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CMX969P4

MOTIENT/ARDIS RD-LAP MDC4800 Modem

MX-COM Inc  

MOTIENTSM/ARDISSM RD-LAPTM MDC4800 Modem

9

CMX969 Advance Information

4.2 Modem - µC Interaction

In general, data is transmitted over-air in the form of messages, or 'Frames', consisting of a 'Frame Preamble'

followed by one or more formatted data blocks. The Frame Preamble includes a Frame Synchronization

pattern designed to allow the receiving modem to identify the start of a frame. The following data blocks are

constructed from the 'raw' data using a combination of CRC (cyclic redundancy checksum) generation,

Forward Error Correction coding and Interleaving. Details of the message formats handled by the modem are

given in Section 4.4 and Figure 5 and Figure 6.

To reduce the processing load on the associated µC, the CMX969 modem has been designed to perform as

much as possible of the computationally intensive work involved in Frame formatting and de-formatting and -

when in receive mode - in searching for and synchronizing onto the Frame Preamble. In normal operation the

modem will only require servicing by the µC once per received or transmitted block.

Thus, to transmit a block, the controlling µC has only to load the - unformatted - 'raw' binary data into the

modem's Data Block Buffer then instruct the modem to format and transmit that data. The modem will then

calculate and add the CRC bits as required, encode the result as 2 or 4-level symbols (with Forward Error

Correction coding) and interleave the symbols before transmission.

In receive mode, the modem can be instructed to assemble a block's worth of received symbols, de-interleave

the symbols, translate them to binary - using the FEC coding to correct as many errors as possible - and

check the resulting CRC before placing the received binary data into the Data Block Buffer for the µC to read.

The modem can also transmit and receive un-formatted data using the T4S, T24S, R4S, T8B, T40B and R8B

tasks described in Section 4.5.4. These are normally used for the transmission of Symbol and Frame

Synchronization sequences. They may also be used for the transmission and reception of special test

patterns.

4.3 Binary to RD-LAP 4-Level Symbol Translation

Although the over-air signal, and hence the signals at the modem TXOUT and RXIN pins, consists of 4-level

symbols in RD-LAP mode, the raw data passing between the modem and the µC is in binary form. Translation

between binary data and the 4-level symbols is done in one of two ways, depending on the task being

performed.

Direct: the simplest form, which converts between 2 binary bits and a single symbol, such as the 'S' Channel

Status symbol.

Symbol

+3

+1

-1

-3

MSB

1

1

0

0

LSB

1

0

0

1

This is expanded so that an 8-bit byte translates to four symbols for the T4S, T24S and R4S tasks described

in Section 4.5.2.

Bits:

Symbols:

MSB

7

6

a

sent first

5

4

b

3

2

c

LSB

1

0

d

sent last

With FEC: This is more complicated, but essentially translates groups of 3 binary bits to pairs of 4-level

symbols using a Forward Error Correcting coding scheme for the block oriented tasks THB, TIB, TLB, TSID,

RHB, RILB and RSID described in Section 4.5.4.

¤2001 MX-COM, Inc.

www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054

Doc. # 20480211.002

4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All trademarks and service marks are held by their respective companies.

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