When to use serial cables to connect routers?

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This is a bit of a novice question but when do you use serial cables for conecting routers, i thought ethernet cat cables and fibres would always be better, for example in packet tracer you often see the routers connected via seriel like below, with the red cable being the serial, some of the ref topologies in packet tracer suggest that you connect over huge distances via serial?

 

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Hi zr79,

Serial links are indeed commonly used over WAN links, largely for historical reasons and the fact that it can be used over a whole variety of different infrastructure mediums and methodologies. Regardless of how the WAN operates, which is usually not under your control, the routers at either end just see a straightforward serial link between them.

Hence the labs you are doing, and the image posted, are usually simply representing a WAN link using a back-to-back serial cable. The fact that there could be 10,000 miles and a variety of 3rd party networks between those two sites matters little - it still appears to each end as a straightforward serial connection.

At the risk of confusing the issue, whilst the example image represents what is likely to be two sites separate by a WAN link the implementation of it differs from reality (i.e. a real WAN connection) in that a serial connection requires one end to act as the DTE, and one the DCE (which provides the clocking amongst other things). The WAN provider provides the DCE hence your router at each end would both act as DTE's. In the lab case you don't have a real WAN hence one end must take on the role of the DCE and one the DTE. The end result is near enough the same, but it is still worth pointing out this difference as compared to what it is trying to emulate.

Does that help?

Mathew