Computer Networks Fundamentals Quiz

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Questions and Answers

What does the variable 'E' represent in the given equation?

The average time taken for transmissions

The expected value of successful transmissions (correct)

The probability of a successful slot

The total number of nodes used

From the equation, which factor does NOT influence the probability that a given node has successful transmissions?

The variable 't'

The success probability in a single slot

The number of nodes in total

The duration of the time slot (correct)

What does the term 'p(1 - p)N-1' represent in the context provided?

The overall probability of all nodes succeeding

The probability of at least one node succeeding

The probability that any node has at least one transmission (correct)

The probability of a single node succeeding

Considering the 'prob that given node has a transmission', what does the '37%' signify?

The fraction of time nodes are successfully transmitting Signup and view all the answers

What does the expression 'sin(p)' imply when looking at the computation of successful transmissions?

It denotes a non-linear relationship with the success probability Signup and view all the answers

What is the primary function of an adapter in a network context?

To implement the link layer in communication Signup and view all the answers

Which layer does the Ethernet card specifically implement?

Link layer Signup and view all the answers

What is added to a datagram on the sending side for error checking?

Error checking bits Signup and view all the answers

How does the receiving side handle incoming data?

It extracts the datagram and checks for errors Signup and view all the answers

Which component is responsible for encapsulating a datagram in a frame?

Network interface card Signup and view all the answers

What does the physical layer primarily deal with?

Transmission of raw bitstreams over a physical medium Signup and view all the answers

What is the role of flow control in network adapters?

To manage data flow and prevent overflow Signup and view all the answers

What type of cards implement the link layer functionality within network communication?

Ethernet and 802.11 cards Signup and view all the answers

What is the primary characteristic of a fully decentralized multiple access protocol?

It does not depend on any special node to manage transmissions. Signup and view all the answers

What is a key benefit of random access protocols in a multiple access environment?

They allow nodes to recover from collisions. Signup and view all the answers

When multiple nodes are active in a channel, what is the average transmission rate each node can achieve in ideal conditions?

$R/M$ bps Signup and view all the answers

Which of the following is NOT a characteristic of channel partitioning protocols?

They allow nodes to share the channel's capacity fully. Signup and view all the answers

In the context of multiple access protocols, what is meant by 'taking turns'?

Nodes with more data to send can have a longer duration for their turns. Signup and view all the answers

What type of multiple access protocol is characterized by dividing a channel into smaller, exclusive pieces?

Time-Division Multiple Access (TDMA) Signup and view all the answers

Which protocol allows for transmission without a strict structure, often resulting in collisions?

Random access Signup and view all the answers

In an ideal scenario, what happens to the transmission rate when only one node is active?

It remains at the total channel rate R. Signup and view all the answers

What is the primary responsibility of the data-link layer?

Transferring datagrams from one node to another over a link Signup and view all the answers

Which of the following best describes 'framing' in the link layer?

Encapsulating a datagram into a frame and adding headers Signup and view all the answers

How do MAC addresses differ from IP addresses?

MAC addresses identify network interfaces, while IP addresses identify networks. Signup and view all the answers

Which protocol is commonly associated with wired links in local area networks?

Ethernet Signup and view all the answers

What service does link layer addressing primarily provide?

Identifying source and destination nodes on the same link Signup and view all the answers

Which of the following is NOT a function of the link layer?

End-to-end connection establishment Signup and view all the answers

What does the acronym 'ARP' stand for in the context of link layer services?

Address Resolution Protocol Signup and view all the answers

Which of the following describes multiple access protocols in the link layer?

Protocols manage how multiple nodes share a common channel. Signup and view all the answers

What is the role of switches in local area networks (LANs)?

To connect nodes and manage data packets between them Signup and view all the answers

Which of the following statements about link layer services is true?

Different link protocols offer different services depending on the network type. Signup and view all the answers

What is the primary purpose of using RTS packets in the CSMA/CA protocol?

To minimize the chance of collisions before sending data frames Signup and view all the answers

What happens after a sender transmits an RTS packet?

The receiver broadcasts a clear-to-send (CTS) message Signup and view all the answers

Which statement is true regarding RTS and CTS packets in collision avoidance?

CTS packets are sent in response to RTS packets Signup and view all the answers

What is a potential issue that can still occur with RTS packets?

RTS packets can collide with each other Signup and view all the answers

Which of the following best describes the effect of a CTS message in the CSMA/CA protocol?

It notifies all nodes to defer their transmissions Signup and view all the answers

Why does CSMA/CA use short RTS packets instead of allowing random access of data frames?

To specifically manage and avoid long data frame collisions Signup and view all the answers

What is the result of a successful RTS-CTS exchange?

All nodes are informed and wait for their turn Signup and view all the answers

How does CSMA/CA contribute to network efficiency?

By utilizing short reservation packets to avoid collisions Signup and view all the answers

What does the preamble in an ARP packet primarily do?

Synchronizes receiver and sender clock rates Signup and view all the answers

What type of network communication does Ethernet utilize?

Unreliable and connectionless Signup and view all the answers

What happens to data in dropped Ethernet frames if a higher-layer protocol is not used?

The data is lost Signup and view all the answers

Which of the following describes Ethernet's Medium Access Control (MAC) protocol?

Unslotted CSMA/CD with binary backoff Signup and view all the answers

Which of the following protocols is mostly indicated by the type field in an ARP packet?

IP Signup and view all the answers

What is the primary role of the CRC in an Ethernet frame?

To check for errors in the received frame Signup and view all the answers

What does the 'binary backoff' in Ethernet's collision handling describe?

A random exponential increase in wait time after collisions Signup and view all the answers

What condition makes the Ethernet network unreliable?

Connectionless nature with no handshakes Signup and view all the answers

What is the significance of the address fields in an Ethernet frame?

They identify the source and destination of the frame Signup and view all the answers

How does the absence of acknowledgments in Ethernet affect data transmission?

Can lead to lost data Signup and view all the answers

What is the size of the preamble in an Ethernet frame?

7 bytes Signup and view all the answers

What mechanism does Ethernet employ to handle collisions?

Exponential backoff Signup and view all the answers

What is the function of the CRC in data transmission?

To check for errors during transmission Signup and view all the answers

What happens during a collision in an Ethernet network?

The involved frames are dropped and retransmission occurs Signup and view all the answers

Study Notes

Computer Networks - Link Layer

Goals: Understanding link layer services, error detection/correction, broadcast channel sharing, link layer addressing, local area networks (LANs), and implementation of various link layer technologies.

Terminology

Nodes: Hosts and routers.
Links: Communication channels connecting adjacent nodes.
Wired Links: Physical connections.
Wireless Links: Wireless connections.
Layer-2 Packet (Frame): A layer-2 packet encapsulates a datagram, adding a header; used in data-link layer.

Data-Link Layer Responsibility

The data-link layer is responsible for transferring datagrams from one node to a physically adjacent node over a link

Link Layer Services

Framing: Encapsulates datagrams into frames, adding headers, channel access (for shared medium), and use of MAC addresses in frame headers.
Reliable Delivery: How adjacent nodes deliver data reliably, typically not necessary on low bit-error links like fiber, while crucial for wireless links with high error rates.
Error Detection: Methods for detecting signal attenuation noise; receivers detecting error; signals for retransmission or frame drops.
Error Correction: Methods allowing receivers to identify and fix bit errors without retransmission.
Half-Duplex/Full-Duplex: Half-duplex allows transmission in one direction at a time, while full-duplex allows simultaneous transmission in both directions.

Link Layer Context

Datagrams are transferred over different link protocols (e.g., Ethernet on one link, 802.11 on another).
Each link protocol provides distinct services.
Link layer context discusses how a single datagram can use different link protocols to traverse over different links.

Where Link Layer is Implemented

Link layer implemented in adapters (network interface cards or NICs), Ethernet cards, or chipsets.
Integrated into system's bus.
Combination of hardware, software, and firmware.

Error Detection

EDC (Error Detection and Correction) bits: redundancy to protect data by checking, often including header fields; used for detection (not guaranteed 100% reliable).
Larger EDC fields often enable better detection and correction.

Parity Checking

Single Bit Parity: Detects single bit errors.
Two-Dimensional Bit Parity: Detects and corrects single bit errors.

Internet Checksum

Detects errors (flipped bits) in transmitted packets (used at the transport layer, not link).
Sender computes a checksum from segment contents.
Receiver computes the checksum, comparing it with the received value, determining presence of errors.

Cyclic Redundancy Check (CRC)

More powerful error detection coding.
Data bits (D) are viewed as a binary number.
A generator (G) is chosen.
CRC bits (R) are chosen so that (D + R) is exactly divisible by G (modulo 2). Useful in practice, for example in Ethernet, 802.11.

CRC Example

Given data bits and generator, calculated CRC value for error detection and correction during transmission.

Multiple Access Protocols

Techniques for multiple nodes to share a single broadcast channel: interference, collisions if nodes transmit at same time.
Examples: Point-to-Point, Broadcast (shared wire or medium), 802.11 wireless LANs.

MAC Protocols: Taxonomy

Channel Partitioning: Dividing channel into smaller pieces (time slot or frequency) to allocate each node a piece for exclusive use.
Random Access: Channel is not divided, collisions allowed, and nodes recover from collisions. Taking turns: nodes take turns, but some can take longer than others.
Includes TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access).

Random Access Protocols (e.g., ALOHA and Slotted ALOHA)

ALOHA: a simple random access protocol, where nodes transmit without coordination, but collisions are common.
Slotted ALOHA: nodes transmit only at the start of time slots, reducing the chance of collisions by transmitting at specified times. More efficient.
Efficiency is a function of P - probability a node transmitting.

Pure Unslotted ALOHA

Simpler than slotted ALOHA but with lower efficiency.

CSMA (Carrier Sense Multiple Access)

Nodes sense the channel before transmitting.
If channel is busy, they wait.
Deferral: the act of waiting to transmit a frame if the media is busy.
Uses for preventing collisions.

CSMA/CD (Collision Detection)

Detect collisions while transmitting entire frame.
Collisions aborted if detected, minimizing wasted transmission time.
Includes propagation delay which impacts collision detection.

CSMA/CA (Collision Avoidance)

Sends short reservation packets (RTS/CTS) to reserve the medium. Avoids collisions and wasted data.

Taking Turns Protocols (Polling and Token Passing)

Polling: Master node "invites" slave nodes to transmit in turn, typically with "dumb" devices, potentially causing overhead.
Token Passing: Token passed sequentially which allows one node to transmit at a time.

Ethernet

Dominant wired LAN technology.
Low cost.
First widely used LAN technology.
Supports speeds from 10 Mbps to up to 100 Gbps; uses CSMA/CD.
Physical topology: Star topology (central switch).

Ethernet Frame Structure

Premble; Addresses, Type, Data (Payload), CRC.
Uses 7-byte premble (10101010 followed by 10101011) patterns for synchronization.
Contains addresses, type, data, and a cyclic redundancy check.
Unreliable connectionless protocol (no handshaking and acknowledgments).

Ethernet CSMA/CD Algorithm

NICs (Network Interface Cards) sense the channel for activity.
If idle, transmits; otherwise waits.
Collisions are detected, and nodes enter a binary exponential backoff period.

Ethernet Switch

Link layer device enabling multiple simultaneous transmissions.
Stores and forwards frames based on MAC addresses.
Self-learning: builds a table of MAC addresses and their ports.
Transparent: Hosts unaware of switch's presence.

Interconnecting Switches

Switches connect to each other.
A switch learns how to reach the destination host on another switch by forwarding the frame until it reaches a port that’s connected directly to the destination host.
This process is sometimes referred to as flooding.

ARP (Address Resolution Protocol) Protocol

Protocol used to resolve IP addresses to MAC addresses.
A broadcast ARP request is sent out, and the corresponding destination host responds with its MAC address.
ARP tables are cached to store information for future access.

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Description

Test your knowledge on the principles of computer networks with this quiz covering key concepts such as successful transmissions, error checking, and the roles of various network components. Explore factors influencing node probabilities and the functionality of adapters within the network architecture.