Adda, Mo

Early, G.

Peart, Amanda

English

Portsmouth University Research Portal (Pure)

High Priority Traffic in HCF on Wireless Networks 
 
Mo Adda, Amanda Peart, Gordon Early 
School of Computing, University of Portsmouth, Lion Terrace, Portsmouth, UK 
{mo.adda, amanda.peart, gordon.early }@port.ac.uk 
 
Abstract 
 
In this paper we analysis and compare the performance of the high priority traffic in 
HCF using fragment-to-fragment acknowledgement and block-frame 
acknowledgment in a noisy wireless environment. We will propose a mechanism for 
the block-frame acknowledgement that will implement on the IEEE 802.11 standard 
to support quality of service, with no major upheaval.  The analysis is valid for all 
type of traffics low, medium and high priority classes. 
 
Keywords 
Wireless Networks, WLAN, Hybrid Coordination Function (HCF), Enhanced 
Distributed Channel Access (EDCA), 802.11, Quality of Service. 
 
 
1 Introduction 
 
Industry have adopted the Internet Protocol (IP) standard to route data over their 
corporate network, this enables them to maintain a single infrastructure, which in turn 
reduces their maintenance costs. The implementation of Wireless Fidelity (WiFi) 
networking has allowed users to access the corporate network whilst still maintaining 
mobility. Applications such as Voice over IP (VoIP) has primarily been used on wired 
networks, implementing this on a WiFi network would give greater flexibility to the 
user. This type of implementation would enable the corporate user to roam and collect 
calls from the corporate telecoms network. The drawback of a wireless implementation 
is that VoIP demands a good quality of service (QoS) for it to achieve an acceptable 
level of performance for the end user. 
 
It is the type of VoIP application as discussed above that necessitates a classification of 
high priority network traffic providing a good QoS delivery. This paper will analysis 
and compare the performance of the high priority traffic in a noisy wireless environment 
employing HCF using fragment-to-fragment acknowledgement and block-frame 
acknowledgment. A mechanism for the block-frame acknowledgement that will be 
compatible with the IEEE 802.11 standard [1] to support quality of service [2], with no 
major upheaval will be proposed.  The analysis is valid for all type of traffics low, 
medium and high priority classes. 
 
 
2 Notations 
 
We refer to the size of the frame generated by the MAC-sub layer protocols as L (bits).  
We also refer to the size of the fragment by F (bits).  The number of fragments 
transmitted consecutively is therefore L/F.  The transmission rate of the NIC (Network 
Interconnection Card) is R (bits/sec). The positive acknowledgment ACK and the 
negative acknowledgment NAC are of equal size.  We label the number of back off slots 
before a transmission takes place by S.   
 
DIFS = 34sec, SIFS = 16sec, we propose another inter-frame spacing IIFS 
(Immediate Interframe spacing) equals to the length of the time slot = 9sec. The length 
of the time slots = BFS = 9sec. The length of the RTC and CLS are 34Bytes each.  
 
The probability that a frame will be corrupted is called p, and the probability that an 
acknowledgment or a negative acknowledgement will be corrupted is called q. In this 
analysis we will consider single hop transmission, as shown in figure 1. 
 
 
 
 
 
 
Figure 1:  EDCF part of the HCF, the MAC protocol.  The value of the DIFS depends on the 
priority classes. 
 
Table 1, below shows the notations are used to simplify the equations of the efficiency 
for each scheme. 
 
 
 
 
 
 
Table 1. Notations used to simplify equations of the efficiency for each scheme 
 
The probability that k retransmissions are required after a fragment is corrupted is 
define as Prob = p
k-1
 (1-p).  The expected number of retransmissions would be 1/(1-p).  
The probability that k negative acknowledgement retransmissions are required after a 
fragment is corrupted is define as Prob = p.q
k-1
 (1-q).  The expected number of 
acknowledgements retransmitted would be p/(1-q). 
 
 
3 Fragment-by-Fragment Acknowledgment  
 
As shown in figure 2, the IEEE 802.11 standard, for the reliability each fragment is 
acknowledged.  If a fragment is corrupted a negative acknowledge is send back.  In the 
case where, the negative acknowledge is lost, the sender times out and retransmit the 
fragment.  We assume that the length of the time out period is equal to the 
acknowledgment time Ta.  Other possibilities can be considered. 
 
 
 
 
 
 
  
 
 
 
 
Figure 2:  Transmission of sequence of Fragments with Errors 
 
DIFS 
S 
RTS CLS F ACK F ACK F ACK 
SIFS Ta Tf 
F ACK F ACK Fx NAC
K 
Fx NAC
K 
Fx NAC
K 
Fx NAC
K 
Fx ACK F ACK Fv NAC
K 
Fv+1 NAC
K 
Fv+2 NAC
K 
Fv+3 NAC
K 
Fv+4 NAC Fv+5 NAC Fv+6 NAC
K 
Fv+7 NAC
K 
Fv ACK F ACK 
Tb = S*BFS = 9S  (back off time) 
Tp = DIFS + RTS/R + CLS/R + 2*SIFS  (Protocol time) 
Ta = 2*SIFS + ACK/R  (Acknowledgment time) 
Ta
’
 = IIFS+ SIFS + NAC/R 
 
After k retransmissions, when the fragment is corrupted, there have been p/(1-p) retrial 
periods of (Ta + F/R).  After k retransmissions, when the negative acknowledge has 
been corrupted, there have been p/(1-q)*(Ta + F/R). 
 
Therefore the efficiency of would be (1): 
 











)
11
)((1
1
q
p
p
p
R
F
TTT
L
R
T
F
R
E
abpa
  (1) 
 
 
4 Block-Frame Acknowledgment 
 
An acknowledgement is sent when all the fragments generated from the same frame 
have been received.  From the IEEE 802.11 standard, each fragment has a sequence 
number and a flag indicating the end of the transmission. We suggest that in this case 
when a fragment is corrupted a negative acknowledgment is generated after a frame 
space called IIFS, which is less than the SIFS the space of the acknowledgment, as 
shown in figure 3. 
 
After k retransmissions, when the fragment is corrupted, there have been p/(1-p) retrial 
periods of (Ta
’
 + F/R).  After k retransmissions, when the negative acknowledge has 
been corrupted, there have been p/(1-q)*(Ta’ + F/R).  The sender receives the corrupted 
negative acknowledgment as noise, it waits until the channel is clear, then after SIFS it 
transmits next fragment, unaware of the previous unsuccessful transmission. 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3: Block Frame Acknowledgement with a Sequence of Corrupted Fragments. 
 
The efficiency of would than be (2). 
 













)
11
)(()
*
(**1
1
'
q
p
p
p
R
F
TTT
L
R
FL
FL
SIFSRT
L
R
E
abpa
   (2) 
 
 
5 Results 
 
Figures 4,5 and 6 demonstrate graphically the results of the efficiency proposed. The 
efficiency percentage for non-fragmented (M=1), Fragment-by-Fragment ACK (F/F-A), 
and Block Frame ACK (B/F-A) for noiseless channel (p=q=0), as a function of the 
frame length is shown in figure 4. The noiseless channel parameter is (p=.5, q=0), as a 
Fv+4 NAC Fv+5 NAC Fv+6 NAC
K 
Fv+7 NAC
K 
Fv F ACK 
F F Fx NAC
K 
Fx NAC
K 
Fx NAC
K 
Fx NAC
K 
Fx F Fv NAC
K 
Fv+1 NAC
K 
Fv+2 NAC
K 
Fv+3 NAC
K 
function of the frame length in figure 5. Here you can see that there is minimal effect on 
Fragment-by-Fragment ACK (F/F-A), and Block Frame ACK (B/F-A) but there is 
significant impact on the efficiency on the non-fragmented frame. The graph in figure 6 
increases the probability that negative acknowledgement will be corrupt at 0.5 on the 
noiseless channel. Here, there is a slight improvement of the efficiency of the non-
fragmented frame. 
  
Efficiency
60.00
70.00
80.00
90.00
100.00
51
2
15
36
25
60
35
84
46
08
56
32
66
56
76
80
87
04
97
28
10
75
2
Frame (Bytes)
E
M=1
F/F-A
B/F-A
 
Figure 4: The efficiency (%) for Non-fragmented (M=1), Fragment-by-Fragment ACK (F/F-A), 
and Block Frame ACK (B/F-A) for noiseless channel (p=q=0), as a function of the frame length. 
 
 
 
Efficiency
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
51
2
15
36
25
60
35
84
46
08
56
32
66
56
76
80
87
04
97
28
10
75
2
Frame (Bytes)
E
M=1
F/F-A
B/F-A
 
Figure 5: The efficiency (%) for Non-fragmented (M=1), Fragment-by-Fragment ACK (F/F-A), 
and Block Frame ACK (B/F-A) for noiseless channel (p=.5, q=0), as a function of the frame 
length. 
 
 
Efficiency
20.00
30.00
40.00
50.00
60.00
70.00
80.00
90.00
100.00
5
1
2
1
5
3
6
2
5
6
0
3
5
8
4
4
6
0
8
5
6
3
2
6
6
5
6
7
6
8
0
8
7
0
4
9
7
2
8
1
0
7
5
2
Frame (Bytes)
E
M=1
F/F-A
B/F-A
 
 
Figure 6: The efficiency (%) for Non-fragmented (M=1), Fragment-by-Fragment ACK (F/F-A), 
and Block Frame ACK (B/F-A) for noiseless channel (p=.5, q=0.5), as a function of the frame 
length. 
 
 
6 Future work 
 
We will consider an intelligent sender that will use the noise from the negative 
acknowledgments as indication to a wrong transmission.  The receiver will have to send 
negative acknowledgment to overcome duplications.  The sender has a time out 
mechanism to retransmit non-acknowledged fragments.  We believe that this will 
improve equation 2 further.  We will also look at the impact of multi-hops on the 
efficiency of the proposed scheme. 
 
 
References 
 
[1] IEEE Draft Std 802.11e, Medium Access Control (MAC) Enhancements for Quailty of Service 
(QoS), D4.4, June 2003  
[2]  Anastasi, G., Lenzini, L., QoS provided by the IEEE 802.11 wireless LAN to advanced data 
application: a simulation analysis. Wireless Networks, (6): 99-108,2000. 
  


High priority traffic in HCF on wireless networks

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