alog information signal at regular time intervals and coding of the measured ampli- tude into a series of binary values, which are transmitted by modulation of a pulsed, or intermittent, carrier. PCM is a common method of speech digitizing by using 8-bit codewords, or samples, and a sampling rate of 8,000 times per second. PCS (Personal Communication Services) A digital service that operates in the 1.8GHz to 2GHz band and uses both microcell and picocell architectures. PDC (Personal Digital Cellular) Also known as Japanese Digital Cellular (JDC), a 2G standard for digital wireless communications deployed widely in Japan. PDH (Plesiochronous Digital Hierarchy) The first generation of digital hierarchy, which defines the available digital transmission rates and number of channels. PDH is used by tele- communications operators and implemented according to three standards: T-carrier in North America, E-carrier in ITU-T countries, and J-carrier in Japan. PDH is defined by the ITU-T in its G.703 standard. PDN (public data network) A generic term for the collection of packet-switching net- works that provide public data services. Well-known examples are the Internet and X.25. PDU (protocol [or packet] data unit) A message of a given protocol, comprising payload and protocol-specific control information, typically contained in a header. PDUs pass over the protocol interfaces that exist between the layers of protocols (per the OSI model). peering agreement An arrangement in which operators agree to exchange with one another the same amount of traffic over high-speed lines between their routers so that users on one network can reach addresses on the other. peer-to-peer The interaction by which computers and other network devices communicate with each other as equals and on their own initiative (as opposed to a client/server environment). performance A major factor on which the total productivity of a system depends. Perfor- mance is largely determined by a combination of several other factors: throughput, latencies, respon