Balance DICOM's Bandwidth Demands

Instructions for how to manage a deluge of digital-image traffic

Modern MRIs, X-rays, ultrasounds, and CT and PET scans are stored and moved as digital images. Studies can be huge. Typical MRIs contain 10 files of 20 to 30 megabytes (MB) each. Digital images swell to consume most of the bandwidth in limited-capacity WAN links without regard for other applications. Multiple locations, separate imaging facilities, and remote clinicians exacerbate the problem, as images must traverse WAN links multiple times.

DICOM (Digital Imaging and Communications in Medicine) is the global industry standard for transfer of radiological images such as MRI, CT, PET, ultrasound, and mammography. DICOM enables heterogeneous diagnostic and therapeutic equipment and systems to exchange images.

DICOM images are important and must traverse the healthcare network dependably and consistently. However, the images can't be allowed to dominate the network and impose poor performance on all other traffic, including equally important but much more time-sensitive applications. Effectively managing DICOM is a balancing act.

Steps:

1. Identify and differentiate digital image traffic.
   
   Enable traffic discovery. PacketWise automatically identifies DICOM, NetBios-IP, NFS, FTP, and RSH, all commonly used in image transfers.
   
   If your image transfers don't use DICOM or another autodiscovered type of traffic, create your own traffic class using your own criteria (for example, port number or server). Make sure you have traffic classes for your image traffic in both the Inbound and Outbound branches of the traffic tree.
   
   If you want to distinguish one DICOM application from another (if you wanted, for example, to treat their traffic differently), then you can divide your DICOM traffic class into multiple, separate classes for different applications.
   
   The remainder of this recommendation references DICOM only. If the bulk of your image data is in another class, or if you are managing one component of DICOM, the same concepts apply.
   
   For background information, see Traffic Tree Overview and/or Traffic Classification Overview.
   
   
2. Analyze the impact of DICOM traffic on your network.
   
   Look at the Top Ten classes. See if DICOM is there. What percentage of the traffic is DICOM?
   
   Consult the Monitor Traffic window to look at DICOM traffic data and get an idea of bandwidth trends. Observe the measurements for current rate, one-minute average, and peak rate. Compare the peaks to other traffic classes.
   
   Create utilization graphs for your DICOM classes that cover the last week. Is there sustained or frequent high bandwidth usage?
   
   
3. Decide an appropriate amount of bandwidth to reserve for DICOM images even during times of much network demand. This will be the minimum size for your DICOM partition. If DICOM doesn't need this whole amount at any time, the excess will be loaned to other traffic (it's never wasted). And if DICOM is very active while there is little demand for other applications, DICOM will be able to access more bandwidth than this minimum indicates.
   
   The minimum you choose depends on the nature of your other traffic, the volume of your other traffic, your image volume, and other factors. An appropriate size might be 20 percent of your network capacity.
   
   For background information, see Sizing a Static Partition.
   
   
4. Determine a maximum size for your DICOM partition, probably the whole link. DICOM will be able to access this additional bandwidth depending on the presence and urgency of other traffic. After all, you probably don't want to contain DICOM to a minority percentage of the link if there is little or no other traffic.
   
   

5. Create a partition to set the total amount of bandwidth for each DICOM traffic class. Use the minimum percentage you determined earlier, select burstable, and either leave the limit blank or enter the maximum you determined in the previous step.
   
   For example, you might enter 20%, burstable, blank.
   Remember to create partitions for both the Inbound and Outbound branches.
   
   For background information, see Partition Overview.

6. Set a rate policy on each of your DICOM classes. A rate policy will indicate the relative importance of your image traffic so that PacketWise knows how to distribute excess bandwidth. In addition, your DICOM traffic will gain the benefits of TCP Rate Control and have fewer retransmissions that waste bandwidth.
   
   In the rate policy, use a guaranteed rate of 0 and a priority of 2 or 3 with no limit.
   
   For background information, see Policy Overview and Priority Guidelines.
   
   
7. Turn on PacketWise's compression. This feature offers lossless compression for DICOM and other types of network traffic by creating compression tunnels between compression-enabled PacketShapers. No manual configuration is required, as the PacketWise application-specific plug-in architecture automatically selects the algorithm that will yield the best compression ratio for DICOM and every other compressible application or service type.
   
   The compression feature also allows you to define which hosts are allowed to send data through the Xpress tunnel (such as hosts in radiology labs), or specify which PacketShaper units can be a tunnel partner (tunnel discovery partner).