Computer Networks: Link Layer and LANs

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Questions and Answers

What happens when an adapter detects signal energy from other adapters while transmitting?

It aborts the transmission and waits a random amount of time. (correct)

It switches to a different frequency to avoid interference.

It increases its transmission speed to avoid collision.

It continues to transmit the frame until completion.

In the binary exponential back off algorithm, how is the value of K determined after n collisions?

K is chosen randomly from the range {0, 1, 2, ..., 2n - 1}. (correct)

K is selected based on the number of nodes present in the network.

K is chosen randomly from the range {0, 1, 2, ..., n}.

K is chosen deterministically based on previous transmission times.

What is the desired characteristic of the back off time when the number of colliding nodes is small?

The back off time has no impact on collision outcomes.

The back off time should be long to prevent further collisions.

The back off time should be short to quickly resume transmissions. (correct)

The back off time should be fixed regardless of conditions.

What is the maximum value that n can take in the context of the binary exponential back off algorithm?

10 Signup and view all the answers

How is the efficiency of CSMA/CD defined?

As the long-run fraction of time frames are transmitted without collisions. Signup and view all the answers

What is true about broadcast channels in link-layer communications?

They are used in networks like wireless LANs and satellite systems. Signup and view all the answers

Which of the following is NOT a characteristic of nodes running link-layer protocols?

They utilize point-to-point communication methods exclusively. Signup and view all the answers

What does a medium access control (MAC) protocol accomplish?

It specifies rules for sharing a broadcast link among multiple nodes. Signup and view all the answers

Which service is primarily designed to enhance reliability over error-prone links?

Reliable delivery services. Signup and view all the answers

What is the primary function of a frame in a link-layer protocol?

To encapsulate datagrams for transmission over the link. Signup and view all the answers

Why are different types of links important in link-layer networking?

They enable datagrams to be transmitted over diverse communication paths. Signup and view all the answers

In the context of link-layer services, what does error detection aim to achieve?

Ensure error-free transmission of data frames. Signup and view all the answers

What is the primary function of appending additional bits, R, to the data D in the context of Cyclic Redundancy Check (CRC)?

To ensure the resulting bit pattern is divisible by G Signup and view all the answers

In the division operation using modulo-2 arithmetic in CRC, what is the significance of doing addition and subtraction without carries?

It simplifies the computation process Signup and view all the answers

Given the formula R = remainder(G) in the context of CRC, what does G represent?

The generator polynomial Signup and view all the answers

What is the maximum number of burst errors that can be detected using CRC with r additional bits?

r + 1 bits Signup and view all the answers

What is a common example of a broadcast link-layer technology?

Ethernet Signup and view all the answers

How does the point-to-point link differ from a broadcast link?

It is limited to a single sender and receiver rather than multiple nodes. Signup and view all the answers

What does the multiplication by $2^r$ in the CRC process signify?

It left shifts the bit pattern by r places Signup and view all the answers

Which statement best describes the importance of the remainder R in the CRC calculation?

It validates the integrity of the transmitted data. Signup and view all the answers

What is the probability of detecting a burst error longer than r + 1 bits using CRC?

1 - 0.5^r Signup and view all the answers

What is the maximum efficiency of the pure ALOHA protocol?

$1/(2e)$ Signup and view all the answers

What probability represents the chance that all other nodes do not begin a transmission when node i is transmitting?

$(1 - p)^{N-1}$ Signup and view all the answers

In CSMA/CD, what happens after a collision is detected?

Both nodes abort their transmission. Signup and view all the answers

Why do collisions occur in CSMA despite nodes sensing the channel?

Signals from other nodes take time to propagate. Signup and view all the answers

What does CSMA stand for in networking protocols?

Carrier Sense Multiple Access Signup and view all the answers

During a transmission by node B, when can node D begin to transmit?

After sensing an idle channel at any time. Signup and view all the answers

What is a significant characteristic of the ALOHA protocol?

Nodes do not check for ongoing transmissions before sending. Signup and view all the answers

What is one potential drawback of the ALOHA protocol?

High collision rates in heavily loaded networks. Signup and view all the answers

What occurs first when an adapter prepares to send data in a CSMA/CD environment?

It senses whether the channel is idle. Signup and view all the answers

How is the performance of CSMA/CD improved compared to ALOHA?

By avoiding full transmissions of damaged frames. Signup and view all the answers

What is the main reason the efficiency of the slotted ALOHA protocol can be less than one?

Nodes refrain from transmitting due to idle slots. Signup and view all the answers

What defines the maximum efficiency of slotted ALOHA as the number of active nodes approaches infinity?

The formula yields 1/e. Signup and view all the answers

In slotted ALOHA, what is the probability that a given node successfully transmits given N nodes and probability p?

$p(1 - p)^{N - 1}$ Signup and view all the answers

What occurs immediately after a transmitted frame experiences a collision in pure ALOHA?

The node retransmits with probability p. Signup and view all the answers

What is the role of the parameter p in the context of the ALOHA protocol?

It indicates the probability of a node transmitting in any given frame. Signup and view all the answers

Which of the following statements about the performance of slotted ALOHA is correct?

It suffers from wasted slots due to transmission collisions. Signup and view all the answers

How does a node ensure a successful transmission in the pure ALOHA protocol?

By ensuring no other nodes transmit during the frame time. Signup and view all the answers

What is the effect of using a probabilistic transmission policy in ALOHA?

It can result in empty slots due to nodes deferring transmission. Signup and view all the answers

What mathematical expression represents the total probability of success for any one of the N nodes in slotted ALOHA?

$N p(1 - p)^{N - 1}$ Signup and view all the answers

Study Notes

Computer Networks: The Link Layer and LANs

The link layer connects multiple hosts in networks like wireless LANs, satellite networks, and hybrid fiber-coaxial (HFC) access networks.
The second type of link-layer channel is point-to-point communication, connecting devices like routers and computers to switches.
A node is any device running a link-layer protocol, including hosts, routers, switches, and Wi-Fi access points.
Links are the communication channels connecting adjacent nodes.
Data is transferred between hosts by moving it over individual links along the path.
A transmitting node encapsulates a datagram into a link-layer frame and sends the frame over the link.

Link Layer Services

Framing: The link layer protocol defines the structure of the frame.
Link access: Medium access control (MAC) protocols outline the rules for transmitting frames on a link. In broadcast links, multiple access issues arise.
Reliable delivery: A reliable delivery service is important for links prone to high error rates, like wireless links. This service helps correct local errors.
Error detection and correction: Signal attenuation and electromagnetic noise lead to bit errors. Error detection is commonly hardware-based, while correction pinpoints and corrects these errors in the frame.

Link Layer Implementation

Much of the link layer controller functionality is hardware-based.
Ethernet capabilities are integrated into the motherboard chipset or a low-cost dedicated Ethernet chip (Network Interface Controller (NIC)).
Part of the link layer is implemented in software to handle functions like assembling link layer addressing information andactivating the controller hardware.
Link layer software handles receiving side interrupts, error conditions, and sending datagrams to the network layer.

Error-Detection and -Correction Techniques

Data protected includes datagram and link-level addressing, sequence numbers, and header fields.
At the sending node, error-detection and correction bits (EDC) are added to the data (D).
At the receiving node, received data (D') and its EDC (EDC') are compared for differences resulting from in-transit bit flips.
Undetected bit errors are possible even with error detection bits.

Parity Checks

A single parity bit is the simplest error detection method.
Even parity ensures an even number of 1s in the data bits plus the parity bit.
Two-dimensional parity calculates parity bits for rows and columns, increasing error detection and correction capability.
Two errors in a packet are detectable but not correctable by two-dimensional parity.

Checksumming Methods

Data bits are treated as sequences of k-bit integers.
Summing these integers produces the error detection bits.
TCP and UDP use Internet checksums encompassing header and data fields.
IP uses checksum only for its header.
Although these methods are lightweight, error protection is relatively weak.

Cyclic Redundancy Check (CRC)

CRC codes, often referred to as polynomial codes, consider the bit string as a polynomial.
Sender and receiver agree on an r+1 bit generator pattern (G) where the most significant bit of G is 1.
Data (D) is appended with R bits to make the combined d+r bits exactly divisible by the generator G (modulo-2 arithmetic).
International standards define 8-, 12-, 16-, and 32-bit generators.
CRCs can detect burst errors of fewer than r + 1 bits.
They can detect an odd number of errors.

Multiple Access Links and Protocols

A point-to-point link has one sender and one receiver.
A broadcast link has multiple senders and receivers sharing a channel.
Ethernet and wireless LANs are examples of broadcast link technologies.
Multiple access protocols organize access of multiple nodes to a shared broadcast channel.
Human-based protocols for coordinating broadcast communications provide helpful illustrative examples.

MAC Protocols

Collision is a problem in multiple access.
MAC collision occurs when more than two nodes transmit at the same time, leading to garbled signals and lost data.
Channel partitioning protocols, random access protocols, and taking turns protocols categorize multiple access protocols.
Channel partitioning divides the shared channel's bandwidth among nodes (TDM, FDM, CDMA).
Random access protocols (ALOHA, CSMA) allow simultaneous transmissions and handle collisions (CSMA/CD).
Taking-turns protocols (polling, token-passing) allocate transmission time in a coordinated manner, solving collisions.

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Description

This quiz explores the vital concepts of the link layer in computer networks, focusing on its role in connecting hosts through various types of networks like wired and wireless LANs. Understand the services provided by the link layer, including framing, link access, and reliable delivery methods. Test your knowledge on how data is managed across these connections.