Hello Atila
The terms Channel Service Unit/Data Service Unit (CSU/DSU), Data Terminal Equipment (DTE) and Data Communication Equipment (DCE) are all terms used to refer to equipment that has been traditionally installed at or near the demarcation point of the customer’s premises.
This equipment was necessary primarily because the WAN technologies (serial, DS1, T1, E1, OC-X) used by telcos were different from the LAN technologies used by the customer. The CSU/DSU was used to convert the WAN technology to LAN technology such as Ethernet.
The DTE is typically a customer device, on the customer side of the demarcation point, such as a router.
The DCE, which is typically a telco-owned device, is the device on the telco side of the demarcation point. It is a CSU/DSU that converts from WAN technology to LAN technology.
Now today, these terms are slowly falling out of use because we have more and more cases where the WAN and LAN technologies are the same (i.e. Metro Ethernet, fiber optics, wireless links etc…). So although the demarcation point still exists, the equipment that may or may not be there depends highly upon the arrangement that you have made with the telco.
So you can have many different arrangements such as:
- a single fiber optic cable coming onto your premises that connects to your router’s SFP port and delivers Ethernet
- a DSL modem (that can be considered a CSU/DSU, DTE and DCE all in one device) that delivers wired and wireless communication on the customer premises
- a more traditional serial connection with the necessary DTE unit
- a wireless microwave link that may need specialized equipment to terminate the microwave link before delivering Ethernet internally
Specifically, in the case you mention, if you have a fiber optic leased line, you can terminate that on your own equipment, so you wouldn’t need any other telco-owned equipment in such a case.
You can, if that is the arrangement you have with your ISP.
They are scarce nowadays but are mentioned here for completeness as they may be mentioned in an exam. But remember, there are still some places in the world where these technologies still exist, especially in remote and rural areas.
I hope this has been helpful!
Laz