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I am doing some CCNA training videos and using packet tracer and they showed using a serial cable to connect two routers. Why are serial cables used instead of Ethernet cables? Does it depend on the router or is it just a Cisco thing?

The cable connection in question is dce to dte.

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  • What kind of serial cable do you mean? In networking, there are several different ones (such as RS-232 and HSSI). Are you talking about running RS-232 serial over a Cat5 cable to the router's console port? – chrylis -cautiouslyoptimistic- Jul 31 '13 at 19:20
  • I don't know the technical name, but it was refereed to as dce to dte cable – Jason Pass Jul 31 '13 at 20:57
  • Oh, when you say "serial cables used over Ethernet cables", are you meaning "serial via Ethernet" or "serial instead of Ethernet"? – chrylis -cautiouslyoptimistic- Jul 31 '13 at 21:02
  • @JasonPass, Cisco simulators can use either an ethernet or serial cable; the decision to use one or the other is normally arbitrary unless there is a specific link encapsulation required (such as frame-relay or hdlc) – Mike Pennington Jul 31 '13 at 21:13
  • chrylis, Serial cable instead of using Ethernet. MikePennington So its more based on the need or personal preference? – Jason Pass Jul 31 '13 at 21:44
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One of the jobs of a router is to connect networks that have physically different transmission media. Twisted-pair Ethernet is extremely common these days, but fiber (Ethernet, SONET/OC, and others) is used for most high-speed or long-distance runs, and different types of layer-2 and layer-1 networks have been used in the past and are still in use today, such as T1/T3 lines, ISDN, and Token Ring.

For historical reasons, serial cables such as HSSI may be used to connect equipment such as T1/T3 terminating devices. These will show up as interface SerialN, and you can get detailed information on the hardware installed in a router with show controllers serial.

Unfortunately, "DCE/DTE" technically applies to a number of different hardware signaling schemes. In the context of networking, it's most likely to refer to a serial interface that's used to connect to a WAN drop.

In the case of your simulator, you should practice with both types of interfaces. In most business networks, the "inside" interfaces will be Ethernet, while the WAN interface will be either serial or Ethernet depending on what service the telco is providing, and you'll need to be able to configure either one.

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For the sake of completeness: Serial links are not solely used for gateway-WAN connections.

For instance, there are companies which host your network equipment and link your device via serial link to their infrastructure. Although your device is online with a public address, it still is not a DTE-like topology.

Another even more LAN-related serial link may be used on a daily basis by yourself and your colleagues: USB. Since USB does not only connect an end device to a storage device, but is also hosts to networks. An example for a host-to-LAN connection would be a tablet user currently being dislocated from a WAP. Because tablets usually do not have an Ethernet interface, a work-around is necessary. An example for a host-to-WAN connection would be field personnel accessing mobile broadband services via a pluggable antenna.

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Historically WAN links were almost always serial connections, to connect to service providers csu / dsu normally a serial cable was run from the router to the device sp. Cisco 200-355 VCE

Ethernet is used internally within the organization for the LAN, so the router has an ethernet port for that purpose.

Nowadays there is less use of serial interfaces, since WAN connections use the ethernet derivations of service providers for MPLS, Ethernet Metro, etc. There is usually a fiber device installed by the service provider and an Ethernet cable from which it goes to the router. So using serial interfaces for new connections is now less common, they are still loads out there though.

As for why PT default to a serial link between two routers, I guess that's what the developers chose as a default. Probably because connecting two routers together is usually a WAN connection. And ICND1 and 2 cover a lot of serial interface material, and Frame Relay, which also runs in series.

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we can classify devices into two.. 1.DCE 2.DTE to connect similar device Ex. DCE and DCE ..use crossover cable similarly, to connect DTE and DTE use cross over cable

To connect different devices DTE and DCE or DCE and DTE use Straight through cable.

(check which devices are DTE and which devices DCE)

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A serial interface on a Wan Interface Card (WIC) card is common. The configured framing for it varies depending on implementation but could be hdlc, ppp, etc. The DTE/DCE specifies your cable pinouts.

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