Digital Data / Information Transmission Systems - Computer Networks and Protocols

Digital Data / Information Transmission Systems

Computer Networks

A computer network consists of interconnected computers and accessories (e.g., printers).
These components share data or resources through interconnection channels.
Each computer or accessory is a node with a unique address.

Computer Network Types

Different networks cater to different needs.
Common types:
- LAN (Local Area Networks)
- MAN (Metropolitan Area Networks)
- WAN (Wide Area Networks)
- Internet
- VPN (Virtual Private Networks)

Local Area Networks (LAN)

Typically used for computers in close proximity (e.g., university campus, company premises).
Useful for sharing resources like printers and internet connectivity.
Employ different physical network topologies.
Topology: the arrangement of computer nodes on the network.

LAN - Network Topology Types

Point to point
Bus
Ring
Mesh (full and partial mesh)
Tree
Star
WLAN (Wireless LAN)

LAN - Network Topology Types: Star

Most common topology today.
Each network host connects to a central hub, switch, or router.
Easy to add new nodes.
Can be extended or distributed.

LAN - Network Topology Types: Extended Star

Uses repeaters to extend the network.
A repeater can be a switch or hub.

LAN - Network Topology Types: Distributed Star

Involves multiple switches without a central switch.

LAN - Network Topology Types: Wireless LAN

Wireless distribution typically through access points or routers.

Metropolitan Area Network (MAN)

More extensive than LAN but less extended than WAN.
Example: covers a city.

Wide Area Networks (WAN)

Wide area coverage, even beyond cities.
Connects LANs in different, far-away locations without using the open Internet.
Examples: connecting different campuses of a university or branches of a bank.
Useful for ensuring privacy of information flow when LAN locations are distant.

Internet

Global integrated network.
A global network of ISP (Internet Service Provider) networks.
Individuals and organizations connect through ISPs.
Serves as a public global information superhighway.

Internet Service Providers in Nigeria:

Swift Networks
IPNX
Smile
Spectranet
Mobile Network Operators

VPN (Virtual Private Networks)

Networks involving interconnection between private networks (e.g., LAN, WAN) or even between individual computers through a public network (e.g., Internet).
Useful for securely connecting to a remote network over the public Internet.
Example: employees accessing the office LAN through the Internet.
VPNs use two main concepts: Tunneling and encryption to secure data.
- Tunneling: Encapsulating data (IP address related data) to be transmitted in an outer wrapper. This protects the information from unauthorized viewing and involves sending requests from a different system.
- Encryption: Converting messages to cipher-text that can be decoded only by the intended recipient.

Internetworking

When a network communicates with another network through gateways/routers, using a suitable communication protocol.
Information is routed from one network to another.
Internet is a shortened form of Internetworking.

Communication Protocol

Protocol refers to 'how' the communication is technically achieved (the communication logic).
For the Internet, the TCP/IP (Transmission Control Protocol/Internet Protocol) stack is commonly used.
Besides the TCP/IP stack, the OSI model is also used.
TCP/IP stack is more widely used, while the OSI model is referenced to explain the networking process.

The OSI Model

The OSI (Open System Interconnection) model divides interconnections into 7 layers.
Layer conveys the idea that there are multiple levels of interconnection.
Each layer accommodates specific communication protocols.
A layer serves the layer above it and is served by the layer below it.

OSI Model Layers

Application Layer (Layer 7)
- Function: Provides end-user processes and application services.
- Examples: Email (SMTP), File transfers (FTP), printer access (Internet Printing Protocol), Web browsing (HTTP/S), Virtual terminal (Telnet), and various network service-related APIs (Application Programming Interfaces) (e.g., JSON).
Presentation Layer (Layer 6)
- Function: Format data to be presented to the application. Translate data between network service and application.
- Examples: Character encoding, data compression, and encryption/decryption (e.g., TLS protocol for encryption). The external data representation (XDR) protocol specifies a standard representation of various data structures.
Session Layer (Layer 5)
- Function: Establish connection handshake between applications across the network. Manage and terminate such connections. Also enables authentication and authorization.
- Examples: NetBIOS (Network Basic Input/Output System), TCP/IP Sockets.
Transport Layer (Layer 4)
- Function: Complete/reliable transfer of data between network end points.
- Examples: TCP, UDP.
- TCP transports data packets in data segments. UDP transports data packets in datagrams.
- Functions: Segmentation, flow control and error control.
Network Layer (Layer 3)
- Function: Routing data in packets from one network node to another. Involves logical addressing, path determination, and routing.
- Examples: IP addressing, ICMP (Internet Control Message Protocol), bandwidth management.
Data Link Layer (Layer 2)
- Function: Reliable transfer of data in frames from one node to another over the physical layer.
- ARP (Address Resolution Protocol) maps IP address to Physical address.
- IEEE 802 divides the data link layer into two sub-layers: Media Access Control (MAC) and Logical Link Control (LLC). Physical addressing is based on the MAC address.
- MAC controls right to data on the network. LLC controls frame synchronization, flow control, and error checking. IEEE 802 protocols are typically used over Ethernet, 802.11 Wi-Fi physical layers.
- High-Level Data Link Control protocol used for frame synchronization.
Physical Layer (Layer 1)
- Function: Convey data bit streams using suitable physical hardware and networking equipment. Bit streams can be electrical impulses, light, or radio signals.
- Hardware examples: Ethernet cards, Wi-Fi cards, Bluetooth pc cards, Fiber media converters.

Data Units in Different Layers (Protocol Data Units)

Application: Data
Transport: Data Segment
Network: Data Packet
Data Link: Data Frame
Physical: Bits

Physical Layer Connectivity Examples

Dial-up
- Connects to the Internet via a regular telephone line using a modem (modulator/demodulator).
- $56 \newline Kbps$ limit.
ISDN (Integrated Services Digital Network)
- Internet service over power lines.
- $128 \newline kbps$ maximum.
- A single terminal adapter enables the transmission of analog and digital signals over Public Switched Telephone Networks (PSTN).
DSL (Digital Subscriber Line)
- Connects to the Internet via a regular telephone line using a DSL Filter.
- Voice and data are sent using the same cable at different frequencies.
- The DSL Filter separates voice from data.
- DSL can be Asymmetric (ADSL) or Symmetric (SDSL).
- Maximum speed by service providers is typically about $10 \newline mbps$ , but technically can be up to about $100 \newline Mbps$ .
Coaxial Cable
- Uses TV cable for Internet provision.
- Maximum speed by service providers is typically about $100 \newline mbps$ , but technically can be up to about $250 \newline mbps$ .
Fibre Optic
- Uses thin strands of glass (optical fibres) rather than metallic cable as a signal transmitter.
- Each strand is less than 1/10th of the thickness of human hair.
- Several hundreds can be packed inside a cable.
- Uses light fired by lasers rather than electricity for signal pulses, similar to Morse code.
- Optical transmitters convert electric signals to light pulses. Received light is converted back to electrical signals using photoelectric cells (optical receiver).
- Faster than electrical signals passing through cables.
- Download speed can be up to $1 \newline Gbps$ .
Wireless Broadband
- Radio (Broadcast from Masts)
- VSAT (Very Small Aperture Terminal)
- WiMAX (Worldwide Interoperability for Microwave Access)
Mobile Internet
- Evolves through generations from 1G to 5G.

Common Internet Protocols – A Focus on TCP/IP Stack

IP (Internet Protocol)
- Each addressable computer or device on the Internet has an allocated Internet Protocol (IP) address.
- This protocol is at OSI Layer 3.
- Two versions of IP in use:
  - IPv4 (IP version 4)
  - IPv6 (IP version 6)
- IPv4
  - IPv4 addresses are 32 bits long and written in four groups of decimals separated by dots (e.g., 192.168.1.100).
  - Maximum value for each number is 255.
  - Total addressable = $2^{32} = 4,294,967,296$ .
- IPv6
  - IPv6 addresses are 128 bits long and in its full notation are written in eight groups of hexadecimal quartets, separated by colons (e.g., fe80:0000:0000:0000:981b:99c8:54ec:8a98).
  - The letters ‘a’ ‘b’ ‘c’ ‘d’ ‘e’ ‘f’ are always written in small cases.
  - Total addressable = $2^{128} = 3.4028236692093846346337460743177 \times 10^{38}$ . (340 undecillion)
  - Practically inexhaustible number of addresses.
  - Compressed Formats
    - Recommended in textual representation.
    - Compression rules:
      - Use the symbol "::" to represent a set of zero quartets when there are more than one of such that appear contiguously.
        Example: fe80:0000:0000:0000:981b:99c8:54ec:8a98 becomes fe80::981b:99c8:54ec:8a98
        Example: fe80:0000:3a56:2001:981b:99c8:54ec:8a98 remains as is.
      - Suppress leading zeros.
        Example: fe80::981b:09c8:54ec:0a98 becomes fe80::981b:9c8:54ec:a98
  - More syntax
    - Zone identifier may be specified with percentage- Zone id addresses are reserved for node communications that are internal and not external to the internet.
    - Zone IDs may be represented in the following format:
      - fe80::981b:9c8:54ec:a98%eth0
      - fe80::981b:99c8:54ec:8a98%11
  - More syntax
    - For use in URL, put the IPv6 address in square bracket.
      - http://[fe80::9049:2f72:36bc:42e9]
      - http://[fe80::9049:2f72:36bc:42e9]:80
TCP (Transmission Control Protocol)
- An OSI Layer 4 Protocol
- Internet data to be transported are organized in segments and sent using this protocol
- The Protocol is layered on IP Addressing of Layer 3. Hence the term TCP/IP.
Application Layer Protocols based on TCP/IP
- HTTP (Hypertext Transfer Protocol): Used for accessing and transporting published documents on the Internet
  - Default port is 80 for HTTP and 443 for HTTPS
  - HTTPS is HTTP over TLS (Transport Layer Security)
  - HTTP related Software systems
    - Web Server (Used for publishing HTML documents) e.g. Apache, Microsoft Internet Information Services (IIS), Nginx, etc.
    - Browser e.g. Firefox, Chrome, etc.
    - URL (Uniform Resource Locator): System for identifying and locating published documents
- FTP (File Transfer Protocol) is used for interactive file transfer.
  - Default port is usually 21
  - FTPS is FTP over TLS (Transport Layer Security)
  - FTP related software systems
    - FTP Server: Used for publishing files in directory structure on the Internet e.g. vsftpd on Linux, Filezilla
    - FTP Client: Used for accessing published files e.g. browser
    - URL: Used for locating and identifying published files
- Email Protocols
  - SMTP (formerly port 25 now port 587) (Simple Mail Transfer Protocol): For sending email
  - POP (port 110) for receiving mail (Post Office Protocol), IMAP (port 143) (Internet Message Access Protocol: For reading email
  - Popular versions are POP3 and IMAP4
  - Email related software systems
    - Email Server e.g. Microsoft Exchange, Sendmail, etc.
    - Email Client e.g. Outlook, Thunderbird, Webmail like Gmail.com, yahoomail.com
Presentation Layer Protocols based on TCP
- TLS (Transport Layer Security)
  - Enables encryption of data transmitted across the internet.
  - A website needs to obtain a TLS certificate to become secure.
  - The certificate is issued by a certificate authority.
- XDR (External data representation)
  - standard representation of various data types.
Session Layer Protocols based on TCP
- Following the OSI model, session layer receives requests from presentation layer and issues service requests to transport layer.
- Transport layer protocols (e.g. TCP), specify both source and destination ports and segment sequence numbers in their header information.
- TCP sessions use different ports for different TCP based applications
- Port which is represented by a number indicates the communication endpoint/application (a running process) on a computer or device.
  - A port number is 16-bit integer (65536 possibilities)
  - Examples of default ports
    - HTTP port 80; HTTPS port 443; FTP port 21; Telnet port 23; SMTP port 25 and 587 for secure email, etc.
A briefer TCP/IP model existed before OSI model with only 4 layers.