NAND Gate as NOT Gate
If both inputs of a NAND Gate are combined, we get single output device
resulting in NOT Gate.
Truth table
A
|
Y
|
0
|
1
|
1
|
0
|
NOR
If we put an NOT Gate
at the output of OR Gate we obtain a
very useful combination called NOR
Gate. A NOR Gate has two or more
inputs but only one output. The NOR
function is the complement of OR
function.
It produces a value of true if and only if both operands are false. In other words, it produces a value of false if and only if at least one operand is true.
| A |
B |
Y |
| T |
T |
F |
| T |
F |
F |
| F |
T |
F |
| F |
F |
T |
If A and B are two
input then output is given by
Truth table for NOR Gate
INPUT
|
OUTPUT
|
|
A
|
B
|
Y = A
NAND B
|
1
|
1
|
0
|
1
|
0
|
0
|
0
|
1
|
0
|
0
|
0
|
1
|
SYMBOL
NAND and NOR Gate are called Universal Gate because we may obtain
all the possible Gates by using these individual Gates as basic building.
NAND and OR Gate may be used to
realize the basic logic functions OR,
AND, and NOT.
NAND
When we put NOT Gate in the output of an AND
Gate, we get NAND gate. A NAND gate has two or more inputs but
only one output. The NAND function is the complement of AND functions.
It produces a value of false if and only if both of its operands are true. In other words, it produces a
value of true if and only if
at least one of its operands is false.
- If A and B are two input then output is given by
| A |
B |
Y |
| F |
F |
T |
| F |
T |
T |
| T |
F |
T |
| T |
T |
F |
Truth
table for NAND Gate
INPUT
|
OUTPUT
|
|
A
|
B
|
Y = A
NAND B
|
0
|
0
|
1
|
0
|
1
|
1
|
1
|
0
|
1
|
1
|
1
|
0
|
SYMBOL
MESH TOPOLOGY
In a Mesh Topology, every node has a dedicated
point-to-point link to every other node. Messages sent on a mesh network can
take any of several possible paths from source to destination. A fully
connected mesh network has n (n-1)/2 physical links to link n devices.
Advantages of Mesh Topology
§ The use of large number of links eliminates network
congestion.
§ If one link becomes unusable, it does not disable the
entire system.
Disadvantages of Mesh Topology
§ The amount of required cabling is very large.
§ As every node is connected to the other, installation
and reconfiguration is very difficult.
§ The amount of hardware required in this type of
Topology can make it expensive to implement.
TREE TOPOLOGY
A Tree Topology combines
characteristics of linear bus and star topologies. It consists of groups of
star-configured workstations connected to a bus backbone cable.
Advantages
of Tree Topology
§ The distance to which a signal can travel increases as
the signal passes through a chain of hubs.
§ Tree Topology allows isolating and prioritising
communications from different nodes.
§ Tree Topology allows for easy expansion of an existing
network, which enable organizations to configure a network to meet their needs.
Disadvantages of Tree Topology
§ If the backbone line breaks, the entire segment goes
down.
§ It is more difficult to configure and wire than other
Topologies.
STAR TOPOLOGY
In Star Topology devices are
connected via a centralised network component known as hub or concentrator. The
hub acts as a central controller and if a node wants to send data to another
node, it boosts up the message to the intended node. This topologies commonly
uses twisted pair cable, however, coaxial cable or fiber optic cable can also
be used.
Advantages of Star Topology
§ Star Topology is easy to install and wire.
§ The network is not disrupted even if a node fails or
is removed from the network.
§ Fault detection and removal of faulty parts is easier
in Star Topology.
Disadvantages of Star Topology
§ It requires a longer length of cable.
§ If the hub fails, nodes attached to it are disabled.
§ The cost of the hub makes the network expensive as
compared to bus and Ring Topology.
RING TOPOLOGY
In Ring Topology, computers
are placed on a circle of cable. Every node has exactly two neighbors for
communication purposes.
All messages
travel through a ring in the same direction (clockwise or counterclockwise)
until it reaches its destination. Each node in the ring incorporates a
repeater. When a node receives a signal intended for another device, its
repeater regenerates the bits and passes them along the wire.
Advantages of Ring Topology
§ Ring Topology is easy to install and reconfigure.
§ Every computer is given equal access to the ring.
Hence, no single computer can monopolies the network.
Disadvantages of Ring Topology
§ Failure in any cable or node breaks the loop and can
take down the entire network.
§ Maximum ring length and number of nodes are limited.
BUS TOPOLOGY
Bus topology uses a common bus or backbone (a single
cable) to connect all devices with terminators at both ends. The backbone acts
as a shared communication medium and each node (file, server, workstations, and
peripherals) is attached to it with an interface connecter.
Whenever a message is to be transmitted on the network, it is passed
back and forth along the cable, past the stations and between the two
terminators, from one end of the network to the other. As the message passes
each station, the station checks the message’s destination address. If the
address in the message matches the station’s address, the station receives the
message. If the address does not match, the bus carries the message to the next
station, and so on.
Advantages of BUS Topology
§ Connecting a computer or peripheral to a linear bus is
easy.
§ This topology requires least amount of cabling to
connect the computers and, therefore, less expensive than other cabling
arrangement.
§ It is easy to extend a bus since two cables can be
joined into one longer cable with a connector.
Disadvantages of Bus Topology
§ Entire network shuts down if there is a failure in the
backbone.
§ Heavy traffic can slow down a bus because computers on
such networks do not coordinate with each other to reserve time to transmit.
NETWORK TOPOLOGY
Network topology is the
study of the arrangement or mapping of the elements (links, nodes, etc.) of a network, especially the physical (real) and logical (virtual)
interconnections between nodes. The term
topology refers to the way a network is laid out, either physically or
logically. A topology can be considered as the network’s shape.
There are five basic topologies: Bus, Ring, Star, Tree, and Mesh.
- BUS TOPOLOGY
- RING TOPOLOGY
- STAR TOPOLOGY
- TREE TOPOLOGY
- MESH TOPOLOGY
WIDE
AREA NETWORK
Wide Area Network (WAN) is a computer
network that covers a broad area (i.e., any network whose communications
links cross metropolitan, regional, or national boundaries).
Several options are
available for WAN connectivity:
|
Option:
|
Description
|
Advantages
|
Disadvantages
|
Bandwidth range
|
Sample protocols used
|
|
Point-to-Point connection between two computers or Local Area
Networks (LANs)
|
Most secure
|
Expensive
| |||
|
Less Expensive
|
Call Setup
|
28 Kb/s - 144 Kb/s
| |||
|
Devices transport packets via a shared single point-to-point or
point-to-multipoint link across a carrier internetwork. Variable length
packets are transmitted over Permanent Virtual Circuits (PVC) or
Switched Virtual Circuits (SVC)
|
Shared media across link
| ||||
|
Similar to packet switching, but uses fixed length cells instead
of variable length packets. Data is divided into fixed-length cells and then
transported across virtual circuits
|
best for simultaneous use of Voice and data
|
Overhead can be considerable
|
Transmission rate usually range from 1200 bits/second to 6 Mbit/s,
although some connections such as ATM and Leased lines can reach speeds greater
than 156 Mbit/s. Typical communication
links used in WANs are telephone lines, microwave links & satellite
channels.
