CONTINUING EDUCATION Computer Networking In Nuclear Medicine Michael K. O'Connor Department of Radiology, The Mayo Clinic, Rochester, Minnesota to the possibility of not only connecting computer systems Objective: The purpose of this article is to provide a com­ from different vendors, but also connecting these systems to prehensive description of computer networks and how they a standard PC, Macintosh and other workstations in a de­ can improve the efficiency of a nuclear medicine department. partment (I). It should also be possible to utilize many other Methods: This paper discusses various types of networks, network resources such as printers and plotters with the defines specific network terminology and discusses the im­ nuclear medicine computer systems. This article reviews the plementation of a computer network in a nuclear medicine technology of computer networking and describes the ad­ department. vantages and disadvantages of such a network currently in Results: A computer network can serve as a vital component of a nuclear medicine department, reducing the time ex­ use at Mayo Clinic. pended on menial tasks while allowing retrieval and transfer­ WHAT IS A NETWORK? ral of information. Conclusions: A computer network can revolutionize a stan­ A network is a way of connecting several computers to­ dard nuclear medicine department. However, the complexity gether so that they all have access to files, programs, printers and size of an individual department will determine if net­ and other services (collectively called resources). In com­ working will be cost-effective. puter jargon, such a collection of computers all located Key Words: Computer network, LAN, WAN, Ethernet, within a few thousand feet of each other is called a local area ARCnet, Token-Ring. network (LAN). LANs that are remote from each other can J Nucl Med Techno/1994; 22:4~55 be interconnected by telephone or satellite to form a wide area network (WAN). A network can be organized as a server-based system or This is the second article in a four-part series on computers as a peer-to-peer system. In a server-based system, one or in nuclear medicine. Upon completion, the technologist will more computers act as servers. These servers make the be able to (1) describe a computer network, (2) understand network resources available to any client computer on the network terminology and (3) have an overview of a currently network. A client computer can only talk to another client operational network used in a large nuclear medicine labo­ computer through the server. Hence, the server essentially ratory. controls the network and decides what resources to make available to a given client computer. In a peer-to-peer sys­ Computer networks are becoming an integral part of the tem, there is no server or "master" system. Instead, any way we move information around our department, our insti­ computer can communicate directly with any other com­ tution and between institutions. While the ability to network puter on the network and has access to the hard drive and nuclear medicine computer systems has been available for other resources (e.g., printers) available on that computer. several years, many of these network schemes were propri­ etary and nonstandard and only permitted communication NETWORK TERMINOLOGY between systems from the same vendor. More than any other area of computers, networking As more and more vendors have moved toward the use of abounds with confusing acronyms, such as AFP, TCP/IP, off-the-shelf computer hardware (e.g., Sun computers or Ma­ CSMNCD, etc. Before entering into a full discussion of cintosh systems), with minimal proprietary hardware, they networking, the following definitions may clarify some of the have also made use of the standard networking hardware and confusion surrounding networks. Let us consider the cre­ software available for use on such systems. This now leads ation of a very simple network of two PCs. The connection of these two desktop computers requires two network inter­ face adapters (generally called interface cards or adapter For correspondence or reprints contact: Dr. M. K. O'Connor, Section of Nuclear Medicine, Charlton 2, Mayo Clinic, Rochester, MN 55905. cards). Each computer will need one printed circuit card YOLUMI! 22, NUMBI!R 2, JUNI! 1994 48 (Fig. 1) plugged into the computer's motherboard. These adapter cards are connected to each other by a cable, such as a coaxial cable or a twisted-pair telephone wire. Finally, each computer will need to run a software program to permit the computers to communicate with each other. This pro­ gram typically runs in the background and provides users with access to the hard drive and printers of the other com­ puter, as if they were directly attached to their system. Currently, the majority of computers are either IBM-com­ patible 286-, 386- or 486-based personal computers (PCs) or Macintosh systems (Macs). Therefore, the greatest variety of hardware and software products for networking has been developed for these systems. Many of the more powerful computers, such as the Sun Microsystems SPARC 10 or DEC Alpha systems, are UNIX-based and come with networking as a standard feature of the system. A description of the major hardware and software components of a network follows. FIGURE 1. Token-Ring (top) and Ethernet (bottom) interface adapter cards. These cards are designed to plug into the mother­ board of a PC. NETWORK ADAPTER CARD There are several types of adapter cards available. Gen­ lowest numbered station becomes the controller of the net­ erally, a card can only transmit data using a specific scheme. work. This controlling station sends a token to each station The three most commonly used schemes are Ethernet, ARC­ in numerical order. When each station receives the permis­ net and Token-Ring. A simple analogy to these schemes is a sion token, it either sends its message or remains silent. The radio station that broadcasts in AM or FM. Clearly, an AM controlling station then sends a permission token to the next receiver cannot pick up an FM transmission. Likewise, an station in numeric sequence. ARCnet cards can be pur­ Ethernet adapter card cannot communicate with a Token­ chased for as little as $50 and provide a very economical Ring adapter card. The choice of adapter card is based on network architecture. cost and speed requirements. The cheapest and slowest is Token-Ring ARCnet, while Ethernet is potentially four times faster for In a Token-Ring scheme, a special toke~ (software mes­ data transkr over a network, with only a slight increase in sage) continuously circulates through all the systems on the cost. Token-Ring is proprietary IBM technology and is the network. Unlike ARCnet, it has no controlling station. Only most expensive of the three, with a speed intermediate be­ a system that currently has the token is allowed to transmit tween Ethernet and ARCnet. Typical costs in 1993 for messages on the network. When it has finished transmitting, adapter cards ranged from $100 to $300. it passes the token to the next system on the network. There Ethernet is one potential problem with Token-Ring that is not present with either Ethernet or ARCnet; each system must actively In an Ethernet card, the card checks the network to see if pass the token to the next system in line, hence if one the network is busy (i.e., another system is transmitting). If adapter card on the network fails, it can bring down the it doesn't hear the transmitted signal of another computer, entire network. Although this is not a common problem, it the card broadcasts its signal to all other systems on the can be catastrophic. Token-Ring is primarily an IBM prod­ network. When the inevitable happens and two computers uct and is expensive and should be considered if direct broadcast simultaneously, a special software protocol built access to mainframe computers is required. into the Ethernet card detects the collision and each card stops transmitting and goes into its internal program to de­ NETWORK TOPOLOGY termine a randomly selected time for retransmission. This protocol is called carrier-sense multiple access with collision Phrases such as Star, Ring, or BUS topology refer not to detection (CSMNCD). Ethernet was one of the first LAN the type of network, but rather to how computers on the architectures and is still the most popular. It provides high­ network are physically or logically connected. In a logical speed transmission at an economical price and offers a broad sense, computers on an Ethernet LAN can be considered as base of support for a large array of PCs, mini-computers and a string of systems daisy-chained together (BUS topology), mainframes. whereas ARCnet LANs work on a star topology with a central controlling system. Obviously, Token-Ring works on ARCnet a ring topology. Physically, computers can be connected An ARCnet card uses a different scheme from Ethernet. using any of these topologies, however in the real world, The user can set an identification number on an ARCnet card only Star and BUS topologies are used, each with its own from 1 to 255, using switches on the back of the card. When advantages and disadvantages. A BUS (daisy-chain) topol­ activated, the adapters broadcast their numbers, and the ogy uses less cable, does not need a central wiring hub, but 50 .JOURNAL OF NUCLUR MEDICINE TECHNOLOGY one bad connection can bring down the network. The Star operating system. Some vendors have combined it with topology requires that each system be connected to a central TCP/IP to give their network software greater flexibility. wiring hub. This is more expensive than the BUS topology, NFS: The Network File System was developed by Sun but is easier to install and more reliable.