4 Network Media and Topologies

Media and network layout

Network media carry signals between devices, while topology describes how the devices and their connections are arranged. These are related but distinct: the same topology can use different media, and a network's physical layout may differ from how data logically travels through it. Media characteristics are defined at the physical layer of the OSI model.

Transmission media

Copper

Copper cables carry data as electrical signals. Twisted-pair cable, commonly used for Ethernet connections to computers, phones, and access points, contains insulated copper wires twisted together to reduce interference. Unshielded twisted pair (UTP) is widely used. Shielded twisted pair (STP) adds shielding to help protect against electrical noise, but is generally more costly and harder to install. Coaxial cable is another copper medium used in some broadband and other communications systems.

Copper can be practical and cost-effective. Ethernet over copper can also supply power to compatible devices. Its reach and performance depend on the cable type and networking standard.

Fiber

Fiber-optic cable carries data as pulses of light through glass or plastic fibers. It supports high data rates and longer link distances than many copper connections, and the cable itself is immune to electromagnetic interference. Multimode fiber is commonly used for shorter links, such as within buildings or data centers. Single-mode fiber is suited to longer links because it limits light to a narrower path. Fiber is often used for high-capacity links between switches, buildings, or network facilities.

Wireless

Wireless networks transmit radio signals through the air rather than through a cable. Wi-Fi is commonly used to connect mobile devices to a local network. Wireless allows mobility and flexible placement, but signal quality and performance can vary with distance, obstacles, interference, and the number of devices sharing the radio channel. A wireless link still connects into a physical network, often through a wired access point.

Bandwidth and throughput

Bandwidth describes a link's data-carrying capacity and is commonly expressed in bits per second (bps), such as megabits per second (Mbps) or gigabits per second (Gbps). Throughput is the data rate actually achieved. It is often lower than the link's rated bandwidth because of factors such as network load, signal quality, distance, and protocol overhead.

For example, a link rated at $1\,\text{Gbps}$ has a maximum capacity of $1$ billion bits per second, but a file transfer may run more slowly. The rate is also limited by the slowest link in the path: connecting a fast computer through a slower switch port does not make the complete path faster than that port.

Physical and logical topologies

A physical topology shows the actual layout of cables, radios, and devices. A logical topology describes how data moves or how devices share access to the network. For example, a physical star may carry traffic as individual switched links or as shared wireless communication.

Common topology forms

• Star: Devices connect to a central switch or access point. This is common in modern wired LANs. A failed endpoint cable usually affects only that endpoint, while failure of the central device can affect many connections.

• Bus: Devices share one main cable. This layout is simple, but a fault in the shared cable can disrupt the network. It is mainly of historical importance in Ethernet LANs.

• Ring: Each device connects to neighboring devices in a loop. Traffic may travel around the ring; some designs use a second path for resilience.

• Mesh: Devices have multiple interconnections. A full mesh connects every device directly to every other device; a partial mesh provides selected redundant paths. Mesh designs can improve resilience but require more links and planning.

• Tree (hierarchical): Groups of star networks connect through higher-level devices, forming layers. This supports organized expansion across floors, buildings, or departments.

• Hybrid: A network combines topology types, such as star-shaped office LANs connected in a hierarchy.

Topology choices involve trade-offs among cost, performance, ease of expansion, and resilience. A home network may use a star centered on a Wi-Fi router, while an organization may connect access switches in a hierarchy with redundant links between key devices.