Why are serial ports used in routers?

What are the uses of having serial interfaces in routers? I know they're used to connect routers together but aren't they too slow? or are they not used anymore in modern cisco routers?
are serial ports less expensive than fast ethernet or gigabit ports?
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GJHopkinsConnect With a Mentor Commented:
No they are not slower than always slower than Ethernet , how about a 10Gbps Serial port


SDH OC-92/STM-64 for example

Its a question of what you want to do with the router, say you have 1000 branch offices and each has a E1 or T1 tail circuit and you want to bring them together at the core of your network. Well you can't get a router with a 1000 x T1 interfaces but you can use your SDH network to bring them together using multiplexers into  an STM-1 -  84 x T1 and then mux that up to STM-64 and plug into a router on a single interface - service provider stuff granted.
Dave BaldwinFixer of ProblemsCommented:
Serial ports are used to configure the routers, not to pass network traffic.
Istvan KalmarHead of IT Security Division Commented:

The older cico routers has Serial ports, it is not depends on the IP routing...
The newr cisco devices x9xx series has USB connector, CIsco wants to use USB on the future!
As Dave said it use for confiure....
All professional devices has SERIAL ports.....

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I believe you are referring to serial interfaces, not console ports.  Serial interfaces are used to connect T1's and such lines to your network.  They are used to connect WANs together.  They shouldnt be used in a LAN environment, except for testing.  They are incredibly slow (1.555mbps), but they dont need to be fast since the speed of a T1 maxes out at 1.544mbps.
I think he is refering to actual serial connection ports such as DCE/DTE type connections (ex.HWIC-1t.)

Yeah serial ports are slower but they are usually used now for T1 connections or an incoming connection from an ISP. For slower WAN connections such as these the speed limitations of serial interfaces will do.

MistralolConnect With a Mentor Commented:

If you are talking about the console ports. Thats so you can get access to them without being on the network (a common requirment) eg engineer visiting a site who isnt permitted to plug his laptop into somebody else's network. Or remote access by dial up modem etc..

If you are talking about the serial interfaces that a router will use to send data then you are probably making an unfair comparison to a pc's serial port (crap hardware) eg a pc port will do 115,000 baud or 115kbit. The poorest cisco serial will do 256/512kbit (typically async port). The high speed serial ports will do 2mbit or some 2 port cards will do 8mbit. So a router with multiple port really isn't so slow after all.

Also most other companies eg telecoms will present a serial connection when dealing with leased lines when are around 2mbit or so. It's either that or they would present you fibre when its not really required. So they are really used because other things will interface with them.

vidularandunuAuthor Commented:
I am indeed talking about serial interfaces (sorry for my mistake at the title, but as the question says)..so the serial interfaces are used to connect to t1 lines?, can't anything else be used for that purpose?
Istvan KalmarHead of IT Security Division Commented:

Some ISP not able to solve ethernet connection, a lot of ISP system build by older PDH/SDH system!

PDH system contains nx64bit timslots:


MistralolConnect With a Mentor Commented:

You can also connect isdn30. Which is like E1 / T1 but you can choose to break up the channels into voice / data which makes things more complicated.

But sure if you dont want to use a T1 you can pay for a new cable for the 50+ miles run that your doing between to location instead of using existing infrastructure.

vidularandunuAuthor Commented:
guess i have a lot to learn about these..thanks alot for the answers..
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