Day2: Computer network overview-computer network performance indicators: packet delay, loss and throughput

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2. Computer network performance

1. Indicators: rate, bandwidth, delay/latency (delay or latency), delay bandwidth product, packet loss rate, throughput/rate (Throughput)

1.1 Rate

Rate is data rate or data transmission rate or bit rate

• The amount of information (bits) transmitted per unit time (second) is the most important performance indicator in computer networks. Units: b/s (or bps), kb/s, Mb/s, Gb/s
• Among them, k=10 ³ , M=10 ⁶ , G=10 ⁹ (different from the kMG of calculation group, programming course, etc.

Speed ​​often refers to rated speed or nominal speed

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1.2 Bandwidth

"Bandwidth" originally refers to the bandwidth of a signal , that is, the difference between the highest frequency and the lowest frequency, in Hertz (Hz) (such as the bandwidth of a sound )

• The "bandwidth" of the network is usually the ** "highest data rate" that the digital channel can transmit, unit: b/s (bps)**
• Commonly used bandwidth units:
  kb/s (10 ³ b/s)
  Mb/s (10 ⁶ b/s)
  Gb/s (10 ⁹ b/s)
  Tb/s (10 ¹² b/s)

1.3 Delay/latency (delay or latency)

Q: Consider a question first, where does Delay come from and where does packet loss come from?
A:
1. The packet is queued in the router buffer ;
2. The arrival rate of the packet exceeds the output link capacity ;
3. The packet is waiting to be queued to the head of the queue and be transmitted

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• Four packet delays
  • d_proc: node processing delay (nodal processing delay)
     Error detection
     Determine the output link
     Usually <msec
  • d_queue: queueing delay
     The time to wait for the output link to be available
     Depends on the congestion level of the router
  • d_trans: transmission delay
    L: packet length (bits)
     R: link bandwidth (bps)  d_trans = L/R
     store and forward delay
  • d_prop: propagation delay
    d: physical link length
     s: signal propagation speed (~2×10 ⁸ m/sec) (0.7c, 0.7 times the speed of light)
     d_prop = d/s

As shown in the figure:


Let's look at another example to illustrate the four kinds of delays: the fleet passes the toll gate

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Finally, look at a graph of queuing delay . The flow queuing delay depends on the flow intensity



 Under Windows system

•  Tracerert hostname
   如 Tracerert www.gucas.ac.cn
• A more complete
  example tracert [-d] [-h maximum_hops] [-j computer-list] [-w timeout] target_name
   See help: http://www.linkwan.com/gb/broadmeter/article/ trace-help.htm
•  Test URL :  www.traceroute.org  www.linkwan.com
  

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1.4 Delay bandwidth product (bandwidth delay product)

Delay bandwidth product = propagation delay * bandwidth = d_prop * R (bps) as shown in the figure:

The delay bandwidth product of the link is also called the link length in bits

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1.5 Packet loss rate

• Reasons that cause packet loss: the
  queue buffer capacity is limited; the
  packet will be discarded when the queue reaches the full queue (ie packet loss); the
  discarded packet may be retransmitted by the pre-order node or source (or may not be retransmitted) as shown in the figure
  

Therefore, the packet loss rate is: packet loss rate = number of packets lost/total number of packets sent

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1.6 Throughput/rate (Throughput)

What is certain: indicates a data transfer rate (b / s) between the sending and receiving ends. Two indicators: instant and average throughput.
 Instant Throughput : given moment rate
 Average throughput : a period of time the average rate

As shown in the figure, the server on the left sends information to the host on the right , the bandwidth of the left link is Rs , and the right is the so-called bottleneck link of Rc : on the end-to-end path, the link that limits the end-to-end throughput is shown in the figure:

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End of this article.