Basic knowledge and terminology of computer network (2)---Performance indicators

1 rate

• Rate: Refers to the digital channel speed of hosts linked to a computer networkThe rate at which data is sent or received(The number of bits of data transmitted per unit time). Also called data transfer rate, data rate, bit rate .

unit:

• When referring to the signal frequency band range, the unit is Hertz (Hz)
• When referring to the maximum rate, the unit is bits/second (b/s).

For unit conversion in bits/second:

• When describing rate units
  
• When describing storage capacity

2 bandwidth

• Bandwidth: ideally what can be achieved on this channelThe maximum rate at which signals can be transmitted. Used to describe the channel transmission capability, indicating the maximum amount of data that can be transmitted per unit time.

• Units: bits/second (b/s), kb/s, Mb/s, Gb/s.

• Bandwidth refers to the link entranceThe rate at which data is sent, not the rate of propagation on the link. Data is transmitted over the link in the form of electromagnetic waves.

Bandwidth originally refers to the frequency bandwidth of a signal, that is, the amount of data that can be transmitted per unit time.
In analog signal systems, bandwidth is used to represent the frequency width occupied by a transmitted signal, also known as bandwidth.

In digital devices, bandwidth represents the amount of data that can be transmitted per unit time, usually measured in bits per second (b/s).
Bandwidth is commonly used in computer systems to describe the theoretical maximum rate at which a network or line can transmit data.

Bandwidth can be classified differently according to different standards

• According to the application scope, it can be divided into WAN bandwidth and LAN bandwidth
• According to the working mode, it can be divided into uplink bandwidth and downlink bandwidth.
• According to technical standards, it can be divided into Ethernet, Token Ring and FDDI.

3 throughput

• Throughput: refers toThe amount of data passing through a certain network (interface, channel) per unit time. Also called actual rate, it is used to describe the actual transmission rate.
• Unit: bit/second (b/s), or byte per second (Bps ¹ ) as the unit, where 1 byte = 8 bits
• Throughput is limited by network bandwidth or network rated rate
• Throughput = sum of all link transmission rates (as shown below)
• Category: average throughput, instantaneous throughput.
  

bottleneck link

A bottleneck link refers to an end-to-end path,Links that limit throughput。

• Classification:
• 1. End-to-end single link (only one exclusive link, usually an ideal situation)
  
• 2. There is an end-to-end link reuse situation (actual reality)
  where link R is multiplexed by multiple links.
  Then the end-to-end of each connectionThroughput=min{Rc, Rs, R/10 }

In fact , due to R>>max{Rs, RC}, Rc or Rs is often the bottleneck .

Tight Link

In the end-to-end path, the link with the smallest effective bandwidth is called the tight link.

4 delays

4.1 Definition

• Delay ( delay or delay ): refers to the transmission of data (messages/packets/bit streams) from the network (or link)The time it takes for one end to propagate to the other end
• Unit: second (s)
• Total node delay = accumulation of four delay types

4.2 Classification

4.2.1 Sending delay (transmission delay)

• Definition: The time it takes from the first bit of the packet to be sent to the last bit. (put allThe time used to push data to the channel）

High-speed links can increase transmission speed and reduce transmission delay.

• Calculation: sending delay = data length /sending rate

Sometimes the channel bandwidth is used as the transmission rate, but in fact the rate of the bandwidth cannot be reached.

4.2.2 Propagation delay

• Definition: dataTime spent transmitting on the channel, (from the last bit leaving the upper end and entering the channel to the first bit contacting the next end)
• Limitations: DependsElectromagnetic wave propagation speed，link length，Channel environment(communication media) etc.
• Location: Occurs on the channel
• Calculation: Propagation delay = channel length/propagation rate of electromagnetic waves on the channel

4.2.3 Queuing delay

• Definition: Queuing delay refers to the time when data packets are transmitted in a computer network.Due to network congestion or insufficient resources, etc.,lead toThe time a packet waits in the queue for transmission
• Limitations: The size of queuing delay is affected by many factors, including network utilization, network bandwidth, router cache size, data packet size, etc.

4.2.4 Processing delay

• Definition: Processing delay refers to the delay in the processing of data packets due to limitations in the processing capabilities of the host or router when data packets are transmitted in a computer network.Check for errors and the time required to find the next link。
• Limitations: The processing delay is affected by many factors, including network bandwidth, packet size, host performance, router cache size, etc.

5 Delay-bandwidth product

• Definition: Delay-bandwidth product refers to the response speed of the network
• Calculation: Delay-bandwidth product = propagation delay (s) x bandwidth (b/s)
• Unit: bit.

Delay-bandwidth product is often referred to asLink length in bits, represents the number of bits transmitted in the link. That is, " how many bits does a certain link currently have? "

In computer networks,The larger the delay-bandwidth product, the faster the response speed of the network., but it also meansThe greater the likelihood of network congestion。

6 Round trip delay RTT

• Definition: Round trip delay (RTT) isThe total time required to transmit data from one end of the network to the other and then receive confirmation from the receiving end (the receiving end sends an acknowledgment immediately after receiving the data)

RTT includes:

• Round trip delay =propagation delay * 2
• terminal processing time

The larger the RTT, the more data can be sent before receiving confirmation, and the more laggy the game and other running programs will be.

7 Utilization

• Definition: The utilization of network channels

include:

• Channel utilization:
  time with data passing/ (with + without) data passing time

• Network utilization
  Weighted average of channel utilization

Legend
Utilization is very low:
Utilization is high:

Relationship between utilization and delay:
Explanation: Excessive utilization will cause a large amount of channel resources to be occupied, greatly increasing the delay and reducing the data transmission rate.

Extension:

Bits

• Bit (BIT, Binary digit) is a unit of information, which is transliterated from the English BIT. It is also a bit in a binary number, a unit of measurement for the amount of information, and isThe smallest unit of information。
• Unit: bit。
• In word bits, bitsThere are only two symbols 0 and 1. Because the physical properties of the electronic hardware itself determine that bits have only these two symbols 2.

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1. bps: bits per second ↩︎