Channel Service Units (CSU) and Data Service Units (DSU) Information

Channel Service UnitChannel Service Units (CSUs) and Data Service Units (DSUs) are digital interface devices that connect end user data communications equipment to digital access lines and provides framing of sub-64 kbps customer access channels onto higher rate data circuits. Network types serviced by CSUs and DSUs include DDS, ATM, E1/FE, E3, frame relay, SMDS, T1/FT1, and T3. Digital Data Service (DDS) was the first digital service made available to the general public. It operates at different rates from 2.4 kbps to 64 kbps. Asynchronous Transfer Mode (ATM) is a high-speed network technology that uses fiber optic or CAT-5 copper cables. E1, or Fractional E1, is the European format for digital transmission, similar to the T1 system used in the United States. E3 is the European designation for T3, a long-distance, point-to-point communications circuit service that operates at 44 Mbps and can carry 672 channels of 64 kbps. Frame relay is a packet switching protocol for connecting devices on a Wide Area Network (WAN). Switched Multimegabit Data Services (SMDS) is a high-speed switched data communications service offered by telephone companies that enable organizations to connect geographically separate Local Area Networks (LANs) into a single WAN. T3 is a dedicated phone connection supporting data rate of about 44 Mbps.


Form factor is important to consider when searching for CSUs and DSUs. Choices include chip, board, and module. Some devices are configured to be stackable. Rack mountable is another common mounting feature. Important performance specifications to consider include:

  • line rate
  • framing
  • line code
  • power requirement

Line rate is the network transmission speed in bits/second. Data framing defines the general structure of a data packet or frame. A packet, in general, consists in a preamble section, one or more sub frames, error checking information, data link information, and other sections that are a function of the type of network. Framing choices include SF/D4, ESF, SF/ESF, and auto detection. Line code (or encoding formats) refers to the methods used to represent the zeros and ones of digital systems for transmission. The zeros and ones are transmitted as two different voltage levels. A zero is a space and a one is a mark. Power requirement choices include 230 VAC, 115 VAC, 48 VDC, and 24 VDC.


Physical interface parameters to consider for CSUs and DSUs include choices for network interface and DTE interface. Network interface choices are RJ-48C, DB-15, DB-25, and BNC. DTE interface choices include V.35, RS442, RS530, DSX-1, X.21, HSS, ISDN port, and SCSI.  Common features include integrated diagnostic tables and included management utilities. Utilities are used to operate and manage the network such as Simple Network Management Protocol (SNMP), remote management utilities, predictive failure analysis, system monitoring, etc.

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