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From my Cisco books (CCNA level) I learned that in a WAN connection one router must be configured as the DCE and the other as the DTE. In real life I have a DS-3 between two routers. There are no CSU\DSUs (just coax out) or DCE\DTE configuration on the serial interfaces. There are no clock rate commands in the configuration, only dsu bandwidth XXX. The routers are using c-bit framing if that matters.

Everything is configured and works but I am just wondering why there are no DCE\DTE configurations. Is CCNA teaching old school technology?

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up vote 17 down vote accepted

The whole dce/dte thing is still correct, in theory. However, today's devices generally insulate you for those details.

I bet your ds3 cable connects directly to your router, right?

It used to be that the only way to connect to a router was a serial cable. Slightly more modern routers got ethernet. It was a long time before anything else was available. If your connection wasn't a serial cable or ethernet then you needed a csu/dsu to convert it to serial.

With Ethernet you had standardized signaling. For the most part, just plugging in an Ethernet cable was all you needed to do to "configure" the physical layer.

Serial cables, on the other hand, provide no information about the timing characteristics of the signals on them. Since the serial port had to support a bunch of different timing possibilities to be able to connect to a variety of different interface types, and the serial cable didn't provide any hints as to the signaling format, configuration was needed in the router so it knew what to expect coming from the csu/dsu.

Now fast forward a bit more and you start seeing routers with built-in ISDN and t1 ports. These connection methods carry with them more assumed parameters (such as the clock rate and signaling format) so those options disappear. For example, a t1 has a fixed bit rate, so that option disappears. And since the csu/dsu is built in to the router, all of the options about cable distance and equalization disappear.

So this directly applies to your ds3. There is only one clock rate possible on a ds3, so you don't need that. Is the ds3 clock provided by the telco? If so, then you don't need any clock settings because the router will detect the clock and latch on to it.

In fact, the bandwidth number is probably unnecessary. First of all, that number doesn't actually limit the bandwidth of the port. It is used by other processes in the router, such as routing and qos, to make flow decisions. What is it set at? If its the full bandwidth of a ds3 then it's unnecessary.

Tl;dr: today's routers accept wan connections directly instead of going through a csu/dsu to convert it to a serial cable. The interface type (isdn, ATM, t1, ds3, oc1, etc.) implies a lot of things, such as clocking, and therefore do not need to be configured. Your study materials are teaching you about converting things to serial first, which is becoming more and more rare these days.

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worth mentioning - any TDM-based interface (56k -> OC768, inclusive of T1/T3 etc) by default will derive its clock source externally but can be set to clock internally, which is generally useful if in a back-to-back configuration with another router (..which is set to derive from line) – rnxrx Aug 16 '12 at 3:06
Thanks. I thought T1 and T3 (could have) the same physical layer (coax). I didn't think there was a "ds3 card", I thought it could be used for a T1 or T3 connection in which case how would it know what standard to use? Back in the CSU\DSU days could you set the clock to whatever you wanted or did the telco still provide it? If so then I suppose setting clocking is only useful in the lab and for QoS as you said. – user974896 Aug 17 '12 at 0:28
they might have the same physical connection, but other properties of their layer 1 are different, such as electrical characteristics and clock timing. – longneck Aug 17 '12 at 13:17
also, you seem to be using bandwidth and clock rate interchangeably. they are not the same or equivalent. the clock rate is how fast the digital clock on the line ticks. bandwidth is how many bits per second a logical channel can handle. with t1, the clock rate is more "bits per second" than the available bandwidth due to signaling overhead. with the type of serial connection that cisco routers use, the clock rate could actually be fewer "bits per second" than the bandwidth because the serial port has multiple serial links in the cable and sends multiple bits simultaneously in parallel. – longneck Aug 17 '12 at 13:28
You will still see CSU/DSU connections around, so it's still relevant. As far as the clock, you have to negotiate with the telco but usually they require you to take clocking from the line (the default). Mismatched clocks are difficult to troubleshoot since the errors you see in the IP layer are inconsistent. – Alo Aug 23 '12 at 21:40

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