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Usually, customer receive only a single device when they buy an Internet connection from an ISP. This device is connected to the ISP via fiber or a DSL line, has several Ethernet ports and provides wireless access. I have always called that a customer premise equipment.

DCE and DTE are as far as I understand the two ends of a serial line, and usually the DCE is a DSL modem that provides connectivity toward an ISP and the DTE is most of the time a router. Does that mean that a CPE is a single box combining the functions of both the DCE and the DTE, like in the following picture?

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DCE and DTE are purely serial-interface concepts (where the DCE is the modem ie. line interface and the DTE is the upper layer device ie. router or PC). They don't apply to DSL or fiber.

DCE and DTE are terms from the terminal age, predating more modern network layering. I'm not sure if there's a single formal definition but here is says for DTE

  1. An end instrument that converts user information into signals for transmission or reconverts the received signals into user information.
  2. The functional unit of a data station that serves as a data source or a data sink and provides for the data communication control function to be performed in accordance with link protocol.

And for DCE

  1. In a data station, the equipment that (a) performs functions, such as signal conversion and coding, at the network end of the line between the data terminal equipment (DTE) and the line, and (b) may be a separate or an integral part of the DTE or of intermediate equipment.
  2. The interfacing equipment that may be required to couple the data terminal equipment (DTE) into a transmission circuit or channel and from a transmission circuit or channel into the DTE.

If you take the user information literally the DTE covers up to OSI layer 7 while the DCE covers layer 1. However, there's a large gap where layers 2 through 4 are located.

DSL uses a modem as CPE and a DSLAM as COE. Passive fiber uses an ONT as CPE and an OLT as COE. Very often, the CPE integrates a router.

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  • Not entirely; ethernet continued the DCE/DTE legacy when 10bT came about. (there's a dedicated pair for RX and TX, so which end pays which roll mattered. Gigabit ethernet finally killed it -- all four pairs are used for TX and RX) [not that that's the only distinction between the two]
    – Ricky
    Sep 2 '19 at 20:17
  • And DSL and xPON most certainly do still have DCE and DTE rolls. (ignoring the never standardized SDSL) A DSL/xPON/DOCSIS modem is DTE; they cannot talk to each other, only DCE equipment (DSLAM, OLT, CMTS) (D3.1 FDX may be physically capable of, but it's not designed to work that way)
    – Ricky
    Sep 2 '19 at 20:23
  • @RickyBeam Of course, you can stretch the definition but it isn't the same. Applying it to 10/100BASE-T in contrast to 1000BASE-T is pretty far fetched though...
    – Zac67
    Sep 2 '19 at 20:39
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Usually, customer receive only a single device when they buy an Internet connection from an ISP. This device is connected to the ISP via fiber or a DSL line, has several Ethernet ports and provides wireless access.

You are looking at this from a home networking perspective, and that is not usual for a business. Typically, a business will provide its own networking equipment, and that is the CPE that connects to the DMARC (demarcation point), which can come in as any of various WAN technologies, depending on what the business ordered from the ISP. xDSL is less common for businesses than for residential users because it is too unpredictable for many business uses.

DCE and DTE are as far as I understand the two ends of a serial line, and usually the DCE is a DSL modem that provides connectivity toward an ISP and the DTE is most of the time a router.

DCE can be something like a CSU/DSU, modem, ethernet switch, etc. That is a communication device, while DTE is an end-device, such as a PC, router, etc. DTE connects to DCE. The most common example is a PC with an ethernet connection (DTE) that connects to an ethernet switch (DCE). Connecting two like devices (DCE-to-DCE or DTE-to-DTE) used to require a crossover cable, although most modern devices have built in the hardware and logic to detect this situation and adjust one end or the other so that a crossover cable is no longer necessary.

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