atp edition Einsatz robotergeführter Patientenliegen (Vorschau)

FIGURE 1: Old (A)
and new (B)
communication
concept in OPC UA
FIGURE 3: Delay for an OPC UA request-response
FIGURE 2: OPC UA proxy server
FIGURE 4: Average delay on Raspberry and Android
uses pure HTTP only in combination with the XML
encoding, which is currently not supported by the
OPC UA Java stack.
Typically XMPP uses also a binary TCP transport mechanism
– but with the necessity to use a base64 encoding
in order to wrap the binary data into an XML container.
This and the additional hop over the XMPP server (on the
same PC) make XMPP slower than OPC.TCP. In comparison
to HTTPS this approach is faster – which may be
because it re-uses previously established connections.
XMPP can be used in two secure modes: XMPP-SSL uses
a (deprecated) approach by applying a SSL connection to
the XMPP server, while XMPP-SEC uses an XMPP internal
mechanism to provide a secured transport (with endto-end
encrypted body). Both approaches have a similar
delay in this scenario. XMPP-BOSH uses a BOSH based
transport. BOSH itself uses an HTTP-long polling approach.
The communication over base64+XMPP+BOSH
has the highest latency – but also the ability to work over
HTTP proxies, NATs and firewalls.
The approach using WebSockets (WS) transports the
data like native OPC.TCP – but with the bidirectional
communication strategy. This results in a similar delay
during the communication: The unsecure approaches
need 0.25 ms WS compared to 0.21 ms OPC.
TCP; the secure approach WS (SSL) is also comparable
to OPC.TCP (SEC) with enabled signing and encryption
mechanisms.
2.2 Platform comparison
In this scenario an Android device (Samsung Galaxy
Tab S2) or a Raspberry Pi was used as an example for a
field device gateway on an embedded device. While the
Raspberry was connected using a 1Gbit LAN, the Android
device was connected over a Wi-Fi access point
in the same LAN. In the case of XMPP the XMPP server
was installed on the PC. The results in Figure 4 give an
approximation of the expected delay:
Usually the embedded device was used as the local
instance (which receives requests from the PC in the
role of a remote/cloud instance). In some cases also the
“backward-direction” was tested (marked with “BW”)
atp edition
7-8 / 2014
37