Data communication 2

Delay Model (Queueing)

Node 之間傳packet 總時間=>delay time

D trans= packet(bit) / BW (bps)
D process= 處理packet的time (以目前科技來看 近乎等於零)
D distance =網路線長 (也是幾乎等於零)
*D queueing 等待時間  -----------------這節重點~~~~

Queueing time

1. FIFO (first-in-first-out)
2. TDM (Time-Division Multiplexing)
3. FDM (Frequency Division Multiplexing)
4. SM (Statistical Multiplexing)

1.不需要多做解釋吧~就是字面的意思XD

2想像一條線路，被切成好幾段(線路的切割)，每一段都放不同的packet

3想像一條很粗的線路，線路中塞了很多packet(口徑的切割)

4類似FIFO，不過會根據傳輸packet的大小作queueing (使用效率高，但缺點是packet過大時會造成嚴重Delay)

使用時機:n大的時候用2, n小的時候用3, packet 小的時候用 4

再來要開始進入計算的部份了，在這之前來定義一下名詞~~~

N = avg of customers (packet) in the system

T = avg delay for customer 

λ=  customer arrival  rate

M =  customer service rate

N(t) = Number of customers in the system at time t

α(t) = Number of customers who arrived in the interval [0, t]

Ti = Time spent in the system by the ith(第i個) arriving customer

Little's Theory

* N=λT

Little's Theorem expresses the natural idea that crowded systems (large N) are associated

with long customer delays (large T) and reversely. For example, on a rainy day, traffic

on a rush hour moves slower than average (large T), while the streets are more crowded

(large N). Similarly, a fast-food restaurant (small T) needs a smaller waiting room

(small N) than a regular restaurant for the same customer arrival rate.