Random Assignment Protocols

Transcription

1 Random Assignment Protocols Random assignment strategies attempt to reduce problem occur in fixed assignment strategy by eliminating pre allocation of bandwidth to communicating nodes. Random assignment strategies do not exercise any control to determine which communicating node can access the medium next. Furthermore, these strategies do not assign any predictable or scheduled time for any node to transmit. All backlogged nodes must contend to access the transmission medium. Collision occurs when more than one node attempts to transmit simultaneously. To deal with collisions, the protocol must include a mechanism to detect collisions and a scheme to schedule colliding packets for subsequent retransmissions.

2 ALOHA ALOHA is a simple random assignment protocol developed to regulate access to a shared transmission medium among uncoordinated contending users. Channel access in pure ALOHA is completely asynchronous and independent of the current activity on the transmission medium. A node is simply allowed to transmit data whenever it is ready to do so. Upon completing the data transmission, the communicating node listens for a period of time equal to the longest possible round-trip propagation time on the network.

3 This is typically the time it takes for the signal to travel between the two most distant nodes in the network. If the node receives an acknowledgment for data transmitted before this period of time elapses, the transmission is considered successful. The acknowledgment is issued by the receiving station after it determines the correctness of the data received by examining the error check sum. In the absence of an acknowledgment, however, the communicating node assumes that the data are lost due to errors caused by noise on the communication channel or because of collision, and retransmits the data. If the number of transmission attempts exceeds a specified threshold, the node stops retransmitting the data and reports a fatal error.

4 ALOHA is simple protocol that requires no central control, thereby allowing nodes to be added and removed easily. Furthermore, under light-load conditions, nodes can gain access to the channel within short periods of time. The main drawback of the protocol, however, is that network performance degrades severely as the number of collisions rises rapidly with increased load.

5 To improve the performance of pure ALOHA, slotted ALOHA was proposed. In slotted ALOHA, all communication nodes are synchronized and all packets have the same length. Furthermore, The communication channel is divided into uniform time slots whose duration is equal to the transmission time of a data packet. Contrary to pure ALOHA, transmission can occur only at a slot boundary. Consequently, collision can occur only in the beginning of a slot, and colliding packets overlap totally in time.

6 CSMA CSMA operates both in continuous time, unslotted CSMA, and in discrete time, slotted CSMA. Furthermore, the class of CSMA protocols can be divided into two categories, 1. non persistent CSMA and 2. persistent CSMA, depending on the strategy used to acquire a free channel and the strategy used to wait for a busy channel to become free.

7 In nonpersistent CSMA protocol, when a node becomes ready to transmit a packet, it first senses the carrier to determine if another transmission is in progress. If the channel is idle, the node transmits its packet immediately and waits for an acknowledgment. In the absence of an acknowledgment, before a timeout occurs, the sending node assumes that the data packet is lost due to collision or noise interference. The station schedules the packet for retransmission. If the channel is busy, the transmitting node backs off for a random period of time after which it senses the channel again. Depending on the status of the channel, the station transmits its packet if the channel is idle, or enters the back-off mode if the channel is busy. This process is repeated until the data packet is transmitted successfully.

8 CSMA/CD CSMA/CA scheme are susceptible to two problems, commonly referred to as the hidden- and exposed-node problems. A hidden node is defined as a node that is within the range of the destination node but out of range of the transmitting node.

9 Hidden Node

10 Hidden Node Node B is within the transmission range of nodes A and C. Furthermore, assume that nodes A and C are outside their mutual transmission ranges.. Given this network configuration, assume that node A needs to transmit a data packet to node B. According to the CSMA protocol, node A senses the channel and determines that it is free. Node A then proceeds to transmit its packet. Assume now that before node A completes its transmission to node B, nodec decides to transmit a data packet to node B. Using the CSMA protocol, node C senses the channel and also determines that the channel is free, since node C, which is outside the transmission range of node A, cannot hear the signal transmitted by node A. As a result, both transmissions collide at node B, thereby causing the loss of both data packets.

11 An exposed node is a node that is within the range of the sender but out of the range of the destination.

12 Exposed Node where node B is within the transmission range of nodes A and C, nodes A and C are outside their mutual transmission ranges, and node D is within the transmission range of node C. Assume that node B wants to transmit a message to node A. Node B executes the CSMA protocol to sense the channel, determines that the channel is free, and proceeds to transmit the data packet to node A. Assume now that node C needs to send a packet to D. Node C follows the CSMA rule and first senses the channel. Due to the ongoing transmission between nodes B and A, node C determines that the channel is busy and delays the transmission of its packet to a later time.