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

27-10 Industrial Communication Systems
In the area of streaming multimedia data, asynchronous transactions are not used for the transfer
of multimedia, but they are used for control purposes. IEC 61883-1 uses asynchronous transactions for
setup and teardown of isochronous connections, which also includes the isochronous resource management
[IEC61883-1]. Higher level control protocols like AV/C are based on IEC 61883-1 and use asynchronous
transactions to communicate. Real-time transmission of audio and video data is well-standardized
by the series of IEC61883. IEC 61883-1 defines general things like a common format for isochronous
packets as well as mechanisms for establishing and breaking isochronous connections [IEC61883-1].
This also includes timestamps, which are used for synchronization [WZ03].
Transmission of data in the digital video (DV) format [IEC61834-2] (typically used on handheld
digital video cameras) is specified in IEC 61883-2 [IEC61883-2]. An advantage of this format is the
high quality and a low complexity of encoding and decoding because it does not use the sophisticated
interframe compression techniques known from MPEG-2. On the other hand, it imposes rather high
bandwidth requirements (about 25.Mbps for an SD format DV video stream, together with audio and
other information it yields about 29.Mbps).
For transmission of audio data, IEC 61883-6 is used [IEC61883-6]. It is used by consumer electronic
audio equipment like CD players, amplifiers, speaker systems, etc. Usually, audio is distributed as
uncompressed audio samples at 44,100 or 48,000.Hz sample rate, but other data formats are supported as
well. But audio distribution is definitely more of interest in home environments than in industrial ones.
Transmission of MPEG-2 transport streams [MPG2] over IEEE 1394 is specified in IEC 61883-4
[IEC61883-4]. This allows the distribution of digital television over IEEE 1394, since both DVB and
ATSC use MPEG-2 transport streams. MPEG-2 provides a good trade-off between quality and utilized
bandwidth. A typical PAL TV program requires about 6.Mbps.
In addition to the two possibilities based on IEC 61883 mentioned above to transmit video in DV
or MPEG-2 data format, there is another protocol that is primarily targeting industrial environments
(albeit there are some Web cameras using this protocol as well): the Instrumentation & Industrial
Digital Camera (IIDC) protocol [IIDC1394]. This is a very simple protocol (not based on IEC 61883)
that transmits uncompressed digital video over isochronous FireWire channels. Therefore, it requires
significantly more bandwidth, but does not require encoding and decoding.
27.4.2 IP-Based Networks
IP, the Internet protocol, is a very widely used network protocol. The main reason is that it can be used on
top of almost every lower layer networking system (Ethernet, token ring, IEEE 1394, etc.) [AST03]. These
networks were not designed for the purpose of multimedia transmission and, therefore, have some weaknesses
with regard to real-time data transmission. QoS is very limited. There are mechanisms for QoS
in IP networks, but they provide only soft guarantees. The resource reservation protocol (RSVP) is the
standard protocol to reserve resources in IP networks [WZ01]. In fact, RSVP is only a signaling protocol
for resource reservation requests and does not by itself reserve resources. IP networks only offer best effort
services, and therefore, QoS is usually only achieved by oversizing the network and providing much more
performance than needed in average to ensure good performance even in worst-case scenarios.
Besides the so-called integrated services approach with per-stream reservations, there is also the differentiated
services approach, which does not provide per-stream reservations, but has advantages in
scalability [WZ01]. Below IP, on the Ethernet layer, IEEE 802.1p also provides some kind of QoS mechanism.
But this is very weak. It just allows defining priorities of individual Ethernet packets. It does not
reserve resources and does not provide any guarantees. One problem of IP networks is that transmission
of audio and video is not very well-standardized. There is a wide variety of data and transmission
formats, open and proprietary ones, which are not compatible with each other.
In IP-based networks, many different protocols are commonly used for real-time transmission of multimedia.
The most primitive one is HTTP, the hypertext transfer protocol. There are implementations that
use HTTP to stream real-time audio data. But besides some advantages (it is very simple and works well
© 2011 by Taylor and Francis Group, LLC