৪৯তম বিসিএস ⎯ তথ্য ও যোগাযোগ প্রযুক্তি [২৮১]

The Data Link Layer (Layer 2 in the OSI model) is responsible for:

Framing: Dividing data into frames for transmission.
Error detection: Identifying errors in transmitted frames (using techniques like CRC).
Flow control: Preventing a fast sender from overwhelming a slow receiver.
Medium Access Control (MAC): Determining which device gets to use the physical medium.

It does not perform routing (Layer 3 task), encryption (usually done at Layer 6 or 7, or in VPN protocols), or host addressing (IP addressing is done at Layer 3).

Source: Computer Networks (4th Ed) by  Andrew S. Tanenbaum

The Data Link Control (DLC) sublayer is part of the Data Link Layer in the OSI model and is responsible for:

Organizing data into frames → Encapsulation of network-layer packets into frames.
Detecting transmission errors → Using error detection techniques like CRC.
Regulating data transmission speed → Flow control to prevent buffer overflow.

Determining the path for packet delivery is not part of the DLC's job; this is done by the Network Layer, which handles routing based on logical addresses (e.g., IP).

Source: Behrouz A. Forouzan is the author of Data Communications and Networking (5th Edition)

In Data Link Control (DLC), flow control ensures that the sender does not overwhelm the receiver by sending data faster than it can be processed or stored.

• Without flow control, a fast sender could overflow the receiver’s buffer, causing data loss.
• Flow control works by coordinating data transmission rates, often through protocols like Stop-and-Wait or Sliding Window.
• Encryption is related to data security, not DLC.
• Routing packets is a Network Layer function.
• Addressing hosts is handled in the Network Layer (logical addressing) and partially in Data Link Layer (physical addressing).


Source: Behrouz A. Forouzan is the author of Data Communications and Networking, 5th Ed.

At the Data Link Layer, Stop-and-Wait Automatic Repeat reQuest (ARQ) is a simple protocol that ensures reliable transmission by:
1. Sending one frame at a time.
2. Waiting for an acknowledgment (ACK) before sending the next frame.
3. If no ACK is received within a set timeout period, the sender retransmits the frame.

• TCP ensures reliability but works at the Transport Layer, not the Data Link Layer.
• IP is an unreliable Network Layer protocol.
• FTP operates at the Application Layer.

Source:  Behrouz A. Forouzan is the author of Data Communications and Networking, 5th Ed.

• Selective Repeat ARQ is an error control method in which only the frames found to be erroneous (or lost) are retransmitted, not the entire sequence after the error.
• This makes it more efficient than Go-Back-N ARQ, where all frames after the erroneous one must be resent.
• Stop-and-Wait ARQ retransmits the current frame if no acknowledgment is received.
• Parity check is only an error detection technique, not a retransmission protocol.

Key Points:
Efficiency: Saves bandwidth by avoiding unnecessary retransmissions.
Mechanism: Receiver stores correctly received frames and requests retransmission of only the missing ones.
Usage: Often used in high-speed, high-latency links where retransmitting many frames would be costly.

Source: Behrouz A. Forouzan is the author of Data Communications and Networking, 5th Ed.

♦ IMAP (Internet Message Access Protocol) is an Application Layer protocol used for retrieving and managing email from a mail server.
♦ NAT (Network Address Translation) operates at the Network Layer.
♦ ARP (Address Resolution Protocol) works at the Data Link Layer (or between Data Link and Network).
♦ UDP (User Datagram Protocol) is a Transport Layer protocol.

Note:
Application Layer protocols handle user-level communication tasks, such as email, file transfer, and web browsing.
Examples: HTTP, SMTP, FTP, IMAP, POP3, DNS.
IMAP allows accessing emails from multiple devices without downloading them permanently to the client.

Source: Data Communications and Networking-5th Ed. by  Behrouz A. Forouzan  

IEEE 802.2 LLC is the upper sublayer of the Data Link Layer in the OSI model.
It provides a standard interface between the network layer and different LAN technologies (Ethernet, Token Ring, Wi-Fi).

In the IEEE 802.2 LLC (Logical Link Control) protocol, the SSAP field is 8 bits long:
First 7 bits → Identify the Source Service Access Point address (i.e., the source protocol or process).
Last bit (LSB) → Indicates whether the frame is a Command or a Response:
0 → Command frame
1 → Response frame

This bit is essential for the receiver to correctly interpret the intent of the LLC PDU (Protocol Data Unit).

Source:  Computer Networks (4th Edition) by Andrew S. Tanenbaum 

The 10BASE-T Ethernet standard supports a star topology, but when you specifically say "passive-star topology", it refers to the wiring layout where all connections meet at a central passive hub or patch panel.

10BASE-T is part of the IEEE 802.3 standard for Ethernet over twisted-pair cables.
It uses a physical star topology (all nodes connected to a central point such as a hub or patch panel) but a logical bus topology for data transmission.
In passive-star topology, the central device doesn’t regenerate signals (unlike active hubs or switches).
This configuration is simple and cost-effective for small networks but is mostly replaced today by active star setups.

Source: Computer Networks - 4th Ed. by  Andrew S. Tanenbaum

CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is a media access control method used in traditional Ethernet (IEEE 802.3) networks to avoid and handle data collisions.
It works by checking if the transmission medium is idle before sending data, and if a collision occurs, the devices stop, wait for a random time, and then retransmit.
Token Ring (IEEE 802.5) uses a token-passing method, FDDI uses token passing over fiber optics, and Wi-Fi uses CSMA/CA (Collision Avoidance).

Source: Computer Networks - 4th Ed. by Andrew S. Tanenbaum

IEEE 802.11 defines standards for Wireless Local Area Networks (WLANs), commonly known as Wi-Fi.
IEEE 802.3 → Ethernet (wired LAN)
IEEE 802.5 → Token Ring
IEEE 802.15 → Wireless Personal Area Networks (WPANs), e.g., Bluetooth.

The IEEE 802 is a family of networking standards developed by the Institute of Electrical and Electronics Engineers (IEEE) for Local Area Networks (LANs) and Metropolitan Area Networks (MANs).

Source: Data Communications and Networking-5th Ed. by  Behrouz A. Forouzan

Wi-Fi (IEEE 802.11) transmits data using radio waves, typically in the 2.4 GHz and 5 GHz frequency bands. These bands allow wireless devices to communicate without physical cables, making Wi-Fi a form of wireless LAN.

Source: Data Communications and Networking, 5th Ed. by Behrouz A. Forouzan

Ethernet is a widely used LAN technology, and its specifications are defined by the IEEE 802.3 standard.
IEEE 802.3 covers physical layer and data link layer specifications for wired Ethernet.
It includes speed standards like 10 Mbps (Ethernet), 100 Mbps (Fast Ethernet), 1 Gbps (Gigabit Ethernet), and higher.

Note:
IEEE 802.5 → Token Ring
IEEE 802.11 → Wi-Fi (Wireless LAN)
IEEE 802.15 → Wireless Personal Area Networks (Bluetooth, etc.

Source: Computer Networks - 4th Ed. by Andrew S. Tanenbaum

TCP (Transmission Control Protocol) is a connection-oriented protocol, meaning it establishes a virtual connection between sender and receiver before data transfer begins.
TCP (Transmission Control Protocol):
Connection-oriented: Establishes a virtual connection between sender and receiver before data transfer begins.
Reliable: Ensures data is delivered in order, without duplication, and without loss.
Error Detection: Detects errors in transmitted data.
Retransmission: Lost or corrupted packets are retransmitted.
Acknowledgments (ACKs): Receiver confirms receipt of packets.
Flow Control: Prevents sender from overwhelming the receiver.
Congestion Control: Prevents network congestion by regulating the rate of data transmission.

Source: Computer Networks (4th Ed.) by Andrew S. Tanenbaum 

In LAN protocols (e.g., Ethernet), control information is primarily handled at the Data Link Layer.

This is done using:
• MAC addresses → Identify the source and destination devices within the same network segment.
• Frame headers → Contain control information such as addressing, frame type, and error-checking fields (e.g., FCS — Frame Check Sequence).

IP addresses operate at the Network Layer and are not the main method for LAN-level control.
Port numbers are used at the Transport Layer for application-level communication, not LAN control.

Source: Data Communications and Networking (5th Ed.) by Behrouz A. Forouzan.

TCP (Transmission Control Protocol) is connection-oriented and ensures data is delivered in order.
The Sequence Number field in the TCP header identifies the first byte of data in a segment.
This allows the receiving side to correctly reassemble segments even if they arrive out of order.

Other fields:
Checksum → Used for error detection.
Acknowledgment number → Indicates the next byte expected from the sender.
Source port → Identifies the sending application process.

Source: Data Communications and Networking (5th Ed.) by  Behrouz A. Forouzan 

TCP Sliding Window is a flow control mechanism that allows the sender to transmit multiple segments before needing an acknowledgment, but limits the number based on the receiver’s capacity.
This prevents overwhelming the receiver and ensures efficient data transmission.
The “window” size determines how much unacknowledged data can be in transit.

Note:
Encrypt data → Handled by protocols like TLS/SSL, not TCP’s sliding window.
Route packets → Done by network layer protocols like IP.
Error detection → Done with TCP checksum, not sliding window.

Source: Data Communications and Networking (5th Ed.) by Behrouz A. Forouzan 

In TCP, the unit of data that is transmitted is called a segment. It contains both the TCP header and the application data.
In networking, each OSI or TCP/IP layer has its own Protocol Data Unit (PDU) name which is given bellow 

Source: Data Communications and Networking (5th Ed.) by Behrouz A. Forouzan

Malicious software (malware) refers to programs intentionally created to cause harm.
Its typical actions include:
• Stealing sensitive data (passwords, financial info).
• Damaging files or corrupting systems.
• Disrupting network operations or system performance.
• Gaining unauthorized control over systems.

Source: Computer Networks (4th Ed.) by Andrew S. Tanenbaum.

In TCP, each byte (octet) in the data stream is assigned a sequence number.
 • TCP uses a 32-bit Sequence Number field in its header.
 • This means numbering goes from 0 to 2³² − 1 (0 to 4,294,967,295).
 • When the maximum number is reached, it wraps around to 0 again — this is modulo 2³² numbering.

​Note:
• It allows reliable tracking of data order even for very large data transfers.
• It helps with duplicate detection and retransmission handling.

Source: Computer Networks by Prof. Dr. Md. Mozammel Huq Azad Khan and Data Communications and Networking-5th Ed. by  Behrouz A. Forouzan.

• Social engineering is a manipulation technique used to trick individuals into revealing confidential information or performing actions that compromise security.
• In network security, attackers or hackers often use it as a non-technical method to bypass security controls by exploiting human psychology instead of system vulnerabilities.

Examples:
• Phishing emails
• Fake tech support calls
• Impersonating trusted contacts

Source: Data Communications and Networking—5th Ed. by Behrouz A. Forouzan.

The Domain Name System (DNS) acts like the phonebook of the internet.
Its primary purpose is to translate human-readable domain names (e.g., www.example.com) into machine-readable IP addresses (e.g., 192.0.2.1), so that computers can locate and communicate with each other.
Without DNS, users would have to remember long numerical IP addresses instead of easy-to-remember names.

Example:
When you type www.facebook.com into a browser, DNS resolves it to the corresponding IP address, enabling your device to connect to the correct server.

Source: Computer Networks (4th Ed.) by Andrew S. Tanenbaum.

The Domain Name System (DNS) is a hierarchical, distributed database that maps human-readable domain names (e.g., www.example.com) to their numerical IP addresses (e.g., 192.0.2.1).

• DNS acts as a translator between client applications (like web browsers) and the IP addresses of servers.
• When a user enters a domain name, the client sends a request to a DNS server, which resolves the name into an IP address so the client can connect to the server hosting the website or service.
• This allows users to use easy-to-remember names instead of having to remember long IP numbers.
• It also works in reverse (reverse DNS lookup), mapping IP addresses back to domain names.

Source: Computer Networks (4th Ed.) by Andrew S. Tanenbaum

The DNS record type used to map a domain name to an IPv4 address is the A record (short for Address record).
• A record → Maps a domain name to an IPv4 address (e.g., 192.168.1.1).
• For IPv6 addresses, the equivalent is the AAAA record.

So if you want example.com to point to an IPv4 address, you would create an A record in your DNS configuration.

This allows clients to access servers using human-readable names instead of numeric IP addresses.

Note:
Other DNS record types serve different purposes:
MX record (Mail Exchange record)  → directs email to mail servers
CNAME record (Canonical Name record) → aliases one domain name to another
PTR record (Pointer record) → used for reverse DNS, mapping IP to domain

Source: Data Communications and Networking-5th Ed by Behrouz A. Forouzan