What is WiMax technology?

Worldwide inter-operability for microwave access or wimax is standard developed for wireless communications that has been designed so as to deliver data rates of 30-40 mbps. The update in the technology in the year 2011 upgraded the technology to provide around 1 gbps for the stations that were fixed. 

- The Wimax forum is responsible for naming the technology as Wimax. 

- This forum was formed in the year of 2001 for the promotion of the inter-operability and conformity of this standard. 

- The Wimax has been defined by the forum as the technology based up on standards that enable the last mile wireless broadband delivery as alternative for the DSL and the cable thing. 

- The IEEE 802/ 16’s interoperability implementations are referred to as the WiMax. 

- The wimax forum has ratified this family of standards. 

- By virtue of the certification provided by this forum, the vendors are able to sell mobile and fixed products that are wimax certified. 

- This is done for ensuring that a level of inter-operability is maintained at par with the other products that have been also certified for the same profile. 

- The ‘fixed wimax’ is the name given to the original IEEE 802.16 standards.

- ‘Wifi on steroids’ is the term used to refer to WiMax sometimes. 

It has got a number of applications such as in:

Ø  Broadband connections

Ø  Cellular back-haul

Ø  Hot spots and so on.

- This technology shares some similarity with the Wifi technology however, this one is more capable of transmitting data at greater distances.

It is because of its range and bandwidth that the WiMax is suitable for the following applications:

Ø  Provides services such as the IPTV services and VoIP (telecommunications services).

Ø  Provides mobile broadband connectivity that is portable across the cities and countries and that can be accessed via different kinds of devices.

Ø  Provides an alternative for DSL and cable in the form of wireless last mile broadband access.

Ø  Acts as a source of internet connectivity.

Ø  Metering and smart grids.

- This technology can be used at home for providing internet access across the countries. 

- This has also caused a rise in the market competition. 

- The WiMax is even economically feasible. 

- Mobile wimax has been used as a replacement for the technologies like CDMA, GSM that are cellular phone technologies.  

- The technology has also been used as an overlay for increasing the capacity.  

- The fixed wimax is now used for 2g, 3g and 4g networks as a wireless back-haul technology in almost all the nations whether they are developed or developing.  

- In some states of North America, this technology is provided through a numbered of copper wire line connections. 

- On the other hand, the technology is back hauled via satellites in case of the remote cellular operations.  

- While in other cases even microwave Links are used. 

- The bandwidth requirements of the WiMAX demand more substantial back-haul when compared to other legacy cellular applications. 

- In some of the cases, the sites have been aggregated by the operators by use of Wireless Technology.  

- The traffic is then introduced to the fiber networks as per the convenience.  

- The technologies that provide triple play services are directed compatible with the WiMAX.  

- These services might include multi-casting and quality of service. 

- WiMax has been widely used for providing assistance in the communications. 

- The Intel Corporation has donated the hardware for WiMax technology for assisting the FCC (federal communications commission) and FEMA etc.

- The subscribers’ stations or SS are the devices which are used for connecting to a WiMAX Network. 

- These devices might be portable such as the following:

      > Handsets and smart phones

      > PC peripheral such as USB dongles, PC Cards and so on. 

      > Embedded devices in notebooks.

Application areas of leaky bucket algorithm and token bucket algorithm

In this article we discuss about the applications of the leaky bucket algorithm and the token bucket algorithm.  

Applications of Leaky Bucket Algorithm

- The leaky bucket algorithm is implemented in different versions. 

- For example, the generic cell rate algorithm is a version of this algorithm which is often implemented in the networks using ATM (asynchronous transfer mode).  

- The algorithm is applied at the user interfaces in the usage/network parameter control in order to provide protection to the network from the problems of congestive collapse or excess traffic. 

- An algorithm equivalent to the generic cell rate algorithm might be used in shaping the transmissions made by the network interface card to a network using ATM. 

There are two major applications of the leaky bucket algorithm. 

- The first is using it as a counter only for checking whether the events or the traffics confirm to the defined limits or not.

- Whenever a packet arrives at the check point, the counter is incremented. 

- This is same as adding water to the bucket in an intermittent way. 

- In the same way, the counter is decremented as the water leaks out at a constant rate. 

- Because of this, the conformance of the packet to the burstiness and bandwidth limits is indicated by the value of this counter whenever a packet arrives. 

- Or if an event occurs, the counter checks whether it confirms to the peak and average rate limits. 

- So, when the packets arrive or an event occurs, water is added to the bucket and then leaks out. We call this version of the leaky bucket algorithm as a meter.

- Another application of the leaky bucket algorithm involves its use as queue implemented for controlling the flow of traffic. 

- This queue maintains a direct control over the flow. 

- When the packets arrive, they are put in to the queue. 

- This is same as adding water to the bucket. 

- The packets are then removed in the order they arrived at a constant rate. 

- This is same as water leaking out. 

- As a result of this, there is no jitter or burstiness in the traffic flow.

Applications of Token Bucket Algorithm

- The token bucket algorithm finds its application in the telecommunications and packet switched computer networks.

- This algorithm is implemented for checking whether the data transmissions confirm to the burstiness and bandwidth predefined limits. 

- The token bucket algorithm used in traffic policing and traffic shaping. 

- In the former, the packets that are non-conformant are discarded or assigned low priorities. 

- This is done for the management of the downstream traffic. 

- On the other hand, the packets are kept in delay unless they are conformed in traffic shaping. 

- Both of these are used in protecting the network against the burstiness of the traffic. 

- Bursty traffic gives rise to congestion problems. 

- These algorithms help in managing the bandwidth as well congestion of the network. 

- Network interfaces commonly use the traffic shaping process for preventing the discarding of the transmissions by the network’s traffic management functions. 

- This algorithm is based up on the analogy of a bucket with fixed capacity. 

- Tokens are added to this bucket at a fixed rate and represent a single packet of a fixed size. 

- When the packet has to be checked whether it confirms to the predefined limits or not, first the bucket is checked if it contains sufficient tokens. 

- If sufficient tokens are there, tokens equal to the number of bytes in the packet are removed and the packet is transmitted. 

- If sufficient tokens are not there, the packet is said to be non-conformant and the number of tokens in the bucket remain unchanged.

What is the difference between leaky bucket algorithm and token bucket algorithm?

- Telecommunications networks and the packet switched computer networks make use of the leaky bucket algorithm for checking the data transmissions. 

- This check is carried out in the form of packets. 

About Leaky Bucket Algorithm

- This algorithm is used for determining whether the data transmissions confirm to the limits that have been defined for the burstiness and bandwidth. 

- Leaky bucket counters also use the leaky bucket algorithm for detecting the peak or the average rate of the stochastic or random events and processes and if they are exceeding the predefined limits. 

We shall take analogy of a bucket for explaining this algorithm.

- Consider a bucket having a hole in its bottom through which the water it has will leak away. 

- The rate of leakage is constant if it is not empty. 

- We can intermittently add water to it that is in short bursts. 

- But if a large amount of water is added to it in one go, the water will exceed the bucket’s capacity and overflow will occur. 

- Hence, it is determined using this leaky bucket algorithm that whether or not adding water to it will make up the average rate or will exceed it. 

- Leak rate sets the average rate of adding the water and depth of the bucket decides the amount of water to be added. 

- Asynchronous transfer mode networks use the generic cell rate algorithm which is one of the versions of the leaky bucket algorithms. 

- At the user network interfaces, these algorithms are used in the usage/ network parameter control. 

- The algorithm is also used in network-network interfaces and inter-network interfaces for protecting networks from the overwhelming traffic levels through the connections in the network. 

- A network interface card can be used on a network using ATM for shaping the transmissions. 

- This network interface card might use an equivalent of the generic cell rate algorithm or this algorithm itself.

- The leaky bucket algorithm can be implemented in two different ways both of which are mentioned in the literature. 

- It appears as if there are two distinct algorithms that are together known as the leaky bucket algorithm.

About Token Bucket Algorithm

- At an interval of every 1/r seconds the token bucket algorithm adds a token to a bucket. 

- The maximum number of tokens that can be handled by a bucket are b. 

- Any token above this limit is rejected by the bucket. 

- When the bucket receives a packet from the network layer consisting of n bytes, the n numbers of tokens are taken out from the bucket and then the packet is transmitted in to the network. 

- If number of tokens available is less than n, the packet is treated as being non-conformant. 

- A bucket with a fixed capacity is associated with some virtual user and the rate at which it leaks is fixed. 

- No leakage occurs if there is nothing in the bucket. 

- Some water has to be added to the bucket in order to make the packet conform-ant. 

- No water is added to the bucket if adding this amount of water will cause the bucket to exceed its capacity. 

- Therefore, we can see that one algorithm adds something constantly to the bucket and removes also for conforming packets. 

- The other algorithm removes something constantly and adds something for confirming packets. 

- Both the algorithms are same in effectiveness and this is why the two see each the same packet as non-confirming or confirming. 

- The leaky bucket algorithm is often used as meter. 

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. 

Comparison between Virtual Circuit and Datagram subnets

Difference #1:

- In virtual circuits the packets are allowed to contain in them the circuit number rather than storing the full address of the destination. 

- This reduces the requirement for a much larger memory and bandwidth. 

- This also makes it cheaper in cost. 

- On the other hand, the data-grams have to contain the full destination address rather than a single circuit number.

- This causes a significant overhead in the data-gram sub nets. 

- Also, this leads to wastage of the bandwidth. 

- All this implies that the data-gram sub nets are more costly when compared to the virtual circuits. 

Difference #2:

- A set up phase is required for the virtual circuits. 

- For establishing this phase a lot of resources are required along with a lot of time. 

- Data-gram sub net in contrast does not require establishment of set up phase. 

- Hence, there is no requirement of resources.

Difference #3:

- In virtual circuits, for indexing purpose the circuit numbers are used by the router. 

- These numbers are stored in a table and are used for finding out the destination of the packet. 

- This procedure is quite simple when compared with the one used by the data-gram sub nets. 

- The procedure used in data-gram sub nets for determining the destination of the packet is quite complex. 

Difference #4:

- Virtual circuits allow for reserving the resources in advance on the establishment of the resources.

- This has a great advantage which is that the congestion is avoided in the sub net. 

- However, in the data-gram sub nets, it is quite difficult to avoid congestion. 

Difference #5:

- If a crash occurs in a router, then it will lose its memory. 

- Even if it backs up after sometime, all the virtual circuits that pass via it must be aborted. 

- This is not a major problem in the case of the data-gram sub nets. 

- Here, if the router crashes, the only packets that will have to suffer will be the ones that were queued for that router at that instant of time. 

Difference #6:

- The virtual circuits can vanish as a result of the loss or fault on the current communication line.

- In data-gram sub nets, it is comparatively easy to compensate for the fault or loss on the communication line. 

Difference #7:

- In virtual circuits there is one more cause for the traffic congestion. 

- This cause in the use of the fixed routes for the transmission of the data packets throughout the network. 

- This also leads to the problem of unbalanced traffic. 

- In data gram sub nets the routers are given the responsibility of balancing the traffic over the entire traffic.

- This has been made possible because it is allowed to change the routers halfway between the connections. 

Difference #8:

- Virtual circuits are one way of implementing the connection-oriented services. 

- For various types of data gram sub nets, a number of protocols are defined by the internet protocol. 

- Internet protocol provides the data-gram service at the internet layer. 

- In contrast with the virtual circuits, data gram sub nets are connection-less service. 

- It is the best effort message delivery service but at the same time is very unreliable. 

- There are a number of high level protocols such as TCP that are dependent up on the data gram service of the internet protocol.

- This calls for additional functionality. 

- The data gram service of IP is even used by the UDP. 

- The fragments of a data gram might be referred to as the data packets. 

- The IP and UDP both provide unreliable services and this is why both of them are termed as data grams. 

- The fragments of TCP are referred to as TCP fragments to distinguish it from data-grams. 

What is a virtual circuit? What are the advantages of virtual circuit?

The term VC or virtual circuit is synonymous with the term virtual channel or connection in the field of computer networks as well as in the telecommunications. 

- These are the connection oriented communication service.  

- The packet mode communication is the means through which this service is delivered. 

- After the establishment of a virtual circuit or connection between the two application processes or nodes, the stream of bytes or bit stream can be delivered between the two. 

- The higher level protocols are allowed by the virtual circuits in order to avoid the task of unnecessary dealing with the data division.

- This task may involve dividing data in to frames, packets or segments. 

- The virtual circuits bear a resemblance to the circuit switching mode under the fact that both of them are connection oriented.

- This means that both of them require correct order of delivery of the data. 

- Also, they both require signaling overhead during the establishment phase of the connections.

- The difference between the two is in the terms of the latency and bit rate. 

- It is constant in circuit switching and may vary in virtual circuits. 

This happens because of the following 3 major causes:

1. Varying length of the packet queues in the nodes.

2. Varying bit rate as generated by the application.

3. Varying load generated by the users who share the same resources on the network through statistical multiplexing. 

- A number of virtual circuit protocols are known for providing reliable services for communication but not all. 

- These services are provided by the means of data re-transmission because of the ARQ (automatic repeat request) and error detection. 

- Data-gram represents an alternate configuration for the virtual circuit. 

There are two types of virtual circuits namely:

Layer 4 virtual circuits: Data-link protocols such as TCP which are connection oriented and include segment numbering and thus reordering on the receiver’s side use this kind of virtual circuits. Thus out of order delivery is prevented. 

Layer 2/3 virtual circuits: The virtual circuit protocols of the data link  layer and the network layer are based up on the packet switching that is connection oriented. this implies that the delivery path of the data is always the same. 

Advantages of Virtual Circuits

There are several advantages of this kind of virtual connections:

1. They support the bandwidth reservation while the connection is being established.  This in turns increases the possibilities of QoS (quality of service). 

2. They produce less overhead. This is because of the fact that there is no individual packet routing and exclusion of the complete addressing info from the packet header. Each packet contains only a small VI or virtual channel identifier. The remaining routing info is provided to the network nodes during the establishment of the connection. 

3. Theoretically speaking, the nodes here have high capacity and are faster because their only task is to carry out routing. On the other hand, the network nodes in a connection-less network carry out routing for every packet individually. In switching, it requires only to look up the VCI in the table instead of analyzing the full address. Implementation of the switches is quite easy in the ASIC hardware but the complexity of the routing increases and demands software implementation. But as we know there is a huge market of the IP routers and layer 3 switching is supported by the advanced IP routers. 

Below mentioned are some protocols that provide VC facility:

- TCP or transmission control protocol

- SCTP or stream control transmission protocol

- X.25

- Frame relay

- ATM or asynchronous transfer mode

- MPLS or multi-protocol label switching

What are five key assumptions in dynamic channel allocation?

Putting the available bandwidth in operation of the cellular telephone system to efficient use is an important problem to be considered for providing good service to the largest number of customers possible. The problem has gained a critical status owing to the rapid growth of the cellular telephones users. 

- A communication channel is nothing but a band of frequencies which a number of users can use simultaneously if they are residing far apart from each other. 

- There is a minimum distance at which no interference occurs between the users and it is known as the channel reuse constraint. 

- A cellular telephone system divides the service area in to a number of regions commonly known as the cells. 

- Each of the cells has its own base station for handling the calls concerned with that cell. 

- The bandwidth of the communication channel is partitioned in to many channels permanently. 

- The cells are then allocated these channels in such a way that the channel reuse constraint is not violated by the calls. 

- There are a number of ways for allocating the channels. 

- Few of them are better than the others when it comes to reliably making channels available to all the cells. 

Few examples of channel allocation methods are:

1. Fixed assignment method
2. Dynamic allocation method
3. Reinforcement learning method

About Dynamic Method Allocation

- One type of dynamic method allocation is the BDCL or the borrowing with directional channel locking. 

- Out of all the above mentioned channel allocation methods, the dynamic allocation is considered the best one according to some studies conducted. 

- It is somewhat of the heuristic kind. 

- In dynamic allocation, the channels are allocated in the same way as in the fixed assignment method but it permits borrowing channels from the other cells whenever required. 

- It then arranges those channels in a specific order in each of the cells and this ordering is used in determining the channels for borrowing and reassigning the calls dynamically within the cells.

- There are static allocation techniques also but those don’t seem to work as well as the dynamic allocation techniques. 

In dynamic channel allocation 5 assumptions are always made which we have discussed below:

Station model: 

- There are N independent stations in the model and one frame is generated by each of the stations one at a time. 

- It is blocked until the successful transmission of the previous frame. 

- This means a station cannot queue multiple frames for transmission. 

- For example, a transmission gap of 100 bits is required during the transmission of the consecutive frames.

Single channel assumption:  

- The same medium is shared by all the stations. 

- Through it all the stations can receive and transmit.

Collision assumption: 

- A collision occurs whenever at the same time two frames are transmitted. 

- The two frames that collide have to be re-transmitted.

Transmission model: 

- There are 2 types namely, the continuous time model and the slotted time model. 

- In the former type transmission can be started at any given time. 

- In the latter model, transmission starts with a time slot.

Carrier sense: 

- It can also be classified in to 2 categories namely carrier sense and no carrier sense. 

- Stations can know if a channel is occupied prior to using it. This is called carrier sense.

- In no carrier sense, the stations cannot know whether the channel is occupied or not before transmission.

- Also, it gets difficult for the dynamic allocation method for setting up the favorable usage patterns as the calls start saturating the system. 

How to execute Performance Tests?

Performance testing involves executing the same test case multiple times with data variations for each execution, and then collating response times and computing response time statistics to compare against the formal expectations. Often, performance is different when the data used in the test case is different, as different number of rows are processed in the database, different processing and validation come into play, and so on. By executing a test case many times with different data, a statistical measure of response time can be computed that can be directly compared against a formal stated expectation.

Network sensitivity tests are variations on load tests and performance tests that focus on the Wide Area Network (WAN) limitations and network activity. Network sensitivity tests can be used to predict the impact of a given WAN segment or traffic profile on various applications that are bandwidth dependent. Network issues often arise at low levels of concurrency over low bandwidth WAN segments. Very chatty applications can appear to be more prone to response time degradation under certain conditions than other applications that actually use more bandwidth. For example, some applications may degrade to unacceptable levels of response time when a certain pattern of network traffic uses 50% of available bandwidth, while other applications are virtually un-changed in response time even with 85% of available bandwidth consumed elsewhere.

This is particularly important test for deployment of a time critical application over a WAN. Also, some front end systems such as web servers, need to work much harder with dirty communications compared with clean communications encountered on a high speed LAN in an isolated load and performance testing environment.

Performance Issues of Tertiary storage

There are three aspects of tertiary-storage performance :
- Speed : There are two aspects of speed in tertiary storage are bandwidth and
latency.
Bandwidth is measured in bytes per second.
* Sustained bandwidth – average data rate during a large transfer; # of bytes/transfer time.Data rate when the data stream is actually flowing.
* Effective bandwidth – average over the entire I/O time, including seek or locate, and cartridge switching. It drive’s overall data rate.

Access latency is the amount of time needed to locate data.
* Access time for a disk – It moves the arm to the selected cylinder and wait for the rotational latency; < 35 milliseconds.
* Access on tape requires winding the tape reels until the selected block reaches the tape head; tens or hundreds of seconds.
* Generally say that random access within a tape cartridge is about a thousand times slower than random access on disk.
The low cost of tertiary storage is a result of having many cheap cartridges share a few expensive drives. A removable library is best devoted to the storage of infrequently used data, because the library can only satisfy a relatively small number of I/O requests per hour.

- Reliability : A fixed disk drive is likely to be more reliable than a removable
disk or tape drive. An optical cartridge is likely to be more reliable than a
magnetic disk or tape. A head crash in a fixed hard disk generally destroys the data, whereas the failure of a tape drive or optical disk drive often leaves the data cartridge unharmed.

- Cost : The main memory is much more expensive than disk storage. The cost per megabyte of hard disk storage is competitive with magnetic tape if only one tape is used per drive. The cheapest tape drives and the cheapest disk drives have
had about the same storage capacity over the years. Tertiary storage gives a cost savings only when the number of cartridges is considerably larger than the number of drives.