WJEC Computer Science GCSE Unit One Communication Full Study Notes
Introduction to Computer Networks
A network consists of a number of computer systems connected together.
The primary purpose of a network is to allow the sharing of resources and data.
Advantages of Networks: - Share hardware: Multiple users can access hardware like printers or scanners. - Share software: Applications can be stored centrally. - Share data/files: Information is accessible to multiple users. - Facilitates internal communication: Users can send emails or instant messages. - Central backup: Data can be backed up in one central location rather than on individual machines. - Monitoring: It is easier to monitor network activity and user behavior. - Centrally controlled security: Security updates and firewalls can be managed from a single point. - Accessibility: Data can be accessed from any computer connected to the network.
Disadvantages of Networks: - Management: A network manager may need to be employed, which is expensive. - Security risks: Files sent between computers could spread a virus. - Hacking: Hackers can gain access to data more easily if the network is compromised. - Dependency: If the server goes down, all workstations on the network are affected. - Initial cost: The cost of servers, communication devices, and cabling can be high.
Factors Affecting Network Performance
Bandwidth: The amount of data that can pass between two network devices per second. It is measured in bits per second ().
Transmission Media Quality: The physical material (copper, fiber) affects how well data travels.
Distance: Performance degrades as the distance between devices increases.
Interference: Signals from other electronic devices can disrupt data transmission.
Shared Bandwidth: Bandwidth must be shared between all users of the same network; more users can lead to lower individual connection speeds.
Network Types: LAN and WAN
Local Area Network (LAN): - A network where computer systems are located relatively close to each other (e.g., in the same building or on the same site like a school).
Wide Area Network (WAN): - A network where computer systems are located relatively distant from each other (e.g., in different buildings across a country or in different countries). - The Internet is a prime example of a WAN. - Multiple LANs can be linked together using a WAN.
Protocols: Computer networks use agreed-upon protocols (common methods of sending data and consistent data formats) to allow systems to communicate.
Network Topologies
A network topology is the theoretical layout of computer systems on a network.
Bus Network: - All nodes are connected to a single cable called the "bus." - Requires terminators on each end to ensure the network functions correctly. - Packets travel along the cable; each computer checks the destination address. If it matches, the data is processed; if not, it is ignored. - Advantages: Easy to implement, quick to set up (good for temporary networks), cost-effective due to less cabling. - Disadvantages: Difficult to troubleshoot, limited cable length, performance degrades as more stations are added, reliance on the main cable (if it breaks, the whole network fails), low security as all computers see all data, requires proper termination, high chance of data collisions (two computers sending packets at the same time).
Ring Network: - Computer systems are connected in a loop. - Packets travel in one direction from one system to the next. - Advantages: Quick data transfer without bottlenecks, simple transmission (one direction), adding nodes has little impact on bandwidth, prevents collisions. - Disadvantages: If one system fails, the ring is broken and data transmission is hindered, a break in the main cable takes the whole network down, difficult to troubleshoot, network must be shut down to add new nodes.
Star Network: - Each system is connected to a central node, such as a hub or switch. - Advantages: High performance/speed, easy to set up, systems can be added without taking the network down, non-central failures do not affect the rest of the network, minimal collisions, better security. - Disadvantages: Expensive to install (more cabling), requires extra hardware (hub/switch).
Mesh Network: (Identified as a core topology type in standard network arrangements).
Connectivity and Transmission Speeds
Network Interface Card (NIC): A hardware component required to connect a computer to a network.
Physical Connections: - Copper Cable: Typical transfer speeds between and . - Fibre-Optic: High-speed connections ranging from to .
Wireless Connections (Wi-Fi): - Typical transfer rates of . - Performance is severely affected by distance and atmospheric conditions (e.g., heavy rain).
Circuit Switching vs. Packet Switching
Circuit Switching: - Provides a temporary but dedicated link between two nodes. - No other data can be transmitted along the route during the connection. - Example: The landline telephone system. - Advantages: Reliable, fast once established, generally error-free. - Disadvantages: Takes time to establish the connection, vulnerable to failure (if one point on the route fails, the connection breaks).
Packet Switching: - Breaks data into small packets sent via different routes. - Process: Data is split, packets are transmitted (often taking different paths), and reassembled at the destination. - Example: The Internet.
Data Packets
A packet is a collection of data transmitted over a packet-switched network.
Contents of a Packet: 1. The source address. 2. The destination address. 3. Information for reassembly (to put data back in original form). 4. Tracking information. 5. The actual data. 6. A checksum (used to check if the data has been corrupted during transmission).
Packet Sniffing: An activity where packets transmitted over Wi-Fi are intercepted. Analysis of these packets can potentially break encryption. This is why agencies like the National Security Agency (NSA) forbid Wi-Fi devices on their networks.
Routing
Definition: The method of selecting paths for packets on a network.
Hardware: Routers, switches, bridges, firewalls, and gateways.
Routing Tables: Stored in the memory of these devices; they contain the best paths to reach specific destinations.
Preferred Routes: Routers usually use the quickest route (e.g., Router A to Router B to Router E). Poorly constructed tables might lead packets through longer routes (A to C to D to E).
Multipath Routing: Techniques that allow packets to be sent using multiple alternative paths simultaneously to ensure delivery if one path (like Router B) fails.
Addressing: MAC and IP Addresses
MAC Address (Media Access Control): - Also called a physical or hardware address. - A unique hexadecimal number (e.g., ) given to every communication device. - Stored in the device's ROM. - Use of hexadecimal allows for over combinations. - Stored in routing tables because they are permanent. - MAC Address Spoofing: Changing a MAC address via software to trick systems into providing data.
IP Address (Internet Protocol): - Allocated to a system on a network, often by a DHCP (Dynamic Host Configuration Protocol) server. - Example: . - Uniquely identifies systems on a network to allow communication. - Routing tables map IP addresses to unique MAC addresses and update them as needed.
Internet Domain Name System (DNS)
Definition: A distributed database that matches IP addresses to computer system resources, specifically human-friendly domain names.
Example: Mapping to
www.google.co.uk.Hierarchy: - If a local DNS server cannot find a requested address, it passes the request up to a higher-level DNS (e.g., the ISP's DNS). - If still not found, it moves to the DNS responsible for an entire zone (e.g., the
.co.ukzone).Other Uses: Used by systems joining a network to find resource servers, such as the logon server for usernames and passwords.
Fact: In 2015, YouTube users uploaded of video every minute, totaling of video per hour.
Network Protocols
A protocol is a set of rules allowing two devices to communicate.
Key functions include: - Handshaking: Devices establish readiness to communicate. - Signaling the end of a message. - Acknowledgment of received messages. - Defining error checking methods. - Agreement on data compression methods.
Standard Protocols: - TCP/IP: Two protocols for network communication. IP sets the packet format and addressing; TCP manages sending and receiving. - HTTP (Hypertext Transfer Protocol): Used to transfer multimedia web pages over the Internet. - FTP (File Transfer Protocol): Used for copying files between locations; allows resumption of broken transfers, which is useful for large files.
The TCP/IP 5-Layer Protocol Stack Model
A protocol stack is a hierarchy of layers; each layer supports the one above and uses the one below.
5 Layers: 1. Application Layer: Interfaces with software (email, FTP, WWW) to allow network use. 2. Transport Layer: Ensures reliable, error-free data transfer and correct ordering of data. Exists in sending/receiving computers, but not in intermediate routers. 3. Network Layer: Responsible for addressing and routing of data. Routers operate here, using logical addresses and determining paths to minimize congestion. 4. Data Link Layer: Sends data from the network layer to the physical layer. Divides data into "data frames" (link layer header + packet). Handles bit pattern analysis and acknowledgments. 5. Physical Layer: Transmits raw data. Includes hardware like switches, routers, and cabling. Manages the physical link between computers.
Data flow: From Host 1, data flows down the 5 layers, through routers (using only the bottom 3 layers: Network, Data Link, Physical), and finally up the 5 layers of Host 2.
Specific Wireless and Email Protocols
Ethernet: At the Data Link layer, it describes how devices format data using frames and packets. At the Physical layer, it defines standards for cabling and speeds.
Wi-Fi Protocols: Uses electromagnetic waves instead of cables. Standards include .
Security Protocols: Wi-Fi Protected Access (WPA) provides privacy comparable to wired networks.
Wireless Application Protocol (WAP): Standardizes how wireless devices access the Internet.
Bluetooth: Protocol for short-range wireless communication.
IMAP (Internet Messaging Access Protocol): Stores emails on a mail server, allowing users to manage them as if stored locally (creating folders, drafts, etc.).
POP3 (Post Office Protocol 3): Receives email and stores it in a single file on the mail server. Once the client logs in, the email is transferred to the user's computer and deleted from the server.
SMTP (Simple Mail Transfer Protocol): Delivers email from a sender to a server or between servers. Can only be used to send emails.