Digital Equipment Corporation

Company Background

Digital Equipment Corporation (DEC) was founded in 1957 by three graduates of Massachusetts Institute of Technology (MIT) operating out of an old mill in Maynard, Massachusetts. One of the original three founders, Ken Olsen, went on to lead the corporation and has been the most significant person to shape Digital. The fact that the corporate headquarters remains in Maynard is representative of the company's paradoxical commitment to both innovation and tradition.

During its early years, Digital produced specialized logic modules. Although it operated in a data processing world dominated by large, mainframe-style computers, Digital had the vision to pioneer minicomputers by introducing the PDP-1 in 1960. The revolutionary aspect of the PDP-1 was its interactive video terminal capability--the first such implementation of this now commonplace technology.

In 1970, Digital released the PDP-11, a 16-bit machine that became the most popular design of the PDP series. Used most often in manufacturing and technical environments for process and instrument control, the PDP-11 family remained in demand until it was discontinued in 1991. Following the growth of the PDP-11 market, Digital released a networking architecture in 1976 called the Digital Network Architecture (DNA). The set of products and services defined by DNA is more commonly known as DECnet. The PDP-11 design also served as the launching platform for another product: Digital's general-purpose, 32-bit Virtual Address Extension (VAX) computer. Introduced in 1977, the VAX gradually became Digital's premiere product--a building block for other products offering performance and disk capacity similar to that of mainframes. Among these products are VAX clusters, a computing environment released in 1983 and based on multiple VAX computers sharing a common repository of disk space; and the mainframe-level VAX 9000 Series, announced in 1989. In 1992, Digital released the Alpha AXP architecture, based on 64-bit RISC microprocessors. Today, the Alpha runs on all Digital machines, from PCs to high-end symmetric multiprocessing and clustered servers. Digital also offers a line of high-end Pentium Pro products, including the four-processor Prioris server and Celebris workstation.

Product Line Overview

Digital maintains a comprehensive manufacturing operation that designs and builds most of its products. Having paved the way for the so-called midrange computer market, Digital has expanded its product line up into the mainframe domain and down to the PC playground. In support of these offerings, Digital produces a complete line of terminals, printers, and assorted networking devices.

Digital's VAX family of computers ranged from the MicroVAX desktop system, all the way up to high-end symmetric multi-processing (SMP) servers. Every member of the VAX family supported the same operating systems and applications. The VAX family was based on CMOS V technology and was highly scalable, up to symmetric multiprocessing and clustering configurations. A number of different clustering options were possible under the VAX architecture, including clustering in a single office, or clustering systems that were geographically separated. However, the 32-bit nature of the VAX architecture was quickly hitting the wall, primarily because of address limitations.

In 1988, Digital formed a task force to explore ways to preserve its existing VAX VMS customer base through the coming decade, and in 1992, released the Alpha AXP architecture. The Alpha AXP architecture retains many of the VAX's attributes while offering significantly more power. The 32-bit VAX architecture was based on complex instruction set computing (CISC), whereas the 64-bit Alpha AXP architecture was reduced instruction set computer (RISC)-based. Digital subsequently ported its OpenVMS operating system to the Alpha AXP architecture to enable OpenVMS applications to take advantage of RISC's performance advantages.

The Alpha AXP architecture was designed for high-performance computing. In fact, OpenVMS AXP applications outperform OpenVMS VAX applications by a factor of 3.59 to 1. Digital built easy migration capabilities to enable customers to move from the VAX to the Alpha AXP architecture without significant recoding of their applications. Digital's Alpha AXP processors run multiple operating systems and have the ability to run native programs translated from VAX and MIPS architectures, thereby preserving their customers' existing VAX and MIPS investments.

Digital's goals in developing the Alpha AXP architecture were to provide:

To operate OpenVMS AXP, DEC OSF/1 AXP, and Microsoft Windows NT operating systems, Digital adopted some technology from their PRISM design. Under this model, a set of sub-routines (PALcodes) with controlled entry points were established for each operating system. For running VAX and MIPS binary images, Digital uses binary translation.

The Alpha AXP architecture is based on a shared-memory model. The first implementation was the DECchip 21064 microprocessor. At the time of its release in 1992, it was the world's fastest single-chip microprocessor--even listed in the Guinness Book of Records as such. The latest implementation, the 21164 Alpha microprocessor, runs at a blazingly fast 300MHz, and power can be increased further through symmetric multiprocessing or clustering. The Alpha AXP chip is capable of running multiple operating systems, and can run native programs translated from VAX and MIPS architectures.

The Alpha AXP architecture is now used throughout Digital's product line. The Alpha AXP's 64-bit architecture is designed with an eye towards high performance, and continues Digital's focus on multiple processors. The powerful 64-bit Alpha architecture is capable of bringing high-end features to smaller systems. The Alpha systems can run Digital UNIX, OpenVMS, and Windows NT; for sophisticated functions such as data warehousing, they can address massive files greater than 2G in size.

Digital released a 64-bit version of its Digital UNIX operating system in March, 1996. Digital UNIX 4.0 integrates the now pervasive Common Desktop Environment (CDE) GUI, which establishes a common look and feel between all major UNIX implementations. Digital UNIX 4.0 supports POSIX threads, real-time standards, and X11R6. Additionally, it conforms fully to the Single UNIX Specification (Spec1170) administered by the X/Open organization. The 64-bit nature of version 4.0 now enables Digital UNIX to run high-end applications such as data warehousing.

Digital Equipment Terminals

The Digital line of terminals is perhaps the most widely emulated line of character-oriented displays. The reasons are straightforward: The following generations of products compose the fundamental line of Digital video terminal (VT) devices: In most cases, Digital terminals transmit characters as they are pressed on the keyboard. Issues such as buffering and transmission optimization are handled by the computer system or by the devices between the terminal and the computer system. These considerations will be discussed later in this chapter.


The terms personal computer and Digital Equipment Corporation are not as synonymous as one might expect. It is not that Digital has been unable to apply its innovative talents in this arena, or that the resulting products did not come to market. The real problem rests with timing. Before IBM's release of the first PC, Digital had also been working on a small, personal microcomputer. At that time there was no single dominant hardware architecture or dominant operating system on the market (although the Control Program/Microprocessor [CP/M] was doing well at that time).

When IBM released the first PC in 1981 (and subsequently set the standards for today's market), Digital redoubled its efforts to get its own products to market. Digital produced not one but three possible contenders for the low-end market. Each had its own distinct advantages and disadvantages, and each was given an opportunity to prove itself.

The three products that resulted were the DEC Rainbow, the DECmate, and the Professional 300. Of these three, the DEC Rainbow was the only product that came close to being a clone of the IBM PC; the Rainbow ran Microsoft's MS-DOS operating system but did not feature the Basic Input/Output System (BIOS) and hardware-level compatibility with the IBM machines.

In contrast, the DECmate and the Professional 300 lines were corporate-oriented computers. For example, the DECmate featured sophisticated word-processing capabilities and could be networked easily with Digital's larger machines. The Professional 300, on the other hand, was intended to be a desktop version of the Digital's PDP computer.

None of these three products have any inherent, glaring faults, but all three were released in the shadow of the IBM PC (and the emerging clone market), which fell on them like a wet wool blanket. Although it is certainly unfair to call any of the products a market failure, none achieved the celebrity status earned by the IBM PC. Digital's resigned mindset was further evidenced by the release of the VAXmate, a follow-up product to the IBM PC/AT. Based on the same Intel 80286 microprocessor chip as the IBM AT, the VAXmate had limited expansion capabilities and was marketed only to existing Digital customers.

At the end of the 1980s, Digital was ready to take another chance on the IBM PC-compatible market. This time, however, it turned to Tandy Corporation (Radio Shack; Fort Worth, Texas) to manufacture a line of PC-compatible products for Digital. The resulting products were the DECstation 200, DECstation 300, and DECstation 400 lines--based on the Intel 80286, 80386, and 80486 processors, respectively. Although Digital downplayed the fact that it had out-sourced the product to Tandy, this fact was not lost on the market.

Finally, in 1990, Digital released the applicationDEC 433MP, a multi-user UNIX system based on the Intel 80486 microprocessor. This product was targeted at the small-business market and features support for multiple 486 processors and connectivity for up to 96 concurrent users. Its roots in PC and Personal System/2 (PS/2) technology are evident by its support for either the Extended Industry Standard Architecture (EISA) or Micro Channel Architecture (MCA) bus. If nothing else, the 433MP represents an interesting convergence of PC, engineering workstation, and minicomputer technologies.

Digital's latest innovation, the Digital Personal Workstation, is designed around a processor-independent architecture, and offers the user a choice between Pentium, Pentium Pro, or Alpha processors. Users can move from Intel to Alpha with the change of a single card. Both processors are optimized to run Windows NT, and can be used to run many of the high-end applications, such as CAD or GIS, that had been limited to higher-end workstation products. It offers PCI or ISA Ethernet or token ring options for networking. The Celebris XL is based on Intel technology and features either single 100, 120, or 133 MHz; or dual 100 or 133 MHz Pentium processors. The Alpha XL runs 233 MHz and 266 MHz implementations of the Alpha 21064 processor. Several native Windows NT applications have been ported to the Alpha system as a result of an alliance between Digital and Microsoft.

Engineering Workstations

Digital's original response to the engineering workstation market was to couple high-resolution displays with various VAX processors. Capable of running either the VMS operating system or ULTRIX (Digital's implementation of UNIX), the product line was termed the VAXstation.

In general, as the capabilities of the mainstream VAX models grew, so did those of the VAX-stations. The VAXstation 100/500 line introduced in the early 1980s was replaced by the VAXstation II in 1985, a line that included a model with an independent graphics coprocessor. On the heels of the VAXstation II came the VAXstation 2000-7000, released in 1987 and 1988.

The VAX 7000 was designed as a high-performance system, suitable for high-volume transactions or distributed networks. Many mainframe-based applications could run on the VAX 7000, making it usable as a backbone platform for supporting business-critical applications. The VAX 7000, which was expandable to six CPUs, was built on Digital's CMOS technology and ran the OpenVMS operating system.

In 1989, Digital revamped its product line and stirred some new ingredients into the mix. The first ingredient was the use of a RISC architecture for some of the machines. Another was that the basic RISC processor used by Digital was, in fact, manufactured by MIPS Computer Systems, a third party supplier. Finally, these new RISC-oriented machines could run only ULTRIX (and thus were shunned from the VAX community).

Presumably to reduce confusion between the VAX-based and RISC-based products, Digital named the new line DECstations (a named shared by Digital's MS-DOS computers). Digital targeted this new lineup as a sweeping, low-end desktop blitz and paraded it in front of the public.

The product lineups consisted of the VAXstation 3000 Series (based on Digital complementary metal oxide conductor, or CMOS, VAX processor technology); the DECstation 5000 Series (based on the MIPS RISC architecture); and the DECstation 200, 300, and 400 Series (based on the Intel 80286, 80386, and 80486 microprocessors). While the low-end, PC-oriented DECstations were not presented as heirs to the engineering workstation throne, the association (by name alone) of the DECstation 3000 Series with these units raised both eyebrows and confusion.

The AlphaStation 255 and 500 families of UNIX and Windows NT workstations are the latest additions to Digital's line of midrange workstations. Digital's AlphaStations support Digital's new 64-bit operating systems (Digital UNIX and OpenVMS), as well as Windows NT.

The 500 supports dual-fast and wide SCSI-2 channels, Ethernet and Fast Ethernet, and can accommodate up to 512M of RAM. All products in the AlphaStation line include multimedia capabilities.

The PowerStorm PCI-based workstation graphics option is available for all Digital workstations. PowerStorm delivers superior graphics performance, using the common OpenGL API. PowerStorm was designed for 2-D and advanced 3-D applications that require high performance, such as motion and texture mapping.

Digital's RISC-based AlphaStations offer superior performance for applications such as modeling, imaging, animation or videoconferencing. The new 64-bit architecture can directly address up to 1G of real memory, making it easier to handle very large files without disk-swapping. The newest AlphaStations run the 64-bit RISC Alpha 21164 microprocessor, at speeds up to 300 MHz. Like the other Digital products running Alpha technology, it can run UNIX, OpenVMS or Windows NT.

The AlphaStation 250 is well-suited to mechanical or electrical CAD applications, and also offers collaborative computing facilities. The 200 is a somewhat lower-cost, entry-level product, but also offers the 64-bit computing environment, at speeds up to 233 MHz. On the high end are the AlphaStation 600 systems, which are the fastest and most ideal for high-end scientific research projects that involve complex visualization and calculation.

The AlphaStation 500 (see Figure 2.1) is well-suited for higher-end CAD applications or memory-intensive and CPU-intensive multimedia projects. The system runs the Alpha 21164 processor at 333, 400, or 500 MHz.

Midrange Offerings

The PDP-11 line, until it was discontinued, filled the lower end of the spectrum, if for no other reason than by virtue of being a 16-bit machine. The 32-bit VAX, on the other hand, was offered in a broad range of models, starting from the diminutive MicroVAX extending to the mainframe-oriented VAX 9000.

Digital Equipment Alpha 500 Workstation

Both lines shared the dubious honor of using multiple bus designs. The three most common bus architectures are as follows:

Because of its roots as an interactive multiprocessing system, the PDP-11 supported a variety of operating systems. The RSX line (RSX-11M, RSX-11M-PLUS, and RSX-11s) provided multiprocessing capabilities under priority-driven and/or real-time constraints. In contrast, RT-11 was at the low end, offering multiprocessing in a single-user environment.

While the PDP-11 maintained a respectable position at Digital until it was discontinued, the favorite offspring is still the VAX. The first VAX offering was the VAX-11 line, released in 1977. Whereas the PDP-11 was oriented more to the technical market, the VAX was suited better for the commercial and business markets. The primary operating system, VMS (Virtual Memory System), is a multiprocessing, interactive environment well suited to multi-user business applications. The secondary operating system, ULTRIX, is Digital's implementation of UNIX for the VAX. A real-time operating system, VAXELN, is also available (primarily as a migration tool for PDP-11 users).

The VAX product line contains two segments: the Q-Bus MicroVAX line and the VAXBI line. The MicroVAX line starts with the relatively old MicroVAX I and progresses to the MicroVAX 2000 (which, unlike other MicroVAX systems, uses its own bus structure); the MicroVAX II and the MicroVAX 3300, 3400, 3800, and MicroVAX 3900. These machines are targeted to smaller businesses and departments and are also a bridge from PDP to VAX technology.

With an eye to replacing the MicroVAX name and line, Digital introduced the VAX 4000 Series in 1990. These machines are positioned between the MicroVAX line and the low end of the VAX line, and functioned as uniprocessing departmental servers or high-end CAD workstations. From a performance perspective, the VAX 4000 Series offers a price/performance ratio that is significantly better than the high end of the MicroVAX line, while providing performance similar to the low end of the VAX series. This overlap is part of Digital's strategy to reposition the low end of its product line with its higher performance offerings.

In the big leagues, VAXBI systems start with the VAX 8200 Series, and then advance into the 8300, 8500, 8600, 8700, 8800, and 8900 Series. In recent years, Digital introduced the VAX 6200, 6300, and 6400 Series as the de facto replacements for the low end of the 8000 Series (the 8200 and 8300). In fact, the 6000 Series was designed to accommodate the multithreading of online transaction processing (OLTP). Additional requirements for OLTP are addressed by the VAXft 3000, a fault-tolerant system based on dual-processor MicroVAX 3000 technology.

At the top of the VAXBI line are the VAX 8600, 8700, 8800, and 8900 Series; the 9000 Series; and VAXclusters. While the high-end models of the older 8000 Series approached mainframe capabilities, they had some difficulty maintaining fast access to large capacities of memory and disk storage. These deficiencies were corrected in the VAX 9000.

Most of the VAX series are upgradable to the newer Alpha AXP architecture, and capable of running the OpenVMS or Digital UNIX operating systems. The Alpha AXP architecture afforded a new 64-bit architecture that can directly address up to 1G of real memory.

The AlphaServer 2000 series (see Figure 2.2), based on Digital's new 64-bit RISC technology, offers highly compact SMP servers with up to four processors; they are used primarily as departmental or LAN servers. The 2000 series runs the DECchip 21064 CPU at 150MHz, and supports DEC OSF/1 AXP, OpenVMS AXP, and Windows NT. The 3000 Series, running the 21064 CPU at 175MHz, offer extensive memory and storage capabilities, and is an excellent choice as a server for X Window terminals.

Digital Equipment AlphaServer Family

The 64-bit design of the Alpha AXP architecture provides much more power to the midrange, as well as added networking capacity and more file storage.

Top-End Offerings

Announced in 1989, the 64-bit VAX 9000 Series is Digital's best performing system, offering mainframe-level performance. Using a combination of CISC and RISC technology, the new processor and bus design (XMI) gave the 9000 a significant increase in data processing speed. To achieve compatibility with the mainstream midrange machines, the VAX 9000 also supports the VAXBI bus interface.

In addition to the main CISC/RISC system processor, the VAX 9000 supports specialized vector processors for parallel operations. The Star Coupler CI interface was enhanced for direct support of the XMI bus, and the resulting interface, termed CIXCD, brings higher throughput and new configuration options to VAXclusters. In the most general sense, the VAX 9000 was designed with one eye on compatibility between VMS and the existing VAX line, while the other eye focused on optimization and mainframe-class performance.

The RISC-based AlphaServer 8000 line has the ability to run up to 12 300-MHz 21164 Alpha processors, and combines the benefits of the mainframe with those of an open, client/server system. It can accommodate a 10-terabyte cluster storage, and is usable as a high-availability database server or OLTP server. This line includes the 8200, which is scalable up to six 300 MHz processors, and can run both Digital UNIX and OpenVMS. The 8200 and 8400 are the high end of the 8000 line, and are the first of the AlphaServer 8000 series which Digital plans to extend through three generations of products.

In designing the 8000 series, Digital had several goals, including support for legacy I/O subsystems and DEC 7000/10000 AXP compatibility. The 8000 series doubles the performance of the 7000/10000 AXP server line while providing a viable and fairly straightforward upgrade path. In designing the 8000, Digital's design team conducted a thorough analysis and performance study of the 7000/10000 systems. After the analysis, Digital decided that system data bandwidth and memory read latency goals were critical to their new design.

Read latency is the amount of time required for a CPU to read a piece of data into a register in response to a load instruction. Cache memory is a common way to minimize read latency. Latency tends to degrade as the number of processors is increased, and latency will improve as the amount of available bandwidth is increased. Comparable systems from HP and IBM do not stress low-memory latency in the design of the RISC System/6000 or the PA-8000 SMP systems. Although the older 7000/10000 AXP systems were comparable to the RS/6000 and PA-8000 in terms of latency, the AlphaServer 8000 was developed with an emphasis on low-memory read latency. The 7000/1000s had a latency of 560 nanoseconds, comparable to the ratings of the RS/6000 and PA-8000. On the 8000 platform, however, Digital achieved a read latency of 200 nanoseconds. In its design of the 8000 series, Digital pinpointed low latency as a major factor in high system performance.

Symmetric Multiprocessing and VMSclustering

Previously known as VAXclustering, VMSclustering is executed with Digital's OpenVMS Cluster Software. The software establishes an integrated OpenVMS environment, which can be distributed over multiple Alpha and VAX CPUs. VMSclusters can include both VAX and Alpha CPUs in the same cluster system.

To obtain high performance and greater computing capacity, Digital engineered two optional processing configurations: symmetric multiprocessing (SMP) and clustering. Both offer significant processing improvements.

SMP uses multiple processors sharing resources within a single VAX or Alpha AXP system. The SMP architecture enables the system to break up functions and process them independently. Thus, a disk access can be handled by one of the processors while another performs a CPU-intensive calculation. The number of processors available depends on the base model (note that not all models are capable of being used in SMP mode).

Although SMP does provide some benefits based on its architecture alone, applications must be specifically written (rewritten, or DEComposed as Digital calls it) for the SMP environment. One final advantage of the SMP approach is that it makes the system more fault-tolerant; if one processor malfunctions, the system can be restarted and instructed to ignore the faulty processor. While this is far from an online recovery operation, it sure beats being down until the hardware engineer arrives.

In contrast to SMP, clusters enable multiple systems to share disk storage in a highly opti-mized manner. Clusters use a common piece of hardware, called a StarCoupler, to provide a physically common point of contact between the multiple VAXs or Alpha AXPs and the disk subsystem (see Figure 2.3). A VMScluster system can contain an unlimited number of star couplers; the number of star couplers connected to a CPU is limited only by the number of adapters provided by the CPU. A single Star Coupler can interface multiple systems to one or more Hierarchical Storage Controllers (HSC) which, in turn, control multiple physical disk drives. The interface between the computers and the Star Coupler is referred to as the interconnect, a high-speed link designed for fault tolerance. If there is an interconnect failure, the VMScluster software will automatically resort to an alternate interconnect. The VMScluster software will support any combination of these interconnects:

Sample VAXcluster

CI and DSSI are special-purpose interconnects, designed for CPUs and subsystems in the VMSclusters. A maximum of 16 CPUs can be connected to a star coupler; a maximum of four CPUs can be connected to a DSSI. The SCSI bus is not used for CPU-to-CPU communications; therefore, CPUs connected to a multihost SCSI bus also require another interconnect to provide this capability. The purpose of the SCSI bus is to provide multihost access to SCSI storage devices.

Every aspect of a clustered system has built-in redundancy. And, the hardware-level design can be enhanced further by doubling (or tripling in some cases) the number of computers or amount of disk storage required. Overall, this makes for a highly efficient, highly fault-tolerant hardware architecture. The cluster design provides a self-contained backup system as well, and is an ideal solution to gradual expansion because the cluster can be built one system at a time.

However, no degree of hardware tolerance can salvage the data damaged by an application crashing or running amuck. And because the purpose of a cluster is to provide concurrent access to a large pool of information, the integrity of that information is of paramount importance--after all, there is no point in sharing a pool of garbage (unless you're a goat).

To preserve the integrity of information, clusters can (and usually do) implement the following safeguards:

On a much broader level, another technique to help maintain a high degree of data integrity and availability is disk shadowing. With this technique, you attach two disks to at least one (but preferably two) hierarchical storage controllers in a cluster so identical information is maintained on both disk drives. Please note, however, that you should use shadowing with the above recovery mechanisms to handle situations in which the application corrupts both copies of the information.

As previously discussed, VAXclusters initially were developed to help Digital compete in the higher end of the market. The advent of high-end systems from Digital, however, will not make the cluster obsolete. Instead, systems like the Alpha 8000 Series will be used in clusters to maximize and even increase their high performance.

To sum up the salient difference between SMP and clusters, SMP is an architecture that enables multiple applications within a computer to run in parallel (to a degree), sharing common memory and disk resources. Clusters, on the other hand, enable multiple applications on separate computers to share the same information stored on disk. With clusters, there is no need to redesign or DECompose programs to take advantage of the shared disk facility. Finally, SMP can be implemented within a cluster to achieve maximum performance (provided the computers in the cluster support multiple processors).

Strategy for Connectivity

Digital Equipment Corporation's strategy for connectivity is built upon DECnet. DECnet is a set of products and services designed and implemented based on the Digital Network Architecture (DNA). DECnet was originally implemented using proprietary protocols, but in the early 1990s, Digital shifted DECnet into the open network environment by incorporating support for both OSI protocols and for TCP/IP.

The shift toward open networking was the fifth major "phase" (revision) in the DECnet specifications. The original DECnet phase (Phase I) was introduced in 1976, and provided simple services for file transfer and program-to-program communication between PDP-11 machines running the RSX-11M operating system. At the time, this was fairly revolutionary, as software portability was almost non-existent. The subsequent phases added more networking and interoperability services, bringing DECnet up to the level of where it is today.

Phase II was released in 1978, and brought the capabilities of Phase I to all major operating systems (including VMS). Additional features included remote file access, network management tools, and support for point-to-point network configurations. Phase III, released in 1980, introduced a dynamic adaptive routing algorithm, which automatically calculated the best route to a message's destination. Under Phase II, a direct connection had to be created between systems. Phase III also offered support for X.25 packet switching networks as a method to connect systems, record-level access over the network, and downline loading. A Phase III network could include up to 255 nodes.

Introduced in 1982, Phase IV was a major jump in technology. Under this phase, Digital came to support ethernet, large LANs and WANs, virtual terminal services, and various communications servers and gateways.

Digital's Phase IV approach to combining DNA with Ethernet (resulting in DECnet) was the most revolutionary and sweeping networking architecture since IBM's Systems Network Architecture (SNA). Ethernet, the result of a collaboration among Digital Equipment, Intel Corporation, and Xerox Corporation, defines a physical interface that supports a data rate of 10 million bps over a shielded coaxial cable. Up to 1,024 nodes (addressable attachments) are supported. One of the major benefits of Phase IV was conformance with Open Systems Interconnect (OSI) standards.

The second important aspect of Ethernet is the collision-detection mechanism. Because any node on the network could transmit at any time, a regulation mechanism had to be created to recover from the inevitable collisions (which result in garbled information). The technique Digital and Xerox put in place was termed the Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

Using the CSMA/CD technique, a node with information to transmit first listens to the network. If no one else is transmitting, that node goes ahead and sends. If, however, another node is transmitting on the network, that node waits for a predetermined length of time before attempting to transmit again. If two nodes transmit simultaneously, the collision is detected and both nodes wait for a random time period before attempting to retransmit.

The CSMA/CD technique is also used in the IEEE 802.3 network implementation. In fact, Ethernet and IEEE 802.3 are similar enough that nodes of both types can coexist on the same physical network (although the two implementations are also different enough that one type of node can't "understand" the other type's information).

Phase V was made available in 1991. It made significant changes to accommodate TCP/IP as an alternative to OSI standards for open networking. Digital had three goals in releasing Phase V: to permit a network to grow up to a million systems, to incorporate both OSI and TCP/IP standard protocol suites to provide for a higher level of system integration, and to support a distributed mode of operation. Although a Phase IV network could accommodate up to 64,000 nodes, the industry's move towards distributed, client/server computing soon limited Phase IV's viability. Phase V introduced a new routing algorithm, which can potentially support millions of nodes. This algorithm has since been adopted as a routing standard for both OSI and TCP/IP networks. Phase V also set out to provide a distributed networkwide naming service, and to permit nodes to generate their own addresses and register themselves automatically.

The Digital Distributed Name Service (DECdns) provides the following features: distribution (so naming information does not have to be stored in a single location), replication, dynamic updating, automatic updating, and a hierarchical naming structure. Adding a new node to a Digital Equipment network is remarkably simple when using DECnet with DECdns. The combination of these two products permit full autoconfiguration. Where DECdns assigns names to computer systems, X.500 names extend to naming individuals within a naming framework.

DECnet can be implemented on a LAN using Ethernet or over a WAN using a variety of routers and/or gateways to bridge the distant LANs. Technicalities notwithstanding, the primary function of DECnet is to deliver the following capabilities:

Understanding the importance of DECnet is the key to understanding Digital's approach to data processing. It is much rarer to find a single, isolated, non-networked VAX than to find two or more Digital nodes communicating via ethernet.

Digital uses an X.500-based Directory Service, a general-purpose distributed directory similar to Banyan's StreetTalk and Novell's NetWare Distributed Services. Digital goes further than these two products in making the service accessible from any Web browser or messaging system. Digital's service can be used to hold any information that an organization wants to make public; sensitive data can be restricted. The directory can be divided between multiple directory servers, with each server having the ability to pass directory tasks to other servers that possess the relevant data. Digital X.500 Directory Service supports Internet and X.400-based networks, and can function as a superset of other directory services to form a global directory for messaging and other applications. The service is integrated with Digital's MAILbus 400 backbone, which acts as a bridge between SMTP and other enterprise mail systems. Developers can write applications to access the directory using the industry standard X/Open XDS API. With the Digital X.500 Directory Synchronizer, directory information can be exchanged and synchronized in a multivendor environment. Directories from many popular e-mail products can be synchronized, and it can also support legacy e-mail, Internet mail, and custom systems.


Digital accommodates enterprisewide messaging in a multivendor environment with a range of products that adhere to industry standards. Digital's messaging strategy comprises a three-tier, client/server architecture, consisting of the enterprise backbone, departmental system, and the desktop.

The messaging infrastructure lives at the Enterprise Backbone Server tier, and provides mission-critical transport and directory services for heterogeneous mail environments, as well as management of the messaging network. Connectivity to X.400 and the Internet is also achieved at this level.

At the Departmental Server tier, a mail or groupware server is deployed, and access to the enterprise directory is achieved through a variety of servers, including MailWorks (a client/server messaging system for LAN mail connectivity), Microsoft Exchange Server, and ALL-IN-1. At the Desktop tier. E-mail clients can select from various products such as Microsoft Mail, Lotus cc:Mail, TeamLinks, and Windows 95.

The Digital and Microsoft Alliance
Recently, Digital entered into a messaging alliance with Microsoft to develop a joint enterprise mail infrastructure. The result of this alliance will be the eventual integration of ALL-IN-1 and MailWorks, with the Microsoft Exchange Server through MAILbus 400 and Digital X.500 Directory Service. MAILbus 400 is a store-and-forward message transfer agent (MTA) that connects with other systems and services. MAILbus is based on the X.400 standard, and can therefore connect to a wide variety of other X.400 MTAs. DEC EDI (Electronic Data Interchange) provides connectivity between a company and its customers or suppliers.

Digital and Microsoft's alliance further ensures the interoperability of all Digital and Microsoft products. The Microsoft Exchange Server mail technology will be integrated with Digital's ALL-IN-1 and MailWorks products, and Digital's mail backbone will support Exchange Server. Digital will also support Microsoft's Windows Open Systems Architecture (WOSA) API in OpenVMS, which will permit application developers to write to both operating environments more easily. The alliance will contribute significantly to the integration of Windows NT into the enterprise. 

Application/User Relationship

Although Digital offers a variety of different operating systems and hardware architectures, this section focuses on the most popular combination--the OpenVMS operating system on Alpha AXP.

OpenVMS is an interactive operating system, originally developed for the VAX, that gives each user the illusion that he or she is the only user on the computer. Each user can run programs and access files independently of other users. If shared services are required for access to a common data base, for example, the services are handled at a system level, invisible to the user. After a user is logged onto a given computer in the system, the further interaction between that user and the computer is termed a session.

Within a session, the interaction between the user and the application is typically character-oriented. Although Digital terminals and programs do support block-oriented transmission formats for data entry, the application usually reads single (or small groups of) characters from the keyboard as they are being typed.

For more sophisticated applications such as word processing, this reading method enables the terminal and the application to interact as a PC keyboard interacts with a PC--each key pressed can be interpreted as appropriate for that context. (This similarity is one of the reasons that PC software vendors migrate toward DEC equipment. Conceptually, they are very similar in intent and implementation.)

Although many other computer systems use a character-oriented, interactive interface (for example, the HP 3000 under its MultiProgramming Executive, or MPE), Digital has so refined this interface that it has become an integral part of Digital's applications development strategy.

Digital has also developed its own graphical interface for terminals and PCs. This interface, termed DECwindows, provides an alternative interface between the user and the applications. As X Window graphic terminals become more available, this method will become the preferred alternative to the traditional character-oriented interface. DECwindows can be used with the OpenVMS, ULTRIX, and MS-DOS operating systems.

Terminal Attachment Philosophy

Terminals can attach physically to the network through one of the following two devices: The simplest connection consists of a terminal directly connected to a Digital host via a simple, asynchronous line (see Figure 2.4). With this attachment, the host responds directly to the terminal's activities without using any complicated protocol. By necessity, the terminal must activate a session on the attached host to access any local or remote application. The relationship between the terminal and host is a simple one-to-one connection (one terminal connecting to one host port).

Direct Attached Terminal

In the more normal case, however, terminals connect to terminal servers called DECservers (see Figure 2.5). Although the interaction between the terminal and its server is identical to the interaction between the terminal and a directly connected host (specifically, no complicated protocol is required), the interactions between the terminal server and the computers are quite different.

Terminal Server Connection

Terminal servers talk to Digital host computers using the LAT protocol. LAT operates independently from other DECnet protocols and provides two significant benefits:

More complicated are the cases in which a terminal directly connected to a host wants to access another host, or when a terminal on a terminal server wants to access a host that is not locally attached to the ethernet network but is attached to the wide area Digital network. In both instances, another protocol, CTERM (Command Terminal), comes into play. A DECnet host uses the CTERM to shuttle information between a terminal and another host (see Figure 2.6). The drawback to this technique, however, is that it consumes resources in the host sponsoring the remote link (that is, the one initiating the connection).

Regardless of the type of connection, the terminal normally uses the XON/XOFF mechanism to control the flow of data. With this technique, the terminal sends an XOFF character to the host when it wants the host to stop transmitting and then sends an XON character when it is ready for the host to resume transmitting.

Peer-to-Peer Relationships

DECnet, by its very design and intent, promotes peer-to-peer relationships among its computing nodes. This peer-oriented relationship is at the core of the DNA that underlies products like the VAXcluster and even DECnet; to obtain true distributed processing, a network of peer processes (and therefore processors) must be established.

You can see how peer-to-peer relationships contribute to peer-to-peer processing and communications by looking at remote file handling and task-to-task communications. Regarding network file access, the entity requesting the remote file could be an application program, the Network File Transfer (NFT) utility, or just a standard copy command. In the case of an application program requesting a remote file, Digital supplies a set of routines called the Network File Access Routines (NFARs) to assist the program by performing some of the lower levels of the exchange. In the case of user commands, the interface is handled via Record Management Services (RMS).


On the other side of the equation (that is, the computing node with the file to be accessed), resides a utility called the File Access Listener (FAL). FAL listens for network requests for files on its node and translates the network request into a local operation. FAL communicates with the requesting entity via the Data Access Protocol (DAP), which is part of the DNA. In truth, DAP actually handles most of the file transfer, another sign of how deep the peer-to-peer relationship is embedded in DECnet.

The issue of task-to-task communications is, however, a bit more complex. A task might have to communicate with another task that might not be running; or maybe it is running, but is unable to respond. Therefore, programs performing this type of network communications must follow some basic rules.

These rules, or task-to-task communications, begin with one program requesting a logical link to another program and identifying the location (node) and name of the target program. If the program is not known on the remote node or if the program is not able to receive communications, the network will reject the link request. If, on the other hand, the program is available and ready, then the link request will be delivered to the program, and then the program will accept or reject the request.

After the link has been requested and accepted, the programs can exchange data. Information can be sent one or both ways--it is a function of the application, not the network. When the communications are complete, one of the programs requests a termination of the link, and that logical link is disassembled.

In 1990, Digital refined the marketing of its peer-to-peer relationships with the introduction of Network Application Support (NAS). Targeted to compete against IBM's Systems Application Architecture (SAA), NAS is offered as a set of software products for developing and implementing distributed processing systems or client/server applications. Behind the marketing hype is Digital's solid peer-to-peer technology.

PC Integration Strategy

Although Digital Equipment's PC product line has some shaky history, there is nothing shaky about its PC integration strategy. Because Digital has such a strong LAN foundation with its DECnet strategy, the integration of PCs is simply a matter of interfacing them with the existing network. Furthermore, because of the wide presence of Digital midrange computers in the general marketplace, PC LAN vendors such as Novell have developed special products that enable Digital machines to participate in their third-party network architectures.

Digital Equipment provides two approaches to integration (see Figure 2.7).

PC-DECnet Integration with DECnet-DOS and PCSA

To integrate PCs into the standard DECnet architecture, Digital provides DECnet-DOS, which enables a PC to function as a DECnet computing node. DECnet-DOS offers the following fundamental capabilities:

A DECnet-DOS node can be connected to the network with an Ethernet or asynchronous DECnet connection, using the PC's COM port as a physical link. The system to which the PC is attached must be a DECnet Phase IV, full-function node, supporting asynchronous DDCMP. For read-only operations, multiple DECnet-DOS nodes can simultaneously access the same network disk.

The other approach is Digital's Pathworks, formerly Personal Computing Systems Architecture (PCSA), which is a set of services that includes DECnet-DOS. Its benefits (in addition to those already listed for DECnet-DOS) are as follows:

Both of Digital's approaches work for any PC-compatible device (in other words, the solutions do not run only on Digital PCs). Also, both approaches require an Ethernet interface card (from any of a variety of vendors) in the PC.

Pathworks permit PC users to run non-PC applications and access data beyond the PC LAN. Pathworks' Network Connect interface permits PC users to access multivendor file and print services. Further host connectivity is offered with services that include terminal emulation, an X Window System server, and e-mail.

Several third-party PC LAN companies have been attracted to Digital's market. This attraction, coupled with Digital's impressive installed base and the relative ease with which software can be ported from the PC to Alpha, has spawned some interesting products, such as Windows NT versions of OpenVMS and Polycenter Manager.

Office Automation

Digital's offering in this area is the VAX/OpenVMS resident product ALL-IN-1. ALL-IN-1 is a menu-driven system that, in its most basic form, provides electronic mail, calendar functions, and general information management functions. Furthermore, ALL-IN-1 can be enhanced to include sophisticated word processing and spreadsheet functions.

The basic package provides mail distribution and retrieval services to all participating ALL-IN-1 users. The calendar functions provide for individual (or group, in some cases) scheduling as well as general time-management functions. The information functions, however, are quite sophisticated and include forms-management tools to make data look attractive. Information entered into the facility can be used to create an online reference facility, a public notice facility, or any other file-oriented application.

More often than not, ALL-IN-1's electronic mail function is tied to the advanced word processing offered by the optional WPS-PLUS (usually pronounced "WIPS-PLUS"). WPS-PLUS is a word processor that uses the advanced capabilities of the standard VT200-style keyboard to provide word processing features similar to those found on dedicated word processors or PCs running high-end word processing software. These capabilities include word wrap, cut and paste, find and replace, headers, footers, multiple font types, and so on. To add even more sophistication, WPS-PLUS supports optional modules for spell and grammar checking.

Another option to the basic ALL-IN-1 package is 20/20, a VAX-based spreadsheet from Access Technology Inc. (Indianapolis, Indiana). Conceptually similar to the well-known Lotus 1-2-3 spreadsheet, 20/20 offers a range of spreadsheet functions and the capability to display business graphics. Some of the more advanced features of 20/20 include project management (scheduling and tracking), as well as file interchange with Lotus 1-2-3 (via a conversion utility).

Besides ALL-IN-1, Digital's Alpha machines come equipped with the following:

Network Architecture

Any DECnet LAN of reasonable size begins with a backbone. Individual connections or tributaries of connections are dropped from the backbone, providing a relatively understandable network topology that is simple to change or grow.

The backbone can be either a baseband (for computer data only) or a broadband (for combined voice, video and data) medium. If a broadband medium is used, a converter extracts the information from the broadband medium and converts it into the more readily used baseband facility. The Ethernet baseband medium conforms to the IEEE 10BASE5 standard.

Connections to the baseband cable are made through transceivers (see Figure 2.8). Transceivers convert between the baseband medium and the ThinWire coaxial medium and/or a 15-wire medium. The most common transceivers are the H4000 series manufactured by Digital.

Sample DECnet LAN

The cabling from the transceiver normally connects directly to an interface card within a system or to another type of network device. The type of interface card used depends on the bus of the host system. Some common interfaces that support connection via transceiver and their matching buses include DELUA for Unibus (PDP/VAX) systems, DELQA for Q-Bus (PDP/VAX) systems, DEBNA for VAXBI (VAX) systems, and DESVA for the MicroVAX 2000.

Another common network device that connects to a transceiver is the Digital Ethernet Local Network Interconnect (DELNI). DELNI connects up to eight devices (via transceiver cables) to form a small LAN. DELNI can also connect to a baseband Ethernet cable (via an H4000 transceiver) to turn the other connected devices into tributaries of the backbone. (In smaller networks, DELNI is used alone or with other DELNIs to form a LAN that has no backbone.)

To connect PCs, IBM PS/2s, or other devices that operate in locations (or in numbers) that make standard transceiver connections difficult, Digital Equipment supports the use of a ThinWire medium. ThinWire is a thin coaxial medium that conforms to the IEEE 10BASE2 standard. ThinWire devices are linked on the coaxial cable via a T-shaped connector for each attached device. Common thin-wire devices include the following:

Terminals normally connect through terminal servers. Terminal servers can connect to the backbone via a transceiver or DELNI and interface to terminals via standard RS-232C asynchronous connections that can operate up to 19.2Kbps. Almost all terminal servers use the LAT protocol for terminals accessing computing nodes.

In addition to providing connectivity for terminals, terminal servers also allow connectivity to serial printers and modems. Two Digital terminal servers are:

Bridges, routers, and gateways can attach to Ethernet via transceivers. These highly specialized devices (normally computers in their own right) are used to implement WANs or to provide interfaces with IBM equipment. Refer to Chapter 7, "LANs and WANs," for more information on these devices.

Also, a Digital Data Communications Message Protocol (DDCMP) link can connect Digital systems together over traditional telephone or serial links. DDCMP is an integral part of DECnet that is often essential in implementing networks that are low cost or non-PSDN (packet-switching public data networks). In a nutshell, DDCMP is a system-to-system communications method.

Finally, because most of the components offered by Digital conform to the IEEE 802.3 standards as well as the Ethernet standards, a functional, mixed-vendor network can be constructed with them. Such a network could use Digital Equipment systems and networking devices but run alternative network software such as TCP/IP. (In fact, because the equipment conforms to both standards, such alternative networking software as TCP/IP and HP's NS can run concurrently with DECnet on the same physical network.)

Digital is the leader in Fiber Distributed Data Interface (FDDI) switching, with its GigaSwitch product. FDDI is a high-speed fiber optic networking technology, often used as a backbone for joining together servers or multiple LANs. GigaSwitch is an FDDI switch that can significantly extend the useful lifetime of a LAN by increasing the amount of effective bandwidth available.

Digital is embracing virtual LAN (VLAN) technology with its enVISN architecture ( for more information on virtual LANS, see the section "VLAN Technology" in Chapter 11, "Network Management"). Under enVISN, policy agents are created to ensure that the physical switch carries out quality of service, security, and membership policies relative to the VLAN architecture. Quality of Service features permit certain types of traffic to receive higher priority than others. For example, a system with quality of service features might grant the highest priority to transaction processing, and the lowest to file transfers.

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