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TM
COMSPHERE 3800Plus Modems
Synchronous Data Compression Feature
Document Number 3980-A2-GZ40-20
November 1996
Overview
The Paradyne proprietary Synchronous Data Compression (SDC) feature allows an increase of throughput in data
transmission, and has an error correcting procedure so that transmitted data is less sensitive to channel
disturbances.
The ITU-T recommendations V.42bis and V.42 have data compression and error correcting (EC) procedures for
DCEs connected to asynchronous DTEs. These
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DCE DATA ERROR COMPRESSION CORRECTION FUNCTION FUNCTION INTERCHANGE SIGNAL CIRCUITS CONVERTER SYNCHRONOUS FUNCTION LINE DTE CONTROL FUNCTION 495-14785 Figure 2. Synchronous Data Compression Installing the SDC Feature The SDC feature can be installed using AT commands or the DCP. Installing the SDC Feature Using the DCP To install SDC using the DCP, use this procedure. 1. Press the key until Control appears. Press F1. 2. Press the key until Install appears. Press F1. 3. If the SDC option is
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10. Perform a power reset: • For a standalone modem, turn the Power switch to the Off position, then to the ON position, or • For a carrier-mounted modem, release the lock, slide the modem part of the way out of the carrier, then replace it and lock it in place. 11. Set the SDC configuration options suitable for your environment. See Tables 1 and 2. Installing the SDC Feature Using AT Commands To install SDC using AT commands, use this procedure. 1. Attach a DTE to the modem. 2. Load the Async
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SDC User Interface When SDC is enabled, the following configuration options are available to set up the SDC feature. Some of the configuration options are not displayed unless SDC is enabled. V.42/MNP/Buffer (see Table 1) • Sync Comp Mode • SDC Negotiation • SDC Idle Fill • SDC Bit Encoding • Sync DTE CRC • SDC Delay Min • Sync Flow Control • TX Buff Disc Delay • RX Buff Disc Delay • Max Frame Size • Cellular Enhance DTE Interface (see Table 2) • Sync DTE Rate • Tx Clock Source 4 November 1996 3
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Table 1 (1 of 3) V.42/MNP/Buffer Configuration Options for SDC Sync Comp Mode: DirectMode Nxt DirectMode HDLC/SDLC This configuration option only appears if Async/Sync Mode is configured for Sync. Synchronous Compression Mode. Enables and disables synchronous data compression. DirectMode – Disables the Synchronous Data Compression mode. HDLC/SDLC – Enables the Synchronous Data Compression mode The AT command for Sync Comp Mode is #SCn. See Table 3, page 9. SDC Negotiation: LAPM_Buffer Nxt
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Table 1 (2 of 3) V.42/MNP/Buffer Configuration Options for SDC SyncDTE CRC: Ignore Nxt Ignore CRC16 This configuration option only appears when the Async/Sync Mode configuration option is set to Sync and Sync Comp Mode configuration option is not set to DirectMode. Synchronous DTE CRC. Determines whether the CRC of the DTE frame is transmitted from modem to modem. Ignore – No assumption is made about the CRC type. The CRC is transmitted along with the other data in the frame. 16 CRC16 – The
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Table 1 (3 of 3) V.42/MNP/Buffer Configuration Options for SDC TXBuffDiscDelay: 10sec Nxt 10sec Disable 60sec This configuration option only appears when Async/Sync Mode is configured for Async, and Error Control Mode is not configured for DirectMode, or when Async/Sync Mode is configured for Sync and Sync Comp Mode is not configured for DirectMode. Transmit Buffer Disconnect Delay. Determines how long the modem continues to transmit data stored in its Transmit buffer when the modem is co
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Table 2 DTE Interface Configuration Options for SDC Sync DTE Rate: 128000 Nxt 128000 115200 112000 96000 76800 72000 64000 57600 56000 48000 38400 28800 19200 14400 9600 4800 2400 1200 This configuration option does not appear when: • Async/Sync Mode configuration option is set to Async. • Sync Comp Mode configuration option is set to DirectMode. • Tx Clock Source configuration option is set to External. • The SDC feature is disabled. Synchronous DTE Data Rate.
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Table 3 AT Commands for SDC DCP LCD AT Description Command Command Sequence #DMn SDC Delay Minimization. Configure\Edit\ V42/MNP/Buffer Minimizes the delay introduced by SDC by allowing the modem to transmit a DTE frame to its DTE in a discontinuous way. #DM0 No delay minimization. The modem does not begin transmitting a frame to its DTE until the entire frame has been received. #DM1 Delay minimization. The modem begins to transmit data from a DTE frame to its DTE even if the end of the DTE fram
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Table 4 (1 of 2) S-Registers for SDC DCP LCD S-Register Description Command Sequence S28 SDC Negotiation. Configure\Edit\ V42/MNP/Buffer Controls negotiation between two modems attempting to connect using SDC. Register has the following values: 0 = LAPM_Buffer 1 = LAPM_Discon The factory setting is LAPM_Buffer. S29 SDC Idle Fill. Configure\Edit\ V42/MNP/Buffer Determines whether the modem uses an HDLC flag or mark to fill the time between DTE frame transfers. Register has the following values: 0
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Table 4 (2 of 2) S-Registers for SDC DCP LCD S-Register Description Command Sequence S31 SDC Bit Encoding. Configure\Edit\ V42/MNP/Buffer Determines the type of bit encoding used when transmitting synchronous DTE data. Register has the following values: 0 = NRZ 1 = NRZI The factory setting is NRZ. S32 SyncDTE CRC. Configure\Edit\ V42/MNP/Buffer Determines whether the CRC of the DTE frame is transmitted from modem to modem. Register has the following values: 0 = Ignore 1 = CRC16 The factory setti
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Synchronous Data Format The SDC feature supports the High-level Data Link Control (HDLC) and Synchronous Data Link Control (SDLC) type protocols for synchronous DTE data. An HDLC frame is composed of an information field containing the data to transmit, plus an overhead (address, control, and FCS fields) used by the DTE protocol. Flags opening and closing the HDLC frame serve as frame delimiters (Figure 3). FLAG ADDRESS CONTROL INFORMATION FCS FLAG 495-14783 Figure 3. Synchronous Data Format A 3
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Data Synchronization Data exchanged between the synchronous DTE and the modem is synchronized by two clock signals. Data coming from the DTE is synchronized by the transmit clock and data received by the DTE is synchronized by the receive clock. The transmit clock is provided by the modem on circuit 114 or by the synchronous DTE on circuit 113. The receive clock is provided by the modem on circuit 115. When the transmit clock is provided by the modem, the transmit and receive clocks are locked
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Flow Control Method The data transmitted from the DTE is sent to the DCE at a rate determined by the transmit clock frequency. When SDC is active, this data is then compressed in the DCE and sent to the remote DCE at the line rate less than the rate between the DTE and DCE (usually twice, three, or four times less). If the data sent by the DTE is not compressible, or bad line conditions cause retransmissions between the two DCEs, then the transmitting DCE must indicate to the transmitting DTE it
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Table 5 SDC Negotiation Configuration Scenarios Originating Modem Answering Modem Negotiation Results SDC enabled with LAPM_Buffer SDC enabled. SDC connection. negotiation. SDC enabled with LAPM_Discon SDC enabled. SDC connection. negotiation. SDC enabled with LAPM_Buffer SDC disabled. Buffer mode connection.* negotiation. SDC enabled with LAPM_Discon SDC disabled. Disconnect.** negotiation. SDC disabled. SDC enabled with LAPM_Buffer negotiation. Buffer mode connection.* SDC disabled. SDC enable
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Both groups have DTE Rate, Compression Ratio, and Line Efficiency measurements. Each measurement is an average of the last ten seconds’ data, and is updated every second. See Table 6. Table 6 SDC Performance Measurements Group DTE Rate Compression Ratio Line Efficiency TX The number of bits per second The ratio between the number of The ratio between the number of received from the DTE. This bits in the input and output of the bits per second sent on the line, number takes into account only comp
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Modem Configuration There are also configuration options in the V42/MNP/Buffer branch of the 3800Plus modem which affect SDC efficiency. (LAPM) Max Frame Size. Except in environments where line impairments are known to be unusually high, set this to the maximum (256). Sync DTE CRC. If the CRC type of the DTE frame is the 16-bit CRC for HDLC frames as defined by ITU-T, set this to CRC16. SDC Delay Min. If clamping of the receive clock (circuit 115) is accepted by your DTE, then set this to Rx_Clo
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2. SOFTWARE USE a. The Software (1) shall be disabled or deleted when no longer used in accordance with this License Agreement, or when the right to use the software is terminated; and (2) shall not be removed from a country in which use is licensed. 3. TERM This license is effective until terminated. You may terminate it at any time by destroying the documentation and requesting that Paradyne disable or delete the Software on your modem. It will also terminate automatically if you fail to compl
