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A local area network ( LAN) is a sum of connected computers where extend in limited area. Local may be a netwrk with one or more rooms, a build or near builds. For example, the network of corporation, which has storage area, countinghouse and other services in the same build, constitute a local net.
As wireless network is characterized the telecommunication network, usually telephonic or computers networks which uses, radio wave as carrier information. Data carried through electromagnetic waves with carrier frequency which, is depended every time by the bit rate of data where is required to supports the network. The wireless communication, contrary to wired, not uses some type cable as transmission way. In wireless networks is subsumed the mobile networks, the satellite communications, the Wide Wireless Area Networks (WWAN), the Wireless Metropolitan Area Networks (WMAN), the Wireless Local Area Networks (WLAN) and the Wireless Personally Area Networks (WPAN).
Hidden Node Problem.
In wireless network the transmittance and receiving range of every terminal is specific and finite, with result, every terminal has a different reason for the range of middle and the sum of terminals which is connected in its. This problem which is feature of wireless networks is called hidden station problem. So, a terminal - transmitter can't detect the collision which probably happens in the place of terminal - receiving through of contemporaneous transmittance by other hidden for terminal transmitter.
Like showed in follow picture:
Picture 1. The hidden station problem, the terminal C is visible by terminal B, but hidden by terminal A.
RTS/CTS (Request to Send / Clear to Send).
RTS/CTS: is the selective mechanism which used by the wireless networking protocol (802.11) to minimize the collisions of frame inducted by the hidden station problem. Also the RTS/CTS includes ACKs (acknowledge) which sent by receiver when the data received by him.
Request To Send is a signal which is sent by the transmitting station to the receiving station requesting permission to transmit. Contrast with CTS.
RTS threshold is the threshold which is used by RTS/CTS mechanism. Defines the size of data which are sent. If the data size is smaller than this threshold then, the data are not transmitted. It used for collisions avoidance. The process is:
â-º A station send RTS in the receiver.
â-º Rts includes a prediction for transmit time and the data size.
â-º Receiver compares the data size with rts threshold.
â-º If rts threshold is higher than data size, then, the data are not sent.
â-º If rts threshold is lower than the data size, then, the data are sent.
Clear To Send is a signal which transmitted between nodes and required for begin data transmission. The CTS message silences all wireless stations in its adjacency to avoid collision and enables the sender of the RTS message to begin the data transfer.
If a node wanting to send data then, begin the process by sending a Request to Send frame (RTS). The foreordination node responds with a Clear To Send frame (CTS). The other node(s) which receiving the RTS or CTS frame avoid to sending data for a definite time. The time which required for complete the transmission and the acknowledge is included in both the RTS and the CTS frame. The RTS and CTS was designed for the same transmission range of nodes.
Typically, sending RTS/CTS frames does not occur unless the packet size exceeds this threshold. If the packet size the node wants to transmit is larger than the threshold, the RTS/CTS handshake gets triggered. Otherwise, the data frame gets sent immediately .
Access Point (AP).
Wireless access point (AP or WAP) are specially configured nodes on wireless local area networks (WLANs). Access points act as a central transmitter and receiver of WLAN radio signals. 
Is one more complex topology of wireless netting. In this topology the wireless network has cellular form, textural by a number of cells. In every cell there is an access point (AP) and a number of wireless stations which are served by AP and for this, is called clients. The cell is called by the terminology of model BSS (Basic Service Set), comprise by a number of wireless stations and an access point (AP). BSS is the fundamental essential element of wireless network.
The basic service set (BSS) is the basic building block of an IEEE 802.11 wireless LAN (according to the IEEE 802.11-1999 standard). In Infrastructure mode a single access point (AP) together with all associated stations (STAs) is called a BSS. An AP acts as a master to control the stations within that BSS. 
ESS (Extended Service Set).
ESS is the sum of one or more BSS connected and local networks (LAN) which appear like one BSS for anyone station which is related with some from these BSS.
â-º The BSS allowed to operates in the same frequency or in different frequency so as to increases the transit.
â-º The actuation becomes always through the AP (access point).
â-º The AP communicated one with other for to boost the actuation.
Aim of ESS is to increase the range of wireless coverage and the network capacity. It comprised by a number of BSS, where the APs are connected between its with a network structure DS (distribution system) transmission. With this way the movement of a wireless station by a BSS in other is feasible. The DS comprises the network body of WLAN and may be it constructed with wired LAN or wireless network. Typically, the DS is a thinness in every AP which determine the destination of data. So the DS is this which decides if the data which received by a BSS station will return in the same BSS or if it will boosted in other AP through the DS, or it will transmitted through DS in destination out of ESS, namely, off the network.
SIFS - Short Interframe Space
SIFS are found in IEEE 802.11 networks. They are used for the highest priority transmissions enabling stations with this type of information to access the radio link first.
2. LAB 1: HIDDEN NODE SCENARIO.
In this lab it will studied the hidden station problem, which, as referred in the abstract, is the problem where a transmitter can't detect that the receiver is 'busy' and transmits the data, this, result in to increase the collisions in the receiver. So, the data destroyed and the information not received correctly.
It will observed:
â-º The differences where arises when is used RTS/CTS mechanism and when not used this mechanism.
â-º The influences where arises by the hidden station problem.
For this aim it will used the OPNET and specify the OPNET IT Guru.
The OPNET IT Guru provides a different work environment which allow us to perceived the WLAN characters. As is showed in the below picture:
Picture 2. OPNET IT Guru work environment.
The work environment presents three stations:
â-º Node A.
â-º Node B.
As it observed, there are two circles which represent the signal range of the two stations:
â-º Node A.
â-ª The signal range representation by light green circle.
â-º Node B.
â-ª The signal range representation by gray circle.
The receiver can receive data both of node A and node B in this position.
The three stations are occurred in the common place between these two circles which is represented by both gray and green color. In this position the two stations (node A, node B) can detect each the other. Namely, if the node A transmits, the signal arrives in node B, so, node B detects the transmittance of node A, therefore detects the presence of node A. Of course, the vice versa takes effect.
In node A there is a white arrow, this arrow represents the way which will follow this node when time increases. In this way, node A will occurred out of node B range for some time period and then will occurred again into node B range.
For this Lab all the figures is separated in three regions:
â-º Region 1 - node A is in the range of node B (and vice versa):
â-º Region 2 - node A is out of range of node B (and vice versa).
â-º Region 3 - node A is in the range of node B (and vice versa).
Each figure is explained in base of these regions.
It required to be perceived the hidden station problem. For this aim it followed the next steps:
In this step we will do settings.
â-º It selected the node B. The attributes window is opened and we observe the inter - arrival time parameters and packet size parameters.
Picture 6. Attributes window.
For Traffic Generation Parameters:
Traffic Generation Parameters: Includes the traffic formula parameters which generated by this source.
Start time: Defines the time (in seconds) from whom starts the generation traffic. If this time is defined zero ('0'), then, the node(s) doesn't generate any traffic. Is constant.
On state time: Defines the time (in seconds) for whom are generated the packets. The generation of packets is came off only in on state time. Is constant.
Off state time: Defines the time (in seconds) for whom doesn't generated packets. Is constant.
For Packet Generation Arguments:
Packet Generation Arguments: Includes the parameters of the packet generation rate and the size of these packets.
Inter - arrival time: Defines the time between subsequent packets in state 'on state time'. Is exponential.
Packet size: Defines the size of generated packets (in bytes). The value of packet size is defined as:
â-ª minimum 1500 bytes.
â-ª maximum 2300 bytes.
Segmentation size: Defines the segments size, in this case set 'no segmentation', therefore, each generated packet sent in lower level (lower layer) whose size is defined in Packet Size attribute.
From the above arise that, the packets start generated in five ('5') seconds, after, the packets generated for three ('3') seconds (on state time) and generation stop for three ('3') seconds (after stop the on state time). It observed that the on state time is equal with off state time. The first packet is generated in three ('3') seconds by on state time, but, because the start time not starts the node(s) doesn't generate any traffic. Therefore, the first packet which generates traffic is generated in six ('6') seconds (on state time + off state time), one ('1') second after the start time.
In this step we do simulation, are presented and are analyzed the results.
â-º While it make sure that the settings of attributes window are these which want, then, click on the DES button. The follow picture is showed:
Picture 7. Configure/Run DES: 1332_WLAN_ref-hidden_node.
â-˜ The duration is 1000 seconds, it means that the simulation becomes for 1000 seconds.
After the run, click on the Hide/Show Graph Panels button to see the results.
The results are presented below in figure 1 (figure 1 includes 4 different figures):
Figure 1. Results for simulation for hidden station problem.
We will continue with analysis for each figure by above figure 1.
In the next figure is presented two graphs:
â-º Node A Wireless Lan Data traffic sent - received (bits/sec).
â-º Node B Wireless Lan Data traffic sent - received (bits/sec).
The green vertical line represent the time (timer).
In x - axis is presented the time (seconds).
In y - axis is presented the data (bits).
Region 1 (until timer):
Figure 2. Wireless Lan Data Traffic Sent (bits/sec) for Node A - B.
The node A sends and receives data until approximately 5 min and 40 seconds (340 seconds). Until this time, the data which receives node A is the data which send the node B, the transmittance - receiving becomes normally. This happens because the nodes can detect each other, so, when finishes the transmittance of anybody node, the other node can transmits. The data traffic sent - received approximately fluctuate between 300 bits/sec - 450 bits/sec.
Region 2 (between previous - below timer):
Figure 3. End of time for whom node A was hidden by node B.
In this region, node A only sends data. It happens because, node A is out of range of node B, the signal of node B not arrive in node A antenna, so, node A becomes hidden node to node B and vice versa. In this case, both of nodes send data in the receiver because detect the middle empty. In the receiver which receives both signals by nodes happens collisions and the data is destroyed. This happens for approximately 5 minutes and 20 seconds (320 seconds), like is showed below, figure 2:
In this region is perceived that the data which sent, increase. It happens because, the station not hear the other station and send the data continuity, with result increase the quantity of data. The received data in nodes are imaginarily because each node is out of range of the other node, so the signal not arrives in the antennas.
Region 3 (after the timer of figure 3):
Node A stop to be hidden by node B and comes in again in node B range and the transmittance - receiving becomes normally, like region 1.
In the next figure is presented:
â-º The average bits/sec for node A - node B.
â-º The average throughput (bits) in the receiver.
Figure 4. Average bits/sec for node A - node B and average throughput (bits) in the receiver.
In this figure it observed that the average throughput in the receiver is double than the average bits/sec in the nodes. The average is approximately:
â-º 310 bits/sec for node A.
â-º 305 bits/sec for node B.
â-º 610 bits throughput for the receiver.
The average bits/sec for nodes represents the data which received and sent by each node.
In the next figure is presented the WLAN delay (sec):
Figure 4. Delay start (sec).
The delay is fixed (approximately 0,0022 sec). The initial delay is fixed because the nodes hear each other, therefore, first hear and after transmit. Wait the end of transmittance and after transmit.
Figure 5. Delay stop (sec).
The increase of delay starts when the node A not detected by node B (and vice versa), namely, each node is out of range by the other node (hidden node). In this point the nodes send data continuity in the receiver, so, the data from both of nodes collide and destroyed there. Therefore, the receiver (which not receives only one signal) not send acknowledge in the nodes that receives the data right. For this reason, the nodes retransmission the data with result the wireless LAN is presented delay.
The delay starts increase in 6 minutes (360 seconds) and finishes in 10 minutes and 14 seconds (614 seconds). Its duration is 4 minutes and 14 seconds (254 seconds). Exactly the time where nodes not receive data.
Region 3 (after the figure 5 timer):
The delay takes again the previous value because the node A is in the range of node B, therefore, can hear it and transmit after.
In the last figure is presented the WLAN retransmission. Like is showed below:
Figure 7. Start of retransmission.
In this region it observed that when there isn't hidden node problem, the number of retransmission remain low because each node can "hear" other.
Figure 5. Delay stop (sec).
Figure 6. WLAN retransmission.
In above figure it perceived that, in the time that there is delay and the node A is out of range of node B, there is retransmission. In this time both of two nodes send data (in packets) in the receiver due to hidden node phenomenon, therefore, in the receiver arrives both of two signals. Consequently, the data collide between its and destroyed. So, the receiver not receive either signal by node A neither signal by node B. For this reason, the acknowledge by receiver not sent in nodes (node A, node B). If the receiver not send acknowledge that receive the data right in SIFS time then, the sender assumed that there is collision. So, create a random number and starts decreases it. Until nihilism this time (backoff timer) the node can't transmit. When backoff time becomes zero then, the node starts transmission (retransmission). Must be referred that the data comprised by packets. This retransmission is presented in above figure (figure 6).
Before and after delay time the node A is in the range of node B, therefore, each node can "hear" the other. The nodes send data in different time, for this, not observed increased retransmissions number.
In the above two steps it observed the WLAN behavior in hidden node problem. It described and explained the follow:
â-º Node A - B Wireless LAN Data Traffic Sent - received (bits/sec).
â-º Average bits/sec for node A - B and throughput (bits) in the receiver.
â-º Wireless LAN delay (sec).
â-º Wireless LAN retransmission.
Next step is to be observed the WLAN behavior when select "hidden node rts cts" scenario.
This scenario includes the RTS/CTS mechanism which explained in introduction.
For observation the behavior of this scenario we should follow the next steps:
In this step we do settings.
â-º We must select "hidden_node_rts_cts" from scenarios. After, choose select similar nodes and select edit attributes, the below window is appeared:
Picture 6. Node B attributes.
Rts threshold must be lower than the minimum size of packets which generated. The mechanism (RTS/CTS) is activated only if the packet is bigger than threshold.
â-º Minimum size of packets which generated is: 1500.
â- So, threshold is selected to be 1024 (bytes).
In this step we do simulation, are presented and analyzed the results.
â-º For results appearance must choose Configure/Run Discrete Event Simulation button. The follow window is appeared:
Picture 7. Configuration/Run DES.
â-ª The duration is 1000 seconds, this means that the simulation becomes for 1000 seconds.
After the run, click on the Hide/Show Graph Panels button to see the results.
The results are presented below in figure 7 (figure 7 includes 4 different figures):
Figure 7. Results of simulation for hidden_node_rts_cts scenario.
We will continue with analysis for each figure by above figure 7.
In next figure is presented the WLAN data traffic received (bits/sec):
Figure 8. WLAN data traffic received (bits/sec) for node A.
The waveform of figure 8 has many common ground with figure 2.
Each of nodes which wants to transmit data, send request in the receiver (RTS), the receiver sends a message (answer) in all nodes (CTS). This message informs the node if allows or excludes the data transmittance. So, each node knows the presence of the other node, for this reason, the nodes not send data together in the receiver. Clear to send (CTS) message includes information which informs the nodes who node transmits. When the transmittance finishes, then, the CTS informs the other node that it can transmit (and previous node that the other node transmits), so, the collisions are avoided.
Generally, the RTS/CTS control the data traffic and inform all nodes who node transmit.
Approximately until 5 minutes and 40 seconds (340 seconds) node A receives data without problem because the nodes are in the same circle (therefore, each node is in the range of the other node), there isn't hidden node problem. Nodes can detects the presence of other node, the receiving becomes normally.
In this region node A becomes hidden node by node B for approximately 5 minutes and 20 seconds (320 seconds). Node A not received data (in packets) because is out of range by node B.
After this time, node A came in the range of node B again, for this reason received data normally (like region 1 in this figure).
Generally, for this figure it can observed that, the use or not of RTS/CTS mechanism has not significant different.
In the next figure is presented the data traffic sent for node A:
Figure 9. WLAN data traffic sent for node A.
As long as there isn't hidden node problem the traffic data sent for node A is the same if it used or not RTS/CTS mechanism. The data transmittance becomes normally, because the nodes can detects each other.
In this region difference it can perceived. When use the RTS/CTS mechanism then, the data traffic is kept fixed. This happen because the RTS includes prediction for transmission time. This prediction becomes when the node sends RTS in the receiver and entered in counter which is called NAV.
â-º Until nihilism NAV counter not allowed in the node to transmit.
â-ª So, the channel is charged.
The time of prediction includes the sum of time where is required for all data transmission and acknowledge receipt.
The data quantity (data traffic) is kept fixed because becomes channel charge. The transmission becomes based in initial prediction where this prediction happens when the node was in the range of node B (no hidden node problem).
Without RTS/CTS mechanism there isn't channel charge and the node A sends the more data which can sends when is out of range by node B. Therefore, increase the data traffic sent.
This region is the same like region 1. Node A is in the range of node B and the transmission - receiving becomes normally.
Next figure is the wireless LAN delay:
Figure 10. WLAN delay.
In this figure it observed two waveforms:
a) WLAN delay with RTS/CTS.
b) WLAN delay without RTS/CTS.
a) For the first part, the delay is presented increased but fixed (approximately 0.003 seconds). It happens because the RTS/CTS mechanism, receives and transmissions data which determine the WLAN data traffic:
â-º Who node starts first the transmission.
â-º The sum of time of transmission.
â-º Who node will start the transmission after the end of previous transmission.
These operations increase the time delay.
But, like is presented in figure 10, the delay is fixed when there is the hidden node problem. This fixity happen because, during transmission not allowed in other node to transmit. Thereupon, each node transmits in different time and not together. So, avoided the collisions and delay kept fixed.
b) In this part, the delay has lower value when nodes can hear each other (until approximately 340 seconds). When there is hidden node problem (for approximately 320 seconds) delay presents increase and the reason for this, is that the nodes can't hear each other and both of nodes send data together in the receiver. So, the receiver not send acknowledge in nodes since the data which arrive in him collide between its and destroyed. Therefore, the data is transmitted again and delay increases. When node A returns again in the range of node B the delay is decreased.
Next figure presents the retransmission for node A:
Figure 11. Retransmission for node A.
The number of retransmission when is used RTS/CTS hasn't high different with retransmission number when not used RTS/CTS because nodes can hear each other.
In this region of figure becomes clear that when used the RTS/CTS mechanism the number of retransmission is reduced sufficiently. Due to the channel is charged by node which transmits for definite time, presents less collisions. This entails, lower delay and less retransmissions. As we can see the maximum packets which collide between its, with RTS/CTS, are little smaller than the minimum number of packets which collide when not used RTS/CTS.
Like as region 1, the nodes can hear each other and the retransmissions is avoided. Must referred that in region 1 - 3 there are retransmissions but are very little.
In this point is completed the presentation and analysis of results for lab 1.
3. Lab 2: INFRASTRUCTURE ESS.
An ESS WLAN network comprises by two or more BSS WLAN network. For studied an ESS WLAN network first of all must be structured and studied a BSS WLAN network. For this reason this lab is separated in two parts:
Part a) BSS WLAN network.
Part b) ESS WLAN network.
In principle, it will structure an BSS WLAN network. It will defines some properties for the WLAN stations (nodes) and after simulation it will presents and analyzed the results. Thereinafter, it will structured more BSS WLAN network which will connect between its, so, will be created an ESS WLAN network.
The environment where will be used is a building which has 4 level. In the first level occurred the server and switch and in the other levels occurred the BSS WLAN networks. For study the results must be applied the next two steps:
In this step it will presented the environment work and we will do the desired settings.
â-º We select "infrastructure_BSS" from scenario menu and the next window is showed:
Picture 8. Environment work for BSS and ESS WLAN network.
We select edit attributes for all stations and the next window is showed:
Picture 9. Part 1 by attributes window.
Picture 10. Part 2 by attributes window.
We do the next settings:
â-º Set "1" in value of "Number of Rows".
â-ª With this setting is added one row.
â-º Set "Wlan_engineer" on the "Profile Name".
â-ª Is defined name for profile.
â-º Set "1 Mbps" in "Data Rate".
â-ª Is defined data rate.
As is showed in above picture 6 - 7.
Thereinafter, select "Choose Individual DES Statistics" and continue the settings.
â-º Remote Login.
â-º Wireless LAN.
As it presented in below picture:
Picture 11. Settings of Choose Individual DES Statistics.
After these settings continue in step 2.
In this step we do simulation, are presented and analyzed the results.
â-º Click the DES button, in the window which is appeared click run.
â-˜ Must be referred that the duration is 10 minutes contrary to Lab 1 where the duration was 1000 seconds.
After the simulation run, click on the Hide/Show Graph Panels button to see the results. The results are appeared in follow figure:
Figure 12. Results of simulation for BSS WLAN network.
We will continue with analysis of the results for each figure by above figure 12.
In follow figure is presented the response time of applications:
Figure 13. Response time for applications: Email - Ftp - HTTP - Remote Login.
As it perceived in figure 13, the response time for upload application (Ftp) is higher than the other applications. The Ftp application starts after the other in a higher value and remain higher for all simulation time. The other applications fluctuate in low value, the differences between its, are very small. As it showed in figure, the classification of applications is:
Email - HTTP
These two applications have approximately the same response time. It isn't clear by figure which application has the higher response time. Email is download application.
Ftp is upload application.
Table 1. Classification of applications.
In the next figure is presented the total traffic received - sent:
Figure 15. Total traffic received - sent (bytes/second)
It observed that the total traffic sent is higher than the total traffic received. The maximum value for receiving is less higher than 1 Mbps (1 Mbps is defined by us, as, the maximum data rate, picture 7) and occurred in start of time because the WLAN is empty by applications. Also, it perceived that, the traffic values not take random values but specifics values. We can say that there are four values level in total. The traffic received fluctuates between two levels and traffic sent fluctuates among four levels.
The traffic frequency is fixed.192, 384, 512, 1024?
The next figure presents the WLAN global statistics:
Figure 16. WLAN global statistics.
This figure is separated in three parts:
Wireless LAN data dropped (bits/sec).
Wireless LAN Media Access delay (bits/sec.
In this part it observed that the WLAN drops data. This happen because the buffers are replete, therefore, the WLAN presents saturation. Buffers overflow and empty continuity. But it isn't enough because the WLAN network data are much more than the data where buffers can process. The total data of WLAN network increase, so, drops data increase too.
In this part is presented the throughput. It perceived that the throughput becomes high from the first time and remains high for all simulation time. The reason is that the WLAN data is more than the data which can process the WLAN network buffers. This mean that there is saturation in WLAN network and the WLAN drops data as it perceived above in part a. The maximum value is 800,000 bits/sec.
As it referred in two above parts the WLAN network presents saturation, so, is senses that there is delay. Delay is created when users want to send (or receive) together data in one AP. So, the throughput presents saturation because the AP capacity is full. Therefore, the WLAN buffers are saturate and present delay in processing data. The applications operations and generally all the WLAN network be hung up by the delay. The value (of delay) remains high for all simulation time (this is logic because the WLAN is saturated for all simulation time).
We must refer that the AP (in level 2) transmits not only in the same level, but transmits at all levels.
In this point the presentation and analysis of results for BSS WLAN network ended.
We continue with part b.
In this part we will created BSS WLAN network for each level (level 3, level 4) and will connect between its. So, it created an ESS WLAN network. Thereinafter, we do simulation and finally are presented and analyzed the results. For this aim we do the next two steps:
In this step we create BSS WLAN network for each level which connect between its. Also, we adjusted the parameters of BSS identifier and run the simulation.
â-º For creation an ESS WLAN network select "Infrastructure_ESS " from Scenarios/Duplicate Scenario. After, we copy the access point (AP) of level two and paste in other levels. The structure is presented below:
Picture 12. ESS WLAN network.
We select "Edit Attributes" for each AP and in "BSS Identifier" of "Wireless LAN/Wireless LAN Parameters" set the value of the level which belong. In the next picture is showed the adjustment for AP at level 2:
Picture 13. BSS Identifier for level 2.
We do the same for each station per level. Next table presents the value for each AP and each station per level.
In this step we do simulation and the results are presented and analyzed.
â-º Click the DES button, in the window which is appeared click run.
â-˜ Duration is 10 minutes.
â-ª choose"1332_infrastructure_ess" from Scenarios / Scenario Components / Importâ€¦ / Analysis Configuration.
After, click on the Hide/Show Graph Panels button to see the results. The results are appeared in follow figure:
Figure 16. Results of simulation for ESS WLAN network.
In follow figure is presented the average (in Ftp traffic Received (bytes/sec)):
Figure 17. Average (in Ftp traffic Received (bytes/sec)) for infrastructure_BSS - ESS.
In this figure it observed the difference of traffic received between two infrastructures. Is senses that when used more than one BSS network the average traffic increases. Network cover more users and provide more "ways" for data, also, the network used together by many users and this increase the traffic.
In next figure is presented the average delay - data dropped for BSS - ESS:
Figure 18. Average WLAN Delay (sec) and average WLAN data dropped (bits/sec) for BSS - ESS.
The above figure is separated in two parts:
â-º Part a) - average WLAN delay (sec) for BSS - ESS.
â-º Part b) - average WLAN data dropped (bits/sec) for BSS - ESS.
In part a) it perceived that the delay for BSS is higher than the delay for ESS. The reason which happen this, is that the more APs which are used in ESS can share the clients, so, the WLAN capacity increases. The buffers are more and can process the data more quickly than BSS. The WLAN network not presents saturation, therefore, the delay is lower (in ESS).
In part b) it observed a big difference between BSS - ESS in data dropped. In BSS the WLAN network presented saturation and for this dropped the data. In ESS the WLAN network not presents saturation due to used more APs and clients are shared. More APs entail higher capacity and more WLAN buffers. So, the WLAN buffers can process the data and not dropped it. Like is showed in above figure, the average of data dropped for ESS is very low and remains low, contrary to BSS where increases linear.
In the next figure is presented the average upload response time in Ftp:
Figure 19. Average (in Ftp Upload Response Time (sec)) for BSS - ESS.
Due to there are more APs (than BSS) the WLAN network becomes in total more quickly. The capacity increases and the application can "run" faster. Therefore, is senses that the Ftp upload response time presents reduction (in ESS). As it observed, the response time in start of time is showed low and after approximately 7 minutes is showed fixed. This happen because all the WLAN network operations are fixed after this time.
Next figure presents the average throughput in WLAN:
Figure 20. Average throughput (bits/sec) in WLAN.
As made clear by above figures, the throughput in ESS is higher than BSS because are used more APs. So, each AP adds additional capacity in WLAN with result the increase of total capacity. The users in ESS are more than BSS and for this reason the data which are sent by them are more in ESS than BSS. Therefore, the throughput is higher.
In this point the presentation and analysis of results for this workshop ended.
In this assignment, in Lab 1, is observed the hidden node station problem. We observed that when there is hidden node problem both of nodes transmit together and the data in the receiver are destroyed. So, each node retransmits the data since not get acknowledge by the receiver, coherence of these is the increase of delay and retransmissions too. In the case where each node can "hear" other, there isn't problem because transmit one after another. We use RTS/CTS mechanism and observe that the retransmissions are reduced contrary to delay which is fixed. Also, perceived that the traffic sent remain fixed due to channel charge.
In Lab 2 we constructed BSS infrastructure, where one AP (access point) was serving 4 levels. We perceived that:
â-ª Traffic send is higher than traffic received.
â-ª Ftp has increased response time than other applications.
â-ª Throughput becomes maximum from the first time.
â-ª The WLAN data dropped increase due to the buffers are saturated.
Effect of above is the increase of delay.
In ESS infrastructure where in each level there is one AP, we perceive that:
â-ª Ftp traffic in ESS infrastructure is higher than BSS infrastructure.
â-ª Delay is smaller than BSS, like the Ftp response time.
â-ª Throughput is presented increased.
Generally, we can tell that when add APs, the WLAN capacity increases.