3.2 Network Interoperability

The telephone industry is moving toward an all-digital network that integrates voice and data over a single telephone line from each user to the telephone company equipment. Integrated services digital networks (ISDNs) are being implemented with the intent of providing worldwide communications for data, voice, video, and facsimile services within the same network. The basic principles and evolution of ISDN are outlined in CCITT recommendation 1.120 (1984). One of the principles is that a layered protocol structure should be used to specify the access procedures to an ISDN, and that the structure can be mapped into the OSI model. However, in essence, ISDN ignores levels 4 to 7 of the OSI model. Standards already developed for OSI-applications can be used for ISDN, such as X.25 level 3 for access to packet switching networks. In addition, whenever practical, ISDN services should be compatible with 64-kbps switched digital connections. The 64-kbps frame is the basic building block of ISDNs that expect to use the plant and equipment of existing telecommunications systems.

Interactive television and telemedicine applications require transfer rates that exceed the original ISDN specifications. Broadband ISDN (BISDN) addresses this need [11]. With the advent of BISDN, the original concept of ISDN is referred to as narrowband ISDN. The new BISDN standards are based on the concept of an asynchronous transfer mode (ATM), which will include optical fiber cable as a transmission medium for data transmission. BISDN standards set a maximum length of 1 km per cable, with expected data rates of 11, 155, or 600 Mbps.

ATM is a member of a set of packet technologies that relay traffic through nodes in an ISDN via an address contained within the packet. Unlike packet technology such as X.25 or frame relay, ATM uses short fixed-length packets called cells [11]. This type of service is also known as cell relay. An ATM cell is 53 bytes long, with the first 5 bytes called a header, and the next 48 bytes called an information field. The header contains the address and is sometimes referred to as a label. In contrast, frame relay uses a 2-byte header and a variable-length information field.

The X.25 protocol was developed for use over relatively noisy analog transmission facilities and addresses only the physical, data link, and network layers in the OSI model [11]. It is a protocol that was developed to ensure reasonably reliable transport over copper transmission facilities. To accomplish this, every node in an X.25 network goes through a rigorous procedure to check the validity of the structure of the message and its contents and find and recover from detected errors. It can then proceed with or abort communications, acknowledge receipt, or request retransmission, before passing the message along to the next node where the process is repeated. In total, it can be a relatively slow and time-consuming process, constraining throughput.

The extensive checking of the X.25 may be replaced by one or two simple checks: address validity and frame integrity. If the frame fails either check, it is discarded, leaving the processors at the ends of the connection to recover from the lost message. Frame relay's strength (i.e., robust error checking and recovery) is ultimately its weakness. It is suited for data communications, but not flexible enough to cope with the variety of multimedia traffic expected to be running on an ISDN. In the future, traffic on an ISDN will be a mixture of transactions devoted exclusively to data, voice, or video and transactions embedded with combinations of data, voice, video, and static images.

The following paragraphs compare performance of cell and frame relay schemes in network scenarios. If the interval between voice samples varies too much, problems (called jitter) arise with reconstructing the signal. If the intervals become very large, echo becomes a problem. ATM's cell length is chosen to minimize these problems. It will be necessary to ensure that data messages do not impose overly long delays to voice traffic (or similar services that count on periodic transmission of information). Frame relay is just as likely to insert a several-thousand-byte data frame between voice samples as it is a 64-byte frame, playing havoc with the equipment trying to reconstruct the voice traffic. The short, fixed cell size ensures that the cells carrying voice samples arrive regularly, not sandwiched between data frames of varying, irregular length. The short cell length also minimizes latency in the network, or end-to-end delay. The ATM cell size is a compromise between the long frames generated by data communications applications and the needs of voice. It is also suitable for other isochronous services such as video. Isochronous signals carry embedded timing information or are dependent on uniform timing information. Voice and video are intimately tied to timing.

Frame relay also suffers from a limited address space. Initial implementations provide about 1000 addresses per switch port. This is adequate for most corporate networks today, but as needs expand beyond corporate networking to encompass partner corporations and home offices, more flexibility will be needed. Speed and extensibility (expandability) are also issues with frame relay as a solution to network interoperability. As network transmission capabilities grow, so will transmission speeds, which, in turn, will lead to faster switching speed requirements. As the network expands, the work and functions of the switches will grow. The increased requirements dictate very fast, extensible switches. Network switches, however, do not cope well with the variable-length messages of the frame relay. They require identical fixed-length messages, preferably short ones.

The ATM standard does not include parameters for rates or physical medium. Thus, different communications networks can transport the same ATM cell. With ATM, a cell generated from a 100 Mbits/sec LAN can be carried over a 45 Mbits/sec T3 carrier system to a central office and switched into a 2.4 Gbits/sec SONET system.

3.3 Telemedicine Applications Compatibility

A telemedicine application can be a complex collection of a variety of objects. In general, it can be an integrated session containing text, rich text, binary files, images, bitmaps, voice and sound, and full-motion video. The utility of these applications will in part depend on their ability to accommodate these different types of data. The development and use of standards which allow interoperability between systems and applications developed by different manufacturers is a critical part of this process.

Health Level Seven (HL7)

Health Level Seven (HL7), which dates back to 1987, is a standard for exchanging clinical, administrative, and financial information among hospitals, government agencies, laboratories, and other parties. The HL7 standard covers the interchange of computer data about patient admissions, discharges, transfers, laboratory orders and reports, charges, and other activities. Its purpose is to facilitate communication in health-care settings. The main goal is to provide a standard for exchange of data among health-care computer applications and reduce the amount of customized software development. The HL7 standard focuses primarily on the issues that occur within the seventh level of the OSI.

The standard is organized to address the following issues:

• Patient admission, discharge, transfer, and registration

• Patient accounting (billing) systems

• Clinical observation data such as laboratory results

• Synchronization of common reference files

• Medical information management

• Patient and resource scheduling

• Patient referral messages for referring a patient between two institutions

In the future, the HL7 Working Group expects to undertake development of standards in the following special interest areas:

• Decision support

• Ancillary departments

• Information needs of health-care delivery systems outside of acute care settings.

Digital Imaging and Communications in Medicine (DICOM)

The American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA) developed DICOM to meetthe needs ofmanufacturers and users ofmedical imaging equipment for interconnection of devices on standard networks [12]. DICOM, which is described in detail in the chapter "Medical Image Archive and Retrieval," has multiple parts that facilitate expansion and updating and allow simplified development of medical imaging systems. DICOM also provides a means by which users of imaging equipment are able to exchange information. In addition to specifications for hardware connections, the standard includes a dictionary of the data elements needed for proper image identification, display, and interpretation. The future additions to DICOM include support for creation of files on removable media (e.g., optical disks or high-capacity magnetic tape), new data structures for X-ray angiography, and extended print management.

Although the HL7 standard enables disparate text-based systems to communicate and share data and the DICOM standard does the same for image based systems, efforts to link compliant systems have met with limited success. The goal of an initiative known as Integrating the Healthcare Enterprise (IHE) is to foster the use of HL7 and DICOM and ensure that they are used in a coordinated manner. The initiative is a joint undertaking of the Healthcare Information and Management Systems (HIMS) and the Radiological Society of North America (RSNA). The major impact of DICOM is expected to be on PACS because it can serve in many interfacing applications. For example, DICOM may be used as the interface standard among CT and MR imaging units, and printer systems.

4 Conclusion

During the early 1900s, the evolution of the radio, which exploited electromagnetic waves, initiated a series of technology developments that form the foundation for modern telemedicine systems. Facsimile first attracted attention in 1924 when a picture was sent from Cleveland to the New York Times. Nevertheless, it came into widespread use in medical applications only during the past 30 years. Several clinical applications have experimented with telemedicine systems that use evolving technologies such as high-resolution monitors, interactive video and audio, cellular telephones, the public switched telephone network, and the Internet.

Clinical applications that have experimented with telemedi-cine include radiology, pathology, cardiology, orthopedics, dermatology, pediatrics, ophthalmology, and surgery. As telecommunication technologies improve and become available, reliable, easy to use, and affordable, interest in telemedicine systems in clinical applications will increase. Today, few health-care organizations routinely use the entire range of capabilities resident in computer-based telemedicine systems to integrate, correlate, or otherwise manage the variety of multimedia data available for patient care. Use of the wide-range of capabilities inherently available in modern telemedi-cine technology is probably more commonplace in biomedical education and research applications than in clinical applications focusing on patient care.

Patient care places emphasis on reliability of data and accuracy and timeliness of diagnosis, decision-making, and treatment. The technical and clinical issues in telemedicine systems include quality of images, videos, and other data, the percentage of the patient examination that can be accomplished using existing telemedicine technologies, and the integration of telemedicine service with current clinical practices.

Basic technological components that affect the transmission, storage, and display of multimedia data presented in this chapter indicate the complexity of telemedicine systems. Further research is required to identify performance and interoperability requirements for telemedicine systems that will assist the care provider in achieving better outcomes for the patient.


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