Computer Networks: Link Layer and LANs

The lowest layer in the TCP/IP model, responsible for transmitting data between adjacent nodes over a physical link.

A communication channel where multiple hosts can send and receive data at the same time. Examples include wireless LANs, satellite networks, and cable TV networks.

A communication channel that connects two devices directly. For example, a direct connection between two routers or a computer and an Ethernet switch.

A process of dividing data into smaller units (frames) with specific headers and trailers. The link layer protocol defines the frame structure.

A set of rules that determine how multiple devices on a shared link can access the medium to transmit data without collisions.

A situation where multiple nodes on a shared broadcast link attempt to transmit data simultaneously, leading to collisions and data corruption.

A service offered by the link layer to guarantee reliable delivery of data over links prone to errors, such as wireless links. It involves resending data if errors are detected.

A mechanism used in data communication to verify the integrity of transmitted data. It involves adding a specific set of bits (CRC) to the data before transmission. The receiver then calculates the CRC for the received data and compares it with the received CRC. If they match, it indicates that the data has arrived without errors.

The number of bits appended to the original data to form the CRC. It determines the strength of the CRC.

A specific, predetermined pattern of bits used in the CRC calculation. It is used to generate the CRC value for the data.

The original data that is to be transmitted. It is the data that needs to be protected by the CRC.

The calculated CRC value, which is added to the original data 'D' before transmission. It helps detect errors in the data.

The combination of the original data 'D' and the calculated CRC value 'R'. This is sent over the network.

The process of ensuring that a combination of data bits and CRC bits is exactly divisible by the generator 'G'. It involves adding the CRC to the data such that their combination is divisible by the CRC generator.

A type of communication link where multiple devices can send and receive data simultaneously over a single, shared channel.

A method in which computers on a network wait for a silence on the shared channel before transmitting data, thereby avoiding collisions.

An algorithm used in Ethernet to handle collisions by increasing the waiting time exponentially with each collision.

The maximum time it takes for a signal to reach any point on the network, used to determine the minimum frame size for CSMA/CD.

The fraction of time that a network successfully transmits data without collisions, especially for transmissions between a large number of active nodes.

The number of times a frame has been transmitted and collided before being successfully sent.

The efficiency of a protocol is the proportion of successful transmission slots over a long period where many nodes constantly have frames to send.

A collision happens when two or more nodes attempt to transmit data at the same time in a shared channel.

A slot refers to a specific time interval in a slotted ALOHA protocol where nodes are allowed to attempt transmissions. It helps coordinate transmission attempts.

In slotted ALOHA, nodes attempt to send data at the beginning of predefined slots. This synchronization reduces collisions compared to pure ALOHA where transmission can start at any time.

In pure ALOHA, nodes transmit data immediately when they have a frame, without waiting for a specific time slot. This can lead to frequent collisions.

The probability of a node successfully transmitting a frame in a slotted ALOHA system is calculated by considering its own transmission probability and the probability of all other nodes NOT transmitting during the same slot.

The maximum efficiency of slotted ALOHA is achieved when the number of nodes is large, and each node has a high probability of transmitting. This maximum efficiency reaches approximately 37%.

In ALOHA, after a collision, a node retransmits its frame with a probability 'p'. This helps avoid constant retransmissions but introduces the risk of further collisions.

The probability of a node transmitting a frame in a given time interval is denoted by 'p' in the ALOHA protocol. This probability determines the likelihood of a collision.

In ALOHA, a node waits for a frame transmission time (the time it takes to transmit a frame) before attempting to retransmit, if its previous attempt failed. This helps avoid immediate collisions.

The probability that a node's transmission is successful in pure ALOHA is calculated by multiplying the probability that the node itself transmits successfully (p) with the probability that no other node transmits during that interval (1-p)^(N-1).

In the pure ALOHA protocol, nodes transmit whenever they have data, without considering the channel status or potential collisions. This can lead to overlapping transmissions and collisions.

The maximum efficiency of the pure ALOHA protocol, achieved when the transmission rate is optimized, is limited to approximately 18 percent (1/(2e)). This means only about 18 percent of the channel capacity is used for successful transmissions.

In CSMA, a node listens to the channel before initiating transmission, ensuring that it doesn't transmit while another node is actively sending data. This reduces the likelihood of collisions, but doesn't completely eliminate them.

The time it takes for a signal to travel from the sender to the receiver is called propagation delay. In CSMA, even if a node senses the channel to be idle, another node's transmission might begin during the propagation delay, resulting in a collision.

In CSMA/CD, nodes listen for collisions while transmitting, and immediately stop transmitting if a collision is detected, preventing the transmission of a completely useless frame.

The CSMA/CD protocol involves a series of steps to ensure efficient data transmission: preparing the frame, sensing channel idle, initiating transmission, and detecting collisions.

After detecting a collision, nodes in CSMA/CD wait for a random amount of time before attempting to retransmit, reducing the probability of immediate re-collisions.

The maximum transmission distance that can be supported in a CSMA/CD network depends on the transmission speed, the propagation delay, and the minimum frame size. This is because a collision must be detected before the frame is fully transmitted.

The CSMA/CD protocol operates within a specific range of transmission distances and frame sizes to ensure efficient collision detection and resolution. If these limits are exceeded, collisions might not be detected effectively, leading to reduced network performance.