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COURIER HIGH SPEED MODEMS APPENDIX A. LINK NEGOTIATION (HANDSHAKING) AND ERROR CONTROL This appendix includes detailed information on how USRobotics HST and V.32 bis modems negotiate with remote modems for the rate and other characteristics of each connection. You may find it helpful if you are having difficulty connecting with another modem. In addition, you'll find information on error control and, especially useful, some statistics and guidelines on using the modem for the best throughput. The following text relies on familiarity with two terms used in this manual, ARQ and DTE. ARQ (automatic repeat request) designates a connection under error control. DTE (data terminating equipment) means the computer or terminal to which the modem is attached. LINK NEGOTIATION (HANDSHAKING) During high-speed link negotiation, timing and procedures depend on whether the modem uses HST or V.32 bis modulation, as follows. HST Mode DTE rate: 38.4K/19.2K/9600 bps (38.4K or 19.2K required for 14.4K calls) Modem settings: B1, &B1, &H1/&H2/&H3, &M4/&M5, &N0 Link Negotiation and Error Control A-1 COURIER HIGH SPEED MODEMS 1. The modems begin handshaking by training (synchronizing) at 2400 bps and exchanging information on their error control, data compression, and speed capabilities. 2. Once the modems enable error control, handshaking proceeds as follows. If the remote modem is operating at 2400 bps or lower, the Courier stays at 2400 bps or falls back to match the remote modem's slower rate. If the remote modem is HST-compatible and operating at a higher speed, both modems shift up and train at 9600 bps. From there, if possible, they shift up again to 12K bps and again to 14.4K bps. Or they fall back to 7200 bps and again to 4800 bps, if the remote modem is operating at one of those lower speeds. Initially, the high-speed channel is allocated to the answering modem, the 450-bps channel to the calling modem. This allocation reverses if the calling modem has more data to transmit than the answering modem. Specialized High-Speed Connect It's possible for two modems in HST mode to connect directly at 9600, 12K or 14.4K bps, without 2400-bps handshaking. However, this is only recommended for faster connections in specialized installations where the computer and software manage error control. The reason is that there can be no error control because HST modems negotiate A-2 Link Negotiation and Error Control COURIER HIGH SPEED MODEMS error control during 2400 bps handshaking. Both modems must be set as follows. High speed handshaking requires a fixed link rate: the modem only connects if the remote modem is operating at the same link rateΩ9600 (&N6), 12K (&N7), or 14.4K bps (&N8). DTE rate: 38.4K/19.2K/9600 bps (38.4K or 19.2K required for 14.4K calls) Modem settings: B1, &B1, &H1/&H2/&H3, &M0, &N6/&N7/&N8 HST Mode, Error Control Disabled If error control is disabledΩthat is, the Courier HST is set to &M0 or the remote modem isn't set for error control and the Courier HST reverts to &M0Ωthe Courier operates in one of the following ways. 1. If set to &N0, it only connects at 2400 bps or lower. This ensures that there will not be data transfer at high speeds unless the call is protected by error control. Courier HST modems negotiate error control during 2400 bps handshaking. If they cannot connect under error control, they stay at 2400 bps. Or, if the remote modem is operating at a lower rate, the Courier HST switches to the lower rate. 2. If set to &N6, &N7 or &N8, the Courier will handshake at the higher speed, 9600, 12K or 14.4K bps, as Link Negotiation and Error Control A-3 COURIER HIGH SPEED MODEMS shown in the previous configuration example, Specialized High-Speed Connect. However, that configuration example should only be used in the special situation described, where the system takes care of error control. NOTE: Without error control, there are no high speed line reversals. The high speed channel is always allocated to the answering modem. Also, there is no online fallback to protect the connection on impaired lines. V.32 bis Mode The handshaking described immediately following this paragraph adheres precisely to the CCITT recommendation. In addition, USRobotics V.32 bis modems, when connecting with each other, use an enhanced proprietary handshaking procedure which allows them to connect faster and to retrain faster during calls. They must, however, establish a V.42 error control connection. This proprietary handshaking is described after the next section, under USRobotics V.32 bis to V.32 bis. V.32 bis NOTE: High speed calls are highly vulnerable to errors unless the data is protected by error control. The opera- tions described below take place even if one of the modems is not set for error control, thereby prohibiting error control for the call. A-4 Link Negotiation and Error Control COURIER HIGH SPEED MODEMS DTE rate: 38.4K/19.2K bps Modem settings: B0, &B1, &H1/&H2/&H3, &M4/&M5, &N0 WARNING: Answering V.32 bis modems shift their DTE rate up to 19.2K bps (for 14.4K bps connections) if the calling V.32 bis modem dials at 14.4K or 12K bps. The answering V.32 bis modem then sends data to its DTE at 19.2K bps. If your DTE is limited to a rate of 9600 bps, disable high-speed modulation by setting Register S34 to a value of 3 (ATS34=3). The calling modem will then shift down to 9600 bps for a V.32, not V.32 bis, connection. 1. The answering modem first sends out a 2100 Hz answer tone identifying it as a V.32 or V.32 bis modem. If the calling modem recognizes the tone, the modems connect at the highest possible speed, usually 14.4K bps. (If the calling modem is a V.32 modem, its maximum speed is 9600 bps, and the modems may connect at that rate. But see paragraph 3.) 2. If the calling modem doesn't recognize the answer tone, the answering modem then sends a 2250 Hz answer tone used by 2400, 1200 or 300 bps modems. If the calling modem is operating at one of those rates, the modems connect at the calling modem's rate. 3. If the modems don't connect as in paragraph 1 or 2, the answering V.32 bis modem then sends extra V.32 answer tones. If the calling modem Link Negotiation and Error Control A-5 COURIER HIGH SPEED MODEMS is a V.32 modem, the answering V.32 bis modem shifts its speed down to 9600 bps and the modems connect at that speed. This extended V.32 handshaking ensures connections with V.32 modems if they did not connect as in paragraph 1. It may even be necessary to extend the length of these tones if the modem is to answer older, "dumb" V.32 modems. Register S28 is used to modify the duration of these extra V.32 tones. See S28 in Appendix B. USRobotics V.32 bis to V.32 bis 1. The modems first negotiate a V.42 error control connection. Speed negotiation is part of the V.42 detection/LAPM sequence. If they fail to make a V.42 connection, these modems follow the procedures described under V.32 bis, above. 2. The Receiver of each modem dictates its modem's highest speed, with negotiation beginning at 7200 bps and proceeding upward to the 14.4K bps maximum. This means that your modem's receiver may be operating at a different speed from your modem's transmitter, which operates at the same rate as the remote modem's receiver. In other words, the rate at your modem's Originate frequency may be different from the rate at your modem's Answer frequency. Because of this asymmetrical design, one frequency may suffer line hits or other impairments, and fall back to a A-6 Link Negotiation and Error Control COURIER HIGH SPEED MODEMS slower speed, while the other does not. The result is more efficient line operation. If the impairments are severe, the modems use normal retraining. Dual Standard Handshaking Dual Standard modems always connect with each other, either in HST mode or in V.32 bis mode. However, we recommend that Dual Standard modems be set to B0 and to &N0, so that they can connect with V.32 bis, HST and lower-speed modems, in both Originate and Answer modes. A Dual Standard modem set to B1 (HST) does not send out any V.32 bis or V.32 answer tones, and therefore cannot answer calls in those modes. When it is calling, however, and set to B1, it switches to V.32 bis mode if it detects the V.32 bis/V.32 answer tone. It attempts to connect at 14.4K bps (V.32 bis) or drops to 9600 bps (V.32). The mode shift is for that call only. Similarly, a Dual Standard modem set to B0 will switch to HST mode for that call only if the other modem is operating in HST mode. ERROR CONTROL AND THROUGHPUT Overview Error control is available for calls at 1200 bps and above. It can be disabled, although high speed calls (above 2400 Link Negotiation and Error Control A-7 COURIER HIGH SPEED MODEMS bps) should always be under error control. The operations defined in an error control protocol include the following: ∙ Establishment of compatibility ∙ Data frame formatting ∙ Error detection through Cyclic Redundancy Checking (CRC) ∙ Retransmission of corrupt data frames The Courier is set at the factory to &M4, causing it to try for an error control connection and, if that isn't possible, to proceed with the call in Normal mode. The modem first tries for a V.42 connection, then an MNP connection. The following information is based on the Courier's setting of &M4. A-8 Link Negotiation and Error Control COURIER HIGH SPEED MODEMS CCITT V.42 Handshaking This international standard includes a two-stage handshaking process: ∙ A Detection phase that is based on an exchange of predefined characters. ∙ LAPM (Link Access Procedures for Modems) Negotiation. In this phase, the modems identify their capabilities concerning maximum data block size and the number of outstanding data blocks allowed before an acknowledgment is required. MNP Handshaking This protocol is supported by the CCITT V.42 Recommendation. It was originally developed by Microcom, Inc. and is now in the public domain. MNP is based on special protocol frames. If the remote modem doesn't recognize an MNP Link Request, error control isn't possible. (In HST asymmetrical mode, USRobotics modems use a proprietary scheme similar to MNP.) Data Compression If the modems successfully establish a V.42 connection, they also negotiate for V.42 bis data compression. If the they successfully establish an MNP connection, they negotiate for MNP5 data compression. The type of compression for a call, if any, is reported in the ATI6 display. Link Negotiation and Error Control A-9 COURIER HIGH SPEED MODEMS Modems using V.42 bis compression negotiate the following options and report them in the ATI6 display. ∙ Dictionary size, that is, the amount of memory available for compression table entries. (Entries are codes devised for redundant data. The data is packed into shorter data units, called code words, and unpacked by the receiving modem.) Possible sizes are as follows. USRobotics modems use 11-bit, or 2048-entry dictionaries, but drop down if the A-10 Link Negotiation and Error Control COURIER HIGH SPEED MODEMS remote modem uses a 512- or 1024- entry dictionary. The size of the dictionary for a call is reported in the ATI6 display. Bits Entries 9 512 10 1024 11 2048 ∙ Maximum string length of each entry. As the dictionary fills, the modem deletes the oldest unused strings. V.42 bis compression is more efficient than MNP5 compression in part because it dynamically deletes unusable strings. In addition, it works better with files that are already compressed. These include .ARC files downloaded from many Bulletin Boards and 8-bit binary files, which seem to the modem to be compressed. MNP5 compression should not be used with such files because it adds data to them, which lessens throughput. (The additional data is stripped when the file is decompressed by the remote modem.) When transferring such files, it's best to set the modem to &K3: this allows V.42 bis compression to work dynamically with the compressed data, but disables MNP5. Flow Control Flow control of data from the computer is required under error control for two reasons: Link Negotiation and Error Control A-11 COURIER HIGH SPEED MODEMS 1. The transmitting modem buffers a copy of each frame it transmits to the remote end until it is acknowledged by the receiving modem. 2. If errors are encountered, retransmission activity can cause a steady stream of data from the computer to overflow the buffer. Throughput Guidelines The following guidelines should help to make the most of the modem's advanced performance features. In many instances, experimentation and experience will indicate what works best for your applications. 1. Optimal throughput is attained under the following conditions: ∙ The communications software allows fixing the DTE rate higher than the link rate, e.g., setting the soft- ware to 38.4K or 19.2K bps and setting the modem to &B1. If the software automatically switches bit rates to follow the link rate, the modem's DTE rate must be also set to follow the link rate for each call, &B0, and throughput will be limited. Installations with specialized software may want to enable a fixed DTE rate for ARQ calls and a variable DTE rate for non-ARQ calls. See the &B2 command in Chapter 4. A-12 Link Negotiation and Error Control COURIER HIGH SPEED MODEMS ∙ The call is under data compression. ∙ The data is comprised of text files rather than binary files such as .EXE or .COM files. See the table at the end of this appendix. 2. MNP5 compression is disabled for files that are already compressed and 8-bit binary files, that appear to the modem to be already compressed. MNP5 is disabled by setting the modem to &K3. 3. The file transfer is not slowed down by a file-transfer protocol. Many non-text files require a file transfer protocol, but the results vary. For example, certain public domain file transfer protocols have the following effects: Kermit Throughput is severely reduced due to Kermit's short block lengths (under 128 bytes) and acknowledgment turn-around time. Xmodem Throughput may be reduced if your version uses short block lengths (128 bytes). Some versions user larger blocks (1K blocks). Throughput is also reduced by overhead (error control protocol information). Ymodem There is an improvement over Xmodem, due to larger block lengths (1K bytes), but throughput is still reduced by the protocol's error control overhead. Link Negotiation and Error Control A-13 COURIER HIGH SPEED MODEMS The above protocols further reduce throughput when an error control connection is established. The accuracy of the data is checked twice, by the file transfer protocol and the modem. To avoid redundancy, use the above protocols only for non- ARQ connections, and only at speeds of 2400 bps and below. For the best throughput performance, we recommend Ymodem-G and ZMODEM as better choices, Ymodem-G being the most efficient. These protocols assume the modems are handling error control: overhead is minimal, with results almost equal to those when there is no file transfer protocol. Achievable Throughput The table below indicates the maximum throughput, in characters per second (cps), that can be expected under the following optimal conditions. ∙ DTE rate set at 38.4K bps; modem set to &B1 ∙ Connection (link) rate of 14.4K bps ∙ V.42 bis compression negotiated for the call, and the default size 11- bit, 2048-entry dictionary ∙ Straight data (e.g., not already compressed, no file-transfer protocol) ∙ Transmission from a fast (e.g., 286) computer Throughput (cps) File Type MNP5 V.42 bis A-14 Link Negotiation and Error Control COURIER HIGH SPEED MODEMS Assembler or Compiler listing 28 80 3840 Text file 2325-2625 3400-3840 Binary file: .EXE 2175-2400 2030-2600 Binary file: .COM 2100-2250 2050-2300 .ARC files (common on BBS's)* 1500- 1650 1700 Random binary 8-bit* 1460-1575 1 700 * These files are already compressed or appear to the modem to be compressed. Additional MNP5 compres- sion causes throughput lower than what can be expected using MNP without compression. We recommend setting the modem to &K3 when transferring these files, to allow V.42 bis but disable MNP5. Throughput, MNP5 and V.42 bis Link Negotiation and Error Control A-15