Network topology
1. The term topology refers to the way a
network is laid out, either physically or logically. Two or more devices
connect to a link; two or more links form a topology. The topology of a network
is the geometric representation of the relationship of all the links and
linking devices (usually called nodes) to each other.
2. In the following discussion of network
topologies, two important terms are:
a. Node. A node is
a device that is connected to the network. For our purposes here, a node is the same as a computer.
Network topology deals with how the nodes of a network
are connected to each other.
. b. Packet. A packet is
a message that is sent over the network from one node to another node. The packet includes the
address of the node that sent the packet, the address of the node the packet is being sent to, and
data.
Types of Topology
3. Star Topology. In a
star topology, each device has a dedicated point-to-point link only to a
central controller, usually called a hub. The devices are not directly linked
to each other. A star topology does not allow direct traffic between devices.
The hub acts as an exchange. If one device wants to send data to another, it
sends the data to the controller, which then relays the data to the other
connected device.
4. Mesh Topology. All devices have a dedicated point-to-point link to every
other device. The term dedicated means that the link carries traffic only
between the two devices it connects. A mesh guarantees that each connection can
carry its own data load, thus eliminating the traffic problems that can occur
when links must be shared by multiple devices. All of the data that is
transmitted between nodes in the network takes the shortest path. In case of a
failure or break in one of the links, the data takes an alternative path to the
destination.
5. Bus Topology. Each node is connected to a single cable. Each
computer or server is connected to the single bus cable. A signal packet from
the source travels in both directions to all machines connected on the bus
cable until it finds the intended recipient. If the machine address does not
match the intended address for the data, the machine ignores the packet.
Alternatively, if the data matches the machine address, the data is accepted.
Since the bus topology consists of only one wire, it is rather inexpensive to
implement when compared to other topologies.
6. Ring Topology. A network topology that is set up in a circular fashion in
which data travels around the ring in one direction and each device acts as a
repeater to keep the signal strong as it travels. A packet is passed along the
ring in one direction, from device to device, until it reaches its destination.
Each device incorporates a receiver for the incoming signal and a transmitter
to send the data on to the next device in the ring.
7. Tree Topology. Tree topologies integrate
multiple star topologies together onto a bus. In its simplest form, only hub
devices connect directly to the tree bus and each hub functions as the
"root" of a tree of devices.
OSI Reference Model
8. Over the past couple of decades many of
the networks that were built used different hardware and software
implementations, as a result they were incompatible and it became difficult for
networks using different specifications to communicate with each other. To
address the problem the International Organization for Standardization (ISO)
researched various network schemes and recognised there was a need to create a Network
Model that would help vendors create interoperable network implementations. As
an output the Reference Model for network is created. The model is widely known
as OSI (Open System Interconnection) reference model.
9. The Open Systems Interconnection (OSI)
reference model is a descriptive network scheme. It ensures greater
compatibility and interoperability between various types of network
technologies. The OSI Reference Model is composed of seven layers, each specifying
particular network functions. The process of breaking up the functions or tasks
of networking into layers reduces complexity. Each layer provides a service to
the layer above it in the protocol specification. Each layer communicates with
the same layer’s software or hardware on other computers.
a. Application
Layer. The application layer provides a means for
the user to access
information on the network through
an application. This layer is the main interface for users to interact with the application and therefore
the network.
b. Presentation
layer. Ensures that the information that the
application layer of sender has
forwarded is readable by the application layer of another system. It also
ensures encryption and compression
of data in different forms (e.g. JPEG,
MPEG, HTML etc)
c. Session
layer. The session layer controls the
connections (sessions) between computers.
It establishes, manages and terminates the connections between the local and remote application.
d. Transport
layer. The transport layer provides transparent
transfer of data between end users, thus relieving the upper
layers from transfer concerns while providing reliable data transfer. The transport layer controls the
reliability of a given link through flow control, segmentation/ desegmentation, and error control.
e. Application
Layer. The network layer provides the means of
transferring data sequences from a source
to a destination by using one or more networks while maintaining the quality of service requested by the
Transport layer. The Network layer performs network routing functions, and might also perform
segmentation/de-segmentation and report delivery errors.
f. Presentation
Layer. The data link layer provides the
means to transfer data between network entities and to detect and
possibly correct errors that may occur in the physical layer. It arranges bits from the physical layer into logical chunks
of data, known as frames.
g. Physical
Layer. The physical layer defines all the
electrical and physical specifications
for devices. This includes the layout of pins, voltages, and cable
specifications.
10. Working Principle of OSI
reference Model. The basic idea of the OSI reference
model is
a. each
layer is in charge of some kind of processing and each layer only talks to the
layers immediately below and
above it. For example, the sixth layer will only talk to the seventh and fifth layers, and never directly with the
first layer.
b. When
your computer is transmitting data to the network, one given layer will receive
data from the layer above,
process what it is receiving, add some control information to the data that this particular layer is in charge of, and
sending the new data with this new control information added to the layer below.
c. When
your computer is receiving data, the contrary process will occur. One given
layer will receive data from the layer
below, process what it is receiving, removing control information from the data that this particular layer
is in charge of, and sending the new data without the control information to the layer above.
d. What
is important to keep in mind is that each layer will add (when your computer is
sending data) or remove (when your
computer is receiving data) control information that it is in charge of.
TCP/IP model
11. The TCP/IP model (Transmission Control
Protocol/Internet Protocol) is a descriptive framework for the Internet
Protocol Suite of computer network protocols created in the 1970s by United
States Department of Defense. The TCP/IP Model is sometimes called the Internet
Model
12. The TCP/IP model describes a set of
general design guidelines and implementations of specific networking protocols
to enable computers to communicate over a network. TCP/IP provides end-to-end
connectivity specifying how data should be formatted, addressed, transmitted,
routed and received at the destination. The layers of a TCP/IP model are as follows:
a. Application
Layer
b. Transport
layer
c. Internet
Layer
d. Network Access Layer
d. Network Access Layer
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