Data Communications Networking Devices - 4th Ed.pdf

4.8 LIMITED-DISTANCE MODEMS ___________________________________________________ 487Operational featuresbe completely familiar with this particular type of hardware or the tariff structurefor the service to support limited-distance transmission. It is also important thatthe proposed limited-distance modem conforms to the speci®cations set forth in theBell System publication 43401 entitled `Transmission Speci®cations for PrivateLine Metallic Circuits'. This publication describes the signal level criteriaobjectives for private line metallic circuits cable pairs without signal battery orampli®cation devices). In addition, the publication notes that the telephonecompanies have no obligation to provide private line channels on a metallic basis.Most manufacturers of limited-distance modems clearly specify that their equipmentoperate in accordance with the previously mentioned publication. If it is notexplicitly stated, you may encounter delays and additional cost to ensure that thetransmitter of the device is modi®ed to comply with the speci®cations.Most limited-distance modems utilize a differential diphase modulation schemeand permit internal transmit timing or external derived timing from the associateddata terminal. These devices act similarly to a line driver, and most willaccommodate four-wire half/full duplex and two-wire half-duplex/simplex datatransmission. Data rate switches on some asynchronous units provide selectabledata rates ranging from 110 to 28 800 bps, while the selectable rates of mostsynchronous units inlude various increments of 2400 bps. Users often select atransmission speed only to realize that by the time the equipment is installedchanging requirements may indicate a different speed; therefore, the ease ofadjustment of the unit should be investigated.Some manufacturers state that their limited-distance modems can be insertedinto a transmission line to serve as a repeater, as illustrated in Figure 4.70, toFigure 4.70 Typical point-to-point applications. If a limited-distance modem is planned tobe used as a repeater, care should be taken to ensure that shelter, a power source, andaccess for diagnostic testing and maintenance is available