• Definition
  • Voice vs. Data
  • Lines vs. Trunks
  • The 1/0 Digital Cross-Connect Switch
    • Components
      • Encoders
        • Voice Encoders
        • Waveform Encoders
        • Hybrid Encoders
      • Multiplexor
  • Analog to Digital Conversion
    • General process
      • Sampling
      • Quantizing
      • Coding
    • Pulse Code Modulation
    • Differential Pulse Code Modulation
    • Adaptive Differential Pulse Code Modulation

A Digital Signalling level zero (DS0) connection is the smallest and simplest digital signalling connection. The DS0 is the basic level of communication upon which all the other Digital Signalling levels (DS1, DS2, DS3 etc.) are built. The DS0 is often erroneously called the local loop. The local loop is only the analog portion of a voice circuit between the central office and the subscriber. A DS0, being digital, is not actually part of the loop because it is on what is referred to as the trunk side of a Digital Cross Connect Switch (DCCS). The local loop is on the line side of the DCCS. DS0's are used to physically connect calls between end users through a Digital Cross Connect Switch (DCCS). If the user is communicating by voice on a telephone line, then the encoding on the signal that the DCCS receives is analog. That analog signal is converted to digital pulse codes at the switch and then multiplexed into a higher speed link such as a DS1 or DS3.
Voice vs. Data
Residential voice calls start at your phone as an analog call. The analog signal is converted by your phone into an electrical signal on a copper wire. This wire carries the voice portion of your call are encoded into the central office where they are converted to digital pulse codes. DS0 as digital pulse codes. However, it is still possible to encode computer data into a DS0 if you have a device called a Data Service Unit (DSU). The DSU accepts a serial connection from a computer system and can encode that data into a DS0. A DS0 cannot carry voice and data concurrently but it can carry one or the other. This is changing, but not because the telephone system is changing, but because users are purchasing equipment that utilizes what is called IP Telephony which encodes both data and voice into a single multiplexed communications path.
Lines vs. Trunks
Lines are the analog voice connections to end phones. Trunks, also called tie trunks (or sometimes tie lines), are digital. However, trunks carry multiple DS0's and are designed to carry traffic from a large number of subscribers. DS0's are not trunks, though they do exist on the trunk side of the DCCS.
Digital Cross-Connect Switch (DCCS)
This device has several names which depend upon the vendor. Called variously a Digital Access Crossconnect Switch (DACS) or a Digital Cross-Connecting switch (DCCS) the switch connects calls between end users. It performs A/D encoding, circuit switching of calls as well as multiplexing and demultiplexing trunks into individual DS0 circuits.



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