[Computer network] calculation of various delay

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Latency: is the amount of time that data (a message or packet, or even a bit) is transferred from one segment of the network or link to the other.

Send delay : Is the time taken from the first bit of the sending data frame to the last bit of the frame to be sent.

Transmit delay = data frame Length (b)/channel bandwidth (S/b) [channel bandwidth is data rate]

propagation delay : The time it takes for the electromagnetic wave to propagate a certain distance in the channel.

Propagation delay = Channel Length (m)/propagation rate of electromagnetic waves on the channel (/s)

processing delay : The time at which the host or router processes the received packet.

Queue delay : Queue delay is formed when packets are queued for processing in the input queue and waiting for forwarding in the output queue.

Total Delay = Transmit delay + propagation delay + processing time delay + queue delay

Example 1:

Try to compare circuit switching and packet switching under the following conditions. The message to be transmitted is a total of x (bit). From the source point to the end of the total K-segment link, the propagation delay of each link is D (s), the data rate is B (b/s). The settling time of the circuit during circuit switching is s (s). The packet length is p (bit) at the time of packet switching, and the queuing time for each node is negligible. Ask in what kind of condition, the minimize circuit of the packet exchange is exchanged to be small. (Hint: Draw a sketch to observe that there are several nodes in the K-segment link.) ）

A: In the circuit exchange, the establishment of the circuit delay is s, the message transmission delay is x/b, message propagation delay is: KD, so the total delay is: x/b+kd+s;

In packet switching, the transmission delay of the message is (x/p) * (p/b); K-segment transmission, there is (k-1) the storage and forwarding delay (p/b) * (K-1), transmission delay is KD, so the total delay is: (x/p) * (p/b) + (p/b) * (k-1) +kd=x/b+kd+ (p/b) * (k-1);

When s> (k-1) * (p/b), the circuit-switched minimize packet-switched delay is large when x>>p, on the contrary.

Example 2:

In the packet switching network above, the message length and packet length are respectively x and (P+h) (bit), where p is the length of the data portion of the packet, and H is the fixed length of the control information with each packet, regardless of the size of p. The two ends of the communication pass through the K-segment link. The data rate of the link is B (/s), but the propagation delay and the queueing time of the node are negligible. If the total delay is intended to be minimal, the length of the data part of the question group P should be taken as much. (Tip: Refer to the packet switching section of Figure 1-12 to see which parts of the total delay are composed.) ）

Answer: Total delay: y= (x/p) * ((P+H)/b) + (k-1) * ((P+H)/b) +KD

Both sides of the derivative, and make its value of 0 p=[(XH)/(k-1)]^0.5