Development of the DECbridge 500 Product By Robert C. Kochem, James S. Hiscock, and Brian T. Mayo Abstract Bridges only forward The DECbridge 500 product traffic destined for other connects Ethernet/802.3 LANs; local traffic is local area networks (LANs) confined to its home LAN. to fiber distributed One important function of data interface (FDDI) bridges is the ability, LANs and is, therefore, under network management a fundamental element of control, to block traffic an extended LAN. Developers of selected protocol types of this product encountered or traffic from specific many technical hurdles. The sources. Restricting higher data rate and token unnecessary traffic, ring topology inherent especially multicast or in the FDDI technology broadcast, significantly impose several demands improves the utilization of on any bridging product LAN bandwidth. connected to an FDDI LAN. In this paper we first The differences in formats discuss the role of the and size of frames on the DECbridge 500 product two types of LANs introduce in an FDDI and Ethernet further requirements. The /802.3 extended LAN and development team met these outline the design of the requirements and delivered bridge. We then describe a high-performance product the operation of the bridge that provides seamless by tracing the flow of integration of both LAN LAN traffic through it. types. This description gives Introduction insight into many of Bridges are essential to the complex tasks that a the creation of extended bridge must perform to local area networks (LANs) connect two dissimilar because they provide LANs. Key points of the transparent forwarding development methodology are of traffic between adjacent also presented. LANs.[1] Traffic may be forwarded to or from individual destinations, to groups of destinations (multicast), or to all destinations (broadcast). Digital Technical Journal Vol. 3 No. 2 Spring 1991 1 Development of the DECbridge 500 Product DECbridge 500 Design requirements. File servers Considerations and other common resources The DECbridge 500 device may also be part of the serves as the point of local FDDI LAN. Here, the connection between a new role of the DECbridge 500 family of LAN products product is to provide a based on the fiber path from the local work distributed data interface group to other parts of the (FDDI) technology and a extended LAN via Ethernet large installed base of /802.3 LANs. Ethernet/802.3 LANs. The DECbridge 500 product must meet the requirements of both LANs to provide a smooth migration path for Digital's customers. Note that Ethernet and 802.3 have media access control (MAC) frame formats that may be used on the same 10-megabit (Mb)-per- second LAN. Throughout this paper, the expression Ethernet/802.3 is used to identify such LANs and to distinguish them from 100- Mb-per-second FDDI LANs. The terms Ethernet, 802.3, and FDDI are used when discussing the specific MAC frame formats. System Description Two typical extended LAN applications involving DECbridge 500 devices are shown in Figure 1. The backbone application employs an FDDI LAN to provide a high-bandwidth interconnect of multiple Ethernet/802.3 LANs. The DECbridge 500 device is the point of connection between the Ethernet/802.3 LAN and the FDDI backbone LAN. In the work group application, FDDI LANs provide localized connectivity of users, such as DECstation 5000 workstations, that have high throughput 2 Digital Technical Journal Vol. 3 No. 2 Spring 1991 Development of the DECbridge 500 Product In either application, the bridge must perform the following functions: o Forward traffic between nodes residing on two different LANs o Prevent (i.e., filter or not forward) traffic between nodes on the same side of the bridge from getting to the LAN on the other side of the bridge o Be responsive to host-based network management, provided by, for example, Digital's extended LAN management software (DECelms) and Digital's management control center (DECmcc) product o Be a proper participant as an end station on both LANs to which it is connected o Interact with other bridges in the topology of the extended LAN to prevent redundant paths or loops[2] Hardware Description Figure 2 shows a block diagram of the DECbridge 500 hardware design. The applications processor (AP), a subsystem based on a 68020 microprocessor, performs initialization and maintenance of the bridge hardware as well as some steps involved in processing frames. The AP also acts as the management entity for the bridge. Digital Technical Journal Vol. 3 No. 2 Spring 1991 3 Development of the DECbridge 500 Product The operating programs Each chip set checks for the AP as well as two every incoming frame for other processors, the queue integrity. Also, some manager and the translation rudimentary identity, processor, are stored in or address, tests are a nonvolatile electrically applied. Frames that erasable programmable read- meet the integrity and only memory (EEPROM). At identity requirements initialization, the AP are then placed in a distributes the programs packet memory. The bridge to random access memory maintains a table of (RAM) in the other two learned MAC addresses. processors' subsystems. The table contains data The AP executes much of for each address that its own program directly is used to decide if a from the nonvolatile frame should be forwarded memory, although some high- or filtered. The queue performance operations are manager is a subsystem executed from static RAM. dedicated to checking each frame received in FDDI The DECbridge 500 device packet memory against the may have an entire information contained in new operating program the learned address table. downloaded over the Based on this information, network and stored in the the queue manager decides nonvolatile memory. This whether to filter the allows rapid updates of frame, forward the frame functionality without the to the Ethernet/802.3, or need to perform a hardware deliver the frame to the upgrade on-site. Program bridge entity for action. updates are received via The FDDI and Ethernet/802.3 either of the attached LANs employ different LANs and stored in an area data link protocols. The of RAM referred to as the translation processor, a "landing pad." The AP then second 68020 subsystem, transfers the new program examines frames to be into the nonvolatile memory forwarded from one side and initiates a firmware of the bridge to the other. reset. Each frame is reformatted The FDDI and Ethernet/802.3 to the appropriate outbound chip sets and some analog protocol and moved from the interface circuitry provide incoming packet memory to connection to the two LANs. the outbound packet memory. The bridge represents a The two chip sets examine single attachment station their respective packet (SAS) on the FDDI ring. memories for outbound On the Ethernet/802.3 side frames and transmit them of the bridge, switch- onto their LANs. selectable ThinWire and Physical Description The attachment unit interface DECbridge 500 product is (AUI) connections are shown in Figure 3. The provided. hardware is approximately 4 Digital Technical Journal Vol. 3 No. 2 Spring 1991 Development of the DECbridge 500 Product An exploded view of the bridge is shown in Figure 4. The electronics is implemented by the following four logic modules: 7 inches high by 17 inches o AP, the applications wide by 14 inches deep processor and may be rack-mounted or o QM, the queue manager installed on a tabletop. It subsystem including the operates over the range of learned address table 100 to 240 voltage AC (VAC) o FI, the FDDI chip set at 50 or 60 hertz (Hz). and the FDDI packet memory Figure 3, a photograph, o NI, the Ethernet (DECbridge 500 Product) /802.3 chip set and is inserted here in the packet memory and the bound version only. translation processor External signal connectors frequency interference (EMI are located on the front /RFI). edge of the two network interface cards, FI and Operation NI. Each module has light- emitting diodes (LEDs) for As mentioned previously, various status functions the DECbridge 500 device and also diagnostics. In forwards traffic between addition, the AP has a bank two different LAN types. of switches for setting Consequently, the product certain bridge operating development team faced functions. several challenges beyond The power and packaging those encountered in were designed to simplify previous bridges that the swap out of field- connect similar Ethernet replaceable units (FRUs). /802.3 LANs. The principal The four logic module FRUs sources of these new can be replaced through the challenges were: front of the box, without o Higher data rates on the opening it. By taking out FDDI LAN. Ethernet/802.3 only two screws, the outer operates at 10Mb per shell of the case can be second and has a minimum removed. This gives access MAC frame size of 64 to the three other FRUs, bytes. The maximum frame namely, the power supply, arrival rate is 14,880 the passive backplane, and frames per second (fps). a fan assembly. The five- FDDI operates at a rate sided design of the outer of 100Mb per second and shell results in a product has a minimum MAC frame that is mechanically strong size of 17 bytes. The and provides shielding from maximum frame arrival electromagnetic and radio rate is 446,429 fps, a Digital Technical Journal Vol. 3 No. 2 Spring 1991 5 Development of the DECbridge 500 Product rate 30 times greater itself, and frames to be than that of Ethernet discarded. /802.3. To comply with Digital's o Different frame formats. bridge architecture Ethernet, 802.3, and specification and the FDDI have different MAC IEEE standard 802.1d for frame formats. Traffic bridges, the bridge must entering an FDDI LAN examine all incoming from an Ethernet/802.3 frames.[2] It must LAN must be properly identify, set aside, and translated to an FDDI process frames of each frame format. This protocol type directed translation must be to itself, in the order performed in such a way received. To meet product that passage through a performance requirements, second bridge back to the bridge must be able a different Ethernet to forward frames at the /802.3 LAN results in a full Ethernet/802.3 rate. frame that recovers its A best effort must be made original frame format. to buffer frames received o Different frame sizes. in bursts exceeding that Ethernet and 802.3 rate. Frames should not differ from FDDI in be erroneously discarded. both maximum and minimum Results of this compliance frame sizes. FDDI visible to the network user frames shorter than are: the Ethernet and 802.3 o Transparency. Nodes minimum must be padded. across the extended LAN FDDI frames longer operate as if they were than the Ethernet and connected to the same 802.3 maximum cannot LAN. be forwarded, with the o Stability. The paths in exception of special the LAN remain constant protocol types, which yet can reconfigure must be broken into around equipment changes multiple, smaller with a minimum loss of frames. connectivity. Frames Objectives are not duplicated; nor The bridge can only forward are they received out of frames from the FDDI LAN to order. the Ethernet/802.3 LAN at o Manageability. Network the maximum rate accepted management can always by that LAN, i.e., 14,880 observe and control fps. Yet the arrival rate the components of the of frames from the FDDI LAN extended LAN. may be in excess of 440,000 fps. The incoming frames consist of an unknown mixture of frames that need to be forwarded, frames directed to the bridge 6 Digital Technical Journal Vol. 3 No. 2 Spring 1991 Development of the DECbridge 500 Product The operation of the DECbridge 500 device is best described by examining the progress made through the bridge by frames received from the FDDI LAN. Tracing this flow of traffic also gives insight into many of the challenges faced by the product's development team. The subsystems that process these frames and the flow of frames through logical queues in these subsystems are shown in Figure 5. The operation of the subsystems as the frames progress through them is described sequentially in the following sections. Digital Technical Journal Vol. 3 No. 2 Spring 1991 7 Development of the DECbridge 500 Product Receiving FDDI Frames and used by the queue manager and the translation The FDDI chip set in processor subsystems. the bridge places all Queue Manager Process received frames in the FDDI packet memory on the The queue manager subsystem receive queue. Frames in operates on all frames the FDDI packet memory can in the receive queue to be accessed by subsystems determine if they should in the bridge by using a be discarded, forwarded virtual address method. A to the Ethernet/802.3 LAN, page table memory is used or received and processed to assign a physical 512- by the bridge management byte buffer to each of 16K entity. Discarded frames virtual buffers. Queues are returned to the receive contain sequential sets queue; the remaining frames of virtual buffers. Data are placed on the forward frames are "moved" from one or bridge queues. The queue queue to another by moving manager constantly makes the virtual address buffer updates to the table of pointers from one queue to learned addresses based on another. source addresses observed Frames received from the on the FDDI LAN. FDDI LAN may be as long as The queue manager's 4500 bytes. Frames longer operational decisions are than 512 bytes are chained, based on the following that is, stored in multiple data: buffers. Each buffer has o The frame descriptor an associated descriptor containing assorted longword containing status status information such information about the frame as transmission errors such as error conditions, and frame length frame length, and flags indicating the start o The frame control field and end of multibuffer specifying the type of frames. The ability of frame the bridge to chain small o The type and quantity buffers to handle frames of of frames previously various sizes increases the received (used to efficiency of the packet prevent a flood of memory by minimizing the any one type of frame amount of unused buffer from blocking out other space. (Statistically, types) LAN traffic has a higher o A learned database content of shorter frames.) containing addresses Thus, more buffers are made indicating on which available to handle bursts side of the bridge of traffic. Additional each MAC is located and information about buffer special filtering status status is contained in the information assigned to page table memory. This each address by network information is generated management 8 Digital Technical Journal Vol. 3 No. 2 Spring 1991 Development of the DECbridge 500 Product The gate array burster address is found, the TLU (GAB) allows the queue presents that status to the manager to access the queue manager processor. FDDI packet memory. This Otherwise, the TLU gives application-specific the queue manager processor integrated circuit (ASIC) a programmable status is a specialized direct indicating whether to memory access (DMA) device. forward or to discard It is capable of moving the frame. A second TLU selected fields or large port allows the TLU and sections of frames into the table address RAM to or out of FDDI packet serve as slaves to the memory. The objects may be AP. Thus, destination moved either into internal address filtering for holding registers for traffic received from the examination by the queue Ethernet/802.3 LAN and manager engine or directly table maintenance can be to destinations such as performed. registers in the table To keep up with the lookup engine (TLU). Note packet arrival rate, the that the GAB used in the queue manager subsystem queue manager subsystem makes extensive use of is the same device used in pipelining. The queue the translation processor, manager engine operates which is discussed later concurrently on six in this paper. These packets. The TLU unit two subsystems have many performs three searches similar requirements, concurrently: one each but each also has unique for the source and requirements. Using destination addresses on one GAB design for both FDDI packets and one source subsystems reduced the or destination search on overall development effort. Ethernet/802.3 packets. The table address RAM Discarding and Keeping and the TLU are key Frames components of the queue manager. The RAM contains The decision to discard a a table of up to 16K 48- frame is based principally bit addresses. Each address on the frame's address or also has status bits that its contents. The following determine what action are typical of frames that the bridge should take are discarded: when a frame's source or o Frames destined for destination address matches nodes that the bridge a particular address. The recognizes as not on TLU is an ASIC with a port the Ethernet/802.3 that is a slave to the side of the LAN. Also, queue manager engine. network management may The queue manager engine specify addresses to be inputs an address to the discarded regardless TLU which scans the RAM of location in the for that address. If the topology. Digital Technical Journal Vol. 3 No. 2 Spring 1991 9 Development of the DECbridge 500 Product o Frames of either a transmit additional SMT reserved or undefined frames on the FDDI LAN. frame control type. Counters o Frames that are either Each frame type is too long or too short. guaranteed a minimum When a frame is discarded, amount of processing its buffers are returned time by the bridge. If to the end of the receive at any time the bridge queue by reassigning them holds too many of any one in the page table. frame type, it discards Frames that are kept further frames of that are placed on either the type. The queue manager forward or bridge queues. uses allocation counters to Frames ultimately destined keep track of the number of for the Ethernet/802.3 LAN forwarded, FDDI SMT, bridge are placed on the forward management, spanning tree, queue. Frames placed on and error frames. the bridge queue, to be The queue manager also has processed internally by counters that summarize its the bridge, are of the activity. These counters following types: are periodically dumped o FDDI station management to the AP and are used to (SMT) frames calculate LAN utilization o Digital's extended LAN statistics required by management sofware network management. (DECelms) frames or Translation maintenance operation Bridges operate at and protocol (MOP) frames below the data link o Spanning tree frames, level in the seven-layer containing messages used International Standards to determine the network Organization (ISO)/Open topology and turn System Interconnection individual bridge ports (OSI) reference model shown on or off to eliminate in Figure 6. The data link path redundancy layer is divided into a o Frames containing errors lower MAC sublayer and an o Frames placed on upper logical link control the bridge queue (LLC) sublayer. The LLC not forwarded to the protocol is specified Ethernet/802.3 LAN. in ANSI/IEEE standard 802.2.[3] However, after receiving and processing these frames, the bridge may generate one or more frames on either or both LANs. For example, received SMT frames are never forwarded, but a given SMT frame may cause the bridge to 10 Digital Technical Journal Vol. 3 No. 2 Spring 1991 Development of the DECbridge 500 Product When forwarding frames 802.3 MAC protocol. IEEE from one LAN to another, standard 802.1 defines a the DECbridge 500 device mechanism for translating converts the outgoing frame Ethernet frames into an to the MAC frame format IEEE 802.2 format (as of that LAN. This process is used on FDDI LANs). is called translation. Figure 7 illustrates how Also, when a frame is Ethernet frames and two generated by the DECbridge types of 802.3 LLC frames 500 product on either LAN, are translated into three the data link frame format different types of FDDI of that LAN is employed. data link frames. By performing translation, Maximum and minimum frame the DECbridge 500 product sizes of the LANs also complies with the IEEE impose requirements on 802.1d requirements for the translation process. transparent bridging. Ethernet and 802.3 MAC This enables end nodes protocols require a minimum to communicate across data field length of the extended LAN as if 46 bytes. The FDDI MAC the nodes are directly protocol supports zero- connected to the same length data fields. When LAN. An alternative a bridge forwards frames to translation, called that originated at nodes on encapsulation, is possible, an FDDI LAN to an Ethernet but it does not comply /802.3 LAN, the translation with the IEEE 802.1d process must add padding requirements. Further, (null bytes) to any short using encapsulation data fields to bring them puts restrictions on up to the 46-byte minimum the configuration of the size. network. FDDI has a maximum frame The Process size of 4500 bytes. The Ethernet/802.3 maximum Ethernet, 802.3, and FDDI frame size is 1518 bytes. have different MAC frame Frames received from the formats. When Ethernet or FDDI ring that are longer 802.3 frames are bridged than 1518 bytes after to an FDDI LAN, they are translation are discarded reformatted to the FDDI with the exception of MAC frame format. The frames discussed in the original MAC type (Ethernet text that follows. or 802.3) is indicated by The internet protocol setting information in the (IP) is a widely used, LLC header. If the frame network-layer protocol. passes through a second Nodes on FDDI rings may FDDI-to-Ethernet/802.3 generate IP frames longer translation at another than the Ethernet/802.3 bridge, the LLC information maximum size. The DECbridge is used to determine if 500 product performs one the bridge should translate function beyond the process the frame into Ethernet or Digital Technical Journal Vol. 3 No. 2 Spring 1991 11 Development of the DECbridge 500 Product of transparent bridging. The bridge breaks up large IP packets into smaller ones. This function is supported by IP and is called fragmentation. Without fragmentation, the queue manager would discard these long IP frames, preventing communication between nodes on separate FDDI rings that are linked by Ethernet/802.3 LANs. 12 Digital Technical Journal Vol. 3 No. 2 Spring 1991 Development of the DECbridge 500 Product Since the translation The translation processor process alters frames, the consists of principally original cyclic redundancy a GAB and a translation check (CRC) field is no engine (based on a 68020 longer valid. In the subsystem). The GAB DECbridge 500 device, and translation engine the translation process interactively copy frames concurrently verifies the from FDDI frame memory received CRC, translates to Ethernet/802.3 frame the frame, and generates a memory. Concurrently, new CRC. This concurrent the translation engine processing results in makes changes to the a high degree of data frame format, and the GAB integrity. calculates both the CRC of the incoming frame, Address Bit-ordering using old bit-ordering, The bits of the destination and generates the CRC of and source addresses are the translated frame, using transmitted in the reverse both new bit-ordering and order on FDDI from that on new frame format. Ethernet/802.3 data links. Frames from the forward Digital's FDDI chip set queue in FDDI frame memory performs a bit-reversal are thus translated and operation on receive and moved to the forward queue transmit for only the MAC in Ethernet frame memory. frames' destination and Frames from the bridge source address fields. queue are separated into Since these MAC fields management and spanning are stored inside the tree queues in Ethernet bridge in IEEE 802.1, /802.3 frame memory. The canonical bit-ordering, translation processor only one version of each returns buffers from the address needs to be kept in forward and bridge queues the forwarding database. to the free queue in FDDI Also, when generating frame memory. The queue management messages, this manager returns buffers method of frame storage from the free queue to the allows the bridge to receive queue, making them move an address from the available to store newly source or destination received frames. field of a received frame NI-side Processing into the data field of the management message Frames placed in output without modification. queues by the translation When calculating the CRC processor are processed on incoming packets, or next by the AP. Frames generating a new CRC on in the spanning tree and forwarded packets, the management queues are translation process must destined for the bridge take into account the bit- as a manageable entity ordering. on the extended LAN. The Implementation AP processes these frames Digital Technical Journal Vol. 3 No. 2 Spring 1991 13 Development of the DECbridge 500 Product and may generate response The frames in the forward traffic on either the FDDI queue that pass the or the Ethernet/802.3 LAN. protocol and source address Frames in the forward queue filters are placed on the are subjected to additional transmit queue of the match or nonmatch filtering Ethernet/802.3 chip set. by the AP. These frames The AP must merge these are checked against a list frames into the transmit of protocol types loaded queue with management by network management traffic that the AP has (e.g., TCP/IP and AppleTalk generated for the Ethernet protocols). Protocol /802.3 LAN. filtering is often a NI-to-FDDI Forwarding useful mechanism to The bridge processes prevent all frames of traffic received from the one or more protocol Ethernet/802.3 LAN in much types from propagating the same way as FDDI LAN across the extended LAN. traffic processing was The AP also uses the described in the preceding table lookup engine to material. It is essentially check frames against a a mirror-image process, list of source addresses but a few significant loaded in the address differences exist. table RAM for filter /forward requirements. The lower arrival rate of Source address filtering frames from the Ethernet may be used to contain /802.3 LAN does not traffic from nodes with require a dedicated frame- an unusually high transmit processing engine such rate. In additon, this as the queue manager. filtering may be used Thus, destination address as a form of security to filtering is performed deny access to nodes that by the AP, which shares are masquerading, that the TLU engine and table is, transmitting by using address RAM with the queue another node's address. manager. The bridge checks frames Also, allocation counters against protocol and source are not used on Ethernet address lists after the /802.3 traffic. The AP frames have been filtered directs all incoming by destination address traffic into different in the queue manager. The queues at full rate. rate of frames here is Unusually high bursts of a lower, not exceeding the particular frame type could Ethernet/802.3 LAN rate. overflow a given queue. Performing such checking on Another difference is a all incoming traffic from requirement for stations the FDDI LAN would require placing traffic on the significant additional FDDI ring. On token ring computational work by the networks, the transmitting queue manager subsystem. station is responsible for 14 Digital Technical Journal Vol. 3 No. 2 Spring 1991 Development of the DECbridge 500 Product removing its own frames electrical, firmware, and from the ring. A typical mechanical and power. station knows which frames System-level Design to strip by recognizing its own address as the source The AP and the Ethernet address. When a bridge /802.3 packet memory of transmits a frame, the the DECbridge 500 product source address is that of correspond approximately the originating node. In to the processor and the DECbridge 500 product, memory of its most recent the stripping function predecessor, the LAN Bridge is handled in the FDDI 200. The high FDDI packet chip set. A bridge strip rate required the use algorithm is implemented of a separate processor that generates frames to filter incoming FDDI marking the end of a block traffic. Also, another of transmitted frames and dedicated processor was also makes use of counters necessary to perform the for sent and stripped translation function. frames. (Ethernet-to-Ethernet bridges do not require a Development Methodology translation function.) The resulting increase in the It was important to get rate at which a processor the DECbridge 500 product accesses frame data to market in as short a required the development time as possible. The of a separate packet memory solidification of the for the FDDI LAN. ANSI FDDI specifications, Electrical Design coupled with the appearance of products from different The Ethernet interface vendors, created a finite and packet memory designs window of opportunity. again were borrowed from At the same time, the the LAN Bridge 200 product, requirements to be met but several extensions were significant. The next were needed. The AP design three sections present is very similar to the brief descriptions of some design of the processor elements of the development in the LAN Bridge 200, methodology that were but it has several new employed to meet the features, namely, a down- design requirements while line, loadable program optimizing the development memory, a bus system schedule. for communicating over Utilization of Existing the backplane with other Technology modules, and a distributed interrupt system. The queue The DECbridge 500 manager, the translation developers combined processor, and the FDDI technology from existing chip set with packet products with their own new memory are new designs. technology in the following The additional circuitry design areas: system level, Digital Technical Journal Vol. 3 No. 2 Spring 1991 15 Development of the DECbridge 500 Product resulted in a multimodule test bed for the FDDI chip system with a backplane. set. The test bed design Firmware Design was expanded midstream so that separate modules could The DECbridge 500 be added, turning it into product uses the same a breadboard for either operating system as other a DECconcentrator or a Telecommunications and DECbridge device. The two Network products. Much of DECbridge modules contained the firmware associated the queue manager, the with the bridge entity translation processor, and and with Ethernet-side the Ethernet/802.3 chip processing was modified set and packet memory. The from the LAN Bridge 200 test bed provided the FDDI product. The queue manager interface, an FDDI packet and translation processor memory, and an application required all new code. processor, as well as a Mechanical and Power power/packaging platform. Designs While evaluation of the breadboards was still Previous products typically taking place, activities consisted of a single were accelerated to develop module mounted in a box. the products. The DECbridge 500 device Technical Risk Analysis required developing a multimodule system with Different approaches a backplane. The initial were adopted for various goals were to install parts of the bridge design two, or at most three, based on technical risk. logic modules. To minimize Completely new technology, the risk to the module e.g., the queue manager development schedule, a and the translation four-board approach was processor, were simulated, adopted, which closely breadboarded, and tested. follows the block diagram Areas that were understood shown in Figure 2. The but still new, e.g., box, the backplane, and the packet memory designs, power supply are all new were evaluated largely designs. by gate-level simulation. Integration of FDDI High-confidence areas, Products and Chip Set such as designs taken from Development previous products, were evaluated in the prototype A strategy was adopted to products. maximize the commonality of The DECbridge 500 product effort in the development employs three processors. of the DECbridge 500 Thus, a lot of the bridge and the DECconcentrator functionality was in 500 products, and in the firmware, and changes could evaluation of the FDDI chip be made with relatively set. When a product set little impact on the was defined, plans were in schedule. Also, in several place to develop a hardware 16 Digital Technical Journal Vol. 3 No. 2 Spring 1991 Development of the DECbridge 500 Product instances, deficiencies corrections to recognized found in the system-level problems. A process was design could be corrected developed whereby new in the firmware. releases were tested for Use of Parallel Activities a few days each in RQT and at internal field test Several parts of the usual sites and then released to development process were external field test sites. overlapped to minimize The down-line-upgrade time. A combined functional ability was instrumental and design specification in allowing us to use this was generated instead process. of going through two serial stages to produce Conclusions separate specifications. In the hardware design, Differences in frame module layout started format, frame length, and once a confidence factor transmission speed place was achieved through requirements on an Ethernet simulation. Design reviews /802.3-to-FDDI bridge were held concurrently that are not encountered with module layout, and in bridges between like performance simulation data links. The DECbridge continued throughout the 500 product met these process. There was a close requirements by dedicating interaction of the printed one processor subsystem circuit board layout to the translation group and the electrical process and another to designers. the process of filtering In product qualification, and sorting incoming FDDI a pipelined system of frames. By adhering to the reliability qualification requirements of the IEEE testing (RQT), process standard for transparent qualification testing bridging, the DECbridge 500 (PQT), and internal device allows the problem- /external field test was free interconnection of set up to accommodate a FDDI LANs to the large phased release of firmware. existing base of Ethernet RQT and PQT started with /802.3 LANs. a functional, subset The development team release of the firmware. concluded that by Hardware confidence grew. performing risk analysis After electrical design and having backup plans verification testing, in place, several parts of firmware with the minimal the standard design process functionality for field could be compressed or test was tested briefly overlapped. Fundamental to in RQT and PQT and then the design was the ability shipped to the field. New to make remote, nonvolatile firmware releases were upgrades to the product's developed with increased operating firmware. functionality as well as Digital Technical Journal Vol. 3 No. 2 Spring 1991 17 Development of the DECbridge 500 Product Acknowledgments A large number of people Management, and contributed to the success Publications. of the DECbridge 500 project. It is not possible References to name all of these important contributors o F. Backes, "Transparent in the space provided. Bridges for Therefore, the authors wish Interconnection of IEEE to acknowledge that the LANs," IEEE Network, DECbridge 500 product could vol. 2, no. 1 (January not have been developed 1988). without significant o Local Area Network contributions from members MAC (Media Access of the Telecommunications Control) Bridges, IEEE and Networks (T&N) Standard 802.1(d) (New Communications Systems York: The Institute Engineering Group of Electrical and responsible for the Electronic Engineers, diagnostics, firmware, Inc., 1990). hardware, product o Logical Link Control, assurance, and software ANSI/IEEE Standard and from the following 802.2-1985, ISO/DIS T&N organizations: 8802/2 (New York: The Architecture, Customer Institute of Electrical Services Systems and Electronics Engineering, Marketing, Engineers, Inc., 1985). Manufacturing, Product 18 Digital Technical Journal Vol. 3 No. 2 Spring 1991 ============================================================================= Copyright 1991 Digital Equipment Corporation. Forwarding and copying of this article is permitted for personal and educational purposes without fee provided that Digital Equipment Corporation's copyright is retained with the article and that the content is not modified. This article is not to be distributed for commercial advantage. Abstracting with credit of Digital Equipment Corporation's authorship is permitted. All rights reserved. =============================================================================