responses to client requests, a NOS server
requires a powerful CPU to execute its tasks or programs.
Single processor systems with one CPU can meet the needs of
most servers if the CPU has the necessary speed. To achieve
higher execution speeds, some systems are equipped with more
than one processor. Such systems are called multiprocessor
systems. Multiprocessor systems are capable of executing
multiple tasks in parallel by assigning each task to a
different processor. The aggregate amount of work that the
server can perform in a given time is greatly enhanced in
multiprocessor systems. Since servers function as central
repositories of resources that are vital to the operation of
client systems, these servers must be efficient and robust. The
term robust indicates that the server systems are able to
function effectively under heavy loads. It also means the
systems are able to survive the failure of one or more
processes or components without experiencing a general system
failure. This objective is met by building redundancy into
server systems. Redundancy is the inclusion of additional
hardware components that can take over if other components
fail. Redundancy is a feature of fault tolerant systems that
are designed to survive failures and can be repaired without
interruption while the systems are up and running. Because a
NOS depends on the continuous operation of its server, the
extra hardware components justify the additional expense.
Server applications and functions include web services using
Hypertext Transfer Protocol (HTTP), File Transfer Protocol
(FTP), and Domain Name System (DNS). Standard e-mail protocols
supported by network servers include Simple Mail Transfer
Protocol (SMTP), Post Office Protocol 3 (POP3), and Internet
Messaging Access Protocol (IMAP). File sharing protocols
include Sun Microsystems Network File System (NFS) and
Microsoft Server Message Block (SMB). Network servers
frequently provide print services. A server may also provide
Dynamic Host Configuration Protocol (DHCP), which automatically
allocates IP addresses to client workstations. In addition to
running services for the clients on the network, servers can be
set to act as a basic firewall for the network. This is
accomplished using proxy or Network Address Translation (NAT),
both of which hide internal private network addresses from the
Internet. One server running a NOS may work well when serving
only a handful of clients. But most organizations must deploy
several servers in order to achieve acceptable performance. A
typical design separates services so one server is responsible
for e-mail, another server is responsible for file sharing, and
another is responsible for FTP. The concentration of network
resources, such as files, printers, and applications on
servers, also makes the data generated easier to back up and
maintain. Rather than have these resources distributed on
individual machines, network resources can be located on
specialized, dedicated servers for easy access and back up.
Interactive Media Activity PhotoZoom: Server
Components In this PhotoZoom, the student will view components
inside a server.
Content 6.1 Workstations and
Servers 6.1.3 Client-server relationship The
client-server computing model distributes processing over
multiple computers. Distributed processing enables access to
remote systems for the purpose of sharing information and
network resources. In a client-server environment, the client
and server share or distribute processing responsibilities.
Most network operating systems are designed around the
client-server model to provide network services to users. A
computer on a network can be referred to as a host,
workstation, client, or server. A computer running TCP/IP,
whether it is a workstation or a server, is considered a host
computer. Definitions of other commonly used terms are:
- Local host – The machine on which the user
currently is working.
- Remote host – A system
that is being accessed by a user from another system.
- Server – Provides resources to one or more clients
by means of a network.
- Client – A machine that
uses the services from one or more servers on a network.
An example of a client-server relationship is a File
Transfer Protocol (FTP) session. FTP is a universal method of
transferring a file from one computer to another. For the
client to transfer a file to or from the server, the server
must be running the FTP daemon or service. In this case, the
client requests the file to be transferred. The server provides
the services necessary to receive or send the file. The
Internet is also a good example of a distributed processing
client-server computing relationship. The client or front end
typically handles user presentation functions, such as screen
formatting, input forms, and data editing. This is done with a
browser, such as Netscape or Internet Explorer. Web browsers
send requests to web servers. When the browser requests data
from the server, the server responds, and the browser program
receives a reply from the web server. The browser then displays
the HTTP data that was received. The server or back end handles
the client's requests for Web pages and provides HTTP or WWW
services. Another example of a client-server relationship is a
database server and a data entry or query client in a LAN. The
client or front end might be running an application written in
the C or Java language, and the server or back end could be
running Oracle or other database management software. In this
case, the client would handle formatting and presentation tasks
for the user. The server would provide database storage and
data retrieval services for the user. In a typical file server
environment, the client might have to retrieve large portions
of the database files to process the files locally. This
retrieval of the database files can cause excess network
traffic. With the client-server model, the client presents a
request to the server, and the server database engine might
process 100,000 records and pass only a few back to the client
to satisfy the request. Servers are typically much more
powerful than client computers and are better suited to
processing large amounts of data. With client-server computing,
the large database is stored, and the processing takes place on
the server. The client has to deal only with creating the
query. A relatively small amount of data or results might be
passed across the network. This satisfies the client query and
results in less usage of network bandwidth. The graphic shows
an example of client-server computing. Note that the
workstation and server normally would be connected to the LAN
by a hub or switch. The distribution of functions in
client-server networks brings substantial advantages, but also
incurs some costs. Although the aggregation of resources on
server systems brings greater security, simpler access, and
coordinated control, the server introduces a single point of
failure into the network. Without an operational server, the
network cannot function at all. Additionally, servers require
trained, expert staff to administer and maintain them, which
increases the expense of running the network. Server systems
require additional hardware and specialized software that adds
substantially to the cost.
Content 6.1
Workstations and Servers 6.1.4 Introduction
to NOS A computer operating system (OS) is the software
foundation on which computer applications and services run on a
workstation. Similarly, a network operating system (NOS)
enables communication between multiple devices and the sharing
of resources across a network. A NOS operates on UNIX,
Microsoft Windows NT, or Windows 2000 network servers. Common
functions of an OS on a workstation include controlling the
computer hardware, executing programs and providing a user
interface. The OS performs these functions for a single user.
Multiple users can share the machine but they cannot log on at
the same time. In contrast, a NOS distributes functions over a
number of networked computers. A NOS depends on the services of