Ref: 99980049 Title: Internetworking Date: 11/28/88 Copyright 3Com Corporation, 1991. All rights reserved. .h1;Introduction Computer networks are very much a part of today's workplace. Local area networks (LANs) or wide area networks are used extensively to solve data communications problems. They allow a variety of computing devices ranging from personal computers and workstations to minicomputers and mainframes to communicate. As organizations grow, so do their computing and networking needs. Departments swell, divisions mushroom, and branch offices open in various locations around a city or across the country. It is common for one organization to have many networks, widely ranging in size and performing different functions. In an office, a PC network enables users to share files and printers. In the engineering division, network connections provide workstation-to-host access for computer-aided design applications. Elsewhere, a remote manufacturing site's network suports computer-aided manufacturing functions. With the wide acceptance of computer networks, a new requirement is clearly evident - internetworking. Internetworking, quite, simply, is the ability to connect multiple networks. The most obvious benefit to creating an internetworking system is expanding the network size. Users on one network can access resources on another network. Communications among divisions or remote locations can increase dramatically. Other benefits are not as apparent. For example, segregating networks can ease management and directing traffic flow can achieve higher data throughput. Once linked, the various networks become subnetworks to the newly created larger network called the internetwork. The internetwork is the communications foundation for the entire organization. It provides the computing connectivity necessary to ensure high efficiency and productivity. Internetworking products enable interconnection of both similar and dissimilar networks. Interconnection of dissimilar networks brings up a particularly complex set of issues. Networks use a variety of communications protocols such as XNS, TCP/IP, or OSI, as well as different technologies such as token ring, Ethernet, and the forthcoming FDDI. In addition, the same technology can support different media. For instance, Ethernet can use thin coaxial cable, thick coaxial cable (baseband or broadband), twisted-pair wiring, or fiber-optic cable. In spite of the difficulties arising from this wide variety of technologies, however, solutions to these connectivity problems do exist. An array of internetworking products - repeaters, bridges, routers, and gateways - is currently available to network designers. Each serves a different purpose. To select the appropriate internetworking product requires a thorough understanding of the application. There continues to be much confusion as to the benefits of and difference among these products, especially in the case of bridges and routers. The primary purpose of this tutorial is to clarify the functions and appropriate applications of each internetworking product type. After a brief review of the four major product classifications, there is a more detailed discussion of two internetworking products - bridges and routers. These products require further examination due to their rising popularity and the confusion that surrounds their respective application. Descriptions of their unique benefits, the problems they best solve, and how they can coexist on the same internetwork are all discussed. To provide greater understanding, there is also an explanation of how these devices work. Finally, this tutorial offers guidelines for network managers to consider when choosing among these devices for particular applications. .h1;How To Use This Tutorial The major sections of this article can be read consecutively or as independent units depending on the level and type of information needed. For a more complete view of internetworking, read the entire guide. For a brief summary, read the Internetworking Solutions section. Should you encounter any unfamiliary terms, refer to the glossary. .h1;Internetworking Solutions Internetworking products fall into four broad categories - repeaters, bridges, routers, and gateway. Each handles different functions that directly correspond to the ISO layer at which the internetworking function is performed. To understand these functions, it is necessary to be familiar with the role of each layer in the ISO model. The first diagram briefly describes this model. The next diagram illustrates the ISO layer at which each internetworking product operates. ISO Reference Model ------------------------------------------------------------------ Open System Interconnect Layer Function 7 Applicaton Specialized functions such as file transfer, virtual terminal, electronic mail 6 Presentation Data formatting and character code conversion 5 Session Negotiation and establishment of a connection with another node 4 Transport Provision for end-to-end delivery 3 Network Routing of packets of information across multiple networks 2 Data Link Transfer of units of information, frames, and error checking 1 Physical Transmission of raw data over a communications channel Internetworking at Multiple Levels ------------------------------------------------------------------ 7 Application <--------- 6 Presentation | Gateway 5 Session <--------- 4 Transport 3 Network <--------- Router 2 Data Line <--------- Bridge 1 Physical <--------- Repeater Repeaters Repeaters extend the geographic coverage of a local area network by interconnecting multiple segments. For example, the Ethernet standard specifies a maximum length of 500 meters for a single segment; but with repeaters interconnective five segments, an Ethernet network can reach a maximum distance of up to 2,500 meters. Repeaters can also interconnect segments using different physical media such as thick Ethernet, thin Ethernet, or fiber- optic cbles. Repeaters are hardware devices that operate at the Physical layer of the ISO model repeating all electrical signals from one segment to the other. They do not provide any type of traffic isolation. Bridges Bridges interconnect local or remote networks at the media access (or MAC) sublayer of the Data Link layers of the ISO model. Bridges are transparent to high-level protocols such as XNS, OSI, or TCP/IP. Their main purpose is to partition traffic on each of the interconnect segments. Bridges forward only traffic addressed to the other subnetworks, increasing the effective throughput of the entire network. Although all segments interconnected by a bridge form a single logical network, they are elctrically isolated from one another. Local bridges may connect similar networks such as Ethernet or broadband or dissimilar networks such as Ethernet to broadband. Remote bridges use data communications links such a T-1 lines to join physically isolated networks. Routers Routers perform packet routing and forwarding functions. Operating at the Network layer of the ISO model, they link networks that share the same network layer. Most routers on the market today interconnect networks that use the same high-level protocol suites. But there are routers, recently introduced, that work in multiprotocol environments. Due to their recent arrival and their support of mixed protocols, these routers are respectively less mature and more complex than those which function in single-protocol applications. Routers create a number of logical subnetworks, allowing large internetworks to be organized into different administrative domains. Routers, for example, may interconnect Ethernet segments either locally or remotely over point-to-point lines. Another example is an applicaton where routers interconnect token ring and Ethernet networks over an X.25 wide area network. Gateways The most complex of internetwork products, gateway interconnect networks that have totally different communications architectures. Since the network facilities and addressing schemes are incompatible, the gateway must provide complete conversion from one protocol stack to the other without altering the data that needs to be transmitted. Currently available gateways make such interconnections as TCP/IP to SNA or TCP/IP to X.25. A TCP/IP-to-SNA gateway gives users on a multivendor TCP/IP local area network access to IBM hosts through the SNA protocols. Gateways between local area networks and X.25 wide area networks either connect LAN users to X.25 hosts or large databases on an X.25 Public Data Network (PDN) or they connect users attached to a Packet Assembler Disassembluer (PAD) on the PDN to LAN devices. .h1;Bridges The relatively new concept of MAC-layer bridging offers powerful solutions for a number of internetworking problems. Since a bridge operates at the Data Link layer, a low level of the ISO model, it can transparently pass traffic running different high- level protocols. Bridges are thus a flexible and cost-effective choice for heterogeneous network environments. A bridge can interconnect networks running a wide variety of applications using such protocols as XNS, TCP/IP, or OSI, though only devices running the same protocols can communicate with one another. For example, traffic from a TCP/IP device is understood only by other devices running TCP/IP. In addition to interconnecting networks running different protocols, bridges can connect networks using different media such as coaxial cable (baseband or broadband), fiber-optic cable, or twisted-pair. For instance, an internetwork might consist of several coaxial cable networks or a mixture of coaxial, twisted- pair, and fiber-optic cable networks. The interconnected networks can also use different access methods, for example, CSMA/ CD or token passing. One of the limitations on such internetworks is the possible difference in maximum frame size supported by the various subnetworks. It is essential that the high-level protocols passing through the bridge do not violate the maximum packet size on any network segment. Along with protocol transparency, another important benefits of a bridge is to provide simple traffic isolation between physical segments or cabling systems that make up the internetwork. The bridge accomplishes this with a learning algorithm that alerts the bridge as to which packets should stay on the segment and which should be forwarded to another segment. These steps in the traffic isolation process are known as learning, filtering, and forwarding. There are two main categories or bridges: local and remote. A local bridge interconnect two or more directly attached local networks. A remote bridge connected multiple physically isolated networks by means of long-haul data communications links. Typically, these are point-to-point links with speeds ranging from 9600 bps to T-1. Basic Principles For the learning algorithm to function, the bridge listens to all traffic on the attached segments. It then checks the source addresses of all packets and the location of their sending station. The bridge progressively organizes these addresses into a table that it uses to determine whether a packet should be discarded (filtered) or forwarded. If a packet is forwarded, the bridge consults its address table to select the appropriate destination for the packet. Filtering and forwarding are relatively simple in the case of a local bridge connecting only two networks, but become increasingly complex for local or remote bridge interconnecting multiple networks. As does any other node on the local segment, the bridge regenerates each packet it receives. Therefore, the number of nodes on the segment or the distance the packet travels before reaching the bridge has no effect on the quality of packets being forwarded to another segment. Any delay related to distance, however, must meet the requirements of the high-level protocols involved. The forwarding and learning process assumes that the topology of the overall network is a tree or that there is only one path between any two nodes located on LANs separated by bridges. If active loops (parallel paths) exist, problems may occur, such as packets being duplicated or traveling endlessly throughout the internetwork. In order to deal with this problem, some bridges implement intelligent algorithms to detect loops and shut down alternate paths. The Spanning Tree Algorithm is one example. If the active path fails, one of the inactive paths takes over automatically. For remote bridges, parallel lines do not constitute a loop, so bridges can balance the internet traffic among multiple lines. This allows planners to design networks with some level of redundancy. Topology The most common topologies for bridges are cascaded networks or backbone networks for local applications and star topologies for remote applications. A choice among these depends on the number of computing devices networked and how much partitioning they require. In a case where the performance of the network is no longer satisfactory due to traffic bottlenecks, bridges can divide the network into segments - forming a cascaded network. The bridges control and monitor inter-segment traffic, restoring the efficiency of each segment. For example, a bridge might isolate a group of PCs or workstations sharing the same file server. Whatever the case, a cascaded network should probably include no more than five or six segments. Otherwise, the delays introduced by successive bridges may become excessive for the higher-level protocols, as well as intolerable to users. A high-speed backbone is a reasonable alternative in cases where many segments need to be linked. One prime advantage is that such a configuration allows systematic network growth. In contrast, many cascaded networks are the result of pressure from unplanned growth. Another benefit of backbone configurations is improved performance, since inter-segment traffic only passes over one intervening segment between the source and destination segments (unlike a cascaded network where traffic must traverse all intervening networks). A backbone topology is extremely efficient in an office tower with many floors. In this case, an Ethernet backbone - either coaxial or fiber-optic cable - runs the full height of the building. Ethernet ribs extend from the backbone onto each floor. Bridges partition the traffic among the floors, maximizing the performance of each segment. The star topology is the most common choice for remote applications. It allows remote sites to be interconnected with a minimum number of intervening segments and without loops. For instance, a large corporation with several divisions and remote sales offices can solve its connectivity problems with bridges in a star configuration. Divisions with high network bandwidth requirements are connected to the headquarters through high-speed T1 links and a 56 Kbps back-up link, whereas smaller remote offices are interconnected with lower speed lines. Network Management Given the complexity of an internetwork and its many components, it is important to manage these resources for most efficient use. It is also critical to manage these resources from a central location. In this area, bridges are privileged devices since they see all traffic on each attached segment. Some intelligent bridges can collect information and display it on an attached console terminal or they can send it to a central network management station for further analysis. A strong network management scheme enables a network manager to display bridge-supplied status information and statistics on a global or per node basis. Reports may include network utilization as seen in the accompanying figure, number of frames transmitted or received, collisions, or alignment errors. This information helps the network manager troubleshoot the network and make future decisions regarding the network design. For example, if network utilization levels reach 50 or 60 percent, the manager might consider adding another bridge or changing the topology from cascaded to backbone. Security and Protection Intelligent bridges can also help the network manager control internetwork security. With special filters, stations or network segments can be individually protected from specific stations or packet types. This type of filtering is different from the learning filtering described earlier in that it allows the network manager to prevent certain kinds of packets from crossing the bridge. The network manager can do this by specifying a particular pattern anywhere within an Ethernet frame. This pattern can correspond to a protocol type, a protocol header, a source address, a destination address, or any traffic pattern. For example, a bridge can filter all XNS traffic or can prevent network nodes on a segment from accessing a large computer located on another segment. Performance Another important area requiring careful study is that of performance. A bridge filters all the packets transmitted on each of the attached networks. In practical terms, this means that the bridge must be able to receive and check packets at a rate corresponding to the maximum anticipated usage for each network segment. If the bridge cannot accommodate this traffic load, it will lose packets, causing the end stations to retransmit them This results in performance degradation and possibly session disconnections. The maximum possible usage of a network is often much less than its theoretical limit. A 10 Mpbs Ethernet network can theoretically carry small-size (64 byte) packets at a rate of 14,880 packets per second. Because of the access method used by Ethernet, 100 percent efficiency can only be achieved by a single station of synchronized transmitting stations. Otherwise, too many collisions occur and overall network throughput is negatively affected. Even very large Ethernet networks rarely exceed 50 percent utilization for a long period of time. In practice, a local bridge that can handle an aggregate filtering rate of 19,000 packets per second is appropriate for virtually all traffic conditions. Another parameter that bears examination is the forwarding rate. Bridges partition the load on the different segments so that the amount of inter-segment traffic is relatively limited. For instance, users and their file server should be on the same network segment. Aggregate rates of about 6,000 packets per second are sufficient for all applications, and are within the range of existing local bridges. .h1;Routers Routers are devices that interconnect multiple networks, primarily, running the same high-level protocols. They operate at the Network layer of the ISO model. With more software intelligence than bridges, routers are well-suited for complex environments or large internetworks. In particular, they support redundant paths and allow logical separation of network segments (so each segment can have its own network number). Routers also better solve the problems associated with interconnected network segments using different media such a token ring and Ethernet. For instance, the packet size is controlled by the Network layer and is identical on both sides. The problem of address resolution is also addressed in a better way in that routers, unlike bridges, do not pass MAC layer addresses from end-to-end, but rather each router knows the MAC layer address of the next router in the path. This approach avoids the delicate problem of converting MAC layer addresses from one format to the other before transmitting a packet. Since routers impose no topology constraints, they provide sophisticated routing or flow control as well as traffic isolation. How routers perform these functions depends largely on the network protocol they use and the particular implementation thereof. Unlike bridges, routers do not require full participation of sending and receiving stations in addressing packets. Routers give network managers the ability to define boundaries for administrative control. Using a hierarchical addressing scheme, a network manager can divide a large internetwork into small administrative domains. A good example is a university campus with a backbone network linking different departments. With routers, each department's network can remain logically separate and under the administrative control of the department. Basic Principles In contrast with a bridge which makes a simple forward or discard decision, a router is able to choose the best route for each packet. It can do this because the address of the final destination network is attached to the packet. Based on this information, it looks in a routing table to disconver the best path. In this process, it is important to understand the fundamental difference between a bridge and a router: as it was described in a previous section, a bridge looks at all the packets sent on its attached network, whereas a router receives only the packets that are addressed to it directly by either an end station or another router. Each node on the path knows how to go to the next hop in order to get a certain network. The routing process is handled on a step-by-step basis. Because routers selectively forward packets, loops are allowed in the internetwork topology. In addition, most routers implement a time-to-live program for packets. This process, which consists of destroying packets that have traveled too long or through too many routers, prevents bad packets from congesting the network. Routing Mechanisms The information used by routers to direct packets is updated either statically or dynamically. With static routing, the network manager configures specific paths to the different segments in the internet. With dynamic routing, the router itself creates and updates paths as changes occur on the internet. The router does this by constantly monitoring internet activity. Any time an important event occurs, for example, a station is removed or added, the router revises internet paths to reflect such a change. Dynamic routers usually also have static- routing capabilities. In a pure static-routing applicaiton, direct route control is up to the network manager. For this reason, it is not well-suited for large internetworks where the topology is subject to change. Dynamic routing relies on specific protocols to convey the routing information throughout the internetwork. Using RIP (Routing Information Protocol), a component of both the XNS and TCP/IP protocol suites, routers can broadcast their own routing information. RIP makes routing decisions using an algorithm based on the number of hops between two networks. As a result, the router ensures that traffic travels over the shortest possible path. To assist in this process, the network manager can manually change the number of hops associated with specific routes to modify the preference for one route over another. Topology Routers are often the foundation of large internetworks. They impose no constraints on network topology and they provide a way to divide the internetwork into domains or subnetworks. Since the maximum size of an internetwork often depends on the time required for a packet to go from one end to the other, routers' ability to select the shortest paths allows larger configurations. Routers also permit systematic development of complex networks as an oranization grows. .br;Load Sharing By spreading the traffic load over the internetwork, routers minimize the possibility of congestion at a single point. A remote router with parallel links can choose at will whichever link is currently providing the most rapid packet delivery. Another way to provide load balancing is to statically assign groups of stations to different routers, preventing traffic from focusing on a single router. This reduces delay in the routers and ultimately improves network throughput. Congestion Control Internetworks sometimes face congestion problems similar to those on highways in densely populated areas. This situation usually arises from processing limitations of internetwork products or from speed mismatches between LANs or the longhaul links that interconnect them. Routers can handle these problems in different ways. In the rare case of traffic overload, routers can drop packets and then cause them to be regenerated later. This is consistent with the "best effort" approach in datagram networks. Such a method is often counterproductive, however, since it does not solve the funamental problem of sources sending more packets than the routers cna realistically handle. To overcome this problem, some more sophisticated routers can send special packets (i.e., ICMP source quench messages in the IP protocol) to the sources asking them to slow down. Upon receiving such a quench message, the source learns the destination is congested so it slows or even stops traffic to that node. Network Management To provide the best possible service, routers need good network management capabilities. For example, a network manager trying to identify communications problems between two end nodes must be able to trace the exact paths packets are following. In another case, to take advantage of a new interconnection the manager may want to change routes in the router or even modify the preference levels among routes. Whatever the situation, accurate statistics are essential for measuring and maintaining the quality of the interconnection service. Other prime requirements are an ability to perform all management functions from a central location and an audit trail that automatically collects statistics. With these features, the network manager can monitor traffic and then anticipate and control any problem. Currently available routers offer most or all of these network management capabilities, enhancing their attractiveness as internetworking solutions. .h1;Bridges vs. Routers:Selection Guidelines Network planners must completely understand the computing and networking requirements of their environments before they can design an efficient internetwork system. In particular, the choice between a bridge or a router depends on a thorough analysis of the application. To simplify this decision-making process, the following section compares the relative strengths and weaknesses of bridges and routers in specific area. Computing Environment In environments with a variety of computer resources (PCs, workstations, mainframes) there are varying requirements for network bandwidth. For instance, clustered minicomputers, diskless workstations, or PCs sharing a file server place a great burden on a network because of numerous file transfers. When such traffic begins to seriously affect performance, the most efficient way to deal with the problem is to use a bridge to subdivide the network into two segments, thereby partitioning the traffic. Networking Environment In multiprotocol environments, bridges provide, at this time, a more flexible and mature solution. They are transparent to high- level communications protocols and accommodate many different applications. For instance, bridges would be a logical solution in a case where a user wants to interconnect networks supporting a mix of protocols such as DECnet, XNS, and TCP/IP. Over the past few years, an increasing number of computer and networking vendors have introduced products using the TCP/IP protocol suite, making it somewhat of a de facto standard. Therefore, multivendor, single-protocol environments are more and more common. In these cases, routers are a possible solution. Network complexity will probably determine whether a bridge or router is used. In simple configurations, bridges are a sensible choice. As complexity increases, so does the need for the traffic isoltion and control capabilities or routers. A combination of bridges and routers, though, can solve particularly complex internetworking problems. Bridges are used to isolate the traffic between the different departments within each site. Routers are used to interconnect remote locations to allow each site to have a separate logical network. Network Topology Bridges impose some restrictions on network topology. Active loops cannot exist on a bridged internetwork. Routers, however, support all network topologies. Here the size of the proposed internetwork and the level of redundancy required is extremely important. The majority of simple cascaded networks function well with bridges. As more networks and alternate paths are added, though, routers become the better choice. This is particularly apparent in cases where there is a need for redundant data paths and load sharing. Router keep up-to-date on the changes in the topology and are able to determine the shortest path between two points. Network Administration Bridges and routers offer equal capabilities in the areas of network statistics and monitoring. Both can provide automatic audit trail and, along with a network management station, sophisticated analysis of such information. How an organization wishes to administer its internetwork will most likely determine what internetworking product it uses. On one hand, if the network manager wishes to administer the entire internetwork from a central location, a bridge makes good sense. A bridged internetwork acts as a single logical network. For example, the network manager must take into account all stations on a bridged internetwork when adding or removing a station. On the other hand, an organization may want or need decentralized network management, thus making a router the better choice. An internetwork connect by routers allows each segment to be logically independent. Internetworks that have many distant sites or that are relatively large may require several network managers. For instance, each department in a university may want to retain control of its own network while still retaining access to other networks on the same or other campuses. For ease of installation and maintenance, bridges offer definite advantages over routers. Bridges require no intervention from the network manager. They can make extremely basic routing decisions themselves. Routers, however, are more sophisticated devices. Particularly if static routing is used, they require network manager intervention to establish and maintain the desired configuration. Performance Because the operate at a low level of the ISO model and perform relatively simple software tasks, bridges provide higher throughput than routers. Routers, though, offset their slower performance by performing sophisticated software tasks such as dynamic routing. .h1;Conclusion It is important to remember that each computing and networking environment has its own characteristics. In some cases, the decision regarding which internetworkings solution to implement is easy. In others, the choice is less obvious. Those responsible for network planning must carefully consider current computing resources and how to best internetwork them. There must also be thought for the future. How is an organization likely to expand and what kind of internetwork could best support it? For example, do your networks run one or many different protocols? Bridges, currently, have the distinct advantage over routers in multiprotocol applications. What sort of network management is best for your organization? Routers allow internet segments to act as individual networks, while bridges function as a single logical network. Routers allow internet segments to act as individual networks permitting independent administration of these segments. Does your internet have two or three segments or does it have twenty? Bridges efficiently interconnect a relatively small number of networks, whereas routers can capably handle more complex topologies. Once this kind of analysis is complete, one of the other of these internetworking products will emerge as the appropriate choice for the particular application. In some cases, the best configuration might include both bridges and routers, or you may start out with bridges and add routers as your network expands. The implications of this concept are particularly important, especially in relation to growth. In any event, no matter what internetworking solutions you choose today, your future options are unlimited and your current investment is protected. .h1;Glossary Access Method Way to determine which workstation or PC will be the next to use the LAN. A set of rules used by network software and hardware that direct traffic over the network. Examples of access methods are token passing and collision detection. Address A set of numbers identifying the location of a node on the network. Each node must have a unique address on that network. Audit Trail A function of a network management system that provides a list of information about connections and disconnections and reasong for these disconnections and excessive errors on the network. Baseband An electrical signaling technique used to transmit information. Baseband signaling uses unmodulated signals. The carrier is present only when data is being transmitted. The entire frequency range of the channel is used during this tranmission. Broadband An electrical signaling technique used to transmit information. Broadband signaling involves modulation of the signal before transmission. Broadband networks typically divide the total bandwidth of the communication's channel into multiple subchannels, so different types of information can be transmitted simultaneously using different frequencies. Broadband signaling is used when mixing multiple types of information such as video, voice, and data. CCITT The initials for the French International Telegraph and Telephone Consultative Committee. This organization defines standards or recommendations (e.g., X.25) for international networking. Coaxial Cable A physical tranmission medium with two conductors. The center conductor carries the information signals. The outer conductor (electrostatic shielding) acts as a ground. Collision (See CSMA/CD) Detection Communications A hardware and software device that allows Server devices such as terminals, host computers, or printers to access a network without having to implement the communications protocol in the device itself. The communications server communicates with the device using standard protocols built into the device. Connection A communications path between two devices that allows the exchange of information. Other terms used to refer to a connection are session or circuit. CRC Abbreviation for Cyclical Redundancy Check. This is a method of detecting errors in a message by performing a mathematical calculation on the bits in the message and then sending the results of the calculation along with the message. The receiving network station performs the same calculation on the message data as it receives it and then checks the results against those transmitted at the end of the message. If the results do not match, the receiving end asks the sending end to send the entire message again. CSMA/CD Abbreviation for Carrier Sense Multiple Access with Collision Detection. It is an access method that allows many nodes to share a single channel of a communications medium. If more than one signal is transmitted at the same time, the signals collide and are retransmitted at a later randomly calculated time. Datagram A transmission method in which sections or a message are transmitted in scattered order and the correct order is reestablished by the receiving workstation. DECnet Digital Equipment Corporation's proprietary communications protocol. Ethernet A local area network that utilizes baseband signaling at 10 Mbps. The development of the Ethernet specification was a joint effort by Xerox, DEC and Intel and is the predominant local area network standard. FDDI Abbreviation for Fiber Distributed Data Interface. FDDI is an emerging standard for a 100 Mbps fiber-optic LAN. It uses a "counter-rotating" token ring topology. It is compatible with the standards for the Physical layer or the ISO model. Flow Control The hardware or software mechanisms employed in data communications to turn off tranmission when the receiving workstation is unable to store the data it is receiving. Frame A group of bits sent over a communications channel, usually containing its own control information, including address and error detection. The exact size and format of a frame depends on the protocol used. HDLC Abbreviation for High-level Data Link Control, which is the ISO procedure for data link control. HDLC uses a specific series of bits rather than control characters for transmitting and receiving data. High-Level A protocol that allows network users to carry Protocol out functions at a higher level than merely transporting streams or blocks of data; for example, reliably transmitting data, formatting data, establishing a connection, transferring a file. IEEE 802.2 A Data Link layer standard used with the IEEE 802.3, 802.4, and 802.5 standards. IEEE 802.3 A Physical layer standard specifying a LAN with a CSMA/CD access method on a bus topology. Ethernet following the 802.3 standard. IEEE 802.4 A Physical layer standard specifying a LAN with a token passing access method on a bus topology. Used with Manufacturing Automation Protocol (MAP) LANs. IEEE 802.5 A Physical layer standard specifying a LAN with a token passing access method on a ring topology. Used with IBM's token ring hardware. ISO Abbreviation for International Standards Organization. (See also ISO Model) ISO Model ISO has developed the Reference Model for Open Systems Interconnection, which divides a complex set of communications functions into self-contained modules. LAN Abbreviation for Local Area Network. A LAN is a communications network that provides high-speed data transmission over a small geographic area. LLC Abbreviation for Logical Link Control. Upper sublayer of the Data Link layer of the ISO model. MAC Abbreviation for Media Access Control. The lower sublayer of the Data Link layer of the ISO model. The MAC layer supports medium- dependent functions. Network The overseeing and maintaining of a network. Management The duties performed by a network management system include installing and configuring the network, maintaining an operation log, monitoring network performance, and statistics. Network Geography of a network or a set of networks. Topology Node Point in a network where service is provided, service is used, or communications channels are interconnected. Sometimes used interchangeably with station. Packet A block of data handled by a network in a well-defined format. Packet The internal operations of a communications Switching network that uses software to dynamically route packets from a source to a destination. Packet switching allows the sharing of a single communications channel among several connections. PAD The abbreviation for Packet Assembler and Disassembler. A PAD is a device that allows asynchronous terminals to have access to a Public Data Network. PDN A network that provides data transmission services to the public. Typically, a Public Data Network uses packet switching technology. Protocol A strictly defined procedure and message format that allows two or more systems to communicate over a physical transmission medium. Due to the complexity of comunications between systems and the need for different communications requirements, protocols are divided into layers. Each layer of a protocol performs a specific function, such as routing, end-to-end reliability, and connection. SNA Abbreviation for Systems Network Architecture. The network architecture developed by IBM. Subnet A portion of a network that is partitioned by a router. T-1 Carrier A digital transmission system developed by AT&T that sends information at 1.544 megabits per second. T-1 links can transmit voice or data. TCP/IP Abbreviation of Transmission Control Protocol/ Internet Protocol. A set of de-facto networking standards commonly used over Ethernet or X.25 networks. It was originally developed by the U.S. Government and is now supported by many equipment manufacturers. It defines high-level protocols such as Telnet (terminal connection), FTP (file transfer), and SMTP (electronic mail). Thin Ethernet A ligher (0.2" diameter, black coating) variation of Ethernet cable that saves cable and installation costs, but is restricted in effective distance. This type of cabling is specified under the IEEE 802.3 10Base2 standard. Token Ring A technology developed by IBM whereby a token is used to direct traffic on the network. There is only one token on the ring (the network) at a time. It is either free or busy. A node must wait for a free token to tranmit data; it marks the token as busy, then transmits a frame of data onto the ring. Data collisions cannot occur as only one node can transmit at any one time. Twisted Pair A form of wiring commonly used for telephone installations. Standard networks such as Ethernet can operate over such wiring. This method is economical, but poses distant limitations unlike coaxial cable. VAX A trademark name for a family of computers manufactured by Digital Equipment Corporation. Virtual Circuit A facility in a packet switching network in which packets passing between a pair of devices are kept in sequence. This is a "virtual circuit" because it appears there is an actual point-to-point connection. WAN Abbreviation for Wide Area Network. A data communications network designed to serve an area of hundreds or thousands of miles. A WAN can be public or private. X.25 A CCITT standard that defines the standard communications protocol by which mainframes access a public or private packet switching network. These networks are often referred to as X.25 networks. XNS Abbreviation for Xerox Network Systems. A protocol family specifically designed to run on Ethernet. It contains the Internetwork Datagram Protocol (level 3) and the Sequenced Packet Protocol (level 4).