wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Industrial Multimedia 27-11
with proxies and firewalls), it also has many disadvantages. It does not support multicast, only unicast.
Therefore, if more than one node is listening to the stream, several connections have to be established that
use the corresponding multiple of the necessary bandwidth. Furthermore, HTTP is based on TCP, which
is a reliable protocol, that is, it does perform retransmission of packets that were not transmitted correctly.
This is completely inadequate for real-time transmission because the retransmitted data will often be late
and, therefore, useless. In fact, the use of HTTP is just an extension of progressive download (the presentation
of a multimedia file starts while downloading), but it was never designed for real streaming.
A much more suitable protocol for transmission of multimedia data is real-time transport protocol
(RTP) as proposed in RFC1889, which is usually based on UDP and in principle supports multicast as well
as unicast. It also includes timestamps, which are used for synchronization purposes; but it has to deal
with the fact that in IP networks there is no global time base that is synchronized automatically. Real-time
streaming protocol (RTSP) can be used for setting up the transmission. There are also proprietary protocols
like Microsoft media server (MMS) or Real delivery transport (RDT) and others. They have similar
features and are used by the tools of the corresponding companies (Microsoft, RealNetworks, etc.).
Many different (open and proprietary) formats for audio and video exist and can be transmitted in an
RTP stream [W05]. Concerning audio, the most important open formats are uncompressed audio with 16,
20, or 24 bit resolution and MPEG compressed audio streams. For video, RFC2250 specifies how to transmit
MPEG video elementary streams. If audio and video are to be transmitted in one stream, MPEG-2 transport
streams can be used, but the concept of RTP is to transport each media in a separate stream rather than using
multiplexes. Digital video in the DV format of IEC 61834 [IEC61834-2] can be transported in RTP according
to RFC3189 [IEC61883-2]. However, it should be mentioned that this is a format commonly used on IEEE
1394 [IEC61883-2] but not on IP networks. Important proprietary formats are RealAudio and RealVideo.
IPv6, the successor of IP version 4, has some advantages concerning multimedia streaming. Multicast
has been integrated into IPv6, so every compliant implementation supports multicast. There are features
(especially the flow labels should be mentioned) that make the provision of QoS for streams easier.
The functionality of Internet Group Management Protocol (IGMP) is integrated into ICMPv6 (Internet
control message protocol). The “multicast listener discovery” (MLD) used for the signaling between
routers and hosts is a subprotocol of ICMPv6.
27.5 Conclusions
Multimedia has a wide range of applications in the industrial area. These applications also have a wide range
of requirements, like resolution, frame rates, image quality, latency, etc. Industrial communications networks
provide the necessary infrastructure for many applications. In this chapter, industrial multimedia applications
and their particular requirements are discussed. Compression technology, which is needed in many
applications due to bandwidth or storage limitations, is briefly reviewed. Finally, two different examples of
network and the associate protocols for transmission of multimedia data are presented.
Acknowledgment
This work was partially supported by de MCYT of Spain under the project TSI2007-66637-C02-02.
References
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[AST03] A. S. Tanenbaum. Computer Networks. Paerson Education International, 4th edn., Upper Saddle
River, NJ, 2003. ISBN 0-13-038488-7.
[CSE00] C. Christopoulos, A. Skodras, and T. Ebrahimi. The JPEG2000 still image coding system: an
overview, IEEE Transactions on Consumer Electronics 46(4), 1103–1127, 2000.
© 2011 by Taylor and Francis Group, LLC