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?

<p>The fraction of time nodes are successfully transmitting (A)</p> Signup and view all the answers

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

<p>It denotes a non-linear relationship with the success probability (C)</p> Signup and view all the answers

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

<p>To implement the link layer in communication (A)</p> Signup and view all the answers

Which layer does the Ethernet card specifically implement?

<p>Link layer (B)</p> Signup and view all the answers

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

<p>Error checking bits (B)</p> Signup and view all the answers

How does the receiving side handle incoming data?

<p>It extracts the datagram and checks for errors (B)</p> Signup and view all the answers

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

<p>Network interface card (C)</p> Signup and view all the answers

What does the physical layer primarily deal with?

<p>Transmission of raw bitstreams over a physical medium (A)</p> Signup and view all the answers

What is the role of flow control in network adapters?

<p>To manage data flow and prevent overflow (B)</p> Signup and view all the answers

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

<p>Ethernet and 802.11 cards (D)</p> Signup and view all the answers

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

<p>It does not depend on any special node to manage transmissions. (C)</p> Signup and view all the answers

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

<p>They allow nodes to recover from collisions. (A)</p> 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?

<p>$R/M$ bps (D)</p> Signup and view all the answers

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

<p>They allow nodes to share the channel's capacity fully. (A)</p> Signup and view all the answers

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

<p>Nodes with more data to send can have a longer duration for their turns. (A)</p> Signup and view all the answers

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

<p>Time-Division Multiple Access (TDMA) (C)</p> Signup and view all the answers

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

<p>Random access (D)</p> Signup and view all the answers

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

<p>It remains at the total channel rate R. (B)</p> Signup and view all the answers

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

<p>Transferring datagrams from one node to another over a link (D)</p> Signup and view all the answers

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

<p>Encapsulating a datagram into a frame and adding headers (D)</p> Signup and view all the answers

How do MAC addresses differ from IP addresses?

<p>MAC addresses identify network interfaces, while IP addresses identify networks. (D)</p> Signup and view all the answers

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

<p>Ethernet (A)</p> Signup and view all the answers

What service does link layer addressing primarily provide?

<p>Identifying source and destination nodes on the same link (D)</p> Signup and view all the answers

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

<p>End-to-end connection establishment (B)</p> Signup and view all the answers

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

<p>Address Resolution Protocol (D)</p> Signup and view all the answers

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

<p>Protocols manage how multiple nodes share a common channel. (A)</p> Signup and view all the answers

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

<p>To connect nodes and manage data packets between them (A)</p> Signup and view all the answers

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

<p>Different link protocols offer different services depending on the network type. (A)</p> Signup and view all the answers

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

<p>To minimize the chance of collisions before sending data frames (C)</p> Signup and view all the answers

What happens after a sender transmits an RTS packet?

<p>The receiver broadcasts a clear-to-send (CTS) message (C)</p> Signup and view all the answers

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

<p>CTS packets are sent in response to RTS packets (C)</p> Signup and view all the answers

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

<p>RTS packets can collide with each other (B)</p> Signup and view all the answers

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

<p>It notifies all nodes to defer their transmissions (D)</p> Signup and view all the answers

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

<p>To specifically manage and avoid long data frame collisions (A)</p> Signup and view all the answers

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

<p>All nodes are informed and wait for their turn (B)</p> Signup and view all the answers

How does CSMA/CA contribute to network efficiency?

<p>By utilizing short reservation packets to avoid collisions (B)</p> Signup and view all the answers

What does the preamble in an ARP packet primarily do?

<p>Synchronizes receiver and sender clock rates (C)</p> Signup and view all the answers

What type of network communication does Ethernet utilize?

<p>Unreliable and connectionless (C)</p> Signup and view all the answers

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

<p>The data is lost (B)</p> Signup and view all the answers

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

<p>Unslotted CSMA/CD with binary backoff (A)</p> Signup and view all the answers

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

<p>IP (C)</p> Signup and view all the answers

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

<p>To check for errors in the received frame (C)</p> Signup and view all the answers

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

<p>A random exponential increase in wait time after collisions (D)</p> Signup and view all the answers

What condition makes the Ethernet network unreliable?

<p>Connectionless nature with no handshakes (D)</p> Signup and view all the answers

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

<p>They identify the source and destination of the frame (A)</p> Signup and view all the answers

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

<p>Can lead to lost data (C)</p> Signup and view all the answers

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

<p>7 bytes (D)</p> Signup and view all the answers

What mechanism does Ethernet employ to handle collisions?

<p>Exponential backoff (B)</p> Signup and view all the answers

What is the function of the CRC in data transmission?

<p>To check for errors during transmission (D)</p> Signup and view all the answers

What happens during a collision in an Ethernet network?

<p>The involved frames are dropped and retransmission occurs (B)</p> Signup and view all the answers

Flashcards

Adapter

A network interface card (NIC) or a specialized chip that handles the physical and link layers of network communication. It connects to the system bus and communicates with the network.

Link layer

The layer of the network model responsible for transmitting data packets (frames) over a physical connection. It defines how data is formatted and transmitted.

Datagram

A small, independent packet of data containing information for routing and delivery on the network. It includes data and control information.

Encapsulation

The process of adding header information to a datagram to create a frame, which can then be transmitted over the physical link. This also includes adding error detection and flow control mechanisms.

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Reliability Mechanisms

Error detection, retransmission, and flow control mechanisms that ensure reliable data transmission over the network.

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Decapsulation

The process of removing the header information and extracting the original datagram from a received frame.

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Adapter Communication

The ability of an adapter to communicate with different types of network nodes, regardless of their physical connection or protocols.

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Network layer

The layer of the network model responsible for addressing, routing, and packet forwarding. It manages the flow of data between different networks.

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Data Field in Ethernet Frames

In the context of Ethernet frames, the data field carries the actual payload, which is the data to be transmitted by the network layer.

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Preamble in Ethernet Frames

The preamble in an Ethernet frame serves to synchronize the sender and receiver clocks, making sure they're running at the same pace.

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Type Field in Ethernet Frames

The Type field in an Ethernet frame identifies the higher-level protocol that will handle the data payload, like IP or IPX. It tells the receiver which protocol to use for interpreting the data.

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CRC in Ethernet Frames

The CRC (Cyclic Redundancy Check) at the end of an Ethernet frame is used for error detection. It helps guarantee that the data received is the same as the data sent.

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Ethernet's MAC Protocol: CSMA/CD

Ethernet's MAC protocol is called CSMA/CD, which stands for Carrier Sense Multiple Access with Collision Detection. It's a way for multiple nodes on a network to share the same communication medium effectively.

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CSMA/CD Access Method

CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection. It's an access method that allows multiple devices to share a communication channel.

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Carrier Sense in CSMA/CD

In CSMA/CD, the "carrier sense" part means that before transmitting, a node listens to the channel to see if anyone else is sending data. This helps avoid collisions.

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Collision Detection in CSMA/CD

In CSMA/CD, the "collision detection" means that if two nodes send data at the same time, they both detect the collision and stop sending. This prevents data corruption.

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Backoff Period in CSMA/CD

In CSMA/CD, a node waits for a random amount of time, or a "backoff period", after a collision. This helps reduce the likelihood of subsequent collisions.

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Binary Backoff in CSMA/CD

The "binary backoff" method in CSMA/CD involves doubling the backoff period each time a collision occurs. This helps ensure that eventually a node will get a chance to send its data.

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Connectionless Communication

Connectionless communication means that no handshaking or connection establishment is required between sending and receiving nodes. Data is sent without prior setup.

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Unreliable Communication

Unreliable communication means that the sending node doesn't receive acknowledgments (ACKs) or negative acknowledgments (NACKs) from the receiving node. This means dropped data packets may not be retransmitted automatically.

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Contention Window in CSMA/CD

A contention window is a range of time during which a node can attempt to transmit its data after a collision. The backoff period is chosen randomly within this window.

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Exponential Backoff Algorithm in CSMA/CD

The exponential backoff algorithm in CSMA/CD uses a variable called 'k' to calculate the maximum backoff delay. The value of 'k' increases with each successive collision.

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Collision Window in CSMA/CD

In CSMA/CD, the collision window is a time-frame during which a collision is possible. When the backoff period is longer than the collision window, collisions can be prevented.

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Switch

A device that connects two or more networks or network segments, typically operating at Layer 2 of the OSI model. It learns MAC addresses and forwards frames based on destination MAC address.

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Framing

The process of enclosing a datagram within a frame. This adds a header to the datagram containing link-layer information, such as the sender and receiver's MAC addresses, and other control information.

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Frame

A packet transmitted over a link layer. It is used for communication between directly connected nodes on a shared media network.

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Multiple access protocol

A protocol used to determine which device on a shared network can transmit data at any given time. Examples include CSMA/CD and token passing.

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MAC Address

Unique identifier for a device on a network. It is typically used for addressing devices within a local network.

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Error Detection and Correction

A technique used to detect and correct transmission errors in data packets. This ensures data integrity during transmission.

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IP Address

Unique identifiers used to address devices on the internet. They are used for global routing of data packets.

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LAN

A local area network (LAN) is a network that connects devices within a limited geographical area, such as a home, office, or school.

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Probability of Success

Measures the probability of a node successfully sending data in a single transmission slot, taking into account the probability of collisions.

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Probability of Node Success

Represents the probability that any node within the network will successfully transmit data in a given slot.

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Channel Success

Represents the probability that a node successfully transmits data in the current slot and that no other node successfully transmits in the same slot.

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Channel Utilization

A network channel utilized for successful data transmission. It represents the portion of the network available for transmitting data in a particular time slot.

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Shared Wire

A communication channel where all nodes share the same physical medium, like a cable or wireless signal, for transmitting and receiving data.

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Shared RF

A communication channel where all nodes share the same physical medium, like airwaves or a satellite signal, for transmitting and receiving data.

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Cocktail Party Problem

A group of people talking at the same time, creating a mix of voices that can be difficult to understand.

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Random Access

A network access method where a network node transmits with the hope that the message will reach its destination without colliding with transmissions from other nodes.

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Taking Turns

A network access method where a network node takes turns transmitting data, ensuring that only one node is active on the communication medium at a time.

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Collision

When transmissions between multiple nodes on a shared medium collide, resulting in a loss of data.

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Collision Recovery

A protocol that provides a way for nodes to recover from collisions by retransmitting data that was lost during a collision.

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Channel Partitioning

A type of network access method where the shared communication channel is divided into smaller pieces, allowing different nodes to transmit at different times or frequencies.

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CSMA/CA

A mechanism that allows multiple devices to share a communication channel by having them listen before transmitting, which minimizes collisions.

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RTS (Request to Send)

A small packet sent by a device before transmitting a large data frame, reserving the channel for transmission.

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CTS (Clear To Send)

A small packet sent by the receiver in response to an RTS, indicating the channel is clear to transmit.

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Reservation Collision

A collision that occurs between RTS packets.

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Backoff Period

A technique where a device waits for a random amount of time before retrying transmission after a collision. This helps reduce the possibility of further collisions.

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Binary Backoff

The process of doubling the backoff period after each collision, ensuring that eventually a device will be able to send its data.

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Collision Avoidance Using RTS/CTS

The use of RTS/CTS packets to reserve the channel for transmission, preventing conflicts between sending large data frames.

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Slotted ALOHA

A method to reduce collisions in data transmission by dividing time into small slots and allowing only one device to transmit in each slot.

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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.
  • The data-link layer is responsible for transferring datagrams from one node to a physically adjacent node over a link
  • 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.
  • 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.
  • 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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CPSC 441 Computer Networks - Chapter 6 PDF

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.

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