Transmission of Multimedia Data over Wireless Ad-Hoc Networs

Transmission of Multimedia Data over Wireless Ad-Hoc networs

A. Berk Barutlu

Abstract

Ad-hoc networking has been of increasing interest and therefore recognized as an important research area in recent years. This paper is about transmission of multimedia data over wireless ad-hoc networks, the problems and the related issues.

1. Introduction

1.1 Ad-Hoc Networks

An ad-hoc network is a collection of wireless mobile nodes that will dynamically form a temporary network without the use of any existing network infrastructure.

The technological developments that have enabled ad-hoc networks are divided into two major groups; the first group is the devices that are small and light enough to carry them around like printers, laptops, PDA's,... etc., and the second group is wireless network interfaces, like WirelessLAN, Bluetooth, GSM,... etc., that are available for these types of devices . Since no pre-installed base stations are required, ad-hoc networks can be deployed quickly in cases such as conventions, search & rescue, disaster recovery situations and battle field communications. For example, in case of conventions, meetings in which the laptops and the other devices are connected in a wireless network in order to exchange data and share resources, like a printer,... etc.. Another example is the occurrence of natural disaster, like an earthquake, which results in the destruction of existing network facilities. Ad hoc networks fill the communications void created in such a situation to coordinate the efforts of rescue.

Transporting multimedia data over ad hoc networks is a challenging problem. At that point, before the investigation of that problem, first we have to mention the multimedia data and it's characteristics.

1.2 Multimedia Data

The word multimedia is made up of the two Latin words " Multi"= "numerous" and"Media"="medium"= "intermediary" .In information field, Multimedia means "multiple intermediaries" between the source and sink of information or "multiple means" by which information is stored, transmitted, presented, and perceived.Digital multimedia is the field concerned with the computer controlled integration of text, graphics, still and moving images, animation, sounds, and any other medium where every type of information can be represented, stored, transmitted, and processed digitally.

1.2.1 Characteristics of Multimedia Data:

A multimedia object is usually a composition of several data objects. These data objects can be in different data formats, such as text, image, video, audio,... etc. The objects are ordered spatially, by linking various data objects into a single entity with dealing object size, rotation, and placement within the entity, or temporally, creating a multimedia object according to temporal relationship with the requirement of the synchronization of the data objects, to create composite multimedia objects. The synchronization can be continuous or point. Continuous, like the video conferencing, where audio and video signals are created at a remote site, transmitted over the network, and synchronized continuously at the receiver site when presented, it requires constant synchronization of lengthy events. On the other hand, in point synchronization, a single point of a media block coincides with a single point of another media block, such as a slideshow with blocks of audio allotted to each slide.

Multimedia data is often classified according to if it is non-temporal or temporal. Non-temporal data is often called static, such as text, images and graphics, and temporal data is called dynamic, like audio, video and animation.

A multimedia object can consist of both static and dynamic data objects. An example of this would be a document describing the weather, with text and a video on the movement of clouds in the area. Dynamic data divides into two subgroups: Non-changing dynamic data, non-real-time audio, video, animation, and changing dynamic data, which includes real-time audio, video, answers to database quaries and other data changing according to given input or input time includes real-time audio, video.

2. Literature Survey

Common features to all ad-hoc systems are the use of a shared medium for data transfer and of multi-hop communications to reach a distant destination. These characteristics pose challenging issues, especially related to traffic routing and acsess control.

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3. Discussions

It is a great challenge

to provide multimedia service in ad-hoc networks. Due to it's real time nature, real time multimedia transport has stringent bandwidth, delay, and loss requirements. Such networks are characterized with frequent link failures, as well as congestion. In fact, a wireless link usually has higher transmission error rate because of shadowing, fading, path loss, and interference from other transmitting users. An end-to-end path found in ad-hoc networks has an even higher error rate since it is the concatenation of multiple wireless link. Moreover user mobility makes the network topology costantly change. In addition to this, ad-hoc networks also reconfigure when users join or leave the network. An end-to-end route in ad-hoc networks may only exist for a short period of time. The frquent link failures and route changes cause packet losses and reduce the received video quality. This is different from wireline networks, where packet loss is mainly caused by congestion and buffer overflow. To provide an acceptable received video quality in ad-hoc networks, there should be effective error control to reduce packet losses to a certain level. Traditional error control techniques, including Forward Error Correction (FEC) and Automatic Repeat Request (ARQ) have been adopted to take link failures into consideration. Even though some packet loss is generally tolerable, the quality of reconstructed video or audio will be impaired and errors will propagate in the following frames because of the dependency introduced among consecutive frames at the encoder. However, the current best-effort network architecture does not offer any Quality of Service (QoS) guaraantees for video transport.The Transmission Control Protocol (TCP) is mainly designed for reliable data traffic. It is not suitable for real time multimedia