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 A cable made up of one or more separately insulated twisted-wire pairs, none of which is arranged with another to form quads.
 
  
 The structure of data, address, and control information in a packet. (188) Note: The size and content of the various fields in a packet are defined by a set of rules that are used to assemble the packet.
 
  
 The process of routing and transferring data by means of addressed packets so that a channel is occupied during the transmission of the packet only, and upon completion of the transmission the channel is made available for the transfer of other traffic.
 
  
 Pulse-amplitude modulation - Modulation in which the amplitude of individual, regularly spaced pulses in a pulse train is varied in accordance with some characteristic of the modulating signal. The amplitude of the amplitude-modulated pulses conveys the information.
 
  
 A test that determines whether the number of ones or zeros in an array of binary digits is odd or even. Odd parity is standard for synchronous transmission and even parity for asynchronous transmission.
 
  
 In communications, patches may be made by means of a cord, i.e., a cable, known as a
 
  
 In automated systems, patches may be made electronically.
 
  
 Pulse-code modulation - Modulation in which a signal is sampled, and the magnitude (with respect to a fixed reference) of each sample is quantized and digitized for transmission over a common transmission medium. In conventional PCM, before being digitized, the analog data may be processed (e.g., compressed), but once digitized, the PCM signal is not subjected to further processing (e.g., digital compaction) before being multiplexed into the aggregate data stream. PCM pulse trains may be interleaved with pulse trains from other channels.
 
  
 Protocol data unit - In layered systems, a unit of data that is specified in a protocol of a given layer and that consists of protocol-control information of the given layer and possibly user data of that layer.
 
  
 In layered systems, one of a set of entities that are in the same layer or the equivalent layer of another system.
 
  
 Layer 1. The lowest of seven hierarchical layers. The Physical layer performs services requested by the Data Link Layer. The major functions and services performed by the physical layer are: establishment and termination of a connection to a communications medium participation in the process whereby the communication resources are effectively shared among multiple users, e.g., contention resolution and flow control and, ersion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel.
 
  
 Phase modulation - Angle modulation in which the phase angle of a carrier is caused to depart from its reference value by an amount proportional to the instantaneous value of the modulating signal.
 
  
 In a communications network, a point at which signals can enter or leave the network en route to or from another network.
 
  
 Layer 6. This layer responds to service requests from the Application Layer and issues service requests to the Session Layer. The Presentation Layer relieves the Application Layer of concern regarding syntactical differences in data representation within the end-user systems. Note: An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.
 
  
 The dispatching, for reception elsewhere, of a signal, message, or other form of information. In communications systems, a series of data units, such as blocks, messages, or frames.