Computer Crime Research Paper Example | Topics and Well Written Essays - 750 words

Computer Crime - Research Paper Example In July 2012 a major security breach took place against Yahoo that resulted in 450,000 usernames and passwords to be disclosed (Gonsalves, 2012). This event raised many questions regarding the precautionary measures taken by Yahoo to protect the security of its users. Instead of storing the passwords cryptographically they were stored as plain texts which made it very easy for hackers to gain access to this confidential data. Usually usernames and passwords are stored using the cryptography technique which encrypts the data thereby hiding the information. This encryption prevents hackers from deciphering the data. Yahoo refused to give an interview but confirmed the breach saying that the data was stolen via its Contributor Network, which is one of the websites owned by Yahoo. Yahoo Contributor Network is a Yahoo website for freelance content writers that write articles for another of its website Yahoo Voice. Although only five percent of the stolen data had valid passwords, Yahoo an nounced that they were taking prompt action to fix the susceptibility that led to the exposure of the passwords. Yahoo notified the other companies about the accounts that had been hacked including LinkedIn, Gmail, Hotmail, AOL, and so on (Gonsalves, 2012). The hacker group called D33Ds Company admitted to be responsible for the breach through a statement that they published on their website. The hackers said that the security breach was meant to be a warning for Yahoo and not a threat. They also alleged to having used the program SQL injection which is normally used to send instructions using the search field or a URL to breach a badly secured website. The SQL injection allowed the hackers to gain access to the database containing the usernames and passwords (Gonsalves, 2012). The event happened just before the annual shareholder’s meetings at Yahoo and the temporary CEO Ross Levinsohn said that they were looking for a transparent strategy in order to bring back their invest or confidence (Chaykowski & Robertson, 2012). According to Jordan Robertson in an interview given to Bloomberg (Yahoo Investigating Security Breach), it is not very embarrassing for a company like Yahoo because the stolen usernames and passwords were of mostly old and inactive accounts and the number 450,000 was not huge because it represented only a small fraction of people who were affected. A spokesperson from Yahoo also previously mentioned that these numbers only made up about only one percent of the total Yahoo active users (Chaykowski & Robertson, 2012). Robertson said that the humiliating factor was the susceptibility of a company like Yahoo through a method called SQL injection. This is because it only involved very basic security measures to be prevented and Yahoo’s inability to create such a security protocol was embarrassing. As a result of the whole security breach event, Yahoo closed at $15.69 in New York and its shares slid down by 2.7% in the same year (Chayko wski & Robertson, 2012). The Yahoo event did not only affect Yahoo Mail users but because its users signed up for the content writing website Contributor Network, the breach also resulted in retrieval of password for email accounts other than Yahoo such as Gmail, Hotmail, and AOL mail. One way this event could have been

Design Of Wifi Based Tdma Protocol Information Technology Essay

Design Of Wifi Based Tdma Protocol Information Technology Essay Time division multiple access is a multiple access method for shared the channel by dividing the signal into different time slots. TDMA is successful works in cellular mobile communication for several years ago. Recently has been combined with OFDM to introduce OFDMA. TDMA also ensure fairness between nodes in the network. In vehicular scenario, we proposed TDMA protocol to work with CSMA/CA to mitigate and cope with some of the challenges in vehicular communications. In this chapter we will discuss the design of the protocol, connection messages, protocol flow and cross intercommunication between the new TDMA sublayer with CSMA/CA and PHY layers. In section 3.2 a general explanation of proposed TDMA protocol, the design of Wi-Fi-based 802.11p is discussed in Section 3.3. In Section 3.4, implementations of TDMA protocol in the simulation environment is presented. Simulation problems and implementation improvements is discussed in Section 3.5. The chapter summarization is given in Sec tion 3.6. 4.2. EXPLANATIONS OF TDMA PROTOCOL The TDMA protocol is representing as a provider client protocol, which means the protocol is centralized. The other possibility is to define a distributed or an ad hoc protocol as it is done in (Fan Yu, 2007) and (Katrin, 2009). We mean by centralized that the provider will be the only one handles the information that has given in both channels. This does not mean that all communication is going to be only unidirectional (from the provider to the client), but sometimes is going to be bidirectional communication. The provider in our case here is the RSU (Road Side Unit) and the client/station is OBUs (Onboard Units). Form now we may always use the term RSU to provider or centralized node and the OBU to client or mobile station. Here we need to implement frame of 10 ms, those frame consist of two main time slots, one of them for the control and the second one for the service or data channel, and both are using different duration. Why we chose value of 10 ms because this is currently used in many TDMA implementations i.e. WIMAX. 4.3. DESIGN OF WI-FI BASED TDMA PROTOCOL IEEE802.11 has two modes DCF and PCF. Distributed Coordination Function (DCF) relies on CSMA/CA distributed algorithm and an optional virtual carrier sense using RTS and CTS control frames (IEEE Std 802.11, 1999). If the channel is busy during the DIFS (DCF Interframe Space) interval, the station defers its transmission. Point Coordination Function (PCF) is used for infrastructure mode, which provides contention-free frame transfer for processing time-critical information transfers (W. Wang, 2003). PCF is optional in the standard and only few vendors implemented it in their adapters. Two different periods defined at PCF mode: contention-free period (CFP) and contention Period (CP). CFP uses contention free-poll frames to give stations the permission to transmit. However, PCF has many drawbacks and limitations in long distance applications (i.e. up to tens of kilometers) this due to sensitivity of the acknowledgement (ACK) messages to propagation delay which is designed for contention -free local area networks purposes. Also, once a station reserves the access to the medium, it may occupy the medium for long time without any station can interrupt its transmissions even in the high priority traffics case; i.e. if the remote station has lower data rate due to the distance, then it will take long time to release the channel (Pravin, 2003). Consequently, it has been shown that (S. Sharma, 2002) (Sridhar, 2006) TDMA based MAC is suitable for long distance propagation delay. Most of the implemented solution for long distance Wi-Fi-based network was used WiMAX like TDMA frame for conducting the PMP scenario. However, using WiMAX/TDMA above Wi-Fi is increasing the system complexity and overhead since the WiMAX/TDMA has been built for the licensed-based and Wi-Fi is built with unlicensed environment. In this research a design of TDMA over the 802.11 is presented. The function of the proposed TDMA is to disable the contention behavior of 802.11 (CSMA/CA) for contention-less MAC. In this research a new cross layer design is introduced between CSMA/CA and new logical TDMA layer, which the Wi-Fi MAC frame is encapsulated in a logical TDMA header before forwarded to IP layer. The proposed protocol stack is shown in Figure4.1. The CSMA/CA peer-to-peer protocol is disabled and replaced with TDMA peer-to-peer protocol as shown with the dot-lines. Figure.4.1. Protocol flow of the TDMA-based PMP The logical TDMA header is added between IP header and MAC header. The function of the new header is to disable the random access feature of the CSMA/CA in 802.11 and replace it by logical TDMA function, which is maintains the synchronization of the local timers in the stations and delivers protocol related parameters. The frame is shown in Figure 4.2. The proposed TDMA header contains BCCH (broadcast control channel), FCCH (frame control channel) and RA (random access). BCCH: contains general information i.e. timestamp through time_stamp_update(), SSID, BS-node capabilities and random access time interval ra_interval(). All this parameters (except the RA time interval) is prepared and copied from the beacon frame (using beacon_content()) from the Wi-Fi MAC device driver. The BCCH information helps the APs in the sleep, wakeup, transmitting and receiving times. Figure.4.2. Additional TDMA header is added to Wi-Fi frame FCCH: carries the information about the structure and format of the ongoing frame i.e. scheduler () and time_slot_builder(); containing the exact position of all slots and Tx/Rx times and guard time between them and scheduling. RACH: contains a number of random access channels (RCH). This field is uses when no schedule has been assigned to the APs in the UL fields. Non-associated APs use RA for the first contact with an AP using slot_time_request(). The flow diagram of logical control and data channels is shown in Figure 4.3. Figure 4.3: the flow of the virtual channels for the TDMA frame, First, the RACH frame is receiving if there any connection request from APs to BS. Then, BCCH, FCCH and AGCH broadcast their information, then transmit and receive users payload. Timer is controlling all the transmitted and received signals. Although, the new TDMA header is introduced at the cost of the performance due to the overhead, however, in the long distance applications with point-to-multiple-point infrastructure scenarios usually the numbers of stations are not too high compared with end-user part. In our scenario we consider 4 remote access points and one central access point (BS-node). By implementing TDMA_module() each APs would assigned with time slot within the TDMA frame. TDMA also saves power because each STA only needs to wake-up during these time slots in each frame. If new node (AP) wants to join the network it listens to the BCCH frame to get the initial parameters from the BS-node. Then it uses the RA period to send time_slot_request() request to the BS-node to r equest for time slot. The BS-node uses the FCCH field to update the new scheduling table in scheduler(). The TDMA_module() assigns time slots for APs by taking copy of the NAV (network allocation vector) information (NAV_update()) from the Wi-Fi MAC layer and modifying it according to the schedule scheme. NAV is considered as virtual carrier sensing which is limits the need for contention-based physical carrier sensing. This is done by setting new back_off_counter() and NAV_new() in the TDMA_module() which indicates the amount of time that medium will be reserved for each time slots. The BS-node set the NAV value to the frame length time plus any other necessary messages to complete the current operation to make sure that no station (AP) will access the channel during the frame transmission. Other stations count down from the NAV to 0. When the NAV has nonzero value, the scheduler () send back to the Wi-Fi MAC that indication that the medium is busy; before the NAV reaches 0, the ba ck_off() and NAV_new() update the Wi-Fi MAC with the new NAV. The destination address (DA) and source address (SA) in the MAC frame header and in the SSID is modified according to the new NAV and RR scheduling information. Figure4.4 shows illustrate the flow of the process in cross-layer concept, which is consisting of three layers: TDMA source code, wireless driver and hardware abstraction layer (HAL). The cross layer is performed between wireless driver and the source code. HAL is different for each hardware platform. The procedure of this approach is also below: Core Module: Repoint the WiFi_MAC_SAP to TDMA_MAC Point the MAC-TDMA_SAP to IP TDMA_module() { //modify the NAV vector for virtual (fake) busy network busy If NAV() not_equal_to_zero then { //copy the NAV value to new place to use it for new AP Network_entry Copy CSMA/CA/NAV() to CSMA/CA/NAV_old() Copy TDMA()/NAV_new() to CSMA/CA/NAV() } If NAV()=0 then { // call NAV_update() TDMA/NAV_update() Set back-off counter() Send the NAV_new() to scheduler() } Scheduler(){ //using round robin queue scheme Round_robin() } //time_slot_builderà ¢Ã¢â€š ¬Ã‚ ¦ Time_slot_builder(){ random_access(){ // See if there are any time slot request If time_slot_request(){ Time_slot()++ } else traffic(); } } //add the new TDMA header // send the broadcast control channel (BCCH) bcch(){timestamp(); ra_interval; SSID;BS-node capability}; //for the RA using the same etiquettes used by contention period (CP) at the MAC level fcch() { slots_time_builder() Set frame_format(){ Slot_time_interval; } 4.3.1 TDMA Protocol Flowchart In Vehicular Environment The RSU sends the beacon frame periodically with the free slots available in the TDMA frame. The OBU scans for the RSU beacon. If more than one RSU respond, comparisons are made on their (received signal strength indicator) RSSIs and the best one is selected after which the order of merits are applied on the other RSUs as first, second, etc candidates according to their RSSI signal strength. The OBU uses the beacon to synchronize its frame with the RSU after which the OBU sends the data in the free slots (in the coming uplink frame). A check is performed to find if the RSSI Figure.4.4. Implementation and incorporating TDMA in 802.11p protocol stack. OBU scan for the RSUs Beacon frame More than one Beacon frame Are received No Compare the different RSSIs and select the best and > threshold Yes Synchronization and clock Exchange with RSU Send data in the free slots In the UL sub-frame RSSI No Yes Figure4.5 TDMA Protocol Flowchart The TDMA frame structure is shown in Figure4. 3. The TDMA frame encapsulates the 802.11 frames in the payload subsection. The frame is repeated periodically for every 20msec (which is the length of the frame). Each frame contains the beacon filed, i.e. the broadcast control channel (BCCH) which comprises timestamp, SSID, and BS-node capabilities. The frame also contains the frame control channels which carries information on the structure and format of the ongoing frame, i.e. the slots scheduler which contains the exact position of all the slots and the Tx/Rx times and guard times. The GACH and the RACH are used for random access channel when the OBU needs to join the WBSS. The RACH is the channel that the OBUs use for association request. The GACH is the grant access channel that contains all the OBUs accepted for transmission in the next frame. The TDMA is using transmission opportunities (TXOPs) mechanism originally provided by the IEEE 802.11e to calculate the DL and UL time slots duration. The TXOP is a predefined start time and a maximum duration for the station to access the medium. A RSU will set its own NAV to prevent its transmission during a TXOP that has been granted through the OBU (Figure4. 6). Rather than categorizing the data traffic based on the voice, the data and the video as in the 802.11e, the data traffic priority categories are based on the OBUs channel quality. The RSU gives high priority to vehicles with high speed to send more frames before it leaves the WBSS. The vehicle with the high channel fading will get more number of slots. Of course, this mechanism will introduce performance anomaly, however, we can use any of the solutions available in the literature for the performance anomaly (Tavanti, 2007) (IEEE P802.11p/D3.0,2007). DCD feedback TDMA DCF EDCA PMD and PLCP Figure4.6 Channel fading parameters feedback for vehicles transmission priority and TXOP setting. 4.4. IMPLEMENTATIONS OF TDMA PROTOCOL IN THE SIMULATION ENVIRONMENT In the previous section, we theoretically describe the main characteristics of the protocol which we want to design. This section explains how we implemented the ideas of this thesis by the modification Code of C++ in the Network Simulation. Although the protocol that we want to design is basically a protocol of MAC, we have to put in mind that is not only changes to be made in the MAC layer will be done. We will also have to deal with the physical and application layer. From the point of view of the provider of MAC layer is the one which is responsible to handle the various types of packages (from service and control channel). In fact, the MAC layer in the side of the provider is the one which carries the multiplexing of TDMA between the two channels (and also between various services to the interior of the channel of the service). The application, in this case, will produce the packages which will be presented in each channel. From the point of view of the client layers, MAC and application are simple. The MAC layer basically responsible; to send to application the packages which the client wants to receive (packages which belong to the channel or from desire service) or throwing the packages that not requested by the client (packets from a broadcast or unicast service the client is not interested in). The application layer will be the one that which produces the packages of the request from clients to send to the provider when they are interested by a service unicast. Well as we described in detail the characteristics of the MAC protocol designed earlier, we may carry out this idea described theirs corresponds to the last version of our protocol, to reach this execution that we programmed and examined before simple versions, the changes of the force and improvements made between the versions are related to the definition of the various services offered in the section of channel service: The first version only considered a unidirectional communication between provider and client. The reason was that we only define broadcast services in the service channel. A second version consisted in defining unicast services and hence introducing a bidirectional communication between provider and client. This new version was more complex than the previous one so we decided to divide the application we had until now (called pbc3) into two sides: the application in the provider side (pbc3) and the application in the client side (pbc3Sink). This idea of defining two sides of an application or protocol layer (to simplify its implementation) is already used in other applications or protocol layers included in the simulator as TCP. The third and last version consisted in implementing the algorithm which handles the access of more than one client to the same unicast service. This was not considered in version two. When we are programming our MAC protocol, several problems raised, which are not only in our side, but also of the limitations or the restrictions of the Network Simulation. Well, in the next section4.5 we describe and explained all, it is worth to mention a principal limitation in so much it deeply influenced the execution of our protocol. The limitation comes like more parallel flow of the data in the same node started to appear (the node can be client or provider). In our case we decided to have only one provider, this provider will produce the data of both services. This means that the provider will have more application to the function in parallel and more file test in the MAC layer (see Figure 3.2), each one associated to a different data flow. That is the reason why we are interested in parallelizing. The problem is, in so far as we know, installing parallel flows in same node; is not a task easy to make in the simulator. The most common solution is composed to use nodes as parallel flows as much of an idea used when a protocol stack is defined on the two aircraft or more (like the plan of the data and management represented on Figure 2.3). This is Explained in (GMPLS, 2006), or when we wish to have a node of multi-interface, in (NS2 Notebook). The idea is: if we cannot have more than one application function in parallel in the same node, what would be the possible solution? The answer to that, like also accentuated in section4.5, is to have only one application to function in the provider, which produces various types of packages according to the time execution. This approximation also solves the problem to have more than one queue (in parallel) in the MAC layer. We will not need various queues to store various packages due to these packages arrive at the MAC layer already in the order; they must be sent. This solution simplifies the definition of the MAC layer but made the definition of the application layer to be more complex. Although the solution taken could seem rudimentary; the fact is that the difference between the theoretical solution and rectifies is not also large particularly when to think that what we want with once examine with the protocol is implemented. After the mention and explanation of this problem we can now specify how the protocol was made. We will start to explain how application is defined in all both, client and provider. Both sides application have two principal functions: one is responsible for creation and sending of the packages to the lower layers and the other is responsible to receive the packages of the lower layer. The application in the side of the provider calls the pbc3 and has two principal functions: one for send and other for receive frame. While sending the frame we basically have to create a package (by defining its title) and send it. The provider will send various types of packages according to the execution time. Basically we will have two types of packages: the packages of management in the control channel of inspection mark and the packages of the data in the excavation of service of the channel timeslot. These packages will have various headers. In case of the packages of the zones information of the header are as in Figure 4.7: Type Service_id Time_slot seqNum lastPacket node_id Send_time Payload Figure 4.7: Fields of the application header for data frames. Service_id: Field used by the provider to indicate the service whose payload is included in the frame. Time_slot: Field that shows the subtime slot when the service is offered. SeqNum: Sequence number of the packet sent. Nowadays is only used in data packets which belong to unicast services, it is used by the provider when more than one client want to receive the same private information. LastPacket: This field is related to seqNum. It is used to indicate that the packet sent is the last one. In case of management frames the header is defined by the following fields in Figure 4.8. Type Services_ Num_services node_id Send_time Payload Information Figure4.8. Fields of the application header for management frames. services_information: It is only used in management frames. It is a vector which contains the basic information about the services offered by the provider. This basic information is defined by three fields: the first field is the identifier of the service, the second field is the subtimeslot identifier and the third field is the type of the service (as we said already before the type of the service means if the service is broadcast or if it is unicast). These three fields must be defined for each service available in the provider, the Figure 4.9 shows that. Service 1 Time Slot where Type of Service 1 Service 2 Time Slot when Type of service 2 Identifier Service 1 is offered Identifier Service 2 is offered Figure 4.9: Example of the services_information buffer when two services offered. Although in our implementation the identifier of the service and the identifier of the timeslot is the same (which means the service whose identifier is the number one will be offered in the subtime slot number one), we decided to define two variables because they would have different values in future versions of the protocol. Num_services: Value used to indicate the total number of services which are going to be offered by the provider during the service channel timeslot. Once we have explained how our application works in both sides (provider and client) we must explain the main changes done in the MAC layer. When we download the NS2.33 version there was already included an implementation of IEEE802.11a protocol. We didnt want to make use of this code because it was totally oriented to guaranty the CSMA/CA with virtual carries sense mechanism, we are not interested in. there was also simple TDMA implementation included. We decided to adapt into our requirements. We basically had to change the definition of the TDMA frame and to set up both data and management MAC headers. In contrast to the application layer there are no variables defended to make use of the MAC layer through the Tcl script. If we concentrate on the physical layer, we will see that in our version of Network Simulation NS2, there were already two physical layers for wireless communications applied: the first one called the wirelessPHY and the second one called WirelessPhyExt. We are interested to use this last version of the physical channel basically it presented an important concept for us: it supports multiple arrangements of the modulation. WirelessPhyExt leaves the function with BPSK, QPSK, QAM16 and QAM64 as it is described in (Qi Chen, 2008). The modulation influences certain important characteristics such as the rate of header information and minimum sensitivity of the receiver, according to the indications the Figure 3.3 of, and consequently the period of the data of transmission and SINR necessary to receive it and to decode it. The only problem is that this new version of the wireless channel must be used together with an extension of the MAC layer called Mac802_11Ext. We were not interested in using that one for the same reason we were not interested in using the Mac802_11 version; and for that we decided to introduce the multiple modulation schemes in the WirelessPhy layer. Another important point when working low layers of the WAVE protocol stack is to think about how the channel is modelled in NS2. There are four different types of channel propagation defined and include in NS2.33, The free space model, the Two-Ray Ground reflection model, the Shadowing model and the Nakagami model. The first three models are well described in (The NS manual, 2008). 4.5 SIMULATION PROBLEMS AND IMPLIMENTATION IMPROVEMENTS Here we explained the reason for which we are interested to study the technology of TDMA in the V2I communications and the process followed to define and apply our protocol. By creating a new protocol, sometimes it is not possible to design the theoretical idea that we had because of some limitations presented by the simulator. It is also possible that our execution could be improved at the points given. We must realize that although the protocol seems to be complex sometimes, many improvements could be made to obtain the best and more specific results. The idea of this section is just to explain the main problems found when elaborating our protocol and to suggest some future improvements. If we refer first to the problems found when we were working we must clarify that most of them are not really problems (in the sense of bugs found when executing the protocol) but limitations the simulator has which do not allow us to define the protocol as we wanted to. There are three main limitations we want to point out: The first is already mentioned in the previous section. The problem is related to the parallel data flows in a node. We were interested by this fact of being able not only to have more application to the function in parallel in the same node (as explained in the section 4.4) but to also define the two planes of protocol, data and planes of management, in the same node (that we can see Figure 2.3) although this last idea was thrown it required of much work to make. In section 4.4 we adopted easy and the fast solution which does not have affected the results obtained. But there is other solutions, simplest is composed to define nodes as many, in the code of TCL, as data flows we need and link these nodes through a router. To explain it easily: we will have one node per each data queue (see Figure 3.3) and one router that handle the information from each node. This solution is based in the actual implementation of the Diffserv queues in the NS (Definition of physical queues) where virtual and physical queues are used (Implementing multiqueue). In our case we will need at least two nodes: one for the data of the control channel and the other for the data of the service channel, in case only one service is offered. We must realize this solution involves changes in the Tcl code which leads to a simple C++ implementation. Another solution, which could be considered as an improvement is to have only one application running on the provider that generates different types of packets but instead of doing it as a function of the execution time, it could generate them randomly and give the work of organize them to the link layer. In this case we will need to define an algorithm in charge of finding the desired packets in the unique queue that exists in the link layer and sending them in the correct order to the MAC layer. There is a third solution which allows having an implementation closer to the one specified in the standards ((IEEE 802.11, 2007) and (Implementing multiqueue)). The idea consists of adapting the definition of the queue done nowadays in the implementation 802.11e standard which is included in the simulator. As we can see in (Design and verification, 2003) this implementation requires changes in the definition of the class queue which allows having multiples queues by creating them in an array (Evaluation of IEEE 802.11e). The source code of this new type of queue can be found in (Evaluation of IEEE 802.11e). The second limitation is related to the synchronization of the nodes. Those nodes can be an OBU or a RSU. The NS is a simulator based in events controlled by timers. The fact is, as it is pointed out in IEEE 1609.4 standard in (Yunpeng, 2007) all the nodes require to be synchronized before communication. The synchronization is especially important when using TDMA technology and it is a process which will be carried on when any OBU enters in the communication area of a new RSU in a centralized system. The fact is that in the NS tool all the nodes implemented (in the Tcl code) have the same time basis which means they do not need any synchronization because they are already synchronized. If we are interested in defining the synchronization process we should first desynchronize the nodes by manipulating their timers. In our case we will consider the RSU time basis to be the one the other nodes must to synchronize on. Each OBU will have to follow a synchronization process before receiving data frames from the RSU. The idea could be the following: the first time an OBU receives frames from a new RSU it gets the timestamp of the RSU and, after adding the delay produced by the propagation of the frame to this timestamp, adjusts its timers. Calculate the delay or time difference between the RSU and the OBU is not complicated. The only idea which does not seem clear is how to set up different time basis in the nodes. The third and last limitation is related to the anti-collisions mechanism used in the MAC layer mainly based in CSMA/CA algorithm. We detected the problem when executing our code: we found there were collisions between request frames when a considered number of OBUs were interested in receiving information about the same unicast service. Those collisions should not take place if we keep in mind each node is supposed to be able of sense the medium to see whether it is busy before sending any kind of frame. Why this type of collisions is produced? As we explained earlier, it is necessary to present intervals of guard to the end of each time slit to avoid collisions between the reinforcements produced by various devices (OBUs and RSU in our case) but in this case the collisions due are produced to different OBUs send braces of request really narrowly in time and, because of them cannot detect if the medium is with vacuum or not, a collision is produced and detected by the RSU. When working with the NS tool we are not able to do all the things we want to, not only because of some restrictions or limitations the tool has (as we explained before) but also because of time we did not implement all the ideas which came to our mind and we must simplify and focus our work. Because of this lack of time there exist a lot of points in our protocol which could be improved. Some of these points are explained in the following paragraphs. If we focus in the implementation of the control channel the most important improvement which could be done is to introduce critical frames and implement the process each node has to follow when receiving those frames. Introducing those types of frames will be really interesting because we can see how to handle both types of information (critical and no critical) and we would make a better use of the control channel than we do in our actual implementation. If we pay attention to the service channel there are some things which could be improved, the ideas are summarized in the following points: In the actual implementation there is not any prioritization between nodes, which means when two or more nodes want to receive the same information (which is unicast) the first who ask for it is the first which receives the data. One possibility is to define the priority as a function of the position of each OBU with respect to the RSU. It sounds coherent to give higher priority to the nodes that are closer to the RSU because of their small latency (time necessary to consume a service). Basically the latency is smaller because the propagation time (one of the terms

Abscisic Acid and Stomatal Closure :: essays research papers

Abscisic Acid and Stomatal Closure Abscisic acid is a single compound unlike the auxins, gibberellins, and cytokinins. It was called "abscisin II" originally because it was thought to play a major role in abscission of fruits. Though ABA generally is thought to play mostly inhibitory roles, it has many promoting functions as well. In 1963, abscisic acid was first identified and characterized by Frederick Addicott and his associates. They were studying compounds responsible for the abscission of fruits (cotton). Two compounds were isolated and called abscisin I and abscisin II. Abscisin II is presently called abscisic acid. ABA is a naturally occurring compound in plants. It is a sesquiterpenoid (15-carbon) which is partially produced via a certain pathway (mevalonic pathway) in chloroplasts and other plastids. Because it is sythesized partially in the chloroplasts, it makes sense that biosynthesis primarily occurs in the leaves. The production of ABA is accentuated by stresses such as water loss and freezing temperatures. It is believed that biosynthesis occurs indirectly through the production of carotenoids. Carotenoids are pigments produced by the chloroplast which have 40 carbons. Breakdown of these carotenoids occurs in a complex mechanism which produces ABA. The transport of ABA can occur in both xylem and phloem tissues. It can also be translocated through paranchyma cells. The movement of abscisic acid in plants does not exhibit polarity like auxins. ABA is capable of moving both up and down the stem. The various roles of ABA are †¢ Stimulates the closure of stomata (water stress brings about an increase in ABA synthesis). †¢ Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots. †¢ Induces seeds to synthesize storage proteins. †¢ Has some effect on induction and maintanance of dormancy. (This information taken from http://www.plant-hormones.info/abscisicacid.htm) Stomatal Closure †¢ Addition of ABA to the growth medium (a mixture of vermiculite and peat moss) causes the closure of the stomates within 3 h and an increase in the Pos of the protoplasts of the aba1 plants to 50 mm s-1. †¢ Arrival of ABA in the leaves appears to signal stomatal closure as well as a change in the Pos of the plasma membranes. †¢ We hypothesize that under non-stress conditions ABA is required to maintain a population of actively functioning aquaporins at the plasma membrane.

Introduction Internet Protocol Suite Essay

The Internet protocol suite is the set of communications protocols used for the Internet and similar networks, and generally the most popularprotocol stack for wide area networks. It is commonly known as TCP/IP, because of its most important protocols: Transmission Control Protocol(TCP) and Internet Protocol (IP), which were the first networking protocols defined in this standard. It is occasionally known as the DoD model due to the foundational influence of the ARPANET in the 1970s (operated by DARPA, an agency of the United States Department of Defense). TCP/IP provides end-to-end connectivity specifying how data should be formatted, addressed, transmitted, routed and received at the destination. It has four abstraction layers, each with its own protocols. From lowest to highest, the layers are: The link layer (commonly Ethernet) contains communication technologies for a local network. The internet layer (IP) connects local networks, thus establishing internetworking. The transport layer (TCP) handles host-to-host communication. See more: introduction paragraph example The application layer (for example HTTP) contains all protocols for specific data communications services on a process-to-process level (for example how a web browser communicates with a web server). The TCP/IP model and related protocols are maintained by the Internet Engineering Task Force (IETF). SRI First Internetworked Connection diagram Layers in the Internet protocol suite Two Internet hosts connected via two routers and the corresponding layers used at each hop. The application on each host executes read and write operations as if the processes were directly connected to each other by some kind of data pipe. Every other detail of the communication is hidden from each process. The underlying mechanisms that transmit data between the host computers are located in the lower protocol layers. Encapsulation of application data descending through the layers described in RFC 1122 The Internet protocol suite uses encapsulation to provide abstraction of protocols and services. Encapsulation is usually aligned with the division of the protocol suite into layers of general functionality. In general, an application (the highest level of the model) uses a set of protocols to send its data down the layers, being further encapsulated at each level. The â€Å"layers† of the protocol suite near the top are logically closer to the user application, while those near the bottom are logically closer to the physical transmission of the data. Viewing layers as providing or consuming a service is a method ofabstraction to isolate upper layer protocols from the nitty-gritty detail of transmitting bits over, for example, Ethernet and collision detection, while the lower layers avoid having to know the details of each and every application and its protocol. Even when the layers are examined, the assorted architectural documents—there is no single architectural model such as ISO 7498, the Open Systems Interconnection (OSI) model—have fewer and less rigidly defined layers than the OSI model, and thus provide an easier fit for real-world protocols. In point of fact, one frequently referenced document, RFC 1958, does not contain a stack of layers. The lack of emphasis on layering is a strong difference between the IETF and OSI approaches. It only refers to the existence of the â€Å"internetworking layer† and generally to â€Å"upper layers†; this document was intended as a 1996 â€Å"snapshot† of the architecture: â€Å"The Internet and its architecture have grown in evolutionary fashion from modest beginnings, rather than from a Grand Plan. While this process of evolution is one of the main reasons for the technology’s success, it nevertheless seems useful to record a snapshot of the current principles of the Internet architecture. RFC 1122, entitled Host Requirements, is structured in paragraphs referring to layers, but the document refers to many other architectural principles not emphasizing layering. It loosely defines a four-layer model, with the layers having names, not numbers, as follows: †¢Application layer (process-to-process): This is the scope within which applications create user data and communicate this data to other processes or applications on another or the same host. The communications partners are often called peers. This is where the â€Å"higher level† protocols such as SMTP, FTP, SSH, HTTP, etc. operate. †¢Transport layer (host-to-host): The transport layer constitutes the networking regime between two network hosts, either on the local network or on remote networks separated by routers. The transport layer provides a uniform networking interface that hides the actual topology (layout) of the underlying network connections. This is where flow-control, error-correction, and connection protocols exist, such as TCP. This layer deals with opening and maintaining connections between Internet hosts. Internet layer (internetworking): The internet layer has the task of exchanging datagrams across network boundaries. It is therefore also referred to as the layer that establishes internetworking, indeed, it defines and establishes the Internet. This layer defines the addressing and routing structures used for the TCP/IP protocol suite. The primary protocol in this scope is the Internet Protocol, which defines IP addresses. Its function in routing is to transport datagrams to the next IP router that has the connectivity to a network closer to the final data destination. Link layer: This layer defines the networking methods within the scope of the local network link on which hosts communicate without intervening routers. This layer describes the protocols used to describe the local network topology and the interfaces needed to effect transmission of Internet layer datagrams to next-neighbor hosts. (cf. the OSI data link layer). The Internet protocol suite and the layered protocol stack design were in use before the OSI model was established. Since then, the TCP/IP model has been compared with the OSI model in books and classrooms, which often results in confusion because the two models use different assumptions, including about the relative importance of strict layering. This abstraction also allows upper layers to provide services that the lower layers cannot, or choose not, to provide. Again, the original OSI model was extended to include connectionless services (OSIRM CL). For example, IP is not designed to be reliable and is a best effort delivery protocol. This means that all transport layer implementations must choose whether or not to provide reliability and to what degree. UDP provides data integrity (via a checksum) but does not guarantee delivery; TCP provides both data integrity and delivery guarantee (by retransmitting until the receiver acknowledges the reception of the packet). This model lacks the formalism of the OSI model and associated documents, but the IETF does not use a formal model and does not consider this a limitation, as in the comment by David D. Clark, â€Å"We reject: kings, presidents and voting. We believe in: rough consensus and running code. † Criticisms of this model, which have been made with respect to the OSI model, often do not consider ISO’s later extensions to that model. 1. For multiaccess links with their own addressing systems (e. g. Ethernet) an address mapping protocol is needed. Such protocols can be considered to be below IP but above the existing link system. While the IETF does not use the terminology, this is a subnetwork dependent convergence facility according to an extension to the OSI model, the internal organization of the network layer (IONL). . ICMP & IGMP operate on top of IP but do not transport data like UDP or TCP. Again, this functionality exists as layer management extensions to the OSI model, in its Management Framework (OSIRM MF) . 3. The SSL/TLS library operates above the transport layer (uses TCP) but below application protocols. Again, there was no intention, on the part of the designers of these protocols, to comply with OSI architecture. 4. The link is treated like a black box here. This is fine for discussing IP (since the whole point of IP is it will run over virtually anything). The IETF explicitly does not intend to discuss transmission systems, which is a less academic but practical alternative to the OSI model. The following is a description of each layer in the TCP/IP networking model starting from the lowest level. Link layer The link layer is the networking scope of the local network connection to which a host is attached. This regime is called the link in Internet literature. This is the lowest component layer of the Internet protocols, as TCP/IP is designed to be hardware independent. As a result TCP/IP is able to be implemented on top of virtually any hardware networking technology. The link layer is used to move packets between the Internet layer interfaces of two different hosts on the same link. The processes of transmitting and receiving packets on a given link can be controlled both in the software device driver for the network card, as well as on firmware or specialized chipsets. These will perform data link functions such as adding a packet header to prepare it for transmission, then actually transmit the frame over a physical medium. The TCP/IP model includes specifications of translating the network addressing methods used in the Internet Protocol to data link addressing, such as Media Access Control (MAC), however all other aspects below that level are implicitly assumed to exist in the link layer, but are not explicitly defined. This is also the layer where packets may be selected to be sent over a virtual private network or other networking tunnel. In this scenario, the link layer data may be considered application data which traverses another instantiation of the IP stack for transmission or reception over another IP connection. Such a connection, or virtual link, may be established with a transport protocol or even an application scope protocol that serves as a tunnel in the link layer of the protocol stack. Thus, the TCP/IP model does not dictate a strict hierarchical encapsulation sequence. Internet layer The internet layer has the responsibility of sending packets across potentially multiple networks. Internetworking requires sending data from the source network to the destination network. This process is called routing In the Internet protocol suite, the Internet Protocol performs two basic functions: †¢Host addressing and identification: This is accomplished with a hierarchical addressing system (see IP address). †¢Packet routing: This is the basic task of sending packets of data (datagrams) from source to destination by sending them to the next network node (router) closer to the final destination. The internet layer is not only agnostic of application data structures at the transport layer, but it also does not distinguish between operation of the various transport layer protocols. So, IP can carry data for a variety of different upper layer protocols. These protocols are each identified by a unique protocol number: for example, Internet Control Message Protocol (ICMP) and Internet Group Management Protocol (IGMP) are protocols 1 and 2, respectively. Some of the protocols carried by IP, such as ICMP (used to transmit diagnostic information about IP transmission) and IGMP (used to manage IP Multicast data) are layered on top of IP but perform internetworking functions. This illustrates the differences in the architecture of the TCP/IP stack of the Internet and the OSI model. The internet layer only provides an unreliable datagram transmission facility between hosts located on potentially different IP networks by forwarding the transport layer datagrams to an appropriate next-hop router for further relaying to its destination. With this functionality, the internet layer makes possible internetworking, the interworking of different IP networks, and it essentially establishes the Internet. The Internet Protocol is the rincipal component of the internet layer, and it defines two addressing systems to identify network hosts computers, and to locate them on the network. The original address system of the ARPANET and its successor, the Internet, is Internet Protocol version 4 (IPv4). It uses a 32-bit IP address and is therefore capable of identifying approximately four billion hosts. This limitation was eliminated by the standardization of Internet Protoc ol version 6 (IPv6) in 1998, and beginning production implementations in approximately 2006.

Eugenics: the Artificial Selection

S R August 26, 2008 Biology 340 Eugenics: The Artificial Selection In the 1800’s, well-known biologist, Charles Darwin enlightened us with his theory of evolution and natural selection. In short, natural selection states that random genetic changes transpire within an organism's genetic code, such changes are preserved because they are valuable for survival. Darwin’s ideas came from economics applied to biology. By the late 1800’s Francis Galton, Darwin’s cousin, had thoroughly studied his cousins findings and disclosed his beliefs in biology, which he related to human beings.His philosophy was known as Eugenics. Eugenics was an idea was based on ways to control reproduction so that human race can better succeed, in other words, a revised sequel of Darwin’s natural selection, artificial selection. Galton understood that good advancement of mankind was let down by generous outreach to the underprivileged when such hard work motivated people to have m ore children. Galton sought after expanding his eugenics idealism from science to a policy and religion. This science was a form of perfecting the human race through improved reproduction.That alone should have scared people, however, it began to evolve, as Galton desired. Eugenicist aspired the development of advantageous characteristics and abolition of the adverse ones. Eugenics was seen as a means to resolve the combined problems because it located the cause in the flawed germ cells within the embryo of individuals of certain ethnic groups instead of focusing on the structure of society. Eugenicist alleged that inherited disorders with basic modes of inheritance could be construed from derivations of inheritance contained by families, such as polydactyl.Psychiatric disorders, such as manic depression, were also considered when researching inherited disorders. Geneticist understood that behavioral personality had the utmost impact on society. They assumed that people inherited a trait that made them have an affinity to be poor. This was termed as shiftlessness. Other predispositions that were deemed inherited were alcoholism and sexual immorality, which was a notion to be part of the cause to criminal behavior.In the 1920’s restrictive immigration laws were passed because eugenicist believed that undesirable traits were linked to racial and ethnic groups, which was detrimental to society. Furthermore, eugenics was incorporate by Adolf Hitler’s attempt at a master race. His barbaric behavior and massacre of Jews made the initiative of developing a master race became exceedingly ostracized. However, eugenics should not be considered eradicated from society. Some believe that the advancement and encouragement of birth control is considered a major eugenic success.It was seen as a eugenic success because of a sudden increase in population and frenzy to control this. I understand that eugenics may be intended to promote reproduction among the genet ically advantaged. Therefore, in that sense I find it to be acceptable to some extent. The practices that follow this type of positive eugenics, nowadays, is slowly being accepted in society. Such as, in vitro fertilization, a couple may find this option as a gift in life. However, others may argue that adoption is always the best option when infertility is an issue.The fact that there are millions of children that do not have parents, would make me opt for child adoption. As previously mentioned, birth control is considered a eugenic success. In this area I would agree with that generalization. I find that women should take care of themselves form of eugenics that I consider eugenics negative when the attempt is to subordinate fertility amongst the genetically underprivileged. Such an example would be to have an abortion knowing that your child will have a disability.Overall, this subject matter is more complicated and thought provoking that it may seem when once introduced. Howeve r, the evolution of society has made modifications to the early eugenic standards.Work Cited EugenicsArchive. org Image Archive on the American Eugenics Movement http://www. eugenicsarchive. org/eugenics/list3. pl Adams, Mark, ed. The Wellborn Science: Eugenics in Germany, France, Brazil and Russia (New York, Oxford: Oxford University Press, 1990 Neo Eugenics: http://neoeugenics. home. comcast. net/~neoeugenics/ Future Generations: http://www. eugenics. net/links/othrlink. html