Networking

Network Models

• ISO/OSI Model (Theoretical)

• TCP/IP Model (Practical)

ISO/OSI Model (Theoretical)

international standards organisation (ISO) - open systems interconnection model (OSI)

conceptual framework that describes how communication happens over a network

7 layers

used for understanding and teaching how different network functions interact

universal reference model

OSI Layers

Application - your program

Presentation - encoding, encryption, compression

Session - connections are established and managed

Transport - reliable transmission, segmentation of large data

Network - routing between multiple devices to a destination

Data Link - between two connected devices

Physical - electrical transmissions

Why is OSI Theoretical?

designed as a universal reference to explain how computers should communicate over a network

doesn’t specify real protocols or software

like a textbook model or ideal standard, useful for teaching, documentation, and conceptualising how networks should work

TCP/IP Model (Practical)

used on the internet

real-world implementation of how data is transferred over networks

typically 4 layers:

1. Application - your program

2. Transport - end to end links and persistent connections

3. Internet - between different networks/routers

4. Network Access (link) - physical, local network

Why is TCP/IP Practical?

used in real world communication systems, especially the internet

includes real protocols

actually implemented in OS, routers, and web servers, what makes the internet work everyday

OSI vs TCP/IP

Feature	ISO/OSI Model	TCP/IP Model (Protocol)
Purpose	Theoretical/Reference	Practical/Implementation
Layers	7	4
Developed by	ISO	US department of Defence
Protocols Included	None (only structure)	Yes (TCP, IP, HTTP, etc)
Usage	Teaching, documentation	Real-world internet and LANs
Flexibility	More detailed and layered	Simplified for performance

Network Delay - Latency

the time it takes for data (a packet) to travel from the source (sender) to the destination (receiver) over a network

can be very small (milliseconds), but even small delays matter a lot in real-world applications

Types of Network Delays and How to Reduce Them

Delay Type	Description	Affected by	How to Reduce
Processing Delay	Time to examine the packet header and determine where to send it next, happens at routers or end devices	Router performance	Faster processors
Queueing Delay	Time the packet waits in queue before being transmitted, happens when many packets are waiting to be sent	Traffic congestion	Better scheduling
Transmission Delay	Time to push all the packets bits onto the link	Packet size, bandwidth	Faster links
Propagation Delay	Time for a signal to travel from sender to receiver across the medium	Distance, medium	Shorter paths

Total Nodal Delay

to find the total delay at a single router (or node), we sum all 4 delays:

total delay = processing delay + queueing delay + transmission delay + propagation delay

Real-World Examples of Delay

Application	Delay Sensitivity	Example
Online Gaming	High	Even 50ms delay can cause lag
Video Calls	Medium to high	Causes echo, talking over
Email or File download	Low	Delay is less noticeable

Reducing Delay

• use faster links - higher bandwidth

• optimise routing paths

• avoid congestion e.g. through load balancing

• use Content Delivery Networks (CDNs) to reduce distance