ISDS 409 NEWEST ACTUAL WITH QUESTIONS AND

ISDS 409 NEWEST ACTUAL / WITH QUESTIONS AND

ANSWERS. VERIFIED /GRADED A+

Cyclic redundancy check - ----Answers---Popular error- checking scheme

-Adds 8,16,24, or 32 bits to the message.

-A message is treated as one long binary number, P. Before transmission, the data link layer ( hardware device) divides P by a fixed binary number, G, resulting in a whole number, Q, and a remainder, R/G. So P/G = Q + R/G

-Example if P= 58 and G = 8, then Q = 7and R = 2. G is chosen so that the remainder, R, will be either 8 bits, 16 bits 24 bits, or 32 bits.

• The remainder R is appended to the message as the error-

checking characters before transmission. The receiving hardware divides the received message by the same G, which generates an R. The receiving hardware checks to ascertain whether the received R agrees with the locally generated R. If it doesn't the message is assumed to be in error.

Commonly used CRC codes - ----Answers--- CRC-16 ( a 16 bit version) -CRC-CCITT ( 16 bit) -CRC-32 ( 32 bit) 1 / 4

• Probability of detecting errors is 100% for all errors of the

same length as the CRC or less.

-Ex, CRC-16 is guaranteed to detect errors if 16 or fewer bits are affected.

-If the burst error is longer than the CRC, then CRC is not prefect but is close to it.

-CRC-16 detect about 99.9998% of all burst errors longer than 16 bits

-CRC-32 will detect about 99.99999998% of all burst errors longer than 32 bits.

Error correction via Retransmission - ----Answers--- Simplest, most effective, leas expensive and most commonly used method for error correction is retransmission.

-A receiver that detects an error simply asks the sender to retransmit the message until it is received without an error.This is called Automatic Repeat reQuest. there are two types of

ARQ: stop-and-wait and continuous.

Stop-and-wait ARQ - ----Answers---The sender stops and waits for a response from the receiver after each data packet. 2 / 4

After receiving a packet, the receiver sends either an acknowledgment (ACK) if the packets was received without an error, or a negative acknowledgment (NAK), if the message contained an error.

-If it is an ACK, the sender continues with the next message.Stop-and-wait ARQ is by definition a half-duplex transmission technique.

Continuous ARQ - ----Answers---Sender does not wait for an acknowledgment after sending a message; it immediately sends the next one.

-Although messages are being transmitted, the sender examines the stream of returning acknowledgments. IF it receives an NAK, the sender retransmits the needed messages. The packets that are retransmitted may be only those containing an error ( Called link Access protocol for modems [LAP-M]) or may be the first packet with an error and all those that followed it ( Called Go-Back-N ARQ)

-LAP-M is better because it is more efficient.

-Continuous ARQ is a full-duplex transmission technique, because both the sender and the receiver are transmitting simultaneously ( Sender is sending messages, and the receiver is sending ACKs and NAKs.

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-Sometimes called sliding window because of the visual imagery the early network designers used to think about continuous ARQ.

• As a message is sent, the window expands to cover it,

meaning that sender is waiting for ACK for the message.

-As an ACK is received for a message, the window moves forward, dropping the message out of the bottom of the window, indicating that it has been sent and received successfully.

-Continuous ARQ is important for providing flow control, means ensuring that the computer sending the message is not transmitting too quickly for the receiver.

-Using ACKs and NAKs, the receiver can control the rate at which it receives information.

-With stop-and-wait ARQ, the receiver does not send an ACK until is is ready to receive more packets.

-In continuous ARQ, the sender and receiver usually agree on the size of the sliding window. Once the sender has transmitted the maximum number of packets permitted in the sliding window, it cannot send any more packets until the receiver sends an ACK.

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