Cisco LAN2LAN Software

This document provides some tips and items to consider when you use a modem with LAN²LAN:

A. LAN²LAN makes minimum demands of the modem/DSU to which it is attached and provides all the outgoing signals required by normal modems and DSUs.

• LAN²LAN activates output signal DTR (TR for RS422) continuously from initiation time.
• LAN²LAN requires the presence of the incoming modem signal DSR (DM for RS422) before it activates the line. When DSR is present, LAN²LAN activates output signal RTS (RS for RS422), begins to send flags to the line, and starts the line protocol.
• LAN²LAN also requires modem-supplied transmit and receive clocks to clock data in and out. All other incoming modem signals are ignored.
• LAN²LAN dynamically senses the line speed by means of periodic test transmissions. LAN²LAN adjusts its protocol timer according to the measured line speed.

Lines between LAN²LANs can be implemented with a primary leased line and a backup dialup line. A typical example might be a leased line that operates at 56 Kbps, backed up with a 9600 bps dial line. The switch between primary and secondary can be activated by external equipment. In this situation, assuming a switch-over period of a few seconds, LAN²LAN detects a short line outage and continues operation on the new line with the line protocol protecting against data loss or corruption. LAN²LAN adjusts its protocol timer for the different line speed at the next periodic speed sample.