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

Industrial Multimedia 27-9
is necessary, or even in wide network inspection, the use of lineal cameras (lineal cameras are devices
with a sensor of only one line of pixels forming the image with the concatenation of consecutive lines).
27.4 Image Transmission
Since processing of image/video data typically does not take place in the camera that captures the data,
the data has to be transmitted over some kind of data network. There are transport protocols that define
how multimedia data has to be put into data packets that are sent on the network [W06]. This typically
also involves some timestamps that are used for synchronization purposes. There are also protocols
necessary to set up and start streaming of the data over the network. These protocols are usually tightly
coupled with the transport protocols. Each kind of network has different kinds of data formats that are
transported by the transport protocol. This depends on the special features (like available bandwidth)
of the network. Two kinds of network will be described more in detail here: IEEE 1394 and IP-based
networks (typically based on Ethernet) [W05].
27.4.1 IEEE 1394
IEEE 1394 [IEEE1394], also known as FireWire or i.Link, is a serial bus designed to connect up to
63 devices (like camcorders, CD players, satellite tuners, but also personal computers and peripherals
like hard disks, scanners, etc.). The original version of this bus supports up to 400.Mbps, has an automatic
configuration mechanism allowing hot plugging of devices, and is very well suited for streaming of multimedia
content [A99]. It supports 4.5.m links and up to 16 hops between two devices leading to a maximum
distance of 72.m between two nodes. This clearly shows that it is suited for small area environments like
homes or automobiles, but it is used in industrial environments as well. IEEE 1394a [IEEE1394a] introduced
some new features, mainly to increase the bus efficiency (reduction of idle times), but the main
characteristics were preserved. The newer version IEEE 1394b supports up to 1600.Mbps and has architectural
support for 3200.Mbps [IEEE1394b]. It introduced some new physical media in addition to shielded
twisted pair (STP) copper cables and also increased the maximum distance between devices (up to 100.m
using optical fibers or CAT-5 UTP). The transmission model and coding of data is completely different to
IEEE 1394a; however, compatibility is maintained by introduction of bilingual nodes.
On the physical layer, a FireWire bus is a tree consisting of point-to-point connections between the
devices. However, the physical layer transforms this tree into a logical bus, where each node (except the
node that is currently transmitting a packet) acts as repeater. The higher layers (link layer and transaction
layer of the three-layer design used in IEEE 1394) have a view as a bus, where all packets are received
by every node, provided the used speed is supported on all links in between.
Two different data transfer modes are supported: asynchronous and isochronous data transfer. The
isochronous mode has been designed to meet the requirements of multimedia streaming. There
are 64 isochronous channels. Such channel can be regarded as a multicast group. In fact, the packets are
transmitted to each node on the network, but the hardware inside the node usually filters and discards
packets of isochronous channels that are not of interest. There is a resource management mechanism:
bandwidth and channel can be reserved at the isochronous resource manager (a functionality provided
by one of the nodes in the network) and are then guaranteed. This means that quality of service is provided.
The requirement for the guarantee of isochronous bandwidth is that all nodes have to obey the
rules of IEEE 1394 and reserve bandwidth and channel before they are used. Since this is not enforced
by hardware or software, nodes violating these rules can compromise QoS. The source can send one
isochronous packet every 125.μs (leading to 8000 isochronous cycles/s). The allowed size of the packet
corresponds to the reserved bandwidth. In case of errors, no retransmission occurs, so timing is guaranteed,
but not delivery. On the other side, the asynchronous transfer mode is used for delivery of variablelength
packets to explicitly addressed nodes. There is an acknowledgment and a retry procedure in case
of errors. Therefore, the delivery of integer data can be guaranteed, but timing is not guaranteed.
© 2011 by Taylor and Francis Group, LLC