TM8000 Mobile Radios: TN-855-AN TM8000 and TB7100 Data Modem Facilities

Overview

A DTE connects to the radio modem via a serial connection and will send and receive serial data in a standard asynchronous fashion. The radio then processes this data and sends it over-air using a synchronous protocol, sending data in a continuous bit stream until there is no more data available to be sent.

The radio enters THSD mode either via a Computer Controlled Data Interface (CCDI) command, when programmed to do so at powerup or via a programmed input line.

When a radio is in THSD mode, the radio keys up the transmitter as soon as it detects data from the DTE. It then formats the data according to the configuration (layer-1 or layer-2, Forward Error Correction (FEC) or no FEC) and after the programmed lead-in delay, passes that data to the modem to be encoded for over-air transmission.

The receiving radio decodes the modulated data and passes it on to the control-software. Here the data gets processed according to the configuration (layer-1 or layer-2, FEC or no FEC) and user data is then passed on to the connected DTE. When the sending radio has no more data to send, it automatically keys down the transmitter.

Modulation Scheme

The THSD modem uses a CP-4GFSK modulation scheme.

Like C4FM, this is a four-level FSK scheme, but can achieve higher data rates while maintaining compliance with ETS300–113 ACP. This scheme is a form of minimum shift keying, and hence is a subset of MSK.

A good explanation of CPFSK can be found in chapters 6.5 to 6.10 of “Communication Systems 4th Edition" by Simon Haykin.

[Diagram: Eye diagram showing signal quality for 4FSK modulation.]

Over-Air Protocols

Layer 1

Layer 1 mode requires a good understanding of transmission timing. The nominal terminal port speed (asynchronous) must be at least 20% higher than the over-air data speed (synchronous). Depending on the amount of data and the programmed lead-in delay, it may also be important to enable terminal flow control. Refer to “Flow Control" on page 14.

If at any stage the transmitting radio runs out of data to transmit, idle characters are inserted. Idle characters may also be appended to fill the last THSD frame.

As the data has no format, the receiving radio cannot strip out the inserted idle characters, as it cannot distinguish between valid data and idle data. It is up to the DTE to handle detection and the removal of the idle characters.

Layer 1 with FEC

Protocols using forward error correction (FEC) provide better over-air data integrity than protocols without FEC. Data is packetised into lots of 12 bytes and a 9-byte calculated parity is added to each packet to form one FEC code word. The receiving radio receives a code word and uses the data and parity information to correct any detected errors. Again, idle characters may be inserted into and/or appended to the data stream and passed on to the receiving terminal. Usually the last code word contains idle data to fill up the 12 byte data block.

Layer 2

Layer-2 adds a header containing a start-of-data sequence and a data-length byte. This header is pre-pended to a block of data of the given data length.

When the receiving radio detects the start-of-data sequence it examines the length and then passes the indicated number of bytes to the terminal. It then ignores all incoming data until the next valid start-of-data sequence is detected. So in essence, layer-2 is a data filter that strips out non-user data.

The transmitting radio packetises only data that it already holds in its input buffer from the terminal. If the buffer runs empty, idle data is sent, which the receiving radio removes and does not pass on to the terminal. As a result, a slower terminal speed is no longer an issue as it is with fully transparent mode. However, if data arrives at the sending radio too slowly, the radio may decide that there is no more data and key down the transmitter. As soon as it detects new data the transmitter is keyed up again and data transmission commences after the lead-in delay.

Layer 2 with FEC

This combines the above two features: FEC provides a high level of data integrity and layer-2 filters out non-user data at the receiving radio. Layer-2 uses the first of the 12 data bytes as a length byte (1 to 11 data bytes).

THSD Over-Air Data Format

Basic Modem Data Structure

A THSD frame consists of sync sequences and data blocks. The frame time is 64.4ms at 19.2kbps (wide-band) or 103ms at 12kbps (narrow-band).

Data Format: No Layer-2, no FEC

The format includes a sync sequence, user data, another sync sequence, more user data, and idle characters to fill the remainder of the frame. The effective user-data rate is 95%.

Data Format for Layer-2, no FEC

This format includes a layer-2 header with a start-of-data indication and a data length, followed by user data blocks. The effective modem user data rate ranges from 6.3% to 88% depending on the data block size.

Data Format with FEC

A THSD frame with FEC contains multiple codewords, each including data and parity bits for error correction. The effective user-data rate is 54.4% without layer-2 and 49.8% with layer-2.

[Diagram illustrating the structure of THSD frames and data formats with and without FEC and Layer 2.]

Scrambling

In The THSD implementation, data is scrambled before it is packed and sent over-air. This introduces a controlled randomness to aid decode performance.

Synchronisation

Synchronisation (sync) sequences are added into the data stream. The sync sequence that occurs every 588 symbols is the same as the start sync sequence.

Synchronisation allows the receiving radio to synchronise its symbol clock with the incoming data. This reduces the possibility of data errors when the transmitting and receiving radios are not exactly on the same frequency due to frequency drift or ageing.

The sync sequence is inverted if the modem is in FEC mode which tells the receiving radio to expect data packets with FEC data added.

Rate Changer

The rate changer has the ability to detect changes within the VCXO frequency when receiving a signal, and can adjust the DSP accordingly so that a constant 48K sample rate is achieved.

This effectively tracks the VCXO frequency to offset the effects of ageing, temperature and “Birdie” channels, providing a reliable data decode.

Overview

A 1200 baud modem is also provided as standard for data transfer. The modulation scheme for the 1200-baud modem is FFSK. Data is represented by one cycle of 1200 Hz (logic 1) or 1.5 cycles of 1800 Hz (logic 0), and is transmitted at rates of 1200 bps.

It is important to note that while the FFSK format is widely used, it is unlikely that a third party FFSK modem is compatible with the Tait 1200 baud FFSK implementation. This is mainly due to the proprietary way in which the data is packed and sent over-air.

Data is presented to the modem in the same fashion for THSD as it is for FFSK. Refer to “DTE Considerations” on page 5.

Over-Air Protocol

The FFSK over-air protocol includes a Lead In Delay, Preamble Field, Sync Field, Length Field, Data Block, Dummy CRC, and Lead Out Delay.

[Diagram: Timing diagram for FFSK transmission, showing the transmitter key-up, data transmission period, and lead-out delay before key-down.]

Physical Connections

TM8000: Requires an RS-232 options board (TMAA01-02) for standard RS-232 connection. The front microphone serial port can also be used for programming.

TB7100: Features a 9-way D-range socket providing Standard RS-232 levels. The "Aux" port is recommended for serial connection; "Mic" and "Internal options" ports have limitations.

Baud Rate

Serial port baud rates can range from 1200 to 28800bps. Considerations include matching the serial port rate to the over-air rate to avoid delays or buffer overflows.

Flow Control

Provides a mechanism to stop serial data transmission from the DTE during lead-in delay or when the serial port buffer is full. The DTE can also use flow control to stop data from the modem.

TM8000: Supports Hardware (RTS/CTS) and Software (Xon/Xoff) flow control.

TB7100: Supports Hardware flow control (CTS/RTS) via resistor links; does not support software flow control.

Serial Port Buffer

TM8000 and TB7100 have a 512-byte buffer for data received from the DTE and a 128-byte buffer for data received over-air.

Uart Write Delay

A programmable option (0-500ms) to buffer received serial data, creating a constant stream and mitigating small gaps that can occur during data conversion.

Serial Port Protocol

Data is sent with start bit, 8 data bits, stop bit, no parity. Uses Tait Proprietary CCDI protocol with Command Mode and Transparent Mode.

Command Mode: Allows driving the radio and receiving status via ASCII commands (e.g., "g0223D2"). Limited support on TB7100.

Transparent Mode: Data is sent and received without specific formatting. Suitable for higher-level protocols. Can use 1200 baud FFSK or THSD.

[Diagram illustrating transparent mode data flow between a DTE and a radio modem via serial cables.]

Configurations

Mobile to / from Mobile: DTE to DTE via TM8000 modems. Supports Point to Point/Multipoint, Semi Duplex. Suitable for FFSK or THSD.

[Diagrams showing system configurations: Mobile to Mobile.]

Mobile to / from Base Station (RS-232 out): Uses TB7100 base for higher duty cycle and transmit power. Recommended for direct data routing. Supports Point to Point/Multipoint, Semi Duplex. Suitable for FFSK or THSD.

[Diagrams showing system configurations: Mobile to Base Station.]

Data Repeater (serial loop): Data looped back from TB7100 receiver to transmitter. Supports Point to Point/Multipoint, Semi Duplex. Suitable for FFSK or THSD. Note potential data echo.

[Diagrams showing system configurations: Data Repeater (serial loop).]

Data Repeater (Talk Through voice and data): TB7100 acts as a talk-through repeater. NOT suitable for THSD. Supports voice and FFSK data, Semi Duplex, Point to Point/Multipoint.

[Diagrams showing system configurations: Data Repeater (Talk Through voice and data).]

Throughput

Compares effective data rates for THSD and FFSK modems, noting that FEC reduces effective throughput. Tables detail various test parameters and their impact on system and over-air bit rates.

Turnaround times

Provides data on polling operations, comparing turnaround times for different modems and configurations. Tables show time taken for poll requests and responses.

Voice and Data systems

THSD: Not suitable for simultaneous voice and data. Requires switching back to voice mode.

FFSK: Can be used for voice and data on the same channel. Suggests separating voice and data channels for better performance.

Bit error rates (BER)

Compares BER for FFSK and THSD modems under various conditions (channel spacing, FEC). THSD with FEC shows improved performance. Includes sensitivity data and BER graphs.

[Graphs comparing Bit Error Rate (BER) versus carrier level for THSD modems with and without FEC, and for FFSK modems.]

Retries

Neither modem performs automatic retries for missed or incorrect packets; this functionality must be handled by the DTE via a higher-level protocol.

Error Checking and Correction

FFSK: No true error checking/correction; relies on third-party applications. TM8000 has a "Check Packet Length" option for limited error checking.

THSD: Provides FEC (Reed-Solomon algorithm) for improved performance at lower signal levels, though it reduces effective throughput.