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<td bgcolor="#336666"><b><font face="Arial" size="2" color="#FFFFFF">6.1</b></font></td>
<td bgcolor="#336666"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
<td bgcolor="#336666" width="100%"><b><font face="Arial" size="2" color="#FFFFFF">
<strong>Workstations and Servers</strong> </font></b></td>
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<td bgcolor="#669999" height="25"><b>
<font face="Arial" size="2" color="#FFFFFF">6.1.2</font></b></td>
<td bgcolor="#669999"><img border="0" src="../../images/transdot.gif" width="10" height="1"></td>
<td bgcolor="#669999" height="25" width="100%"><b>
<font face="Arial" size="2" color="#FFFFFF">Servers</font></b></td>
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In a network operating system environment, many client systems access
and share the resources of one or more servers.
<img border="0" src="../../images/1.gif" align="absmiddle" width="12" height="12"> Desktop client systems
are equipped with their own memory and peripheral devices, such as a
keyboard, monitor, and a disk drive. Server systems must be
equipped to support multiple concurrent users and multiple tasks as
clients make demands on the server for remote resources.
<img border="0" src="../../images/2.gif" align="absmiddle" width="12" height="12"><p>
Network operating systems have additional network management tools and
features that are designed to support access by large numbers of
simultaneous users. On all but the smallest networks, NOSs are
installed on powerful servers. Many users, known as clients, share
these servers. Servers usually have high-capacity, high-speed disk
drives, large amounts of RAM, high-speed NICs, and in some cases,
multiple CPUs. These servers are typically configured to use the
Internet family of protocols, TCP/IP, and offer one or more TCP/IP
services.</p>
<p>Servers running NOSs are also used to authenticate users and
provide access to shared resources. These servers are designed to
handle requests from many clients simultaneously. Before a client can
access the server resources, the client must be identified and be
authorized to use the resource. Identification and authorization is
achieved by assigning each client an account name and password. The
account name and password are then verified by an authentication
service to permit or deny access to the network. By
centralizing user accounts, security, and access control, server-based
networks simplify the work of network administration. </p>
<p>Servers are typically larger systems than workstations and have
additional memory to support multiple tasks that are active or
resident in memory at the same time. Additional disk space is also required on servers to
hold shared files and to function as an extension to the internal
memory on the system. Also, servers typically require extra expansion
slots on their system boards to connect shared devices, such as
printers and multiple network interfaces.</p>
<p>Another feature of systems capable of acting as servers is the
processing power. Ordinarily, computers have a single CPU, which
executes the instructions that make up a given task or process. In
order to work efficiently and deliver fast 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.
<img border="0" src="../../images/3.gif" align="absmiddle" width="12" height="12"></p>
<p>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.
<img border="0" src="../../images/4.gif" align="absmiddle" width="12" height="12"></p>
<p>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).
<img border="0" src="../../images/5.gif" align="absmiddle" width="12" height="12"></p>
<p>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.
<img border="0" src="../../images/6.gif" align="absmiddle" width="12" height="12"></p>
<p>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.</p>
<p>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.</p>
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<p><font color="#808080">
<a href="javascript:void(0)" onClick="window.open('../zoom6_1_2/zoom6_1_2.html','Interactivity','width=740,height=390,toolbar=0,resizable=yes,scrollbars=0,screenX=26,screenY=38,left=26,top=38');">
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Interactive Media Activity</a></b></font></p>
</font><font FACE="Arial" SIZE="2" COLOR="#808080">
<p>PhotoZoom: Server Components</p>
<p>In this
PhotoZoom, the student will view components inside a server.</p>
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<p><IMG alt="" height="2" width="1" src="../../images/s.gif"></p></TD>
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