Data Transmission

What type of messages does a Digital Communication System (DCS) handle?

A finite set of discrete messages or symbols (e.g., 0 and 1).

What is the main difference in signal type between analog and digital communication systems?

Analog uses continuous signals; digital uses discrete symbols.

In DCS, what is the receiver's primary goal?

To detect the most likely sent symbol from a finite set, not to reproduce the exact waveform.

What technologies does digital communication depend on?

Microelectronics and software development.

Why are analog fundamentals still important in DCS?

Because analog waveforms are used to transmit digital data, especially over radio channels.

What does digital telecommunication enable?

The remote use of computing systems, combining communication and IT.

What makes DCS more robust than analog communication in noisy environments?

DCS is based on symbol detection, which is more resistant to noise than continuous waveform reproduction.

Give an example of a real-world system that uses digital communication.

The Internet, mobile phones, and Wi-Fi are all based on DCS.

What does DTE stand for, and what is its role?

Data Terminal Equipment; generates or receives data (e.g., a computer), sends it to DCE.

What does DCE do in a communication system? Give some examples.

Data Circuit Equipment handles the actual transmission over the network and interfaces with the DTE. It provides the physical path for communication. It applies transmission protocols.

Ex. Modems, routers, NICs

What does a modem do?

Converts digital signals to analog (for transmission) and analog back to digital (for reception).

What are the four interface categories between DTE and DCE?

Mechanical, Electrical, Functional, Procedural.

What does a communication controller do?

Manages data flow and ensures efficient, stable transmission between devices and networks.

What is the role of the Transmission Control Unit (TCU)?

Manages signaling, flow control, and data timing during transmission.

Why are digital signals easier to regenerate than analog signals?

Because they consist of discrete pulses (e.g., 0s and 1s), making it easy to detect and recreate using regenerative repeaters.

What two mechanisms distort transmitted signals?

1. Non-ideal transfer functions of the transmission media, and 2. Noise/interference.

What is a regenerative repeater?

A device that detects, cleans, and retransmits a digital signal to remove distortion.

Why are digital circuits cheaper and more reliable than analog ones?

Because they’re easier to manufacture, have fewer sensitive components, and don’t degrade as easily.

What makes digital systems more flexible?

Digital hardware and software can be easily updated or reprogrammed, unlike analog systems.

How are different signal types treated in digital systems?

All data is converted to bits, allowing for uniform transmission and switching.

What feature allows digital systems to ensure signal accuracy?

Error detection and correction techniques like parity.

What is the main disadvantage of digital communication?

It typically requires greater bandwidth than analog transmission.

What does the formatting block do in digital signal processing?

It transforms the source information into digital symbols compatible with the channel and processing system.

What is the purpose of modulation in a DCS?

It converts digital symbols into waveforms compatible with the transmission channel.

What is the purpose of encryption in digital signal processing?

To secure the message from unauthorized access and prevent false message injection.

What is multiplexing in communication systems?

The process of combining multiple signals to share the same communication channel.

What happens during demodulation?

The analog waveform is converted back into digital symbols.

What is analog modulation?

A process of superimposing a low-frequency signal on a high-frequency carrier for transmission.

What are the three main types of analog modulation?

Amplitude Modulation (AM), Frequency Modulation (FM), and Phase Modulation (PM).

What does AM change in the carrier signal?

Amplitude of the carrier.

In Frequency Modulation, what part of the carrier is altered?

The frequency.

Which type of modulation alters the carrier’s phase?

Phase Modulation (PM).

What do +v and –v represent in a digital signal?

They are significant voltage levels used to represent binary values (e.g., +v = 1, –v = 0).

What are “significant moments” in a digital signal?

The transition points where the signal changes level — used for timing and decoding.

Define the elementary interval (Te).

The shortest time between two consecutive transitions in a digital signal.

Why is a clock signal important in digital communication?

It provides synchronization so that the receiver knows when to read each bit.

What’s the benefit of using multilevel digital signals?

Higher data rates without requiring more bandwidth.

What is data rate?

The number of bits transmitted per second; measured in bps.

What is signaling rate (baud rate)?

The number of symbols (signal changes) per second.

What is the formula connecting data rate and signaling rate?

D = V_m * log₂ N

If N = 8, how many bits are carried by each symbol?

log₂(8) = 3 bits per symbol.

Why use multilevel signals (N > 2)?

To transmit more bits per symbol and increase data rate without increasing bandwidth.

Care sunt cele doua domenii principale ale studiului comunicatiilor?

Retele de calculatoare & Protocoale de Comunicatie

What are the 3 main forces that drive the evolution of data communication?

1. Growth of communication traffic

2. Development of new services

3. Advances in technology

Why is point to point communication not always practical?

Devices are too far apart. Large set of devices would need impractical number of connections.

What is networking?

Networking deals with the technology and architecture of the communications networks used to interconnect communicating devices. This field is generally divided into the topics of local area networks (LANs) and wide area networks (WANs).

Ce sunt rețelele punct la punct?

Rețele în care două calculatoare sunt direct conectate printr-un cablu, fără intermediari.

Ce sunt rețelele de difuzare (broadcast)?

Rețele în care toate stațiile sunt conectate la un canal comun, iar mesajele pot fi recepționate de toate calculatoarele.

Cum este transmisă informația într-o rețea punct la punct?

Direct între două noduri, fără a implica alte calculatoare.

Ce este o adresă unicast?

O adresă care identifică un singur calculator ca destinatar al unui mesaj.

Ce este o adresă de difuzare (broadcast)?

O adresă folosită pentru a trimite un mesaj către toate calculatoarele din rețea simultan.

Ce face un calculator dintr-o rețea de difuzare când primește un pachet?

Verifică adresa de destinație. Dacă e destinatarul, procesează datele; altfel le ignoră.

What is PAN, LAN, MAN, WAN?

Personal Area Network

Local Area Network

Metropolitan Area Network

Wide Area Network

+Internet (largest by distance)

Ce este Server-ul in Sistemele Client/Server?

Program care ofera un serviciu prin retea. Serverul primeste cereri si returneaza un rezultat solicitantului.

Ce este Client-ul in Sistemele Client/Server?

Program care utilizeaza un serviciu oferit de un server.

Descrie Peer to Peer

• Partenerii de comunicatie sunt considerati egali neavand atribuite roluri prestabilite (client, server);

• Conexiuni realizate intre orice pereche de calculatoare;

• Stabilirea unei intregi retele de conexiuni, ca de exemplu File Sharing

What is the typical geographical scope of a LAN?

Who usually owns the devices and infrastructure in a LAN?

The same organization

What is the purpose of a LAN?

To connect PCs/workstations for data exchange and resource sharing

What are typical LAN data transmission speeds (wired)?

10 Mbps → 100 Mbps → 1 Gbps → 10 Gbps

How has wireless LAN speed evolved?

11 Mbps → 54 Mbps → hundreds of Mbps

What is the typical message delay in LANs?

Around 10 milliseconds

In a bus topology, how is data sent and received?

All stations receive the message, but only the destination processes it

In a star topology, what role does the central station play?

Forwards messages between nodes

What devices are used in star topology?

Hub (broadcast) or Switch (point-to-point)

What makes ring topology vulnerable?

A failure in one node or link can break the entire ring

Ce este o rețea WAN?

O rețea care acoperă o arie geografică foarte mare și interconectează LAN-uri și MAN-uri.

Are topologie neregulata, adaptata la nevoi. Este formata din statii interconectate punct la punct.

Ce tipuri de tehnologii de comutare sunt folosite în WAN?

Comutare de circuit și comutare de pachete.

Care este WAN-ul global dominant în prezent?

Internetul.

Care este raza tipică de acoperire a unei rețele wireless locale (WLAN)?

Între 10 și 100 metri.

Ce este circuit switching?

O cale de comunicare dedicată este stabilită pentru durata conversației.

Care sunt cele 3 etape ale circuit switching?

1. Setup (alocarea resurselor dedicate)
2. Transmiterea datelor (utilizarea resurselor)
3. Terminarea circuitului (eliberarea resurselor)

Cum funcționează packet switching?

Datele sunt trimise în pachete mici, fiecare pachet fiind transmis prin rețea de la nod la nod. La fiecare nod pachetul este primit, stocat si trimis la urmatorul nod.

Store & forward: buffering

Ce înseamnă store-and-forward în contextul packet switching?

La fiecare nod, pachetul este primit complet, stocat temporar (buffered), apoi transmis mai departe.

Care este avantajul packet switching față de circuit switching?

Folosește mai eficient resursele, deoarece nu rezervă o cale dedicată permanentă.

How do individual hosts and LANs connect to the Internet?

Through an ISP (Internet Service Provider) via a Point of Presence (POP).

What does CPE stand for and what is it?

Customer Premises Equipment; communication devices like modems located onsite with the user.

Name the different levels of ISPs.

Local ISP, Regional ISP, Backbone ISP.

Through what do Backbone ISPs connect to each other?

Network Access Points (NAPs).

Care sunt cele două soluții pentru implementarea protocoalelor?

Soluția 1: un program complex; Soluția 2: set de programe mici specializate (acceptată acum).

Ce avantaje și dezavantaje are soluția cu un program complex?

Avantaj: schimb eficient pentru o aplicație. Dezavantaj: lipsă de flexibilitate, greu de adaptat.

Ce este un protocol de comunicare?

Un set de reguli și acorduri între procese/aplicații pentru a comunica corect.

Care este cea mai cunoscută arhitectură de protocol?

TCP/IP și modelul OSI cu 7 straturi.

What does ISO stand for?

International Standards Organization – ISO

What does OSI stand for?

Open Systems Interconnection

What are the stages of TCP/IP?

fizic, acces rețea, internet, transport, aplicație

What are the stages of OSI?

fizic, data link, rețea, transport, session, presentation, aplicație

What is a Public IP Address?

A unique numerical label assigned to a device on the Internet for global identification and communication. Assigned by ISP, visible to wider Internet.

What is a MAC Address?

A 12-digit hexadecimal number, unique identifier assigned to a device's Network Interface Card (NIC) during manufacturing. Used by the local network.

Which OSI layer sends raw data bits?

Physical Layer (Layer 1)

What are the two sub-layers of the Data Link Layer?

Logical Link Control (LLC) and Media Access Control (MAC).

What is the main responsibility of the Network Layer?

Data transmission over long distances and between heterogeneous sub-networks.

What is a Router?

A device that connects different networks and routes data packets between them. Operates at Layer 3.

What are the two widely used protocols at the Transport Layer?

TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

What are the key services provided by the Session Layer (Layer 5)?

Dialogue control (transmission direction), token management, synchronization points (for restarting transmission).

What is the primary task of the Presentation Layer (Layer 6)?

Displaying data in a common, universally recognized format (data translation/encoding/decoding).

It provides network services directly to the end-user app.

Ex.: HTTP, FTP, SMTP

What is the main function of the Application Layer (Layer 7)?

Provides standard protocols directly usable by applications/systems.

What replaced conventional, relay-based controllers?

Programmable Logic Controllers (PLCs).

Define "Industrial Communication Network."

A network connecting devices for factory automation & process control, facilitating communication and data exchange among sensors, controllers, etc.

What two types of distribution do industrial networks often require?

Geographical distribution (of I/O/sensors) and Functional distribution (of applications).