Practical Industrial Data Networks:Design, Installation and ...

Appendix B – Basic terminology 327superimposed on the same physical circuit. Commercial solutions to this problem includemodems for use on phone lines or one of the broadband alternatives.B.6.3B.6.4Digital signals via digital methodsNo special conversion techniques are needed. The transmission channel could be any ofthe dial-up or leased alternatives such as T1 or ATM.Analog signals via digital methodsHere the channel expects digital information and for this reason the analog signal first hasto be converted to a digital format. A typical solution is pulse code modulation (PCM) ascovered elsewhere in this chapter. In the case of voice, the analog signal is typicallysampled at 8000 times per second and each sample is then converted to an 8-bit binarynumber giving a 64 kbps data rate.B.7 Dial-up vs leased accessB.7.1B.7.2Dial-up accessDial-up access refers to connecting a device to a network via a public switched telephonenetwork and a modem. The procedure is the same as for two people establishing contactvia telephone, the difference being that two computer devices are communicating viamodems.The connections are not always good, causing modems to drop out or fail to establishconnections, and the data transfer rates are limited to 56 kbps or less, depending on themaximum frequency that can be negotiated by the modems. Provided that multipleconnections are supported at both the user and the service provider (e.g. throughMultilink PPP), multiple parallel dial-up connections can be established for high volumesof data.An alternative to a normal telephone connection is an integrated services data network(ISDN) connection. One ISDN BRI (basic rate interface) can handle 128 kbps of data,and if the necessary services are installed, multiple ISDN connections can be establishedto handle high data volumes for example, in the case of videoconferencing.Leased accessA leased line (also referred to as a dedicated or a private line) is a permanent line installedbetween locations, usually to a user’s premises, by a telephone company. Unlike a dial-upconnection, it is always active. Leased lines are available in 2 and 4 wire versions, whichhas a bearing on the line quality and obviously also on the line rental costs. Leased linesprovide faster throughput of data and better quality connections than dial-up, but at ahigher cost.Examples of leased line services are:• T-1/E-1 lines. These provide 1.544 Mbps for T-1 (for example, in theUnited States and Japan or 2.048 Mbps for E-1 in Europe and Mexico).• Fractional T-1 lines. These build up in units of 64 kbps up to 768 kbps.• 56/64 kbps lines. In Europe, these lines provide 64 kbps, In the US andother countries 8 kbps is used for control overhead so that only 56 kbps isavailable to the user.