Introduction to Network Simulator

To investigate network performance, researchers can simply use an easy-to-use scripting language to con? gure a network, and observe results generated by NS2. Undoubtedly, NS2 has become the most widely used open source network simulator, and one of the most widely used network simulators. Unfortunately, most research needs simulation modules which are beyond the scope of the built-in NS2 modules. Incorporating these modules into NS2 requires profound understanding of NS2 architecture. Currently, most NS2 beginners rely on online tutorials.

Most of the available information mainly explains how to con? gure a network and collect results, but does not include su? cient information for building additional modules in NS2. Despite its details about NS2 modules, the formal documentation of NS2 is mainly written as a reference book, and does not provide much information for beginners. The lack of guidelines for extending NS2 is perhaps the greatest obstacle, which discourages numerous researchers from using NS2. At this moment, there is no guide book which can help the beginners understand the architecture of NS2 in depth.

The objective of this textbook is to act as a primer for NS2 beginners. The book provides information required to install NS2, run simple examples, modify the existing NS2 modules, and create as well as incorporate new modules into NS2. To this end, the details of several built-in NS2 modules are explained in a comprehensive manner. NS2 by itself contains numerous modules. As time elapses, researchers keep developing new NS2 modules. This book does not include the details of all NS2 VIII Preface modules, but does so for selected modules necessary to understand the basics of NS2.

For example, it leaves out the widely used modules such as wireless node or web caching. We believe that once the basics of NS2 are grasped, the readers can go through other documentations, and readily understand the details of other NS2 components. The details of Network AniMator (NAM) and Xgraph are also omitted here. We understand that these two tools are nice to have and could greatly facilitate simulation and analysis of computer networks. However, we believe that they are not essential to the understanding of the NS2 concept, and their information are widely available through most of the online tutorials.

This textbook can be used by researchers who need to use NS2 for communication network performance evaluation based on simulation. Also, it can be used as a reference textbook for laboratory works for a senior undergraduate level course or a graduate level course on telecommunication networks o? ered in Electrical and Computer Engineering and Computer Science Programs. Potential courses include “Network Simulation and Modeling”, “Computer Networks”, “Data Communications”

This layer supports several higher-level protocols such as HTTP (Hypertext Transfer Protocol) for World Wide Web applications, SMTP (Simple Mail Transfer Protocol) for electronic mail, TELNET for remote virtual terminal, DNS (Domain Name Service) for mapping comprehensible host names to their network addresses, and FTP (File Transfer Protocol) for ? le transfer. Transport Layer The objective of a transport layer is to transport the messages from the application layer of the source host to that of the destination host. To accomplish this goal, two well-known protocols, namely, TCP and UDP (User Datagram Protocol), are de? ed in this layer. While TCP is responsible for a reliable and connection-oriented communication between the two hosts, UDP supports an unreliable connectionless transport. TCP is ideal for applications that prefer accuracy over prompt delivery and the reverse is true for UDP. Generally, control information related to ? ow control and error control need to be embedded into the messages. Also, before adding any header, fragmentation is usually performed to break a long message into segments. For this reason, the protocol data units in this layer are normally called segments.

Network Layer This layer provides routing services to the transport layer. Network layer is designed to deliver the data units, usually called packets, along the paths they are meant to traverse from a source host to a destination host. Again, to facilitate routing, headers containing information such as source and destination network addresses are added to the transport protocol data units to formulates network-layer data unit. 1. 2 System Modeling 5 Link Layer The packets are generally routed through several communication links and nodes before they actually reach the destination node.

To successfully route these packets all the way to the destination, a mechanism is required for nodeto-node delivery across each of the communication links. A link layer protocol is responsible for data delivery across a communication link. A link layer protocol has three main responsibilities. First, ? ow control regulates the transmission speed in a communication link. Secondly, error control ensures the integrity of data transmission. Thirdly, ? ow multiplexing/demultiplexing combines multiple data ? ows into and extracts data ? ows from a communication link.

Choices of link layer protocols may vary from host to host and network to network. Examples of widely-used link layer protocols/technologies include Ethernet, Point-to-Point Protocol (PPP), IEEE 802. 11 (i. e. , WiFi), and Asynchronous Transfer Mode (ATM). Physical Layer The physical layer deals with the transmission of data bits across a communication link. Its primary goal is to ensure that the transmission parameters (e. g. , transmission power, modulation scheme) are set appropriately to achieve the required transmission performance (e. g. , to achieve the target bit error rate performance).