The DECconcentrator 500 Project

Interconnect (CI) bus. All Digital's decision stations were to be dual to implement the fiber attachment stations (DASs) distributed data interface and the interconnections (FDDI) physical topology between the stations were with a dual ring of to be wired directly, trees, as opposed to a without patch panels or dual ring only, resulted similar structured wiring in the development of schemes. the DECconcentrator Using this dual ring 500 product. The dual topology is feasible for ring of trees topology a small number of stations provides high availability, in a single, tightly manageability, and support controlled room. However, for building wiring soon the emphasis of FDDI standards. The function shifted primarily to local of the concentrator area networks (LANs). A demanded that the product LAN consists of many nodes, be reliable, provide for spread over a large area remote management and and with potentially many control, and allow a low individuals able to connect cost per connection. The and disconnect stations. use of common FDDI hardware To accomodate LAN topology and software components requirements, ANSI chose developed by Digital helped to add the concept of the product team to meet concentrators midway these goals. in the development of the FDDI standard.[1] Concentrators in the ANSI FDDI In the simplest case, a X3T9.5 Standard concentrator is a device In the initial stages that attaches to a dual of its development, the ring (via A and B ports) American National Standards and provides additional Institute (ANSI) standard ports (M ports) to which for the fiber distributed stations can be connected data interface (FDDI) by means of radially wired technology was intended cables. These additional for a computer room system stations can be single interconnect, similar attachment stations (SASs) to Digital's Computer with a single port (S port) rather than the pair of Digital Technical Journal Vol. 3 No. 2 Spring 1991 1 The DECconcentrator 500 Product Also, concentrators continuously perform ports required by a DAS. link error monitoring This simple topology was (LEM). Each active link is soon generalized, allowing monitored for data errors, concentrators to be nested and a link found to have to any depth in a dual ring excessive data errors of trees. Concentrators is disabled. In this way may be singly attached concentrators ensure that (by using an S port plus the ring error rate, and M ports rather than A and therefore the packet loss B ports plus M ports), and rate, remains acceptably DASs may be connected to low. concentrators. Figure 1 Given the many choices illustrates the basic FDDI for concentrator topologies. interconnection allowed FDDI concentrators by the ANSI standard, are more than wiring it is possible to hubs, unlike certain construct highly complex other LAN technologies. topologies, including They also perform two many that have "bad" functions that are key properties. When a station to network integrity. When is physically plugged in a station's connection and that connection is to a concentrator is operating properly, the activated, a multistep station should be able initialization procedure to communicate with all called physical connection other stations in its own management (PCM) takes network. This property is place, using physical layer often stated as "Physical (PHY) signaling. In this connectivity equals procedure, the station and logical connectivity," the concentrator exchange or, in other words, "Being some topology information, plugged in implies being and a link confidence test able to communicate." is performed to verify that In bad topologies, this the data integrity on the property does not hold. link is acceptable. Once Such topologies are very the PCM initialization is confusing to network complete, the connection managers and are therefore becomes part of the ring. undesirable. The ANSI standard specifies therefore depends to some some topology rules to extent on the competence reduce this problem. of the network manager to However, the decision to avoid bad topologies. accept or reject offered One special case allowed connections is based by the topology rules is only on local knowledge called dual homing, as (i.e., information held shown in Figure 2. With a locally in each station or dual homing configuration, concentrator), and it is the A and B ports of a not possible to detect all concentrator or a DAS the bad topologies. FDDI 2 Digital Technical Journal Vol. 3 No. 2 Spring 1991 The DECconcentrator 500 Product are connected to M ports, FDDI networks has been usually of two different recognized by many of concentrators. In this our key customers. case, the B to M connection Digital chose the tree becomes active, and the /dual ring of trees A to M connection remains architecture implemented in a "standby" state. The with concentrators over a standby connection is not purely dual ring approach part of the ring, but it because this architecture can quickly change to the offers: active state if the B to o Support for industryM connection breaks. In standard radial wiring this way, connectivity is practices maintained when failures occur. o Manageability We next present the reasons o Configuration Digital chose the dual flexibility ring of trees topology o A definable demarcation over the dual ring topology point between the end and examine the resultant user and the backbone need for a concentrator. o Scalability A detailed discussion of the development of the Although the specific DECconcentrator 500 product behavior of an FDDI follows. concentrator is relatively new, the concept is not. Digital's Choice of the Dual Most system vendors and Ring of Trees Topology users of large systems have adopted the use of A dual ring topology manageable hubs (multiport consisting of dual repeaters) for Ethernet attachment stations may be networks and media access appropriate in very small units (a type of passive networks that do not use concentrator composed of structured and permanent bypass relays) in token cable plants. But a dual ring networks. ring of trees topology, Initially, cost was a major using single attachment concern in the decision stations as the end-user to implement a tree-type devices and concentrators architecture. Some users as hubs, provides the saw the addition of the optimal solution for a concentrator as an added highly flexible, reliable, cost burden. However, available, maintainable, the cost increase, which and robust FDDI LAN. As can be amortized over described in the preceding the entire network, is section, the concentrator greatly outweighed by the is the cornerstone of added advantages. With a an FDDI network. Its concentrator, stations significance in building can be separated into large, robust, and most two categories, end-user importantly manageable devices and backbone Digital Technical Journal Vol. 3 No. 2 Spring 1991 3 The DECconcentrator 500 Product devices. This is especially inadvertent disruption important in large networks caused by the end user. where the functions of network administration As shown in Figure and users of the LAN are 3, the actual number totally disparate. The of components that are concentrator becomes a tool required to connect eight that simplifies the role FDDI stations, whether into of network managers. As a the dual ring or in the demarcation point between tree, is approximately the end-user devices and the same; only the distribution backbone, the concentrator of these components is protects the backbone from different. In both topologies we have network, made possible by eight physical connections. the concentrator. A physical connection, A large network of whether in the tree or in many stations has a the dual ring, is a pointhigh probability of to-point, full-duplex path disruptions. Although a between adjacent physical DAS and a concentrator are layers. The initial fundamentally different, reasoning for a dual, they have in common the counter rotating ring was role of controlling the to create a bidirectional network topology through data path between adjacent PHY-level signaling. In stations in which the the case of a disruption, secondary path's main whether an operatorpurpose is to assist in initiated function (i.e., startup, initialization, a station powered down, and reconfiguration of the installed, or removed) or primary ring. a failure of the station Either topology requires or cable, the token path roughly the same number is modified according to of components, i.e., the station management PHY entities, optical /connection management (SMT transceivers, cables. /CMT) algorithm to maintain With the tree, the PHYs a continuous logical ring. are rearranged so that The main difference between conceptually we have taken the two approaches to a PHY entity from each DAS, solving the disruption in conjunction with a data problem is in the way a path switch, to create the station is bypassed. With concentrator. This approach the dual ring approach does incur the additional shown in Figure 4, when cost of the power and a disruption occurs, the packaging. However, from stations adjacent to the the perspective of the disruption bypass the network administrator, offending station(s) and this small incremental reconfigure the ring by cost is offset by the wrapping the secondary and increased ability to primary rings to form a manage and control the new single continuous ring. 4 Digital Technical Journal Vol. 3 No. 2 Spring 1991 The DECconcentrator 500 Product This provides a degree of fault tolerance but is limited to only a single be limited to the stations disruption. remaining on the portion In the case of multiple of the ring to which the failures or disruptions, management station was all dual attachment directly attached. stations adjacent to the An FDDI concentrator faults reconfigure, thereby provides fault tolerance creating multiple disjoint in a different way, as rings. Even though the illustrated in Figure 5. majority of the stations When a station connected to in the network might be a concentrator is removed operational, they would or powered off, the failure operate over several is bypassed through the disjoint networks. The concentrator data path potential loss of the switch at the PHY level. service access point would Any one or all of the effectively leave the stations can be effectively network nonoperational bypassed through the from the client/server concentrator without perspective. Management of affecting the connectivity such a situation would also of the other stations or be an ordeal, since access the global topology of the to fault information would FDDI network. Digital Technical Journal Vol. 3 No. 2 Spring 1991 5 The DECconcentrator 500 Product demarcation between enduser devices and backbone is required to maintain the Structured Wiring integrity of the backbone To fully appreciate and to minimize disruption the benefits of the to, or manipulation of, the concentrator, let us backbone cabling. consider an FDDI network The Telecommunication implemented in an office Industries Association building environment in (TIA), together with the conjunction with structured Electronics Industries wiring. Association (EIA), is A typical building defining a commercial environment includes building wiring standard; wiring between offices draft EIA/TIA 568 has the and equipment rooms on the framework as designated same floor and in between in Figure 6.[2] According floors. The wiring is to this standard, endpermanent and involves a user devices in offices relatively large number should be wired from a of end-user devices as telecommunications closet well as backbone devices (TC). All closets in each over moderate distances. building then connect to Moreover, frequent adds the intermediate cross /moves/changes occur in connect (IC) or to the this environment, and building hub for that the ability to move from building. In turn, all one location to another building hubs connect to a without manual intervention single main cross connect or network disruption (MC) or to the campus hub is desirable. A clear in the campus. Single Attachment Stations so that a disruption in an The tree topology, as end-user device, such as illustrated in Figure 7, disconnecting a station, facilitates structured does not affect the (or radial) wiring as operation of the network. prescribed by the draft The concentrators in the EIA/TIA 568 standard. various closets connect The end-user devices, to root concentrators implemented as SASs or in the building hubs. If DASs, connect to the other backbone devices are concentrators located in present in the building telecommunications closets, hubs or communication which are maintained closets, such as bridges, and controlled by the they can also be connected network administrator. When to concentrators in the concentrators are used, building hubs or the campus the most cost-effective hub, as appropriate. The user stations are SASs. tree topology offers Connection to concentrators the fault tolerance as keeps the end-user devices well as configuration separate from the backbone flexibility required in 6 Digital Technical Journal Vol. 3 No. 2 Spring 1991 The DECconcentrator 500 Product a structured wiring system. reconfiguration of the Also, it allows for adds backbone is dependent /moves/changes without on 200 devices. The disrupting or manipulating probability of having two the backbone cabling. or more disjoint rings Dual Attachment Stations is much higher in the latter case. Also, with End-user devices directly DAS stations, the network attached to the dual ring, administrator is faced with however, are not easy to the impractical task of isolate from the backbone directly controlling 200 LAN. Both the end-user devices. devices and the backbone devices are part of the Fault-tolerant Configuration same physical loop, as Options shown in Figure 8. To a network administrator, Two fault-tolerant management and control of configuration options the backbone becomes an are available: bypass ever-increasing ordeal relays and dual homing. because each end-user Bypass relays may be station is now considered used with DASs directly part of the backbone. attached to the dual Even though rules for the ring to provide fault end-user behavior can be tolerance in addition to established, they cannot the single fault protection easily be enforced. provided by wrapping to The availability of the the secondary ring. Dual backbone is increased by homing is an alternative the use of concentrators, mechanism which allows dual since these are the only attachment devices to have devices that form the dual a redundant connection to a ring backbone. This benefit concentrator when installed is very important for a in a tree topology. These large network. For example, two alternatives are in a network supporting 200 examined in this section. end-user stations on a dual Bypass Relays ring of trees topology, if To avoid the aforementioned 8-port concentrators are reconfiguration problems used to connect them to the with DAS implementations, dual ring backbone, only the FDDI standard offers 25 concentrators reside an option of using an on the dual ring backbone. optical bypass relay. The reconfiguration of While such relays are the backbone is dependent envisioned to alleviate on only 25 devices. Also, some of the reconfiguration the network administrator problems, they may induce needs to control only 25 more problems than they devices. In contrast, if solve. The inclusion of the same 200 stations are relays in the network means DASs directly attached added cost of components, to the dual ring, the cables, and connectors, Digital Technical Journal Vol. 3 No. 2 Spring 1991 7 The DECconcentrator 500 Product As shown in Figure 9, the end-user stations A, B, and C are dual attachment with bypass relays, and loss of optical power, station B has failed. With reduction in interstation B bypassed, stations A distance, and an additional and C become adjacent. The failure mechanism. These total link loss between factors limit the use of these two stations, using such relays to possibly a fiber-optic cable of very small, physically 62.5-micron core diameter collocated work group and 125-micron cladding LANs and make the relay an diameter, must not exceed unattractive solution for a 11 decibels (dB) to comply large network environment. with the FDDI standard. Let us assume that the For some applications a fiber-optic cable has tree or dual ring of trees a loss of 1.5 dB per may not meet the customer's kilometer (according to requirements. The dual the EIA/TIA-492), and homing topology, shown the distance between in Figure 2 and described the communication closet earlier in the paper, has and each station is 50 none of the limitations meters.[3] Since the cable or problems that can be length between stations imposed by the dual ring. A and C is 200 m, the This topology is beneficial power loss is 0.3 dB. in a large campus to A connector has a loss connect remote buildings of 0.7 dB (according to into the FDDI backbone. the EIA). Since there are It allows standard radial eight connectors (labeled wiring practices, with 1 through 8 in Figure 9) up to 2 km between the between A and C, the power campus hub and any given loss is 5.6 dB. A bypass building through multimode relay has a loss of 2.5 fiber (MMF) links. The dB. Since there are three dual homing topology is relays between A and C, especially useful with long the power loss is 7.5 distance links utilizing dB. The total link loss single mode fiber (SMF), between stations A and which span distances of C, therefore, is 13.4 dB, up to 40 km. With the dual which is in excess of the ring topology each span maximum allowed by the FDDI has to be counted four standard. Note that with times towards the fiberthe use of optical bypass optic path length maximum relays, the effective of 200 km allowed by the distance is limited to standard, in the event less than 200 m, which is that a wrap occurs. For a far below the maximum of 2 link of 40 km, 160 km has km allowed by the standard. to be subtracted from the Dual Homing maximum of 200 km, thus leaving only 40 km for the 8 Digital Technical Journal Vol. 3 No. 2 Spring 1991 The DECconcentrator 500 Product rest of the network. Using simplicity and reliability a tree topology, the span enabled us to keep product is counted twice as it has transfer cost to a minimum only one active fiber pair. and to nearly meet the time-to-market goal. And Product Development finally, the relatively small and tightly knit The following sections development team stayed examine the DECconcentrator together from conception 500 product development through field test and process from beginning first product shipment. to end, elaborating on Hardware Partitioning details of the product functionality as they were As the hardware block refined along the way. diagrams were developed, Several key factors several concepts for provided a smooth product partitioning the hardware development process. First, into modules were the architecture for the evaluated. The electrical, DECconcentrator 500 product mechanical, and power was chosen as a subset of supply designers worked the generality allowed by closely together to choose the ANSI FDDI standard. a suitable partitioning. Several features which The initial high cost would have significantly of the FDDI fiber-optic complicated the product transceivers led the without greatly enhancing designers to select modular functionality were not hardware partitioning. included. Examples of A modular design allows these are "roving MAC" ports to be added according and the ability to allow to the customer's needs, stations to select the ring thereby minimizing both (primary or secondary) the initial cost and to which to attach. the number of unused Second, product management ports. Since Digital's established a clear list of networking products have priorities, requirements, traditionally used side-toand goals that allowed the side airflow for cooling, development team members a card cage that supported to focus their efforts. horizontal modules was The absence of significant chosen. Four ports per changes in architecture module was considered to or product requirements be a reasonable number during the development by which a customer could helped the team stay on increment a system. This schedule. The priorities module, referred to as the selected were to design port module, contains four for low cost and high sets of the FDDI physical reliability, provide for layer chip sets, one status ease of firmware upgrades, light-emitting diode (LED) and strive for quick time per port, one module fieldto market. The emphasis on replaceable unit (FRU) Digital Technical Journal Vol. 3 No. 2 Spring 1991 9 The DECconcentrator 500 Product fault LED, and a small amount of support logic. One function of the DECconcentrator 500 product end-user configurations. is to provide support for Two basic configurations network management; this are supported in the requires data link layer DECconcentrator 500 hardware. In addition, product. A concentrator the DECconcentrator 500 configured with one to device must connect to three port modules (4 to the FDDI dual ring; this 12 ports) can support a requires the A and B port standalone work group but types. The DECconcentrator cannot connect in the dual 500 management module was ring of trees topology. designed to meet these A concentrator configured needs. By combining the with a management module data link and A and B PHY and one or two port modules port hardware, we ensured supports the dual ring that any DECconcentrator of trees topology and is 500 device installed in a remotely manageable. dual ring of trees would be manageable. Another goal of the The DECconcentrator 500 hardware team was to product also includes a eliminate the use of microprocessor to execute cables within the box. diagnostic and operational This goal was consistent firmware. To minimize with minimizing cost and the number of modules, maximizing reliability. we specified that the The use of modular port microprocessor and its and management cards led support logic fit on the team to believe that the backplane. The use the power supply could also of an active backplane be modular and plug into eliminates the need for the backplane in a similar a separate control module manner. To avoid potential in the card cage, thereby safety hazards, the power reducing both cost and supply module is not the vertical height of accessible without opening the box. The backplane up the box; however, the also provides the token interconnection of the ring data path function supply with the backplane which interconnects any is achieved with the same allowable configuration type of connector used on of port and management the logic option modules. modules. The number of The only cable used in the modules supported by the DECconcentrator 500 device backplane is based on our provides power to the fans. evaluation of customer Figure 10 is a diagram of need. We decided that 8 to the various modules that 12 ports per concentrator compose the DECconcentrator is sufficient for most 500 hardware. 10 Digital Technical Journal Vol. 3 No. 2 Spring 1991 The DECconcentrator 500 Product Power and Packaging Tradeopen area in each side of offs the box. When prototypes were tested, we found the Once we decided that three improvements in cooling modules could support followed predictions. The 8 to 12 end users, we modification also yielded focused on the selection considerable reduction of packaging. Two basic in acoustic noise levels, proposals were examined. which allowed the use of The first was to modify off-the-shelf ball bearing the Digital's NAC common fans with good reliability. box which has been used in Mechanical and electrical many of Digital's products. requirements could not be The second proposal was met by any of Digital's for a new box design, existing power supplies. which allowed improved The power supply height serviceability via quick was limited; a unique access to all FRUs (fieldconnector was required to replaceable units). The interface to the backplane; existing common box design and the available area was chosen to minimize was determined by the size risk to the product of the card cage on top development schedule. The of which the supply was necessary modifications mounted. Electrically, the were the addition of a supply had to provide a card cage for the port relatively large amount and management modules, of minus5.2-volt power provision for mounting the to support the emitterpower supply and fans, and coupled logic (ECL) in the improvement of the airflow FDDI PMD. Fortunately the characteristics. total power requirement Cooling was also seen was similar to that of as a potential problem Digital's Ethernet-toin product development. Ethernet bridge product, The bipolar logic used in the LAN Bridge 200. The the FDDI physical medium power supply group agreed dependent layer (PMD) to modify this existing dissipates a considerable high-volume supply to amount of power in a small meet the requirements area. Evaluation of the of the DECconcentrator NAC common box showed that 500 product. The use of the grill area in each an existing design was side had approximately 35 expected to result in fewer percent open area. Analysis bugs, and this proved to showed that significant be the case. Only one bug improvements in airflow was found in system stress could be achieved by testing, and it was easily increasing this percentage. corrected with a minor Mechanical rigidity was design change. traded off against airflow Card Handles improvement to reach a compromise of 50 percent Digital Technical Journal Vol. 3 No. 2 Spring 1991 11 The DECconcentrator 500 Product The module handle design radiation. A "waveguidewas probably the single beyond-cutoff" structure most complex part of the was evaluated. This mechanical design, and structure is a rectangular it had the potential to extension to the handle impact the cost of the with an opening the size product. The mechanical of the FDDI connector. design team recommended The design attenuates all modifying a handle design emissions below the cutoff from an existing Digital frequency and was predicted product. This essentially to provide excellent meant stretching the handle attenuation performance and making openings as for all harmonics of the required for I/O connectors FDDI signals. Testing and LED displays. Handles of prototypes verified for both DECbridge 500 the emissions problem and DECconcentrator 500 created by the connector products are processed opening and proved the from the same mold since effectiveness of the only I/O connector and waveguide structure in switch/indicator openings eliminating the emissions. are different for the two This structure was then products. included in the design of Traditionally, these the handles for each FDDI handles have been made port. from a plated cast alloy. Logic Design Plastic offered the The logic design team potential of significant developed prototype cost savings and weight hardware as quickly as reduction. However, there possible so that the was concern about the diagnostic, operational quality of plating with firmware, and common node plastic, as well as about software (CNS) firmware the structural strength. teams could proceed with The decision was made hardware-based debugging. to start the development First-pass prototypes of effort for plastic handles the controller/backplane while using machined metal module and the fourparts for the interim. When port module were in the the final plastic product laboratory within six was available, it met all months of the start of the requirements. project. The GenRad HILO Another critical simulation software was factor in handle design used in the module design was electromagnetic process. interference (EMI). Each One type of bug was FDDI duplex connector discovered in the firstrequired a large, 1.4 pass prototypes that was cm by 3.8 cm, opening in not caught in simulation. the handle which posed A through-hole component the risk of emitting body was used in schematic 12 Digital Technical Journal Vol. 3 No. 2 Spring 1991 The DECconcentrator 500 Product capture instead of a Concentrator Port. An surface mount body. As a FDDI concentrator consists result, the layout was of a group of serially wired according to the connected ports, each through-hole pinout. This of which implements the error was not caught by FDDI PHY functionality. any of the software that Key to our product was checks design rules. Digital's PHY chip, which Thus, the controller implements the physical /backplane modules required layer functionality and engineering change order supports the physical (ECO) wires to mount a connection management component onto the back of requirements for station the module in a "dead bug" management. In addition to fashion. the PHY chip, the CDC chips On the four-port module, (one each for transmit and the differential ECL receive) provide the serial signal detect lines from /parallel conversion, clock the fiber-optic receiver recovery on the receive to the clock and data side, and nonreturn to zero conversion receiver (CDCR) inverted (NRZI) encoding component were crossed. /decoding. The fiber-optic This logic was not included transceiver, however, had in the simulation due to be purchased from an to its analog nature. outside vendor. Since many The problem in this case mechanically incompatible was an inconsistency devices are on the market, in nomenclature between we tested the products of the CDCR and fiber-optic many vendors. Fortunately receiver chip bodies used for the product development in schematic capture. teams, our optics group was able to identify pinThe strategy of building compatible transceivers first-pass hardware from two vendors. Dual prototypes as quickly as sourcing of the tranceivers possible to support early used in our concentrator firmware debugging was ports reduced the risk of successful. Simulation shipping products based played a key role in on this relatively new guaranteeing functionally fiber-optic technology. correct designs. When Internal Token Ring second-pass prototypes Data Path. The FDDI PCM were tested, only a single process provides fault ECO wire was required coverage and topology in the product, and the rule checking between any product shipped with the two connected FDDI ports. second-pass designs on This is essential to ring all modules. The following stability since a token sections examine several ring is made up of a series areas of the logic design of physical connections, that are unique to the any one of which can bring concentrator function. down the entire ring. Digital Technical Journal Vol. 3 No. 2 Spring 1991 13 The DECconcentrator 500 Product Because connections between the mechanisms built ports within a concentrator into SMT. are not specified by the o Fault detection FDDI standard, they are a /isolation. The critical design area for controller/backplane ring availability. The and the PHY chip DECconcentrator 500 design design allow the addressed this aspect of DECconcentrator the product as described 500 device to offer below. continuous service in o Data path (port-tothe presence of hardware port) integrity. faults by isolating Adjacent ports in the the faulty hardware concentrator interface from the data path. with a dual-symbol wide The diagnostics that data path of 10 bits are invoked at power plus parity for a total up or on command from of 11 bits. This PHY-tofirmware (as in the PHY interconnection is SMT trace function) the same interface used have the ability to to connect PHY to media isolate faults very access control (MAC) close to the component in a station. Parity level. The fault report checking was added to is then passed to Digital's PHY chip to initialization firmware ensure that intermittent which configures the or hard faults could DECconcentrator 500 easily be detected. If product so that the a parity error in this faulty hardware is not data path occurs, the included in the data DECconcentrator 500 path. Two levels of device is taken off bypassing are provided, line to ensure that the one at the port level entire token ring is and one at the option not corrupted. Without module level. Bypassing parity protection, a is always performed hard failure on this one level of hardware internal path stops ring away from the detected traffic altogether, in fault. Thus if a fault which case the SMT trace is detected at the CDC function might isolate component level (using the fault. However, a CDC loopback test), an intermittent fault then tha single port in a concentrator's is bypassed in the internal data path that PHY chip. If a fault is not protected with is detected at the parity could arbitrarily PHY level or between reduce ring performance PHY chips within a and increase the risk module, the entire of undetected data module is bypassed on errors and would not the backplane. Note that be isolated by any of power-up diagnostics 14 Digital Technical Journal Vol. 3 No. 2 Spring 1991 The DECconcentrator 500 Product do not provide a PMD It also provides ring external loopback test scrubbing in case the except in a special MAC should have to leave manufacturing mode. or enter the ring while Fault coverage of this the ring is operational. hardware is provided Upgradeable Nonvolatile by the SMT PCM process, Program Memory. To support which prevents a faulty firmware upgrades over the connection from being network, the FDDI products included into the require electrically ring. Fiber loopback erasable programmable readconnectors are included only memory (EEPROM). All with each product for code in the DECconcentrator isolating media faults 500 product is executed between the fiber-optic directly out of EEPROM transceivers and the since the microprocessor's fiber-optic cables. clock rate is relatively o Internal MAC attachment. slow. In the first-pass A MAC is not required design we used conventional for an FDDI concentrator 32K by 8 dual in-line to function; however, package (DIP) devices as it is included as an they were qualified within option to provide remote Digital. In order for the management. The internal controller/backplane module MAC can be thought to accomodate sufficient of as a "management program memory, we needed a station" attached to one denser technology. At the of the concentrator time, component engineering ports whose job is was evaluating flash to provide control EEPROM technology. Flash /status of the local devices became available concentrator function. in surface mount packages This attachment is with a density of 128KB internal to the that met our needs. The concentrator, but flash memory proved to be a must provide the same robust technology; however, basic service as a development of a flash physical connection programming algorithm was to an external station. challenging and required This service is provided extensive testing. The by logic in the data older EEPROM technology had path referred to as the built-in logic to handle "null PHY." The null the details of the erasing PHY provides a means of and programming steps, bypassing the internal but with the flash memory MAC if diagnostics these details were directly should find a fault controlled by software. in any of the hardware In order to upgrade [MAC, CAM, ring memory firmware over the network, controller (RMC), and a well-controlled procedure packet memory] related was developed. A firmware to the data link layer. image plus the flash Digital Technical Journal Vol. 3 No. 2 Spring 1991 15 The DECconcentrator 500 Product programming code is to identify ways to shorten transferred over the the software development network through multiple cycle. Code that could packets and stored in potentially be shared packet memory. This downamong the products, and line upgrade is provided code that could be ported by a network device upgrade from previous projects was (NDU) utility that was identified. These early developed for the FDDI efforts resulted in the products. Once the entire common use of the real image is received in time operating system packet memory, it is (RTOS), common FDDI chip checked against a cyclic diagnostics, diagnostic redundancy check (CRC) error logger, diagnostic included in the image. dispatcher, and common If the CRC is correct and node software (CNS). In the firmware image is of addition to the code that the correct type (destined is common among the FDDI for this product), the product set, much time was DECconcentrator 500 product saved by porting portions takes itself off line. of the remote bridge The 68000 microprocessor management software (RBMS) then executes the flash responder and maintenance programming firmware operation protocol (MOP) directly from packet memory from the LAN Bridge 200 to load the new image. Once product. The management the load is complete, the model for the data link firmware forces a reset, and physical layer entities and a power-up self-test for both the DECbridge 500 is run that includes a CRC and DECconcentrator 500 check of the contents of products was developed to flash memory. ensure commonality between Software Design Issues the two products. Essential to the completion Field Test. The field of the development test provided valuable process was the use information regarding the of common software and quality of the products. the field testing of Several of the sites chosen the DECconcentrator 500 to field test the FDDI product. backbone products were technically knowledgeable Common Software. Early in about networking. They the development process were able to perform it was clear that the specific testing while aggressive time-tomonitoring their networks. market goals for the As a result, detailed FDDI product set would test information was require the development provided to engineering. team to be resourceful. One engineer was assigned In the beginning of to each field test site the development cycle a as a "site parent." The significant effort was made site parents monitored 16 Digital Technical Journal Vol. 3 No. 2 Spring 1991 The DECconcentrator 500 Product their sites and channeled o Providing a thorough the information back to testing process for engineering. This structure both hardware and for supporting the field software which tested test enabled engineering the product in realistic to react quickly to the environments with a needs of the sites as process in place to well as act on problems correct problems and found. This testing and verify fixes quickly. feedback, coupled with We had quite robust the capability to load new products at the time of firmware revisions over first customer shipment the network, was crucial to as a result of our test achieving quality prior to /fix/verify process. first customer shipment. Acknowledgments Conclusions People from many The dual ring of trees organizations worked topology is well suited together to achieve the for all FDDI environments development of the product that require fault on time. The key members of tolerance, scalability, the engineering design team and flexibility of were Dave Benson, Gerry configuration. This Capone, Stan Chmielecki, topology is the right Paul Ciarfella, Alison choice for managing the Coolidge, Janis Roth ever-growing local area Cooper, Joe Dagdigian, Tom networks through the 1990s. Ertel, Cheryl Galvin, Dave Several factors proved to Hyre, John Iannarone, Bill be crucial to meeting both McCarthy, Charlie McDonald, functional and time-toDick Muse, Luc Pariseau, market requirements with a Dave Sawyer, and Karen quality DECconcentrator 500 Shay. product.o EstablishingReferences architecture and producto FDDI Station Managementfeature requirements(SMT) Preliminaryearly and maintainingDraft Proposed Americanthese with minimalNational Standard,ANSIchanges.X3T9/90-078, Rev. 6.2 o Establishing and(May 1990).maintaining a close-knito Building Wiring Standardproduct development teamfor Industrial andwith good communicationCommercial Use, EIAchannels./TIA 568, PN1907o Leveraging whereverB (Washington, DC:possible from provenElectronics Industriesand available designs,Association, Engineeringmaking incrementalDepartment, forthcomingimprovements as needed.1990). Digital Technical Journal Vol. 3 No. 2 Spring 199117 The DECconcentrator 500 Product o Detailed Specificationfor 62.5-m Core Diameter/125-m Cladding DiameterClass Ia Multimode,Graded Index OpticalWaveguide Fibers,ANSI/EIA/TIA-492AAAA-1989 (Washington, DC:Electronics IndustriesAssociation, EngineeringDepartment, 1989). 18 Digital Technical Journal Vol. 3 No. 2 Spring 1991=============================================================================Copyright 1991 Digital Equipment Corporation. Forwarding and copying of thisarticle is permitted for personal and educational purposes without feeprovided that Digital Equipment Corporation's copyright is retained with thearticle and that the content is not modified. This article is not to bedistributed for commercial advantage. Abstracting with credit of DigitalEquipment Corporation's authorship is permitted. All rights reserved.=============================================================================