ions to other nodes in the system. Microwave links. Systems which use the relatively short microwave frequencies to broadcast from one point to another. Modem. Contraction of the term modulator-demodulator. A de- vice to convert one form of signal to another form for facility com- patibility. For example, a modem is used to convert a digital signal from a computer into an analog signal so that it may be transmitted over the network. GLOSSARY 125 Modulator. The equipment or apparatus which modifies some characteristic of a signal. Monitor jack. A jack that provides access to communications cir- cuits for the purpose of observing the signal conditions on the circuit without interrupting the service. Mutiplexer. Electronic device for combining several signals into a composite stream for economic transmission. Techniques employed are frequency division (FDM) and time division (TDM). Multipoint line. Refers to a single link connecting several terminals or nodes. Narrowband. A channel whose bandwidth is less than that of a voice grade channel. Commonly used for communication at speeds of less than 300 bits per second. Network. A series of points interconnected by communications channels, often on a switched basis. Networks are either common to all users or privately leased by a customer for his own use. Nodes. Those points on a communications network at which a message can be entered into the network or received from it. Packet assembler/disassembler (PAD). Equipment providing packet assembly and packet disassembly facilites. Packet switching. A method of routing standard size groups of in- formation called packets. Patch panel. An arrangement for connecting channels and termi- nals in which plug cords are inserted into jacks corresponding to the channels or terminals to be interconnected. Port. Entrance or access point to a computer, multiplexer, device, or network where signals may be supplied, extracted, or ob- served. Pulse code modulation (PCM). The form of modulation in which the modulating signal is sampled, and