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.

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. 

How can the problem of congestion be controlled?

Networks often get trapped in the situation of what we call network congestion. For avoiding such collapses, congestion avoidance and congestion control techniques are often used by the networks nowadays. 

In this article, we discuss about how we can control the problem of network congestion using these techniques. Few very common techniques are:

1. Exponential back off (used in CSMA/ CA protocols and Ethernet.)
2. Window reduction (used in TCP)
3. Fair queuing (used in devices such as routers)
4. The implementation of the priority schemes is another way of avoiding the negative effects of this very common problem. Priority schemes let the network transmit the packets having higher priority over the others. This way only the effects of the network congestion can be alleviated for some important transmissions. Priority schemes alone cannot solve this problem.
5. Another method is the explicit allocation of the resources of the network to certain flows. This is commonly used in CFTXOPs (contention – free transmission opportunities) providing very high speed for LAN (local area networks) over the coaxial cables and phone lines that already exist.

- The main cause of the problem of network congestion is the limited capacity of the network. 

- This is to say that the network has limited. 

- The resources also include the link throughput and the router processing time. 

- Congestion control is concerned with curbing the entry of the traffic in to the telecommunications network so that the problem of congestive collapse can be avoided. 

- The over-subscription of the link capabilities is avoided and steps are taken to reduce the resources. 

- One such step is reducing the packet transmission rate. 

- Even though if it sounds similar to flow control, it is not the same thing. 

- Frank Kelly is known as the pioneer of the theory of congestion control. 

- For describing the way in which the network wide rate allocation can be optimized by the individuals by controlling their rates, he used two theories namely the convex optimization theory and the micro economics theory. 

Some optimal rate allocation methods are:

Ø  Max – min fair allocation

Ø  Kelly’s proportional fair allocation

Ways to Classify Congestion Control Algorithm

There are 4 major ways for classifying the congestion control algorithms:

1. Amount as well as type of feedback: This classification involves judging the algorithm on the basis of multi-bit or single bit explicit signals, delay, loss and so on.
2. The performance aspect taken for improvement: Includes variable rate links, short flow advantage, fairness, links that can cause loss etc.
3. Incremental deployability: Modification is the need of sender only, modification is required by receiver and the sender, modification is needed only by the router, and modification is required by all three i.e., the sender, receiver and the router.
4. Fairness criterion being used: It includes minimum potential delay, max – min, proportional and so on.

Two major components are required for preventing network congestive collapse:

1. End to end flow control mechanism: This mechanism has been designed such that it can respond well to the congestive collapse and thus behave accordingly.
2. Mechanism in routers: This mechanism is used for dropping or reordering packets under the condition of overload.

- For repeating the dropped information correct behavior of the end point is required. 

- This indeed slows down the information transmission rate. 

- If all the end points exhibit this kind of behavior, the congestion would be lifted from the network. 

- Also, all the end points would be able to share the available bandwidth fairly. - Slow start is another strategy using which it can be ensured that the router is not overwhelmed by the new connections before congestion can be detected. 

What is optimality principle?

A network consists of nodes which require communicating with other on various grounds. This communication is established via communication channels that exist between them. The communication involves data transfers. In a network a node may or may not have a link with every other node in the network. 

Applications that require communicating over a network include:

1. Telecommunication network applications such as POTS/ PSTN, local area networks (LANs), internet, mobile phone networks and so on.

2. Distributed system applications

3. Parallel system applications

- As we mentioned above, each and every node might not be linked with every other nodes since for doing so a lot of wires and cables are required which will the whole network more complicated. 

- Therefore, we bring in the concept of the intermediate nodes. 

- The data transmitted by the source node is forwarded to the destination by these intermediate nodes. 

Now the problem that arises is which path or route will be the best to use i.e., the path with the least cost. 

- This is determined using the routing process. 

- The best path thus obtained is called the optimal route. 

- Today, we have a number of algorithms available for determining the optimal path. 

These algorithms have been classified in to two major types:

1. Non – adaptive or static algorithms
2. Adaptive or dynamic algorithms

Concept of Optimality Principle

- This is the principle followed while determining the optimal router between the two routes. 

The general statement of the principle of optimality is stated below:

“An optimal policy has the property that whatever the initial state and initial decision are, the remaining decision must constitute an optimal policy with regard to the state resulting from the first decision.”

- This means if P is an optimal state that results in another state say Q, and then the portion of the original from that state to this state i.e., from P to Q must be optimum. 

- This only means the optimality of the part of the optimal policy is preserved. - The initial state and the final state are the most important parts of the optimum. 

- Consider an example, suppose we have problem with 3 inputs and 26 states. - Here, the state is associated with the optimum and the total cost is associated with the optimum policy.

- If brute force method is used for 3 inputs and 100 stages we have the total number of computations as 3¹⁰⁰. 

- That means for solving this problem, a super computer is required.

- Therefore, the approach used for solving this problem is a parallel processing approach. 

- Here, for the each state the least step is computed and stored during the programming. 

- This reduces the number of possibilities and hence reducing the amount of computation.

- The problems become complex if the initial and the final states are undefined. - It is necessary for the problem to follow the principle of optimality in order to use the dynamic programming. 

- This implies that whatever the state may be, the decisions that follow must be optimal in regard with the state obtained from the previous decision. 

- This property is found in combinatorial problems but since they use a lot of time and memory, this method is inefficient for them. 

- These problems can be solved efficiently if some sort of best first search and pruning technique is applied.

- In regard to the routing in networks, it follows from the optimality principle if a router B lies between router A and C which lie on an optimal path, then the path between the router B and C is also an optimal path and lies on the same path. 

- Sink tree is formed as a result of all optimal routes which is the ultimate goal of all the routing algorithms.

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

Sliding Window Protocols? - Part 3

In the third part of this article we shall discuss about the types of sliding window protocols and how these protocols can be extended?

1. Stop and Wait: 

- This one is the simplest type among all the sliding window protocols. 

- Under this type, we have the stop–and–wait ARQ protocol as the simplest implementation.

- Both the transmit window and the receive window is 1 packet and the number of possible sequence numbers required is 2 i.e., 1+1 = 2. 

- The packets sent by the transmitter are marked alternatively as odd and even. 

- Therefore, the ACK packets are in the series of odd, even, odd, even and so on. 

- Now, suppose the transmitter sends an odd packet and immediately without waiting for an odd ACK sends the next even packet. 

- In such a case, it would receive an ACK saying that an odd packet is expected. 

- This leaves the transmitter in a state of ambiguity i.e., whether the receiver got both the packets or none of them.

2. Go-Back-N ARQ: 

- This sliding window protocol has a fixed w_r  (fixed at 1) and w_r  which is always greater than one. 

- Here, the receiver will not accept any packet other than the expected one from the sequence. 

- If the packet gets damaged or lost during the transmission, then the packets following the lost ne will not be accepted by the receiver until and unless it receives the lost one after re-transmission. 

- This ensures minimum loss of 1 RTT (round trip time). 

- This is why it results in inefficiency in using this protocol on the links where the packet loss is quite frequent. 

- Suppose a 3 bit sequence number is being used as in typical HDLC. 

- This means number of sequence numbers is 8 starting from 0 to 7. 

- This also means we have 8 possibilities. 

- Enough ACK information is required by the transmitter for distinguishing between those packets. 

- If 8 packets are sent back to back by the transmitter without stopping for ACK, then it will find itself in the same doubt as in the stop-and-wait case.

3. Selective repeat ARQ: 

- This one is the most general case of the sliding window protocols. 

- It works with a receiver that is more capable of accepting packets having the sequence numbers greater than what the current n_r is and storing them till the gap is filled. 

- The advantage is that discarding data before re-transmission is not necessary.

Ways to extend these protocols

1. The above types of the sliding window protocols don’t talk about reordering the packets after receiving them all. This will ensure that they don’t appear in wrong order. If the long distance can be bounded, the protocols can be extended to support this feature. The maximum mis- ordering distance can be used to expand the sequence number modulus N.
2. Not acknowledging every packet is also possible after the sending an ACK up on not receiving packets. For example, every 2^nd packet is acknowledged in TCP.
3. Informing the transmitter immediately about the presence of gap in the packet sequence is quite common and so HDLC uses a packet called REJ packet for this purpose.
4. During the communication, the window sizes may change if their sum remains in the limit defined by N. usually the transmit window size is reduced for slowing down the transmission in order to keep with the speed of the links and preventing congestion or saturation. 

Sliding Window Protocols? - Part 2

As discussed in part 1 of this article, the sliding window protocol is a type of the packet based data transmission protocol. The sliding window protocols are used in regulating the reliability factor of the data transmission. 

In this second part we discuss about the motivation behind this protocol and how it actually operates. 

- There are a number of communication protocols based up on the automatic repeat request for regulating the error control. 

- In such protocols, it becomes necessary for the receiver for acknowledging about the packets it received. 

- If the receiver does not send an ACK to the transmitter within a specified time period, then the transmitter assumes that the packet might have got lost, re-transmits it. 

- It is obvious that if a transmitter does not receives an ACK for the packet it had sent cannot actually know if the packet got delivered correctly. 

- If, suppose corruption is detected during error detection process on the receiver’s side; the receiver will simply ignore this packet and hence, will not send any ACK to the transmitter. 

- Now in the same way, the receiver also does not know whether the ACK it sent was received by the transmitter or it got lost or damaged during the transmission.

-  In such a case, the re-transmission must be acknowledged by the receiver in order to prevent the continuous re-sending of the data by the transmission. 

- In other cases it is simply ignored.

How the protocol operates?

- The current sequence numbers say n_t and n_r is assigned to transmitter and receiver respectively. 

- Both of them have their window sizes say w_t and w_r respectively.

- In simple implementations of the protocols these sizes are fixed however, they may vary when it comes to the larger and complex implementations. 

- For making any progress, it is necessary that the size of the window must be more than zero. 

- In a typical implementation, n_t denotes the packet to be transmitted. 

- Similarly n_r denotes the packet not received. 

- Both of these numbers increase with the time monotonically. 

- The receiver also has to keep an eye on the highest sequence number that has not been received yet. 

- We have another variable called n_s which is one number greater than the highest sequence number that has been received. 

- There are simple receivers which accept the packets only in the order of w_r = 1 which is nothing but same as the n_r. 

- But in some cases it can exceed 1. 

Now we can say that:

1.     Below n_r no packets have been received.

2.     Above n_s no packets have been received.

3.     It is only between n_r and n_s that some packets have been received.

- Whenever a packet is received, its variables are updated appropriately by the receiver and at the same time an ACK is transmitted with the updated value of n_r. 

- Similarly we have variable n_a used by the transmitter for tracking the highest ACK it has received. 

- Below n_a all the packets have been received but there is uncertainty about the packets between n_s and n_a i.e, the n_r. 

- There are certain rules that are always obeyed by the sequence numbers:

Ø  N_a ≤ n_r: The highest ACK the transmitter has received cannot exceed the highest n_r recorded by the receiver.

Ø  N_r ≤ n_s: The partially received packets’ end cannot be greater than the span of those fully received.

Ø  N_s ≤ n_t: The highest packet sent is always greater than the highest packet received.

Ø  N_t ≤ n_a + w_t: The highest ACK received and the window size set the limits for the highest packet sent.

Sliding Window Protocols? – Part 1

- There are many types of data transmission protocols of which one type is the packet based data transmission protocols. 

- These protocols have a feature called the sliding window protocol.

- The sliding window protocols are a great help wherever the in-order delivery of the data packets demand reliability. 

- For example, the Data link layer of the TCP (transmission control protocol) model and OSI model demand such reliability and thus use window sliding protocol. 

- According to the concept of the sliding window protocols, a consecutive number which is unique is assigned to each and every portion of the transmission i.e., the packets.

- These numbers are used by the receiver for placing the packets it will receive in their correct order. 

- Also, with the help of these numbers, the missing packets can be identified and the duplicate packets can be removed. 

- One problem regarding the sliding window protocols is that it has kept no limits for the size of these numbers that are required. 

- An unlimited number of data packets can be allowed to be communicated at any instant of time if limits are placed on the number of packets involved in transmission or reception. 

- By this, we mean using the sequence numbers of fixed size. 

- By term window we refer to the transmission side. 

- It actually represents the logical boundary or limit of the number of packets that the receiver has to acknowledge. 

- The transmitter has to be informed by the receiver for each ACK (acknowledgement) packet regarding the maximum size or the window boundary of the current receiver buffer. 

- For reporting the window size of the received buffer, a 16 bit field is used in the TCP header. 

- The maximum limit or boundary of the window that we can have is 2¹⁶ i.e., 64 KB. 

- When operating in the slow start mode, the counting of the transmitter begins with a low packet count.

- Gradually, the number of packets involved increases in every transmission after the ACK packet has been received. 

- Whenever it receives an ACK packet, the window slides logically by one packet for the transmission of a new packet. 

- On reaching the window threshold, one packet is sent by the transmitter for every one packet of ACK received. 

- Suppose the limit of the window is 10 packets and the transmitter is in slow start mode. 

- Then, first one packet will be transmitted followed by another two. 

- Between these two transmissions, it will send an ACK packet also. 

- This process will continue until the limit of 10 has reached. 

- After crossing the limit, the transmission is restricted to one i.e., for every ACK packet received only one data packet is transmitted. 

- When viewed during simulation, it seems as if the window is shifting by distance of one packet whenever an ACK packet is received. 

- For avoiding the traffic congestion, the sliding window protocol works up a great deal.

- In this way the application layer would not have to worry about transmission the next set of data packets. 

- It can continue to do so since the sliding windows of the packet buffer will be implemented on both the sides i.e., the receiver’s and the sender’s side by the TCP. 

-However, the network traffic influences the window size dynamically to a great extent. 

- In order to achieve the highest possible throughput, care should be taken for not forcing the transmitter to stop the transmission before one RTT or round trip delay time by the sliding window protocol. 

- The bandwidth delay product of the links in the communication should be less than the limit of the data amount that can be sent before sending ACK packet. - If this condition is not met, the links’ effective bandwidth will be limited by the protocol.