Frame Relay WAN clocking on a DTE interface When a router is connected to a Frame Relay wide area network (WAN) using a serial DTE (Data Terminal Equipment) interface, how is the clock rate for the link determined?

Introduction / Context:On serial WAN links, one side provides clocking (timing) and the other side follows that timing. In Frame Relay service from a carrier, the customer premises router typically connects via a CSU/DSU (Channel Service Unit/Data Service Unit). Understanding who provides the clock is essential for stable, error-free communications and to avoid slips or framing errors.

Given Data / Assumptions:

• The router interface is configured as DTE on a Frame Relay WAN.
• A carrier-provided CSU/DSU or provider edge device is present.
• The link uses synchronous serial signaling that requires a reference clock.

Concept / Approach:

In a service-provider-provided WAN, the DCE side (CSU/DSU or provider edge) generates the line clock. The customer router, acting as DTE, recovers timing from the carrier and does not set its own clock rate. The IOS clock rate command is only used when the router is acting as DCE (for example, lab back-to-back serial connections).

Step-by-Step Solution:

Verification / Alternative check:

On Cisco IOS, show controllers serial reveals DTE/DCE. If the router detects DTE, the clock rate command is not accepted or has no effect; timing originates from the CSU/DSU.

Why Other Options Are Wrong:

• By the far end router: routers as DTEs do not provide network timing to each other through the carrier cloud.
• By the clock rate command: used only when the interface is DCE (for example, lab cable), not on a Frame Relay DTE link.
• By the Physical layer bit stream timing: vague; the Physical layer timing ultimately comes from the DCE (CSU/DSU), which is precisely option A.
• Negotiated by LCP during PPP setup: PPP authentication/negotiation does not set physical clocking.

Common Pitfalls:

• Trying to set clock rate on a DTE interface; it will not solve timing issues.
• Confusing DCE/DTE roles; only the DCE supplies clock.

Final Answer:

Supplied by the CSU/DSU