Extron Electronics VN-MATRIX User Guide - Manual (Page 132)

VN-MATRIX User Guide Appendix C: Browser Configuration
Page 132 I458GB issue 6
Comparison between UDP and TCP
The two different transport layer protocols are compared in the following table:
UDP TCP
Connectionless Connection
oriented
Datagrams must be formatted in application Automatically generates datagrams from
bitstream
Multiple applications using ports Multiple applications using ports
Unreliable (best effort) communication Reliable (guaranteed) communication
No flow control (must be in application if
required) Flow control (deals with out-of-order data and
error corrections)
No error recovery Error recovery
Multicast possible (one to many) One to one only
Minimum latency Significant latency
Clearly the most significant difference is that UDP is “best effort” and TCP is “guaranteed delivery”.
TCP is used in most networks for such tasks as exchanging file information between nodes, because
here absolute accuracy is required and because the delivery requirement is usually “point to point”.
The table implies several different methods of transmission, for example:
ƒ Broadcast where a message goes to all nodes on the network. Must be UDP.
ƒ Unicast where a message goes from one node to another. Must be TCP.
ƒ Multicast where a message goes from one node to many nodes, each assigned to a
multicast address group. Must be UDP.
The table has great significance in respect of transmitting images over networks. A few of the points
that arise from it are as follows:
ƒ Many if not most applications will involve the transmission of an image from one node to
many nodes, i.e. multicast operation is required, mandating the use of UDP.
ƒ Many if not most applications will require minimum latency, again indicating the use of UDP.
ƒ But UDP does not provide “reliable” communication, so any application has to take into
account the effect of lost data packets, out-of-order packets, and errors.
ƒ Time critical data like video is often sent using an additional protocol called RTP (Real Time
Protocol). This “time stamps” the packets and can be used to endow UDP with some measure
of flow control.
These are not trivial issues. If a data bit goes missing in a dedicated digital video link like SDI the
result is not catastrophic – its effect is at a single pixel level within a single video frame. If a data bit
goes missing in an MPEG stream it can prevent the reconstruction of an entire group of images.