Computer network class4 (delay, delay-bandwidth product, RTT and utilization)

time delay

1. Refers to the data (message/packet/bit stream) required to transmit from one end of the network (or link) to the other

   time. Also called delay or delay. The unit is s.

   • A message is a data unit exchanged and transmitted in the network , that is, a [data block] to be sent by a station at one time. The message contains the complete data information to be sent, and its length is very inconsistent, and the length is unlimited and variable.
   • Most computer networks cannot continuously transmit arbitrarily long data, so in fact the network system divides the data into small blocks and sends them one by one. This small block is called a packet .

2. Classification

   • Transmission delay: Calculated from the first bit of a packet being sent to the time required for the last bit of the packet to be sent.
     • Sending delay = data length / channel bandwidth (sending rate)
   • Propagation delay: depends on the propagation speed of electromagnetic waves and the length of the link.
     • Propagation delay = channel length / propagation rate of electromagnetic waves on the channel
   • Queuing delay: waiting for outgoing/incoming links to become available
   • Processing Latency: Error Detection, Exit Finding

​High-speed link : channel bandwidth (transmission rate) is increased, and transmission delay is reduced. It has nothing to do with the propagation delay, because the propagation rate of electromagnetic waves on the channel does not change much.

Note: Both the queuing delay and the processing delay are carried out by the router. There are queuing delays and processing delays when entering the router and leaving the router. It is like entering the subway and subway security check.

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delay-bandwidth product

Delay Bandwidth Product = Propagation Delay * Bandwidth

The delay-bandwidth product is also called the link length in bits , that is, " how many bits are there in a certain link now ".

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Round trip time delay RTT

The total delay experienced from the time when the sender sends data to the time when the sender receives the confirmation from the receiver (the receiver sends the confirmation immediately after receiving the data).

• The larger the RTT, the more data can be sent before an acknowledgment is received
• RTT includes
  • Round trip propagation delay = propagation delay *2
  • end processing time

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Utilization

1. Channel utilization: data passing time/(with/without) data passing time.
2. Network Utilization: The weighted average of channel utilization.
   

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