Networks

What is a computer network and why are networks needed?

A network is a group of devices connected together to enable communication and resource sharing between them.

Define interoperability and explain how computers achieve it.

Interoperability is the ability of different devices or systems to exchange information.

It is achieved through standards and protocols that define how communication occurs.

What are standards and protocols, and why are they necessary?

Standards define how data is represented and transmitted, while protocols specify precise rules for communication.

They are necessary so different computers can understand each other unambiguously.

What is a computer network and why are networks needed?

A network is a group of devices connected together to enable communication and resource sharing between them.

Define interoperability and explain how computers achieve it.

Interoperability is the ability of different devices or systems to exchange information, achieved through standards and protocols.

What are standards and protocols, and why are they necessary?

Standards define how data is represented and transmitted, while protocols specify rules for communication, necessary for different computers to understand each other.

What is abstraction in networking and how is it used?

Abstraction hides complex implementation details, allowing networks to be used without knowing how they work, often through layering functionality.

What is internetworking and why is it important?

Internetworking is the interconnection of multiple networks for communication, enabling large-scale systems like the Internet.

What are the two main types of networks and how do they differ?

Public networks are owned by service providers and open to paying public users;

private networks are restricted to specific organizations.

Compare circuit switching and packet switching.

Circuit switching establishes a dedicated path before communication;

packet switching breaks data into packets sent over shared links without a dedicated path.

What is a packet and how is it used across network layers?

A packet is a unit of data transmitted over a network, with each layer encapsulating data from the layer above by adding its own header.

What are network layer models and why are they used?

They are frameworks (e.g., OSI and TCP/IP) that describe how data moves through a network, improving design and understanding.

Describe the TCP/IP model layers and their functions.

Physical - transmits bits as signals;

Network Interface - local delivery;

Internet - addressing and routing;

Transport - end-to-end communication;

Application - application-level protocols.

List the OSI layers in order and state their purpose briefly.

1. Physical - signal transmission;

2. Data Link - framing;

3. Network - routing;

4. Transport - reliable delivery;

5. Session - session management;

6. Presentation - data representation;

7. Application - user communication.

How do layers interact when sending and receiving data?

Each layer provides a service to the layer above; data is encapsulated down the stack when sending and headers are removed when receiving.

Distinguish between connection-oriented and connectionless services.

Connection-oriented services establish a connection before data transfer; connectionless services send data without prior connection.

Explain Layer 3 (IP) addressing and its structure.

Addresses are split into network and host parts (e.g. 128.243.226.103) and must be assignable as devices move networks.

Explain Layer 2 (Ethernet) addressing and its structure.

Ethernet uses a unique 48-bit MAC address for each device on a local network.

The first part identifies the manufacturer; the second identifies the network card.

What type of addressing is used at the Application layer?

Human-readable names such as domain names..

How does packet delivery differ for local vs remote machines?

Data is sent directly if machines are on the same network; if on different networks, data is sent to a router for forwarding

What problem does ARP solve and why is it needed and how does it do this

ARP maps IP addresses to MAC addresses, allowing IP packets to be delivered over Ethernet.

A machine broadcasts an ARP request asking who has the IP. The matching machine replies with its MAC address, allowing direct communication.

What is an ARP translation table and how are repeated requests reduced?

It stores IP-MAC mappings in memory, allowing machines to check the table before broadcasting and nd cache replies to reduce network traffic.

What is data communication and what are its main components?

Data communication is the transfer of information from a source to a destination over a physical medium,

including the:

information source

physical medium

destination.

What is the role of the physical medium in data communication?

It provides the path over which data signals travel between devices.

What is RS-232 and what is it used for?

RS-232 is a serial communication standard used to transmit data between two devices over a point-to-point connection.

How are two devices connected using RS-232?

Tx of one device connects to Rx of the other, and Ground connects to Ground.

What parameters must both ends of an RS-232 link agree on?

Identical communication settings such as baud rate, number of data bits, stop bits, and parity.

How does RS-232 represent binary data physically?

Using voltage levels: +15V represents 0, -15V represents 1.

Why is RS-232 only suitable for short distances?

Voltage levels degrade over long cables, causing signal loss and distortion.

Compare serial and parallel transmission. Why is serial transmission preferred for longer distances?

Parallel transmission sends multiple bits simultaneously using multiple wires;

serial transmission sends bits one at a time using one wire,

preferred for longer distances due to fewer wires and less susceptibility to interference.

What is meant by bit order in data transmission? Do all systems use the same bit and byte order?

The order in which bits are sent: least significant bit (LSB) first or most significant bit (MSB) first.

No, different systems may use different orders, such as Ethernet using big-endian byte order.

Distinguish between little-endian and big-endian transmission.

Little-endian sends LSB first;

big-endian sends MSB first.

What is asynchronous transmission?

How does asynchronous transmission indicate the start and end of data?

A transmission method where sender and receiver are not continuously synchronized.

data is sent whenever it is ready using start bits and stop bits around each character.

Why does RS-232 use start and stop bits? And What should happen after eight data bits are sent in RS-232?

To allow the receiver to detect the beginning and end of each character.

The signal should return to 1 (the stop bit) to mark the end of the character.

What is a framing error in RS-232 and how is it handled?

A framing error occurs when the signal does not return to the expected stop bit level.

The receiver must resynchronise using the next valid start bit.

How can you identify asynchronous transmission from a waveform?

Characters are transmitted individually, with idle gaps between them.

What is synchronous transmission? how does it work

A transmission method where sender and receiver remain synchronised and data is sent as a continuous bit stream.

As the receiver must resynchronise using the next valid start bit. not doesnt requiring start and stop bits

How can you identify synchronous transmission from a waveform?

Bits are transmitted back-to-back with no idle gaps between characters.

Why is synchronous transmission more efficient than asynchronous?

Because it does not require start and stop bits for every character, reducing overhead improving efficiency

What is transmission overhead? and why is it important

Extra control data (like start and stop bits) added to transmissions

it reduces efficiency but improves reliability.

Calculate the transmission overhead when sending 8-bit characters with start and stop bits.

Each character is framed with 1 start bit and 1 stop bit, so 10 bits are transmitted for 8 bits of data.

10 bits are sent per character → 25% overhead. The overhead is the extra 2 bits relative to the data size:

What are the types of duplex transmission? Which duplex mode is more efficient and why?

Half-duplex: Data flows both ways, but only one direction at a time; Full-duplex: Data flows in both directions simultaneously.

Full-duplex is more efficient, because devices can transmit and receive at the same time.

What is a carrier signal?

A continuous wave that is modified to carry digital data.

What is modulation and why is it used?

Modulation is the process of altering a carrier signal to represent digital data, It allows data to be transmitted over long distances and wireless media.

What are the three main types of modulation?

Amplitude Modulation (AM) – changes signal height

Frequency Modulation (FM) – changes signal frequency

Phase Shift Keying (PSK) – changes signal phase

What is the role of a modulator and demodulator?

The modulator converts digital bits into an analog signal.

The demodulator converts the analog signal back into digital bits.

How are bits represented using symbols? Can more bits be transmitted per symbol?

Each symbol represents a specific bit pattern determined by the modulation scheme.

Yes, by increasing the number of symbol states. more bits can be transmitted

What is the advantage and disadvantage of using more signal levels?

Advantage: Higher data rates;

Disadvantage: Greater susceptibility to noise.

How can noise susceptibility be reduced?

Using error correction

filtering

More robust modulation techniques.

What is a shared medium network? Why is it used?

A network where multiple devices connect to a single communication channel instead of dedicated links.

This reduces wiring complexity and improves scalability

How is access to a shared medium managed?

Using Medium-access protocols that control transmission, preventing simultaneous transmissions.

What is packet switching?

Packet switching divides data into packets that are sent independently and reassembled at the destination.

What are the advantages and disadvantages of packet switching?

• + Allows for efficient multiplexing,

• + retransmit only failed packets reducing retransmission costs and cheaper since no esatablished connection.

• - An esablished connection isnt set up leading Queuing delay and out-of-order delivery.

What are the main parts of a packet?

• Header which contains source, destination, sequence number, error detection

• and Payload/Data.

Do packets always arrive in order? and how are they ordered

No, packets may arrive out of order;

• sequence numbers allow the receiver to reorder them using fixed packet sizes or start/end markers.

How are packets delivered to the correct host on a shared medium?

Each packet includes a destination address the hosts discard packets not addressed to them.

What is broadcast delivery? how does ethernet uses this

A packet sent to a broadcast address is processed by all hosts on the network.

• Ethernet uses 48-bit MAC addresses unique on the LAN. The broadcast address is all 1s, and some bits indicate unicast or multicast.

How are transmission errors handled in LANs?

Using error detection (e.g. CRC) and retransmitting only corrupted packets.

What does network topology describe?

The physical or logical arrangement of nodes and links, determining how devices connect and how data flows.

How does a star topology work and what are its pros and cons?

All devices connect to a central hub or switch.

Pros: simple cabling, easy fault isolation.

Cons: central point of failure, extra hardware cost.

What happens if a link fails in a star network?

Only the affected host is isolated; if the central hub fails, the entire network fails.

How does a ring topology operate?

Devices form a closed loop and packets circulate around the ring until they reach the destination.

What are the advantages and disadvantages of a ring topology?

Pros: predictable traffic flow, supports controlled access (token passing).

Cons: a single break can disrupt communication unless bidirectional routing is used.

How does fault tolerance differ in unidirectional vs bidirectional rings?

Bidirectional: traffic reroutes in the opposite direction if a link fails; Unidirectional: a single break disrupts the entire network.

How does a bus topology work and what causes failure in a bus topology?

All devices share a single cable; transmissions are seen by all hosts.

A break in the bus usually disables the whole network.

What must be controlled in a bus network

• Access control (to prevent collisions)

• termination (to prevent signal reflections).

What is a collision? how can they be handled

A collision occurs when two devices transmit simultaneously, corrupting data.

• Handled using detection, avoidance, retransmission, or controlled access.

What are the main access-control approaches in LANs?

• Channelisation: Time Division Multiplexing (TDM);

• Controlled access: Token passing;

• Random access: CSMA/CD, CSMA/CA.

What is Time Division Multiplexing (TDM)? when is it most suitable

TDM divides time into fixed slots, assigning each device a turn to transmit, eliminating collisions.

In controlled environments with predictable traffic patterns.

What are the advantages and drawbacks of TDM?

• + No collisions, predictable access.

• - Wasted slots if idle, fixed delays.

What is CSMA/CD? How does it work?

Carrier Sense Multiple Access with Collision Detection, used in wired Ethernet.

• Devices listen before transmitting and detect collisions to retry later.

What are CSMA/CD pros and cons?

• +Simple, efficient under light load.

• - unrealible since no ACK

• Can lead to collisions and retransmissions under heavy load.

What is CSMA/CA and why is it used?

Carrier Sense Multiple Access with Collision Avoidance, used in wireless networks to prevent collisions using acknowledgements.

What are the advantages and disadvantages of CSMA/CA?

• Advantages: reduces collisions, reliable delivery using ACKs.

• Disadvantages: overhead and the hidden node problem.

Compare TDM, CSMA/CD, and CSMA/CA.

TDM: no collisions, fixed slots, possible wasted bandwidth. CSMA/CD detects collisions and is used in wired Ethernet; CSMA/CA avoids collisions and is used in wireless networks with ACKs.

What is Media Access Control (MAC)?

The rules and protocols that determine how multiple devices share a communication medium for reliable transmission

Why is access control needed on a shared/bus network?

All devices share the same medium; if multiple devices transmit simultaneously, collisions occur, corrupting data.

What is meant by 'eventual packet delivery'?

The protocol accepts that collisions or failures may occur, but ensures packets are eventually delivered through detection, backoff, and retransmission.

What are random access protocols? name 4

Protocols that allow devices to transmit whenever they have data, using randomisation to reduce collision probability.

• ALOHA, Slotted ALOHA, CSMA/CD, CSMA/CA.

How does Slotted ALOHA improve on ALOHA?

Time is divided into slots where devices may only transmit at the start of a slot, reducing collision probability.

What are the advantages and disadvantages of ALOHA?

Advantages: simple, reliable using ACKs.

Disadvantages: high collision rate, inefficient under load.

What is exponential backoff?

After each collision, the device doubles the maximum wait time and chooses a random delay before retrying, reducing repeated collisions.

Is Ethernet MAC layer reliable?

No. Ethernet is not reliable as it does not use ACKs

Why can't Wi-Fi use CSMA/CD?

Wireless devices cannot listen while transmitting, so collisions cannot be detected reliably.

How does CSMA/CA work?

1. Device listens.

2. Waits a random backoff time

3. Transmits.

4. Waits for an ACK.

5. Retries if no ACK is received.

What is the hidden node problem?

Occurs when two devices cannot hear each other but both transmit to the same receiver, causing a collision at that receiver.

What is an acknowledgment (ACK)?

A control packet sent by the receiver confirming successful packet reception, important for reliability, collision detection, loss detection, and retransmission.

What happens if an ACK is not received?

The sender assumes failure and retransmits after a (usually random) delay.

Which MAC protocols are reliable?

Reliable: ALOHA, Slotted ALOHA, CSMA/CA.

Not reliable: CSMA/CD (Ethernet MAC layer).

Compare scheduled access and random access protocols.

• Scheduled access avoids collisions but may waste bandwidth;

• Random access is flexible saving bandwidth but may suffer collisions and delays.

How did Ethernet evolve over time?

Ethernet evolved from a shared bus using CSMA/CD to switched star topologies, eliminating collisions at the MAC layer.

What is an Ethernet frame? what are its main parts it consists of

The basic unit of data transmission on an Ethernet LAN,

consisting of header fields, a payload, and an error-detection code (CRC).

What is the Ethernet preamble and why is it needed?

A 56-bit alternating pattern that allows the receiver to synchronise its clock with the sender

What is the Start Frame Delimiter (SFD)?

An 8-bit value (0xD5) that marks the exact start of the Ethernet frame, distinguishing it from the preamble.

How does a switch use MAC addresses?

A switch examines the destination MAC address and forwards the frame only to the port associated with that address

How does Ethernet know where a frame ends?

Ethernet does not use a length field; the frame ends when carrier transmission stops → followed by a mandatory 96-bit inter-frame gap.