Hardware & Communication

Identify and Describe Hardware and Communication Elements of Computer Systems

A computer system is made up of hardware (physical components) and software (programs and data). The hardware elements can be broadly categorised into processing components, memory, input/output devices, secondary storage, and communication devices.

The major hardware subsystems include:

Subsystem	Purpose	Examples
Processor (CPU)	Fetches, decodes and executes instructions	Intel Core i7, ARM Cortex
Main memory	Stores data and instructions currently in use	RAM, ROM
Secondary storage	Long-term, non-volatile data storage	HDD, SSD, optical disc
Input devices	Allow data to enter the system	Keyboard, mouse, scanner
Output devices	Present processed data to the user	Monitor, printer, speakers
Communication devices	Enable data transfer between systems	Network interface card, router, modem
Buses	Internal pathways that carry data between components	Data bus, address bus, control bus

Communication elements include the network interface card (NIC) which connects a device to a network, routers which direct traffic between networks, switches which connect devices within a local network, and modems which convert between digital and analogue signals.

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Architecture: Main Components Including Von Neumann Architectures

Von Neumann Architecture

The Von Neumann architecture is the foundation of most modern general-purpose computers. Proposed by John von Neumann in 1945, it is based on the stored program concept – both instructions and data are stored together in the same main memory and are accessed via the same bus system.

The main components of a Von Neumann system are:

The Central Processing Unit (CPU) contains:

• Arithmetic Logic Unit (ALU): Performs all arithmetic operations (addition, subtraction, multiplication, division) and logical comparisons (AND, OR, NOT, XOR). It receives operands from registers and sends results back.
• Control Unit (CU): Coordinates and manages the operation of the entire computer. It fetches instructions from memory, decodes them to determine the operation, and generates control signals to direct other components. It controls the timing of operations using a clock signal.
• Registers: Small, extremely fast storage locations within the CPU. Key registers include:

Register	Full Name	Purpose
PC	Program Counter	Holds the address of the next instruction to be fetched
MAR	Memory Address Register	Holds the address of the memory location about to be read from or written to
MDR/MBR	Memory Data Register / Memory Buffer Register	Holds the data that has just been read from or is about to be written to memory
CIR	Current Instruction Register	Holds the instruction currently being decoded and executed
ACC	Accumulator	Holds intermediate results of calculations performed by the ALU

The Bus System connects the CPU to main memory and I/O controllers:

• Data Bus: Carries data between the CPU, memory and I/O devices. It is bidirectional (data flows in both directions).
• Address Bus: Carries memory addresses from the CPU to memory. It is unidirectional (the CPU sends addresses; memory receives them). The width of the address bus determines the maximum addressable memory (e.g., a 32-bit address bus can address 2^32 = 4 GB of memory).
• Control Bus: Carries control signals such as read/write, clock pulses, interrupt requests, and bus request/grant signals. Individual lines are unidirectional but the bus as a whole is bidirectional.

Harvard Architecture

An alternative is the Harvard architecture, which uses separate memory and buses for instructions and data. This allows instructions and data to be fetched simultaneously, improving throughput. Harvard architecture is commonly used in embedded systems, microcontrollers, and digital signal processors (DSPs).

Feature	Von Neumann	Harvard
Memory	Single memory for instructions and data	Separate memories for instructions and data
Buses	Shared bus system	Separate bus systems
Speed	Slower (bus bottleneck)	Faster (simultaneous access)
Flexibility	Programs can modify themselves	Instructions and data strictly separated
Usage	General-purpose computers	Embedded systems, DSPs, microcontrollers

CISC vs RISC

• CISC (Complex Instruction Set Computer): Has a large set of complex instructions, where a single instruction can perform multiple low-level operations. Requires fewer lines of assembly code but each instruction takes more clock cycles. Used in desktop/laptop processors (Intel x86, AMD).
• RISC (Reduced Instruction Set Computer): Has a smaller set of simple instructions, each executing in a single clock cycle. Requires more lines of assembly code but achieves high throughput through pipelining. Used in mobile devices and embedded systems (ARM).

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Different Types of Memory and Caching

Primary Memory

Primary memory is directly accessible by the CPU and is essential for the system to operate.

RAM (Random Access Memory):

• Volatile – loses its contents when power is switched off
• Stores the currently running programs and data the CPU needs
• Can be read from and written to
• Two main types:
  • DRAM (Dynamic RAM): Must be constantly refreshed; cheaper and denser; used for main memory
  • SRAM (Static RAM): Does not need refreshing; faster but more expensive; used for cache memory

ROM (Read Only Memory):

• Non-volatile – retains its contents when power is switched off
• Stores the boot-up instructions (BIOS/UEFI) that initialise the hardware when the computer is first switched on
• Contents are typically written during manufacture and cannot be easily modified
• Variants include PROM (programmable once), EPROM (erasable with UV light), and EEPROM/Flash (electrically erasable)

Cache Memory

Cache is a small amount of very fast SRAM located on or near the CPU. It stores copies of frequently and recently accessed data and instructions from main memory, reducing the time the CPU spends waiting for data.

Cache operates on the principle of locality:

• Temporal locality: Data accessed recently is likely to be accessed again soon
• Spatial locality: Data stored near recently accessed data is likely to be needed soon

Modern CPUs typically have multiple levels of cache:

Level	Size	Speed	Location
L1	32-64 KB per core	Fastest	Built into CPU core
L2	256 KB - 1 MB per core	Fast	On the CPU die, per core
L3	4-64 MB shared	Slower than L1/L2	On the CPU die, shared between cores

When the CPU requests data, it checks L1 first, then L2, then L3, then main memory. A cache hit occurs when the data is found in cache; a cache miss occurs when it must be fetched from slower main memory.

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Parallel Processing

Traditional Von Neumann processors execute instructions sequentially, one at a time. Parallel processing improves performance by executing multiple instructions or tasks simultaneously.

Forms of Parallel Processing

Pipelining: The fetch-decode-execute cycle is broken into stages, and multiple instructions are in different stages at the same time (like a factory assembly line). While one instruction is being executed, the next is being decoded, and a third is being fetched.

Multi-core processing: A CPU contains multiple independent processing cores, each capable of executing its own instruction stream. A quad-core processor can theoretically execute four instructions simultaneously.

Multi-processor systems: Multiple separate CPUs work together on a shared task, connected via a high-speed interconnect.

GPU computing: Graphics Processing Units contain thousands of small cores optimised for performing the same operation on many data items simultaneously (SIMD – Single Instruction, Multiple Data). Used for graphics rendering, machine learning and scientific computing.

Approach	Description	Best For
Pipelining	Overlapping stages of different instructions	General instruction throughput
Multi-core	Multiple independent cores on one chip	Multitasking, threaded applications
GPU computing	Thousands of simple cores	Massively parallel data-level tasks

Not all tasks benefit equally from parallelism. Amdahl’s Law states that the speedup from parallelism is limited by the fraction of the task that must be performed sequentially. If 25% of a task is sequential, no amount of parallelism can give more than a 4x speedup.

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Fetch-Execute Cycle: How Data Is Read from RAM into Registers

The fetch-decode-execute cycle (also called the fetch-execute cycle or instruction cycle) is the fundamental process by which the CPU processes each instruction. It repeats continuously from the moment the computer is switched on until it is shut down.

The Stages in Detail

1. Fetch:

1. The Program Counter (PC) holds the address of the next instruction to fetch.
2. The contents of the PC are copied to the Memory Address Register (MAR).
3. The PC is incremented by 1, so it now points to the next instruction.
4. The address in the MAR is sent along the address bus to main memory.
5. A read signal is sent along the control bus.
6. The data at that memory location is sent along the data bus to the Memory Data Register (MDR).
7. The contents of the MDR are copied to the Current Instruction Register (CIR).

2. Decode:

1. The control unit examines the instruction in the CIR.
2. The instruction is split into the opcode (the operation to perform) and the operand (the data or address the operation acts on).
3. The control unit determines what signals need to be sent and to which components.

3. Execute:

1. The appropriate operation is carried out. This may involve:
   • Performing an arithmetic or logical calculation in the ALU
   • Loading data from memory into a register
   • Storing data from a register to memory
   • Changing the PC to a different address (a branch/jump instruction)

Interrupts

An interrupt is a signal sent to the CPU that requires immediate attention. At the end of each fetch-decode-execute cycle, the CPU checks for interrupts. If an interrupt is detected:

1. The CPU saves its current state (registers, PC) onto the stack.
2. The CPU loads the address of the appropriate Interrupt Service Routine (ISR) into the PC.
3. The ISR is executed to handle the interrupt.
4. Once complete, the CPU restores its previous state from the stack and resumes the original program.

Examples of interrupts include: a key being pressed, a hardware error, a timer expiring, or I/O completion.

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Input/Output: Contemporary Methods and Devices

Contemporary Input Devices

Device	Method of Input	Typical Use
Touchscreen	Capacitive or resistive sensor detects finger contact	Smartphones, tablets, kiosks
Digital camera/webcam	Light is captured by a CCD/CMOS sensor and converted to digital data	Photography, video calls
Microphone	Sound waves converted to electrical signals then digitised by an ADC	Voice input, recording
Barcode/QR scanner	Laser or camera reads patterns of light and dark	Retail, logistics
Biometric scanner	Reads unique biological characteristics (fingerprint, iris, face)	Security authentication
RFID/NFC reader	Radio waves read data from an RFID tag	Contactless payments, access control
Sensor (temperature, pressure, light)	Converts physical quantities into electrical signals	Monitoring systems, IoT

Contemporary Output Devices

Device	Method of Output	Typical Use
LCD/OLED monitor	Pixels emit or filter light to display images	Desktop computing, smartphones
Laser/inkjet printer	Toner or ink deposited onto paper	Document printing
3D printer	Layers of material built up to create a physical object	Prototyping, manufacturing
Speakers/headphones	Electrical signals converted to sound waves by a DAC and driver	Audio output
Actuator	Converts electrical signals into physical movement	Robotics, control systems
Projector	Light projected through or reflected off a display element	Presentations, cinema

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Suitability of I/O Methods in Different Situations

Choosing the right I/O device depends on the context of the application:

Situation	Suitable I/O	Reason
Supermarket checkout	Barcode scanner (input), receipt printer (output)	Fast item identification; printed record for customer
Disabled user	Voice recognition (input), screen reader (output)	Allows hands-free interaction with the system
Warehouse inventory	RFID reader (input), handheld display (output)	Bulk scanning without line-of-sight; portable display
Hospital patient monitoring	Sensors (input), monitor display and alarm (output)	Continuous automatic data collection; immediate alerts
Graphic design studio	Graphics tablet (input), high-resolution monitor (output)	Precise drawing input; accurate colour output
ATM machine	Touchscreen and card reader (input), screen and cash dispenser (output)	User-friendly interface; secure transaction

Factors to consider when selecting I/O methods:

• Speed: How quickly must data be entered or presented?
• Accuracy: How precise does the input/output need to be?
• Volume of data: Is it a single item or thousands?
• Environment: Is it indoors, outdoors, noisy, or hazardous?
• User capability: Does the user have physical limitations?
• Cost: Is the investment justified for the application?
• Automation: Should the process be manual or automatic?

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Secondary Storage: Functional Characteristics of Storage Devices

Secondary storage is non-volatile storage used to hold data and programs long-term, even when the computer is switched off. There are three main types.

Magnetic Storage (e.g., Hard Disk Drive – HDD)

• Data is stored as magnetic patterns on rotating metal platters
• A read/write head moves across the spinning platters to access data
• Capacity: Very high (up to several terabytes)
• Speed: Moderate (limited by physical spinning and head movement – typical 5400-7200 RPM)
• Cost: Low cost per gigabyte
• Durability: Susceptible to damage from physical shock (moving parts)
• Best for: Bulk storage, file servers, backups where cost and capacity matter more than speed

Solid-State Storage (e.g., SSD, USB Flash Drive)

• Data is stored in NAND flash memory chips using electrical charges in floating-gate transistors
• No moving parts
• Capacity: Moderate to high (up to several terabytes for SSDs)
• Speed: Very fast (no mechanical latency; near-instant access times)
• Cost: Higher cost per gigabyte than magnetic
• Durability: Very robust – resistant to shock and vibration
• Lifespan: Limited number of write cycles per cell (though modern SSDs have wear-levelling algorithms)
• Best for: Operating system drives, laptops, portable storage, any application needing fast access

Optical Storage (e.g., CD, DVD, Blu-ray)

• Data is stored as pits and lands on a reflective disc surface, read by a laser
• Capacity: Low to moderate (CD: 700 MB; DVD: 4.7 GB; Blu-ray: 25-50 GB)
• Speed: Slow compared to HDD and SSD
• Cost: Very cheap per disc
• Durability: Resistant to magnetic interference but can be scratched
• Best for: Software distribution, media playback, archival storage

Feature	HDD	SSD	Optical
Capacity	Very high	High	Low-Moderate
Speed	Moderate	Very fast	Slow
Cost per GB	Low	Medium	Very low
Durability	Low (moving parts)	High	Moderate
Portability	Low	High	High
Power usage	Higher	Lower	Low

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Data Storage on Disc: Fragmentation and Defragmentation

How Data Is Stored on a Magnetic Disc

A hard disk platter is divided into concentric tracks, which are further divided into sectors. A group of sectors is called a cluster (or allocation unit), and this is the smallest unit of storage the operating system can manage.

When a file is saved, the operating system allocates enough clusters to hold it. Ideally, the clusters are contiguous (next to each other), allowing the read/write head to read the file sequentially without moving.

Fragmentation

Fragmentation occurs when files are stored in non-contiguous clusters scattered across the disk. This happens over time as files are created, modified, resized, and deleted, leaving gaps between occupied clusters.

Effects of fragmentation:

• The read/write head must move to multiple locations to read a single file
• This increases seek time (the time taken for the head to move to the correct track)
• Overall system performance degrades as file access becomes slower
• More mechanical wear on the drive

Defragmentation

Defragmentation is the process of reorganising the data on a disk so that files are stored in contiguous clusters.

• The defragmentation tool reads fragmented files and rewrites them in consecutive sectors
• Free space is consolidated into larger contiguous blocks
• After defragmentation, file access is faster because the head moves less
• Defragmentation should be performed periodically on HDDs

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Networking: How Networks Communicate

Types of Network

LAN (Local Area Network):

• Covers a small geographical area (single building or campus)
• Hardware is typically owned and managed by the organisation
• High data transfer speeds (typically 100 Mbps to 10 Gbps)
• Connected using Ethernet cables, switches, and wireless access points

WAN (Wide Area Network):

• Covers a large geographical area (cities, countries, or the world)
• Uses third-party communication links (leased lines, telephone networks, satellite links)
• Lower data transfer speeds than LAN (varies widely)
• The Internet is the largest WAN

Other network types:

• MAN (Metropolitan Area Network): Spans a city or large campus
• PAN (Personal Area Network): Very short range, connecting personal devices (e.g., Bluetooth)
• WLAN (Wireless LAN): A LAN using Wi-Fi instead of cables

Network Topologies

A topology describes how devices (nodes) in a network are arranged and connected.

Star topology:

• All devices connect to a central switch or hub
• If one device fails, the rest of the network is unaffected
• If the central switch fails, the entire network goes down
• Easy to add new devices
• Most common topology in modern LANs

Bus topology:

• All devices share a single backbone cable (the bus)
• Data travels in both directions along the bus
• If the backbone cable fails, the entire network goes down
• Prone to collisions as all devices share the medium
• Cheap to install but difficult to troubleshoot

Ring topology:

• Devices are connected in a circular loop
• Data travels in one direction around the ring
• Each device receives data and passes it to the next
• A break in the ring can disable the whole network (unless a dual ring is used)

Mesh topology:

• Every device is connected to every other device (full mesh) or to several devices (partial mesh)
• Very resilient – multiple paths between devices
• Expensive due to the number of connections required
• Used in critical infrastructure (e.g., the Internet backbone)

Client-Server vs Peer-to-Peer

Client-server model:

• A central server provides services (file storage, authentication, email) to client devices
• Easier to manage, back up, and secure centrally
• Server failure disrupts the entire network
• Used in most business environments

Peer-to-peer model:

• All devices are equal and share resources directly with each other
• No central server required
• Harder to manage security and backups
• Used in small home networks and file-sharing applications

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Importance of Networking Standards

Networking standards are agreed-upon rules and specifications that ensure devices from different manufacturers can communicate with each other. Without standards, a computer from one manufacturer would not be able to communicate with a printer from another.

Key reasons standards are important:

• Interoperability: Devices from different vendors can work together on the same network
• Competition: Multiple manufacturers can produce compatible products, driving innovation and lowering prices
• Reliability: Standardised protocols are thoroughly tested and widely understood
• Scalability: Networks can grow by adding any standards-compliant device

Important standards bodies include:

Organisation	Role
IEEE (Institute of Electrical and Electronics Engineers)	Defines standards such as IEEE 802.3 (Ethernet) and IEEE 802.11 (Wi-Fi)
IETF (Internet Engineering Task Force)	Develops and maintains Internet protocols (TCP/IP, HTTP) through RFCs
ISO (International Organization for Standardization)	Developed the OSI 7-layer reference model
W3C (World Wide Web Consortium)	Develops web standards (HTML, CSS, HTTP)

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Protocols: HTTP, FTP, SMTP, TCP/IP, IMAP, DHCP, UDP, Wireless

A protocol is a set of rules that governs how data is formatted, transmitted, and received over a network. Different protocols serve different purposes.

The TCP/IP Protocol Suite

The TCP/IP model is the layered model that underpins the Internet. It has four layers:

Layer	Purpose	Example Protocols
Application	Provides network services to user applications	HTTP, FTP, SMTP, IMAP, DHCP, DNS
Transport	Provides reliable or unreliable end-to-end data delivery	TCP, UDP
Internet	Handles addressing and routing of packets across networks	IP (IPv4, IPv6)
Network Access	Handles physical transmission of data over the network medium	Ethernet, Wi-Fi

Key Protocols Explained

HTTP (HyperText Transfer Protocol):

• Used by web browsers to request and receive web pages from web servers
• Uses a request-response model (client sends a request, server sends a response)
• HTTPS adds encryption using TLS/SSL for secure communication
• Uses TCP port 80 (HTTP) or 443 (HTTPS)

FTP (File Transfer Protocol):

• Used to transfer files between a client and a server
• Supports uploading and downloading files, directory listing, renaming and deleting
• Uses two connections: a control connection (port 21) and a data connection (port 20)
• Credentials are sent in plain text (SFTP or FTPS should be used for security)

SMTP (Simple Mail Transfer Protocol):

• Used to send emails from a client to a mail server or between mail servers
• Uses TCP port 25 (or 587 for authenticated submission)
• Only handles sending – not retrieving email

IMAP (Internet Message Access Protocol):

• Used to retrieve emails from a mail server
• Emails remain on the server and can be accessed from multiple devices
• Supports folders, search, and partial message download
• Uses TCP port 143 (or 993 with encryption)
• Alternative: POP3 (downloads emails and typically deletes them from the server)

TCP (Transmission Control Protocol):

• A connection-oriented transport protocol
• Guarantees delivery, correct ordering, and error checking
• Uses a three-way handshake (SYN, SYN-ACK, ACK) to establish a connection
• Slower than UDP due to overhead
• Used when reliability is essential: web browsing, email, file transfer

UDP (User Datagram Protocol):

• A connectionless transport protocol
• Does NOT guarantee delivery, ordering, or error correction
• Much faster and lower overhead than TCP
• Used when speed matters more than reliability: live video/audio streaming, online gaming, DNS queries, VoIP

Feature	TCP	UDP
Connection	Connection-oriented	Connectionless
Reliability	Guaranteed delivery	No guarantee
Ordering	Maintains order	No ordering
Speed	Slower (overhead)	Faster
Error checking	Yes, with retransmission	Basic checksum only
Use cases	Web, email, file transfer	Streaming, gaming, DNS

DHCP (Dynamic Host Configuration Protocol):

• Automatically assigns IP addresses and other network configuration to devices when they join a network
• Eliminates the need for manual IP configuration
• A DHCP server maintains a pool of available addresses
• Addresses are leased for a set period and can be renewed

Wireless Protocols (IEEE 802.11 / Wi-Fi):

• Define how wireless devices communicate over radio frequencies
• Different standards offer different speeds and ranges (e.g., 802.11ac, 802.11ax/Wi-Fi 6)
• Use CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) to manage access to the shared wireless medium
• Security protocols: WPA2, WPA3 for encryption and authentication

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Role of Handshaking

Handshaking is the automated process of negotiation between two devices before data transfer begins. It establishes the parameters of the communication channel so both sides can communicate successfully.

The TCP Three-Way Handshake

The most important example at AS Level is the TCP three-way handshake, used to establish a reliable connection:

1. SYN (Synchronise): The client sends a SYN packet to the server, requesting a connection. It includes an initial sequence number.
2. SYN-ACK (Synchronise-Acknowledge): The server responds with a SYN-ACK packet, acknowledging the client’s request and sending its own sequence number.
3. ACK (Acknowledge): The client sends an ACK packet back to the server, confirming the connection is established.

After this three-way handshake, data transfer can begin. When the communication is finished, a similar process (FIN, FIN-ACK, ACK) is used to close the connection gracefully.

What Handshaking Establishes

• Both devices confirm they are ready to communicate
• The transmission speed (baud rate) is agreed upon
• Error-checking method to be used
• Packet size and format
• Sequence numbers for ordering packets
• Flow control parameters to prevent the sender from overwhelming the receiver

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The Internet as a World-Wide Communications Infrastructure

What Is the Internet?

The Internet is a global network of interconnected networks. It is not a single network but a vast infrastructure that connects millions of private, public, academic, and government networks using the TCP/IP protocol suite.

Key Components of the Internet Infrastructure

• Backbone: High-capacity fibre-optic cables (often undersea) that carry the majority of Internet traffic between continents and countries
• Internet Service Providers (ISPs): Companies that provide access to the Internet for homes and businesses. They connect to the backbone and to each other at Internet Exchange Points (IXPs)
• Routers: Direct packets across the Internet, choosing the best path from source to destination
• DNS (Domain Name System): Translates human-readable domain names (e.g., www.example.com) into IP addresses (e.g., 93.184.216.34). DNS servers form a distributed hierarchical database
• IP Addressing: Every device on the Internet has a unique IP address. IPv4 uses 32-bit addresses (e.g., 192.168.1.1) providing about 4.3 billion addresses. IPv6 uses 128-bit addresses to provide a vastly larger address space

How Data Travels Across the Internet

1. Data is broken into packets at the source
2. Each packet is labelled with source and destination IP addresses
3. Packets are sent independently across the network – they may take different routes
4. Routers at each hop examine the destination IP and forward the packet towards its destination
5. At the destination, packets are reassembled in the correct order using sequence numbers
6. If any packets are lost or corrupted, TCP requests retransmission

Internet Services

The Internet supports many services beyond the World Wide Web:

Service	Protocol	Description
World Wide Web	HTTP/HTTPS	Web pages and web applications
Email	SMTP, IMAP, POP3	Sending and receiving electronic messages
File Transfer	FTP/SFTP	Uploading and downloading files
Voice/Video calls	SIP, RTP (over UDP)	Real-time voice and video communication (VoIP)
Streaming media	HTTP, DASH, HLS (over TCP/UDP)	On-demand and live audio/video
Domain resolution	DNS	Translating domain names to IP addresses