Criteria for Network Selection

The telecommunications medium chosen for telepathology programs is dependent by sev-eral criteria, like cost, bandwidth, availability or reliability. In most forms of telemedicine, the choice of the telecommunications medium is essential for the data transfer perform-ance. The final decision often results in a compromise, since considering the advantages and disadvantages, there is a conflict of aims. The most obvious is cost and bandwidth, since both are related with each other. From this point of view the PSTN is lowest and ATM is highest.

Availability, Standardization and Network Costs

Availability

Cost (=bandwidth) PSTN

ISDN

ATM

Figure 12: Choice of telecommunications network for telemedicine applications [Wootton, 1997/(1), p. 399].

58Further network descriptions see: Ausseresses, 1995, p. 150; Wootton, 1997/(1), p. 394; Schwarzmann, 1995, p. 210; Hufnagl, 1999, p. 95. Another schedule of telecommunication link’s capacity can be found at Swett, 1995, p. 594.

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Network Availability

Public switched telecommunications services are almost available at every place all over the world. Yet channel capacity is often small [Schwarzmann, 1992, p. 385]. The same it is with ISDN connections, which are also available at many places and at very moderate cost.

They can be used easily nearly world-wide. In contrast, high bandwidth / ATM are expen-sive and not universally available.

Considering the situation of developing countries, there are often only public switched telephone networks available. That is why at a trial between the United Arab Emirates (Abu Dhabi), the Kingdom of Saudi Arabia (Riyadh) and a consultants at Cambridge, MA, public dial-up telephone lines were tested, in combination with multiplexing technology and image compression for international transmission of both - video and high-resolution digital images. The idea of testing conventional telephone lines was that telepathology services, which would require access to fibre optics or satellite communications, remain unrealistic from both - a practical and a cost-benefit standpoint at many third world coun-tries. It is to assume that most third world countries anyway will not employ dynamic-robotic systems due to the high prices and the high technical complexity (problem of maintenance). Yet, if static telepathology systems are utilized, questions like speed or size of files are not so important, but robust and reliable connections are desirable to provide affordable access to telemedicine services by using existing telecommunications infra-structures [Goldberg, 1994, pp. 1495-1500].

Problem of Standardization

Unfortunately in some countries telecommunications network are not fully standardized yet. They show differences between sophisticated digital communication and standard analogic transport at remote places. In some countries one has to overcome different and often idiosyncratic regulations. In addition sometimes different telephone carriers enhance the problem of coordination and integration, too [Puskin, 1995/(2), p. 126]. That makes telepathology introduction and data exchange complicated. Industrialized countries mostly offer access rooters, which give access to digital public (or private) network, and S.O.

standard interfaces, with a warranted flow of 128 kb/s.

Problem of Cost

”Expense is an important factor in the choice of systems” [Eide, 1994].

Efficiency and cost are important factors of influence for the selection of communication links [Weinberg, 1995, p. 833; Schwarzmann, 1992, p. 385]. Due to the expense of braod-band satellite transmission, and the low performance of copper phone lines, ISDN and fibre-optic networks are mostly the favored modes [Houtchens, 1995, p. 113].

Kind of System and Kind of Service

Further criteria for network selection are the kind of services (primary diagnosis, second opinion consultations, education) and the kind of systems (static / dynamic) which should be used. For example, if primary diagnostic should be executed by dynamic telepathology, high communication bandwidth should be available in order to handle remote controlled features with a satisfying speed. Live images need at least ISDN or T1 lines, with a high speed and a better line quality than standard dial-up phone lines would offer. In opposite, low resolution still images can be transmitted via common telephone lines [Kayser, 1995/(2), p. 197; Schwarzmann, 1992, p. 385].

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Transfer Time and Image Size

Preliminary analyses showed that the average time of 25 seconds for the transmission of one full color image is the upper limit [Dzubur, 1996, p. 269; Oberholzer, 1993, p. 1078;

Barker, 1998, PP: 1411-1416]. The time needed for image transfer is dependent on net-work performance and image size. The relevant parameters for performance measure-ment or communication capacity of networks are bandwidth, bit rate, and digital or ana-logue signaling mode. Bandwidth is expressed as the data transport rate, measured in bits per second (bps), kilobits per second (kbps or kbit/s) or megabits per second (Mbps), depending on the medium.

The size of an image depends on its quality. In general the size of an image compared to an textual file is much higher. That is why the time (seconds) required to transmit a still image, or the bandwidth (bit/second) required to transmit a moving image is quite high⁵⁹. In practice, to keep cost and time associated with saving, moving, and viewing of patho-logical full-motion, moderately high-resolution, color audio-video images as small as pos-sible, and to overcome the bottleneck of transmission speed, the information of an image is often reduced by data compression [Perednia, 1995/(3)]. For most still picture transmis-sion the standard JPEG is used, for moving scenes the standard MPEG. Image comprestransmis-sion algorithms are divided into two categories: lossy and loss-less. Lossy algorithms are capa-ble of up to 20:1 or 30:1 ratios. Most loss-less algorithms only allow compressed images up to 3.1 ratios or less [Mun, 1995]. At this point there occurs a conflict of aims - transmis-sion speed and image quality at live imaging systems.

To overcome this conflict, one possibility is to examine slides by panoramic scanning, since only a small fraction of the rate required to transmit all of the data in a given field has to be transferred [Weinstein, 1987, p. 651]. If there is not enough bandwidth available for robotic telepathology systems, Richter recommends to prepare data files in advance. Dif-ferent image modalities and details could be scanned and recorded before the video confer-ence, so that these can be played in during the conference. In this way the search and focusing for the appropriate image detail could become less time consuming [Richter, 1995, p. 299].

Another possibility is to use low resolution images for the low magnification rough scanning process, but high resolution true color images for the detailed examination of specimen fields of high magnification. A map of the tissue slide with a position marker will indicates from which area of a sample the high magnification field has been selected.

This helps to avoid unnecessary scanning procedures. Hybrid systems used for example in Tromsoe and Basel are already employing this variant. If necessary several ISDN chan-nels could be bundled for a short time interval to provide increased channel capacity [Schwarzmann, 1992, p. 387]. Because focusing of images in microscopy is often time-consuming till the best focus is found, the use of autofocus facilities could also save plenty of time [Schmid, 1996/(2), p. 931; Schwarzmann, 1996]. A comparable alternative is to use intelligent JPEG-compression. JPEG parameters could be implemented as a self adapting procedure, which adjusts the image resolution according to the operators

59 A single medium-resolution image 640 x 480 pixels in 24-bit color has a file size in uncompressed format of approximately 1 MB.

The large size of image files stresses processing capacity, and 16 MB of RAM or more is the current minimum specification for an imaging workstation’s personal computer that uses Windows based software (Microsoft corporation, Redmond, Washington, USA) [O’Brian, 1998, p. 149].

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tion. If the operator scans the slide roughly the image resolution is reduced automatically, and if he examines an interesting field of view for a longer time the image resolution improves again up to the maximum value of the camera resolution [Schmid, 1996/(2), p.

931; Schmid, 1996/(1), p. 480].

However, although JPEG compression was used at a trial with static image in Sweden, average image transmission for low, medium and high resolution images was approxi-mately 5, 20 and 45 seconds with ISDN networks (1 x 64 kbit/s). This was judged as not sufficient by many users [Olsson, 1995, p. 240]. At the system in Vermont, bandwidth of 384 kbps with ISDN appeared to be adequate for the great majority of medical applica-tions. Nevertheless, users claimed to get the ability to change bandwidth, for choosing the most appropriate bandwidth for their specific need [Rici, 1997, p. 204]. (Further chapters discussing network speed see 16.2.2.8, 18.6.3.2, 18.6.6.7, and 19.1.2.2).

Reliability

Another problem of networks is the risk of lost data. Not rarely physicians waited in vain for the demanded images due to technical network problems. For example, at the

BERMED project in Berlin (Germany), loss of data was the reason that five of ten partici-pants stopped the usage of their telemedicine system [Boese, 1997, p. 12]. Never mind which network is used, telepathology systems have to guarantee a secure and professional access and data exchange over the network, with warranted telecommunications reliability [Allaert, 1999].

One criteria of reliability is the connection architecture. The question is, whether the link will be established permanent within a restricted area or between restricted participants, or whether the stations will be embedded in a net of users with changing partners, in the extreme case ‘world-wide’? The most secure connection architecture are the so called point to point connections, with dedicated wide band communication lines. This approach is the most expensive. The alternative is to use existing global networks, such as the switched-digital networks. The Internet may be the least secure network due to its easy accessibility.

In contrast, commercial switched-digital networks provide dependable performance but at much greater cost [O’Brien, 1998, p. 153].

11.2 Interface