbound signaling words for such purposes as directing system users to specific channels.
  • Generating the sub-audible data, which is superimposed on all voice communications and is used to unmute the audio circuitry in receivers authorized to monitor the communications.
  • Generating the base station identification.
  • Monitoring of alarm conditions and reporting through the control subsystem.

Central Controller Programming

The basic operating system requires a certain amount of customer supplied information for proper programming at the factory prior to shipment to the customer. This information is unique to each system and is required to meet FCC regulations and allow correct operation.

• RF channel identifiers: The identity of each channel frequency and the repeater position assigned to it in the trunked system must be programmed into the system Central Controller.
• Base station identification: The system call sign is automatically transmitted in morse code once every 30 minutes in compliance with FCC regulations. The alphanumeric system identification must be programmed into the system Central Controller. (This is sent in Morse Code at 17 Word Per Minute and 900 Hz tone modulation).
• System BSI (Base Identification System) Channel: Such station identification shall be made on the lowest frequency in the base station trunk group assigned the licensee.
• System control channels: The identity of those channels in the trunked group, which can be used for the system control channel, must be programmed into the system Central Controller. Four channels are so designated.
• Automatic midnight rollover: the Central Controller rotates the control channel.
• Auto test: (Optional) the Central Controller resets and does a complete diagnostic test.
• Type of Trunking:
  • Transmission trunking: No hang time.
  • Message trunking: Has a programmable hang time.
• Connect Tone: One of eight selected by Motorola
• Telephone Interconnect: Designated interconnect repeaters
• Console configuration: Analog, ASTRO or Mixed Analog with ASTRO (Type I, Type-II or Hybrid)

Functional Block Diagram

Voice channel to be activated (unmuted) needs connect word and connect tone (105.8hz) handshake. Connect word is binary word that is transmitted withthe audio message at 150 baud rate. Connect word specifies a particular group or possible groups.

In order to demonstrate how Motorola Trunked Central Controllers operate, a single-site trunked MC-6809 Central Controller block diagram, shown below, is used. To understand how an MTC-3600 Central Controller operates, in is necessary to have an understanding of the MC-6809 configuration. Other Controller configurations will be described also.

The major Controller blocks can be divided into three CPU boards and three interface boards. I will present a detailed description of these boards in the MC-6809 Central Controller pages and cover the MTC-3600 within the MTC-3600 Central Controller pages. The System Management Terminal and Alarms communicate attach to the Port Asynchronous Module (PAM) to operate the MCP750 System Board which controls the functioning of the Central Site Controller.

These Central Controller Boards are:

• Control interfaces to the Transmitters and Receivers Channels:
  • Transmitter Interface Board (TIB): Interfaces the Central Controller to the voice and control transmitters.
  • Receiver Interface Board (RIB): Interfaces the Central Controller to the voice and control receivers.
  • Inbound Recovery Board (IRB): Interfaces the Central Controller to the control channel receivers. Each Controller can have only one IRB, which works in conjunction with up to four control channels (The first four system channels can be used as the control channel).

  Each RIB and TIE accommodate up to seven channels. A maximum of 4 TIBs and 4 RIBs are allowed per Controller which provide for up to 27 voice channels and one control channel.

• Controller CPUs are:
  
  • Central Site Controller (CSC): The CSC is the main computing center for the system. The CSC accepts input from the RSC, decides upon the appropriate course of action, and tells the TSC what to do.
  • Receiver Site Controller (RSC): The RSC operates under CSC control, interfacing the CSC with the IRB and RIB(s).
  • Transmitter Site Controller (TSC): The TSC operates under CSC control, interfacing the CSC with the TIB(s).

  Hardware-wise, the three CPUs are identical. All configurations use a basic Site Controller Board (SCB) with different PROM, RAM, and PAL chips inserted. In addition, jumper configurations can vary between the SCBs.

Interface Connections

The Central Controller control data connections are:

• Transmitter Interface Board:
  • TSTAT (Transmitter STATus): 0 volts = No Power, >3 volts = Power
  • TDATA + (Transmit DATA +): Digital signal
  • TDATA -(Transmit DATA -): Return for TDATA+
  • PTT (Push To Talk): 13.8 V = Not Keyed, 0.75 V = Keyed
  • Mute: HI = Unmute, LO = Mute. 'Tickles' a negative 2 volt, one milisecond pulse every 30 seconds to keep base repeaters trunked.
• Receiver Interface Board:
  • RSTAT (Receiver STATus): 0 volts = Squelch, >3 volts = Unsquelch
  • DISC (DISCriminator): Receiver discriminator output (audio)
• Inbound Recovery Board:
  • CCI: Control Channel Indicator - Low level to indicate
  • DISC (DISCriminator): Receiver discriminator output (audio)

Base Station/Repeater Description

The assignments for the Control Channel and Voice Channels are:

• Control Channel:
  • Normally one of the four highest frequencies is used as the Control Channel.
  • The Control Channel is used to transmit and receive data and is always keyed while assigned.
  • All data words on the Control Channel contain sufficient information to address particular radio units and specify actions to be taken.
  • This information is assembled into a coding format that has sufficient error correction and detection capability to ensure reliable communications.
  • The encoded binary data is passed to the transmitter at a baud rate of 3600 where it is filtered and impressed on the carrier.
• Voice Channels:
  • A Voice Channel is used to transfer normal Voice and Data (not control signaling).
  • The selection of Voice Channels for assignment will be made in a numerical sequence based on the next RF interface available.
  • Voice Channels are only keyed up when they are needed. The Central Controller tabulates the key-up times.
  • Continuous sub-audible tones accompany all mobile or portable originated transmissions.
  • Base to mobile transmissions contain a sub-audible digital signal.

Subscriber Description

A subscriber refers to a mobile, portable, or a control station.

• The Operational Differences between a subscriber in a conventional system and a subscriber in a trunked system are:
  • Elimination of the squelch control.
  • Elimination of the need for channel monitoring before transmitting.
  • Generation of a busy tone by subscribers when the system is busy.
  • When a channel becomes available after a busy, the subscriber may produce a talk permit tone.
• The Trunking Logic circuitry carries out the following functions:
  • Seeks out and remembers the identity of the System Control channel.
  • Upon activation of the microphone PTT button, the mobile encodes and transmits inbound service requests to the System Central Controller.
  • Automatically switches to receive mode to decode and respond to outbound commands sent by the System Central Controller.
  • Automatically changes the subscriber unit's channel.
  • Generates sub-audible connect tone to maintain the system connection.
• In response to data and/or signaling from the Central Controller, the subscriber unit will:
  • Generate and sound alert tones
  • Enable and disable its transmitter
  • Mute and unmute its transmitter audio
  • Mute and unmute its receive audio.

Trunked System Configurations

The trunked system configurations are:

Single Site System -
Designed for campus or manufacturing applications:

Single Site Systems: Single site trunked systems are the simplest form of trunked systems. They contain one transmitter and one receiver per channel.

Voting System -
Applicable when a single transmitter site has full coverage:

Voting Systems: RF radio systems suffer "dead spots" in their coverage area - places where a field radio of limited transmitter output power, cannot effectively reach the established network of communications. "dead spots" could be caused by a building, a densely wooded area, a deep valley, or an inconveniently located hill - - in short, by any environmental factor that can adversely effect RE propagation. Another cause of system "dead spots" is the varying talk-out range of system radios. In a Voting system, satellite receiver sites are strategically placed to balance coverage throughout the system coverage area - - removing the "dead spots" that would otherwise be present.

Simulcast System (MTA's choice for Los Angeles, California)-
Up to ten overlapping sites each with identical frequency configuration:

Simulcast Systems: In a simulcast system, identical carrier signals are transmitted from multiple, separately located sites, requiring additional equipment and special control of the transmit frequency and phasing. A simulcast subsystem is used in areas where channels are limited, terrain is rugged, or the service area is large. The system shown above is a simplified simulcast five channel configuration (up to 28 channels available), with base transmitters and receivers installed at different geographic locations.

AMSS System(Automatic Multiple Site Select) -
Up to ten sites with each site having different frequencies:

AMSS Systems: A simplified AMSS (Automatic Multiple Site Switching) has 5 channel (up to 28 channels available) on a system configuration. AMSS differs from simulcast in that different channel frequencies are used at each site. This concept requires less additional equipment and control than the simulcast concept at the expense of frequency usage.

SmartZone System-
Multiple Simulcast sites and/or single sites, all for additional coverage and/or capacity:

SmartZone Systems: SmartZone is a multiple subsystem trunking system expanding on SMARTNET-II features and introducing new trunking features. This system provides extended coverage and channel resource efficiency. The multiple subsystem, or zone, design extends coverage of the entire system. Zones can incorporate different subsystem configurations to cover larger geographical areas, such as a region, county, state, or country. This multiple subsystem design also uses a variable number of repeaters at each of the sites to handle different traffic loading requirements. A low traffic area has fewer repeaters than an area with a high volume of traffic. This feature, along with Dynamic Site Assignment, allocates channel resources efficiently for communications.

OmniLink System-
Multiple SmartZone sites for extended coverage and/or capacity.

SMARTNET can support analog and/or digital voice communications. Three primary counsol configurations are supported:

• Analog only
• Mixed analog and ASTRO (Motorola speak for DIGITAL):
  ASTRO systems rely on new technology that digitizes voice and interleaves it with digital signaling. SMARTNET systems using ASTRO channels add the capabilities unique to ASTRO, such as imbedded signaling, ASTRO multikey secure, consistent audio quality over the coverage area and 12.5 kHz channel width where feasible.
  ASTRO trunked systems are discussed in ASTRO Trunked Systems Module.
• ASTRO only (Another Special Term for Radio Officionados)

Trunked Frequencies

The attributes of the 806 Mhz band are:

• Standard channel Spacing: 25 kHz
• Max. Deviation (voice + data): ±5 kHz
• Transmit/Receive Separation: 45 Mhz
• Low Speed Data Deviation: 1 kHz
• High Speed Data Deviation: 3 kHz
• Frequency Allocation:
  • a) 806.0125-824.98775 Mhz for Mobile Transmit/Base Receive
  • b) 851.0125-869.9875 Mhz for Mobile Receive/Base Transmit
• Repeaters on a system should be 1 Mhz apart.

The attributes of the 896 Mhz band are:

• Channel Spacing: 12.5 kHz
• Max. Deviation: + 2.5 kHz
• Transmit/Receive Separation: 39 Mhz
• Low Speed Data Deviation: 500 Hz
• High Speed Data Deviation: 1.5 kHz
• Frequency Allocation:
  • a) 896.0125-902 Mhz for Mobile Transmit/Base Receive
  • b) 935.0125-941 Mhz for Mobile Receive/Base Transmit
• Repeaters on a system should be 1 Mhz apart.

Assignment of frequencies in UHF and VHF bands for trunking purposes is not a structured process. Prior allocation of VHF/UHF frequencies for conventional use makes it impossible to assign a continuous blocks of transmit and receive frequencies. When a client gets a set of frequencies assigned for use in a trunking system, they may be assigned in one to three blocks, each block separated by several megahertz. The diagram shown is an example of three block assignment.

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