Data Communications Networking Devices - 4th Ed.pdf

480 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTTable 4.30Terminal-to-computer circuit connectionsDirect connection of terminals through the use of wire conductorsConnection of terminals through the utilization of line driversConnection of terminals through the utilization of limited distance modemsConnection of terminals through the utilization of modems or digital service unitsIt is interesting to observe that each of the methods listed in Table 4.30 providesfor progressively greater distances of data transmission while incurring progressivelygreater costs to the users. In this section the limitations and cost advantagesof the ®rst two methods will be examined in detail.Direct connectionThe ®rst and most economical method of providing a data circuit is to connect aterminal device directly to a computer through the utilization of a wire conductor.Surprisingly, many installations limit such direct connections to 50 feet in accordancewith terminal and computer manufacturers' speci®cations. These speci®cationsare based upon EIA RS-232 and ITU V.24 standards. If the maximum50-feet standard is exceeded, manufacturers may not support the interface, yetterminals have been operated in a reliable manner at distances in excess of 1000 feetfrom a computer over standard data cables. This contradiction between operationaldemonstrations and usage and standard limitations is easily explained.If you examine the RS-232 and V.24 standards, such standards limit directconnections to 50 feet of cable for data rates up to 20 kbps. Since the data rate isinversely proportional to the width of the data pulses transmitted, takingcapacitance and resistance into account, it stands to reason that slower terminalscan be located further away than 50 feet from a computer without incurring anyappreciable loss in signal quality. Simply stated, the longer the cable length theweaker the transmitted signal at its reception point and the slower the pulse risetime. As transmission speed is increased, the time beween pulses is shortened untilthe original pulse may no longer be recognized at its destination. This becomesmore obvious when you consider that a set amount of distortion will affect a smallerless wide) pulse than a wider pulse.In Figure 4.66, the relationship between transmission speed and cable length isillustrated for distances up to approximately 3400 feet and speeds up to 40 000 bps.This ®gure portrays the theoretical limits of data transfer speeds over an unloadedlength of 22 American wire gauge AWG) cable. Many factors can have an effect onthe relationship between transmission speed and distance including noise,distortion introduced owing to the routing of the cable, and the temperature ofthe surrounding area where the cable is installed. The ballast of a ¯uorescent®xture, for instance, can cause considerable distortion of a signal transmitted over arelatively short distance.The diameter of the wire itself will affect total signal loss. If the cross-sectionalarea of a given length of wire is increased, the resistance of the wire to current