standardized data encoding, data compression, and
data encryption and decryption as required.
APPLICATION LEVEL/LAYER— The final
level of the OSI reference model is the application
level and it consists of the application layer. This
layer directly supports the users and application tasks.
USING THE OSI MODEL— When you use a
layered architecture (such as the OSI reference model)
to design a communications network, it is possible to
update specific items in the network without having to
replace the entire system.
LAN
TOPOLOGIES— The
physical
arrangement of the components of a LAN is called its
topology. The three basic topologies used in building
a LAN are the linear bus, the star, and the ring.
Hybrid topologies can be created by combining
different features of each.
LINEAR BUS— The linear bus topology
connects all the nodes to a common straight cable.
All the nodes on the network share the common bus.
This topology is very reliable since a failure of one or
more nodes does not affect the bus.
The
disadvantages of the linear bus are the need for
minimum distances between nodes to avoid signal
interference, and the loss of data caused by
the simultaneous transmission by two nodes.
STAR NETWORK— In a star network, each
node is connected directly to the central computer.
All communications between the nodes have to pass
through the central computer. Star networks allow the
network administrator to give selected nodes higher
priority and also allow centralized running of
diagnostic programs.
RING NETWORK— In a ring network, all nodes
are connected to a common cable, and the cable starts
and ends at the network server.
In this type of
network, communications are always in one direction,
and the data being transmitted is passed through each
node in the ring.
A major disadvantage of this
network is that when a node fails, it can completely
halt all communications on the network.
6-15
ACCESS METHODS— Once the topology of a
network is determined, the method by which the
nodes access the network must be determined. In
some cases, the access method is determined by the
topology of the network. Some of the access methods
used in networks are as follows:
carrier sense
multiple access (CSMA), carrier sense multiple access
with collision detection (CSMA/CD), and token
passing.
In the CSMA method, each node monitors the
network line for activity. When the node detects that
there is no activity on the network, it will send its
data. A problem occurs when two or more nodes
attempt to use the network at the same time. This
situation causes a collision of the data packets and a
possible loss of data. In the CSMA/CD method, when
a collision is detected, each node ceases transmission
and retransmits when it senses that activity on the
network is completed.
In token passing, a software token is passed to
each node in an orderly manner.
The method is
similar to the Roll Call mode of operation of the Link-
11 system described in chapter 2. When the node
with the token has no data to transmit, it passes the
token to the next unit. When the node has data, it
transmits it when it receives the token, and when it
completes its transmission, passes the token to the
next node.
LAN SYSTEMS— There are several types of
systems that can be installed in a LAN. The topology
used has a major effect on the system the LAN will be
capable of using.
A few of the LAN systems
available are as follows:
EtherNet, STARLAN,
ARCnet, and the IBM Token Ring. EtherNet is
generally a linear bus network using the CSMA/CD
protocol for network access. STARLAN is a star
topology that also uses the CSMA/CD access
protocol. ARCnet is a distributed star network that
uses a token passing access protocol. The IBM Token
Ring network is a star ring network that uses the token
passing access protocol.
NETWORK
OPERATING
SYSTEM
BASICS— The network operating system has five
basic subsystems to control the operation of the
