SLIP modem considerations

When configuring modems for SLIP, it is important that these changes be made on both ends of the communication link.

Both the local and remote modems must be configured exactly the same.

1. The modem must acknowledge the presence of DTR.

   Referencing the local modem, if DTR is assumed or ignored, the modem can never perform a hang-up. It can only close the line or hang up when it recognizes the loss of carrier from the other end. This means that disconnects can occur only when instigated by the other end. The AT commands &D2 or &D3 are proper settings for most Hayes-compatible modems.

2. The modem must never force, assume, or ignore data carrier detect (DCD).

   DCD must follow or track the real condition. This means that carrier will exist after a bona fide connection to the other end (modem) across the switched telephone line. This also applies to a dedicated line. &C1 is the suggested setting for most Hayes-compatible modems.

3. The modem must never force, assume, or ignore a clear to send (CTS) signal.

   CTS must track or follow request To send (RTS). If CTS is forced true, the port open will fail whenever a getty is put upon the port or when RTS flow control protocol is added to the port.

4. Modems should be configured to turn off automatic repeat request (ARQ) codes if problems arise during slattach dial attempts.

   If, the modems repeatedly fail to make a connection during slattach dial-in attempts, the user should check the modem configurations and turn off the ARQ codes if they are currently on. In most Hayes-compatible modems, this is the &A0 setting.

   Disabling ARQ result codes does not affect error-controlled connections nor does it keep the modem from returning standard CONNECT messages (if result codes are enabled) as needed for the slattach dial string.

5. ECL (Error Checking on the Link) is critical.

   Either BOTH modems or NEITHER modem can use it. Normally, both modems must agree on its usage during the connect session. If ECL is chosen, the physical telephone line must be good enough to allow a recovery from a data error before the TCP/IP timers expire while awaiting an acknowledge packet for the last data sent across the SLIP link.

6. Data Compression across the link.

   It is acceptable to use data compression across the link as long as it is totally handled by the modems. SLIP does not perform any type of compression. If data compression is invoked, it is much better to have two modems of the exact same type; this ensures that each will perform the compression in the same manner and same time frame.