移动通信网络：Mobile Communication Networks

Chapter 1 Introduction引言，第一章

近十年，移动通信在全世界的各个地方开始逐渐普及，而且这些移动通信设备变得越来越精巧，电池的待机时间也变得越来越长，并且机器的机体更加的坚固，更难得的是，通信协议的算法也更强悍。现在是一个无线移动的通信时代，无线技术已经逐步普及，给移动性的设备带来方便。

In recent decade, the usage as well as popularity of mobile communication has widely increased all around the world. So the devices are becoming more and more compact, the standby time of the batteries are being improved for a longer period and communication protocols are getting more strong and providing more sturdy throughput. Now a days, Wireless Technology has taken over the Wire Communication, thus giving the facility of mobility to the users.

The main idea of mobile ad-hoc networking is to support strong and efficient operation in mobile wireless networks, by introducing the routing functionality into mobile nodes. These kinds of networks can have dynamic, sometimes rapidly-changing, multi-hop topologies which are likely composed of relatively bandwidth constrained wireless links.

Implementation Work:
 

The work this thesis covers is the modification of the Routing Protocol that is already implemented for mobile wireless networks. The entire implementation has been rewritten as before and in some aspects added. It is now implemented to comply with the RFC describing the protocol and to be as extensible as possible.

In the thesis the Optimized Link State Routing Protocol is implemented and the extension on the routing protocol is also the part of my thesis.

As the space is limited so the background of the technical aspects will not be included in every part of this thesis. I expect that the reader has some basic knowledge of mobile ad hoc networks, UDP/TCP IP networking and C programming.

1.2 Chapter overview:本章概述：

Chapter 1 is the general Introduction and in Chapter 2, I have given the introduction to Mobile Ad-hoc Networks some basics of wireless data-communication are also introduced in it. I have also included the two routing protocol that are proposed by (IETF). Chapter 3 is for the Core-Functionality of the OLSR. The process of implementing and simulations of OLSR protocol is described in chapter 4. Our proposed model and its extension to OLSR are presented in chapter 5.
 

Chapter 2 Mobile AD-HOC Networks

The basis on which much of the Wireless Technology works, is known as “Point to Point Communication”. Popular solutions like Global System for Mobile communication (GSM) and Wireless Local Area Network (WLAN), use the same approach in which ‘mobile node’ is used that communicates with centralized Access point. Centralization for configuration and operation is required for these types of networks. Multi-Hop approach is opposite to the model that is mentioned above. In Multi-hop if the end point is out of direction, the nodes can communicate with each other by utilizing the other node as relays of traffic. Multiple nodes are may be involved between the communication of two nodes. Sometimes it happens that a node is not been accessed directly. Data or control packets can be sent from one node to another node through several nodes. These types of networks are called mobile ad hoc networks.
 

The multi-hop model is used by Mobile ad-hoc networks (MANET). Mobile ad-hoc networks are infrastructure less where the communication between two nodes is accomplished through wireless media. Node mobility is freely allowed as there is no structured media, which translates into a constantly changing network topology. MANETs can also provide networking connectivity on rough scenarios like “disaster relief areas, battle fields” etc, and considered as best alternative where environmental conditions are another factor that changes the network topology. Using the location of nodes and the availability of direct communication between each pair of nodes the network topology is described.

2.1 Ad-hoc networks:Ad-hoc网络：
 

Centralized networks, such as GSM, cannot be used in all situations. Significant examples of such scenarios where ad hoc networks can be used include establishing survivable, efficient, dynamic communication for rescue operations, disaster relief efforts and military networks. These kinds of networks cannot rely on centralized and organized connectivity and can be considered as applications of MANETs. MANETs has several set of diverse applications which range from ‘small static networks restrained by power sources, to large mobile networks which are highly dynamic’.
 

All nodes should be able to act as routers for each other, in order to enable multi-hop communication in a distributed manner (See Figure 2.1). Nodes help each other in conveying information to and fro and thereby creating a virtual set of connections between each other. Ad hoc network can only exist and operate if its nodes demonstrate a cooperative behavior.

Routing protocol sets up and maintains the Routes. Routing protocols play a vital role in the creation and maintenance of these connections. In contrast to wired networks, each node in an ad-hoc networks acts like a router. As these routers are usually on the move, standard intra-router protocols cannot be immediately adapted to ad-hoc networks. MANET routing protocol design is a complex issue considering the possible rapidly changing topology of such networks.

Hassene Bouhouch, Sihem Guemara EL Fatmi, “QoS In Ad Hoc Networks by Integreting Activity in the OLSR.

Leila Boukhalfa, Pascale Minet and Serge Midonnet, “QoS support in a MANET based on OLSR and CBQ.

Many different types of routing protocols have been developed for ad hoc networks and have been classified into two categories

Reactive

Proactive

In reactive routing protocols, in order to preserve precious node battery, routes are only discovered when required, while in proactive routing protocols routes are established before usage and hence avoid the delays incurred while discovering new routes in the reactive routing protocols. Reactive protocols set up traffic routes on-demand, whilst proactive protocols attempts to dynamically maintain a full understanding of the topology.
 

2.1.1 Wireless communication无线传输

Ad-hoc networks are not restricted to any special hardware. But today such networks are most likely to consist of nodes utilizing so-called WLAN interfaces . These are wireless interfaces operating according to IEEE specifications 802.11a, 802.11b or 802.11g. Throughout this document it is assumed that an ad-hoc network consists of links made up by either WLAN or Ethernet interfaces.
 

Figure: 2-1, A traditional base station scheme compared to ad-hoc multi-hop network.
 

IEEE 802.11 does not support multi-hop communication by it self. Two modes are defined for communication using WLAN devices:

Infrastructure mode: The wireless network consists of at least one access point and a set of wireless nodes. Such configuration is known as Basic Service Set (BSS). A set of two or more BSSs forms an Extended Service Set (ESS).
 

Ad hoc mode: This is a peer-to-peer mode. This configuration is called Independent Basic Service Set (IBSS), and is useful for establishing a network where nodes must be able to communicate directly and without any centralized access point.
 

While setting up a MANET, the mode to be used most certainly is The Ad-hoc mode, but one of the most basic requirements is missing here which is ‘Multi hop’. The Ad Hoc mode enables Traffic to be transmitted to the adjacent nodes only within the radio range, therefore there is a need for MANET routing protocols to set up and maintain traffic paths.
 

2.1.2 Traditional IP routing传统的IP路由

Routing is the primary function of IP. IP datagram’s are processed and forwarded by routers which relay traffic through paths set up by various routing protocols. Routing in today’s fixed networks is based on network aggregation combined with best matching. To maintain the knowledge about other IP Networks and IP Hosts, routing table are used by TCP/IP Hosts. IP address and a subnet mask enables to identify the Network and routes to single hosts are rarely set up. When a packet is to be forwarded, the routing table is consulted and the packet is transmitted on the interface registered with a route containing the best match for the destination. A default route is used if no network matches are found.
 

When a network interface is configured with an IP address, a route to the network address is a member, which is mostly registered on the interface automatically. Since hosts with addresses within this network are assumed to be reachable directly from this interface, the route is not set up with a gateway. This confirms that the job of the traditional IP routing is to maintain an idea of all hosts within the same subnet being on the same link. This shows that on a single one-hop network segment all of the hosts in a subnet are available, typically via routers or switches. The case is entirely different when working on wireless multi-hop networks. So the idea of nodes being available “on the link” must be redefined. In MANETs nodes routes traffic by retransmitting packets on the interface they arrived.