ns-3网络模拟器：随机变量与事件管理详解

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NS-3是一款功能强大的网络模拟器，主要用于研究和开发LTE（Long-Term Evolution）和Wi-Fi等无线通信技术。该手册介绍了ns-3的第3.20版本，发布日期为2014年6月17日，详细阐述了模拟器的关键组成部分和使用方法。 1. 组织结构： ns-3项目采用了模块化的设计，使得用户能够方便地构建和扩展网络模型。它包括多个核心组件，如事件驱动的模拟引擎、随机变量生成、哈希函数、时间管理、回调机制以及对象模型。 2. 随机变量 (Random Variables): - 快速概述：随机变量是模拟中的关键元素，用于模型中的不确定性。ns-3提供了多种类型的随机变量，如类随机变量流（ClassRandomVariableStream），允许用户设置流号以实现独立复制和重复。 - 操作细节：文档详细介绍了如何创建和配置随机变量，以及如何利用伪随机数生成器（PRNG）进行更复杂的随机行为设定。 3. 哈希函数 (Hash Functions): - 基本使用：哈希函数在ns-3中用于数据完整性检查和一致性验证。文档展示了如何使用内置的哈希函数，以及如何引入自定义实现。 - 动态更新：ns-3支持增量哈希，这意味着在数据结构变化时可以高效更新哈希值，提高了性能。 4. 事件与模拟器 (Events & Simulator): - 事件是模拟的核心驱动力，它们代表了网络中的各种行为，如节点通信、定时任务等。用户需要理解如何创建和关联事件到模拟器中。 - 模拟器管理时间线，调度事件执行，确保模拟按照预定的逻辑进行。 5. 回调 (Callbacks): - 回调机制允许用户自定义函数在特定事件触发时执行。文档介绍了如何使用回调API，以及 Bound Callbacks（绑定回调）和 Traced Callbacks（追踪回调）的区别。 - ns-3中的回调位置多样，用户可以根据需求选择合适的回调位置，并了解其内部实现细节。 6. 对象模型 (Object Model): - ns-3采用面向对象设计，提供了基础类和内存管理。对象的行为可以通过继承和封装来扩展，而classPtr是一种特殊的指针类型，用于处理对象所有权和生命周期管理。阅读ns-3 manual对于理解和使用这个强大的网络模拟器至关重要。通过掌握随机变量的使用、哈希函数的应用、事件驱动的模拟流程以及灵活的回调机制，用户能够构建复杂的网络模型并进行深入的性能分析。同时，了解对象模型有助于编写高效的代码，确保模拟的准确性和可维护性。

ns-3 Manual, Release ns-3.20

• Decide whether you are running with a ﬁxed seed or random seed; a ﬁxed seed is the default,

• Decide how you are going to manage independent replications, if applicable,

• Convince yourself that you are not drawing more random values than the cycle length, if you are running a very

long simulation, and

• When you publish, follow the guidelines above about documenting your use of the random number generator.

10 Chapter 2. Random Variables

CHAPTER

FOUR

EVENTS AND SIMULATOR

ns-3 is a discrete-event network simulator. Conceptually, the simulator keeps track of a number of events that are

scheduled to execute at a speciﬁed simulation time. The job of the simulator is to execute the events in sequential time

order. Once the completion of an event occurs, the simulator will move to the next event (or will exit if there are no

more events in the event queue). If, for example, an event scheduled for simulation time “100 seconds” is executed,

and the next event is not scheduled until “200 seconds”, the simulator will immediately jump from 100 seconds to 200

seconds (of simulation time) to execute the next event. This is what is meant by “discrete-event” simulator.

To make this all happen, the simulator needs a few things:

1. a simulator object that can access an event queue where events are stored and that can manage the execution of

events

2. a scheduler responsible for inserting and removing events from the queue

3. a way to represent simulation time

4. the events themselves

This chapter of the manual describes these fundamental objects (simulator, scheduler, time, event) and how they are

used.

4.1 Event

To be completed

4.2 Simulator

The Simulator class is the public entry point to access event scheduling facilities. Once a couple of events have

been scheduled to start the simulation, the user can start to execute them by entering the simulator main loop (call

Simulator::Run). Once the main loop starts running, it will sequentially execute all scheduled events in order

from oldest to most recent until there are either no more events left in the event queue or Simulator::Stop has been

called.

To schedule events for execution by the simulator main loop, the Simulator class provides the Simulator::Schedule*

family of functions.

1. Handling event handlers with different signatures

These functions are declared and implemented as C++ templates to handle automatically the wide variety of C++ event

handler signatures used in the wild. For example, to schedule an event to execute 10 seconds in the future, and invoke

a C++ method or function with speciﬁc arguments, you might write this:

ns-3 Manual, Release ns-3.20

void handler (int arg0, int arg1)

{

std::cout << "handler called with argument arg0=" << arg0 << " and

arg1=" << arg1 << std::endl;

}

Simulator::Schedule(Seconds(10), &handler, 10, 5);

Which will output:

handler called with argument arg0=10 and arg1=5

Of course, these C++ templates can also handle transparently member methods on C++ objects:

To be completed: member method example

Notes:

• the ns-3 Schedule methods recognize automatically functions and methods only if they take less than 5 argu-

ments. If you need them to support more arguments, please, ﬁle a bug report.

• Readers familiar with the term ‘fully-bound functors’ will recognize the Simulator::Schedule methods as a way

to automatically construct such objects.

2. Common scheduling operations

The Simulator API was designed to make it really simple to schedule most events. It provides three variants to do so

(ordered from most commonly used to least commonly used):

• Schedule methods which allow you to schedule an event in the future by providing the delay between the current

simulation time and the expiration date of the target event.

• ScheduleNow methods which allow you to schedule an event for the current simulation time: they will execute

_after_ the current event is ﬁnished executing but _before_ the simulation time is changed for the next event.

• ScheduleDestroy methods which allow you to hook in the shutdown process of the Simulator to cleanup simu-

lation resources: every ‘destroy’ event is executed when the user calls the Simulator::Destroy method.

3. Maintaining the simulation context

There are two basic ways to schedule events, with and without context. What does this mean?

Simulator::Schedule (Time const &time, MEM mem_ptr, OBJ obj);

vs.

Simulator::ScheduleWithContext (uint32_t context, Time const &time, MEM mem_ptr, OBJ obj);

Readers who invest time and effort in developing or using a non-trivial simulation model will know the value of the

ns-3 logging framework to debug simple and complex simulations alike. One of the important features that is provided

by this logging framework is the automatic display of the network node id associated with the ‘currently’ running

event.

The node id of the currently executing network node is in fact tracked by the Simulator class. It can be accessed with

the Simulator::GetContext method which returns the ‘context’ (a 32-bit integer) associated and stored in the currently-

executing event. In some rare cases, when an event is not associated with a speciﬁc network node, its ‘context’ is set

to 0xffffffff.

To associate a context to each event, the Schedule, and ScheduleNow methods automatically reuse the context of the

currently-executing event as the context of the event scheduled for execution later.

In some cases, most notably when simulating the transmission of a packet from a node to another, this behavior is

undesirable since the expected context of the reception event is that of the receiving node, not the sending node. To

14 Chapter 4. Events and Simulator

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