Question: A supplement to the X.25 packet-
A supplement to the X.25 packet-switching standard is a set of standards for a packet assembler-disassembler (PAD), defined in standards X.3, X.28, and X.29. A PAD is used to connect asynchronous terminals to a packet-switching network. Each terminal attached to a PAD sends characters one at a time. These are buffered in the PAD then assembled into an X.25 packet that is transmitted to the packet-switching network. The buffer length is equal to the maximum data field size for an X.25 packet. A packet is formed from assembled characters and transmitted whenever the buffer is full, a special control character such as a carriage return is received, or when a timeout occurs. For this problem, we ignore the last two conditions. Figure 21.10 illustrates the queueing model for the PAD. The first queue models the delay for characters waiting to be put into a packet; this queue is completely emptied when it is filled. The second queue models the delay waiting to transmit packets. Use the following notation:
λ=Poisson input rate of characters from each termina.l
C=Rate of transmission on the output channel in characters per second.
M=Number of data characters in a packet.
H=Number of overhead characters in a packet.
K=Number of terminals.
a. Determine the average waiting time for a character in the input queue.
b. Determine the average waiting time for a packet in the output queue.
c. Determine the average time spent by a character from when it leaves the terminal to when it leaves the PAD. Plot the result as a function of normalized load.
Transcribed Image Text:
Input Queues Output Queue Packets Characters Figure 21.10 Queueing Model for a Packet Assembler/Disassembler (PAD)
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