Computer network - Physical Layer Media and Devices

Network Links and Transmission Media Introduction to Physical Media Networks require a physical medium to connect devices. Transmission media (also called physical media) are the materials and technologies that link devices in a computer network. The three main categories are: Electrical cables (like twisted pair) Optical fiber Free space (for wireless communication) The physical medium itself isn't enough—the Physical Layer and Data Link Layer of the OSI model provide the software and protocols needed to transmit data reliably across these media. Wired Media Twisted Pair Cabling Twisted pair cabling is the most common wired medium you'll encounter. It consists of four pairs of copper wires twisted together. Each pair is twisted around each other, which is crucial to its function. Why twist the wires? When electrical signals travel through parallel wires close to each other, they can interfere with one another. This interference is called crosstalk. The twisting pattern disrupts this interference and also reduces electromagnetic interference from external sources. By the time you've twisted the wires at the right pitch and frequency, signals maintain their integrity over practical distances. Twisted pair is used in wired Ethernet and many other networking standards. It's affordable, relatively easy to install, and works well for most local area networks. Optical Fiber Optical fiber uses a completely different approach: instead of electricity, it transmits data as pulses of light. Lasers at the transmitting end generate these light pulses, and at the receiving end, photodetectors convert them back to electrical signals. Optical amplifiers can boost the signal strength along long distances. Optical fiber comes in two main types: Single-mode optical fiber is designed to carry only one light path (mode) through the fiber core. This makes it very efficient and allows signals to travel dozens to hundreds of kilometers without needing amplification. However, single-mode fiber is more expensive and requires precise alignment when connecting. Multi-mode optical fiber allows multiple light paths to travel through a larger core. This makes it cheaper and easier to connect, but the different light paths travel at slightly different speeds, which causes signal degradation. This limits multi-mode fiber to shorter distances: typically a few meters to a few hundred meters, depending on the data transmission rate. Key takeaway: Single-mode is better for long distances; multi-mode is cheaper and works for shorter distances. Wireless Media <extrainfo> Communication Satellites Communication satellites orbit at geosynchronous orbit, approximately 35,400 kilometers above the equator. At this altitude, they remain stationary relative to the Earth's surface, allowing them to relay voice, data, and television signals continuously to the same geographic region. The tradeoff is higher latency due to the distance signals must travel. </extrainfo> Cellular Networks Cellular networks divide a geographic region into overlapping areas called cells, each served by a low-power transceiver (base station). This allows multiple devices to communicate wirelessly while reusing the same frequencies in different cells. Cellular networks provide mobile voice and data services to your phones and tablets. Wireless Local Area Networks (Wi-Fi) Wireless LANs use high-frequency radio waves to transmit data within a limited area, typically within a building or campus. They employ spread-spectrum techniques, which spread the signal across a wider frequency range to improve reliability and reduce interference. The IEEE 802.11 family of standards defines Wi-Fi, the technology you use every day. These standards specify how devices discover networks, authenticate, and exchange data wirelessly. Different 802.11 versions (like 802.11ac, 802.11ax) offer varying speeds and ranges. Network Nodes and Devices Understanding the different devices that make up a network is essential. These devices operate at different layers of the OSI model and serve different purposes. Network Interface Controllers Every device that connects to a network needs a network interface controller (NIC)—a piece of hardware that does the actual work of connecting your computer to the physical medium. The NIC: Translates data from your computer into signals appropriate for the transmission medium Receives signals from the medium and translates them back into data Handles low-level network operations Media Access Control (MAC) Addresses Each Ethernet NIC has a unique identifier called a Media Access Control address or MAC address. A MAC address is six octets long (48 bits), typically written in hexadecimal notation like 00:1A:2B:3C:4D:5E. Think of a MAC address like a physical address on your house—it identifies where your device is on the local network. The first three octets identify the manufacturer, and the last three uniquely identify that specific NIC. Repeaters and Hubs Repeaters are simple devices that operate at the Physical Layer. When a repeater receives a network signal, it: Removes the accumulated noise Regenerates the signal to its original strength Retransmits it at higher power or over a longer distance This allows signals to travel farther than they could on a single cable segment. However, repeaters introduce a small propagation delay (the time it takes for the signal to travel through the device). Hubs are essentially multi-port repeaters. When a hub receives a frame on one port, it broadcasts (forwards) that frame to all other ports. This means every device connected to a hub sees every transmission—which is inefficient and creates security concerns. Modern networks have largely replaced repeaters and hubs with switches, which are much more intelligent about where they send frames. Bridges and Switches Bridges and switches are much smarter than hubs. While a hub broadcasts to all ports, bridges and switches forward frames only to the ports involved in the communication. Operating at the Data Link Layer, these devices learn the association between physical ports and MAC addresses. When a device on port 1 sends a frame to a device on port 3, the bridge/switch learns this and only forwards to port 3 (plus the source port). This is much more efficient than broadcasting to every port. What happens when the destination is unknown? If a switch receives a frame with a destination MAC address it doesn't recognize, it broadcasts the frame to all ports except the source port. This broadcasts the frame to all machines on the switch until someone responds, allowing the switch to learn the correct port for that MAC address. Collision Domains vs. Broadcast Domains Bridges and switches have an important effect on network architecture: They divide the collision domain (reducing collisions between devices) They preserve a single broadcast domain (all devices can still see broadcasts to the network's broadcast address) This improves network efficiency by reducing unnecessary collisions while maintaining the ability to broadcast when needed. Routers If bridges and switches work at the data link layer using MAC addresses, routers operate at the Network Layer using IP addresses. A router forwards packets between different networks by consulting a routing table—essentially a lookup table that contains information about which direction to send packets based on their destination IP address. Rather than broadcasting, routers use intelligent packet addressing, making them efficient even for very large networks with millions of devices. Think of it this way: a switch asks "which port should this go to?" based on MAC addresses for local delivery. A router asks "which network should this go to?" based on IP addresses for cross-network delivery. Modems A modem is short for modulator-demodulator. It converts digital signals (the 1s and 0s your computer produces) into analog carrier signals suitable for transmission over media not originally designed for digital data—most famously, telephone lines. Modern modems are used for: Digital Subscriber Line (DSL) services that run over telephone infrastructure Cable television systems using DOCSIS (Data Over Cable Service Interface Specification) technology Essentially, a modem lets you use analog infrastructure (like your phone line or cable TV line) to transmit digital data. The modem at one end converts digital to analog; the modem at the other end converts back to digital. Firewalls A firewall is a device or software that controls network security by enforcing rules about which traffic can pass between secure internal networks and external networks (like the Internet). Firewalls work by: Allowing traffic from authorized sources (traffic you've explicitly approved) Rejecting traffic from unrecognized sources (traffic from unknown or suspicious places) A firewall acts as a gatekeeper, sitting between your trusted internal network and the untrusted external world. You configure rules specifying what's allowed in and out, and the firewall enforces those rules.