The Digital Service Unit (DSU) is a stand-alone, rack-mounted, or router-integrated device that connects various data terminal equipment via RS-232, RS-449 or V.35 interfaces with widely available digital services that offer 56/64 Kbps access, including Digital Data Service (DDS) and cell- or frame-based services such as ATM, SMDS, and frame relay. Typical applications for the DSU include LAN interconnection, dedicated Internet access, and remote PC access to local hosts.
Digital Data Service (DDS) operates at speeds of 1.2 Kbps to 56/64 Kbps in support of point-to-point or multipoint applications. Most DSUs have a built-in asynchronous-to-synchronous converter, accommodating asynchronous input devices that operate at speeds of 1.2 Kbps to 57.6 Kbps, as well as synchronous input devices that operate at either 56 Kbps or 64 Kbps. When packaged with Channel Service Unit (CSU) functions, the DSU/CSU device interfaces with T1 services at 64 Kbps and N x 64 Kbps up to 1.536 Kbps.
The DSU converts the binary data pulse it receives from the DTE to the bipolar format required by the network. The DSU also supplies the transmit and receive logic, as well as timing. Any device that connects directly to a digital line (via an external or internal CSU) must perform these functions, or it needs a DSU. Any piece of network equipment that does not have a bipolar port needs a DSU to connect to a CSU. The most common type of access device is the combination unit, which offers DSU and CSU functionality, which eliminates these concerns and reduces the number of devices that must be managed.
Like the CSU, the DSU also provides the means to perform diagnostics. The front panel of the DSU provides a set of LED indicators that show the status of the V.35 DTE interface, various test modes, and loop status. The device responds to standard loopback commands from the service provider or the user side. Included with the remote loopback capability are selectable bit error rate test patterns, the results of which are displayed on the front panel.
Like CSUs, DSUs can be managed either by the vendor’s proprietary network management system or by Simple Network Management Protocol (SNMP) tools. The DSU provides a non-disruptive in-band SNMP management channel over a DDS leased circuit. For frame relay connections, SNMP management is provided with a connection from the DSU’s management port to a router port or external LAN adapter at the remote site. In this case, management frames are sent across the network as data, then routed to the DSU by the remote router. Typically, in-band management is lost in a frame relay network due to the packetizing and switching of the data. This management method allows diagnostic packets to be sent to the distant router and then forwarded to the DSU through the external management connection to the management port on the DSU.
There are now simple versions of DSUs that are not designed to be managed. They will automatically setup on any standard 56/64K DDS facility and provide the correct data rate to the DTE port. Remote control and testing of the distant DSU is accomplished via an external modem connection and ASCII interface.
Modular routers, such as Cisco Systems’ 1600 and 3600 series routers, can be equipped with a WAN interface card that incorporates a fully managed DSU/CSU to facilitate the deployment and management of Internet and intranet connectivity. This integrated solution eliminates the need for external DSU/CSUs and allows all of these components, including the router, to be managed both locally and remotely as a single entity via SNMP or a Telnet session using the Cisco command-line interface.
Last Word
DSUs, as well as CSUs, not only provide an interface between DTE and the carrier’s network, but such devices help network managers fine-tune their networks for performance and cost savings. For example, when using the diagnostic capabilities of a DSU/CSU connected to a frame relay network, the network manager can monitor traffic on each permanent virtual connection (PVC) to set an appropriate committed information rate (CIR) and allowable burst rate on each circuit. In addition, the delay between network nodes can be measured as well as the performance of a line between the user and local carrier to see if the carrier is actually delivering the level of service promised.
