Most computer processing units (CPUs) are able to
execute instructions and basic decision-making steps at
a rate of several million instructions per second. Data
can be transferred between the computer’s memory and
the cpu at these same rates of speed. The ideal network
could keep up with the high speed of the cpu and be able
to transfer data between the stations of the network at
rates close to the rates that data is moved around within
the cpu and memory. However, this is just not possible
with a telephone line linked system, which is limited in
the range of frequencies it can carry. When high-
frequency signals are carried by wire such as twisted-
pair, all sorts of electrical effects come into play. It’s not
sufficient to simply link computer systems with
common wire. Considerable thought must be given to
the electrical characteristics of the connection. The
cable selection must be made during the design phase of
the network to ensure that the decision is not left to be
made during the installation of the network.
NETWORK OPERATING
SYSTEM
A network operating system (NOS) is a software
package that makes it possible to implement and control
a network and enables users to use the resources and
services on that network. A NOS’s tasks include:
l
l
l
l
l
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Providing access to files and resources;
Providing electronic mail (e-mail) services;
Enabling nodes on the network to communicate
with each other;
Enabling processes on the network to
communicate with each other;
Responding to requests from applications and
users on the network; and
Mapping requests and paths to the appropriate
places on the network.
A NOS may be server-based or peer-based. Server
based NOSs are considerably more complex and
powerful than NOSs for peer-to-peer networks. In a
server-based network, the NOS and the server run the
show, and the workstations will generally run a network
shell. By contrast, in a peer-to-peer network any station
can function as file server or as a client for network
services.
Operating systems which have built-in networking
capabilities include the following:
l UNIX®
. Windows NT®
l Novell® DOS 7
In most of these cases, the operating system’s
networking capabilities can be greatly enhanced
through the use of utilities or other third-party
programs. To learn more about these utilities or
programs, check the manuals that come with the
operating system.
FIREWALLS
Firewalls can be used for securing a local area
network from a public network like the Internet.
Firewalls are always a part of a much larger security
plan. Choosing a firewall starts with a clear definition
of the security goals. This includes decisions on what
logging and alarms are needed, what authentication is
acceptable and where security barriers are needed.
Once the policy, philosophy, and service goals are
defined, often only a few products on the market really
fit these needs.
There are several types of firewalls that can be
divided into packet filtering and application layer
firewalls.
Packet Filters
Packet filters operate at a lower level than
application layer firewalls. Packet filters decide
whether to forward an IP packet based on the source or
destination address found at the network layer. Routers
typically implement this type of filtering, but since
packets containing bogus IP addresses can easily be
created, it’s not too hard to gain access through even the
most elaborate set of IP address filters. Although the
router on an Internet link can filter packets, it probably
wasn’t designed to provide the level of control that a
firewall product can. A router examines one packet at a
time and forwards the packet.
Application Layer Firewall
Application layer firewalls, on the other hand, are
designed specifically to control unwarranted access to
your network. They can also deal with some of the
trickier protocols. Application layer firewalls gain
more insight into the data conversations that traverse an
Internet link because they examine packets and
protocols at and above the transport layer, which
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