Tài liệu Data Communication Basic Concepts doc

“Datapro Communications Analyst”Data Communications: Basic Concepts 1001 DNW Data Communications: Basic Concepts Datapro Summary Data communications is an integral part of business. Whether a data network accommodates 10 personal computers on a LAN or 100 nodes in a global network, data communications is the link for greater productivity, efficiency. and cost savings. This report offers an overview of data communications principles, the products and services comprising a data network, and the issues facing this dynamic industry. 43~ Thomas No//e Pwsid& C/MI Corp. Lipdated by Datapro staff . . Basic Principles of Data Communications Data communications is the set of products. concepts, and services that enable the connection of computing systems. In this context, a “computing system” can be a source of information or information processing or a medium allowing an information user to access such a source. The “connection” ma’t’ be one that explicitlv makes one system the “client” and the other the “server,” as is the case with terminai-to-computer relationships. It may also be “peer” in nature, imposing no specific master/slave relationship. A collection of such connections, supported over a common circuit structure and using shared technological components. is called a data network. The elements of data communications are all slaves to the relationship between the business applications for computing, the carrier service and standards infrastructure (which includes regulatory issues), and the technology available. In the past. these factors tended to make data communications and data networks slaves of data processing planning. While it is not yet true that networks drive applications more than computers, it is certainly true that networks must be considered in designing human-system interactions. In the past, it was possible to develop computing applications without anv form of data network. That is uncommon today and will 4 be impossible in the near future. Technical Basis of Data Communications Computer systems and their associated devices store and use information using a binary coding. The number of binary bits that make up a character of information has varied, but the majority of systems today use eight binary bits to a character, or “byte.” This allows 256 different combinations of value. which can be mapped to represent letters, numbers, and special symbols. Any combination of bits can represent anything, as long as the systems using the data agree on the value. But to simplify information storage and retrieval, a standard code set is normallv employed. There are two such code sets in common use todav: the Extended Binary Coded Decimal lntercha~~ge Code (EBCDIC), developed by IBM and used on its large*systems, and the American Standard Code for Information Interchange (ASCII). a formal standard used by most midrange and personal computers. Code sets describe OIIIV how textual or “character” data is stored; there are other standards for the storage of binary numeric data, both fixed point and floating point. A standard representation is necessary whenever information generated by! one system must be read by another. This can occur if the systems exchange a transportable media like magnetic tape. It can also occur if the systems are linked over a communications channel. One computer system or device could communicate data to another over a wire or circuit if two basic sets of conditions were true: l The information channel was capable of transporting binary information with no errors or ambiguities sufficient to interfere with the application. 0 1995 McGraw-Hill, Incorporated Reproduction Prohibited Datapro Information Services Group, Delran NJ 08075 USA May 1995 “Datapro Communications Analyst”Data Communications: Basic IOOlDNW Concepts l Both agreed on the strategy to be used to transfer the information and the structure of the information itself. w Data communications can be viewed as the set of strategies needed to ensure these conditions. Analog and Digital Channels An easy wav to transmit binary data is to simply impress a two-value signal (+5 volts and -5 volts, +8 volts and J 0 volts, etc.) on the channel, with one of the values used to represent the binary I and the other the binary 0. This is called digital encoding, and it requires a circuit capable of transmitting the kind of “square” pulses shown in Figure “Anulog and Digital Coding Techniques.” The most pervasive communications system in the world is the voice telephone system, so it is logical to assume that any practical data communications network would have to rely heavily on this pervasive system for support. Unfortunately, human voice is not binary data, and the voice phone system was designed to pass audio, or analog, data. In fact. it was designed to pass the relatively narrow range of frequencies in which human voice carries most of its intelligence roughly 300 Hz to 4000 Hz. To make analog circuits suitable for digitally encoded information, it was necessary to develop a system to modify or modulate an analog signal or carrier in such a way that changes generated in response to a binary I could be distinguished from those generated in response to a binary 0. This might be done, for example, by transmitting a tone of I kHz for a “1” and 2kHz for a “0.” A device at the other end could then, by separating the tones, recover the digital signal. This modulation system, called frequency shifi keying, is still in use in telegraphy and is illustrated in Figure “Analog and Digital Coding Techniques.” ?rc A digital channel can. therefore, be created by a pair of modulator/demodulator devices linked by an analog channel. The devices became known by the acronym modern. Today’s modems employ enhanced techniques of information coding. increasing reliability and information capacity. However, all modems operate by impressing multiple values onto an analog carrier signal to represent digital data. As the public telephone system advanced, the advantages of integrated circuitry in processing phone signals became clear. Digital signals can be regenerated more easily in the presence of noise, because they can only have one of two possible values. Thus, the phone system moved to a strategy of digital coding of voice data through pulse code modulation, or PCM. Digital sampling of voice information at a rate of 8,000 times per second, with 256 possible values per sample, requires 8,000 x 8 bits or 64K bps capacity. This type of digital channel. called a “DSO,” is the foundation of today’s digital carrier system or “T-carrier” system. Tl, which consists of 24 DSO channels plus a framing bit, is a I S44M bps channel often used in integrated voice/data networks. Digital channels can be used to carry data directly; all that is needed is to connect the data device to the channel in some way. In North America, this is done with a two-step device called a channel service unit/data service unit (CSU/DSU). Protocols: Asynchronous and Synchronous CSU/DSUs, in conjunction with digital channels or modems and analog channels. can transport binary data. satisfying the first of the two requirements for data communications. The second requirement is an agreement on the format and rules for the exchange, called the “protocol.” The most basic element of a protocol is the definition of how data stored in a computer will be transferred to the line, and in what bit order. Another issue is just how the receiving device will divide up the bytes into bits. This is more complex than it sounds; information moving at a rate of 9600 bits per second (bps) would generate a new bit about once every 100 microseconds. There are two major strategies for synchronizing the bit/character timing of communications: asynchronous and synchronous. Figure “S~wchrmous and Asynchr*onozts Transmission Blocking Techniyltes” shows the difference between the two. Asynchronous communications places the bits on the line by framing them in a “start” and “stop” bit, with a predictable value. This allows the receiver to distinguish the start of a character from a condition of an idle channel. Within the character, the sender and receiver must “clock,” or time the bit intervals accurately. This is not a problem for a single eight-bit character. Because of the start and stop bits, however, the asynchronous strategy requires an average of IO bits to be transmitted to send the S-bit character. Synchronous communications is designed to eliminate this waste by grouping all of the characters of a message into a block and sending them together, The block is started with a “sync character,” or “flog,” and ends 0 1995 McGraw-Hill, Incorporated. Reproduction Prohibited Datapro InformatIon Services Group, Delran NJ 08075 USA May -t995 Communications Data Corn Concepts munications: Basic 1001 DNW 3 with an error-checking sequence designed to help the receiver detect block errors. But because blocks are likely to be long, the transmitter and receiver cannot track accurately and might lose count of where characters begin and end (“lose svnc”). To prevent this, synchronous channels provide both sender and receiver a standard clock d signal. Asynchronous communications is inefficient when used with devices that can coHect and configure information into blocks, but it is still very common for simple connections where the characters sent on a line are keyed by, or displayed to, a human operator. Synchronous data Iinks are best implemented where direct human intervention is not possible because of the speed. The ability of a human to view the result of a connection means that little must be done by the computer!terminaI to protect data; the “protocol” is that there is no special procedure. Thus. the term “asynchronous protocol” means that characters are sent as they are keyed or as they are to be displayed, with little or no control dialog. Synchronous blocks. having a potential strategy for block error detection and correction, can justify a more complex set of rules for information exchange. “Synchronous protocols” are therefore more complicated. employing strategies for detecting and correcting errors, controlling the rate of flow, and setting other characteristics of connections. Protocols-The OSI Model From the titne of the first practical data networks in the late 19150s~ “protocols” have been an area of concern, Cotnmunications is not possible between two systems that disagree on the procedures that have different protocols. Computer vendors all invented their own (IBM’s Binary Synchronous Communications or Bisync was an early, popular example). Because each was incompatible, equipment from different vendors could not communicate. In the early 197Os, a group of international communications experts devised a model for the connection of generalized data systems through communications networks. The tnodel was called the “Open Systems Interconnection Basic Reference Model” and became known as the OS1 model, the structure of which is shown in Figure “The CXI Model Networ~k.” The model divides all of the functions of data comtnunications into seven layers. each of which provides a cohesive set of services. International standards for each of the layers were developed in succeeding years, and even vendor-proprietary protocols took on the basic structure. The OS1 model is the basis for higher-level protocols. Communications Standards The need for both parties in a connection to agree on data presentation and dialog controt rules, the protocol, has already been noted. Standards to define these rules are as old as data communications and arise from three major sources: l Vendors thetnsetves whose proprietary protocols may be “open” to support by other vendors because the originating vendor publishes the specifications. IBM’s Systems Network Architecture (SNA) is an example of a proprietary, but “open” protocol. l Trade groups or consortiums, which represent special interests in a given communications market. The Institute of Electrical and Electronic Engineers (IEEE) is a trade group that has protnoted the basic standards for local area networking, IEEE 802. l National or international standards bodies, which formally debate rules and publish standards. In the U.S., the American National Standards Institute (ANSI) and the National Institute for Standards and Technology (NIST, formerly the National Bureau of Standards) are the principal standards bodies. The International Organization for Standardization (ISO) and the International TeIecotnmunications Union-Telecommunications Standardization Sector (KU-TSS. formerly known as the CCITT) are the two international standards groups most involved in data comtnunications. The OSI model is not itself a standard, but a framework wlhich describes the relationship of standards. There are standards for each of the seven OSI model layers, often several at the same layer. Applications for Data Communications 0 1995 McGraw-HI11 , Incorporated Reproductjon Prohi bited. Datapro lnformatton Servtces Group, Delran NJ 08075 USA May 1995 “Datapro nications Analyst” Data Corn Concepts nications: Basic 100’lDNW Data communications and data networks serve the information processing systems used, or contemplated, by a business. All information technology planning is based on estabt ishing a user-to-source relationship and identifying the technology elements needed to support it. Data networks are such an element, and data cotnmunications is the foundation of data networks. Early data communications applications were developed when computer facilities were so expensive that access to a cotnputer had to be given to numerous users via relatively primitive entry/display terminals. The computer processed and fortnatted all information at its central location. But as technology advanced, it created the microprocessor and enabled the development of inexpensive desktop computer systems. These allowed cotnputing power and information storage to be dispersed. In this new environment, four distinct types of data cotntnunications relationships developed: host/terminal, client/server, peer or distributed processing, and internetworking. Figure “C(~nlnlz(17icatior~s Network Relationships and Topologies” shows an example of the way that each type of relationship affects the structure of the network that must support it. HoWTerminal Relationships Normallv, host/terminal applications occur when the process of information entry and display is the major element bf the application, and the goal is to support the fastest rate of acquisition or output of data. In almost all cases, the system interaction is to a human operator, either directly (via keyboard and display) or indirectly (via printout). In host/terminal applications, the speed of the human/mechanical component of the connection is often low enough to limit the information flow rate to a level well below that of channel capacity. Because of this, host/terminal networks often include facilities to share the information channel among multiple terminals to reduce overall cost. These devices are called “terminal servers” or “cluster controllers.” IBM’s popular 3270 family of devices includes the 3 174 cluster controller. When a desktop computer is used to “emulate” a terminal, the interaction between the personal or other desktop computer and its partner system is still considered a host/terminal interaction. Any processing capabilities of the desktop system are “hidden” by the fact that the PC is emulating a dumb terminal. Host/tertninaI applications are forgiving of channel limitations. Their relatively limited speed has already been noted: high-capacity channels can be justified only by sharing them among multiple terminals. Host/terminal applications are also generally immune to delays in the data path, since the human reaction titne is normally tong enough to hide any network transit delay. Networks for support of host/tertninal relationships take on a “tree” structure, as shown in Figure “Comntrnicatiom Netwwk Relationships and Topologies. ” The tertninals are often concentrated via cluster controllers or servers onto a shared trunk, which may be further concentrated to a higher-speed facility. All infortnation paths lead to the computer systetn at the heart of the structure, and there is no connection between users except through that computer. Client/Server Relationships Client/server applications utilize a small computer at the point of human/system interaction and a larger one as a central repository for infortnation and/or information processing power. The client system can provide local services to its user, but it may from time to time require access to information stored at the server or to the server’s specialized processing resources. When this happens, the client reties on a data communications connection to the server. The file sharing and printer sharing done in PC-based local area networks (LANs) is a common example of a pritnitive fortn of client/server computing. Because client/server applications are between computing systems and not between a system and a human, the speed of the exchange of infortnation is not limited to human rates. Thus, the applications utilize a much higher channel capacity for the brief period of the interaction, though the capacity might be wasted during periods when the client svstem was involved in a local user dialog only, Client/server applications, therefore, benefit from strategies for sharing infortnation channels as well. w The extent to which the client and server systetns interact in satisfying a user need varies considerable. Some systems, such as electronic tnail systems, simply deliver a message to a client to be read by its user at ati convenient titne. In this case, the client/server interaction is relatively infrequent and not highly constrained by performance. But if the client system is processing a remote database, each record may be sent over the network. 0 1995 McGraw-Hill, Incorporated. Reproduction Prohtbited. Datapro lnformatron Services Group, Delran NJ 08075 USA May 7995 “Datapro Communications Analyst”Data Communications: Basic Concepts 1001 DNW In the latter case, network performance will have a major impact on the application. Client/server relationships often place information sources farther out toward the network user? often at the points of concentration. To provide access to these resources without loading the host, intermediate cross-connections are often provided, creating a meshing of these concentration points. Peer Relationships A client/server application requires two smart system devices, for example, a desktop computer and a data center system. However, despite the fact that both devices are computers, there is a master/slave relationship inherent in the fact that the server is a source of information often for many clients. Peer applications have no such inherent master. Peer systems are those that have relatively little difference in information storage or processing capacitv and are capable of adopting virtually any sort of relationship with one another according to the momentary ieeds of the application or user. Peer connectivity is the most challenging of all types of connectivity to provide, since there is no preferred information focus among the systems communicating. Without such a focus, any number of connections and flow volumes could be possible, and the capacity and number of channels needed to support them make design of a total network difficult. The unpredictability also makes it difficult to concentrate traffic for efficient use of circuits; there are no consistent partners to create consistent patterns of flow. A peer network, therefore, tends to connect users at all levels. There are few/ true peer applications today, because most companies have central data center resources or other departmental information storage points. Peer networking is most likely to be found in companies that rely on personal computers or desktop UNIX systems. H Internetworking Relationships All of the relationships described so far have been between information systems and have been explained in an application context. The last relationship, internetworking, is not a system relationship at all. but a network relationship affecting all users on the network. Internetworking is most likely to occur when a business that has previouslv planned data communications on a per-application basis begins to consider it as a kev part of its strategic planning. A large part of creating a strategic network is making information access Miithin the firm more universal, something that is often called “building an enterprise network.” reflecting the breakdown of internal network barriers. In a technical sense, inter-networking is the task of building a single, large network by combining existing ones while retaining the application support characteristics of each of the networks. Figure “Communications Net-work Relationships and Topologies” shows an internetwork structure created by linking a series of LANs. It is an area supported by specialized products discussed in the Switching Devices section later in this report. Data Transmission Services Given an application, the goal of data communications is to identify a set of transmission services that can be made to effectively support it and the equipment necessary to provide whatever adaptation is required. There are four major options for data transmission available to users: I. Public carrier services provide raw analog or digital transport capacity, often suitable for other forms of information transfer as well. Dial-up and leased analog and digital lines are examples of this. 2. Public value-added networks, also called “packet switched networks,” are designed to transport data only, and to do so at an attractive price relative to the more general analog and digital lines. 3. Private transmission systems on a single premises, based on local copper wire or other technology, are supported by a central switching device, such as a PBX, They may be called “local data switched services.” If thev are provided bv a shared high-capacity channel, thev are called LANs. 4. Private transmiss4ion systems can be based on radio or’optical technology, which can operate over distances of 50 miles or more, Private microwave is the most common example of this type of system. A “private network” is a collection of transmission services designed to provide user-to-user connectivity at a lower cost than could be achieved through the use of public switched services. Most “private networks” stilt relv J on leased carrier services. 0 1995 McGraw-HtII incorporated. Reproduction Prohibited Datapro Information Services Grou p, Delran NJ 08075 USA May 1995 “Datapro Communications AnalysYData Communications: Basic Concepts 1001 DNW Local Area Networks (LANs) In the late 1970~~ a Ph.D. candidate named Robert Metcalfe suggested that a coaxial cable might be routed around a facility and tapped into by each user connection. The information capacity of such a cable. IO tnillion bits per second, would be high enough to reduce loading and contention frotn this shared use to tolerable levels. This was the first practical definition of a local area network and its subsequent commercial exploitation by Xerox, Intel. and Digital Equipment Ethernet. Ethernet was originally designed to support terminal access, but the LAN quickly became associated with personal computers. A cotnmittee sponsored by the IEEE was charged with developing standards for LANs, and most LANs today adhere to the IEEE 802 standard set. LANs can be classified according to their topology and their access control. Topology refers to the physical configuration: access control to the way in which users are granted access to the LAN to send data. The most popular topologies are the bus, the star, and the ring. The most popular access control strategies are carrier sense multiple access with collision detection (CSMA/CD) and token passing. Ethernet is a CSMA/CD bus LAN; IBM’s Token-Ring is a token-passing ring. Today, LANs offer inexpensive and easy-to-implement communications between PCs, engineering workstations. midrange, and mainframe computers, and allow the delivery of many shared applications and services that were not possible before the advent of the LAN. Early LANs allowed the sharing of printers and disk drives; today’s systetns offer electronic mail, fax and imaging services, access to pools of modems and communications gateways, and the increasingly popular groupware, Wide Area Bandwidth Services Most data comtnunications today is based on carrier circuits designed for use by data. voice, fax, and other information forms because they supply only “bandwidth” or unstructured information capacity. These circuits can be classified according to their capacity, as follows: l Voice grade circuits, analog lines with a data capacity of up to approximately 19.2K bps. Telephone dial lines and leased analog lines are examples of this type of service. l Nar=l~oll~hanclcil~czrits, analog or digital lines with a capacity range of 56K or 64K bps to about I .5M bps (North America’s TI ) or 2M bps (Europe’s El ). Dataphone Digital Service (DDS) is a narrowband service. l Widehand cimri~s, digital lines with capacities from T I /El to 34M bps (Europe’s E3) or 45M bps (North America’s T3). l Broadband c’ircwits, digital lines with capacity in excess of 45M bps. Services can also be characterized as being leased or switched. Leased services are provided to the user continually without the need to dial and link two fixed points. Pricing is based on circuit capacity and distance. Switched services provide connections on request, between points selected from a list of available subscribers. Pricing is based on the type of service, the length of the call, and the distance between the points. A final service classification is terrestrial or satellite. A service built on ground-based facilities is terrestrial. Satellite services employ a geostationary satellite, orbiting approximately 22,500 miles above the equator as a relay between sender and receiver. The cost of satellite service may be lower in some applications, since it requires no intermediary relay points to serve remote areas and is adaptable to applications where one message is broadcast to many users. Satellite service introduces a transit delay, owing to the great distances involved in the relay path. and may impact performance for some applications. Today. voice grade, narrowband, and wideband services are based on the carriers’ own internal structure of digital trunks, called the Tl/El-carrier s)stem. This structure is based on the 64K bps DSO channel discussed earlier. Twenty-four or thirty-two DSOs are combined, with a framing bit, to form the I S44M bps Tl or 2.048M bps E I trunk, respectively. When 28 of these trunks are combined, the result is a T3 trunk, offering a bandwidth of 45M bps. The European equivalent, E3, carries 34M bps of bandwidth. A fiber optic carrier transport architecture, called the Synchronous Optical Network (Sonet), increases capacity beyond T3/E3. Sonet’s basic building block is a 50M bps channel called an OC 1. Sonet defines a hierarchy of channel combinations up to OC48, which would have a capacity of 2.4 billion bits per second. Sonet deployment is moving ahead quickly in carrier networks and services. There are three modern service concepts that may be of special interest to users: l Fractional TI/EI is a carrier service allowing users to lease capacity less than I SM/2M bps usually 256K bps 0 1995 McGraw-Hill , Incorporated. Reproduction Prohibited Datapro InformatIon Servtces Grou p, Delran NJ 08075 USA May 1995 “Datapro Communications Data Corn Concepts cations #: Basic IOOIDNW or higher. They provide the benefits of wideband digital service at a lower cost than full Tl/El. l Switched digital carrier services provide the bandwidth of a fractional or full TlfEI WAN link through multiple, independent 56K/64K bps dial-up connections. Multiplexing equipment owned by the user establishes these dial-up links through a switched digital n etwork and evenly distributes the data, voice, or video transmission across those links. Identical equipment at the destination, receiving those multiple transmissions, resynchronizes and recombines them to restore the original message. l The Integrated Services Digital Network (ISDN) is an ambitious plan to create a fully integrated switched digital network. Basic Rate ISDN, the most economical form, offers two 64K bps user channels and one 16K bps “signaling” channel. Primary Rate ISDN. a higher-capacity service, provides 23 (in the US.) or 3 I (in Europe) user channels and I signaling channel, all operating at 64K bps. A more detailed description of carrier services associated with data transmission, including ISDN and fractional Tl, can be found in other Datapro reports. Value-Added Services All of the services described so far provide the user with unformatted transmission capacitv and can be used with any type of information source/user, including fax, voice. or video. whose demand is Within the capacity of the service. This generality of capabilitv limits the capability of the service to offer specialized benefits to the w data user. A network designed onlv for data can optimize the transmission of data and reduce overall network r’ costs. A Rand Corp. study of the 1960s showed that data can be broken into small “packets” of information and moved through a network of shared trunks and “nodes” to its destination. The sharing of circuits and equipment can result in a lower per-character cost than could be achieved through the use of “ban dwidth” services like fractional Tl /E I. Th is study became the basis of v falue-added, packet s witched data ne tworks. Public packet network services differ from the dial-up or leased bandwidth services described earlier in several important ways: l There is a specific access protocol required to attach to the network and transfer information. Traditional carrier services are protocol independent. l Network services are priced based on usage, meaning the number of characters transmitted from source to destination. The distance between users and the duration of the connection are not normally bit1 ing factors. l The introduction of multiple shared trunks and nodes generates an appreciable delay in information transport, often greater than that generated by a satellite data path. This can impact the performance of some applications. Public packet data services are most useful when an application involves the support of a widely dispersed population of “occasional” users. Packet technology can also be employed in private networks. Because packet network interfaces are based on international standards, such networks are excellent for interconnecting computers from different vendors. Packet standards also form the foundation of the OS1 protocols, discussed briefly earlier in this report. A recent industry development is fast-packet switching a streamlined approach to packet processing providing greater efficiency and lower transit delay for interactive data, voice/video, and multimedia communications. Fast-packet switching assumes that the wide area connection. utilizing fiber optics rather than copper, is virtual ly error-free. It elimi nates error checking, th erefore, at all but the destination node of a transmission. On e type of fast -packet service, frame relay, is ideal for interactive LAN/WAN applications that cannot tolerate delay. It propagates data in variable-length frames across star and mesh networks of any size. Fractional and full Tl /E I frame relay products and services are widely available. Asynchronous transfer mode (ATM) products and services, operating at T3/E3 speeds and higher, process videoconferencing, and multimedia network transmissions in small fixed-length (53.byte) cells. thereby minimizing delay and congestion. ATM switches and carrier service offerings have been emerging rapidly since 1994. Data Networking Equipment The minimum amount of data communications equipment needed to support’s connection is the communicating device, a transmission facility. and the interface equipment needed to connect to it (a modem or CSU/DSU, for 0 1995 McGraw-Hill Datapro InformatIon Incorporated Reproduction ‘Services Group, Delran NJ 0 Prohibi ted. 8075 U SA May 1995 “Datapro Communications Analyst” Data Corn Concepts munications: Basic IOOIDNW example). Most data communications environments are much more complex. The need to utilize concentration to spread the cost of carrier services among multiple users has already been noted. Wideband and broadband services not only have capacity in excess of what most single applications can justify. they often have service interfaces that most computers and terminals cannot directly support. For these reasons, data communications equipment is most often really data networking equipment. Its purpose is to establish a shared set of facilities that users and information sources can use to make connections and to maintain those facilities in correct operation. Interface Devices Most transmission services, and all public carrier services, require some form of interface through which computers and terminals can attach. The interface provides whatever transformation is required between the digital interface on the computer or terminal and the carrier service itself. It also generally provides some status indicators that enable the computer or terminal to determine if the service (and the interface device) is operating properly. These are called “control signals.” in that they provide for the local control of the interface. Modems are the most common service interface, designed to link digital computers and terminals to each other via analog carrier services. Modems can be classified as either synchronous (supporting the generation of the synchronous clock signal) or asynchronous, and by the data transfer rates they support. Modems normally transmit data at 2400, 9600, 14.4K, l9.2K, or 28K bps. CSUs/DSUs are devices that interface communicating equipment to digital carrier services. Lower-speed CSU/DSUs, designed to support narrowband digital services, are much like modems in appearance. The higher-speed CSU/DSU devices, designed for wideband Tl, for example, are normally built into networking devices that use the wideband interface. Both modems and CSU/DSU devices can provide special services, going beyond simple interfacing. Data compression uses one of several algorithms to contract the data stream generated by a data device, thereby decreasing the number of characters being transmitted and increasing the effective throughput. Compression rates of 2: I or 3: I are typical, and some information can be compressed even more. Network management support on interface devices allows a user to monitor the quality of the circuit connecting the devices and to run basic tests as well. Encryption prevents interception of information by encoding it. Backup features allow a leased-line modem or CSU/DSU to dial a backup connection should the leased service fail. Concentration Devices Human-operated terminals are rarely utilized at a rate that taxes even a low-speed transmission facility, yet the minimum dial or leased carrier circuit has a theoretical capacity of 2400 bps or more. Even for voice grade and narrowband services, some form of concentration of multiple terminals onto a single circuit will improve economy. Terminal servers and cluster controllers are devices that are provided by the computer vendor to accomplish this concentration, but there are other devices as well. Multiplexers are the most common form of third-party concentration device. There are several types of multiplexers: l Frequency-division multiplexers separate multiple conversations by allocating each a different carrier frequency to be modulated by the digital data. This type of multiplexing is old and extremely rare outside carrier applications. l Time-division multiplexer+-separate multiple conversations by allocating each a reserved amount of space in a digital “frame” of data, which is transmitted at a regular interval. A TI or El frame consists of 24 or 32 eight-bit bytes plus a framing bit, so Tl/E I is a form of time-division multiplexing. l Statistical time-division multiplexers identify information generated by each conversation through a “heade code that prefixes the data. Capacity is allocated to a conversation when that conversation needs it. 0 Networking multipiexers high-end statistical multiplexers performing any combination of concentration, adaptation, and intelligent routing. Usually supporting several TI /E I aggregates, they enable multiple termina devices and host computers to exchange information over star or mesh networks. l Access tnultiplexers provide access to an enterprise backbone for a remote office environment. Access r” multiplexers usually concentrate data from multiple devices onto single or dual trunk connections leading to the 0 1995 McGraw-Hill, Incorporated Reproductlo n Prohi bited. Datapro information Services Group, Delran NJ 08075 USA May 1995 “Datapro Commun ications Analyst” Data Corn Concepts municatio ns: Basic 7001DNW networking multiplexer. As concentrators, they perform little or no intelligent routing. l Voice/data multiplexers can combine both voice and data switching. l Digital access devices such as inverse multiplexers provide dial-up access to multiple, independent 56K or 64K bps channels, usually over a single network access trunk. This capability provides variable bandwidth for a single application or dynamically shared bandwidth for two or more concurrknt applications. Statistical multiplexing differs from the other strategies in that it dynamically allocates capacity. The benefit this provides is better utilization of the shared communications circuit; fixed allocation of resources wastes capacity because it is assigned to a conversation, even when it is idle. Multiplexers that operate between two points and provide the appearance of a series of dedicated circuits are called “point-to-point” multiplexers. Multiplexers that can be used to create more complex interconnections of circuits and switch data between them are called “networking multiplexers” or “hubs,” Switching Devices The communications processor, usually designed for one or more vendor-specific families of computing and communications equipment, performs iletwork control. intelligent routing, and concentration functions. As a front end to a host computer, it serves as a master processor. relieving the host of the overhead involved in message handling and network control. As an intelligent switch, it routes messages across the network. either under the control of a higher-level communications processor or as a peer of other intelligent switches. Remote concentrators control a community of terminals or distributed application processors. gathering, queuing, and multiplexing their transmissions onto one or more high-speed network trunks. These concentrators also often provide protocol conversion and gateway functions to attached devices. Point-to-point multiplexing of a carrier circuit is useful in sharing costs where there is a co-located community of users who need access to a common resource elsewhere. Unfortunately, this simple situation is not pervasive in business; complex patterns of access are now the rule. Furthermore, higher-capacity circuits often need a number of users to be justified. Figure “Transit Routing and Switding Techniqzres” shows that it is possible to concentrate traffic onto a trunk line by bringing some traffic to one trunk termination from locations even more remote (locations “C” and “D” on the figure). This would also provide users in that remote location with access to ant’ information resource located at the near trunk termination (“A” on the figure). But to support this config&ation, there must be a network device at location “A” that can distinguish between traffic from “C” or “D” which is destined for “A” and then destined for location “B.” This requires a routing or switching function. Switching devices are the core of modern traffic for ecoi jomy ai Id ful I connect ivity to without them. Private data networks can be data networks, because the collateral need support all types of information connectio defined as networks in which user devices s of concentration of ns cannot be readily met provide this switching, and public networks are those where the switching is provided by the carrier as a part of the service. There are four major classes of switching devices: I. Connection switches such as PBXes provide a large population of users (telephone users, terminals, etc.) with access to a more limited number of shared resources (tie lines, computer ports, etc.), by allowing users to select a destination through a dial-like mechanism. 2. Concentrator hubs, such as communications controllers or packet nodes, route information among multiple trunk lines and user connections based on information provided within the protocol used. These devices support multiple protocols making them “protocol independent” and are normally either provided by the computer vendors or conform to a vendor-proprietary or formal standard specification. They are therefore limited to use within networks whose protocols conform to those specifications. 3. Multiplexer hubs, such as TI/EI TDM nodes, route information in a way that is information-format independent. These devices can be used in any tvpe of network, as IOII~ as the capacity requirements for the application can be met with the carrier services and equipment. 4. Internetwork switches connect multiple data networks. These are most often used to link LANs and are further classified according to the OSI protocol level at which they operate (see Figure “The OS/ MO&/ Network”). Bridges are internetworking devices operating at Level 2, the Data Link Layer. Routers operate at 0% Level 3, the Network Layer, and gateways operate at any level above Level 3. 0 1995 McGraw-Hill, Incorporated R eproductio In Prohi bited. Datapro Information Services Grou PI Delran NJ 08075 USA May 1995 “Datapro Communications Analyst" Data Comm Concepts unications I: Basic 1001 DNW 10 Switching devices are often called “nodes” because they form the junctions between routes or trunks in a data network. Because the control of node operation and routing of information will control overall information flow, the “management” of a network is tvpically based on the management of the nodes within it, r’ Network Monitoring and Management When an application uses a carrier circuit to connect users to information sources, the failure of that circuit will cause the user to lose service. Because this is a localized problem. the business may elect to tolerate it for the period needed for the carrier to restore operation. But when a shared circuit or a node in a data network fails, many users may lose their ability to interact with information resources, and company operations may suffer. The more complex the network. and the more shared resources are used, the greater the need for explicit attention to service assurance management. There are two goals to service assurance: restoration of acceptable service in the shortest possible time and recommissioning of the failed facility itself. The former can be accomplished through the use of alternate routes for information. The latter will require identification of the specific component of the network that is faulty and the support of the provider of that component. A broader set of goals, comprising network management as a whole, builds from the service assurance goals listed above to include requirements for capacity planning for future application needs, accounting for network resource usage for billing, and control of access to network and network-projected resources to prevent unauthorized intrusions or information damage. There are three basic types of tools that are applied to the meeting of network management goals: network monitoring systems, test systems, and network management systems. The operation of these devices in a network is shown in Figure “Net-war-k Monitor-ing and Management.” Network Monitoring Equipment The goal of network monitoring is the examination of the protocol exchanges at points within the network, so that conditions there can be compared to normal operating conditions and reasons for differences determined. Depending OJI where this monitoring process occurs, the device used is called either an interface monitor or a data line monitor. Both types of devices are inserted into a connection and report on what passes through them. Interface monitors are used to test the boundary between services and devices, verifying the state of the interface. Interface monitors display the status of the “control signals” mentioned earlier in this report, which advise each element of the interface on the status of the other. Interface monitors normally have LED or LCD indicators to display important status conditions and may also provide a means of manipulating the control signals to force specific conditions. Because these devices “break out” the control lines at the interface, they are often called “breakout boxes.” Most are handheld and inexpensive devices. Data line monitors also show the condition of the interface where they are inserted, and the data flow through the interface. Because this data flow is normally defined by a protocol, most data line monitors include the capability to interpret the protocol and to indicate unusual situations. At the very least, they will normally provide a formatted display of the messages being exchanged on a small CRT display or LCD display. Data line monitors are normally microprocessor based and are considerably more expensive than interface monitors. The use of either a data line monitor or an interface monitor is affected by two facts. First, the monitor must be placed into the circuit at the point being tested. This requires either moving the monitor (and an operator) to that point or providing a “test point” at the location with remote access provisions. Second. the monitor must be used by someone who is skilled at its operation and can interpret the results. Network monitors are gaining greater intelligence by using expert technology to monitor, analyze, and automatically diagnose problems in a communications network. These intelligent protocol analyzers can decode several hundred protocols and recommend problem-solving action. These instruments can also perform trend analysis and display and indicate instantaneous traffic in graphical or tabular formats. Network Testing Devices Monitoring is a passive function; it relies on the interception of messages across an interface where test access is available. Sometimes a communications problem manifests itself by the failure of either partner to attempt 0 1995 McGraw-H/II, Incorporated. Reproductio n Prohibtted Datapro InformatIon Services Group, Delran NJ 08075 USA May 1995 [...]... Reproduction Prohibited Services Group, Delran NJ 08075 USA Data pro I nformatlon May 1995 “ Datapro Communications Analyst” Data Communications: Concepts Basic 1001 DNW 12 Data communications and data networks are more than products and services Personnel must plan select, install, and tnaintain equipment and coordinate services Because data networks project computer information, they are closely...“ Datapro Communications Analyst” Data Communications: Concepts Basic IOOIDNW communication at all, resulting in nothing to monitor When this happens, or where a component of the network is to be examined outside a connection dialog, an active testing device is needed The simplest testing devices generate patterns of data either to display on a terminal or to... conjunction with voice services, data networks are also often linked to telecommunications planning and operations Many businesses have computer and communications organizations that have been inherited from a period when the role of data communications was very different than it is today In some cases, this results in a subordination of data communications issues to data processing interests In others,... While it is often true, the fact is that the network underpinnings may have changed many times in that period 0 1995 McGraw-Hrll Datapro Information Incorporated Reproductron Prohibtted Servtces Group Delran NJ 08075 USA May 1995 “Datapro nications Analyst” Data Communications: Concepts 700lDNW 13 The problems of change are magnified by the fact that new information technology plans often disperse computing... or Flag) Figute 3 The OS1 Model Newark User System “B” User Sy8t2m“A Application /I 1 RWentatiOIl Presentation I session Session hpofi Network 44 Network Network Network DataLink Data Link Data Link Ph@d Physical Physical Figure 4 Communications Network Reiutionships and Topologies Host/Terminal Relationship Peer Relutlonship Client/Server Relationship Intern&work Relationship Location ‘ C’ Figure... r’ improves This report was prepared exclusively for Datapro by Thomas Voorhees, New Jersey, and specializing in strategic planning services are regularly quoted in major trade publications CIMI Corp provides strategic planning, market users, vendors, and communications carriers research, 0 1995 McGraw-Hill Incorporated R eprodu ction Prohibited Datapro InformatIon Services Grou P1 Delran NJ 08075... actual usage (connection time and/or bandwidth allocated) rather than on the peak capacity available New technologies, such as switched digital (n x 64K bps) transmission, frame-relay data transport, Switched Multi-megabit Data Service (SMDS), and ATM, are enabling such services to evolve Broadband networks of the future will probably have a stronger carrier component, much like virtual networks of today,... bandwidth traffic The local distribution strategies are evolving from hub and FDDI principles Somewhere, the two trends must mesh As users plan for the future, the role of carrier “virtual” networks for data communications will also become more important A virtual network offers users the services of a private network without the equipment investment This insulates the user against the problems of change,... very simple protocols, since they tack the intelligence to obey complex rules for information exchange Data line monitors, with microprocessor intelligence, provide active testing capabilities of even complex protocols Called “protocol emulation,” this testing capability allows a user to certify the basic operation of a device and confirm the essential characteristics of a carrier service Testing devices... of a central support desk can be considerable, but so can the technological implications Integration of network management systems into a single platform is valuable where data networks are centrally supported, but it is critical if data network management is to be viewed as an element of user information services management This view is essential to any central support of IT resources In the longer . “Datapro Communications Analyst Data Communications: Basic Concepts 1001 DNW Data Communications: Basic Concepts Datapro Summary Data communications. C/MI Corp. Lipdated by Datapro staff . . Basic Principles of Data Communications Data communications is the set of products. concepts, and services