- Email: [email protected]
Networks in nuclear medicine
Networks in N u c l e a r M e d i c i n e Robert C. Lummis and John P. Wexler Computer network hardware and communication prot o c o l s are commonplace. Commercially a v a i l a b l e nuclear medicine computer systems lag behind in their support of n e t w o r k standards, Network hardware and s o f t w a r e are mature and stable technologies available for all computers and operating systems. Networks make it practical t o optimize the configuration of each computer t o a particular task, such as acquiring, processing, viewing, or storing data. This distribution of
OMPUTER ACQUISITION and processC ing have become an integral component of nuclear medicine imaging. The first computers dedicated to acquiring and processing nuclear medicine data were commercially available more than two decades ago. These devices, hampered by the then-current technology, were extremely limited in their abilities. Memory, storage capacity, display limitations, and slow processors made these computers devices in search of an application. In the early 1970s, several events led to the development of functional computers dedicated to nuclear medicine. The development of minicomputers provided the first available computational devices that were both affordable and powerful enough to deal with the demands of imaging science, not only nuclear medicine images but also computerized axial tomographic images. The development of two techniques, firstpass radionuclide angiography and multigated blood pool cardiac imaging, and their use to noninvasively quantitate left ventricular function and evaluate left ventricular regional wall motion, created a demand for computers capable of performing these examinations. Indus-
From the Departments of Neuroscience and Nuclear Medicine, Albert Einstein College o[ Medicine and Montefiore Medical Center, Bronx, NE. Taken from "Network Infrastructure," in Digital Networks and Communications in Nuclear Medicine, withpermission from The Michener Institute for Applied Sciences, Toronto, Canada. Address reprintrequests to John P. Wexler,MD, Department of NuclearMedicine, Albert Einstein Collegeof Medicine, 1300 Morris ParkAve, Bronx, NY10461. Copyright 9 1994 by W.B. Saunders Company 0001-2998/94/2401-0005505.00/0
66
functions has proven to be economical and o p e r a t i o n ally robust. It is reasonable to expect that within the next few years, all commercially available computer systems for nuclear medicine will provide the s o f t w a r e and h a r d w a r e support t h a t will make networking computers within a d e p a r t m e n t a practical way of sharing the computational resources of the department. Copyright 9 1994 by W.B. Saunders Company
trial response to this demand was rapid, and by the mid 1970s most manufacturers of nuclear imaging devices also were offering reasonably priced dedicated computer systems. As the demand for dedicated nuclear medicine computers grew, the computer industry independently entered into a phase of extraordinary technological development. During the 1980s the function of the minicomputer became increasingly replaced by the development of the microcomputer. Simultaneously, larger and faster computers, with increased processing power and display abilities, became available. In the early 1980s the first single-photon emission computed tomographic (SPECT) camera was described. This camera required the use of a computer to control acquisition, processing, and display o f its images. As SPECT imaging proved its efficacy, the majority of new imaging systems purchased from the mid-1980s on included a dedicated image processing computer. Today, the modern nuclear medicine laboratory is populated by gamma cameras, each of which is controlled by a sophisticated computer system. With the computer becoming a central device in the nuclear medicine laboratory, the ability to exchange information between computers has become progressively important. The linear process of acquiring, processing, and then viewing data using the same computer is not efficient. Most manufacturers have chosen to create proprietary means for storing and processing their data. The diversity in hardware and software has confounded the easy interchange of nuclear medicine studies between computer systems. A mechanism for sharing nuclear images between computers has become essential. This article focuses on the specific issue of using networks to establish connectivity be-
Seminars in Nuclear Medicine, Vol XXIV, No 1 (January), 1994: pp 66-74
NETWORKS iN NUCLEAR MEDICINE
67
tween various devices within a nuclear medicine department.
THE INTERNATIONAL STANDARDS ORGANIZATION REFERENCE MODEL
LOCAL AREA NETWORKS
The terminology associated with networks is based on the International Standards Organization (ISO) Reference Model. 1 In the reference model a network consists of two or more machines, called "hosts," that have a physical component (a cable) in common. The hosts are further described as consisting of several functional "layers." In the ISO Reference Model each layer performs a different function. The reference model describes in words what functions are performed by each layer, but the model does not specify how these functions are to be performed. Thus, the model does not specify a particular communication technology. The ISO layering concept and the terminology derived from the model have been adopted almost universally by other standards' bodies. It is not possible to discuss networking equipment or protocols without understanding the ISO Reference Model. Figure 2 shows the general scheme of the ISO Reference Model. The two hosts can be computers or any other devices. The layers of the model are depicted as being arranged vertically, with the lowest layer being the physical components of the network, such as cables, connectors, and
A sample network of the kind one might install in a nuclear medicine department is shown in Fig 1. The network provides connectivity between heterogeneous devices including computers, printers, scanners, display stations, and storage devices such as tape and optical drives. Unlike stand-alone computer systems where peripheral devices are connected to, and thus devoted to, a single processor, each networked processor or peripheral is connected independently to the network, and each device is accessible by any other device on the network. This type of network is called a local area network or LAN, because the network is typically confined to a restricted geographical area such as one clinical suite or one section of a hospital. Longdistance or wide area networking uses different technologies that are beyond the scope of this article. The principal underlying feature of a network is that the connectivity between all devices is controlled by software. Connection to the network permits any device to communicate with any other device, at any time, without modems, cable patching, dialing, or switching.
Fig 1, A simple I.AN connecting a variety of computers and other devices from different manufacturers. (Reproduced with permission from The Michener Institute for Applied Sciences.)
68
LUMMISANDWEXLER Host A
Host B
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I UserPore I ~
-T-- " ~ Presentation
Apparent Data Path
~IPrefsentatoi nl f
Layer 6 protocol
Layer 5 protocol
>, m
~
Vertical Arrows
~
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I Transport [ Layer 3 protocol
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electronic components, and the highest layer being the user's application program. At each host, each layer communicates only with the layers immediately above and below itself. When an application program sends a message to a program on another host, the message is first passed to a routine within the operating system. This routine acts as if it delivers the message directly to a sister routine on the other host. This is indicated in Fig 2 by the arrow labeled "apparent data path." In reality, the presentation layer cannot communicate directly with another host. The message cascades through routines running on successively lower :layers within the host until finally the information is at the lowest layer, the cable. Because the cable is common to both hosts, the message can then be passed upward through corresponding layers on the destination host until it reaches the top layer. As the message travels down through the layers, additional data are added at each layer. These added data serve to route the message to the correct host as well as the correct program running on that host. The added data also identify the source of the message, add errorcontrol information, and supply other data that are needed for network management and intervendor compatibility.
Layer 2 protocol
LayerI protocol
Fig 2. The ISO Reference Model provides a conceptual model of anetwork.It consists of two or more communicating devices, called hosts, and a common cable. Within each host the network functions take place on a number of layers, each of which may communicate only wIth the layers immediately above and below.A set of rules for c o m m u nicating between the layers constitutes e protocols tack. (Reproduced with permission from The Michener Institute for Applied Sciences.)
Figure 3 illustrates the addition of network control information to a user-level message as it moves down through the protocol stack. In the example the source program sends a fivecharacter message, "HELLO," to another host by calling a subroutine, typically with several additional arguments besides the five characters. The definition of these other arguments constitutes the presentation layer protocol. These arguments might include the names of the target host and identify the application program. The presentation layer combines these arguments plus other information into a header, shown as H6. In addition to the header, a trailer field containing error control information, such as either parity check bytes or a cyclical redundancy check calculation, is appended. The resulting longer message is then passed to the next lower layer. As the information progresses downward, at each layer an additional header and a trailer are added to the message. By the time the message is applied to the cable, it is much longer than the five characters sent by the user's program. This process of adding information to the original data as it passes through layers is called encapsulation.
NETWORKSIN NUCLEARMEDICINE
69
7 ] Application
[Pra~nt~io 5 [ 6
H6,HELLO,T6
]
Session
H5,H6,HELLO,T6,T51 4 I
Transport
H4,H5,H6,HELLO,T6,T5,T4] Fig 3. In this example the user program is sending the five-character message "HELLO" to another host. As the message moves down through the protocol layers, control information is added to it. Each layer "encapsulates" the information received from above with its own header and trailer fields, shown here as "H" and "T." (Reproduced with permission from The Michener Institute for Applied Sciences.)
3 l
Network
H3,H4,HS,H6,HELLO,T6,TS,T4,T3J 2
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Physical
NETWORK PROTOCOLS
Any practical implementation of this layering concept requires detailed rules to govern the operation and interpretation of control information. The same rules must be followed on both the source and destination hosts. The rules applicable at any given layer are described by the "protocol" at that layer. For example, layer 1 is governed by the "physical protocol," layer 3 the "network protocol," and layer 4 the "transport protocol." A complete set of protocols consisting of one protocol for each layer is referred to as the "protocol stack" or "protocol suite." Several different protocol stacks are in common use. The protocols for layers 1 and 2 are named according to the type of cable used and the way that the signal is applied to the cable. Ethernet, Token Ring broadband, Fiber Distributed Data Interface, Token Bus, LocaITalk (Apple, Cupertino, CA), and ArcNet are some of the familiar protocols for layers I and 2. Protocols for layers 3 and higher include TCP/IP, XNS, AppleTalk (Apple, Cupertino, CA), NetBEUI, and OSI. Any physical protocol can be used in combination with any of the
HI,H2,H3,H4,HS,H6,HELLO,T6,TS,T4,T3,T2,T~ Cable
higher-layer protocols. A network can be described in terms of the combinations of suites of protocols used. Thus, TCP/IP over Token Ring, or AppleTalk over Ethernet, describe implementations of the physical, networking, and transport protocols in a specific network environment. THE INSTITUTE OF ELECTRICAL AND ELECTRONIC ENGINEERS STANDARDS
Network users are primarily concerned with application programs that run at the top layer of the network. These programs, along with the operating system under which the application programs must run, are supplied by the vendors of computer or camera and computer systems. The purchaser of a network for a nuclear medicine department is responsible for choosing the specifications of layers 1 and 2 and for managing the network cable and electronics. The physical and electrical characteristics of practically all network equipment are governed by specifications published by the Institute of Electrical and Electronic Engineers (IEEE) 802 Committee and/or by the American National Standards Association. 2,3 Figure 4 relates the
70
LUMMIS AND WEXLER
ISO Reference Model
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LI IEEE standards to the ISO Reference Model. The IEEE standards divide ISO layers 1 and 2 into several sublayers. The highest sublayer (logical link control) is the same for all of the lower sublayers.4 This common upper interface layer is the reason that any of the higher-layer protocol stacks can be used with any physical protocol. The next sublayer down varies with the physical protocol, eg, Token Ring or Ethernet, and the lowest layers vary according to cable type, eg, coax or fiber optic). ETHERNET AND TOKEN RING
Ethernet and Token Ring are most commonly used for LAN protocols. Compared with Ethernet, Token Ring can span somewhat greater distances, and it performs well when there is steady traffic from many stations. Token Ring uses a higher transmission speed than Ethernet, 16 compared with 10 million bits per second (Mbps) respectively, but this usually does not result in faster file transfers in the clinical setting. The Token Ring protocol is considerably more complex than Ethernet, and the equipment is more expensive, can malfunction in more ways, and is more difficult to troubleshoot. Compared with Token Ring, Ethernet is simpler, cheaper, more robust in practice, and
Fig 4. The ISO Reference Model specifies what functions should be performed at each of the layers but not how they are performed. The Institute of Electrical and Electronic Engineers 802 Committee published a series of specifications that provide manofecturing and programming standards for the lowest two ISO layers. The IEEE standards divide the first two ISO layers into several sublayers. The highest of these sublayers, called
Varies with cable type
common interface between the various higher-level protocols that may be used, specified by other standards bodies, and several alternative physical protocols specified by the iEEE. These sublayers and physical protocols have gained almost universal acceptance. (Reproduced with permission from The Michener institute for Applied Sciences.)
easier to troubleshoot. It is well suited to the irregular traffic characteristic of a nuclear medicine department where network performance is typically gauged by the time needed to transfer a file. By this measure, Ethernet performance is usually equal to or better than Token Ring, because Ethernet permits one transmitting station to monopolize the medium a larger percentage of the time. Thus, although either Ethernet or Token Ring can be used for a nuclear medicine network, Ethernet is the better choice for most settings. Ethernet Protocol Each device on an Ethernet network has an Ethernet interface. Each interface has a unique 48-bit physical address, which is defined at the time the interface is manufactured. Manufacturers of Ethernet interfaces are allocated ranges of addresses from which they assign a unique address to each interface at the time of manufacture. The 48 bits used to define the interface address represent a number large enough to insure that no two interfaces will ever have the same address. The header of each frame contains the physical addresses of its sending and destination device. Ethernet can be considered to be analogous to a telephone party line. All devices on an Ethernet network hear every frame on the
NETWORKS IN NUCLEAR MEDICINE
Fig 5. This diagram shows the layout of fields within an Ethernet frame. Time progresses from iett to right. The preamble consists of an alternating bit pattern that generates a 10-MHz square wave on the cable. Each receiving station synchronizes its clock to this waveform. The last t w o bits in the preamble are inverted to signal the start of the information fields. The t w o address fields and the type or length field constitute the layer 1 header, and the CRC field constitutes the layer 1 trailer. The field labeled "data" is the encapsulated information received from the next higher layer. The t w o different frame formats shown here are both in regular use. They differ only in the meaning of the type or length field.
71 Destination Address
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cable. Each device is responsible for accepting and processing only those frames directed to it by detecting its own address in the destination field of each frame. The format of the Ethernet frame is shown in Fig 5. There are two variants of the Ethernet frame; the original Ethernet format and the IEEE 802.3 format. These variants differ only in the meaning of the 21st and 22nd bytes. In the original Ethernet format, the higherlayer protocol is identified in the "type" field, and the end of the frame is detected electrically. Some higher layer protocols, particularlyTCP/ 1P, use this format, which bypasses the data link layer. Newer protocols use the IEEE 802.3 format in which the 16-bit "type" field becomes a 16-bit "length" field that is used to verify that the end of the frame is properly detected. These newer higher-layer protocols invoke the data link layer, which identifies the higher-layer protocol within its own header (H2 in Fig 3). Both formats can be used together on the same cable and can be distinguished by the fact that the minimum value for the frame type is larger than the maximum value that can be assigned to the length. With either format, the "data" field contains all the encapsulated information from higher layers and is between 46 and 1,500 bytes in length. The remaining five other fields constitute the header and trailer of layer 1.
Preamble
The Ethernet signal is an irregular step wave in which a voltage transition in one direction represents a "1" bit and in the opposite direction a "0" bit, with the bit transitions occurring at the rate of 10 million/s (Fig 5). Information is sent serially, one bit at a time, at a rate of 10 Mbps. Information is not sent continuously but rather as variable-length frames. An Ethernet frame begins with a 10-MHz square wave lasting 6.4 microseconds, called the preamble. The last two bits of the preamble are reversed to signal the preamble's end (Fig 6). The preamble's function is to permit receiving stations to synchronize their docks to the sendbit time data bfls - -
1
1
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Fig 6. This diagram shows an idealized Ethernet waveform representing the bit sequence " 0 1 1 O 1." The bits are
represented by voltage transitions from low to high for 1 and high to l o w for 0. The transitions occur at the center of each bit time. This is called Manchester encoding. It has the property that even over a very short time interval the average voltage is independent of the bit stream being transmitted. (Reproduced with permission from The Michener Institute for Applied Sciences.)
72
ing station at the start of every frame so the receiving station can accurately detect the bit transitions throughout the remainder of the frame. When an Ethernet interface is ready to transmit a frame, the interface listens to the medium until the medium is idle and then starts transmitting. While transmitting, the interface continues to listen to determine if another station has started transmitting at the same time. Simultaneous transmission by two or more stations is called a collision. A collision is detectable because the voltage on the cable is higher than it would be if only one station were transmitting. Any station transmitting when a collision is detected immediately stops transmitting, waits a short time, and then starts transmitting the same frame over from the beginning. If two stations collide and then both wait the same time, they will collide again. To prevent recurrent collisions, a random number generator is used by each interface to determine the waiting time. This makes it likely that one station will start transmitting first and be detected by the other before the second starts transmitting. If a second collision occurs, the stations involved each wait a longer random time before retransmission. For each successive consecutive collision, the stations wait a progressively longer time. This lengthening of the waiting time is called the back-off mechanism and is the way Ethernet controls traffic when the cable is busy. A nontransmitting station continuously listens to the cable until a signal is detected. It then synchronizes its receive clock to the preamble and begins decoding the fields. If the destination address decoded is not that of the receiving station, the station simply ignores the frame and resumes listening. If the destination address is correct, the station decodes the remainder of the frame, verifies that the length is valid (64 to 1,518 bytes not including the preamble), computes the CRC value, and compares the computed result with the CRC field in the frame. If the CRC values do not agree, or if any other error is detected, such as an illegal frame length, the receiver ignores the frame (Ethernet format) or in certain cases sends a negative acknowledgment (IEEE 802.3 format). If there is no error, the
LUMMIS AND WEXLER
Fig 7. Ethernet can operate on either of the two topologies shown here. The bus topology may use either thick coaxial cable or thin coaxial cable, and the link topology may use either UTP or fiber optic cable. These are the only topologies end cable types permitted for Ethernet. (Reproduced with permission from The Michener Institute for Applied Sciences.)
frame's data field is passed up to the layer 3 routine specified in the frame's type field (Ethernet format) or in the layer 2 header within the data field (IEEE 802.3 format).
Ethernet Cables and Topology Two cable topologies and four cable types have been defined for Ethernet. Only these topologies and cable types may be used. The two topologies, bus, and link are illustrated in Fig 7. In the bus topology, all hosts are connected to a common coaxial cable, which can be either thick or thin. In the link topology, each host is connected to a hub, also known as a concentrator or multiport repeater. The connections are called links. The link cables may be either unshielded twisted pair (UTP) or fiber optic cable. With either topology and any of the four cable types, every frame that is transmitted on the network is seen by every station. Ethernet cables. There are four types of Ethernet cable: thick coaxial cable, thin coaxial cable, unshielded twisted pair, and fiber optic cable. The first of these four is now called standard
NETWORKS IN NUCLEAR MEDICINE
Ethernet cable. It is also referred to as thick coax or 10Base-5 cable. This cable consists of a 2.17-mm solid copper core and a 1-cm outer jacket with at least one and often two shielding layers. The cable can have a maximum length of 500 m. The next kind of Ethernet cable to become available was thin coax, also referred to as Thinnet or 10Base-2 cable. Compared with standard coax, thin coax is cheap, light, flexible, and easy to install and move. Its availability resulted in an explosive growth in the use of Ethernet. It consists of a 0.89-mm stranded tinned copper core, a single braided shield, and a 4.9-mm outer jacket. Its maximum length is 185 m, and it is limited to 30 connectors. Either type of coaxial cable must be grounded at one point and only one point and must have 50-~ terminating resistance at both ends. Thin coax can be troublesome, and a problem anywhere on the cable affects every station. For example, the dimensions of cables and connectors vary slightly, resulting in imperfect connections that loosen over time. Also, although the standard requires all connectors to be insulated, bare connectors are cheaper and are therefore used almost universally. If a bare connector touches a chassis or building steel, a ground loop can form that may manifest itself as excessive Ethernet collisions and/or invalid frames. A user without diagnostic equipment may not realize that a problem exists, except that unnecessarily slow transmission will be experienced. The third type of Ethernet cable is UTP cable, which is also referred to as 10Base-T. UTP cable consists of four twisted pairs of solid 24- or 26~gauge copper wire with no shields. It is convenient because it is quite thin, light, and flexible. Only two of the four pairs are used. With this cable type a concentrator or multiport repeater is always required as portrayed in the diagram in Fig 7. An important advantage of this arrangement is that a problem on one station or link does not affect the others as it does with coax cable. One hundred meters is commonly considered the maximum length of a UTP link. User connections at the stations are made with eight-pin R J-45 jacks, which look similar to the modular jacks used on telephones. A wide variety of special-purpose connectors and tools
73
are manufactured specifically for this type of wire, and their use permits gas-tight (and therefore corrosion-resistant) connections to be made quickly and reliably by semiskilled personnel working under conditions likely to prevail during construction or renovation. A concern sometimes expressed about the use of UTP cable in a laboratory or clinical setting is that electrical noise will be picked up on the cable and that this will prevent reliable transmission. However, the cable and transceiver properties are specified in such a way that interference is rare. Under most conditions UTP links have one to two orders of magnitude lower bit error rates than the one part in !08 that the standard specifies for thin coax. We have tried to find a situation in which interference impaired UTP Ethernet transmission by constructing experimental wiring setups with UTP cable strung close to various potentially interfering equipment, including fluorescent light fixtures, powerful high-frequency electromagnets, and AC and DC motors and relays. Special-purpose Ethernet monitoring equipment was used to record collisions and errors. To date we have been unable to find any environment in which significant interference could be detected. Major cable manufacturers rate twisted pair data cable according to a classification system consisting of categories 1 through 5, Category 1 is ordinary telephone cable. Category 2 is acceptable for 4-Mbps Token Ring, category 3 is the minimum intended for Ethernet, category 4 may be used for 16-Mbps Token Ring, and category 5 has the most stringent specifications.5 Category 5 cable will carry Ethernet well beyond 100 m. In any permanent installation, it is advisable to use category 5 cable, installed to a maximum length of 100 m for each run, and to use only two of the four pairs in the cable. This will likely make the cable reusable for 100-Mbps Ethernet, which is now being developed. The fourth type of Ethernet cable is fiber optic cable. Its use is specified by the 10Base-F standard or by F O I R L (fiber optic interrepeater link). The standard specifies a 1,000-m maximum link length and a 0.5-microsecond maximum transit delay, but in practice much longer lengths, at least up to 2,000 m, can be used. This is the cable type for long runs, and it is the only
74
LUMMIS AND WEXLER
satisfactory way to carry Ethernet between buildings. Many different types of fiber optic cable are suitable, but the kind most commonly used is the multimode type, which can be driven by a light-emitting diode rather than a laser source. Fiber optic cable must be installed by trained personnel using special-purpose tools, which makes it considerably more expensive to install than the other cable types. One link requires two fibers, one for transmission in each direction. Six-fiber cable is usually the minimum that is installed, because the labor involved in pulling the cable is a large fraction of the total installation cost and is almost independent of the number of fibers. NETWORK ELECTRONICS
When the network expands beyond the limits of a single cable system, in distance or in number of devices, additional electronic devices must be used to extend the distances or to interconnect separate cables. A large variety of standards-conforming repeaters, bridges, routers, gateways, and other devices are available that can implement practically any imagineable network configuration. Most of these will be managed at an institutionwide level and will not be the responsibility of the nuclear medicine department. FILE FORMATS
To this point we have described the physical properties of networks generally and Ethernet specifically. It is apparent that a departmental LAN can be easily established and that almost any nuclear medicine computer could theoretically be connected to the network. Unfortunately, the proprietary nature of the software and hardware used by the manufacturers of nuclear medicine computer systems precludes the possibility of one computer becoming a
terminal to a second of different manufacture without the purchase of specialized software. This is because the application-level software on most commercially available systems has not been written to support this type of access. At present it is possible to transfer image files between some computers of different manufacture either by physical transfer using floppy disk, tape, or interconnection using a modem. Few systems support standardized network file transfer protocols. Transferring image files is relatively simple, because image data are matrices containing count information stored in binary file format. However, even when transfer of image data is possible, this transfer is useless unless the image data are stored in a format that is understandable by the receiving computer. Currently, there is a significant debate within the imaging community about choosing a common data structure for storing nuclear medicine study data. Several different protocols have been proposed, but none has yet been chosen as the standard. One, INTERFILE, is a data structure that is specific for storing only nuclear medicine images. A second, DICOM, would be capable of storing image data from any clinical image such as computed tomography, magnetic resonance imaging, or nuclear medicine data. A third, HL-7, is an all encompassing data base format that would include image data as well as patient information that would be included in a hospital information system. DICOM and HL-7 also include standards for the transmission of information from place to place in the medical center, whereas I N T E R F I L E is simply a file format. Because no standard has been adopted, equipment manufacturers have yet to commit to support for any single data structure, although several currently offer the ability to translate between proprietary data structures and INTERFILE.
REFERENCES
1. StallingsW: Handbookof ComputerCommunications Standards,vol 2. New York, NY, Macmillan,1987 2. IEEE Standards for Local Area Networks: Carrier Sense Multiple Access With Collision Detection (CSMA/ CD) Access Method and PhysicalLayerSpecification.New York, NY, The Institute of Electrical and Electronic Engineers, 1985
3. Martin J: Local Area Networks Architectures and Implementations.EnglewoodCliffs,NJ, Prentice-Hall,1989, pp 63-96 4. IEEE Standards for Local Area Networks: Logical Link Control. New York, NY, The Institute of Electrical and ElectronicEngineers, 1984 5. MarksH: Cablevision.LAN Mag 8:77-90, 1993
OMPUTER ACQUISITION and processC ing have become an integral component of nuclear medicine imaging. The first computers dedicated to acquiring and processing nuclear medicine data were commercially available more than two decades ago. These devices, hampered by the then-current technology, were extremely limited in their abilities. Memory, storage capacity, display limitations, and slow processors made these computers devices in search of an application. In the early 1970s, several events led to the development of functional computers dedicated to nuclear medicine. The development of minicomputers provided the first available computational devices that were both affordable and powerful enough to deal with the demands of imaging science, not only nuclear medicine images but also computerized axial tomographic images. The development of two techniques, firstpass radionuclide angiography and multigated blood pool cardiac imaging, and their use to noninvasively quantitate left ventricular function and evaluate left ventricular regional wall motion, created a demand for computers capable of performing these examinations. Indus-
From the Departments of Neuroscience and Nuclear Medicine, Albert Einstein College o[ Medicine and Montefiore Medical Center, Bronx, NE. Taken from "Network Infrastructure," in Digital Networks and Communications in Nuclear Medicine, withpermission from The Michener Institute for Applied Sciences, Toronto, Canada. Address reprintrequests to John P. Wexler,MD, Department of NuclearMedicine, Albert Einstein Collegeof Medicine, 1300 Morris ParkAve, Bronx, NY10461. Copyright 9 1994 by W.B. Saunders Company 0001-2998/94/2401-0005505.00/0
66
functions has proven to be economical and o p e r a t i o n ally robust. It is reasonable to expect that within the next few years, all commercially available computer systems for nuclear medicine will provide the s o f t w a r e and h a r d w a r e support t h a t will make networking computers within a d e p a r t m e n t a practical way of sharing the computational resources of the department. Copyright 9 1994 by W.B. Saunders Company
trial response to this demand was rapid, and by the mid 1970s most manufacturers of nuclear imaging devices also were offering reasonably priced dedicated computer systems. As the demand for dedicated nuclear medicine computers grew, the computer industry independently entered into a phase of extraordinary technological development. During the 1980s the function of the minicomputer became increasingly replaced by the development of the microcomputer. Simultaneously, larger and faster computers, with increased processing power and display abilities, became available. In the early 1980s the first single-photon emission computed tomographic (SPECT) camera was described. This camera required the use of a computer to control acquisition, processing, and display o f its images. As SPECT imaging proved its efficacy, the majority of new imaging systems purchased from the mid-1980s on included a dedicated image processing computer. Today, the modern nuclear medicine laboratory is populated by gamma cameras, each of which is controlled by a sophisticated computer system. With the computer becoming a central device in the nuclear medicine laboratory, the ability to exchange information between computers has become progressively important. The linear process of acquiring, processing, and then viewing data using the same computer is not efficient. Most manufacturers have chosen to create proprietary means for storing and processing their data. The diversity in hardware and software has confounded the easy interchange of nuclear medicine studies between computer systems. A mechanism for sharing nuclear images between computers has become essential. This article focuses on the specific issue of using networks to establish connectivity be-
Seminars in Nuclear Medicine, Vol XXIV, No 1 (January), 1994: pp 66-74
NETWORKS iN NUCLEAR MEDICINE
67
tween various devices within a nuclear medicine department.
THE INTERNATIONAL STANDARDS ORGANIZATION REFERENCE MODEL
LOCAL AREA NETWORKS
The terminology associated with networks is based on the International Standards Organization (ISO) Reference Model. 1 In the reference model a network consists of two or more machines, called "hosts," that have a physical component (a cable) in common. The hosts are further described as consisting of several functional "layers." In the ISO Reference Model each layer performs a different function. The reference model describes in words what functions are performed by each layer, but the model does not specify how these functions are to be performed. Thus, the model does not specify a particular communication technology. The ISO layering concept and the terminology derived from the model have been adopted almost universally by other standards' bodies. It is not possible to discuss networking equipment or protocols without understanding the ISO Reference Model. Figure 2 shows the general scheme of the ISO Reference Model. The two hosts can be computers or any other devices. The layers of the model are depicted as being arranged vertically, with the lowest layer being the physical components of the network, such as cables, connectors, and
A sample network of the kind one might install in a nuclear medicine department is shown in Fig 1. The network provides connectivity between heterogeneous devices including computers, printers, scanners, display stations, and storage devices such as tape and optical drives. Unlike stand-alone computer systems where peripheral devices are connected to, and thus devoted to, a single processor, each networked processor or peripheral is connected independently to the network, and each device is accessible by any other device on the network. This type of network is called a local area network or LAN, because the network is typically confined to a restricted geographical area such as one clinical suite or one section of a hospital. Longdistance or wide area networking uses different technologies that are beyond the scope of this article. The principal underlying feature of a network is that the connectivity between all devices is controlled by software. Connection to the network permits any device to communicate with any other device, at any time, without modems, cable patching, dialing, or switching.
Fig 1, A simple I.AN connecting a variety of computers and other devices from different manufacturers. (Reproduced with permission from The Michener Institute for Applied Sciences.)
68
LUMMISANDWEXLER Host A
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electronic components, and the highest layer being the user's application program. At each host, each layer communicates only with the layers immediately above and below itself. When an application program sends a message to a program on another host, the message is first passed to a routine within the operating system. This routine acts as if it delivers the message directly to a sister routine on the other host. This is indicated in Fig 2 by the arrow labeled "apparent data path." In reality, the presentation layer cannot communicate directly with another host. The message cascades through routines running on successively lower :layers within the host until finally the information is at the lowest layer, the cable. Because the cable is common to both hosts, the message can then be passed upward through corresponding layers on the destination host until it reaches the top layer. As the message travels down through the layers, additional data are added at each layer. These added data serve to route the message to the correct host as well as the correct program running on that host. The added data also identify the source of the message, add errorcontrol information, and supply other data that are needed for network management and intervendor compatibility.
Layer 2 protocol
LayerI protocol
Fig 2. The ISO Reference Model provides a conceptual model of anetwork.It consists of two or more communicating devices, called hosts, and a common cable. Within each host the network functions take place on a number of layers, each of which may communicate only wIth the layers immediately above and below.A set of rules for c o m m u nicating between the layers constitutes e protocols tack. (Reproduced with permission from The Michener Institute for Applied Sciences.)
Figure 3 illustrates the addition of network control information to a user-level message as it moves down through the protocol stack. In the example the source program sends a fivecharacter message, "HELLO," to another host by calling a subroutine, typically with several additional arguments besides the five characters. The definition of these other arguments constitutes the presentation layer protocol. These arguments might include the names of the target host and identify the application program. The presentation layer combines these arguments plus other information into a header, shown as H6. In addition to the header, a trailer field containing error control information, such as either parity check bytes or a cyclical redundancy check calculation, is appended. The resulting longer message is then passed to the next lower layer. As the information progresses downward, at each layer an additional header and a trailer are added to the message. By the time the message is applied to the cable, it is much longer than the five characters sent by the user's program. This process of adding information to the original data as it passes through layers is called encapsulation.
NETWORKSIN NUCLEARMEDICINE
69
7 ] Application
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H4,H5,H6,HELLO,T6,T5,T4] Fig 3. In this example the user program is sending the five-character message "HELLO" to another host. As the message moves down through the protocol layers, control information is added to it. Each layer "encapsulates" the information received from above with its own header and trailer fields, shown here as "H" and "T." (Reproduced with permission from The Michener Institute for Applied Sciences.)
3 l
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NETWORK PROTOCOLS
Any practical implementation of this layering concept requires detailed rules to govern the operation and interpretation of control information. The same rules must be followed on both the source and destination hosts. The rules applicable at any given layer are described by the "protocol" at that layer. For example, layer 1 is governed by the "physical protocol," layer 3 the "network protocol," and layer 4 the "transport protocol." A complete set of protocols consisting of one protocol for each layer is referred to as the "protocol stack" or "protocol suite." Several different protocol stacks are in common use. The protocols for layers 1 and 2 are named according to the type of cable used and the way that the signal is applied to the cable. Ethernet, Token Ring broadband, Fiber Distributed Data Interface, Token Bus, LocaITalk (Apple, Cupertino, CA), and ArcNet are some of the familiar protocols for layers I and 2. Protocols for layers 3 and higher include TCP/IP, XNS, AppleTalk (Apple, Cupertino, CA), NetBEUI, and OSI. Any physical protocol can be used in combination with any of the
HI,H2,H3,H4,HS,H6,HELLO,T6,TS,T4,T3,T2,T~ Cable
higher-layer protocols. A network can be described in terms of the combinations of suites of protocols used. Thus, TCP/IP over Token Ring, or AppleTalk over Ethernet, describe implementations of the physical, networking, and transport protocols in a specific network environment. THE INSTITUTE OF ELECTRICAL AND ELECTRONIC ENGINEERS STANDARDS
Network users are primarily concerned with application programs that run at the top layer of the network. These programs, along with the operating system under which the application programs must run, are supplied by the vendors of computer or camera and computer systems. The purchaser of a network for a nuclear medicine department is responsible for choosing the specifications of layers 1 and 2 and for managing the network cable and electronics. The physical and electrical characteristics of practically all network equipment are governed by specifications published by the Institute of Electrical and Electronic Engineers (IEEE) 802 Committee and/or by the American National Standards Association. 2,3 Figure 4 relates the
70
LUMMIS AND WEXLER
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LI IEEE standards to the ISO Reference Model. The IEEE standards divide ISO layers 1 and 2 into several sublayers. The highest sublayer (logical link control) is the same for all of the lower sublayers.4 This common upper interface layer is the reason that any of the higher-layer protocol stacks can be used with any physical protocol. The next sublayer down varies with the physical protocol, eg, Token Ring or Ethernet, and the lowest layers vary according to cable type, eg, coax or fiber optic). ETHERNET AND TOKEN RING
Ethernet and Token Ring are most commonly used for LAN protocols. Compared with Ethernet, Token Ring can span somewhat greater distances, and it performs well when there is steady traffic from many stations. Token Ring uses a higher transmission speed than Ethernet, 16 compared with 10 million bits per second (Mbps) respectively, but this usually does not result in faster file transfers in the clinical setting. The Token Ring protocol is considerably more complex than Ethernet, and the equipment is more expensive, can malfunction in more ways, and is more difficult to troubleshoot. Compared with Token Ring, Ethernet is simpler, cheaper, more robust in practice, and
Fig 4. The ISO Reference Model specifies what functions should be performed at each of the layers but not how they are performed. The Institute of Electrical and Electronic Engineers 802 Committee published a series of specifications that provide manofecturing and programming standards for the lowest two ISO layers. The IEEE standards divide the first two ISO layers into several sublayers. The highest of these sublayers, called
Varies with cable type
common interface between the various higher-level protocols that may be used, specified by other standards bodies, and several alternative physical protocols specified by the iEEE. These sublayers and physical protocols have gained almost universal acceptance. (Reproduced with permission from The Michener institute for Applied Sciences.)
easier to troubleshoot. It is well suited to the irregular traffic characteristic of a nuclear medicine department where network performance is typically gauged by the time needed to transfer a file. By this measure, Ethernet performance is usually equal to or better than Token Ring, because Ethernet permits one transmitting station to monopolize the medium a larger percentage of the time. Thus, although either Ethernet or Token Ring can be used for a nuclear medicine network, Ethernet is the better choice for most settings. Ethernet Protocol Each device on an Ethernet network has an Ethernet interface. Each interface has a unique 48-bit physical address, which is defined at the time the interface is manufactured. Manufacturers of Ethernet interfaces are allocated ranges of addresses from which they assign a unique address to each interface at the time of manufacture. The 48 bits used to define the interface address represent a number large enough to insure that no two interfaces will ever have the same address. The header of each frame contains the physical addresses of its sending and destination device. Ethernet can be considered to be analogous to a telephone party line. All devices on an Ethernet network hear every frame on the
NETWORKS IN NUCLEAR MEDICINE
Fig 5. This diagram shows the layout of fields within an Ethernet frame. Time progresses from iett to right. The preamble consists of an alternating bit pattern that generates a 10-MHz square wave on the cable. Each receiving station synchronizes its clock to this waveform. The last t w o bits in the preamble are inverted to signal the start of the information fields. The t w o address fields and the type or length field constitute the layer 1 header, and the CRC field constitutes the layer 1 trailer. The field labeled "data" is the encapsulated information received from the next higher layer. The t w o different frame formats shown here are both in regular use. They differ only in the meaning of the type or length field.
71 Destination Address
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cable. Each device is responsible for accepting and processing only those frames directed to it by detecting its own address in the destination field of each frame. The format of the Ethernet frame is shown in Fig 5. There are two variants of the Ethernet frame; the original Ethernet format and the IEEE 802.3 format. These variants differ only in the meaning of the 21st and 22nd bytes. In the original Ethernet format, the higherlayer protocol is identified in the "type" field, and the end of the frame is detected electrically. Some higher layer protocols, particularlyTCP/ 1P, use this format, which bypasses the data link layer. Newer protocols use the IEEE 802.3 format in which the 16-bit "type" field becomes a 16-bit "length" field that is used to verify that the end of the frame is properly detected. These newer higher-layer protocols invoke the data link layer, which identifies the higher-layer protocol within its own header (H2 in Fig 3). Both formats can be used together on the same cable and can be distinguished by the fact that the minimum value for the frame type is larger than the maximum value that can be assigned to the length. With either format, the "data" field contains all the encapsulated information from higher layers and is between 46 and 1,500 bytes in length. The remaining five other fields constitute the header and trailer of layer 1.
Preamble
The Ethernet signal is an irregular step wave in which a voltage transition in one direction represents a "1" bit and in the opposite direction a "0" bit, with the bit transitions occurring at the rate of 10 million/s (Fig 5). Information is sent serially, one bit at a time, at a rate of 10 Mbps. Information is not sent continuously but rather as variable-length frames. An Ethernet frame begins with a 10-MHz square wave lasting 6.4 microseconds, called the preamble. The last two bits of the preamble are reversed to signal the preamble's end (Fig 6). The preamble's function is to permit receiving stations to synchronize their docks to the sendbit time data bfls - -
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Fig 6. This diagram shows an idealized Ethernet waveform representing the bit sequence " 0 1 1 O 1." The bits are
represented by voltage transitions from low to high for 1 and high to l o w for 0. The transitions occur at the center of each bit time. This is called Manchester encoding. It has the property that even over a very short time interval the average voltage is independent of the bit stream being transmitted. (Reproduced with permission from The Michener Institute for Applied Sciences.)
72
ing station at the start of every frame so the receiving station can accurately detect the bit transitions throughout the remainder of the frame. When an Ethernet interface is ready to transmit a frame, the interface listens to the medium until the medium is idle and then starts transmitting. While transmitting, the interface continues to listen to determine if another station has started transmitting at the same time. Simultaneous transmission by two or more stations is called a collision. A collision is detectable because the voltage on the cable is higher than it would be if only one station were transmitting. Any station transmitting when a collision is detected immediately stops transmitting, waits a short time, and then starts transmitting the same frame over from the beginning. If two stations collide and then both wait the same time, they will collide again. To prevent recurrent collisions, a random number generator is used by each interface to determine the waiting time. This makes it likely that one station will start transmitting first and be detected by the other before the second starts transmitting. If a second collision occurs, the stations involved each wait a longer random time before retransmission. For each successive consecutive collision, the stations wait a progressively longer time. This lengthening of the waiting time is called the back-off mechanism and is the way Ethernet controls traffic when the cable is busy. A nontransmitting station continuously listens to the cable until a signal is detected. It then synchronizes its receive clock to the preamble and begins decoding the fields. If the destination address decoded is not that of the receiving station, the station simply ignores the frame and resumes listening. If the destination address is correct, the station decodes the remainder of the frame, verifies that the length is valid (64 to 1,518 bytes not including the preamble), computes the CRC value, and compares the computed result with the CRC field in the frame. If the CRC values do not agree, or if any other error is detected, such as an illegal frame length, the receiver ignores the frame (Ethernet format) or in certain cases sends a negative acknowledgment (IEEE 802.3 format). If there is no error, the
LUMMIS AND WEXLER
Fig 7. Ethernet can operate on either of the two topologies shown here. The bus topology may use either thick coaxial cable or thin coaxial cable, and the link topology may use either UTP or fiber optic cable. These are the only topologies end cable types permitted for Ethernet. (Reproduced with permission from The Michener Institute for Applied Sciences.)
frame's data field is passed up to the layer 3 routine specified in the frame's type field (Ethernet format) or in the layer 2 header within the data field (IEEE 802.3 format).
Ethernet Cables and Topology Two cable topologies and four cable types have been defined for Ethernet. Only these topologies and cable types may be used. The two topologies, bus, and link are illustrated in Fig 7. In the bus topology, all hosts are connected to a common coaxial cable, which can be either thick or thin. In the link topology, each host is connected to a hub, also known as a concentrator or multiport repeater. The connections are called links. The link cables may be either unshielded twisted pair (UTP) or fiber optic cable. With either topology and any of the four cable types, every frame that is transmitted on the network is seen by every station. Ethernet cables. There are four types of Ethernet cable: thick coaxial cable, thin coaxial cable, unshielded twisted pair, and fiber optic cable. The first of these four is now called standard
NETWORKS IN NUCLEAR MEDICINE
Ethernet cable. It is also referred to as thick coax or 10Base-5 cable. This cable consists of a 2.17-mm solid copper core and a 1-cm outer jacket with at least one and often two shielding layers. The cable can have a maximum length of 500 m. The next kind of Ethernet cable to become available was thin coax, also referred to as Thinnet or 10Base-2 cable. Compared with standard coax, thin coax is cheap, light, flexible, and easy to install and move. Its availability resulted in an explosive growth in the use of Ethernet. It consists of a 0.89-mm stranded tinned copper core, a single braided shield, and a 4.9-mm outer jacket. Its maximum length is 185 m, and it is limited to 30 connectors. Either type of coaxial cable must be grounded at one point and only one point and must have 50-~ terminating resistance at both ends. Thin coax can be troublesome, and a problem anywhere on the cable affects every station. For example, the dimensions of cables and connectors vary slightly, resulting in imperfect connections that loosen over time. Also, although the standard requires all connectors to be insulated, bare connectors are cheaper and are therefore used almost universally. If a bare connector touches a chassis or building steel, a ground loop can form that may manifest itself as excessive Ethernet collisions and/or invalid frames. A user without diagnostic equipment may not realize that a problem exists, except that unnecessarily slow transmission will be experienced. The third type of Ethernet cable is UTP cable, which is also referred to as 10Base-T. UTP cable consists of four twisted pairs of solid 24- or 26~gauge copper wire with no shields. It is convenient because it is quite thin, light, and flexible. Only two of the four pairs are used. With this cable type a concentrator or multiport repeater is always required as portrayed in the diagram in Fig 7. An important advantage of this arrangement is that a problem on one station or link does not affect the others as it does with coax cable. One hundred meters is commonly considered the maximum length of a UTP link. User connections at the stations are made with eight-pin R J-45 jacks, which look similar to the modular jacks used on telephones. A wide variety of special-purpose connectors and tools
73
are manufactured specifically for this type of wire, and their use permits gas-tight (and therefore corrosion-resistant) connections to be made quickly and reliably by semiskilled personnel working under conditions likely to prevail during construction or renovation. A concern sometimes expressed about the use of UTP cable in a laboratory or clinical setting is that electrical noise will be picked up on the cable and that this will prevent reliable transmission. However, the cable and transceiver properties are specified in such a way that interference is rare. Under most conditions UTP links have one to two orders of magnitude lower bit error rates than the one part in !08 that the standard specifies for thin coax. We have tried to find a situation in which interference impaired UTP Ethernet transmission by constructing experimental wiring setups with UTP cable strung close to various potentially interfering equipment, including fluorescent light fixtures, powerful high-frequency electromagnets, and AC and DC motors and relays. Special-purpose Ethernet monitoring equipment was used to record collisions and errors. To date we have been unable to find any environment in which significant interference could be detected. Major cable manufacturers rate twisted pair data cable according to a classification system consisting of categories 1 through 5, Category 1 is ordinary telephone cable. Category 2 is acceptable for 4-Mbps Token Ring, category 3 is the minimum intended for Ethernet, category 4 may be used for 16-Mbps Token Ring, and category 5 has the most stringent specifications.5 Category 5 cable will carry Ethernet well beyond 100 m. In any permanent installation, it is advisable to use category 5 cable, installed to a maximum length of 100 m for each run, and to use only two of the four pairs in the cable. This will likely make the cable reusable for 100-Mbps Ethernet, which is now being developed. The fourth type of Ethernet cable is fiber optic cable. Its use is specified by the 10Base-F standard or by F O I R L (fiber optic interrepeater link). The standard specifies a 1,000-m maximum link length and a 0.5-microsecond maximum transit delay, but in practice much longer lengths, at least up to 2,000 m, can be used. This is the cable type for long runs, and it is the only
74
LUMMIS AND WEXLER
satisfactory way to carry Ethernet between buildings. Many different types of fiber optic cable are suitable, but the kind most commonly used is the multimode type, which can be driven by a light-emitting diode rather than a laser source. Fiber optic cable must be installed by trained personnel using special-purpose tools, which makes it considerably more expensive to install than the other cable types. One link requires two fibers, one for transmission in each direction. Six-fiber cable is usually the minimum that is installed, because the labor involved in pulling the cable is a large fraction of the total installation cost and is almost independent of the number of fibers. NETWORK ELECTRONICS
When the network expands beyond the limits of a single cable system, in distance or in number of devices, additional electronic devices must be used to extend the distances or to interconnect separate cables. A large variety of standards-conforming repeaters, bridges, routers, gateways, and other devices are available that can implement practically any imagineable network configuration. Most of these will be managed at an institutionwide level and will not be the responsibility of the nuclear medicine department. FILE FORMATS
To this point we have described the physical properties of networks generally and Ethernet specifically. It is apparent that a departmental LAN can be easily established and that almost any nuclear medicine computer could theoretically be connected to the network. Unfortunately, the proprietary nature of the software and hardware used by the manufacturers of nuclear medicine computer systems precludes the possibility of one computer becoming a
terminal to a second of different manufacture without the purchase of specialized software. This is because the application-level software on most commercially available systems has not been written to support this type of access. At present it is possible to transfer image files between some computers of different manufacture either by physical transfer using floppy disk, tape, or interconnection using a modem. Few systems support standardized network file transfer protocols. Transferring image files is relatively simple, because image data are matrices containing count information stored in binary file format. However, even when transfer of image data is possible, this transfer is useless unless the image data are stored in a format that is understandable by the receiving computer. Currently, there is a significant debate within the imaging community about choosing a common data structure for storing nuclear medicine study data. Several different protocols have been proposed, but none has yet been chosen as the standard. One, INTERFILE, is a data structure that is specific for storing only nuclear medicine images. A second, DICOM, would be capable of storing image data from any clinical image such as computed tomography, magnetic resonance imaging, or nuclear medicine data. A third, HL-7, is an all encompassing data base format that would include image data as well as patient information that would be included in a hospital information system. DICOM and HL-7 also include standards for the transmission of information from place to place in the medical center, whereas I N T E R F I L E is simply a file format. Because no standard has been adopted, equipment manufacturers have yet to commit to support for any single data structure, although several currently offer the ability to translate between proprietary data structures and INTERFILE.
REFERENCES
1. StallingsW: Handbookof ComputerCommunications Standards,vol 2. New York, NY, Macmillan,1987 2. IEEE Standards for Local Area Networks: Carrier Sense Multiple Access With Collision Detection (CSMA/ CD) Access Method and PhysicalLayerSpecification.New York, NY, The Institute of Electrical and Electronic Engineers, 1985
3. Martin J: Local Area Networks Architectures and Implementations.EnglewoodCliffs,NJ, Prentice-Hall,1989, pp 63-96 4. IEEE Standards for Local Area Networks: Logical Link Control. New York, NY, The Institute of Electrical and ElectronicEngineers, 1984 5. MarksH: Cablevision.LAN Mag 8:77-90, 1993