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带宽与延时的一些问题解答(三)

时延

下面图片来自《数据网络技术》ppt 第一章 81 页。 


下面这一段来自于http://web.mit.edu/dimitrib/www/Queueing_Data_Nets.pdf


In what follows, we will mostly focus on packet delay within the communication 

subnet (i.e., the network layer). This delay is the sum of delays on each subnet link 

traversed by the packet. Each link delay in tum consists of four components.


The processing delay between the time the packet is correctly received at the head 

node of the link and the time the packet is assigned to an outgoing link queue 

for transmission. (In some systems, we must add to this delay some additional 

processing time at the DLC and physical layers.)

The queueing delay between the time the packet is assigned to a queue for transmission and the time it starts being transmitted. During this time, the packet waits 

while other packets in the transmission queue are transmitted.

The transmission delay between the times that the first and last bits of the packet 

are transmitted.

The propagation delay between the time the last bit is transmitted at the head 

node of the link and the time the last bit is received at the tail node. This is 

proportional to the physical distance between transmitter and receiver; it can be 

relatively substantial, particularly for a satellite link or a very high speed link.

按照上面的意思: 

时延 = 处理时延 + 排队时延 + 发送时延 + 传播时延


一些网页上提供网络延时计算: 

算了发送时延 + 传播时延 

算信道最大带宽和交换机缓存大小


QoS

现有网络控制时延和带宽的方式为: 

将路由器中正在排队的数据包放在不同的优先级队列中,优先级高的队列,优先发送。 

这样在传输数据量不变的情况下,耗费时间变了,计算出来的带宽也就变了,同时延时也发生了改变。


从下面这句话理解:FDM 控制带宽的方式为减少采样频率,TDM 控制带宽的方式为增加等待时间。 

In time-division (TDM) and frequency-division multiplexing (FDM) with m traffic 

streams, the link capacity is essentially subdivided into m portions-one per traffic 

stream. In FOM, the channel bandwidth W is subdivided into m channels each with 

bandwidth W /m (actually slightly less because of the need for guard bands between 

channels). The transmission capacity of each channel is roughly C /m, where C is 

the capacity that would be obtained if the entire bandwidth were allocated to a single 

channel. The transmission time of a packet that is L bits long is Lm/C, or m times 

larger than in the corresponding statistical multiplexing scheme. In TOM, allocation is 

done by dividing the time axis into slots of fixed length (e.g., one bit or one byte long, 

or perhaps one packet long for fixed length packets). Again, conceptually, we may view 

the communication link as consisting of m separate links with capacity C /m. In the case 

where the slots are short relative to packet length, we may again regard the transmission 

time of a packet L bits long as Lm/C. In the case where the slots are of packet length, 

the transmission time of an L bit packet is L/C, but there is a wait of (m - 1) packet 

transmission times between packets of the same stream.



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