The Media Access Delay Information Technology Essay

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Introduction

I have deliberated various Medium Access Control (MAC) protocols for Wireless local Area (WLAN) Networks. To choose the best one surrounded by the existing protocols, it is necessary to have a network model scenario and some performance metrics on the basis of which they can be assessed. In this paper, various metrics for comparing the performance of MAC protocols and a network model to carry out simulation is discussed. At the end of this chapter, results obtained from the simulation in the form of graphs will be presented.

DCF

EDCF

Metrics

Picking the correct metrics or parameters in the evaluation of the QoS (Quality of Service) mechanisms is vital to the result and rationality of the evaluation. The metrics used are Throughput, Access Delay, and End to End Delay in case of real-time multimedia traffic like VoIP, Video streaming (Video conferencing), response time in case of Telnet or Remote Login; type applications which cannot tolerate delay and loss of data. Retransmission Attempts in case a station does not get a chance due to internal collision. The following list below is the item of metric’s used:

Throughput

The Throughput for different priority levels shows how well the QoS schemes can provide service differentiation between the various priorities. The Throughput of all stations shows the utilization of the wireless medium. Wireless bandwidth is a scarce resource, so efficient use of it is vital.

Media Access Delay

We measure access delay as the time from when the data reaches the MAC layer until it is successfully transmitted out on the wireless medium. The reason for studying average access delay is that many real-time applications have a maximum tolerable delay, after which the data will be useless. Therefore, it is important to provide low delay for real-time flows.

Retransmission Attempts

Total number of Retransmission Attempts by all Wireless Local Area Network MACs in the network until either packet is successfully transmitted or it is discarded as a result of reaching short or long retry limit. For 802.11e-capable MACs, the Retransmission Attempt counts recorded under this statistic also include retry count increments due to internal collisions. This factor plays important role in Performance of WLAN.

Data Dropped

Data Dropped due to unavailability of access to medium. This factor largely affects the reliability of WLAN.

Simulation Scenario

Creating a simulation scenario that is corresponding to real world scenario is the first step of simulation. In this simulation, the wireless topology consisted of several wireless stations and one base station in the wireless LAN. The base station was connected to a wired node (Figure 2.1) which serves as a sink for the flows from the wireless domain. All wireless stations are located such that every station is able to detect a transmission from any other station, and there is no mobility in the system. This means our results will not be impacted by mobility and phenomenon such as the hidden node problem.

Figure 2.1 Wi-Fi network models

The simulation experiments are carried out using OPNET Simulator version 9.1 on Windows XP SP3. For this simulation, a data rate of 11 Mbps is chosen. Various MAC and PHY (Physical Layer of OSI) parameter values used in our experiment are according to IEEE 802.11e default values given in Table 2.1. We have run the simulation for 5 minutes for each scenario, and then compared the results obtained from them. Figure 2.1 shows a network model for the experiment.

Table 2.1: MAC and PHY parameter values used in Experiment

ATTRIBUTE

VALUE

Physical Characteristics

Direct Sequence

Data Rate (bps)

11 Mbps

Transmit Power (W)

0.005

Buffer Size (bits)

256000

BSS Identifier

Auto Assigned

Channel settings

Auto Assigned

Roaming Capability

Disabled

AP Beacon Interval (secs)

0.02

Large Packet Processing

Drop

Simulation Method

To compare the performance of DCF (Distributed Coordination Function) and EDCF (Enhanced Distributed Coordination Function) two scenarios were created; medium access in first scenario was supported by DCF and in second, EDCF protocol was used at the MAC layer. Network environment factors which were used as a benchmark configured same for both scenarios. Detailed specifications are given in the Table 2.1 showing the MAC and PHY parameters used in experiment. The performance evaluation is done by simulating both scenarios one by one in OPNET simulator and then comparing the graphs obtained.

Results

After choosing metrics, the simulation is done for 5 minutes for a scenario. Then results were gathered.

Analysis of EDCF

In case of EDCF, all four traffic classes were fed into the MAC layer from higher layer, which are corresponding to AC (0), AC (1), AC (2) and AC (3) respectively to check how efficient the new protocol is to provide service differentiation required for real time application. (Note that DCF does not support service differentiation, so no provision of Access category). For this, in the application profile of scenario (for EDCF protocol) different application was configured for different access category. Details are shown in the Table 4.1.

Table 4.1 Access Category corresponding to an application

ACCESS

Throughput of Different Access Categories

Figure 4.2 Throughputs of Different Access Categories

It is observed from figure 4.2 that the Throughput of Access category 3 is way high than the Access category 0 and 1. Throughput for Access category 2 lies in between 3 and 1. It means that Throughput for applications like Voice over IP and Video conferencing, EDCF provides maximum Throughput by providing them more priority over the other services like simple HTTP.

Media Access Delay for Different Access Categories

Figure 4.3 Wireless LAN – Media Access Delay

It is observed from figure 4.3 that the Media Access Delay for Access category 3 is at minimum among all Access categories. Media Access Delay for Access category 2 is just 3 to 4 seconds more than AC (3). It means that the medium is assigned to the application according to the priority. Thus, EDCF provides lesser Medium Access Delay for real-time applications.

Comparative Analysis of DCF and EDCF

Next step is to check the performance of both protocols in terms of Throughput, Media Access Delay, Retransmission Attempts and Data Dropped. These four metrics are determining factors in terms of overall performance of both the protocols.

Throughput

Figure 4.4 Throughput of DCF vs. EDCF

It is observed from figure 4.4 that in the first 30 seconds of simulation, Throughput of both DCF and EDCF is high, but then after that, it decreases with time and stabilizes for both protocols. Throughput in first 30 seconds is high due to less number of Retransmission Attempts (less number of backoff’s). From Graph analysis, one fact is clearly visible, that curve of DCF is marginally higher than that of EDCF. We can conclude that DCF’s overall Throughput is somewhat more than the EDCF.

Retransmission Attempts

Figure 4.5 Retransmission Attempts of DCF vs. EDCF

It is observed from figure 4.5 that in the first 30 seconds of simulation, Retransmission Attempts for both DCF and EDCF are less, but then after that, it decreases with time and stabilizes for both protocols. Retransmission Attempts in first 30 seconds are less due to less number of backoff’s assigned to wireless stations. There is a small noticeable difference between curves of Retransmission Attempts of DCF and EDCF protocol. That small difference implies that the overall Retransmission Attempts made in DCF protocols are a bit lesser than EDCF protocol.

Media Access Delay

Figure 4.6 Media Access Delay of DCF vs. EDCF

In Figure 4.6, for the first minute of simulation the Medium Access Delay for both protocols increases at equal pace, and then after that, DCF suffers somewhat lesser Access Delay than EDCF. The increase in the Medium Access Delay for both protocols is due to increase in the number of nodes competing to gain access of medium.

Data Dropped

Figure 4.7 amount of Data Dropped of DCF vs. EDCF

It is observed from figure 4.7 that the first 30 seconds of simulation, DCF suffers a sudden high Data Drop, but Data Drop in EDCF increases gradually. The reason of varying Data Drop gradually in EDCF is the service differentiation which provides priority based scheme to handle different kind of data. After 2.5 minutes of simulation, curves of Data Dropped of DCF and EDCF remain same for both protocols, EDCF finishes at less Data Dropped than DCF.

Conclusions

Conclusion

The results obtained from simulation shows that Enhanced Distribution Coordination Function provides efficient mechanism for service differentiation and hence provides quality of service to the Wireless LAN. However, this improvement comes at a cost of a decrease in quality of the lower priority traffic up to the point of starvation. The acquisition of the radio channel by the higher priority traffic is much more aggressive than for the lower priority. Higher priority traffic benefited, while lower priority traffic suffered.

In terms of overall performance (under the used simulation conditions in this particular study of QoS of Wireless LAN), DCF performs marginally well than EDCF. This happens due to reason that in EDCF mechanism, each AC function acts like a virtual station for medium access, so more collision will be expected for EDCF scenario. But in terms of Quality of Service for real-time applications (like Video conferencing) EDCF outperforms DCF.

EDCF has been purposed as the medium access control protocol for IEEE’s upcoming standard IEEE 802.11e. Presently, all of the wireless devices use DCF as the default MAC protocol and PCF as the optional functionality.

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