What are uses of Wifi?

- Routers sometimes act as a Wi-Fi access point incorporating a cable modem or a DSL modem.

- These routers are installed in buildings and homes for providing Internet access and other inter networking services to the devices that in turn are connected to a either through a cable or wireless. 

- Similarly, there are routers that are powered by battery and they consist of a Wi-Fi access point and a mobile Internet radio modem. 

- Today smartphones come with this as a built-in capability.  

- However, this feature is disabled by the carriers. 

- The carriers might charge extra money for this. 

- The standalone facilities are provided by Internet packs. 

- The places where there is no network access, wifi is used. 

- Using Wi-Fi, a direct communication link between two computers can be established.  

- There is no intermediate point.  

- This type of transmission is termed as ad hoc wifi transmission. 

- This network mode is now very popular with the multi-player game consoles. Examples are:

       > Nintendo DS

       > PlayStation portable

       > Digital cameras

       > Other consumer electronic devices.

- A citywide Wi-Fi plan has been implemented by a number of the cities around the world.  

- In India, the first city to do so was Mysore.  

- The first city in the world was Jerusalem.

- The first city in United States was Sunnyvale in California to offer city-wide wifi. 

- Another type of wifi implementation is campus-wide wifi.  

- A number of colleges in United States have set up this kind of wifi network.  

- The first university to have it was Carnegie Mellon University. 

- Using wifi, the local area Network can be deployed in very less cost.  

- There are places where it is not possible for the physical transmission medium such as cables to reach. 

- In such places wifi network is of crucial importance.  

- Also, wifi can be easily deployed in historical buildings and outdoor areas.  

- Now, because of the increasing popularity of the Wi-Fi, the manufacturers are developing Wireless Network adapters for most of the notebooks and laptops.  

- This eventually led to a fall in the price of the Wi-Fi chip set. 

- Today, the Wi-Fi chip set is economically feasible and is included in most of the devices.  

- There are many brands of client network interfaces and access-points that are competing with each other.  

- These interfaces are able to inter-operate at a basic level. 

- The Wi-Fi certification for the products is issued by wifi alliance. 

- This makes them backwards compatible with each other. 

- A standard Wi-Fi Device is supposed to work anywhere in the world. 

- The encryption standard that is considered secure is the WPA2 or wifi protected access.  

- But, this would work only if the pass phrase that is being used is strong enough. 

- The Wi-Fi has been made more suitable with the use of new protocols such as quality of service.  

- This has made wifi compatible with latency sensitive applications.  

- Nowadays, for extending battery life power saving mechanisms such as WMM are being used.  

- These are the major uses of wifi technology.

- The usage wifi has been limited because of its limited range. 

- Therefore, in order to cover up a large area several intermediate Wi-Fi access-points have to be set up. 

- The variations in the range can be produced by varying the frequency band.  

- Wifi with a small frequency block works better than wifi with a larger frequency block.

- Wifi with the larger frequency blocks are optionally used. 

- The power of wifi network can be harnessed by using high gain direction antennas instead of using detachable antennas.  

- Another factor limiting the performance of wifi transmission is the local regulations. 

- Wifi also requires high power to operate upon. 

- This is a cause of concern for the devices' batteries.

Differentiate between upward and downward multiplexing?

The process of multiplexing is carried out at the transport layer. Several conversations are multiplexed in to one connection or physical links or virtual circuit. For example, suppose the host has only one network address available for use. Then it has to be used by all the transport connections originating at that host. For multiplexing the following two main strategies are followed:

Ø  Upward multiplexing and

Ø  Downward multiplexing

Upward Multiplexing 

- In upward multiplexing, the different transport connections are multiplexed in to one network connection. 

- These transport connections are grouped by the transport layer as per their destinations. 

- It then maps the groups with the minimum number of network connections possible.

- The upward multiplexing is quite useful where the network connections come very expensive.

Downward Multiplexing 

- It is only used when the connections with high bandwidth are required. 

- In case of the downward multiplexing, the multiple network connections are opened by the transport layer and the traffic is distributed among them. 

- But for using downward multiplexing, it is necessary that this capacity must be handled well by the subnet’s data links.

Another Technique 

- In either of the cases it is not guaranteed that the segments will be delivered in order. 

- Therefore, another technique is adopted. 

- The segments are numbered sequentially. 

- Each octet is numbered by the TCP sequentially. 

- Segments are then numbered based up on the number of the first octet present in that segment. 

- The segments might get damaged in the transition or some may even fail to arrive at the destination. 

- This failure is not acknowledged by the transmitter. 

- However, the successful receipt of the segment is does acknowledged by the receiver. 

- Sometimes, the cumulative acknowledgements might be used. 

- If the ACK triggers a time out interrupt, the re-transmission of the segment is done. 

- Also the re-transmission is done when an ACK is lost. 

- The receiver must have the ability to recognize the duplicate ACKs. 

- If such thing occurs, the receiver assumes by itself the ACK might have been lost.

- This happens when the ACK duplicate is received before the connection is closed. 

- If the duplicate is received after the closure of the connection, the situation is dealt differently. 

- In this case, the sender and receiver are allowed to know about each other’s existence. 

- They negotiate about the parameters and the transport entity resources are allocated based up on some mutual agreement. 

The connection release is of two types:

Ø Asymmetric release: 

This is the one used in the telephone systems. However it does not works well for the network that use packet switching.

Ø  Symmetric release: 

- This is certainly better than the previous one.

- Here, all the directions are released independently with respect to each other. 

- The host continues receiving data after the disconnection TPDU has been sent. 

- But the symmetric release has another problem which is related with indirection levels and fake messages. 

- There are no proper solutions for this problem in case of the unreliable communication media. 

- Note that this has nothing to do with the protocol. 

- Putting a reliable protocol over an unreliable medium can actually guarantee the delivery of the message. 

- Another thing to be noted is that it the time limit within which the message will be delivered cannot be guaranteed by any protocol. 

- Error conditions might prolong the delivery period. 

- Restarting the connections can lead to the loss of all the state info and the connection might remain as half-open. 

- Since no protocol has been designed to deal with this problem therefore one has to go forward with the risks associated with releasing the connections. 

Tracking the URL's used in the application and having the ability to modify them

Any software application would be using many URL's in them. For example, when you take a product, you could have the following URL's in the product:
- A link for the product's home page on the organization site
- Links for Help pages
- Links for Video tutorial
- Links for customer support
- Links for online pages where the workflow is a mix of desktop and online components
- A link where a customer can buy the product
- When the product used 3rd party components, the workflow may be pointing to 3rd party links

There are complications that come in because of the 3rd party links. For example, I have run into cases where the 3rd party component that we were using became like an orphan because the organization that had provided us the component became bankrupt and closed down (was not purchased by another organization, just shut down). In this case, the component still continued to work, but the link was no longer existing. However, the link was the last part of the workflow and there would be a percentage of the users who would still click on the link and thus run into a problem.
For this purpose, the policy that should be followed should be to ensure that all the links that are actually used in the application are redirect links, where the redirect happens on the servers of the organization and the target of the link can be modified. Doing it in such a way will ensure that even in the case of the above situation where the link for the 3rd party does not exist, the modification of the redirect URL can happen on the company server and point to an alternative workflow (such an alternative workflow could even mean just pointing out to the customers that because of business reasons, the final part of the workflow does not work any more and apologize for the same). This is better than being in a situation where the workflow leads to an error, since that makes the customers think that something is not working, and if the error is not rectified, they start wondering about why the organization is not correcting the error.
However, there is a lot of book-keeping that needs to happen for this. All these links, with the date of creation, the target destination and the person who created the links (and of course, any description of what the link was meant for) are essential pieces of information to capture. This ensures that in the future, if the person who created the link is no longer with the organization, the replacing person has a good amount of knowledge about these links.

When is a situation called as congestion?

- Network congestion is quite a common problem in the queuing theory and data networking. 

- Sometimes, the data carried by a node or a link is so much that its QoS (quality of service) starts deteriorating. 

- This situation or problem is known as the network congestion or simply congestion. 

This problem has the following two typical effects:

Ø  Queuing delay

Ø  Packet loss and

Ø  Blocking of the new connections

- The last two effects lead to two other problems. 

- As the offered load increases by the increments, either the throughput of the network is actually reduced or the throughput increases by very small amounts. 

- Aggressive re-transmissions are used by the network protocols for compensating for the packet loss. 

- The network protocols thus tend to maintain a state of network congestion for the system even if the actual initial load is too less that it cannot cause the problem of network congestion. 

- Thus, two stable states are exhibited by the networks that use these protocols under similar load levels. 

- The stable state in which the throughput is low is called the congestive collapse. 

- Congestive collapse is also called congestion collapse.

- In this condition, the switched computer network that can be reached by a packet when because of congestion there is no or little communication happening.

- In such a situation even if a little communication happens it is of no use. 

- There are certain points in the network called the choke points where the congestion usually occurs.

- At these points, the outgoing bandwidth is lesser than the incoming traffic. 

- Choke points are usually the points which connect the wide area network and a local area network. 

- When a network falls in such a condition, it is said to be in a stable state. 

- In this state, the demand for the traffic is high but the useful throughput is quite less.

- Also, the levels of packet delay are quite high. 

- The quality of service gets extremely bad and the routers cause the packet loss since their output queues are full and they discard the packets. 

- The problem of the network congestion was identified in the year of 1984. 

- The problem first came in to the scenario when the backbone of the NSF net phase dropped 3 times of its actual capacity. 

- This problem continued to occur until the Van Jacobson’s congestion control method was implemented at the end nodes.

Let us now see what is the cause of this problem? 

- When the number of packets being set to a router exceeds its packet handling capacity, many packets are discarded by the routers that are intermediate. 

- These routers expect the re-transmission of the discarded information. 

- Earlier, the re-transmission behavior of the TCP implementations was very bad. 

- Whenever a packet was lost, the extra packets were sent in by the end points, thus repeating the lost information. 

- But this doubled the data rate. 

- This is just the opposite of what routine should be carried out during the congestion problem. 

- The entire network is thus pushed in a state of the congestive collapse resulting in a huge loss of packets and reducing the throughput of the network. 

- Congestion control as well as congestion avoidance techniques are used by the networks of modern era for avoiding the congestive collapse problem. 

- Various congestion control algorithms are available that can be implemented for avoiding the problem of network congestion. 

- There are various criteria based up on which these congestion control algorithms are classified such as amount of feedback, deploy-ability and so on. 

What is meant by multi-destination routing?

- So many routing algorithms have been devised to aid in routing under different conditions.
- Effective routing algorithms have been developed that are capable of routing the messages from one source node to a number of receiving nodes i.e., the multiple destination nodes.
- These algorithms are termed as the multi – destination routing algorithms and the process is therefore called as the multi – destination routing.
- This type of routing has been developed for the minimization of the cost of the network i.e., NC (network cost).
- Network cost can be defined as the sum of all the links’ weights that consist of the routing path.
- There are many heuristic algorithms available for determining the NC min path.
- This problem falls under the category of the NP – complete problems.
- Heuristics are available for the traveling salesman problem and MST (minimum spanning tree) variations.
- Global information is used by both of them.
- Another set of such heuristics is available that uses only shortest paths for reaching the destinations.
- The best worst case performance is exhibited by the MST algorithm.
- However, one study revealed that effectiveness of the simpler heuristics is higher.
- The network cost (NC) is often compared with the destination cost (DC).
- Destination cost is the sum of the cost of all the shortest paths that lead to the destination.
- A scheme of algorithms has been developed for trading off between these two costs i.e., the NC and DC.  
- The sender of the transmitted data cannot be taken as a single node in a network where the cooperative communication is supported.
- This asks for the re-investigation of the traditional link concept.
- Any routing scheme thus depending up on this link concept needs to be reconsidered.
- Also, the potential performance gain resulting because of the cooperative communication needs to be exploited.

- Routing often gets complicated for some networks where the selection of the paths is no longer the job of a single entity.
- Rather, a number of entities are involved in the selection of the paths.
- Multiple entities can even select specific parts of a path.
- If these selected paths are chosen by the entities for their own objectives optimization then it can lead to inefficiency or serious complications in the network since they may or may not conflict with the other entities’ objectives.
- This would become clear from the following example, consider traffic moving in a system of roads.
- Now here each driver selects a path that would minimize only his/her traveling time.
- In this kind of routing, there are longer equilibrium routes (i.e., longer than the optimal.) for almost all other drivers.
- This is often termed as the Braess paradox.
- Another example is of the routing the AGVs (automated guided vehicles) by a model on some terminal.
- For prevention of the simultaneous usage of the infrastructure’s same part reservations are made. - This is called as the context – aware routing.
- The internet is divided in to a number of divisions which are nothing but Ass i.e., the autonomous system like ISPs.
- All these systems have control over the routes that lie in their own network at various different levels.

Following steps are involved in multi – destination routing:
1. The BGP protocol is used for selecting the AS – level paths.
2. A sequence of autonomous systems is produced by the BGP protocol via which the packet flow will take place.
3. The neighboring Ass offer multiple paths for each of the AS from which it can choose. Paths are selected based up on the relationships between the neighboring systems.
4. Each selected path refers to multiple corresponding router level paths.

What is meant by flow based routing?

- The routing algorithm that considers the flow in the network is known as flow based routing. 

- It takes into consideration the amount of traffic flowing in the network before making a decision regarding the outgoing link over which the packet has to be sent. 

- The ability to characterize the traffic flow’s nature with respect to time is the key to the successful implementation of the flow based routing. 

- For any given line, if we know what is its average flow or capacity we can very well compute the mean packet delay of the line using the queuing theory. - This is the basic idea behind the implementation of this algorithm. 

- This idea reduces the size of the problem i.e., only the minimum average delay has to be calculated for the sub net and nothing else. 

- Thus, the flow based routing considers the load and topology of the network while other routing algorithms do not. 

- In few networks, the mean data flow existing between two nodes might be predictable as well as relatively stable. 

- There occur such conditions under which the average traffic between the two points is known. 

- In such conditions the mathematical analyzation of the flow is possible. 

- This calculation can be used in the optimization of the routing protocol. 

- The flow weighted average can be straightforward calculated which in turn can be used for the calculation of the mean packet delay of the entire sub-net. 

The flow based routing algorithm requires the following things in advance:

Ø  Topology of the subnet

Ø  Traffic matrix

Ø  Capacity matrix

Ø  A routing algorithm

- Information flow based routing algorithms are commonly used in the wireless sensor networks. 

- These days, the measure of information is being used a criterion for the analyzation of the performance of the flow based routing algorithms. 

- One research has put forward an argument stating that since the sensor network is driven by the objective of the estimation of a 2D random field, the information flow must be maximized over the entire field and the sensor’s lifetime. 

In response to this algorithm two types of flow based routing algorithm have been designed namely:

1. Maximum information routing (MIR) and
2. Conditional maximum information routing (CMIR)

- Both of these algorithms have proved to be quite significant when compared to the exiting algorithm – maximum residual energy path or MREP.

About MREP Algorithm

 

- This proves to be quite effective in conservation of the energy. 

- The battery energy which is limited is taken as the most important resource. - For the maximization of the lifetime, the energy consumption has to be balanced throughout the nodes. 

- This should be done in proportion to the resource reserves. 

- This is better than routing for the minimization of the absolute consumed power.

About MIR Algorithm

 

- The ideology behind the MIR algorithm is that there is inequality between the nodes. 

- For example, two very close nodes might not provide twice as much information provided by a lonely node. 

- Therefore, the nodes that provide more information are only given preference. 

- An additional penalty according to the node’s contribution is added to the node for achieving the above mentioned preference. 

- Dijkstra’s algorithm is used for the computation of the shortest path. 

- This helps in sending the data to the sensor as per both the information of the origin and the power consumed.

About CMIR Algorithm

- This one is a hybrid algorithm and makes use of MIR to some extent and then uses MREP algorithm for the rest of the cycle. 

- This hybrid version is better than the above two standalone algorithms since it runs better. 

What is the idea behind link state routing?

There are two main classes of the routing protocols out of which one is the link state routing protocol on which we shall focus in this article. The other class of the routing protocols is the distance vector routing protocols.

In computer communications, the link state routing protocol is applied in the packet switching networks. 

We can give two main examples of the link state routing protocol namely:

1. IS – IS i.e., intermediate system to intermediate system and
2. OSPF i.e., the open shortest path first

Almost every switching node is capable of performing the link – state routing in the network. Switching nodes can be defined as the nodes that can forward the packets. We call these nodes as routers in the internet.

Idea behind the Link State Routing

 

- A map is constructed by every node concerning its connectivity with the network. 

- This map is actually in a graph’s form. 

- This graph shows all the connections that exist between the various nodes in the graph. 

- The next “best logical path” is then calculated independently by each node to every destination in the network based up on its possibility. 

- The routing table of the node will then be formed by this collection of the best paths.

- This is in total contrast with the second class of the routing protocols.

- In the distance vector routing protocols, the routing table of a node is also shared by its neighbors whereas in the link state routing protocol, only the connectivity related information is passed between the nodes. 

- The simplest configuration of the link state routing protocols is the one that has no area. 

- This implies that each node possesses a map of the whole network. 

- The first main stage involves providing map of the network to each node. 

For doing the following, subsidiary steps are followed:

1. Determination of the neighboring nodes: Each node determines to which all neighboring nodes it is connected to. Also, it needs to know whether over the links through which it is connected are fully working or not. A reach-ability protocol is used for accomplishing this task. This protocol is run regularly but in separation from the neighboring nodes.
2. Distribution of the map information: The link state advertisement is the short message that is made by the node in case of some modification in the connectivity periodically.

- The above obtained set of such link state advertisements is used for the creation of the map of the entire network. 

- The second stage involves production of the routing tables through the map inspection. 

This again involves a number of steps:

1. Calculation of the shortest paths: Shortest path from one node to other nodes is determined by running a shortest path routing algorithm over the entire map. The commonly used algorithm is the Dijkstra’s algorithm.
2. Filling the routing table: The table is filled with best shortest paths obtained in the above step for every two nodes.  
3. Optimizations: We gave a glance of the simple form of this algorithm but in practical applications this form is used along with a number of optimizations. Whenever a change is detected in the network connectivity, the shortest path tree has to be immediately recomputed and then the routing table must be recreated. A method was discovered by the BBN technologies for recomputing only the affected part of the tree.

- Routing loops can be formed if working of the nodes using the exactly same map is not proper. 

- For adhoc networks such as the mobile networks, the optimized form of the protocol i.e., the optimized link state routing protocol is used. 

What are applications of flooding algorithm?

- Flooding algorithm and its many other variants are used as material distributing algorithm.

- This algorithm distributes the messages to all the hosts in the entire graph. 

- This algorithm since it acts like a flood and therefore has been named so. 

- In this simple yet useful distribution or routing algorithm, every packet that a node receives is transmitted to every other outgoing link. 

This algorithm is available in many variants but in every variant the following two things are common:

1. Each node acting as receiver and transmitter.

2. Each node responsible for forwarding received message to all its neighboring nodes except the one from which the message came. 

- Thus, the messages are eventually delivered to the hosts spread across the network. 

- Flooding algorithm might have been more useful if it would have been more complex. 

- Also, then it would have been possible to avoid the duplicate messages and infinite loops that occur because of them. 

Applications of Flooding Algorithm

In this article we list some of the application of the flooding algorithm.

1. Used in computer networking

2. Used in graphics

3. These algorithms are quite useful for solving numerous mathematical problems such as the maze problems.

4. Used for solving problems in the graph theory. 

5. Used in systems which make use of bridging. 

6. Used in systems like usenet.

7. Implemented in peer – to – peer file sharing

8. Flooding algorithms are often implemented as a part of some of the routing protocols as in OSPF, DVMRP and so on. 

9. It is also used in the protocols used in the ad hoc wireless networks.

10.There is a variant of flooding algorithm called the selective flooding which is capable on addressing various issues of flooding algorithm partially by allowing the packets to be sent only in the appropriate right direction. The packets are not sent on each and every line. 

11. Another variant of the flooding algorithm called the similarity algorithm is used graph matching algorithm. 

- This variant of the flooding algorithm is quite versatile and has got an application in the schema matching. 

- Matching the contents of the two data schemas has got an important role to play in many biochemical applications, e – business and other data warehousing applications etc. 

- The similarity flooding algorithm is based on a computation that is fixed and can be used across a number of scenarios. 

- Two graphs are passed to the algorithm as the input parameters. 

- These graphs might be of catalogs, schemas or even data structures etc. 

- The algorithm then produces a mapping between the corresponding nodes of the two graphs as output. 

- It depends on the goal of the matching what subset of the mapping has to be chosen using filters. 

- After the algorithm has been executed, a human tester is expected is expected to check and verify the results. 

- The results might be adjusted by the tester if required. 

- In this method, the accuracy of the algorithm is evaluated by number of the adjustments that are necessary.

- In some cases, an accuracy metric might be used for the estimation of the labor savings that could be obtained by the users by means of this similarity flooding algorithm for obtaining a first matching pair.

- Finally, this algorithm can be deployed as an operator of very high level in a test bed that has been implemented for the management of the output mappings and the information models. 

- There are different types of matching problems and thus each type requires following a different approach. 

- For example, the relational schemas can only be matched using SQL data types.

Flooding - a kind of static algorithm

An algorithm designed for the distribution of the material to each and every part of the graph is referred to as the flooding algorithm. 

- The algorithm has got its name from the concept that involves inundation caused by a flood. 

- The main application of these algorithms is in graphics and computer networking. 

- These algorithms also come very handy in solving a number of mathematical problems. 

- Examples of such problems that are graph theory problems, maze problems and so on. 

- Flooding algorithm though it sounds complicated is quite a simple one. 

- Here, every incoming packet is sent via every outgoing link. 

- Only the link through which the packet arrived is saved. 

This algorithm has many applications in the following:

1. Systems that require bridging, systems like use net.
2. Peer to peer file sharing.
3. Used as a part of the routing protocols such as the DVMRP, OSPF etc.
4. Used in protocols used for the adhoc wireless networks.

Nowadays the flooding algorithm is available with its many variants. The following are two main steps that each variant follows while working:

1. Each node in the network might act as both receiver and the transmitter.
2. The incoming message is forwarded by the receiving node to each of its neighboring nodes except the one which is the source code.

- This causes the message to be delivered to each and every part of the network that is reachable. 

- Algorithms are required as a precaution for avoiding the wastage of the infinite loops and the duplicate deliveries and for allowing the messages for expiring. 

- All these issues are addressed partially by a variant of the flooding algorithm called the selective flooding.

- The usual flooding algorithm sends the packets to all the routers that lie in the same direction. 

- But the selective flooding algorithm does not send the packet to each router rather it selects only few of them which lie in the right direction approximately. 

Advantages and Disadvantages of Flooding Algorithm

Advantages:

1. If it is possible for delivering the packet, it will be delivered but a number of times.
2. In flooding algorithm every path in the network is naturally utilized and so the shortest path is also used.
3. The implementation of the flooding algorithm is quite simple.

Disadvantages:

1. The cost of the flooding algorithm can be very high because a lot of bandwidth is wasted. Even if there is one destination of the message, it will be sent to all the hosts on the network unnecessarily. If in case there occurs a denial of service attack or a ping flood, the reliability of the whole network will be affected badly.
2. In the computer network, the message might get duplicated. This in turn can increase the load on the bandwidth of the network. This will call for increasing the complexity of the processing for rejecting the duplicates of the messages.
3. The packets that are duplicate might keep on circulating forever, if the following precautions are not taken:

Ø  Using a time to live count or a hop count to be included with every packet. The value of the count has to include the number of the nodes through which the packets have to be passed while on the way to destination.

Ø  Each and every node should be used for keeping track of every packet that passes through it and a packet should be forwarded only once.

Ø  The network topology must be enforced without any loops.