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Which of the following protocols is a specification for the interconnection between DTE and DCE at the data link layer?
Cisco may simplify some definitions, and you need to know what they are looking for on the certification exam. Unfortunately, not all Cisco documents are consistent in explaining DTE-DCE relationships. The focus here is on layer 2 interactions, although there can be DTE-DCE interactions at multiple layers.
One good training slide I've seen describes three sorts of relationships:
End host to router (e.g., SDLC- or BSC-attached legacy device)
Router-to-router (e.g., HDLC, PPP, or LAP-B)
Router to service provider (e.g., Frame Relay, X.25/LAP-B, ISDN)
Strictly speaking, the DCE comes from a service provider, and the DTE is from the user side. When answering any Cisco question, look for what appears to be the user side versus the provider side.
ISDN and Frame both share the property of being local access protocols to a provider. Indeed, there are Frame over ISDN services. X.25 is also a local access protocol that runs over a data link layer of LAP-B.
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