Big Idea 4: Computer Systems and Networks

Computer system

A combination of hardware and software that work together to input, process, store, and output data.

Hardware

The physical components of a computer system (e.g., CPU, memory chips, storage devices, routers, cables).

Software

Programs/instructions that tell hardware what to do (e.g., operating systems, apps, protocol implementations).

Abstraction

A design approach that hides lower-level details so users/programmers can work at a higher level.

Operating system (OS)

System software that manages resources such as files, memory, processes, and devices.

CPU (Central Processing Unit)

The hardware component that executes instructions and performs computations.

RAM (Random Access Memory)

Fast, short-term working memory used while programs run.

Volatile memory

Memory that loses its data when power is turned off (RAM is typically volatile).

Storage

Long-term data holding (e.g., SSD/HDD); generally slower than RAM.

Non-volatile storage

Storage that retains data without power (e.g., SSDs/HDDs).

Input/Output (I/O)

How a computer system communicates with the outside world (sending/receiving data via devices).

Network interface

Hardware that sends and receives bits on a network (e.g., Wi‑Fi adapter, Ethernet card).

Network

A set of computing devices (nodes) connected by links so they can exchange data.

Node

Any device on a network, such as a laptop, phone, server, router, or printer.

Link

A communication path that carries data between nodes (wired or wireless).

Internet

A worldwide network of networks that interconnects many separate networks globally.

Protocol

An agreed-upon set of rules for formatting, sending, receiving, and interpreting data.

Circuit switching

A communication method that reserves a dedicated path for an entire session (like classic phone calls).

Packet switching

A method where messages are split into packets that can travel independently and share network links efficiently.

Packet

A small unit of data sent over a network, typically containing a header and a payload.

Payload

The part of a packet that contains the actual data being sent.

Header

Packet metadata needed for delivery (e.g., addressing, ordering info, error-checking info).

Encapsulation

The process of adding headers at different protocol layers as data moves down the network stack (and removing them as it moves up).

IP address

A numerical label for identifying a device on a network; used for addressing and routing packets.

Router

A device that forwards packets between networks, sending them to the next hop toward their destination.

Routing

The process of choosing paths through a network to deliver packets from sender to receiver.

Packet loss

A network problem where some packets never arrive at the destination.

DNS (Domain Name System)

A distributed system that maps domain names (e.g., example.com) to IP addresses.

Domain name

A human-friendly Internet name (like collegeboard.org) that DNS translates to an IP address.

DNS resolver

A service (often from an ISP/organization) that looks up domain names and returns IP addresses, frequently using cache.

Caching (DNS caching)

Storing recent DNS lookup results so repeated requests can be answered faster and with less global load.

Layered protocol stack

A networking design where each layer solves a specific problem and relies on services from layers below it.

IP (Internet Protocol)

A core Internet protocol responsible for addressing and routing packets across networks.

Best-effort delivery

The IP design principle that delivery is attempted but not guaranteed (no guaranteed delivery, ordering, or error-free transmission).

TCP (Transmission Control Protocol)

A transport protocol that supports reliable, ordered delivery by detecting loss, retransmitting, reordering, and managing congestion.

UDP (User Datagram Protocol)

A transport protocol with minimal overhead that typically does not guarantee delivery or ordering; useful when low latency matters and some loss is acceptable.

HTTP (Hypertext Transfer Protocol)

An application-layer protocol defining rules for requesting and delivering web content (request/response).

Bandwidth

The maximum data capacity of a connection per unit time, typically measured in bits per second.

Latency

The time delay for data to travel from source to destination (includes propagation, processing, and queuing delays).

Throughput

The actual achieved rate of successful data transfer, often lower than bandwidth due to overhead, congestion, and retransmissions.

Reliability (networking)

The consistency and correctness of delivery (e.g., whether packets arrive without frequent drops/corruption).

Scalability

The ability of a system/network to handle growth (more users, devices, or traffic) efficiently.

Fault tolerance

The ability of a system/network to keep operating even when some components fail.

Redundancy

Having more than one path/component available so the system can keep working if one fails (improves resilience but adds cost/complexity).

Parallel computing

Performing multiple computations simultaneously (e.g., multiple cores/processors working at the same time).

Distributed computing

Computation spread across multiple networked devices; helps with large processing or storage needs but adds communication/coordination overhead.

Cloud computing

A model where computing resources (servers, storage, services) are provided over a network on demand, supporting rapid scaling.

Load balancing

Distributing user requests across multiple servers to prevent overload and improve scalability/reliability.

Authentication

The process of proving identity so you can confirm you are communicating with the intended party.

Digital certificate

A credential used (e.g., by browsers) to help verify a website’s identity, reducing the risk of connecting to an impostor.