RAD-Star User’s Guide
21
© 2015-2016 Intrepid Control Systems, Inc.
Version 1.2 - June 1, 2016
In this section we provide more details on the wrapper and its exact format for those who want
to know more about the inner workings of the RAD-Star.
Wrapping Process
The RAD-Star does the following with each frame received on its BroadR-Reach ports:
1.
It records the time that the frame was received.
2.
It computes a 10-byte RAD-Star header that includes the timestamp and control
information.
3.
It places the original frame into a buffer, including its header and footer.
4.
It constructs a new wrapper frame consisting of the wrapper’s Ethernet header, followed
by the RAD-Star header, the original frame, and the wrapper’s Ethernet footer.
Figure 19 shows graphically the overall format of a wrapped frame.
14
Wrapper
Frame
Header
RAD
‐
Star
Header
Original
Frame
Header
Data
46
to
1,500
Original
FCS
Wrapper
FCS
10
14
4
4
Original
Frame
Wrapped
Frame
Figure 19: Overall Wrapped Frame Format.
The wrapped frame consists of a wrapper header and footer encapsulating the
original frame (including its header and footer) prefixed by the special RAD-Star header.
Splitting and Reassembly of Long Frames
The addition of the wrapper frame header, wrapper frame footer and RAD-Star header mean
that a wrapped frame is 28 bytes larger than the original. If a frame is received by the RAD-
Star that is close to the maximum size allowed on an Ethernet link, the wrapped frame would
become too large to transmit.
Note:
Jumbo frames (those with lengths exceeding the usual
limits) are not supported by the RAD-Star.
The RAD-Star will encapsulate incoming messages in a single wrapped frame if they are up
to 1,498 bytes in length. If the incoming message is longer, it is split into two pieces: the first
carries the first 760 bytes of the original message (including its header) and the second carries
the remaining bytes (including its footer). The receiving device reassembles the original frame,
guided by values in special fields in the RAD-Star header.
