CSMA Protocols and Collision Avoidance short answer

The purpose of the MAC layer in a broadcast link is to coordinate the access of multiple sending/receiving nodes to the shared link.

The role of Medium Access Control (MAC) protocols is to minimize collisions in order to utilize the bandwidth.

Throughput is the amount of data moved successfully from one place to another in a given time period.

Propagation delay is the amount of time it takes for the head of the signal to travel from the sender to the receiver.

The main tasks of Multiple Access Protocols are determining when a station can use the link, what a station should do when the link is busy, and what the station should do when it is involved in a collision.

Random Access (or contention) Protocols are protocols where no station is superior to another station and none is assigned control over another.

Collisions are a problem in multiple access networks because when two or more nodes transmit at the same time, their frames will collide and the link bandwidth is wasted during collision.

The purpose of the ALOHA scheme is to allow stations to transmit packets on the same radio channel.

The advantage of ALOHA protocols is that a node with frames to be transmitted can transmit continuously at the full rate of the channel if it is the only node with frames.

The disadvantage of ALOHA protocols is that if multiple nodes want to transmit, collisions can occur and the rate allocated for each node will not be on average R/M bps.

The formula for calculating the maximum propagation delay is tprop = tprop = time it takes for a bit of a frame to travel between the two most widely separated stations.

The formula for calculating the timeout in the ALOHA scheme is Time out = 2 * tprop.

The vulnerable time in the pure ALOHA protocol is 2 * Tfr, where Tfr is the frame transmission time.

The maximum throughput in slotted ALOHA occurs at G = 1, which is 37%.

The vulnerable time for CSMA is the maximum propagation time.

The performance of the protocol worsens with longer propagation delay.

The three types of CSMA protocols are Non-Persistent CSMA, 1-Persistent CSMA, and p-Persistent CSMA.

If the medium is idle, a station transmits.

If the medium is busy, a station waits a random amount of time (backoff) and then repeats the process.

The purpose of 1-Persistent CSMA is to avoid idle channel time.

When two or more stations become ready at the same time, collision is guaranteed.

The purpose of p-Persistent CSMA is to reduce the possibility of collisions and reduce channel idle time.

(n x p) must be < 1 for stability, where n is the maximum number of stations.

The inefficiency of CSMA is that if a collision occurs, the channel is unstable until colliding packets have been fully transmitted.

The advantage of CSMA/CD over CSMA is that it reduces channel wastage by stopping transmission if a collision has occurred.

CSMA/CD detects collisions by listening to the medium for collisions while transmitting.

If a collision is detected, the station aborts transmission, transmits a jam signal to notify other stations of the collision, and then backs off for a random amount of time before retransmitting the frame.

Packet transmission time should be at least as long as the time needed to detect a collision (2 * maximum propagation delay + jam sequence transmission time)

Reservation, Polling, Token Passing

Stations take turns transmitting a single frame at a full rate (R) bps. Transmissions are organized into variable length cycles. Each cycle begins with a reservation interval that consists of (N) minislots. One minislot for each of the N stations. When a station needs to send a data frame, it makes a reservation in its own minislot. By listening to the reservation interval, every station knows which stations will transfer frames, and in which order. The stations that made reservations can send their data frames after the reservation frame.

Centralized and distributed polling

One device is assigned as primary station and the others as secondary stations. All data exchanges are done through the primary. When the primary has a frame to send, it sends a select frame that includes the address of the intended secondary. When the primary is ready to receive data, it sends a Poll frame for each device to ask if it has data to send or not. If yes, data will be transmitted; otherwise, NAK is sent. Polling can be done in order (Round-Robin) or based on predetermined order.

No primary and secondary. Stations have a known polling order list which is made based on some protocol. The station with the highest priority will have the access right first, then it passes the access right to the next station.

Listen state and Transmit state

Listen to the arriving bits and check the destination address to see if it is its own address. If yes, the frame is copied to the station; otherwise, it is passed through the output port to the next station.

The station captures a special frame called free token and transmits its frames. The sending station is responsible for reinserting the free token into the ring medium and for removing the transmitted frame from the medium.

The MAC layer in a broadcast link is responsible for framing, MAC address, and multiple access control.

Collisions in multiple access networks waste link bandwidth and reduce the efficiency of data transmission.

The main task of Multiple Access Protocols is to minimize collisions and maximize bandwidth utilization.

Throughput is the amount of data moved successfully from one place to another in a given time period.

$\text{Maximum Propagation Delay} = \frac{\text{Link Length}}{\text{Propagation Speed}}$

In Non-Persistent CSMA, if the medium is busy, a station waits for a random amount of time and then listens again.

The three types of CSMA protocols are 1-Persistent CSMA, Non-Persistent CSMA, and p-Persistent CSMA.

Pure ALOHA allows stations to transmit frames at completely random times, while Slotted ALOHA divides time into slots equal to a frame transmission time and stations can only transmit at the beginning of a slot.

Time out = 2 * t_{prop}

Maximum propagation delay (t_{prop}) is the time it takes for a bit of a frame to travel between the two most widely separated stations.

The random retransmission time is intended to spread out retransmissions and reduce the likelihood of additional collisions between stations.

The maximum throughput occurs at G = 1/2, which is 18%.

The maximum throughput occurs at G = 1, which is 37%.

Carrier Sense allows a user to listen to the medium to see if another transmission is in progress before starting its own transmission.

CSMA/CD (Carrier Sense Multiple Access with Collision Detection) overcomes the inefficiency of CSMA by listening for collisions while transmitting and stopping transmission if a collision occurs.

The three types of CSMA protocols are Non-Persistent CSMA, 1-Persistent CSMA, and p-Persistent CSMA.

In Non-Persistent CSMA, a station senses the medium and transmits if it is idle. If the medium is busy, the station waits for a random amount of time (backoff) and repeats the process.

In 1-Persistent CSMA, a station continuously listens to the medium. If the medium is idle, the station transmits immediately. If the medium is busy, the station continues to listen until the medium becomes idle, and then transmits immediately with probability 1.

In p-Persistent CSMA, time is divided into slots, where each slot typically equals the maximum propagation delay. A station listens to the medium and transmits with probability (p) if the medium is idle. If the medium is busy, the station waits one time unit (slot) with probability (1 - p) and repeats the process.

The condition for stability in p-Persistent CSMA is that (n x p) must be less than 1, where n is the maximum number of stations.

CSMA/CD detects collisions by listening to the medium while transmitting. If a collision is detected (the sender hears its own signal), the sender stops transmission to avoid channel wastage.

The two methods of Controlled Access are Reservation and Polling.

In Reservation Access Method, stations take turns transmitting a single frame at a full rate. Transmissions are organized into variable length cycles. Each cycle begins with a reservation interval that consists of minislots, one for each station. When a station needs to send a data frame, it makes a reservation in its own minislot. By listening to the reservation interval, every station knows which stations will transfer frames, and in which order. The stations that made reservations can send their data frames after the reservation frame.

In Polling, stations take turns accessing the medium. There are two models of Polling: Centralized and Distributed. In Centralized polling, one device is assigned as the primary station and the others as secondary stations. All data exchanges are done through the primary. When the primary has a frame to send, it sends a select frame that includes the address of the intended secondary. When the primary is ready to receive data, it sends a Poll frame for each device to ask if it has data to send or not. If yes, the data will be transmitted; otherwise, a NAK is sent. Polling can be done in order (Round-Robin) or based on a predetermined order. In Distributed polling, there is no primary and secondary. Stations have a known polling order list which is made based on some protocol. The station with the highest priority will have the access right first, then it passes the access right to the next station.

In a Token-Passing network, the station interface is in two states: Listen state and Transmit state. In the Listen state, the station listens to the arriving bits and checks the destination address to see if it is its own address. If yes, the frame is copied to the station; otherwise, it is passed through the output port to the next station. In the Transmit state, the station captures a special frame called free token and transmits its frames. The sending station is responsible for reinserting the free token into the ring medium and for removing the transmitted frame from the medium.

The three channelization protocols are FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access).

FDMA (Frequency Division Multiple Access) assigns a frequency to a transmission channel. It divides the transmission medium into separate frequency bands, and each station transmits continuously on the assigned band. A node is limited to an average rate even when it is the only node with a frame to be sent.

TDMA (Time Division Multiple Access) assigns a fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time. The entire bandwidth capacity is a single channel with its capacity shared in time between M stations. A node must always wait for its turn until its slot time arrives, even when it is the only node with frames to send.

CDMA (Code Division Multiple Access) is a channelization protocol where one channel carries all transmissions simultaneously. Each station codes its data signal by specific codes before transmission. The stations' receivers use these codes to recover the data for the desired station.

The purpose of MAC protocols is to coordinate the access of multiple sending/receiving nodes to a shared link.

The main task of MAC protocols is to minimize collisions in order to utilize the bandwidth.

Throughput is the amount of data moved successfully from one place to another in a given time period.

Collisions waste link bandwidth and decrease network efficiency.

Propagation delay is the amount of time it takes for the head of the signal to travel from the sender to the receiver.

The two types of Random Access protocols are contention protocols and reservation protocols.

In contention protocols, no station is superior to another station and none is assigned control over another.

The vulnerable time for CSMA is the maximum propagation time.

Random delays reduce the probability of collisions because two stations with data to be transmitted will wait for different amounts of time. Bandwidth is wasted if the waiting time (backoff) is large because the medium will remain idle following the end of transmission even if one or more stations have frames to send.

1-persistent stations are selfish. If two or more stations become ready at the same time, a collision is guaranteed.

It reduces the possibility of collisions like non-persistent CSMA and reduces channel idle time like 1-persistent CSMA.

The condition for stability in p-persistent CSMA is that (n x p) must be less than 1, where n is the maximum number of stations.

If a collision has occurred, the channel is unstable until the colliding packets have been fully transmitted.

The time it takes to detect a collision is equal to twice the maximum propagation delay.

Controlled Access is a multiple access method that provides in-order access to a shared medium, allowing each station a chance to transmit. Three methods for implementing Controlled Access are Reservation, Polling, and Token Passing.

In the Reservation Access Method, stations take turns transmitting a single frame at a fixed rate. Transmissions are organized into variable length cycles, with each cycle starting with a reservation interval. During the reservation interval, stations make reservations for their own minislots. The stations that made reservations can then send their data frames after the reservation frame.

In the Polling method for Controlled Access, stations take turns accessing the medium. There are two models: centralized and distributed polling. In the centralized polling model, one device is assigned as the primary station, and all data exchanges are done through the primary. The primary sends select frames to the intended secondary stations and polls each device to check if it has data to send. In the distributed polling model, there is no primary or secondary. Stations have a known polling order list, and the station with the highest priority has the access right first.

In the Token-Passing network method, stations have two states: Listen state and Transmit state. In the Listen state, stations listen to arriving bits and check the destination address. If the frame is for their own address, they copy it; otherwise, they pass it to the next station. In the Transmit state, stations capture a special frame called a free token and transmit their frames. The sending station is responsible for reinserting the free token into the ring medium and removing the transmitted frame.

FDMA (Frequency Division Multiple Access) assigns a frequency to a transmission channel, while TDMA (Time Division Multiple Access) assigns a fixed sending frequency to a transmission channel for a certain amount of time. In FDMA, the transmission medium is divided into separate frequency bands, and each station transmits continuously on its assigned band. In TDMA, the entire bandwidth capacity is a single channel, and its capacity is shared in time between multiple stations.

CDMA (Code Division Multiple Access) is a channelization protocol where one channel carries all transmissions simultaneously. Each station codes its data signal with specific codes before transmission, and the stations' receivers use these codes to recover the desired data.

The three types of channelization protocols discussed in the text are FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access).

The main tasks of Multiple Access Protocols are to coordinate access to a shared medium, avoid collisions between stations, and provide fair and efficient access to all stations.

The purpose of Carrier Sense in CSMA is for stations to listen to the medium before transmitting to detect if it is currently being used by another station. If the medium is sensed as busy, the station defers its transmission until the medium becomes idle.

The purpose of the ALOHA protocol is to allow stations to transmit packets on the same radio channel whenever they have data to transmit.

The maximum propagation delay, denoted as $t_{prop}$, is the time it takes for a bit of a frame to travel between the two most widely separated stations.

The vulnerable time in the Pure ALOHA protocol is given by $2T_{fr}$, where $T_{fr}$ is the frame transmission time.

The advantage of Slotted ALOHA over Pure ALOHA is that time is divided into slots equal to a frame transmission time, and stations can only transmit at the beginning of a slot. This reduces the likelihood of collisions.

The maximum throughput in the Pure ALOHA protocol occurs at a value of $G = \frac{1}{2}$, which is 18%.

The main difference between Pure ALOHA and Slotted ALOHA is that in Pure ALOHA, frames are transmitted at completely random times, while in Slotted ALOHA, time is divided into slots equal to a frame transmission time and stations can only transmit at the beginning of a slot.

The purpose of Carrier Sense in Carrier Sense Multiple Access (CSMA) is for a station with frames to be sent to sense the medium for the presence of another transmission before it starts its own transmission. This helps reduce the possibility of collision.

Non-Persistent CSMA, 1-Persistent CSMA, p-Persistent CSMA

To overcome the inefficiency of CSMA by reducing channel wastage in case of collisions

Transmit immediately

Wait a random amount of time and repeat the sensing process

Collision guaranteed

$np < 1$, where $n$ is the maximum number of stations

While transmitting, the sender listens to the medium for collisions

The MAC layer is responsible for error and flow control, framing, and assigning MAC addresses.

The main task of Multiple Access Protocols is to minimize collisions in order to utilize the bandwidth.

Throughput is the amount of data moved successfully from one place to another in a given time period.

Propagation delay is the amount of time it takes for the head of the signal to travel from the sender to the receiver.

Random Access Protocols are protocols where no station is superior to another station and none is assigned control over another.

Collisions result in wasted link bandwidth and decreased efficiency.

The solution is to use Medium Access Control (MAC) Protocols to coordinate the transmission of active nodes.

The purpose of the ALOHA scheme is to connect computers situated on different Hawaiian islands.

The maximum propagation delay (t_{prop}) is the time it takes for a bit of a frame to travel between the two most widely separated stations.

The formula for calculating the vulnerable time in the Pure ALOHA protocol is T_{fr} = 2 * t_{prop}, where T_{fr} is the frame transmission time.

The advantage of the ALOHA protocols is that a node that has frames to be transmitted can transmit continuously at the full rate of the channel if it is the only node with frames.

In Pure ALOHA, frames are transmitted at completely random times, while in Slotted ALOHA, time is divided into slots equal to a frame transmission time and a station can only transmit at the beginning of a slot.

The purpose of carrier sense in CSMA is to listen to the medium to see if another transmission is in progress before starting its own transmission.

The maximum throughput occurs at G = 1, which is 37%.

Channelization is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between different stations.

The two channelization protocols discussed in the text are FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access).

FDMA stands for Frequency Division Multiple Access. In FDMA, the transmission medium is divided into M separate frequency bands. Each station transmits continuously on the assigned band.

TDMA stands for Time Division Multiple Access. In TDMA, the entire bandwidth capacity is a single channel with its capacity shared in time between M stations. A node must always wait for its turn until its slot time arrives, even when it is the only node with frames to send.

CDMA stands for Code Division Multiple Access. In CDMA, one channel carries all transmissions simultaneously. Each station codes its data signal by a specific code before transmission. The station's receivers use these codes to recover the data for the desired station.

The purpose of channelization is to share the available bandwidth of a link between different stations using methods such as time division, frequency division, or code division.

The advantage of FDMA is that it allows each station to transmit continuously on its assigned frequency band, providing dedicated bandwidth for each station.

The advantage of TDMA is that it allows multiple stations to share a single channel by dividing it into time slots, ensuring fair access to the channel and efficient use of bandwidth.

The advantage of CDMA is that it allows multiple stations to transmit simultaneously on the same channel, using different codes to distinguish their signals. This allows for increased capacity and improved security.