Below is list of terms you need to know for the CCNA exam.

Access Link The serial link (physical) that connects a Frame Relay device (such as a router) to a Frame Relay switch
Back-to-Back Link A serial link connecting two routers without CSU/DSUs. One router used a DTE cable and the other uses a DCE cable. Typically used in labs.
CSU/DSU Channel Service Unit/Digital Service Unit. A device that knows how to use the layer 1 details of telco Layer 1 serial links that connect to devices such as routers.
DTE Data Terminal Equipment. The DTE synchronizes its clock based on the DCE clock. (Layer 1). Otherwise it is a device not within the service provider’s network (like a routher).
DS0 Digital Signal Level 0. 64Kbps line from the telco. Used for single voice calls.
DS1 Digital Signal Level 1. 1.544Mbps line inside a telco. Uses 24 DSO channels of its original speed plus a management/framing channel of 8Kbps. Also known as a T1.
Frame Relay A standard data-link protocol to create frame-switched or packet-switched service that uses DTE devices (such as routers) to connect to other devices using one physical connection.
HDLC High-level Data Link Control. Data-link layer protocol.
Packet Switching Term used to reference services that examine the contents of data to make forwarding decisions.
T1 1.544Mbps line inside a telco. Uses 24 DSO channels of its original speed plus a management/framing channel of 8Kbps. Also known as DS1.
PDU OSI Protocol Data Unit. This includes headers and trailers for the current layer and all the encapsulated data, if any. Often used as L#PDU. For example, L2PDU references the Data Link layer data unit.
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