3Com 5012 Installation Guide - Page 32

32 CHAPTER 4: INSTALLATION OF THE ROUTER Table 17 Transmission rate and transmission distance of V.24 (RS232)/V.35 cable V.24 (RS232) Baud rate (bps) Maximum transmission distance (m) 2400 60 4800 60 9600 30 19200 30 38400 20 64000 20 115200 10 - - V.35 Baud rate (bps) 2400 4800 9600 19200 38400 56000 64000 2048000 Maximum transmission distance (m) 1250 625 312 156 78 60 50 30 CAUTION: The baud rate should not exceed 64kbps when the V.24 cable operates in synchronous mode. ■ DTE and DCE Synchronous serial interface can operate in both DTE and DCE mode. For two devices connected directly, one should operate in DTE mode, and the other should operate in DCE mode. The device at the DCE side provides a synchronous clock and specifies the transmission rate, while the device at the DTE side accepts the synchronous clock and communicates at the specified baud rate. Usually, the router serves as a DTE device. To determine whether the device connected to the router is a DTE or DCE, please refer to the manual that came with the device. Also the following table will be helpful in identifying DTE and DCE. Usually, the PC or Router serves as a DTE device and the Modem, Multiplexer or CSU/DSU serves as a DCE device. In general, the asynchronous serial interface is connected to an external Modem or a Terminal Adapter (TA) to work as the dial-up interface. In this case, it is unnecessary to determine whether the device is DTE or DCE, and just choose an appropriate baud rate. Synchronous/Asynchronous Serial Interface Cable The WAN interface of the router is a DB50 receptacle. Proper connection cable needs to be selected for the protocol applied. By far, nine types of synchronous/asynchronous serial interface cables are available. One end of all the nine types of cables is DB50 connector, the other end (network end) varies with the cable type, which can be: ■ V.24 (RS232) DTE cable: DB25 (male) connector ■ V.24 (RS232) DCE cable: DB25 (female) connector ■ V.35 DTE cable: 34PIN (male) connector ■ V.35 DCE cable: 34PIN (female) connector ■ X.21 DTE cable: DB15 (male) connector ■ X.21 DCE cable: DB15 (female) connector