Network Imaging Techniques Gilbert Held

Transcription

1 Network Imaging Techniques Gilbert Held Payoff Supporting imaging applications does not necessarily require an expensive network upgrade. This article discusses several inexpensive, timesaving techniques for protecting network performance while transmitting images across the LAN. Introduction Many network administrators have migrated a LAN to an Asynchronous Transfer Mode infrastructure or upgraded a Token Ring network to Fast Ethernet in an effort to support imaging applications. Although imaging is bandwidth intensive, there are several alternatives for optimizing their transmission over a LAN. Image transmission and printing can be enhanced by using software or by reconfiguring the LAN infrastructure. This article discusses five techniques for enhancing LAN -based imaging that may help users avoid investing in an expensive network upgrade. Transmitting Compressed Files Most images require a minimum of 50 to 100K bytes. If a 10Base-T Ethernet or a 16Mbps Token Ring network is being used, downloading an image can significantly effect network operations. For example, if a 10Base-T Ethernet LAN is operating at 10M bps with 100 active users, each user obtains approximately 100,000 bps of bandwidth. The average delay associated with a 100K-byte image file is eight seconds. If a network user needs to download multiple images, the activity of the workstation would have to be extended, which would result in an increased number of collisions as other network users attempt to perform other operations. More delays would result because each collision is followed by a jam signal during which no station on the network can attempt a retransmission. Compressing Images Prior to Transmission One method of minimizing the effect on network performance is to compress images prior to their transmission. Exhibit 1 illustrates the use of the program NetZIP developed by Software Builders, Inc. (Atlanta GA). This Windows-compliant program can create ZIP files, which are archived and compressed files, and includes a function that automatically extracts previously compressed and archived files. Other programs that perform similar compression and archiving functions include PKZIP/PKUNZIP from PKWARE, Inc. (Brown Deer WI), and ARJ, developed by Robert Jung and LHA. The latter is a public domain archiving program developed in Japan that can be obtained from several computer bulletin board systems in the US. Exhibit 1 illustrates the creation of an archive file named TIF.ZIP that contains 19 images. Using Software Builder's NetZIP Compression Program Exhibit 2 shows the files compressed in TIF.ZIP. The entries in the inner window include a column labeled percent that indicates the percentage of data reduction obtained through the compression of each file in the archive. The file SCREENOO.TIF shows only a 14%

2 reduction. However, each of the files include TIF images that were stored using compression. The use of NetZIP can further compress previously compressed images. Using the NetZIP Program to Archive Previously Compressed Images Using Lossy Compressed Images NetZIP, which is based on the compression used in the PKZIP program, uses a fully reversible, or lossless, compression. This means that the decompression of a previously compressed image results in the exact restoration of the original image. It is important to use lossless compression on text files. The loss of a small amount of detail in an image may not even be noticeable, however, the same loss in a text file could critically alter its meaning. The use of a lossy compression technique can substantially reduce data storage requirements as well as the amount of time required to transport images from a server to workstations. Joint Photographic Experts Group One of the more popular lossy compression methods, Joint Photographic Experts Group (JPEG), is a standard for full-color and gray-scale images. A group known as Independent JPEG developed two public domain software programs compression JPEG (CJPEG) and Decompression JPEG. Compression JPEG provides the capability to convert Graphics Image Format (GIF) and other image files into JPEG format, and decompression JPEG (DJPEG) decompresses a JPEG file back into a different image format. GIF is an image storage format popularized by CompuServe. A GIF image is stored using a lossless compression method known as Lempel-Ziv-Welch (LZW). By converting a GIF image into a JPEG image, users can save file storage space as well as time required to transmit the image on a LAN (see Exhibit 2). Exhibit 3 illustrates the display of the CJPEG help screen that is generated by entering the command line improperly or simply entering the name of the program without arguments. The quality N switch governs the compression quality of the program. A value of 0 for N results in the worst image quality and a value of 100 results in the best quality. For almost all image conversions, the program's default value of 75 results in an image that the naked eye cannot distinguish from the original. However, the differences in storage between the two can be considerable. To illustrate the potential data storage reduction from converting a Graphics Image Format image to a JPEG image, the CJPEG program was used with the following command entry: CJPEG ML3872F.GIF ML3872F.JPG In the command line, ML3872F.GIF represents a multiple listing image of a home stored in GIF format that was used as input to the program. The string ML3872F.JPG represents the name of the output file that contains the converted JPEG image. Because Disk Operating System only supports a three character extension, the extension.jpg is used by the program. Once the Compression JPEG program was executed, a directory listing indicated the following data storage requirements for the following files: ML3872F.GIF30,393 ML3872F.JPG109,618 The simple conversion of a GIF file to a JPEG file resulted in more than a 70% reduction in the storage requirements. Because many viewing programs have a JPEG file

3 format capability and can be used with World Wide Web browsers, many users may have all the software they need to convert GIF files to JPEG files. The conversion of images from a lossless to lossy compression format saves both time and server storage space. The CJPEG Help Screen C:\JPEG>cjpeg C:\JPEG\CJPEG.EXE: must name one input and one output file usage: C:\JPEG\CJPEG.EXE [switches] inputfile outputfile Switches (names may be abbreviated): -quality N Compression quality (0..100; 5-95 is useful range) -grayscale Create monochrome JPEG file -optimize Optimize Huffman table (smaller file, but slow compression) -targa Input file is Targa format (usually not needed) Switches for advanced users: -restart N Set restart interval in rows, or in blocks with B -smooth N Smooth dithered input (N= is strength) -maxmemory N Maximum memory to use (in kbytes) -verbose or Emit debug output -debug Switches for wizards: -qtables file Use quantization tables given in file -sample HxV[...] Set JPEG sampling factors C:\JPEG> Thumbnail Screens If storage of a large number of images is required, the effect of image transmission can be reduced on a LAN through the use of thumbnail screens. The term thumbnail screen represents a collection of images that are displayed by reducing the number of horizontal and vertical scans that make up each image. A thumbnail screen provides a preview of several images simultaneously. Instead of selecting one image after another from a server, a user can view many images on one screen before making a selection. The use of thumbnail screens to reduce LAN transmission has been successfully applied in the parts distribution and real estate industries, and is appropriate for any organization where users may need to view several images prior to selecting one. In a parts distribution environment, a single thumbnail screen provides the images of a dozen related parts. The user can quickly isolate the part that may be the focus of a customer query and display a full screen image of that part. Without the use of a thumbnail screen, the user may have to scroll through a dozen or more full screens of parts, which would impair network performance. Thumbnail creation programs and viewers are available as shareware from several vendors and can be purchased for as little as $5.00 per user under a site license agreement. Compared with the cost of upgrading to a new LAN infrastructure, the use of software to enhance network performance is a nominal expenditure. Network Segmentation There are certain network environments where the use of compression and thumbnail screens only minimally effect network performance because of the overall transfer of

4 images on the network. For such situations, it may be possible to segment the network if the requirement for image access is limited to a defined subset of office employees. A Segmentation Case Study: The Real Estate Office In a real estate office of 25 employees there are 12 agents and a support staff that includes administrative assistants, advertising clerks, contract document preparers, and managers. The office has a common LAN that agents use to preview images of homes stored on a server whenever a potential customer is brought into the office. The other office employees need to perform word processing, and access spreadsheets and data base programs. A few agents may bring customers into the office and begin to browse through stored home images, which can seriously interfere with the interactive query-response communications in progress. To alleviate the effect of the transmission of images on client/server applications, the network could be segmented. The top of Exhibit 4 illustrates a common Ethernet network prior to segmentation, and the lower portion of that illustration indicates the possible segmentation of the network. In this example the network used Novell's NetWare operating system. Segmenting an Ethernet LAN to Minimize the Effect of Image Transmissions One of the lesser-known features of NetWare is the ability of a file server to perform internal bridging to interconnect two LAN segments. Although Novell refers to this as LAN routing, the file server actually functions as a transparent bridge to interconnect the two network segments. The only hardware required to perform bridging is a second Ethernet adapter card that can be obtained for less than $100. Implementing segmentation is relatively inexpensive. In the segmented LAN illustrated at the bottom of Exhibit 4, the 12 agents that require the downloading of images from the server were placed on a common segment. Because an agent typically converses with the client as images are being displayed on their monitor, a delay of ten seconds or more may hardly be noticeable and should not impair productivity as one agent's image requests compete with those of other agents. Image requests are not the only network traffic, however. Support staff members may be trying to create documents on the server when an agent is initiating an image transfer. Although the Ethernet carrier-sense, multiple access with collision detection (CSMA/CD) protocol acts as an arbitrator to provide each station with equivalent LAN access, the large number of frames required to transport each image can only interfere with the frames carrying interactive query-response traffic used to prepare a document. This interference causes delays for users who are entering data into information fields on the document. By segmenting the Ethernet LAN into two segments, the effect of transmitting images on query-response applications is minimized. Reducing the Collision Window On Ethernet Lans In a Carrier Sense Multiple Access/Collision Detection network, stations listen to the network prior to transmitting. If two stations are distant from one another, the probability that one station fails to hear the network and initiates its' own transmission increases. Thus, as the distance between stations increases, the probability of collisions increases. When the first station detects a collision, it transmits a jam signal that precludes transmissions from

5 other workstations for the duration of the Ethernet frame, which causes LAN performance to suffer. Thus, there is a high correlation between reduced Ethernet performance and the distance between very active workstations. Although it is probably impractical to relocate more than a few very active workstations to reduce the distance between stations, it may be relatively easy to reduce the cable distance between the two most active stations on most networks. The file server and print server are normally the most active devices on a LAN. The top of Exhibit 5 illustrates an Ethernet network where a file server is located at a relatively long cable distance from a print server, resulting in a long collision window. The lower portion of Exhibit 5 illustrates a reduction in the collision window between highly active network stations obtained by simply recabling one server closer to the other. Based on the use of performance monitoring equipment, a reduction in network use between 4 and 7% has been obtained by relocating a few active workstations closer to one another. Reducing the Ethernet Collision Window Conclusion Although the transmission and printing of images on a LAN represents a bandwidthintensive application, it may not require a costly and time-consuming LAN upgrade or replacement. As indicated in this article, there are several techniques the communications manager can implement, either individually or with the support of an organization's applications programming branch. The use of a compression and archiving performing program may be obtainable on a site license basis for less than a few hundred dollars and can reduce the time required to transmit images by half. If the scanner and digital camera software support the storage of images in Joint Photographic Experts Group format, as most do, image data storage and transmission time can be reduced even more than they are by using lossless compression techniques. Similarly, a thumbnail capability is included in many commercial picture viewing software products and its use may not require any additional cost. If software does not provide sufficient relief and LAN congestion remains a problem, hardware solutions should be evaluated. Users should first consider recabling an Ethernet LAN to reduce the collision window. Recabling costs are usually minimal. Another possible solution is network segmentation, however, this is a more costly approach that requires the use of a bridge. Before embarking on an expensive and time-consuming network upgrade, network administrators may want to try implementing one or more of the techniques described in this article for reducing the effect of image transmission on LAN performance, saving time and money. Author Biographies Gilbert Held Gilbert Held is director of 4-Degree Consulting, a Macon GA-based high-tech consulting group. He is an internationally recognized author and lecturer, having written more than 40 books and 300 technical articles. He earned a BSEE from Pennsylvania Military College, an MSEE from New York University, and MBA and MSTM degrees from The American

6 University. He has been selected to represent the US at technical conferences in Moscow and Jerusalem and has received numerous awards for excellence in technical writing.

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