BASE STATION CONTROL METHODS
(Source: Motorola, author unknown)
TONE REMOTE CONTROL
Paging uses a Tone Remote Control sequence called PURC TRC. PURC is an acronym for Paging Universal Remote Control, trademarked by Motorola. The TRC sequence was modified and redefined to allow a transmitter to be keyed on a specific frequency channel and to place the transmitter in the proper mode of operation (analog or binary). The mode of transmitter operation is determined by the information that follows the TRC sequence. The analog mode is assumed when the normal TRC sequence of HLGT, FT, and LLGT is sent. The binary mode of operation is assumed when no LLGT follows the FT, and instead a gap or pause of silence occurs followed by paging modem tones. Often due to noisy wireline environments, this pause gets detected as a signal and thus causes mode switches from analog to binary to not take place. Consequently the binary page goes out in the analog mode and the pager is not alerted.
HLGT is a 2175 Hz frequency tone that indicates the start of a TRC control sequence. Function Tone is used to tell the transmitter on what frequency channel to key or is used to disable a transmitter when sector paging is used. FT values range from 750 Hz to 2350 Hz in 100 Hz steps (excluding 2150 Hz and 2250 Hz). The 1950 Hz function tone is the standard key on channel 1 frequency. LLGT is a 2175 Hz tone used to tell the base station that the controller has access to the channel in the analog mode. A loss of LLGT and activity by the base station for more than 350 milliseconds will cause the transmitter to drop off the air. For more in depth timing information on the operation of PURC TRC consult the PURC Simulcast System Controller and Paging Station Controller instruction manual, part number 6881063E15.
Non-Simulcast Function Tone Key Commands for PURC 5000 stations. This option requires special TRC software and the transmitter needs to be programmed for multiple frequency operation.
Simulcast Station Function Tone Key Commands Standard PURC 5000 station TRC.
FIGURE 2. PURC Tone Remote Control Audio Flow Diagram
DIGITAL REMOTE CONTROL
Unlike TRC, which had limited control functionality, DRC is able to transfer much more information in each transmitted packet of MDC 1200. Due to error detection and correction, DRC offers substantially improved performance reliability over TRC in noise environments. Unlike TRC, which relied on "pauses" and "gaps" of silence to force mode switches, DRC simply sends a specific message identifying the desired mode of operation. DRC is therefore referred to as a positive control scheme. See Figure 3 and 4 below, which illustrate Analog and Binary DRC keying sequences. The sequences shown are for single key and dekey modes. If multiple and alternating key requests were present the dekey message would be substituted with the appropriate MDC keying message. HLGT does not need to be present since the link will remain keyed on activity.
FIGURE 3. Analog and Binary keying sequence for Non-Advanced Control DRC Paging Stations
Figure 3 illustrates the order of control information as well as relative level differences in the information. As shown above, HLGT is only required for an RF control system where it keys link transmitters and repeaters. The duration of HLGT is dependent on the number of repeaters and if Digital Private Line (DPL) is used or not. When in use, DPL is a coded identification that is included in the TRC message intended to key a certain link transmitter to insure private use. The MDC DRC messages are sent at the same level as HLGT. LLGT is sent only during an analog page to identify operation in the analog mode. The base station looks for LLGT and will drop off the air if LLGT is not detected for more than 6 seconds and no audio activity exists. This is required as a fail safe method in the event of a control channel impairment to the station (i.e., receiver, link transmitter, or link repeater failure) prior to decoding an MDC dekey message. Without this fail safe method the station could remain keyed indefinitely with dead or offset carrier. LLGT is transmitted 30 dB down from HLGT and MDC audio level. The base station will reduce the LLGT signal another 30-40 dB via a notch filter to prevent the tone from mixing with a voice page and degrading the audio quality. For an Analog transmission the audio is sent at 5 dB down from HLGT and MDC audio level. For a binary page LLGT is not sent. After a binary key message, FSK modem tones are sent until no more binary pages exist. The station senses FSK audio and will drop off the air if FSK audio is not detected for more than 6 seconds. This check of incoming FSK audio is a fail safe method to prevent a transmitter from being keyed indefinitely if the control channel to the station is impaired (from receiver, link transmitter, or link repeater failure) prior to decoding an MDC dekey message. The FSK modem tones are sent 5 dB down from HLGT and MDC audio level.
ADVANCED CONTROL DRC INFORMATION FLOW AND RELATIVE LEVEL DIFFERENCES
FIGURE 4. Analog and Binary keying sequence for Advanced Control DRC Paging Stations
Advanced Control Paging Stations are designed with a different hardware and software architecture than non-advanced control paging stations. This architecture difference imposes operating differences in the MDC audio level required for optimal station MDC decoding. The only difference between Figure 3 and Figure 4 is the MDC message level relative to HLGT and paging information audio. MDC audio is 5 dB below HLGT. MDC audio level now is at the same level as either analog or binary paging information. In the event a DDC controller is used with Advanced Control Paging Stations, the MDC audio level from the Digital Diagnostic Controller (DDC) should be decreased by 5 dB. This 5 dB reduction in level gives the station additional audio level capability. Non-advanced control stations working under this audio level protocol will be more susceptible to falsing low deviation alarms since the alarm trip point is fixed in hardware and will alarm when MDC is received at 6 dB below the HLGT level. This only gives the channel 1 dB of margin as opposed to 6 dB.
DRC MESSAGE STRUCTURE AND FORMAT
To better understand DRC 1 communications for key control and diagnostic messaging the following three DRC parameters must be discussed: the System ID, the Group ID, and the Individual Station ID. The System ID uniquely associates a paging transmitter with a DRC controller. It has 255 possible values, ranging from 0 to 254. The controller and paging transmitter must be programmed with matching System ID's if they are to communicate. Key control for paging is based on the System ID alone.
The Group ID is another ID used to differentiate the controller from the base station. The Group ID has 1024 possible values, ranging from 0 to 1023. The Group ID was implemented early on in the DRC development program for potential future expansion, however it has never been utilized. The Group ID is set by hardware jumpers on the DRC CPB board, unlike the System ID which is programmed in EEPROM non-volatile memory. A peculiarity between the Group ID and DDC controller does exist however. During diagnostic messaging, the station responses include the status of the CPB board Group ID. If the Group ID does not match the Group ID programmed in the DDC, then the message is ignored. This often creates a problem when someone accidentally changes the DDC Group ID value and then cannot figure out why the station keys and sends diagnostic messages but the controller reports a 'No Response' alarm. The CPB board ships from the factory with a Group ID of 0 and should never be changed. For the Advanced Control, ASC 1000, and ASC 1500 product, the Group ID is not a changeable value and defaults to zero (0). The Group ID concept has been cancelled in these new products. When upgrading system equipment be sure the paging transmitters have group ID's of zero (0) so that diagnostic messaging is possible with these new products.
Individual Station ID is uniquely assigned to each paging transmitter enabling communication with the controller. The Individual Station ID has 1024 values, ranging from 0 to 1023. The ID is set locally at the paging transmitter during installation. Care must be taken not to assign two stations with the same Individual Station ID since diagnostic messaging will not be possible for both transmitters. The Individual Station ID has no bearing on paging key-ups, only diagnostic or individual station testing key-ups.
The DRC 1 format has provisions for transmitters to key up on 4 frequencies and 254 sectors for both the analog and binary paging modes and transmitter test modes. The DRC 1 message format supports the following paging transmitter diagnostic messaging: forward and reflected power readings, delay line setting, station audio gain and inversion, station frequency adjust, wildcard input reads, wildcard output writes, and level 1 alarm information. Level 1 alarms are specific to the PURC 5000 paging transmitter alarm set and include, TX fault, PA fault, synthesizer out of lock, low deviation, battery revert, system timer, external 1, and external 2 alarms. For MICOR DRC stations all diagnostic messaging and alarms apply except for the station frequency adjust capability and PA fault alarm.
To better understand DRC 2 communications for key control and diagnostic messaging the following two DRC parameters must be discussed: the System ID and the Individual Station ID. The System ID has 16 possible values, ranging from 0 to 15. The System ID uniquely associates a paging transmitter with an ASC 1500 DRC controller. The controller and paging transmitter must be programmed with matching System ID's if they are to communicate. Key control for paging is based on the System ID alone. As mentioned in the DRC 1 section, the Group ID parameter has been cancelled for DRC 2 messaging. When converting a system from DDC or ASC 1000 controller operation to ASC 1500 controller, the System ID must be within the 0 to 15 range. Use the original controller to change the station System ID to a value between 0 and 15.
The Individual Station ID is uniquely assigned to each paging transmitter enabling individual communication with the controller. The Individual Station ID has 1024 values, ranging from 0 to 1023. The ID is set locally at the paging transmitter during installation. Care must be taken not to assign two stations with the same Individual Station ID since diagnostic messaging will not be possible for both transmitters. The Individual Station ID has no bearing on paging key-ups, only diagnostic or individual station testing key-ups.
The DRC 2 format has provisions for transmitters to key up on 4 frequencies and 254 sectors, for both the analog and binary paging modes and transmitter test modes. The DRC 2 message format can support all alarm and diagnostic information provided by DRC 1 messaging however it is communicated in a much more efficient manner. The DRC 2 format enables new messaging required for Advanced Control and ASC 1500 systems, which could not be, implemented with the DRC 1 format.
KEY ON DATA
DIRECT DIGITAL CONTROL
Direct Digital Control is currently used for digital satellite distribution channels. The ASC 1500 and Advanced Control paging stations are required for this type of system operation. The ASC transmits MDC in digital form at 1200 baud and then passes or generates paging terminal data in the digital form as well. In this mode the ASC1500 expects data from the terminal at RS232 levels and outputs the data at RS232 levels. Pass through digital transmissions are restricted to baud rates of 300, 600, 1200 and 2400 baud. The ASC when used with an MBPS 2000 terminal can be configured to generate the paging information. In this manner the ASC acts like the terminal output card. Paging data is user selectable for TTL or RS232 output levels. The ASC also provides handshake signals such as Request-To-Send (RT.) and Clear-To-Send (CTS), to be used for interface with the external Digital transmission equipment for the satellite uplink.
|END OF SECTION|
|Home Page||Directory||Consulting||Newsletters||Free Subscription||Products||Reference||Glossary||Send e-mail|