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Health Care: Delivery, Education, Communication

Telemedicine Systems and Telecommunications


5. Information Transmission

All of the above relates to videoconferencing equipment using digital telecommunications. It is also possible to transmit video pictures using the ordinary telephone network, but, because a very high degree of image compression is required, the picture quality is rather poor.

In terms of network coverage, the conventional telephone is now widely available, even in the developing world; it is also a relatively inexpensive form of communication. More sophisticated digital telecommunications, such as ISDN, have more restricted coverage and higher costs, though they have major advantages over conventional telephony in terms of reliability and bandwidth. Satellite communications solve the coverage problem - providing truly global access - but remain expensive.

The choice of transmission method for any telemedicine application is, in practice, a compromise between what one would like and what one can afford. In practice, various trade-offs have to be made, which include:

  • cost,
  • availability of the service (i.e. the coverage),
  • bandwidth, reliability and quality of service.

5.1 Cost

Telecommunications costs generally are falling. In most parts of the world, there is increased competition in the telecommunications industry, because of the privatization of previously government-run departments and deregulation. Another factor in falling prices is technological development. Nonetheless, telecommunications costs may still represent a significant proportion of the cost of a given telemedicine application.

5.2 Availability of the Service

Despite the wide availability of network services, a common problem in telemedicine is the availability of network services over the 'last mile'. The 'last mile' describes the connection from the actual site of practice to the nearest telecommunication access point, such as the local telephone exchange. Providing a connection over this last mile is often where telecommunication becomes very expensive.

5.3 Bandwidth, Reliability and Quality of Service

An important factor in telemedicine is the time frame in which the information is required. For some applications rapid information transfer is needed, for example when telemedicine is being performed for emergency management. However, in less urgent cases information can be stored and transmitted at a slower rate, for later examination.

Bandwidth is the data-carrying capacity of the communication medium used. It is measured in bits per second (bit/s, often abbreviated to bps) and ranges from 1200 bit/s for some types of mobile phones to more than 1000 Mbit/s for transmission through a fibre-optic cable. Successful telemedicine has been carried out using a wide range of bandwidths.

The clinical information to be transmitted will dictate the minimum network bandwidth that can be used.[11] How this bandwidth is obtained between the sites involved in the study depends on a variety of different factors. The main questions will be what infrastructure is already in place at each site, and what communications are possible between the sites (both physically and through local telecommunication companies). Rural areas where patients are often most in need of telemedicine services are often those where network communication may be limited. Providing high-bandwidth connections to such sites can be very expensive.

Generally speaking, the problems of network reliability (in the sense of communications being possible or not), exemplified by the traditional telephone network with electromechanical switching, have been solved by the move to digital telecommunications. In fact, substantial portions of the telephone network are already digital, which is partly why the call quality has improved dramatically over the last 20 years.

A related aspect to the general reliability of a telecommunications link is the quality of service that can be guaranteed for the user. For networks in which the user is provided with a circuit from end to end (such as ISDN or the PSTN) there is little difficulty, but for networks in which the bandwidth is shared (such as the Internet) the bandwidth available to the user can be severely affected by what other users are currently doing. In some networks with shared bandwidth, it is possible to reserve bandwidth for the user at the start of the connection and then release it to the general pool at the end. Such quality-of-service techniques have yet to be standardized.

5.4 The Internet

Most people are now familiar with modem access to the Internet via the telephone network, and this method of communication can be used for telemedicine.[12-15] Such systems have the advantage of accessibility, since anyone who can access the Internet can reach the required sites (limiting access to legitimate users may then be a problem, and security issues may become important). The Internet is particularly useful for situations where a clinician may require access to data from home (such as a radiologist viewing an X-ray image) before giving some clinical advice, e.g. a consultant advising on a fracture seen by a junior doctor in the emergency department. It may also be possible to use these systems from areas where analogue telephone lines are the only ones available, which may be particularly helpful for remote sites where other means of communication are limited.[16]

Web servers on the Internet store data for subsequent distribution to users. This is a client/server architecture, which is a useful approach for telemedicine. The Internet can also be used to provide a virtual private network (VPN), i.e. to provide a private (secure) connection between two sites. A VPN offers a method of securely linking sites. The limitation is that both sites must have connection to the Internet at the desired bandwidth. Many remote sites do not yet have Internet service providers (ISPs) with sufficient bandwidth available, although in cities this type of connection is worth considering. The alternative to a dial-on-demand connection is a dedicated or permanent circuit.

5.5 Standard Telephony

The conventional telephone system is the public switched telephone network (PSTN). The standard analogue telephone line is a readily available form of communication, obtainable almost anywhere in the world. Unfortunately, the bandwidth available to users is limited to a maximum of 56 kbit/s and is likely to be much less if the telecommunications infrastructure is poor. Nonetheless, this type of connection can be suitable for some telemedicine applications that do not require either realtime data transmission or large file sizes. For applications such as teledermatology and telepathology, a modem connection may be sufficient to let two sites view images simultaneously, while discussing them via a separate telephone line.

5.6 Mobile Phones

Mobile phones are becoming commonplace, and can be used to transmit digital data, albeit at low rates of data transfer (similar to the PTSN). For applications where realtime transfer is not needed and access to a standard modem is not possible, they are ideal. Mobile phones may be useful to connect with a specific individual who is travelling, and has access to a mobile phone and perhaps a laptop computer. Computerized tomography (CT) scans[17] and ECG recordings[18] have been reported in this way. In many developing nations, mobile phone networks are being deployed instead of upgrading and expanding the traditional telephone network.

5.7 ISDN

The digital counterpart of the PSTN is ISDN, which is now available in most metropolitan areas in industrialized countries. It is completely different from the PSTN and offers end-to-end digital connectivity. This has two main advantages: greater reliability due to the digital nature of the data traffic, and higher bandwidth per line. Two standard types of ISDN connection are available to customers: a basic-rate line and a primary-rate line.

A basic-rate ISDN line offers the user a bandwidth of 128 kbit/s in two separate 64 kbit/s data channels. Basic-quality videoconferencing can be conducted using commercial equipment over a single basic-rate ISDN line. For higher-quality videoconferencing, multiple basic-rate ISDN lines can be used. For example, three lines aggregated together (providing six data channels) are commonly used, resulting in a bandwidth of 384 kbit/s. Primary-rate ISDN lines offer the user a bandwidth of up to 2 Mbit/s, thus allowing very high-quality video pictures to be transmitted.

ISDN connections are commonly used for telemedicine, because of their security, bandwidth, quality of service and relatively wide availability. Another advantage is the potential for adding capacity to these networks by renting additional lines. The major drawback of ISDN is that line rentals and usage charges can be high, although itemized billing allows the actual cost of telemedicine use to be readily evaluated.

A recent trend is to use existing local area networks (LANs) for telemedicine data transmission using Internet Protocol (IP). The principal disadvantage for realtime work is the problem of guaranteeing bandwidth for the telemedicine application.

5.8 Satellite

Generally speaking, fixed satellite connections are expensive to install and costly to use. They offer similar bandwidth to microwave links and the ISDN, but have the advantage of global coverage. Traditional satellite connections use geosynchronous satellites. Several telemedicine projects have used satellite linkages to connect mobile sites (such as military units or ships at sea) which would be impossible to reach in any other way.[19][20] As costs fall in the future, satellite transmission may become competitive with ISDN, particularly for remote areas. See Lamminen for a recent review.[21]

Another option is the use of a low earth orbiting satellite (LEOS). A LEOS offers the advantage of very inexpensive, hand-held receivers, not much larger than a standard mobile phone. They can be used nearly anywhere in the world, but currently have one significant drawback: very low data rates, of the order of 2.4-3.0 kbit/s. This may be acceptable if you need to send only a small file.

5.9 Leased Lines

Leased lines are an alternative to using ISDN, in which a permanent digital connection is established between two locations. The user pays a fixed rental for the line, which includes the cost of all calls. The line is leased for an agreed term (usually annually) and is thus paid for regardless of use. In contrast, the cost of dial-ondemand services like the PSTN or ISDN depends on the actual usage of the circuit. Clearly, there will be a point at which a leased line becomes cheaper than a dial-on- demand connection, which will depend on the usage. However, the leased line option limits the user to transmission between the two locations that have been connected (e.g. two hospitals), unlike the use of ISDN, in which any other institution with an ISDN connection can be called, just as with a telephone.

5.10 Digital Subscriber Lines (DSL)

DSL technology, which is often referred to as 'broadband', provides an IP connection to the user. DSL connections come in a variety of types, such as symmetric and asymmetric - in the latter case, it is called ADSL. The generic acronym is xDSL. The important thing about DSL is that it uses the existing copper telephone wire that is usually already present in the facility. The main drawback is that the facility must be located within about 5 km of the telephone company's switch. ADSL usually provides more downstream bandwidth (i.e. to the user) than upstream. This is fine for downloading data, but is not necessarily suitable for realtime videoconferencing.

5.11 Cable Modems

Cable TV is common in many metropolitan areas. The cable network represents a robust wiring system, which connects to homes and businesses. The same cable can be modified to act as a gateway to the Internet. Like xDSL, cable uses an inexpensive modem to provide more bandwidth than can be obtained with the PSTN. The principal drawback is that cable systems usually provide access through regional gateways, which means that if many people in a local area are using the service, the available bandwidth per user may decrease. Cable modems - like xDSL - also provide asymmetrical bandwidth to the user. However, their cost and bandwidth make cable and xDSL attractive options for low-cost telemedicine applications.

5.12 Microwave

Microwave connections are expensive to install, but inexpensive to maintain. The bandwidth available using microwave connections is high, 2-10 Mbit/s being common. However, microwave links are only feasible for sites that are relatively close together. Sites require direct line of sight to each other and must be less than about 30km apart (less if the visibility is poor). For longer distances, repeater stations can be used, but the cost rapidly becomes prohibitive. The advantage of a microwave link is that there are virtually no running costs - the bandwidth is essentially free following installation. The disadvantage is that a microwave link connects just two locations, point to point.

5.13 ATM

ATM transmission was designed to take advantage of the characteristics of fibre-optic cables. Very high bandwidth transmission is therefore possible, the latest equipment operating at rates of gigabits per second. However, ATM is used primarily in the 'backbone' of a network. Thus, the major telecommunications carriers use ATM for long-haul transmission. While ATM has advantages in terms of capacity and quality of service, it is very uncommon for ATM to be directly connected at the user level, and few telemedicine applications have used ATM networks directly.

Network options for telemedicine are summarized in Table 4: ITU Protocols (click to view table 4)

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