Introduction to Network Layers, Physical Media, and Data Link Control
Unit Administration and Assessment Details
Assessment One Overview
Assessment one is fundamentally based on weekly practical tasks.
For each week, specific selected tasks from the practical sessions will be utilized for the final submission.
Detailed instructions regarding these submissions, including how and where to submit, are provided by Technical Assistants (TAs) during practical classes.
The specification document released at the start of the term contains high-level details.
The marking rubric includes specific points allocated for engagement, which can be secured by attending and participating in practical sessions.
Project Two Assessment
The specifications for Project Two are scheduled for release on Sunday, August 9.
Consultation Hours
Unit and lecture-related consultations are held on Wednesdays from 11:00 to 12:00.
These sessions are conducted online, and students are encouraged to attend with any questions.
Course Material and Final Exam Scope
Lecture worksheets and notes are available on the learning management system (iLearn) under Week 1.
The final exam covers all content delivered in face-to-face classes, content provided via lecture videos, and all materials worked on during practicals and Small Target Tutorials (STTAs).
Practice questions provided in STTAs or practicals are also examinable.
The only content excluded from the final exam are specific reading topics mentioned for advanced or extra knowledge that were not explicitly covered in class or via video tutorials.
Fundamentals of Network Architecture: Layer 1
The OSI Physical Layer
Layer 1 of the Open Systems Interconnection (OSI) reference model is the Physical Layer.
This layer is responsible for the actual physical transmission of data bits over a medium.
The TCP/IP Network Access Layer
The lowest layer of the TCP/IP protocol stack is generally referred to as the Network Access Layer.
Alternative names occasionally used for this layer include the Data Access Layer or Media Access Layer, though the latter is technically a sublayer function.
The Network Access Layer effectively combines the functions of the OSI Model’s Physical Layer and Data Link Layer.
Logical Paths and Physical Links
Logical Paths: These represent the overall end-to-end route that data takes between two communicating devices (source to destination). A logical path is akin to a route suggested by Google Maps for traveling from Sydney to Canberra; it describes the journey rather than the specific physical infrastructure. Multiple logical routes (bus, train, car) may exist between the same two points.
Physical Links: A physical link is a specific segment of physical medium along a logical path. Using the travel metaphor, physical links are the actual roads, tunnels, and bridges utilized. It represents the actual physical infrastructure the traffic traverses.
Media vs. Medium: "Medium" refers to a single transmission vehicle, whereas "Media" is the plural form.
Network Configurations and Transmission Forms
Point-to-Point Networks
Two devices are directly connected to one another.
The medium is exclusive to those two devices; there is no competition for the channel.
Devices can communicate or transmit at any time without waiting for the medium to become clear.
There is no requirement for addressing because the recipient is always known.
No medium access control is necessary.
Multipoint Networks
The medium is shared among multiple devices.
Devices require unique addresses to identify intended recipients.
Access control mechanisms are essential to manage who speaks when to avoid interference.
Transmission can only occur when the medium is determined to be clear or idle.
Forms of Transmission
Simplex: One-way communication only. Examples include television broadcasts or radio transmissions where the receiver cannot respond through the same channel.
Half-Duplex: Communication is possible in both directions but only in one direction at a time. Examples include walkie-talkies where users must indicate they have finished (e.g., saying "over") to clear the channel for the other party.
Full-Duplex: Communication occurs in both directions simultaneously. Separate channels are used for transmitting and receiving. Telephone and cellular communications are primary examples.
Multiplex: Data streams from multiple senders are combined and placed onto a single physical link for transmission. A device called a Multiplexer (MUX) combines the streams, and a De-multiplexer (DEMUX) separates them at the receiving end.
Communication Media
Guided (Wired) Media
Signals are carried using a physical, tangible medium.
Twisted Pair Cable: Commonly used for data networks and broadband. Inside one large cable, there are eight smaller cables twisted into four pairs (colors: blue, orange, green, and brown). One wire in each pair is solid-colored, and the other is white with a colored stripe. The twisting helps mitigate electromagnetic interference.
Coaxial Cable: Rounded cables often used for television signals; these consist of multiple layers around a central conductor.
Optical Fiber: Used for long-distance communication. It carries data in the form of light pulses rather than electrical voltages.
Unguided (Wireless) Media
The medium is air or space rather than a physical wire.
Radio Waves: Utilized in Wi-Fi, Bluetooth, and cellular networks.
Microwaves: Primarily used in satellite communication and radar systems.
Infrared: Used for very short-distance communication, such as television remotes. It requires a direct line of sight and has a limited range.
Data Coding and Bit Transmission
Coding Definition
Data is stored as bits ($0$s and $1$s), the smallest unit of digital information. Coding is the process of converting these bits into a form the physical medium can understand and carry, such as voltages, radio waves, or light pulses.
Coding uses symbols to represent bits. A symbol is a physical signal activity on the medium.
Bit Rate vs. Baud Rate
Bit Rate: The number of bits transmitted per second.
Baud Rate: The number of symbols transmitted per second.
Relationship Formula:
Increasing the number of bits per symbol allows more information to be carried per signal change. For example, $3$ bits per symbol allows for $2^3 = 8$ different signal levels.
Signals types
Analog (Continuous) signals: These exhibit continuous changes in value and amplitude. Every point on the waveform represents a different value.
Digital (Discrete) signals: These have sudden changes in amplitude and exist in a limited number of states (typically representing $0$ and $1$).
Modem (Modulator-Demodulator)
Computers process digital data, but transmission media often require analog signals.
A modem converts digital signals to analog for transmission across a network and converts analog signals back to digital at the receiving end.
Modulation Techniques
Types of Modulation
Modulation changes specific properties of a signaling waveform to represent digital bits.
Amplitude Modulation (AM): The height (amplitude) of the wave varies to represent different bit values, while the frequency and phase remain constant.
Frequency Modulation (FM): The frequency (how often the wave cycles occur) changes to represent bits. High frequency might represent $1$, while low frequency represents $0$. Amplitude and phase remain constant.
Phase Modulation (PM): The "flow" or phase of the wave is disrupted or shifted. A change in the direction of the wave indicates a bit transition. Amplitude and frequency remain constant.
Data Link Layer: OSI Layer 2
Sublayers of the Data Link Layer
Logical Link Control (LLC): Responsible for organizing the communication process.
Media Access Control (MAC): Handles addressing (MAC addresses), error control, and manages who can access the medium at a given time.
Media Independent Interface (MII): Acts as the interface connecting the MAC sublayer with the physical hardware.
Protocol Data Unit (PDU)
The unit of data at Layer 2 is called a Frame.
MAC Sublayer Responsibilities
Marking the start and end of frames.
Allocating and managing MAC addresses.
Performing error detection and error correction.
Controlling access to the shared medium to prevent data collisions.
Random Access Protocols
Definition
Protocols are sets of rules for communication. Random access protocols allow a device to attempt transmission at any time without asking for permission or following a strict time schedule.
CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
Used in wired Ethernet networks.
Carrier Sense: The device "listens" to the medium. If it is idle, it begins transmitting.
Multiple Access: Many devices share the same medium.
Collision Detection: If two devices transmit simultaneously and a collision occurs, the protocol detects the collision. It forces the devices to stop, wait for a randomized period, and then retry the transmission.
CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)
Used in wireless networks.
Devices listen to the medium. If it is found to be idle, they do not transmit immediately. Instead, they wait for a Random Back-off Time.
This randomness reduces the likelihood that two devices waiting for the channel will transmit at the exact same moment when the channel becomes free.
Error Control through Parity Checks
Error Detection Logic
Error control involves adding extra bits (redundancy) to the data stream based on a pre-agreed rule between sender and receiver. This helps identify if bits were flipped or changed during transmission due to interference.
Single-Bit Parity Checks
Even Parity: The sender adds a parity bit (either $0$ or $1$) to ensure the total number of $1$s in the stream is even.
Example: Data string $110110$ has four $1$s (even). The parity bit is $0$.
Example: Data string $110111$ has five $1$s (odd). The parity bit is $1$ to make the total count even ($6$).
Odd Parity: The parity bit is added to ensure the total number of $1$s in the stream is odd.
Block Parity (2D Parity)
To increase detection accuracy, data can be organized into a block. Parity is then calculated for every row and every column.
This 2D approach can sometimes help identify the specific row and column where an error occurred, though even this is not $100\%$ foolproof if multiple bits change simultaneously.
While more bits improve protection, they decrease efficiency because more of the network bandwidth is used for control information rather than actual data.