WAN Link

jaymeetme

Hello frnds
i m new to this networking field.
i have some questions regarding set up of WAN link
like, When we need DTE cable and DCE cable
and why they are used????
If modem converts the digital to analog then why we need CSU/DSU???
what is the function of CSU/DSU??

redgoblin

A DCE/DTE cable is used to 'simulate' a Leased line connection between routers without having to pay alot of money to a Telco for their circuit. The DCE side needs to provide clocking to the DTE.

A CSU/DSU coverts the data into a form the Telco can use to route through their 'cloud', so essentially, it doesnt matter what type of WAN connection you have to them, they use their own converted data form to route through their cloud. I believe also that the CSU/DSU provides clocking to the (customer) routers so that the WAN link can operate as expected.

Cucumber

Just to complement a bit:

The CSU/DSUs are needed for synchronous WAN links (one CSU/DSU, the master, will provide clocking for the CSU/DSU on the other end, the slave ). The CSU/DSUs are in charge of sending bits to each other even when no meaningfull data is being transferred between them just to keep the WAN link on synch.

You use a CSU/DSU when you get a (digital) leased line from the TELCO. Leased lines provide a fixed data rate (56Kbps,64Kbps,128Kbps,256Kbps,1544Kbps,2048Kbps, etc). The TELCO usually provides the CSU/DSUs. Some DSU/CSUs actually show the data rate at which they are working on a little LCD screen (like 64,128,etc).

A DSU/CSU does not convert between analog and digital signals.

Now for Modems.

You can not connect a modem to a (digital) leased line.

Modems are used for asynchronous WAN links (there is no clocking). Modems will convert an outgoing digital signal into an analog signal and will do the opposite on incoming signals. You use modems when you have analog local loops (analog phone line) from the TELCO on each side of the WAN. One of the modem dials, using DTFM, to call another modem that is also connected to an analog phone line. Then both modems will agree on the speed they will work with (28.8Kbps,33.6Kbps, 44Kbps, etc, up to 56Kbps).

You can not connect a CSU/DSU to an analog local loop.

In summary

CSU/DSUs get connected to (digital) leased lines, modems get connected to analog local loops (analog phone lines).
CSU/DSUs are expensive, modems are cheap.
Modems dial, CSU/DSUs cant dial.
CSU/DSUs provide clocking, modems do not.
CSU/DSUs are synchronous, modems are asynchronus.
CSU/DSUs send bits to keep on synch, modems do not.
CSU/DSUs work at a fixed rate, modems must agree on the data rate they will work with.

Conclusion

CSU/DSUs and Modems are very different animals.


Note: A LOT of time ago there used to be "analog" leased lines that would require a modem, but Im not going into it because it will get things confusing.

jaymeetme

Thanks redgoblin & Cucumber
for ur very nice explanation
now i m clear with WAN link set up