A more economical approach is frequently resynchronized by to use a single communication Communication channels dividing the message into short channel for many pieces of informa- limit the speed at which blocks or characters,each with tion;this technique is referred to as their own synchronization bits. multiplexing.The two basic tech- data acquisition and This is an advantage for short mes- niques are frequency division mul- sages where a quick synchroniza- tiplexing (FDM)and time division control can be performed tion is desired.Thus,the efficiency multiplexing (TDM). is relatively high because of the FDM,in which each piece of information is transmit- low synchronization overhead.Cost is very low,due to ted over a dedicated part of the available communi- the simplicity. cation channel spectrum (see Figure 2).For a The synchronous modem,on the other hand,trans- voice-grade channel,up to 25 separate subchannels mits a synchronizing clock signal along with the data (120-Hz spacing per channel)can be utilized.Addi- stream,so that the receiver is precisely synchronized with the transmitter.This technique allows very long messages and high data rates to be transmitted without any problem with falling out of synchronization.However,it SIGNAL 4 does require a longer period of time to establish synchronization,a disadvantage for short messages because the ratio of SIGNAL 3 overhead to data is high.Synchronous modems are generally available from 2400 bits per second (bps)to 1 megabits per sec- SIGNAL 2 ond (Mbps)and are higher in cost than the asynchronous type (although the price dif- ference is decreasing).A summary compari- SIGNAL 1 son of communications with asynchronous and synchronous modems is shown in Table 1. TIME Both asynchronous and synchronous Figure 2.Frequency division multiplexing modems used for SCADA communications require the use of a voice-grade communica- tion channel.Either dial or a dedicated tional voice grade channels are required for each 25 leased channel can be utilized,although most all SCADA pieces of information;therefore,large systems systems require dedicated leased channels.A voice-grade would require an impractical number of channels channel is defined as a "Type 3002 unconditioned"chan- TDM,in which each piece of information is trans- nel and specifies the channel impedance characteristics. mitted as part of a serial digital message over a maximum attenuation,frequency response (from 300 to separate span of time and demultiplexed by the 3,000 Hz),delay characteristics,and noise characteristics receiver into the individual pieces of information (see Figure 3).The digital message utilizes the entire communica- tion channel spectrum in order to obtain the highest possible data rate. Due to the efficiency and low cost of TDM,it is now utilized for all but the smallest SCADA systems. Two basic types of modems are utilized for transmitting information via a communication channel:asynchronous and synchronous. The asynchronous type utilizes separate tim- ing sources (such as crystals)at each end of a data link to make the receiver demodulate the 2 3 4 12 TIME data at approximately the same rate at which SIGNAL it was modulated by the transmitter.Due to this approximation,the data message must be Figure 3.Time division multiplexing 46 IEEE Computer Applications in Power