2. Network Topology
Each computer or device in a computer is called a node. The way in which the connections are made is called the topology of the network. Network topology specifically refers to the physical layout of the network, especially the locations of the computers and how the cable connection is made between them. Thus geometric arrangement of computer resources is known as network topology. It is important to select the right topology for how the network will be used. A network can be made in one of the five different topologies:
1. Bus Network
2. Ring Network
3. Star Network
4. Star Bus and Star Ring Network
5. Mesh Network
2.1
Bus Network
In a Bus Network each computer is connected to a single communication cable via an interface, and every computer can directly communicate with every other computer or device in the networks as shown in figure. When one computer sends a signal up and down) the wire, all the computers on the network receive the information, but only (the one with the address that matches the one encoded in the message) accepts the information. The rest discard the message. This topology is commonly used along with the local area networks.
Only one computer at a time can send a message, therefore the number of computers attached to a bus network can significantly affect the speed of the network. A computer must wait until the bus is free before it can transmit. Another important issue in bus networks is termination. Since the bus is a passive topology, the electrical signal from transmitting computer is free to travel the entire length of the cable. Without termination, when the signal reaches the end of the wire, it bounces back and travels back up the wire. When a signal echoes back and forth along unterminated bus, it is called ringing. To stop the signal from ringing, you attach terminator at either end of the segment. The terminator absorbs the electrical energy and stops the reflections. Cables cannot be left unterminated in the bus network.
Advantages
of the Bus Network:
1. The bus is simple, reliable in very small networks, easy to use, and easy to understand
2. The bus requires least amount of cable to connect the computers together and is therefore less expensive than other cabling arrangements.
3. It is easy to extend a bus. Two cables can be joined into one longer cable with a BNC barrel connector, making a longer cable and allowing more computers to join the network.
4. A repeater can also be used to expend a bus, a repeater boosts the signal and allow it to travel a longer distance.
Disadvantages
of the Bus Network:
2. Each barrel connector weakens the electrical signal, and too many may prevent signal from being correctly received all along the bus.
3. It is difficult to troubleshoot a bus. A cable break or malfunctioning computer anywhere between two computers can cause them not to be able to communicate with each other. A cable or loose connector will also cause reflections and bring do the whole network, causing all network activities to stop.
2.2
Star Topology
In a star topology, all cables run from the computers to central location, where they are connected by device called hub. Stars are used in concentrated networks when the end points are directly reachable from the central location, when network expansion is expected,when the greater reliability of a star topology is needed.
Each computer on a start network communicates with a central hub that resends the message either to all the computers (in a broadcast star network) or only to the destination computer (in a switched star network).
The hub in a broadcast star network can be active or passive. An active hub regenerates the electrical signal and sends it to all the computers connecter it. This type is often called a multiport repeater. Active hubs and switches require electrical power to run. A passive hub, such as wiring panels or punch-down blocks, acts merely connection point and does not amplify or regenerates the signal. Passive hubs do not require electrical power to run.
You can expand a star network by placing another star hub where a computer might others go, allowing several more computers or hubs to be connected to that hub. This creates a start network.
Advantages
of the Star Topology:
Following are the several advantages to a star topology:
2. The center of a star network is a good place to diagnose network faults. Intelligent hubs (Hubs with microprocessors that implement features in addition to repeating network signals) also provide for centralized monitoring and management of the network.
3. A single computer failure does not necessarily bring down the whole star network. The hub can detect a network fault and isolate the offending computer or network cable and allow the rest of the network to continue operating.
4. You can use several cable types in same network with a hub that can accommodate multiple cable types.
Disadvantages
of the Star:
Following are the several disadvantages to a star topology:
1. If the central hub fails, the whole network fails to operate.
2. Many star networks require a device at central point to rebroadcast or switch network traffic.
3. It costs more to a cable a star network because all network cables must be pulled to one central point, requiring more cable than other networking topologies.
2.3
Ring Networks
In the ring topology, each computer is connected to the next computer, with the last of connected to the first. Rings are used in high-performance networks requiring that bandwidth be reserved for time sensitive features such as video and audio or when even performance needed when a large number of client access the network.
Every computer is connected to the next
computer in the ring, and each retransmits why receives from the previous
computer. The messages flows around the ring in one direction .Since each
computer retransmits what it receives, a ring is an active network and is not susceptible
to the signal loss problems a bus experiences.
There is no termination because there is no end point to the ring.
Access Method: Token Passing
Some ring networks do token passing. A short
message called a token is passed around ring until a computer wishes to send
information to another computer. That computer modifies the token, adds an
electronic address and data, and sends it around the ring. Each computer in the
sequence receives the token and information and passes them to the next computer
until either the electronic address matches the address of computer or the
token returns origin. The receiving computer returns a message to the
originator indicating that cr another token and places it on the network,
allowing another station to capture the token begin transmitting. The token
circulates until a station is ready to send and capture
token. This all happens very quickly, a token
can circle a ring 200 meters in diameter ata 10000 times a second. Some even
faster networks circulate several-tokens at once. Other networks have two
counter rotating tokens that help them recover from network faults.
1. Because every computer is given equal
access to the token, no one computer can monopolize the network.
2. The fair sharing of the network allows the
network to degrade gracefully (continuous function) in a useful, if slower,
manner rather then fail once capacity is exce's more users are added.
Disadvantages of Ring:
1. Failures of one computer on the ring can
affect the whole network.
2. It is difficult to troubleshoot a ring
network.
3. Adding or removing computers disrupts the network.
2.4 Star Bus and Star Ring
2.4.1 Star Bus
The star bus topology combines the bus and the star, linking several star hubs together withbus trunks. If one computer fails, the hub can detect the fault and isolate the computer. If a hub fails, computers connected to it will not be able to communicate, and the bus network will bebroken into segments that cannot reach each other.
2.4.2 Star Ring
In the star ring,
also called the star wired ring, the network cables are laid out much like a
star network, but a ring is implemented in the central hub. Outlying hubs can
be connected to inner hub, effectively extending a loop of the inner ring.
The mess topology
is distinguished by having redundant links between devices. A true me configuration
has a link between each device in the network. As you can imagine, this ex unmanageable
beyond a very small number of devices. Most mesh topology networks are true
mesh networks. Rather, they are hybrid mesh network, which contain some redundant
links, but not all.
Mesh topology
networks become more difficult to install as the number of devices increases because
of the sheer quantity of connections that must be made. A true mesh oſ only devices
would require 15 connections (5+4+3+2+1). A true mesh topology of seven device would
require 21 connections (6+5+4+3+2+1), and so on.
The major
advantage of the mess topology is fault tolerance. Other advantages in guaranteed
communication and full channel capacity and the fact that mesh network are relatively easy to troubleshoot.
Disadvantages
include the difficulty of installation and reconfiguration as well as the cost
of maintaining redundant links.
3. Network Architecture
An architecture in which the client (personal
computer or workstation) is the requesting machine and the server is the
supplying machine, both of which are connected via a Local Area Network (LAN)
or Wide Area Network (WAN). Since the early 1990s, client/server has been the
buzzword for building applications on LANs in contrast to centralized minis and
mainframes with dedicated terminals. A The client contains the user interface
and may perform some or all of the application processing. Servers can be
high-speed microcomputers, minicomputers or even mainframes. A database server
maintains the databases and processes requests from the client to extract data
from or update the database. An application server provides additional business
processing for the clients.
The term client/server is sometimes used to
contrast a peer-to-peer network, in which any client can also act as a server.
In that case, client/server means nothing more than having a dedicated server.The
Network Operating System (NOS) together with the Database Management System (DBMS)
and Transaction Monitor (TP monitor) are responsible for integrity and
security.
Some of these
products have gone through many client/server versions by now and have finally
reached industrial strength.
3.2 Non-Client/Server
In non-client/server architecture, the server
is nothing more than a remote disk drive. The user's machine does all the
processing. If many users routinely perform lengthy searches, this can bog down
the network, because each client has to pass the entire database over the net.
At 1,000 bytes per record, a 10,000 record database requires that 10MB of data
be transmitted.
Two-tier client/server is really the
foundation of client/server. The database processing is done in the server. An
SQL request is generated in the client and transmitted to the server. The DBMS
searches locally and returns only matching records. If 50 records met the
criteria, only 50K would be transmitted. This reduces the traffic in the LAN.'
Many applications lend themselves to
centralized processing. If they contain proprietary algorithms, security is
improved. Upgrading is also simpler. Sometimes, programs are just too demanding
to be placed into every client PC. In three-tier client/server, application
processing is performed in one or more servers.
A type of network in which each workstation
has equivalent capabilities and responsibilities. This differs from
client/server architectures, in which some computers are dedicated to serving the
others. Peer-to-peer networks are generally simpler and less expensive, but
they usually do not offer the same performance under heavy loads.