router serial cable question

Dist · 5 Apr 2009 at 01:10

I'm doing my CCNA course at the moment, and have come onto something that has not been explained compleatly so i am hoping i could find out here.

Basicaly, the course talks about using a serial cable connection to connect a router to a WAN, or between back-to-back between routers in a lab environment. It talks about that when using a serial connection such as this that a device on one side is DTE (a router in this case), and on the other side of the WAN is a DCE which does the signal clocking and such (and in a lab environment will be the job of one of the two routers).

Now my question is this, why does connecting these type of routers require such an interface/cables? why is it not simply possible to connect to the WAN via a fibre optic interface (which i assume is what big companies and such use to provide their internet access). Also, what is the point of such a variabl clock rate system? can it not be simplified like ethernet? why does one device have to generate a clocking signal? The course im on says that on an ethernet connection, the devices use the signal of the transmited data itself as timing.

Finaly i have one last question for now. Currently my course says that cisco routers connect their rj45 console port to a serial port on the PC, now since most modern PCs lack a serial port can i expect cisco to create devices that connect to the console port using USB or an ethernet port? (For all i know cisco already do this, but since this is a CCNA it deals with some old equipment, and so the brand new cisco gear that may have given up on using serial for console connections wouldn't be in my course).

Venares · 5 Apr 2009 at 02:52

Not to sure about your first question but as to your second regarding the console cable you just need a serial to usb convertor, there usualy about £10 or so.
Unfortunatly Cisco still ship the same 9pin serial cable they have done for years (I have a mountain of these now) and it doesnt look like there going to change away from that any time soon.

wij · 5 Apr 2009 at 07:54

Not going to answer your question about serial connections between routers further than saying it is fairly legacy tech. and is mostly just included in the CCNA as it gives you a grounding in where stuff has come from. I think I am right in saying that you are unlikely to find it being used in many production systems these days as most people do just use Ethernet for their connectivity now.

If you want to do serial over IP you might want to consider something like some of the devices made my Moxa (http://www.moxa.com/product/NPort_5110.htm). But they are quite dear - you're looking at £200+ for a simple 1 port device.

Dist · 6 Apr 2009 at 16:31

When using the serial interface on a PC to connect to the console port on a router, does the connection have to be direct, or can there be devices inbetween? For example, can multiple routers have all their console ports connect to a switch and then have that switch connected to the serial port on a PC to have a dedicated PC for controlling the console ports on all the routers, or would each console port need its own serial port on a pc?

wij · 6 Apr 2009 at 17:05

When you use the term 'switch' are you referring to an ethernet switch?
If that is the case then the answer is NO.

Just because it uses an RJ45 plug doesn't mean it is ethernet

You can get serial port switchers though they just allow you to select which device you want to talk to.

pdw8 · 6 Apr 2009 at 17:25

Serial connections have generally been used for PDH connections such as T1,E1 (2Mb), E3 and DS3 bandwidth. This is old technology now but you would be surprised how many of these connections are still out there being used.
When I first started in Telecoms we installed E1's and you have an E1 circuit either on-net or using another providor to get to your location back to the hubsite. First you connected a CSU/DSU which is a device that deals with the PDH signal, framing, timing and timeslots. BAck in the day an E1 was very expensive and many thousands of pounds a month. Not all customers wanted a full 2Mb so the CSU can channelize the E1 in blocks of 64Kb. If the customer wanted only 512Kb then you only configure 8 x 64Kb timeslots. There then comes a 10baseT connection to a router such as a Cisco 1600 varient. The router then obvioulsy brought up the layer 3 protocols and end to end connection.
The clocking is required to syncronise the PDH signal so the bits are sent and received in real time and in the correct formation.
Nowadays we only sell Ethernet ports which is a million times easier to use and setup. Cabling is generally CAT5/6 for 10 - 99Mb and fibre for 100Mb, GE and 10GE.