Service providers are presently seeking to increase their profits through low cost deployment of voice and leased line services over more efficient Ethernet and IP infrastructures. At the same time enterprises are looking for ways to take advantage of the promise of convergence by integrating their voice and data networks while preserving their investment in traditional PBX and TDM equipment. The voice-over-IP (VoIP) approach is maturing, but its deployment requires a certain level of investment in new network infrastructure and/or customer premises equipment (CPE).
TDM-over-IP (TDMoIP) is a technology that enables voice and leased-line services such as video and data to be offered inexpensively over service provider IP networks while retaining the reliability and quality of the public switched telephone network (PSTN). In this article, we'll discuss the technical challenges inherent in transporting TDM circuits over IP networks, how TDMoIP technology meets those challenges, and the standards shaping TDMoIP and related technologies.
PCM
(1) See phase change memory.
(2) See also PMC ( programmable metallization cell).
(3) (Plug Compatible Manufacturer) An organization that makes a computer or electronic device that is compatible with an existing machine.
(4) (Pulse Code Modulation) The primary way analog audio signals are converted into digital form by taking samples of the waveforms from 8 to 192 thousand times per second (8 to 192 kHz) and recording each sample as a digital number from 8 to 24 bits long (see sampling). PCM data are pure digital audio samples, and they are the underlying data in several music and surround sound formats (see WAV, FLAC, AIFF and surround sound).
Sound Cards Support PCM
For output, a sound card's audio-out port provides an analog signal to the speakers. Compressed formats such as MP3 and AAC are first converted to PCM, and the PCM data are then converted to analog (see D/A converter). Sound cards may also output PCM and other digital signals such as Dolby Digital (see S/PDIF). For input, an analog microphone is plugged into the audio-in port, and the sound card converts the analog signals to PCM.
PCM Ports on A/V Equipment
When ports on set-top boxes and Blu-ray/DVD players are labeled PCM or linear PCM (LPCM), they refer to uncompressed audio channels rather than encoded formats such as Dolby Digital, TrueHD, DTS and DTS-HD. PCM can be mono, stereo or have multiple channels for surround sound. See Bitstream mode and linear PCM.
It Started With the Telcos
PCM was introduced in the U.S. in the early 1960s when the telephone companies began converting voice to digital for transport over intercity trunks. See mu-Law.
