Information Technology Reference
In-Depth Information
promise over the other two topologies as it allows relatively high data rates. Also, if a node
goes down, it does not affect the rest of the network. The main disadvantage of this topology
is that it requires a network protocol to detect when two nodes are transmitting at the same
time. It also does not cope well with heavy traffic rates. A typical bus network is Ethernet
2.0.
All computers have access to
a common bus at the same time
Common bus
Figure 25.6
Bus topology
25.3 OSI model
A major problem in the electronics industry is the interconnection of equipment and soft-
ware compatibility. Other problems can occur in the connection of electronic equipment in
one part of the world to another, in another part. For these reasons, the International Stan-
dards Organization (ISO) developed a model known as the OSI (open systems interconnec-
tion) model. Its main objects were to:
•
Allow manufacturers of different systems to interconnect their equipment through stan-
dard interfaces.
•
Allow software and hardware to integrate well and be portable on differing systems.
•
Create a model which all the countries of the world use.
Figure 25.7
shows the OSI model. Data passes from the top layer of the sender to the bottom
and then up from the bottom layer to the top on the recipient. Each layer on the sender,
though, communicates directly the recipient's corresponding layer. This creates a virtual data
flow between layers.
The top layer (the application layer) initially gets data from an application and appends it
with data that the recipients application layer reads. This appended data passes to the next
layer (the presentation layer). Again, it appends it with its own data, and so on, down to the
physical layer. The physical layer is then responsible for transmitting the data to the recipi-
ent. The data sent can be termed as a data frame, whereas data sent by the network or trans-
port layer is typically referred to as a data packet.
The basic function of each of the layers are:
1.
Physical
. Defines the electrical characteristics of the communications channel and the
transmitted signals, such as voltage levels, connector types, cabling, and so on.
