Networking media basics: Which of the following is not a transmission medium for carrying signals between devices?

Introduction / Context:
A transmission medium is the physical path or propagation environment a signal travels through to carry information from a sender to a receiver. Distinguishing between the medium itself and the devices that interface with it helps prevent design and troubleshooting errors.

Given Data / Assumptions:

• Media can be guided (copper, fiber) or unguided (microwave, satellite links).
• Interface devices may condition, convert, or modulate signals for transport.
• We must select the option that is not a medium.

Concept / Approach:
Telephone lines and coaxial cables are guided media. Microwave and satellite systems describe wireless propagation environments (unguided media). A modem, however, is a device that prepares signals for a medium (e.g., modulates for a telephone line) and therefore is not itself a transmission medium.

Step-by-Step Solution:

Verification / Alternative check:
Ask whether the item can exist end-to-end between two sites without active electronics. Media can; a modem cannot—it is an endpoint device, not a path.

Why Other Options Are Wrong:

Common Pitfalls:
Treating system names as devices; mixing up modems (devices) with the copper pair they use; overlooking that “systems” can still denote a medium context here.

Final Answer:
modem.