24.2 Communication systems and networks

Wireless Communication Networks

Communication systems and networks make it possible to send data across distances, whether that's a text message across town or a video stream bouncing off a satellite. For electrical engineers, these systems tie together core concepts like signal processing, circuit design, and electromagnetics into real-world infrastructure.

This section covers wireless and wired communication methods, the techniques used to encode and share data efficiently, and the protocols and security measures that keep everything running reliably.

5G Networks and Wireless Sensor Networks

5G is the fifth generation of cellular network technology, designed for high-speed, low-latency wireless communication. Compared to 4G, 5G offers significantly greater network capacity, faster data rates (peak speeds above 10 Gbps in ideal conditions), and lower latency (targeting under 1 ms for some applications).

5G achieves this through several key technologies:

• Massive MIMO (Multiple Input, Multiple Output): uses large arrays of antennas at the base station to serve many users simultaneously
• Beamforming: focuses radio energy toward specific users rather than broadcasting in all directions, improving signal strength and reducing interference
• Millimeter-wave spectrum: operates at frequencies above 24 GHz, where large bandwidths are available, though signals attenuate more quickly and struggle to penetrate walls

These capabilities enable applications like autonomous vehicles, virtual reality, and large-scale IoT deployments where thousands of devices need to connect in a small area.

Wireless sensor networks (WSNs) consist of spatially distributed sensors that monitor physical or environmental conditions (temperature, pressure, humidity) and wirelessly relay that data to a central node for analysis.

• Applications include environmental monitoring, industrial process control, and healthcare (e.g., wearable patient monitors)
• Each sensor node typically has limited battery life, so power-efficient communication protocols are critical
• Key challenges: battery longevity, scaling the network to many nodes, and securing data transmissions against tampering

Satellite Communications

Satellite communications use artificial satellites as relay stations to link points on Earth that may be too far apart or too remote for terrestrial networks. Common applications include television broadcasting, GPS navigation, internet service in rural areas, and emergency communication during disasters.

Satellites are classified by their orbit:

Challenges in satellite communication include signal attenuation (the signal weakens over long distances), propagation delay (especially for GEO), and rain fade (heavy rain absorbs and scatters signals, particularly at higher frequencies).

Wired Communication Systems

Fiber Optic Communications

Fiber optic systems transmit data as pulses of light through thin glass or plastic fibers. They're the backbone of long-distance and high-capacity networks because they offer:

• High bandwidth: a single fiber can carry terabits per second
• Low attenuation: signals travel long distances (tens of kilometers) before needing amplification
• Immunity to electromagnetic interference (EMI): since the signal is light, not electrical current, nearby power lines or motors don't cause noise

A basic fiber optic link has three components:

1. Transmitter: a laser diode or LED converts an electrical signal into light pulses
2. Optical fiber: the glass or plastic medium that guides the light from transmitter to receiver
3. Receiver: a photodetector converts the incoming light pulses back into an electrical signal

There are two main fiber types:

• Single-mode fiber: has a very small core (~8–10 µm diameter), allowing only one light path (mode). This supports higher bandwidth over longer distances and is used in backbone and long-haul networks.
• Multi-mode fiber: has a larger core (~50–62.5 µm), supporting multiple light paths. It's cheaper and easier to work with but suited for shorter distances, like within a building or data center.

Network Protocols and Software-Defined Networking

Network protocols are standardized rules that govern how devices format, send, and receive data. Without them, devices from different manufacturers couldn't communicate.

Protocols are organized into layers, each handling a specific part of the communication process:

• Physical layer: deals with the actual transmission medium (cables, radio waves)
• Data link layer: manages node-to-node data transfer and error detection (e.g., Ethernet)
• Network layer: handles routing and addressing (e.g., IP)
• Transport layer: ensures reliable end-to-end delivery (e.g., TCP)
• Application layer: provides services to the user (e.g., HTTP for web browsing, FTP for file transfer)

This layered approach is modular: you can change the physical medium (say, from copper to fiber) without rewriting the application software.

Software-defined networking (SDN) is a newer architecture that separates the control plane (the logic that decides where traffic goes) from the data plane (the hardware that actually forwards packets). In traditional networking, both functions live in each individual router or switch. SDN centralizes the control logic in software, which makes the network easier to configure, monitor, and adapt programmatically. OpenFlow is the most widely used protocol for communication between the SDN controller and the network switches.

Data Transmission Techniques

Modulation and Multiplexing

Modulation is the process of encoding information onto a carrier signal by varying one or more of its properties (amplitude, frequency, or phase). You need modulation because raw data signals (baseband) often can't travel far or share a medium efficiently on their own.

Common digital modulation techniques:

• ASK (Amplitude Shift Keying): varies the carrier's amplitude to represent 0s and 1s
• FSK (Frequency Shift Keying): shifts the carrier's frequency between discrete values
• PSK (Phase Shift Keying): changes the carrier's phase to encode data; more bandwidth-efficient than ASK or FSK

Analog modulation includes AM (amplitude modulation, used in AM radio) and FM (frequency modulation, used in FM radio). FM is more resistant to noise than AM, which is why FM radio sounds cleaner.

Multiplexing lets multiple signals share a single communication channel:

• TDM (Time-Division Multiplexing): each signal gets a dedicated time slot in a repeating frame. Think of it like taking turns speaking in a conversation.
• FDM (Frequency-Division Multiplexing): each signal is assigned its own frequency band within the channel. AM/FM radio stations use this principle, with each station broadcasting on a different frequency.
• CDMA (Code-Division Multiple Access): each signal is encoded with a unique spreading code, and all signals transmit simultaneously over the same frequency band. The receiver uses the correct code to extract the desired signal. This is used in some cellular networks.

Error Correction and Network Security

Transmitted data can be corrupted by noise, interference, or hardware faults. Two main strategies handle this:

1. Forward Error Correction (FEC): the transmitter adds redundant bits (extra data calculated from the original message) so the receiver can detect and fix errors without asking for a retransmission. This is essential for satellite links where retransmission delays would be too long.
2. Automatic Repeat Request (ARQ): the receiver checks incoming data for errors and sends back an acknowledgment (ACK) if the data is correct, or a negative acknowledgment (NACK) if it's not. Corrupted data gets retransmitted. TCP uses a form of ARQ.

Network security protects data and infrastructure from unauthorized access:

• Encryption scrambles data so only authorized parties can read it. Symmetric-key encryption uses the same key for both encrypting and decrypting (fast, but you need a secure way to share the key). Public-key encryption uses a pair of keys (one public, one private), solving the key-sharing problem at the cost of more computation.
• Firewalls filter incoming and outgoing network traffic based on security rules, blocking unauthorized connections.
• VPNs (Virtual Private Networks) create encrypted tunnels over public networks like the internet, allowing remote users to securely access a private network as if they were directly connected to it.