What is computer Network? Difference between packet switching and Circuit switching.
A computer
network, often simply referred to as a network, is a collection of
computers and devices interconnected by communications channels that
facilitate communications among users and allows users to share resources.
Networks may be classified according to a wide variety of characteristics. A
computer network allows sharing of resources and information among
interconnected devices.
Computer
networking or Data communications (Datacom) is the engineering
discipline concerned with the communication between computer systems or devices.
A computer network is any set of computers or devices connected to each
other with the ability to exchange data. Computer networking is sometimes
considered a sub-discipline of telecommunications, computer science, information
technology and/or computer engineering since it relies heavily upon the theoretical
and practical application of these scientific and engineering disciplines. The
three types of networks are: the Internet, the intranet, and the extranet.
Advantages
of Computer Networks
- File Sharing: The major advantage of a computer network is that is allows file sharing and remote file access. A person sitting at one workstation of a network can easily see the files present on the other workstation, provided he is authorized to do so. It saves the time which is wasted in copying a file from one system to another, by using a storage device. In addition to that, many people can access or update the information stored in a database, making it up-to-date and accurate.
- Resource Sharing: Resource sharing is also an important benefit of a computer network. For example, if there are four people in a family, each having their own computer, they will require four modems (for the Internet connection) and four printers, if they want to use the resources at the same time. A computer network, on the other hand, provides a cheaper alternative by the provision of resource sharing. In this way, all the four computers can be interconnected, using a network, and just one modem and printer can efficiently provide the services to all four members. The facility of shared folders can also be availed by family members.
- Increased Storage Capacity: As there is more than one computer on a network which can easily share files, the issue of storage capacity gets resolved to a great extent. A standalone computer might fall short of storage memory, but when many computers are on a network, memory of different computers can be used in such case. One can also design a storage server on the network in order to have a huge storage capacity.
- Increased Cost Efficiency: There are many softwares available in the market which are costly and take time for installation. Computer networks resolve this issue as the software can be stored or installed on a system or a server and can be used by the different workstations.
Disadvantages
of Computer Networks
Following are some of the major disadvantages of computer networks.
- Security Issues: One of the major drawbacks of computer networks is the security issues involved. If a computer is a standalone, physical access becomes necessary for any kind of data theft. However, if a computer is on a network, a computer hacker can get unauthorized access by using different tools. In case of big organizations, various network security softwares are used to prevent the theft of any confidential and classified data.
- Rapid Spread of Computer Viruses: If any computer system in a network gets affected by computer virus, there is a possible threat of other systems getting affected too. Viruses get spread on a network easily because of the interconnectivity of workstations. Such spread can be dangerous if the computers have important database which can get corrupted by the virus.
- Expensive Set Up: The initial set up cost of a computer network can be high depending on the number of computers to be connected. Costly devices like routers, switches, hubs, etc., can add up to the bills of a person trying to install a computer network. He will also have to buy NICs (Network Interface Cards) for each of the workstations, in case they are not inbuilt.
- Dependency on the Main File Server: In case the main File Server of a computer network breaks down, the system becomes useless. In case of big networks, the File Server should be a powerful computer, which often makes it expensive.
- Expensive to set up
- All the wires can get in the way
- If something goes wrong, the whole network could go down with it
- Lower security
What is flow control and compare and contrast stop and wait flow control method with sliding window method.
The primary advantage of
the sliding window protocol over the stop-and-wait protocol is efficiency. The
sliding window protocol does not waste network bandwidth, while the
stop-and-wait protocol does waste network bandwidth. When using the stop-and-wait protocol, the sender
must wait after every frame to receive acknowledgement from the receiver before
sending the next frame. This protocol only allows for one frame being on the
channel at a time. However, a sliding window protocol utilizes pipelined
communication so that the full capacity of the bandwidth is used. Thus, the
sliding window protocol offers better performance and higher throughput than
the stop-and-wait protocol.
Explain Leaky Bucket Algorithm.
Policing
• Network
monitors traffic flows continuously to ensure they meet their traffic contract.
The process of monitoring and
enforcing the traffic flow is called policing.
• When
a packet violates the contract, network can discard or tag the packet giving it
lower priority
• If
congestion occurs, tagged packets are discarded first
• Leaky Bucket Algorithm is the most
commonly used policing mechanism o Bucket has specified leak rate
for average contracted rate o Bucket has specified depth to accommodate variations in
arrival rate o Arriving packet is conforming
if it does not result in overflow
Leaky Bucket algorithm can be used to police arrival rate of
a packet stream
Let X = bucket content at last conforming packet arrival
Let ta be last conforming
packet arrival time = depletion in bucket
Leaky Bucket Algorithm
• The
above figure shows the leaky bucket algorithm that can be used to police the
traffic flow.
• At
the arrival of the first packet, the content of the bucket is set to zero and
the last conforming time (LCT) is set to the arrival time of the first packet.
• The
depth of the bucket is L+I, where l depends on the traffic burstiness.
• At
the arrival of the kth packet, the auxiliary variable X’ records the difference
between the bucket content at the arrival of the last conforming packet and the
interarrival time between the last conforming packet and the kth packet.
• If
the auxiliary variable is greater than L, the packet is considered as
nonconforming, otherwise the packet is conforming. The bucket content and the
arrival time of the packet are then updated.
Leaky Bucket Example: - The operation of the leaky bucket algorithm
is illustrated in the below figure.
• Here
the value I is four packet times, and the value of L is 6 packet times.
• The
arrival of the first packet increases the bucket content by four (packet
times).
• At
the second arrival the content has decreased to three, but four more are added
to the bucket resulting in total of seven.
• The
fifth packet is declared as nonconforming since it would increase the content
to 11 , which would exceed L+I (10).
• Packets
7, 8, 9 and 10 arrive back to back after the bucket becomes empty. Packets 7, 8
and 9 are conforming, and the last one is nonconforming.
• Non-conforming
packets not allowed into bucket & hence not included in calculations.
