Are modems needed to connect routers?

Question

I apologize for the stupid question; I'm trying to wrap my head around the basics of networking. Also, please feel free to direct me to the right SE forum, if this is not the right place.

My question is:

Are modems (the devices that map between digital and analog data) needed to connect routers (the devices at the networking level that implement some routing algorithm to move packets around)?

My initial thought is: assuming links are not digital (so, say, some kind of copper wire), communicating routers must either:

• be able to translate waves to bits to read the appropriate headers for addressing, or
• use some kind of a device (a modem?) to do the translation for them.

I would appreciate any insight into the role of modems in routing. As a source I have Kurose & Ross, 6th edition, so a simple pointer to a relevant section of the book could also help me a lot.

Answer 1

Modems are only needed to translate signals between analog and digital circuits. Some devices called modems aren't really modems in the strictest sense. For instance, many variants of ethernet use copper, but ethernet uses digital signals, and you wouldn't call a device which connects to a copper circuit using ethernet a modem, although I know people who do. Modem don't need to connect to routers since you could connect a modem to a switch or other networking device, depending on the network design.

There are many different layer-1 technologies - some use modems, and some don't. I'm not really clear by what you mean about the role of modems in networking because you use modems where they are needed to connect analog circuits to digital circuits. It really depends on the network design. It's almost like asking what is the role of UTP cables in networking; you use UTP where it is needed for a particular network design. The same is true for modems, routers, switches, or any other network component.

Answer 2

Fundamentally a circuit is needed to convert the stream of data packets from the internal form used inside the device into a form suitable for tranmission over the physical medium in use.

Whether that device is called a "modem" varies between different physical layers. The convention seems to be that the term "modem" is used when communicating across a physical layer not originally intended for data transmission (e.g. a phone line or a cable TV cable) but not when communicating across a dedicated data link (the term "transceiver" is more commonly used there"). The term "radio modem" also used to be common but seems to have fallen out of favour.

In the old days there was a sharp distinction between the digital coding schemes used by early ethernet and the tone based approaches used by early voiceband modems but nowadays the technical line is much less sharp.

Answer 3

A router transfers packets between two (or more) different networks. A typical case is a router sitting between the home or office network and the ISP's network. A more complex case might have several separate LANs, a separate VoIP network, multiple ISP connections and several VPNs.

There are cases where the ISP provides a connection which is directly usable by the router (like a plain Gigabit-Ethernet link). In such a case, there will be no obvious modem box (even though Gigabit-Ethernet is actually a modulated signal - the Ethernet transceiver is therefore a modem, just not as visible as a typical obvious modem box used to translate DOCSIS, ADSL, VDSL, SDSL or whatever to Ethernet or whatever else).

There is also the option to set up complex routed networks without physical network hardware (and therefore no modem, not even Ethernet transceivers). Just a bunch of virtual machines, connected to each others via virtual network adapters and virtual switches. A use case for such a setup might be the test of router software, running in one or more of the VMs, with clients and servers running in other VMs.