The Unix Workstation's Components



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The Unix Workstation's Components

It's hard to get the best use out of a tool (such as an automobile) without understanding how it works. With that observation in mind, we attempt here to define the various pieces of a workstation as well as give some of their general speed and size requirements. The workstation is a computer small enough in size and cost to be used by a small group or an individual in a work location yet powerful enough for large-scale scientific and engineering applications. Typically the workstation employs a Unix operating system and has good graphics capability.     

Although the specific speeds, sizes, and components of various workstations are constantly changing, the basic ideas should remain the same. Listing the minimum configuration needed to run Unix well, however, is not a clear-cut matter, since it depends on what the system will be used for and what performance will be expected from it (analogous to the engine requirements of an automobile). Users with ``hot'' systems generally cannot imagine enduring the pain of living with anything less, yet acquiring even a minimal configuration is a great advance for those with inadequate systems. In an additional analogy to the automobile, how fast a CPU or how large a block of memory you purchase will often be determined more by your budget and ego than by the requirements of the software. Even if you try to be Spartan, the new components introduced every year as optional luxury items often become required in the future. So you may well end up buying them but at a time when they have become less expensive.

CPU
The central processing unit (CPU) combined with the floating point unit (FPU) is the brains of the workstation. The CPU's speed is determined by its design and, somewhat independently, by its clock speed, that is, the time it takes the computer to execute the simplest instruction. The relative speeds of two machine with the same CPU is the ratio of their clock speeds.    

The collection of programs known as Unix has grown a lot in power and extent in recent years. This growth, combined with the wide use of CPU-intensive graphics displays and their associated use of the X Window System, means that a Unix workstation requires a fast processor.gif It is easy to benchmark a particular numerical program to see exactly how long it takes a certain machine to turn out the right answer. It is harder, however, to determine ahead of time what the response time of your computer will be in an actual working situation when there are a number of users, a number of programs running in the background, and so forth. Since user productivity is significantly affected by this response time,this is an important consideration.gif      

Let's say your usage includes a good number of X applications, X terminals, graphics packages, symbolic manipulation users, and CAD/CAM users. You may find the response time of the system, and thus its usability and everyone's productivity, increase greatly as you move to a faster machine or a network of machines.

RAM
The random access memory (RAM) is the computer's electronic storage. It is so named because each bit of it can be accessed directly, that is, ``randomly,'' in contrast to the more sequential access of tapes and disks. Typically a commercial Unix system requires 4-8 megabytes (MB) of memory to simply run the base system. It is relatively easy to determine the memory requirement of any major programs which your workstation will run. Just read it from the compiler map. Apart from large numerical codes, X Windows tends to be the largest consumer of memory. Consequently, unless you really prefer a lean, and slow, lifestyle, consider 16 MB the minimum RAM for a Unix workstation running X Windows. If there will be heavy graphics use or intensive use by several people, the memory requirements increase. A single-user workstation with 32 MB of RAM is not uncommon. 

Hard Disk Memory
It seems that no matter how large or fast a hard disk you purchase, after a few months of use it appears to have become too small and too slow. Unix keeps a record of what's on the the hard disk(s) by first dividing the hard disks into logical units called file systems and then treating each file system as an independent unit. Therefore, several file systems may reside on a single, physical hard disk. Conversely, with a feature called volume groups under AIX and OSF/1, several physical disks may be grouped together to make a file system larger than any single disk. All Unix implementations are limited to 2 gigabytes (GB) maximum for any file or file system.gif As CPU speeds and the scope of computations have increased, this limit has become a hardship for some users and may well be changed in the future.       

Hard disks are rated by size and average access speed. The size is usually stated in megabytes for an unformatted disk. You can expect a 15- to 20% decrease after formatting. The smallest (PC) Unix systems require a minimum of 100-150 MB, while a workstation Unix may well require 300 MB minimum. While a 600 MB hard disk is adequate for scientific work, it is not uncommon to see systems with more than 1GB of disk space.

The access speed for a hard disk is the average time it takes the disk to move its ``heads'' to a place where it can read or write your data. Currently slow drives, such as found on laptops, have access times of 50 milliseconds or more, while fast workstation drives have access times of 15 milliseconds or less.

Console
Terminals can be simple serial ones capable of displaying text only or graphical ones capable of displaying both text and pictures at high speeds and in colors.gif. The console is the communications center of the system while booting, the bulletin board for error and informational memos during operation, and should be the place where the system administrator works during system maintenance. From a systems point of view, workstations don't require a graphics display but do require a ``console'' as a device to handle text information. Depending upon the workstation, this could be a simple serial terminal or a small monochrome display.     

Terminals
Terminals previously denoted text-only displays connected to a computer by a serial (RS232) line. Now many ASCII terminals have been replaced by PCs running terminal emulation software and, increasingly, by X terminals. 

System Bus
A bus is a communication channel (bunch of wires) used for transmitting information quickly among parts of a computer. The system bus is the data channel between the CPU and the adapters. The adapters are boards containing the electronics for activities such as networking and graphics. Unless you have some special need for particular high speed I/O adapters, the size and speed of the system bus should not be a concern of yours.      

SCSI Bus
The SCSI or ``scuzzy'' bus has become the most popular standard for attaching hard disks and tape drives to workstations. The SCSI standards require each device on the bus to have its own intelligent controller so that the device and workstation can talk to each other in a high level language. This makes it possible to just plug in a new hard disk from a third-party vendor (not the computer manufacturer), reboot the system in the installation mode, and have it all work. To make sure things aren't so easy as to become boring, there now are several SCSI standards. The orignal SCSI, now called SCSI I, has a slow maximum data transmission rate of 2-5 MB/s. The new standard SCSI II corrects some deficiencies in the original protocol and boosts the bus speed up to 10 MB/s. Other standards, such as ``Fast SCSI,'' have speeds greater than 10 MB/s. 

Another attraction of SCSI is that it allows users to add hard disks or tape drives which are external to their workstations. This means the user can just plug a cable into a port on the backside without having to handle the ``guts'' of the computer. The external device resides in its own case with its own power supply and can be repaired or replaced with minimal disturbance to the system (and since drives are mechanical, they will break down regularly). While this may suggest to you that you need a computer with many ports on the back, that is not necessarily true since additional SCSI devices can be connected to the extra port usually located on a SCSI device. Having a number of such devices connected to each other with cable loops is called a daisy chain. At present, seven devices are the maximum number which can be controlled by a single SCSI bus.   

Network Adapter
A network connects one computer with another or, more commonly now, one local network of computers with many other networks. For the vast majority of workstation users, ``network'' means ethernet, a high speed local area network (LAN) composed of specific cable technology and communications protocols (TCP/IP). The worldwide interconnection of ethernets is called Internet. Ethernets are serial networks which run at 10 MHz and so can transmit approximately 10 MBs of data per second (that is, their bandwidth is approximately ten floppies per second). IBM's token ring networks run at 4 or 16 MHz, while the common fiber optics network FDDI runs at 100 MHz.             

Although ethernets are fast compared to PC networks like Appletalk, they can transmit only one message at a time, a so-called packet. The packet contains the data to be transmitted as well as the address of the destination and a return address. As the number of machines on a network increases, the fact that they all must share this 1 MB bandwidth means that the effective bandwidth for any one computer is decreased since it may have to wait while another machine's packet gets delivered. These packet collisions do not become much of a performance problem until there are many machines on the same network or until the network is used heavily for activities such as Network File System (NFS). If the traffic does become a problem, it may be a good idea to break the network up into subnets joined by routers or bridges   . 

The physical characteristic of your workstation's ethernet is either ``thin ethernet'' or ``thick ethernet.'' Both carry the same electrical signals, yet do so on different size coaxial cables and with different connectors. The thin ethernet cables are lightweight, 75-ohm coaxial cables that get looped from one machine to the next. With thick ethernet, a 15-ohm cable connects the computer to a transceiver box which is securely fastened to a thick coaxial cable. The advantage of thin ethernet is lower cost and ease of cable hookup. The disadvantage is that if any one connection to a machine is broken, the entire network is down. Further, thin ethernet supports fewer connections and can be extended a shorter distance (185 m and 30 nodes for thin ethernet and 500 m and 100 nodes for thick ethernet). Commonly, a local group's ethernet may be thin while a department's ethernet may be thick. 

Tape Drives
Magnetic tapes provide mass storage. Along with floppy disks, they are used to back up programs and data, to exchange programs and data, and to receive updates of presently running software. It is always just a matter of time before any hard disk fails (``crashes''), so the need for backup should be attended to. Magnetic storage devices have also increased in size and speed. The standard for many years had been the 1/4-inch cartridge tape capable of holding some 60-150 MB of data. These are being replaced by 8-mm tape drives using inexpensive standard video tapes and capable of holding over 2 GB of data. There are also drives employing data compression to double the effective storage on a tape. By eliminating unneeded repetitions or space, compression is a good way to get something for nothing. But it does not always give the expected increase, since binary files do not compress as much as text files and since already compressed data files do not compress at all. 



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