**MAC-Layer QoS Evaluation Metrics for IEEE 802.11e-EDCF Protocol over Nodes' Mobility Constraints**

Khaled Dridi1, Boubaker Daachi1 and Karim Djouani1,2 *1Laboratory of Images Signals and Intelligent Systems, Paris-East University 2F'SATI Institute of Technology/TUT University 1France 2South Africa* 

#### **1. Introduction**

62 Advanced Wireless LAN

[15] Thomas Paul and Tokunbo Ogunfunmi, "Wireless LAN Comes of Age: Understanding

[16] Wireless Gigabit Alliance, "WiGig White Paper: Defining the Future of Multi-Gigabit

[17] Ryuta Imashioya, Wahyul Amien Syafei, Yuhei Nagao, Masayuki Kurosaki, Baiko SAI

[18] Chang Soon Choi, Eckhard Grass and Maxim Piz, "Performance Evaluation of Gbps

[19] Chi-han Kao, "Performance of the IEEE 802.11a Wireless LAN Standard over Frequency-selective, Slow, Ricean Fading Channels," Master's Thesis, Sep. 2002. [20] Giuseppe Bianchi, "Performance Analysis of the IEEE 802.11 Distributed Coordination

[21] IEEE Std 802.11n 2009 " Part11: Wireless LAN Medium Access Control(MAC) and

[22] Ha Cheol Lee, "A MAC Throughput over Rayleigh Fading Channel in The

802.11a/g/n-based Mobile LAN," MESH 2011, Aug. 2011

Wireless Communications, July 2010, pp. 2-5

Aspect," ISCIT 2009, pp. 296-301

2008, pp. 28-54

547, Mar. 2000.

2009.

2009.

the IEEE 802.11n Amendment," IEEE Circuits and Systems Magazine, First Quarter

and Hiroshi Ochi, " RTL Design of 1.2Gbps MIMO WLAN System and Its Business

OFDM PHY Layers for 60-GHz Wireless LAN Applications," IEEE Conference,

Function," *IEEE Journal on Selected Areas in Communications*, Vol. 18, No.3, pp. 535-

Physical Layer (PHY) specifications: Enhancements for Higher Throughput," Oct.

Although wireless networks suffer from limit bandwidth, higher bit-error rates (BER's), significant amount of delay, and lower security than wired networks, still they have been emerged as an existing technology for the broadband wireless access, like IEEE 802.11 WLAN. Being fair for sharing medium resources considers the main reason for these weaknesses. Furthermore, Quality of Service (QoS) mechanism, in the recent version of the standard, should improve service differentiation among various types of traffic. It challenged to manage collisions and to support channel variation. Beside, wireless networks are more likely to have higher-ranking on flexibility by allowing easy setting up. If the IEEE 802.11 standard family provides the guarantee for connectivity, sufficient local coverage, required security and enough compatibility with the existing technologies, it is highly expected to carry on real-time applications requirements (Andreadis, 2006). Particularly, the EDCF MAC protocol, which improved a set of parameters, defines the classes of priority for the channel access mechanism during the contention-based period (CP). This can subsequently be declined to a variation of network dynamicity. In fact, when a mobile node crosses the overlay area with other connected nodes, the data transfer can be affected during the handoff intervals. The MAC process fails synchronization and it will be considerably corrupted by generating an amount of packets loss. This effect can be highly intensive depending on the increase of node's mobility rate. Consequently, EDCF protocol loses capacity for QoS delivery and can be reverts to a DCF behaviour reached the threshold limit of stability. Reliability analysis of different traffic classes (video, voice and data), without considering both network topology and node's mobility constraint, is not well appropriated. Dealing with this recent constraint, we propose a study which allows to know how EDCF react facing nodes mobility referring to the MAC protocol stability region. The functional analysis allowed us to follow the mobility of the node and identify the high-rated packets loss areas. To reduce this impact, we specify an algorithm, which operates in different network topology, called multi-coverage algorithm for approving the medium access mechanism. This approach can support overlapping adjacent coverage wireless ranges. For performance evaluation, we studied the most common measured metrics: effective throughput, end-to-end delay and jitter bound. To complete the study, we proposed a mobile scenario, between two adjacent and overlapping wireless stations, within three ranges of mobility rates: low, medium and high. Furthermore, we project to present some issues to improve the behaviour of the protocol by correcting session time BS's hand-off association. Evaluation of the simulation results within three modes of mobility combining the main MAC metrics are detailed and summarised as a user's guide based-on traffic priority scheme.

#### **2. IEEE 802.11 MAC legacy**

The standard WLAN IEEE 802.11 used Best-effort service model built on FIFO queuing mechanism. The access mode is based upon two different access methods; the mandatory Distributed Coordination Function (DCF) operates in Contention Period (CP) and Point Coordination Function (PCF) for the polling during Contention Free Period (CFP) (IEEE Std. 802.11-1999).

#### **2.1 DCF**

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol is used to regulate the access in the shared medium. So, all wireless nodes have simultaneous access to the same channel resources. If a node wants to transmit, it first senses the medium. The frame is transmitted when the medium is idle for at least a DCF inter-frame space (DIFS) period of time. If the wireless medium is busy, the node chooses a backoff time slot, *B*, consisting of a random number within the Contention Window (CW) interval values (0 to *CW*). This counter, according to each station, is decremented by one when the medium is detected idle for at least one DIFS. Now, when the medium is busy, the *B* timer is frozen (the backoff value is paused to the current value till the state of medium will change). It will be reactivated when the medium becomes free for the next DIFS space. The MAC layer frame is transmitted only when the backoff timer reaches the zero bound.

If a node does not receive the acknowledgement (ACK) frame, it is considered that collision has occurred and the contention window, *W*, is doubled, as:

$$W\_n = 2^{c+n-1} - 1 \tag{1}$$

Where *n* is the number of transmission attempts along with the current one for the frame.

*c* is a constant, which defines the minimum contention window, as:

$$c = \log\_2(W\_{\min} + 1) \tag{2}$$

To start a new backoff process, a new backoff time is chosen. Before sending a new frame after a successful transmission, the backoff mechanism is once more activated. When a transmission is successful, the contention window will reset to ����.

#### **2.2 PCF**

This coordination function is related to an Access Point (AP) based network topology. The AP performs as Point Coordinator (PC). So, PCF corresponds to a centralized and polling-

throughput, end-to-end delay and jitter bound. To complete the study, we proposed a mobile scenario, between two adjacent and overlapping wireless stations, within three ranges of mobility rates: low, medium and high. Furthermore, we project to present some issues to improve the behaviour of the protocol by correcting session time BS's hand-off association. Evaluation of the simulation results within three modes of mobility combining the main MAC metrics are detailed and summarised as a user's guide based-on traffic

The standard WLAN IEEE 802.11 used Best-effort service model built on FIFO queuing mechanism. The access mode is based upon two different access methods; the mandatory Distributed Coordination Function (DCF) operates in Contention Period (CP) and Point Coordination Function (PCF) for the polling during Contention Free Period (CFP) (IEEE Std.

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol is used to regulate the access in the shared medium. So, all wireless nodes have simultaneous access to the same channel resources. If a node wants to transmit, it first senses the medium. The frame is transmitted when the medium is idle for at least a DCF inter-frame space (DIFS) period of time. If the wireless medium is busy, the node chooses a backoff time slot, *B*, consisting of a random number within the Contention Window (CW) interval values (0 to *CW*). This counter, according to each station, is decremented by one when the medium is detected idle for at least one DIFS. Now, when the medium is busy, the *B* timer is frozen (the backoff value is paused to the current value till the state of medium will change). It will be reactivated when the medium becomes free for the next DIFS space. The MAC layer frame is

If a node does not receive the acknowledgement (ACK) frame, it is considered that collision

Where *n* is the number of transmission attempts along with the current one for the frame.

To start a new backoff process, a new backoff time is chosen. Before sending a new frame after a successful transmission, the backoff mechanism is once more activated. When a

This coordination function is related to an Access Point (AP) based network topology. The AP performs as Point Coordinator (PC). So, PCF corresponds to a centralized and polling-

�� = 2����� − 1 (1)

� = ��������� + 1) (2)

transmitted only when the backoff timer reaches the zero bound.

*c* is a constant, which defines the minimum contention window, as:

transmission is successful, the contention window will reset to ����.

has occurred and the contention window, *W*, is doubled, as:

priority scheme.

802.11-1999).

**2.1 DCF** 

**2.2 PCF** 

**2. IEEE 802.11 MAC legacy** 

based access mechanism. The condition for the coexistence of both DCF and PCF is the support of PCF. Within a super-frame, to start a CFP, the base station transmits a beacon frame. Once CFP is started, PC maintains a list of the nodes which have demanded to be polled for transmitting data and then it sends poll frames to the nodes. On response, the nodes transmit data packets. A Shorter IFS space is introduced between the PCF data frames to prevent interfering with the DCF mode.

#### **2.3 Quality of service and the IEEE 802.11 standard**

Service differentiation is one of the most required strategies to manage and improve peaktime network congestion of diverse class of traffic which combining voice, video and data flows. As, IEEE 802.11 standard initially provided a wireless transmission operation mode for the closed local network area, it shows very poor performance regarding link utilization during the competitive applications access (Visser & El-Zarki, 1995). To overcome this weakness and satisfy the service performance across the network, Quality of Service (QoS) concept is proposed. It refers to the ability of a network for providing desired handling of the traffic requirements which meets the expectations of the end applications (Mellouk, 2009). If a network supports a set of traffic specifications, as: bandwidth, transmission delay, jitter-bound, data path-loss, etc then it is supposed to support QoS delivery. Several new mechanisms for service differentiation have been proposed (Dongxia & al., 2009) (I-Shyan & Jheng-Han, 2008) (Whe-Dar & Der-Jiunn, 2008). The quality of traffics including video streaming is getting better performance when the characteristics of the wireless networks are taken into account. The earlier IEEE 802.11 standard treats packets of all traffic categories at the same priority level. Therefore, delay-dependent traffics suffer from network congestion and bandwidth variations.

#### **2.3.1 DCF QoS limitation**

DCF coordination function is based on "Best-effort" service model. All nodes compete with the same priority for access to the channel. Moreover, time-bounded multimedia applications require strict bandwidth, delay, and jitter guarantee. Accordingly, there is no service differentiation mechanism to specify and offer better service for prioritized applications than the rest of the traffic.

#### **2.3.2 PCF QoS limitation**

PCF mechanism itself weakens QoS; Firstly, IEEE802.11a creates a delay of 4.9 ms because this access process uses beacons frames to separate the two access modes CP and PCF. Secondly, all types of traffics must pass through AP. This condition causes decrease in bandwidth. Thirdly, Mac Service Data Unit (MSDU) size is affected by the transmission of data in different sizes, which makes QoS uncertain for remaining CFP period.

#### **2.4 Toward supporting QoS in IEEE 802.11**

As discussed above, the original IEEE 802.11 has not the capacity for frames differentiating priority rather it offers an equal chance to all nodes contending for the channel access at the same time. The access method in MAC layer for the new IEEE 802.11e is called Hybrid Coordination Function (HCF), it combines functionalities of both DCF and PCF (IEEE Std. 802.11-1999). Eventually, in order to enhance the contention-based access mechanisms during a CP period of the IEEE 802.11, Enhanced DCF Coordination Function (EDCF) was proposed as well.

**Fig. 1.** shows the super-frame of HCF (Dridi & al., 2008). The great challenge was to make sure that EDCF should be well-matched with the old DCF since large number of devices complying with the old standard had been deployed.

Fig. 1. HCF Super-frame Structure.

The new mechanism classifies the traffic into 8 user classes, with the modified size of contention window (*CWmin*) and the inter-frame spaces. Smaller the contention window then shorter will be the backoff intervals. Therefore, the traffic priority will be greater. A new inter-frame space called Arbitration Inter-frame Space (AIFS) is introduced to start decrementing the backoff timer as in ordinary DCF. Besides, AIFS is used to stop waiting a DIFS period of time before trying the access to the medium. AIFS is associated with each traffic class and is evaluated as a DIFS plus a number of time slots. It implies that traffic using a large AIFS will be assigned lower priority. The following scheme in **Fig. 2** depicts the dissimilarities between the IEEE 802.11 coordination functions depending on with vs. without QoS support mechanisms.

Fig. 2. Advanced QoS improvement mechanism in IEEE802.11e

Toward a better use of the wireless medium, the MAC protocol of IEEE 802.11e should operate in packet bursting mode. It consists of allowing a station to send more frames once it has gained the access to the idle medium through ordinary contention during TXOP-Limit (Dridi & al., 2008). The packet burst is terminated, if a collision occurs or no acknowledgment frame is received, as packet bursting can possibly increase the jitter. The most priority traffic operates with Short Inter-frame Space (SIFS), which is the small time interval between data-frame and Ack-frame.

#### **2.5 IEEE 802.11e-EDCF mechanism**

66 Advanced Wireless LAN

802.11-1999). Eventually, in order to enhance the contention-based access mechanisms during a CP period of the IEEE 802.11, Enhanced DCF Coordination Function (EDCF) was

**Fig. 1.** shows the super-frame of HCF (Dridi & al., 2008). The great challenge was to make sure that EDCF should be well-matched with the old DCF since large number of devices

The new mechanism classifies the traffic into 8 user classes, with the modified size of contention window (*CWmin*) and the inter-frame spaces. Smaller the contention window then shorter will be the backoff intervals. Therefore, the traffic priority will be greater. A new inter-frame space called Arbitration Inter-frame Space (AIFS) is introduced to start decrementing the backoff timer as in ordinary DCF. Besides, AIFS is used to stop waiting a DIFS period of time before trying the access to the medium. AIFS is associated with each traffic class and is evaluated as a DIFS plus a number of time slots. It implies that traffic using a large AIFS will be assigned lower priority. The following scheme in **Fig. 2** depicts the dissimilarities between the IEEE 802.11

coordination functions depending on with vs. without QoS support mechanisms.

Fig. 2. Advanced QoS improvement mechanism in IEEE802.11e

proposed as well.

complying with the old standard had been deployed.

Fig. 1. HCF Super-frame Structure.

To make more efficient for the existing mechanisms of IEEE 802.11, EDCF has been proposed, which aims to enhance the access mechanism by providing the distributed access for the service differentiation. The IEEE 802.11e working group brought an extension to enhance the access mechanisms of earlier standard and provide a distributed access mechanism for service differentiation (Wiethölter & al., 2006). Because a lot of devices have been deployed to improve the DCF, Enhanced DCF (EDCF or EDCA) is the new IEEE appellation. Currently, an intense care aimed to carry on high level of compatibility with the previous generations of the IEEE 802.11 standard.

The MAC protocol of 802.11e standard divides the traffic into eight classes. Each class has different ܹ and interframe space for the transmission of data. If a node, for example, requires higher priority for data transmission, it would be having smaller ܹ and hence shorter backoff. If more nodes have the same *W,* the traffic classes are differentiated by having different inter-frame spaces. An inter-frame space called as Arbitration Inter-frame Space (AIFS) is introduced to avoid waiting a DIFS before accessing the medium or like DCF to decrement backoff timer. As mentioned above that EDCF has eight traffic classes with different AIFS but operates with the same DIFS period of time. **Fig. 3.** displays access mechanisms for DCF and EDCF.

Fig. 3. DCF vs. EDCF access mechanisms.

#### **2.6 Mobility**

With wireless imbedded devices and high requirement for spreading data transferring, offer facilities for several applications aiming to investigate and control WLAN networks.


$$Throughput = \frac{\Sigma\_{\text{n Recdeved Packets}}}{\Sigma\_{\text{t}}Time} \text{ (B/s)} \tag{3}$$

$$E2ED = Actual\,\,\text{Time} - Departure\,\,\text{Time (s)}\tag{4}$$

$$E2ED\_{avg} = \frac{\Sigma\_n RTT\_n}{n} \text{ (s)}\tag{5}$$

$$\text{Jitter} = \text{Max}\, E2ED - \text{Min}\, E2ED \text{ (s)}\tag{6}$$

$$
\delta a\_n = a\_n - a\_{n-1} \text{ (s)}\tag{7}
$$

$$Jitter\_n = \delta a\_n - \delta a\_{n-1} \text{ (s)}\tag{8}$$

$$\text{Jitter}\_n = E2ED\_n - E2ED\_{n-1} \text{ (s)}\tag{9}$$

$$E2ED\_n = a\_n - d\_n \text{ (s)}\tag{10}$$

$$filter\_n = (a\_n - d\_n) - (a\_{n-1} - d\_{n-1}) \text{ (s)} \tag{11}$$

$$filter\_n = (a\_n - a\_{n-1}) - (d\_n - d\_{n-1}) \text{ (s)}\tag{12}$$

$$
\delta liter\_n = \delta a\_n - \delta d\_n \text{ (s)}\tag{13}
$$

$$Jitter\_n = E2ED\_n - E2ED\_{av} \text{ (s)}\tag{14}$$

Fig. 5. Throughput with mobility

We proposed a scenario within hybrid network, which is composed of wireless and wired nodes, which can communicate through Base Stations (BS's, b��& b��). We specified a solution with two BS's observe that what happens when a Mobile Node (m�) moves out from one BS to another (**Fig. 6**). At this time, EDCF mobility behaviour can be well evaluated.

For the different Access Categories (AC's) of service and to avoid TCP control packets exchanges', 4 CBR-traffics based on UDP transmission protocol are used.

Fig. 6. Mobility scenario through multiple BS's handoff

#### **4.2 Environment and simulation parameters**

Within a table, the environement of the proposal scenario and the parameters used for developing the mobility scheme are presented (**Tab. 1**).


Table 1. NS-2 simulation parameters & development tools

#### **5. Results analysis**

72 Advanced Wireless LAN

We proposed a scenario within hybrid network, which is composed of wireless and wired nodes, which can communicate through Base Stations (BS's, b��& b��). We specified a solution with two BS's observe that what happens when a Mobile Node (m�) moves out from one BS to another (**Fig. 6**). At this time, EDCF mobility behaviour can be well

For the different Access Categories (AC's) of service and to avoid TCP control packets

exchanges', 4 CBR-traffics based on UDP transmission protocol are used.

Fig. 6. Mobility scenario through multiple BS's handoff

Fig. 5. Throughput with mobility

evaluated.

To focus only on the impact of mobility, the present study doesn't take care of the fading effect related to the wireless channel and the non-stationary flows in high data-rate. 5 Mbps are supported by the wired links, and 1 Mbps of data-rate in wireless medium is shared by the different classes of traffic.

As our work focuses on MAC layer, so, without making any comparison of network layer protocols, we used the same Destination Sequenced Distance Vector (DSDV) routing protocol for all the simulation scenarios. To appear congestion event, we preferred limiting the size of the queues at 50 packets. This can allow dropped packets generation, as assuming in a real network, specially when the scheduler is out of its own capacity (Dridi & al., 2010).

#### **5.1 First case: EDCF – Behaviour over "low mobility" domain**

#### **5.1.1 Throughput**

During low mobility and connection with the BS1, throughput achieves levels 60 Kbps and 20 Kbps for AC0 and AC1 respectively. These tow traffics share 80% of total bandwidth is depicted in (**Fig. 7**).

During connection with the BS2, the traffic switches to the half level of the top start throughputs (30Kbps & 10Kbps) and increases during the range of mobility. In this class of mobility, there is no rapid saturation event and the EDCF has enough gap to follow the mobility rhythm by increasing the level of the throughput application adequately. By no need of high throughput network resource, VOIP applications can well transmitted without be thresholded by saturation. QoS is maintained and the behaviour remain the same in both ACs traffic categories with a slight rapid has expected in AC1 to reach the top (in the border of 12 m/s).

Fig. 7. Throughput for low mobility

#### **5.1.2 End-to-end delay**

During the connection with BS2, the AC0 stays around 0.2s, this is very convenient for realtime and almost delay-dependent applications, like audio and video streaming. Unfortunately, EDCF shows this capability only for the AC0 class. On the other hand, during connection with BS2 in AC1, EDCF shows some weakness and remains still sensitive to the fact of mobility, as depicted in the graph in (**Fig. 8**), level of 0.8s is reached for several times. The limit of 0.2s for a reasonable VoIP conversation is exceeded. Fortunately, the value of 0.5s is rapidly bounded which can be welcomed for CBR-MPEG video stream applications.

Fig. 8. E2ED for low mobility

### **5.1.3 Jitter**

74 Advanced Wireless LAN

During the connection with BS2, the AC0 stays around 0.2s, this is very convenient for realtime and almost delay-dependent applications, like audio and video streaming. Unfortunately, EDCF shows this capability only for the AC0 class. On the other hand, during connection with BS2 in AC1, EDCF shows some weakness and remains still sensitive to the fact of mobility, as depicted in the graph in (**Fig. 8**), level of 0.8s is reached for several times. The limit of 0.2s for a reasonable VoIP conversation is exceeded. Fortunately, the value of 0.5s is rapidly bounded which can be welcomed for CBR-MPEG video stream

Fig. 7. Throughput for low mobility

**5.1.2 End-to-end delay**

Fig. 8. E2ED for low mobility

applications.

The graph in the **Fig. 9** shows the jitter plotted in two sides of x axis. The negative peaks for packets arrive early, and positive peaks for packets arrive late. The variation of the intensity of jitter in both sides identifies the quality of transmission (ex. degradation quality during a call in VoIP).

For AC0, the jitter is bounded around 0.01s for the all transmissions. An interesting behaviour of EDCF is observed at the negative side. The flow stays quite steady before and after the switching period between the BS's. It shows the ability of the EDCF scheduler to track jitter measurements and adjust buffer size to reduce the jitter impacts. Unfortunately, as we can see in the same graph, this behaviour raises the jitter impact of AC1 (it can reach 0.06s with 0.12s peak-to-peak of variance). Outside this region, EDCF with CBR flow is quite steady.

Fig. 9. Jitter for low mobility

#### **5.2 Second case: EDCF – Behaviour over "medium mobility" domain**

#### **5.2.1 Throughput**

At the start during connection with BS1, the top AC0 and AC1 throughputs stay unchanged. In AC0 traffics with the increase of the node's mobility, the throughput slightly increases. Comparing to the previous mode, the graph for this case (**Fig. 10**) and previous case are much closer and can't grow over 57 Kbps (start of saturation bound). Even the start top level is not reached; EDCF is not capable to increase throughput for the highest priority traffics. In contrast to AC0, AC1 can gain more flexibility and it increases over the top start level even the curves saturation is expected (more than 20Kbps reached in 30m/s).

In this mobility mode, EDCF guarantees for the maximum throughput. This is highly required variation for the throughput sensitive application (VBR flow) (Rong & Xuming, 2009), as they need robustness over user mobility. Comparing the connections with both of the BS's, we can observe that AC1 does not waste the bandwidth between two connections as AC0 does.

Fig. 10. Throughput for medium mobility

#### **5.2.2 End-to-end delay**

AC0 delay gains more stability after connection with BS2 down to 0.1s, and up at ~0.2s. This zone can be reserved for high sensitive traffics for the brief period of time (as in Burst mode). The AC0 in the graph depicted in **Fig. 11**, shows that less than 30000 packets are allowed for one burst. AC1, after establishing the connection (0.5s) attains an average delay and stays steady for the rest of transmission. This is the most important characteristic of this mode that it can bring to high priority traffic in the same transmission with a small shift of delay but granting a maximum stability. We find the best result at 30 m/s.

Fig. 11. E2ED for medium mobility

#### **5.2.3 Jitter**

76 Advanced Wireless LAN

AC0 delay gains more stability after connection with BS2 down to 0.1s, and up at ~0.2s. This zone can be reserved for high sensitive traffics for the brief period of time (as in Burst mode). The AC0 in the graph depicted in **Fig. 11**, shows that less than 30000 packets are allowed for one burst. AC1, after establishing the connection (0.5s) attains an average delay and stays steady for the rest of transmission. This is the most important characteristic of this mode that it can bring to high priority traffic in the same transmission with a small shift of

delay but granting a maximum stability. We find the best result at 30 m/s.

Fig. 10. Throughput for medium mobility

Fig. 11. E2ED for medium mobility

**5.2.2 End-to-end delay** 

As shown by **Fig. 12**, high stability of the negative side of AC0 (before the range of 35000 packet ID) shows that there are no packets coming early. AC1 proves E2ED stability with a reasonable level of jitter (<0.5s) after connection with BS2, with 0.1s peak-to-peak of variance. Almost all of the traffics can support the range of mobility of this mode with a condition of burst (as explained previously).

Fig. 12. Jitter for medium mobility

#### **5.3 Third case: EDCF – Behaviour over "high mobility" domain**

#### **5.3.1 Throughput**

Within this mode, AC0 and AC1 keep the same behaviour independently to the node's mobility, as shown in **Fig. 13**. EDCF lasts the throughput level comparing to the other modes (15 Kb/s, 30 Kb/s for AC0 and AC1 respectively). It cannot follow the rhythm of the mobility to adjust the throughput accordingly. This behaviour is mainly involved to the size of buffer of the scheduler which is not able to support higher speeds (40 m/s as a critical). All of the traffic classes are penalized with fixed level of throughput, depending on the defined buffer parameters (the size of queue and the used queuing scheduling strategy) (Rong & Xuming, ICCSN-2009).

Fig. 13. Throughput for high mobility

#### **5.3.2 End-to-end delay**

AC0 stays in fixed position (0.2s) as the previous mode. AC1 shifted with high delay (0.9s) and stays in the same level independently of the network mobility. As we can see in (**Fig. 14**)

Fig. 14. E2ED for high mobility

#### **5.3.3 Jitter**

AC0 is in the worst case than the medium mode, as stability is decreased (less than 10000 packets for burst). AC1 keeps the same levels of jitter (0.06s with 0.12 peak-to-peak of variance in **Fig. 15**) with random concentration is slightly appeared discerned in the path of mobility.

Fig. 15. Jitter for high mobility

AC0 stays in fixed position (0.2s) as the previous mode. AC1 shifted with high delay (0.9s) and stays in the same level independently of the network mobility. As we can see in (**Fig. 14**)

AC0 is in the worst case than the medium mode, as stability is decreased (less than 10000 packets for burst). AC1 keeps the same levels of jitter (0.06s with 0.12 peak-to-peak of variance in **Fig. 15**) with random concentration is slightly appeared discerned in the path of

Fig. 13. Throughput for high mobility

**5.3.2 End-to-end delay** 

Fig. 14. E2ED for high mobility

**5.3.3 Jitter** 

mobility.

The three metrics are worth studying and discussing where the table (**Tab. 2**) Summarizes the behaviour of EDCF under the three mobility domains according to the traffic aware-MAC metric pertinency.

Table 2. EDCF behaviour for the MAC metrics constrained by three levels of Mobility

### **6. Conclusion**

The proposed study of this chapter focuses on the performance of the EDCF protocol under the node's mobility constrains. The IEEE 802.11 standard reveals high sensitivity to the nodes' position and velocity. These can significantly decrease the standard QoS service ability. By looking for QoS stability region of the MAC protocol, several tests are performed over the main layer-link metrics; throughput, End-2-End delay and jitter in order to quantify the mobility effect. Therefore, different classes of traffic are defined. We ended the study by proposing a benchmark which summarized the impact of these metrics according to three zones of stability. Furthermore, the QoS mechanism behaved differently depending on the rhythm of mobility apply in each scenario using NS2 network simulator. The study of MAC protocol, even the range is limited by PHY layer, allows extension since it can operate easily within cooperation topology. The approved results can help users to identify the borderline of service's steadiness depending on the requirements of the traffic flow. Following this optimistic approach, the study will be expanded for supporting channel fading effect, multipath distortion and BER vs. SNR links' quality within node's cooperative diversity network.

#### **7. References**

