Digital Imaging and Communications in Medicine (DICOM)

7.2 Services and Data 125
simply meant to identify each message during its brief life span. When an
Rsp message is constructed, the message ID from the Rq message is copied
into the (0000,0120) field, this is how DICOM pairs up requesting and responding
messages. Imagine a busy DICOM network on which the same
AE can receive dozens of DIMSE messages each second. The message ID in
this case becomes the only way to distinguish among them, to keep track of
the requesting AEs, and reply to them correctly.
6. The Status field, which is always set to 0000 in the C-Echo-Rsp message, indicates
success. This means that the peer AE has received the C-Echo-Rq,
understood it, and replies back with a success C-Echo-Rsp. We really do
not have other options for C-Echo status; simply receiving a C-Echo-Rsp
reply implies that C-Echo was successful. The C-Echo message is considered
failed only if no response returns during the preset timeout interval
(several seconds). Nonzero values in the Status field are used by the other
DIMSE services to return various warning or error messages in case something
goes wrong. For example, if a DIMSE service tries to print an image
and the image is not available, an error value is reported in the Status field
for that service.
With all this knowledge, take a look at the following Table 22 sample C-Echo-
Rq in its final DICOM encoding. Just like any DICOM command object, it follows
implicit VR data encoding (default DICOM encoding, see Table 5).
The sequence of 68 bytes in the binary row is exactly what will be sent on a
DICOM network from one AE to another as the C-Echo-Rq. Note that this sequence
is in Little Endian order (see 5.2); when in multibyte numbers, the least
significant bytes go first. In our example, bytes number 15 and 16 form element
number e = 0002 = 02 (least significant byte) and 00 (most significant byte).
The C-Echo-Rsp is built in a similar manner and returned as shown in
Table 23. We marked the fields that changed in the C-Echo-Rsp compared
to the C-Echo-Rq. First is the command field (0000,0100), which reflects the
change in the command type. Second is the message ID, which in the C-Echo-
Rsp is stored in (0000,0120) and corresponds to the value in (0000,0110) in the
C-Echo-Rq.
In brief, as you can see, there is really nothing baffling about the C-Echo
DIMSE service; nearly all service parameters are fixed. If you are writing
DICOM software, the only parameter you should generate in the C-Echo protocolvis
the message ID number. It has to be different for each new C-Echo
request, and matched in the C-Echo-Rsp (0000,0120) field. As long as this is
done, you are good to go.