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Page 28
J-Series Data Radio – User Manual
Issue 09-10
Legacy Serial Support
The serial port device servers provide transparent encapsulation
over IP (IP tunnelling) of RS-232 serial data . It redirects raw serial
data to remote hosts via the TCP or UDP IP protocol.
Note: The J-Series terminal servers support Internet Protocol
version 4 (IPv4).
Data arriving at the J-Series serial port is packetized as defined
by the configuration of the packet layer parameters. The raw data
frame is then inserted into the data section of the TCP/UDP packet/
datagram for transmission to the remote host. The remote host
can be either another J-Series terminal server or some third party
product.
The received serial data is extracted from the UDP/TCP datagram
end sent out onto the serial port.
For example, PC applications can communicate with remote
devices, with serial interfaces, by either sending/receiving data via
IP directly to the terminal server at the remote radio, or via the PC
serial port to a local radio which transports the raw serial data via
IP to the remote terminal server as shown in the diagram below.
Alternatively, the entry point to the J-Series network could also be
an RS-232 serial connection as shown below.
Differences between TCP and UDP
TCP is a Point to Point (PTP) connection-oriented protocol, which
means that upon communication it requires connection overhead
to set up and maintain the end-to-end connection. The features of
TCP are:
• Reliable - TCP manages message acknowledgment,
retransmission and timeout. Many attempts to reliably deliver the
message are made. If it gets lost along the way, the server will
re-request the lost part. In TCP, there’s either no missing data, or,
in case of multiple timeouts, the connection is dropped.
• Ordered - if two messages are sent along a connection, one after
the other, the first message will reach the receiving application first.
When data packets arrive in the wrong order, the TCP layer holds
the later data until the earlier data can be rearranged and delivered
to the application.
• Heavyweight - TCP requires three packets just to set up a
socket, before any actual data can be sent. It handles connections,
reliability and congestion control. It is a large transport protocol
designed on top of IP.
• Streaming - Data is read as a “stream,” with nothing
distinguishing where one packet ends and another begins. Packets
may be split or merged into bigger or smaller data streams
arbitrarily.
• Only supports uni-cast point-to-point (PTP) communication.
UDP is a simpler message-based connectionless protocol. In
connectionless protocols, there is no effort made to setup a
dedicated end-to-end connection. Communication is achieved by
transmitting information in one direction, from source to destination
without checking to see if the destination is still there, or if it is
prepared to receive the information. UDP datagrams cross the
network in independent units.
• Unreliable - When a message is sent, it cannot be known if it will
reach its destination; it could get lost along the way. There is no
concept of acknowledgment, retransmission and timeout.
• Not ordered - If two messages are sent to the same recipient, the
order in which they arrive cannot be predicted.
• Lightweight - There is no ordering of messages, no tracking
connections, etc. It is a small transport layer designed on top of IP.
• Datagrams - Packets are sent individually and are guaranteed to
be whole if they arrive. Packets have definite bounds and no split
or merge into data streams may exist.
• Supports Multi-cast protocols – Point to multipoint and multi point
to multipoint.
Most SCADA type applications are well suited to the UDP protocol,
where the application layer provides loss of packet and error
recovery mechanisms.