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Lesson 4 Serial modems
Objective Define a modem's role in the Linux system.

Linux Serial Modems

The traditional role of modems, particularly analog dial-up modems, has become less prominent with the advent of high-speed internet connections. This change is not unique to Red Hat Enterprise Linux (RHEL) but reflects a broader shift in the network infrastructure used worldwide. As broadband, fiber optic, and wireless connections have grown more prevalent and affordable, the use of dial-up modems for internet access has declined significantly.
However, it's important to note that "modem", a term derived from "modulator-demodulator", refers to a category of hardware devices, not just the analog dial-up modems often associated with the term. These devices modulate one or more carrier wave signals to encode digital information for transmission and then demodulate signals to decode the transmitted information. The term is traditionally used to describe hardware that converts digital data from computers into analog signals for transmission over traditional telephone lines, and vice versa.
While the specific role of dial-up modems has been mostly superseded, other types of modems still play vital roles in modern network infrastructure. For example, cable modems and DSL modems, which provide high-speed internet access over cable TV lines and digital subscriber lines respectively, are in widespread use. Similarly, cellular modems, which allow for data transmission over mobile networks, have grown increasingly important with the rise of IoT (Internet of Things) devices and applications requiring out-of-band or remote network access.
So, while the traditional dial-up modem's role has been largely deprecated, the function of modems as a category of networking hardware remains relevant. RHEL, like other modern operating systems, continues to provide support for a variety of modem types, reflecting their ongoing role in network connectivity. It provides tools and drivers for configuring and using these devices, and it's capable of working with additional software, like NetworkManager and ModemManager, to manage complex network setups that involve different kinds of modems.

Legacy Modems

A modem places calls to or receives calls from other computers over conventional telephone lines. When a call is established, the communicating computers negotiate a protocol. Linux supports serial protocols such as Xmodem, Zmodem , and Kermit, as well as Internet protocols such as PPP and SLIP. PPP enables other protocols, such as IP or IPX, to function over a telephone line. PPP provides a peer-to-peer connection and is the most common method of connecting to an Internet Service Provider (ISP).
As illustrated below, the client computer uses its modem to establish a connection to the server computer at your ISP. The client computer must have a properly configured kernel, meaning PPP or SLIP support is available to the kernel. PPP support is modularized and included with Red Hat Linux.
  1. Protocol: Protocols identify the type of content communicated. One of the most common protocols is PPP, for communication to and from an Internet Service Provider.
  2. Xmodem: Xmodem is a popular file-transfer protocol.
  3. Zmodem: Zmodem is similar, but has improved error detection.
  4. Kermit: A communications protocol developed at Columbia University.
  5. PPP: Point-to-Point Protocol. It is used to connect computers with the Internet.
  6. SLIP: Serial Line Internet Protocol. SLIP is very similar to PPP.
  7. IPX: Internetwork Packet Exchange protocol. IPX is a networking protocol.

Connecting with a serial modem
Connecting with a serial modem
More information about how to configure a PPP dial-up connection is included in the Linux Documentation Project (LDP) PPP-HOWTO, available from the Resources page. You can also find additional information in The Official Red Hat Linux Installation Guide. The next lesson discusses uses for multiple network interface cards.
Click the link below to read about the setserial command to query the characteristics of a serial device.
Set Serial Command