What are advantages and limitations of Wi-Fi?

The Wi-Fi has its own set of advantages and limitations. 

Advantages of WiFi

- WiFi makes the deployment of local Area Networks or LANs quite cheap.  

- There are some areas where the cables cannot be installed such as in historical buildings and outdoor areas. 

- But these spaces do not have any problem in hosting a wireless LAN.  

- Wireless Network adapters are being built into almost all the laptops by the manufacturers.

- A basic level of service is provided at which different brands concerning and client network interfaces access points that are competing with each other can inter-operate. 

- The products that have been certified by Wi-Fi alliance show back word compatibility. 

- A standard device for WiFi will work at any place in the whole world unlike our phones. 

- The WPA2 or the WiFi protected access encryption is secure provided a condition that the pass phrase used is quite strong.  

- The new protocols use for WMM i.e., Quality of service increase the suitability of the Wi-Fi regarding its use in latency - sensitive applications. 

- WMM is a power saving mechanism that is used for extending the life of the battery. 

Limitations of WiFi

- Inconsistency of the operation and spectrum assignments poses a problem worldwide.  

- The range all the WiFi networks is limited. 

- A wireless access point typically uses a stock antenna having a range of 100 m outdoors and 25m indoors.

- The frequency band is a major factor for producing variations in the range.  

- The range of Wi-Fi with a 2.4 ghz frequency block is better when compared with the 5.0 ghz frequency block Wi-Fi. 

- Some wireless routers come with detachable antennas. 

- These antennas can be removed for improving the range. 

- In their place upgraded antennas can be fitted. 

- The benefit of these antennas is that they have high directional gain at the remote devices. 

- The local regulations limit the maximum amount of power that can be transmitted by a Wi-Fi. 

- The power consumption of Wi-Fi is quite higher than the other standards.  

- This is so because of the reach requirements of the wireless LAN applications.

- There are technologies available that provide a propagation range that is much shorter. 

- One such technology is Bluetooth and has very low power consumption.  

- Other technologies such as zigbee have low power consumption, a long range but provides low data rate. 

- The most commonly used wireless encryption standard is WEP or wired equivalent privacy. 

- Even this standard has been proven to be breakable even if correct configuration is used. 

- This problem was addressed by WPA or Wi-Fi protected access standard to some extent. 

- By default the wireless access points use the encryption free mode. 

- The wireless security is disabled because of which the LAN can be openly accessed. 

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.

What is inter-network routing?

In this article we shall discuss about inter-network routing. Before moving to that there are certain terms with which you should be familiar:

Ø  End systems: The ISO (the international standards of organization) defines the end systems as the network elements that do not have the ability of forwarding the packets across the networks. Sometimes the term host is used to refer to the end systems.

Ø  Intermediate systems: These are the network elements that have the ability of forwarding the packets across the network. Most common examples are routers, switches, bridges and so on.

Ø  Network: It can be defined as a part of the inter-network infrastructure encompassing various elements including hubs, repeaters, bridges and so on. The networks are bounded by the intermediate systems.

Ø Router: This is one of the intermediate systems that is used for connecting various networks with each other. It might support one protocol (router) or many protocols (multi-protocol router). Its hardware part is optimized especially for performing routing. The software part is responsible for carrying out the routing and takes care of the routing tables.

Apart from these devices, there are 3 types of addresses involved in inter-network routing:

Ø  The inter-network address: The host address and the network address are combined together to form this address. This is used for unique identification of a host over the inter-network.

Ø The host address or host ID: This ID might be assigned by the administrator or might be simply the physical address of the host. It is used for the unique identification of the host on its network.

Ø  The network address or network ID: This is address of a network for identifying it in an inter-network.

All the data packets consist of a network layer header. This network layer header consists of the following when the packet is transmitted from one host to another:

ØThe address of the source inter-network: This address combines the address of the source host and the source network.

ØThe address of the destination inter-network: This address combines the address of the destination host and the destination network.

ØThe hop count: This usually begins at zero and is numerically incremented when the packet crosses a router. Or in the opposite case it might be assigned some maximum value which might be decremented on reaching a router. The purpose of using the hop count is to make sure that the packet does not keeps on circulating endlessly in the network.

- For inter-network routing, two things have to be known.

- Firstly, how do you reach other routers which lie in the same network and secondly, how do you reach other routers which lie in other networks? 

- The answer to the first question is easy as it is the common routing problem among two hosts residing over the same network. 

- This routing is handled by the interior gateway protocol and it is different for different networks since only local routing info is required. 

- In this case, the commonly used protocol is the open shortest path first or OSPF protocol. 

- The routing between two different networks is performed using the exterior gateway protocol. 

- This is actually the problem of inter-network routing. 

- Here, the commonly used protocol is the BGP or the border gateway protocol. 

- The graph for inter-network routing is quite different from the one that is used in the network routing. 

- This is so because the routers which lie in the same network can be thought of as being directly connected to one another for routing across inter-network. - All the networks in an inter-network function as though they are one large unit. 

What are multi-protocol routers?

- There are routers that have the capability to route a number of protocols at the same time. 

- These routers are popularly known as the multi-protocol routers. 

- There are situations in networking where combinations of various protocols such as the appletalk, IP, IPX etc. are used. 

- In such situations normal typical router cannot help. This is where we use the multi-protocol routers. 

- Using the multi-protocol routers, information can be shared between the networks. 

- The multi-protocol router maintains an individual routing table for each of the protocols.

- The multi-protocol routers have to be used carefully since they cause an increase in the number of routing tables that are present on the network. 

- Each protocol is advertised individually by the router. 

A multiprotocol router consists of the following information:

Ø  Routing information protocol (RIP)

Ø  Boot protocol relay agent (BOOTP)

Ø  RIP for IPX

- The multi-protocol routers use this routing information protocol for performing dynamic exchange of the routing info. 

- Routers using RIP protocol can dynamically exchange information with the other routers that use the same protocol. 

- The BOOTP agent is included so that the DHCP requests can be forwarded to their respective servers residing on other subnets. 

- It is because of this, a single DHCP server can process a number of IP subnets. 

- Multi-protocol routers do not require to be manually configured.

- The networking world these days relies totally up on the internet protocol. But there are certain situations where certain tasks can be performed more efficiently by the other protocols. 

- Most of the network protocols share many similarities rather than being different. 

- Therefore, if one protocol can be routed by a protocol efficient, then it is obvious that it can route the other one also efficiently. 

- If we route the non-IP protocols in a network, this implies that the same staff that takes care of the IP monitoring is administering the non-IP routing also. 

- This reduces the need for more equipment and effort. 

- There are a number of non-IP protocols available using which a LAN can work more effectively. 

- Using a number of non-IP protocols, a network can be made very flexible and easier to meet the demands of its users. 

- All these points speak in the favor of multi-protocol routing in an abstract way. 

- But the non-IP protocols to be routed must be selected with care. 

Below we mention reasons why routing non – IP protocols can be avoided:

1. It requires additional knowledge because you cannot master everything. For individual protocol an expert is required who in case of a failure can diagnose it and fix it.
2. It puts extra load on the routers. For every protocol, the router would have to maintain a separate routing table. This calls for a dynamic routing protocol for the router itself. For all this, more memory is required along with high processing power.
3. It increases the complexity. Multi-protocol router even though it seems to be simple, it is quite a complicated thing in terms of both hardware and software. Any problem in the implementation of the protocol can have a negative impact up on the stability of all the protocols.
4. Difficulty in designing: There are separate rules for routing of each protocol, assignment of the addresses and so on. There are possibilities that there might be conflicts between these rules which means it is very difficult to design.
5. It decreases stability. Scaling capacity of certain protocols is not as good as of the others. Some of the protocols are not suited to work in a WAN environment. 

What is meant by flow specification?

- There are many problems concerning the flow specification. 

- There are limited options for the provider for mitigation of the DDoS attacks that take place internally. 

- These can be categorized in to three different categories:

Ø  BGP (border gateway protocol) destination black holes

Ø  BGP src/ uRP

Ø  ACLS

- The basic idea is to make use of the BGP for the distribution of the flow specification filters. 

- This helps in dynamic filtering in the routers. 

- The flow specification rules are encoded according to the BGP NLRI address family. 

- The flow spec NLRI is used by the BGP as its opaque key is used as an entry key for its database. 

- The extended communities are used for specifying the actions such as accepting, discarding it, rate limiting, sampling, redirecting and so on. 

- The source/destination prefix and the source/destination port are matched in combinations according to the packet size, ICMP type/co9de, fragment encoding, DSCP, TCP flag and so on. 

- For example, the TCP ports 80…90 are matched with 192.168.0/24. 

- The flow specification trust model uni casts the routing advertisements for controlling the traffic. 

- Filter is considered as a hole for the traffic that is being transmitted to some destination. 

- Filter is accepted when it is advertised for the destination by the next hop. 

- Filters with various flow specifications are available today.

- The major benefit of the flow specifications is the filters with the fine grain specification which make it easy for deploying and managing the BGP. 

- The trust and the distribution problems are solved by the BGP. 

- ASIC filtering in routers is leveraged. 

- This is another major benefit of flow specifications. 

Apart from the benefits, there are various limitations of the flow specifications as mentioned below:

Ø  There is no update level security in the BGP.

Ø The statistics and the application level acknowledgement are not well defined.

Ø  The flow specifications work only for those nodes for which the BGP has been enabled.

Ø  Beyond routing the BGP payload has to be overloaded.

Ø  There are various operational issues between the security operations and the network operations.

Ø  The threat information cannot be gathered in one place.

- The integration of the flow specifications was announced by various security vendors. 

- The DDoS attacks are experienced by a large number of customers. 

- The DDoS attacks are now massive and have put the network infrastructure at risk apart from the end customer. 

- Congestion problems occur at both the exchange and the backbone. 

- The attacks of long durations add to the cost of bursting and circuit congestion problems. 

- Depending up on the size of the attack the POP has to be isolated.

- VoIP is also affected. 

- These attacks have negative economic effects as the cost of the operations has been increased. 

- This has led to a degradation of the business. 

- Measures such as firewall filtering and destination BGP black-holing have proved to be insufficient in preventing the attacks. 

- These methods are slow since it is required to log-in and configuring the devices. 

- The configuration has to be constantly. 

- The traffic is terminated to some destination. 

- This affects the availability. 

- The black hole routes are removed by constantly changing the configurations. - Earlier version of the flow specifications had many bugs. 

- There were some limitations on the performance. 

- However, it provided arbor support for the actions of the flow specifications. 

- It does not provide multi–vendor support. 

- To some extent it provides the mitigation facility for the attack that occurred at the source. 

- The collateral damage is eliminated for both the carriers and supports the change in the matching criteria. 

What are general principles of congestion control?

- Problems such as the loss of data packets occur if the buffer of the routers overflows.

- This overflow is caused by the problem of the congestive collapse which is a consequence of the network congestion. 

- If the packets have to be re-transmitted more than once, it is an indication that the network is facing the problem of congestion. 

- Re-transmission of the packets is the treatment of only this indication but not for problem of the network congestion. 

- In the problem of congestive collapse, there are a number of sources that make attempts for sending data and that too at a quite high rate. 

- For preventing this problem of the network congestion, it requires mechanisms that are capable of throttling the sending node if in case the problem of network congestion occurs. 

- Network congestion is a real bad thing as it manifests in the network’s performance that the upper layer applications receive. 

- There are various approaches available for preventing and avoiding the problem of network congestion and thus implementing proper congestion control. 

- When the capacity of the network is exceeded by the demands for the resources and too much queuing occurs in the network causing loss of packets, congestion of packets is said to occur. 

- During this problem of network congestion, the throughput of the network might drop down to zero and there might be a high rise in the path delay. 

- Network can recover from the state of congestive collapse using a congestion control scheme. 

- A network can operate in a region where there is high throughput but low delay with the help of the congestion avoidance scheme.

- These schemes keep the network away from falling in to a state of congestive collapse. 

- There is a big confusion over congestion control and congestion avoidance. Most of us think it is the same thing but it is not. 

- Congestion control provides a recovery mechanism whereas the congestion avoidance provides a prevention mechanism. 

- Today’s technological advances in the field of networking have led to a rise in the network links’ bandwidth. 

- In the year of 1970, ARPAnet came in to existence and built using the leased telephone lines that had a 50 kbits/second bandwidth. 

- LAN (local area network) was first developed in the year of 1980 using token rings and Ethernet and offered a bandwidth of 10 mbits/ second. 

- During the same time many efforts were made for standardizing the LAN using the optical fibers providing a 100 mbits/seconds and higher bandwidth. 

- Attention to the congestion control has been increased because of the increase in the mismatching that occurs between the various links composing the network. 

- Routers, IMPs, gateways, intermediate nodes links etc. are the hot-spots for the congestion problems. 

- It is at these spots that the bandwidth of the receiver falls short for accommodating all the incoming traffic. 

- In the networks using the connection-less protocols, it is even more difficult to cope with the problems of network congestion. 

- It is comparatively easy in the networks using the connection-oriented protocols.

- This happens so because in such networks, the network resources are kept under advance reserve during setting up the connection.

- One way for controlling congestion problems is preventing the setting up of new connections if congestion is detected anywhere in the network but it will also prevent the usage of the reserved resources which is a disadvantage. 

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 reverse path forwarding?

- RPF or reverse path forwarding is a common technique used for ensuring that the multicast packets are forwarded without any loops in the modern routers in multicast routing. 

- This technique is also used for the prevention of the IP address spoofing during the unicast routing.

- Multicast RPF or just RPF is not used alone. 

- Rather, it is used along with some multicast routing protocol. 

- There are various multicast routing protocols such as the PIM – SM, PIM – DM, MSDP and so on. 

- This is for ensuring that no loops are formed in forwarding the multicast packets. 

- Source address is used for deciding whether the traffic has to be forwarded or not in multicast routing. 

- On the other hand in unicast routing, this depends up on the destination address instead of source address. 

- This it achieves either through utilization of either the unicast routing table of the router or a multicast routing table that has been dedicated to the purpose. 

- As and when a packet comes to the interface of the router, it searches in the networks list for the networks that can be reached through this interface. 

- This is nothing but the reverse path checking of the multicast packet.  

- If the appropriate routing entry is found for the multicast packet’s source IP address, it is said to pass the RPF check. 

- After this the packet is sent to all the participating interfaces in that particular multicast group.  

- If the packet fails at this RPF check, the packet is simply dropped. 

- Because of this, the packet forwarding has to be decided depending up on its reverse path. 

- Otherwise, the forward path can be used as usual. 

- Only those packets are forwarded by the RPF routers which pass this RPF check. 

- Passing this RPF check means breaking any loop that might otherwise exist. 

- This is of critical importance in the multicast topologies that are redundant. 

- This is so because it is possible for the same packet to come again and again to the same router through a number of multiple interfaces. 

- The RPF check is an integral part of the decision concerning forwarding of the packets. 

- Consider a router forwarding a packet from first interface to the second interface and also from second interface to the first one. 

- Thus, the same packet is received by the two packets, thus creating a common routing loop. 

- This loop will keep on forwarding the packets until the expiry of their TTLs. 

- Even if the TTL expiry is considered, the best thing to do is to avoid the routing loops because they are a main cause of the temporary network degradation.

RPF check has the following underlying assumptions:

1. The given unicast routing table is converged as well as correct.
2. There is symmetry between the path that goes from sender to router and the path that comes back from the router to the sender.

- RPF check uses the unicast routing table as the fallback. 

- Therefore, if the first assumption is not satisfied, the check will fail. 

- But in case the second assumption is false, the multicast traffic is rejected by the RPF check save the traffic on the shortest path that exists between the sender and the router. 

- This results in a multicast tree that is non–optimal.

- The reverse path forwarding will not work if there are uni-directional links present in the network.

Unicast RPF: 

- This type of the reverse path forwarding is based up on the concept that the interface which does not originate traffic must not accept it. 

- It is good for the organizations to not allow private address propagation on their network until and unless they are continuously using it.