User guide

13
Protocols and ports
In order to achieve the feat of sending high
resolutionDVIvideo,audio,RS232serial
andfourUSBstreamsacrossastandard
GigabitEthernetnetwork,ALIFunitsrely
upon a combination of industry standard
protocols. These protocols operate at
Layer 4 (the Transport layer) in the OSI
model, i.e. another level of sophistication
above the Layer 2 and 3 techniques
enacted by the switches and routers
through which it travels.
TCP
TCP (Transmission Control Protocol) is a
fundamental internetworking standard that
allows a reliable data delivery route to be
established between two hosts or devices.
To ensure reliability of data transfer, TCP
employs various techniques: Flow control
to regulate data flow to suit the receiver,
error detection to locate and replace
corrupted packets, and congestion control
to avoid swamping a busy network.
By their nature, the techniques employed
for TCP connections impose a certain
latency to the connections. For this reason,
TCP is used by ALIF to handle the slightly
less time-sensitive, but highly accuracy-
sensitiveRS232serialandUSBdatalinks.
UDP
Like TCP, UDP (User Datagram Protocol)
is a fundamental standard that provides
a data route between hosts and devices.
However, unlike TCP, it does not carefully
regulatethelink.Oncesetup,datais
sent to receiver(s) without knowledge of
either their status or that of sent data.
For this reason, UDP is totally unsuited
for important control signals, however,
it is ideal for data streams that can easily
cope with the odd lost frame: video and
audio. The great advantage of UDP is that
its latency is minimal and it also allows
one sender to communicate with more
than one receiver. As such, UDP is a key
component of multicasting.
Ports used
ALIF units make use of various ports in
order to achieve their various parallel data
transfers:
Data Protocol Port
RS232serial TCP 3030
USB TCP 3040to3050
Control TCP 3000
Video UDP 1237
Audio UDP 3020
If ALIF communications are required to
crossarewallwithinthenetwork,these
ports must be opened to allow access.
ALIF transmier video sengs
EachALIFtransmitterincludescontrols
to help you customise how video data is
transmitted.Whenconguredcorrectlyfor
the application, these can help to increase
dataefciency.
Background Refresh
The transmitter sends portions of the
video image only when they change. In
order to give the best user experience,
the transmitter also sends the whole
video image, at a lower frame rate, in the
background. The Background Refresh
parameter controls the rate at which this
background image is sent. The default
value is ‘every 32 frames’, meaning that a
full frame is sent in the background every
32frames.Reducingthisto‘every64
frames’ or more will reduce the amount of
bandwidth that the transmitter consumes.
Onahigh-trafcnetworkthisparameter
should be reduced in this way to improve
overall system performance.
Colour Depth
This parameter determines the number
ofbitsrequiredtodenethecolourof
every pixel. The maximum (and default)
value is ‘24 bit’. By reducing the value
youcansignicantlyreducebandwidth
consumption, at the cost of video colour
reproduction.
Peak Bandwidth Limiter
The transmitter will employ a ‘best effort’
strategy in sending video and other data
over the IP network. This means it will
use as much of the available network
bandwidth as necessary to achieve
optimal data quality, although typically the
transmitter will use considerably less than
the maximum available.
In order to prevent the transmitter
from ‘hogging’ too much of the network
capacity, you can reduce this setting to
place a tighter limit on the maximum
bandwidth permissible to the transmitter.
Frame Skipping
FrameSkippinginvolves‘missingout’video
frames between those captured by the
transmitter. For video sources that update
only infrequently or for those that update
veryfrequentlybutwherehighdelity
is not required, frame skipping is a good
strategy for reducing the overall bandwidth
consumed by the system.
Glossary
continued