Text uses Unicode (not ASCII). Numbers use
binary. Images, audio, and video are all stored as
bit patterns.
↔️ Data Flow Directions
Data can flow in 3 different directions (modes) across a link:
➡️
Simplex
One direction only — forever.
Example Keyboard→Computer, TV
broadcast, Radio station
↔️
Half-Duplex
Two directions, but one at a time.
Example Walkie-talkie — you talk OR
listen, not both at once
⇄
Full-Duplex
Two directions simultaneously.
Example Telephone — you can talk
and hear at the same time
⚠️ Common Confusion – Half vs Full Duplex
Half-duplex: The link can go both ways, but
not at the same time (takes turns). Think: walkie-talkie — you
press a button to talk, then release to listen.
Full-duplex: Both sides communicate simultaneously.
Think: telephone conversation — no turn-taking needed.
📋 Section 1 Mini Summary
Data communication = exchange of info via transmission medium. 5
components: Sender, Receiver, Message, Medium, Protocol. Messages are
bits. Data flows in Simplex, Half-Duplex, or Full-Duplex modes.
PDF page 4The five components of a data communication system: message, sender,
receiver, transmission medium and protocol.
PDF page 9Data flow: simplex sends one way, half-duplex takes turns,
full-duplex sends both ways at once.
02
Networks
📖 Definition
A network is a set of devices (called
nodes) connected by communication
links.
Node (can be):
Host: computer, printer, phone
Networking device: router, switch, modem
Link (can be):
Wired: cables, fiber optics
Wireless: radio waves, satellite
🔗 Types of Network Connections
Point-to-Point
A dedicated link between exactly two devices. Private,
exclusive channel. Example Two
laptops connected by a direct cable
Multipoint (Shared)
One link is shared among more than two devices. All share the
bandwidth. Example Old bus
network — all computers on one cable
📊 Network Performance Criteria
A network must meet three key criteria:
1. Performance Measured by:
Throughput — amount of data a user can actually
send (want: large)
Delay — time for data to travel from sender to
receiver (want: small)
🎯 Exam Focus – The Contradiction
Throughput and delay contradict each other: when more
users send data (↑ throughput), more traffic builds up on the network
→ ↑ delay. You can't maximize both at once.
Performance depends on: number of users, type of medium, hardware
capabilities, software efficiency.
2. Reliability Measured by:
Frequency of failure — how often the network breaks
down
Recovery time — how quickly a link recovers after
failure
3. Security
Protecting data from unauthorized users and damage
Applying security policies for recovery from breaches and data
losses
🧠 Mnemonic – Performance, Reliability, Security
"Please Rely Securely" — P·R·S — the three network
criteria
📋 Section 2 Mini Summary
A network = nodes + links. Nodes can be hosts or network devices. Links
can be wired/wireless, point-to-point or multipoint. Good networks
balance Performance (throughput ↑, delay ↓), Reliability, and Security.
PDF page 7A point-to-point connection gives the whole link capacity to two
devices.
PDF page 8A multipoint connection shares one link between more than two
devices.
03
Network Topology
📖 Definition
Topology is the way in which a network is physically
built — how devices are interconnected across the network.
There are 4 basic topologies, and a
Hybrid combination:
🔷 1. Mesh Topology
Every device has a
dedicated point-to-point link to every other device.
Number of links = N × (N-1) / 2
NICs (ports) per device = N - 1
📐 Example Calculation
For 5 devices: Links = 5 × 4 / 2 = 10 links. Each
device needs 4 ports.
✅ Advantages
Dedicated links → no congestion
Very secure (private channels)
Robust — if one link fails, others still work
Easy to isolate faulty links
❌ Disadvantages
Difficult to install & update (too much wiring)
Very expensive (cables + NICs)
Does not scale well
⭐ 2. Star Topology
Each device has a
dedicated link to a central controller (hub or
switch) only.
Number of links = N (number of devices)
✅ Advantages
Less expensive than mesh
Easy to install
Easy to add/remove devices
Robust (one device failure ≠ network failure)
Easy to find faulty links
❌ Disadvantage
If the central device fails, the entire network
fails
🚌 3. Bus Topology
All devices connect to a single backbone cable via
drop lines and taps.
Number of links = 1 main cable + N drop lines
✅ Advantages
Easy to install
Less cabling than mesh and star
❌ Disadvantages
Difficult to isolate faults
Difficult to add new devices
A break in the bus stops ALL transmission
🔄 4. Ring Topology
Each device is connected to the two devices on either side of it,
forming a loop. Uses repeaters.
Number of links = N
✅ Advantages
Easy to install
Uses less cable
❌ Disadvantages
Not robust — one break disrupts all
Difficult to add/remove devices
Difficult to isolate faults
🔀 5. Hybrid Topology
A combination of two or more topologies. Common
examples:
Star-Bus: Multiple star networks connected via a
bus backbone
Star-Ring: Multiple star LANs interconnected in a
ring pattern
Used in real-world networks to get the benefits of multiple
topologies.
📊 Topology Comparison Table
Topology
Links Formula
Robust?
Easy Fault Isolation?
Cost
Scalable?
Mesh
N(N-1)/2
✅ Very
✅ Yes
💰 Very High
❌ No
Star
N
✅ (without central)
✅ Yes
💰 Medium
✅ Yes
Bus
1+N drops
❌ No
❌ Hard
💰 Low
❌ Hard
Ring
N
❌ No
❌ Hard
💰 Low
❌ Hard
🎯 Exam Focus
Mesh formula: N(N-1)/2 for links, N-1 NICs
per device
Star: Central device failure = whole network down
Bus: Cable break = all transmission stops
Star and Ring both have N links, but are very different!
📋 Section 3 Mini Summary
4 topologies: Mesh (most robust, most expensive), Star (most popular
today, central failure risk), Bus (simplest, single point of failure),
Ring (loop, not robust). Hybrid combines benefits.
PDF page 11Mesh topology: every device has a dedicated link to every other, so n
devices need n(n-1)/2 links.
PDF page 12Star topology: every device links to a central hub or switch.
PDF page 13Bus topology: devices tap into one shared backbone cable through drop
lines.
PDF page 14Ring topology: each device has a dedicated link to the two devices
beside it.
PDF page 15A hybrid topology: a star backbone joining three bus networks.
04
Network Types (LAN, MAN, WAN)
Networks are categorized by their geographic size:
🏢
LAN
Local Area Network
Covers a room, building, or campus (up to ~1 km).
Today Uses star topology with
switches Past Used bus topology
🏙️
MAN
Metropolitan Area Network
Covers a city (~10 km).
Example STC network in Riyadh
Connects multiple LANs within a city
🌍
WAN
Wide Area Network
Covers country/continent/planet (100s–10,000s km).
Provides long-distance data transmission
Uses routers to connect networks
📏 Distance Scale Reference
Distance
Scope
Network Type
1 m
Square meter
Personal Area Network (PAN)
10–100 m
Room / Building
LAN
1 km
Campus
10 km
City
MAN
100–1000 km
Country / Continent
WAN
10,000 km
Planet
🔗 Connecting Networks (Internetwork)
When LANs and WANs are connected together using
routers, they form an
internetwork (a network of networks). The Internet is
the largest internetwork.
Point-to-point WAN: Two routers connected by a
dedicated link
Routers direct traffic between different networks
🎯 Exam Focus
LAN today uses star topology with switches. Old LAN
used bus topology. A router is what connects networks
together. Two LANs + WAN = an internetwork.
📋 Section 4 Mini Summary
LAN = local (building/campus), MAN = city, WAN =
country/continent/world. Routers connect these networks into
internetworks. The Internet is the ultimate internetwork.
PDF page 16Categories of networks by reach: LAN, MAN and WAN.
PDF page 17An isolated LAN, then and now: a shared coaxial cable versus a
switched LAN.
PDF page 20An internetwork made of two LANs joined by one WAN.
05
The Internet & ISPs
📖 What is the Internet?
The Internet is a collection of different networks that
run the TCP/IP protocol suite. It is made of many WANs
and LANs joined by connecting devices and switching stations.
🎯 To Be "On the Internet," a Host Must:
Run the TCP/IP protocol
Have an Internet address (IP address)
Be able to send IP packets to other machines on the
Internet
🏗️ Internet Service Providers (ISPs) – 3 Tiers
T1
Backbones / International ISPs (Tier-1)
International coverage. Called "Internet backbone networks." Very few
exist. Own the physical links and routers. Connect
directly to other Tier-1 ISPs. Examples Sprint, AT&T, LUMEN
T2
Provider Networks / National or Regional ISPs (Tier-2)
National or regional coverage. Gateways to the international Internet.
Connected to at least one Tier-1 ISP. Examples STC, ITC, Mobily (Saudi
Arabia)
T3
Local ISPs (Tier-3)
Closest to end users ("last hop / access network"). Take services from
Tier-2 ISPs to provide Internet to homes and businesses.
🧠 ISP Tiers – Think of a Building
Tier 1 = the global highways (owned by the biggest
companies, e.g. LUMEN) Tier 2 = national roads (e.g. STC connects Saudi Arabia
to the global Internet) Tier 3 = your street/driveway to your house (your local
ISP connects you)
🔄 How ISPs Connect – Peering Methods
Peering = the interconnection of networks, allowing
them to exchange traffic.
Private Peering Points (Direct Connection)
ISPs connect directly to each other. A group of routers owned and
operated by the ISPs themselves (not a third party). Used by Tier-1 ISPs
(international)
Internet Exchange Points (IXP) – Public Peering
Physical infrastructure managed by a third party. Enables any
ISP to connect with any other ISP. High-speed switching networks that
exchange huge amounts of traffic. Used by Tier-2/National
ISPs Example Saudi Arabian
Internet Exchange
🎯 Exam Focus – Current Trend
International ISPs (Tier-1) → connect at
Private Peering Points
National ISPs (Tier-2) → interconnect using
IXP (Internet Exchange Points)
⚠️ Common Confusion – Private Peering vs IXP
Private Peering: Direct connection between ISPs. They
own the infrastructure. More control, more private.
IXP: A shared, neutral third-party facility where many
ISPs connect. Like a central hub at an airport — many airlines meet
there, but a third party runs the airport.
📋 Section 5 Mini Summary
The Internet = networks running TCP/IP. ISPs come in 3 tiers: Backbone
(T1, international), Provider (T2, national), Local (T3, last mile).
ISPs connect via private peering (T1) or IXP (T2).
PDF page 26Internet structure: tier-1, tier-2 and access ISPs connecting into a
network of networks.
PDF page 29An Internet Exchange Point lets ISPs hand traffic to each other
directly.
06
Internet Standards & Protocols
❓ Why Standards?
In the early 1970s, IBM, DEC, and others developed proprietary
networks that only their own computers could use.
Different company networks couldn't communicate without expensive
translation software.
The solution: all communicating devices must
agree on protocols and standards.
📖 Key Terms
Protocol: A set of rules controlling the exchange of
data between two entities in a system.
Entities: Applications capable of sending/receiving
info (e.g., email, browser, user apps).
Systems: The devices running those entities
(computers, terminals, sensors).
🏆 Advantages of Standards
Ensures a large market for equipment and software
Provides guidelines to manufacturers, vendors, and government agencies
Allows products from
different vendors to communicate (interoperability)
📋 What Standards Must Cover
Standards answer questions like:
How many volts represents a 0 and a 1? (Physical layer)
How to determine the end of a message?
How to handle lost messages?
How many bits for each data type?
How are machines identified in a network?
How to find the path to reach another machine?
How do applications talk to each other through the network?
🏛️ Standards Organizations
Abbreviation
Full Name
Focus
ISO
International Organization for Standardization
General international standards
ITU-T
International Telecommunication Union
UN agency for telecom standards
ANSI
American National Standards Institute
US national standards
IEEE
Institute of Electrical and Electronic Engineers
LAN/MAN standards (e.g. Wi-Fi, Ethernet)
W3C
World Wide Web Consortium
Web standards (HTML, CSS, etc.)
🧠 Remember the Organizations
"I Imagine All IEEE Websites" ISO · ITU-T · ANSI ·
IEEE · W3C
🎯 Exam Focus
IEEE specializes in
LAN/MAN standards (Ethernet, Wi-Fi)
W3C handles World Wide Web standards
ITU-T is a UN agency
Standards enable interoperability between different
vendors
📋 Section 6 Mini Summary
Protocols = rules for data exchange. Standards enable different vendors'
equipment to work together. Key orgs: ISO, ITU-T, ANSI, IEEE (LAN/MAN),
W3C (Web).
Why it matters: Every communication system must have
ALL five. The protocol ensures both sides understand each other, while
the medium physically carries the data.
2. How is text represented in a computer system?
▼
✓ Text is represented as a bit pattern using Unicode.
Note: Not ASCII — the modern standard is Unicode, which supports all
world languages. Numbers use binary, images/audio/video also use bit
patterns.
3. What is the difference between half-duplex and full-duplex?
▼
✓ Half-duplex: both directions possible, but only ONE at a time.
Full-duplex: both directions simultaneously.
Example: A walkie-talkie is half-duplex (press to talk). A telephone
is full-duplex (talk and listen at the same time).
4. In a mesh topology with 6 devices, how many cables (links) and how
many I/O ports per device are needed?
▼
Formula: Links = N(N-1)/2. Ports per device = N-1.
5. Why do high throughput and low delay contradict each other in a
network?
▼
✓ When throughput increases, more data enters the network, causing
congestion and increasing delay.
Think of it like a highway: more cars (throughput) = more traffic jams
(delay). Engineers must find a balance.
6. What is the main disadvantage of a star topology?
▼
✓ If the central device (hub or switch) fails, the entire network
stops functioning.
This is its critical weakness, even though it's the most common
topology used today. Redundant central devices can help mitigate this.
7. What are the three conditions for a machine to be considered "on
the Internet"?
▼
✓ (1) Run TCP/IP protocol, (2) Have an Internet (IP) address, (3)
Be able to send IP packets to other machines.
8. What is the difference between Private Peering and IXP?
▼
✓ Private Peering: ISPs connect directly to each other using their
own infrastructure. IXP: A neutral third-party facility where
multiple ISPs connect to exchange traffic.
Current trend: Tier-1 ISPs use private peering. National/Tier-2 ISPs
interconnect via IXP.
9. Which standards organization specializes in LAN and MAN standards?
▼
✓ IEEE (Institute of Electrical and Electronic Engineers)
IEEE created Ethernet (IEEE 802.3) and Wi-Fi (IEEE 802.11) standards,
which are the backbone of modern LANs.
10. Compare bus and ring topology in terms of robustness.
▼
✓ Both are NOT robust — a single failure disrupts the entire
network. In bus: a break in the main cable stops all transmission.
In ring: a break anywhere in the loop stops all communication.
This is in contrast to mesh (very robust) and star (robust if central
device works).
11. STC is an example of which ISP tier? How does it connect to the
global Internet?
▼
✓ STC is a Tier-2 (National) ISP. It connects to at least one
Tier-1 ISP and uses IXP (Internet Exchange Points) to interconnect
with other national ISPs.
12. What is a protocol and why is it necessary?
▼
✓ A protocol is a set of rules controlling the exchange of data
between two entities in a system.
It is necessary because without agreed-upon rules, devices from
different manufacturers cannot understand each other's data — just
like two people trying to talk in completely different languages.
Formula Summary
Key formulas and what each one is used for.
N(N − 1) / 2Find the number of links in a full mesh connecting N devices.
N − 1Find how many network interfaces or ports each device needs in a full mesh.
NFind the number of links in a ring or star topology with N devices.
1 main cable + N drop linesCount the cable segments in the bus-topology model used in this guide.