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1、3.4 Traffic handling capacity: Erlang performance and cell sizing34 流量控制：爱尔兰厄兰性能和小区尺寸,With the appropriate cell cluster size or reuse parameter C determined, by the SIR required for a given system, the number of channels available per cell is immediately established. For example, the first-generatio
2、n AMPS provides 832/C channels per cell, as already noted. With C = 7, as determined in the previous section, the system can provide 118 channels per cell.,一旦由系统给定的信干比决定的再用 参数C或小区簇的大小确定，有效的信道数 量也就确定了。例如，第一代AMPS系统每 个小区提供832C个信道，这已经在前面 提到过。当C=7时，系统每个小区提供 118个信道。,Given the number of channels available
3、per cell, we can now determine the geographical extent or size of cells. This determination is based on a consideration of the traffic expected in a given region and the system performance, other than SIR, that is to be provided to a typical user.,给定每个小区的有效信道数量，能确 定小区的地理范围和尺寸。在一个给定区 域和系统性能的情况下，主要考虑流
4、量， 而不是信干比，信干比主要是针对每个特 定的用户而言的。,In this introductory section on system performance we focus exclusively（专有地，单独地） on the probability of call blocking as a measure of performance or user satisfaction. The call blocking probability has, historically, been the principal（主要的） measure of performance in tel
5、ephone systems . Simply put, it describes the chance （可能性）that a user, attempting to place a call, receives a busy signal.,在这部分，我们将介绍一下体现了系统 性能和用户满意度的方法之一呼叫阻 塞概率。 历来，呼叫阻塞率一直是电话系统性能 指标的主要测量方式。 简单地说，它是表征一个用户打算通话 但系统告知繁忙的可能性.,In the mobile wireless domain the calculation of the call blocking probabilit
6、y in a given cell depends on the number of channels available and the traffic expected to use that cell.,在无线通信领域，给定一个小 区，呼叫阻塞概率的计算依赖于可以 同时处理的有效信道数量以及此小区 期望的流量.,In the material（素材，资料） following, we first define what is meant by “amount of traffic（流 量） ,or traffic intensity ,or traffic load（负荷） ,using
7、 a simple statistical model, then indicate how the blocking probability is related to it.,在下面，定义了通信总流量，或 者流量密度，或者流量负荷。使用了 一个简单的模型，用其说明阻塞概率 与它的关系。,Traffic intensity is commonly defined to be the product（产物，结 果）of the average number of call attempts per unit time, times the average call length.,流量密度被定
8、义为每单位时间内 呼叫的平均数量，乘以平均呼叫长度.,Let the symbol（符号) represent the average number of call attempts per unit time and 1/ be the average call length. The traffic intensity or load A is then /, in units of Erlangs.,令为每单位时间内的平均 呼叫数量，1是平均呼叫长度。 流量密度或者负荷A为，用 爱尔兰厄兰进行单位表示。,As an example, say there is an average of
9、50 call attempts per minute, with each call, if accepted, lasting 3 minutes. The traffic load is then 150 Erlangs.,举个例子，假如平均每分钟有50 次呼叫，每次呼叫如果成功的话，持 续通话3分钟，流量负荷是150爱尔 兰厄兰。,Calculate the Erlang loads on a system for the following cases: 1. An average call lasts 200 seconds; there are 100 call attempts
10、 per minute. 2. There are 400 mobile users in a particular cell. Each user makes a call attempt every 15 minutes, on the average. Each call lasts an average of 3 minutes.,计算下面系统的爱尔兰厄 兰负荷： 1呼叫平均持续200 s， 每分钟平均100次呼叫。 2小区内有400个移动用 户，每个用户平均每隔15 min呼叫一次。呼叫平均持续 3 min。,Call lengths are assumed in this simp
11、le model to be exponentially distributed, with average value 1/. A simple analysis then shows that, with N channels available, the call blocking probability PB is given by the so-called Erlang-B formula:,呼叫时间长度在单一模型中被假 设为服从指数分布，平均值为1/。 一个简单的分析表明，拥有N个有效 信道，呼叫阻塞概率PB，也就是爱尔 兰厄兰B式为： (3.7),A table or plot
12、 of PB vs A indicates that N A for PB =10%. N A channels are required to have PB = 5% or less.,一个PB为纵轴和A为横轴的表格 或曲线图说明，P=10时，NA。 当P5时，需要NA个信道。,As an example, consider the traffic load A = 150 Erlangs calculated above. For PB = 5%, N =153 channels are found to be required. For a blocking probability o
13、f 1%, 169 channels are required. Small increases in the number of channels made available reduce PB substantially （实质上）.,举个例子，如上所计算的，如果 通信负荷A=150爱尔兰厄兰PB=5 时，需要N=153个信道。对于1的 阻塞概率，需要169个信道。信道数 量只要增加一点，PB的降低也是很明 显的。,Consider the converse now. Say the number of channels is fixed. How does the blocking p
14、robability vary with Erlang load?,反过来，信道数量是固定的，阻塞 概率会随着爱尔兰厄兰负荷发生什么样 的改变呢?,Let N = 100 channels, to be concrete. To keep PB at 1% or less, a load of no more than 84 Erlangs can be allowed. If A were to increase by 13%, to 95 Erlangs, the blocking probability would jump to 5%, a five-fold increase! At
15、 low blocking probabilities, the blocking probability is very sensitive（敏感 的） to changes in load.,取N=100，且不变。若保持 PB1，则只允许至多84个爱尔兰 厄兰的负荷。如果A增加13，到 95爱尔兰厄兰，则阻塞概率将变为 5，增加了5倍。在低的阻塞概率 情况下，阻塞概率对于负荷的变化非 常敏感。,现在来考虑上面讨论的几个量与给 定信道灵活使用之间的关系。 如果每个呼叫平均持续了200s。平 均每隔15 min用户进行一次呼叫，有 100个有效信道，呼叫阻塞概率为 1，则可以容纳378个用户同
16、时 使用(这个数字是怎么得到的?)。如果 平均每个呼叫持续了400 s，那么仅仅 可以容纳189个用户同时使用。,Consider now the significance of these numbers in relation to mobile usage of a given channel. Say a call lasts 200 seconds on the average. Say a typical user makes a call every 15 minutes, on the average. If 100 channels are available and a blocking probability of 1% is desired, 378 users can be accommodated. (How is this number obtained?) If the average call duration（持续，持久） increases to 400 seconds, only
