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I was searching this for quite long time but I haven't found any satisfying result so my question is basically that, if I understand correctly how modems work and when they are used. I have some more questions so here is what I think I know

  1. They work on OSI layer 2 (MAC adresses) and they are connected directly to ISP since they don't know how to work with IP of some webpage which I am requesting (OSI layer 3) (Or they can somehow go up the OSI stack to third layer?)

  2. They are used only when you are accessing internet through telephone cable because they need to modulate/demodulate analog signal for a router or computer to which they are connected.

  3. If number 1 is right and they are connected directly to ISP, what is next? Are packets sent through routers and other networking devices like switches and so?

martin
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3 Answers3

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  1. The role of modems is to modulate and demodulate (hence the word modem) data so that it can flow over the phone line, effectively transforming the data from digital to analog and viceversa. For this reason, they can be considered Layer 1 devices. They cannot work on Layer 3 of the OSI model (routers and Layer 3 switches can do that).

  2. Correct, but they are not necessarily used only with telephone cables. They could also be used with a cable (CATV) line.

  3. Yes, the data flows in the wires to the next network devices. In summary, the next router in the nextwork will receive the stream, deencapsulate it until the layer 3 IP packet inside it is exposed, and will use this information to decide where to send the data. This is repeated until the data arrives at the destination.

And to answer the question in the title: yes, they are still used today. It's just that you don't see them most of they time because they are built-in the routers you buy in the shops.

user1301428
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A modem does not 'care' about networking as it strictly a physical layer device.

As the name suggests, a modem's job is to (de)modulate signals so that they may be transmitted across various media.

Here are four scenarios in which two parties may wish to transfer data. In each case they will require a different type of modem.

  1. Your home router connects to an ISP's access network by using a digital xDSL modem to establish a connection over a twisted pair of Copper cables (your telephone line). Some providers do this over a coaxial cable ( your cable TV line) instead. Older systems typically used analogue 56Kbps dial-up modems. If you are lucky enough to have fibre-to-the-door then you'll be given an optical modem instead.

  2. Your mobile device connects to a wireless router using a wireless modem. This allows a connection to be established using radio waves. Similarly Infra red devices will use an infra red modem.

  3. You are underwater and want to communicate with another vessel. To do this you will transmit data via an acoustic modem.

  4. When you switch your mobile telephone on, it will attempt to connect to a base station (cell tower) using a GSM modem.

The point is there are many different types of modem (satellite-, voice-, soft-, the list goes on), but they all essentially do the same job.

Regarding point 2 in your post. Just because a signal is transmited over a particular (possibly exotic) medium, this does not automatically mean that the signal is analogue.

Regarding point 3 in your post. After leaving your home computer network, your data enters your ISP's access network. The job of the access network is to get your data to your ISP's core network as quickly as possible. Your data will traverse the core network and then exit "out the other side" down another access network until the it reaches it's destination.

The whole thing is analogous to the road network.

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A telephone modem modulates and demodulates digital data onto a analogue phone line (POTS) - a purely physical layer function (OSI L1).

A DSL modem bridges between different network technologies (often ATM over xDSL and Ethernet), so it's a data link layer device (OSI L2).

A cable modem also bridges between different technologies (usually Ethernet framing over DOCSIS and plain Ethernet), also a data link layer device (OSI L2).

A GSM modem is harder to grasp - it's somewhat like a phone modem and pretty much physical layer. Beginning with UMTS and especially with LTE, there are also higher-layer functions, up to the network layer (OSI L3).

Commonly, "modem" is also - incorrectly - used for a (home) router integrating a DSL or cable modem - that would be a network layer device (OSI L3).

Zac67
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