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A Centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs
¨ 58 (2004): 305–309 Int. J. Electron. Commun. (AEU) http://www.elsevier.de/aeue
Letter A Centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs Woo-Yong Choi Abstract: The admission control algorithm that can be performed at the MAC (Medium Access Control) layer in a real-time is proposed for the decision for accepting or rejecting the requests for adding traffic streams to an IEEE 802.11e wireless LAN (Local Area Network). In numerical examples, we apply the proposed admission control algorithm to VOIP (Voice Over Internet Protocol) traffic streams, and obtain the maximum numbers of VOIP traffic streams that can be admitted to IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs for various delay requirements. Keywords: IEEE 802.11, MAC, Traffic Stream, Admission control, VOIP
1. Introduction The wireless LAN (Local Area Network) standards have been developed by IEEE 802.11 WG (Working Group) and ETSI (European Telecommunications Standards Institute) BRAN (Broadband Radio Access Networks). [1– 6] Using the ISM (Industrial, Science, Medical) band at 2.4 GHz, the IEEE 802.11b version and the IEEE 802.11g version, which is the physical layer extension of the IEEE 802.11b version, provide data rates of up to 11 Mbps and 54 Mbps at the wireless media, respectively. And, the IEEE 802.11a version can achieve data rates of up to 54 Mbps at the wireless media using the OFDM (Orthogonal Frequency Division Multiplexing) modulation technique in the unlicensed 5 GHz band. IEEE 802.11 basic MAC (Medium Access Control) protocol based on CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) was designed to support the best-effort services in wireless LAN environments. However, by the growing interest in wireless LANs supporting QoS (Quality of Service) services, IEEE 802.11 WG has continued the standardization activities for the QoS enhanced MAC protocol, which is called HCF (Hybrid Coordination Function) in the draft. [4] According to the HCF protocol in [4], traffic stream addition request and response messages for adding traffic streams to an IEEE 802.11e wireless LAN should be exchanged between QSTAs (QoS Stations) and a QAP (QoS
Received July 15, 2003. Revised December 22, 2003. Woo-Yong Choi, Mobile Telecommunication Research Lab., Electronics and Telecommunications Research Institute, 161 Gajeongdong, Yuseong-gu, Daejeon, 305-350 Korea. E-mail: [email protected]
Access Point) before the traffic stream services are actually activated. The QAP should notify QSTAs of the decision for accepting or rejecting the traffic stream addition requests from QSTAs by sending the response messages. The centralized admission control algorithm is necessary for the QAP to make the decision efficiently considering the service requirements of traffic streams and the currently available bandwidth. In the literature, the call admission control for mobile networks has been studied by many researchers. [7–9] However, the admission control algorithm for traffic streams in IEEE 802.11e wireless LANs has not been addressed in the literature known by us. In this paper, the admission control algorithm that can be performed at the MAC layer in a real-time is proposed for the decision for accepting or rejecting the requests for adding traffic streams to an IEEE 802.11e wireless LAN. The admission control algorithm derives the service rate for satisfying the delay requirement of each traffic stream. And, the decision for accepting or rejecting traffic streams is made based on the service rate derived for each traffic stream and the currently available service rate. In numerical examples, we apply the proposed admission control algorithm to VOIP (Voice Over Internet Protocol) traffic streams, and obtain the maximum numbers of VOIP traffic streams that can be admitted to IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs for various delay requirements.
2. Related works for QoS in wireless LANs Two access mechanisms, which are called EDCF (Enhanced Distributed Coordination Function) and HCF controlled channel access in the draft, were proposed for MAC QoS enhancements in IEEE 802.11 wireless LANs. [4] The former was devised for the relative QoS differentiation among four access categories and is the QoS enhanced version of the legacy DCF. The latter is the enhanced version of the legacy PCF (Point Coordination Function), and can be used to provide the absolute QoS to real-time services such as VOIP. HIPERLAN (High Performance Radio Local Area Network) Type 2 developed by ETSI BRAN can provide QoS services via the fixed capacity agreement mechanism. [5] This mechanism allows the fixed amounts of radio bandwidth to be periodically granted to the traffic streams, which are setup by pre-negotiations between AP and STAs. Two approaches, ACTS (Advanced Communication Technologies and Services) MEDIAN and BRAIN (Broadband Radio Access Integrated Network) systems, were proposed to realize the wireless ATM (Asynchronous Transfer Mode) concept in wireless LAN environments. [10, 11] 1434-8411/04/58/04-305 $ 30.00/0
306 W.-Y. Choi: A Centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs
3. Traffic stream admission control We want to model the input traffic of a traffic stream, TSi by the arrivals of bursts with the constant inter-arrival time, Di (seconds) as shown in Fig. 1. L i (bits) and Pi (seconds) denote the burst size and the length of the burst period, respectively. Many MPDUs (MAC Protocol Data Units) compose a burst. Generally, the burst size and the length of the burst period are variable, and the delay performance of the traffic stream is degraded as the burst size, L i and the length of the burst period, Pi are larger. We want the maximum delay that the traffic stream experiences with the maximum burst size, which is specified in the traffic stream addition request message, to be within the delay bound specified in the traffic stream addition request message for the traffic stream. (Actually, the burst size can be variable in the range of [0, maximum burst size]. But the maximum delay of the traffic stream will be within the delay bound.) For this purpose, the burst size, L i is set to the maximum burst size and the length of the burst period, Pi can be obtained as Pi =
Li PRi
(1)
where PRi is the peak data rate (bits/second) in the traffic stream addition request message for the traffic stream. From the mean data rate, MRi (bits/second) in the traffic stream addition request message for the traffic stream, the inter-arrival time, Di can be obtained as Di =
Li − Pi MRi
no data in the transmission queue, the service rate, Si allocated to the traffic stream can be temporarily used to transmit other traffic such as the best-effort traffic in the basic service set. And, when the traffic stream has data again to transmit, the service rate should be taken back to serve the traffic stream. Actually, the burst period of the traffic stream can be variable, and this phenomenon of switching the allocation of the service rate between the traffic stream and other traffic will happen in an irregular and unpredictable manner. For this reason, while the traffic stream has no data to transmit, it will be appropriate that the service rate is used to transmit the best-effort traffic demanding no guaranteed QoS. If the mean data rate, MRi ≤ Si < the peak data rate, PRi , the queue state, Bi for TSi , which is the number of bits in the transmission queue for TSi , will fluctuate as shown in Fig. 2. (If Si < MRi , the traffic stream, TSi will experience infinite mean delay. And, if Si ≥ PRi , TSi will experience no queueing delay.) The increasing and decreasing slopes of the queue state, Bi are PRi − Si and −Si , respectively. Therefore, the maximum queue state, Bˆi can be obtained as Bˆi = (PRi − Si )Pi
Using the maximum queue state, Bˆ i and (1), we can obtain the maximum delay, Ti that the traffic stream, TSi can experience as follows: Ti =
(2)
We want to provide the traffic stream with the constant service rate, Si (bits/second). While the traffic stream has
(3)
Bˆ i (PRi − Si )Pi (PRi − Si )L i = = Si Si PRi · Si
(4)
From the preceding equation, we can derive the minimum service rate, Si satisfying the delay bound, DBi , which is specified in the traffic stream addition request message for
Fig. 1. Arrival Pattern of Traffic Stream, TSi .
Fig. 2. Fluctuation of Transmission Queue State, Bi for Traffic Stream, TSi .
W.-Y. Choi: A Centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs 307
4. Numerical examples
the traffic stream, TSi , as follows: Si =
PRi · L i L i + DBi · PRi
(5)
If the mean data rate, MRi is larger than Si obtained by the preceding equation, the actual minimum service rate should be adjusted to be MRi . (In this paper, the MAC and physical layer processing delays are ignored because the processing delays are relatively very small compared with the delay bound requirement.) Let us denote by R (bits/second) the total available service rate that can be assigned for transmitting the MPDUs of the traffic streams of QSTAs served by a QAP. Then, we can devise the following admission control algorithm for the QAP to decide accepting or rejecting the requests for adding traffic streams. ALGORITHM STEP 0: Initialize the currently available service rate, A R : A R ← R. STEP 1: If the traffic stream addition request message with the peak data rate (PRi ), delay bound (DBi ), the maximum burst size (L i ), and the mean data rate (MRi ) is received, calculate the service rate, Si as follows: PRi · L i , MRi Si = Max (6) L i + DBi · PRi STEP 2: If A R > Si , accept the traffic stream addition request, and update A R : A R ← A R − Si . Otherwise, reject the traffic stream addition request. If the traffic stream deletion request message is received and the service rate, Sold was assigned to this traffic stream, update A R : A R ← A R + Sold . Go to STEP 1. In the preceding algorithm, STEP 1 calculates the service rate, Si for the traffic stream corresponding to the traffic stream addition request message. And, STEP 2 decides accepting or rejecting the traffic stream addition request and decreases the currently available service rate, A R by the service rate, Si if the request is accepted. If the traffic stream deletion request message is received, STEP 2 takes back the service rate that was assigned to the traffic stream, and increases the currently available service rate, A R by the service rate, Sold assigned to the traffic stream.
Fig. 3. SIFS, Physical Layer Header, and MPDU.
In this section, we will obtain the maximum numbers of VOIP (Voice Over Internet Protocol) traffic streams that can be admitted to IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs by the admission control in the previous section. (IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs are IEEE 802.11a, IEEE 802.11b and IEEE 802.11g wireless LANs with IEEE 802.11e MAC protocol, respectively.) The burst period length, Pi and the burst inter-arrival time, Di of each VOIP traffic stream are, respectively, 1.5 seconds and 1 second, which are from the voice traffic model in [9]. The user payload of each VOIP MPDU is 88 bits. [12] With the IMBE speech coder, the total number of MPDUs generated in the burst period by each VOIP traffic stream is 4.8 Kbps * 1.5 seconds/88 bits = 82 where 4.8 Kbps is the speech coding rate of the IMBE speech coder. [12] UDP (User Datagram Protocol), IP (Internet Protocol) and MAC layer headers are included in each MPDU as shown in Fig. 3. Therefore, the burst size, L i is 4.8 Kbps * 1.5 seconds + 82 * (UDP, IP and MAC header length) = 7,200 bits + 82 * (16 bits + 224 bits + 240 bits) = 46,560 bits where the lengths of UDP, IP and MAC headers were obtained from [4] and [12]. (For convenience of presentation, the CRC field was included in the MAC header.) The actual peak data rate, PRi is L i /Pi = 46,560 bits/1.5 seconds = 31.0 Kbps. The mean data rate, MRi is PRi * 1.5 seconds/(1.5 seconds + 1 second) = 18.6 Kbps. According to the HCF protocol in [4], we can omit ACK frame transmissions for real-time traffic streams like VOIP traffic streams. As shown in Fig. 3, a SIFS (Short InterFrame Spaces) and the transmission time for a physical layer header are necessary for each MPDU transmission. Considering the bandwidth consumption by the SIFS time and the physical layer header transmission time, the actual available service rate, R that can be assigned for transmitting the MPDUs of the traffic streams can be obtained as follows: L MPDU / Rˆ ˆ R= R (7) SIFS + TPHY + L MPDU / Rˆ where Rˆ is the raw transmission rate at the wireless media, TPHY the transmission time for a physical layer header, and L MPDU the MPDU length. According to [4, 12],
308 W.-Y. Choi: A Centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs
Fig. 4. Maximum Numbers of VOIP Traffic Streams for IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e Wireless LANs versus Delay Bound, DBi .
L MPDU = UDP, IP and MAC header length + payload length = 16 bits + 224 bits + 240 bits + 88 bits = 568 bits. Rˆ = 54 Mbps, SIFS = 16 µs and TPHY = 24 µs for IEEE 802.11a and IEEE 802.11g wireless LANs, and Rˆ = 11 Mbps, SIFS = 10 µs and TPHY = 192 µs for 802.11b wireless LANs. [1–3] Therefore, by (7), the actual available service rate, R that can be assigned for transmitting the MPDUs of the traffic streams in IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs is 11.34 Mbps, 2.20 Mbps and 11.34 Mbps, respectively. (R is far less than Rˆ because the VOIP MPDU size of L MPDU = 568 bits is too small, compared with the maximum allowable MPDU size of at least 18,672 bits. [4] In Fig. 3, we neglected the QoS polling frame transmissions because many MPDUs can be transmitted after a QoS polling frame, and the bandwidth consumption by the transmissions of QoS polling frames will be small. [4]) The number of STAs should be taken into account to analyze the DCF and EDCF protocols, which are the contention-based protocols, since the collisions among STAs occur more frequently and the MAC performance becomes worse as the number of STAs increases. But, according to the IEEE 802.11e HCF controlled channel access protocol that is considered in this paper, the access point transmits the separate polling frames to grant the transmission opportunities to the traffic streams that are even from the same STA, and the other STAs keep from sending data frames and no collision can be assumed to occur in the HCF controlled channel access protocol. In the polling frames, the traffic streams for which the poling frames are intended are specified. For this reason, we can treat the traffic streams like the traffic streams are from
the different STAs. Therefore, the number of STAs is not considered and only the number of traffic streams is considered in this paper. Now, we have the values of the traffic parameters of the VOIP traffic streams, (R = 11.34 Mbps (for IEEE 802.11a/e and IEEE 802.11g/e) or 2.20 Mbps (for IEEE 802.11b/e), PRi = 31.0 Kbps, L i = 46,560 bits, MRi = 18.6 Kbps), and are ready to apply the admission control algorithm in the previous section to VOIP traffic streams. For the various delay bounds, DBi = 10 ms, 50 ms, 100 ms, 150 ms, . . . , 600 ms, the maximum numbers of traffic streams that can be admitted to IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs by the admission control algorithm in the previous section are plotted in Fig. 4. Using computer simulations, the maximum numbers of traffic streams in IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs were obtained and are also plotted in Fig. 4. We considered the polling frame transmissions in computer simulations. (Note that in Fig. 3, the polling frames were neglected for the analytical approach.) From Fig. 4, we can see that the analytical and simulation results match very closely although the simulation result values are a little smaller than the analytical values due to the bandwidth consumption by the polling frame transmissions in the computer simulation analysis. The maximum numbers of simultaneous VOIP traffic streams that IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs can support increase almost linearly as the delay bound, DBi gets looser. The VOIP capacity of IEEE 802.11a/e is the same as that of IEEE 802.11g/e. The maximum number of VOIP traffic streams of IEEE 802.11a/e and IEEE 802.11g/e is about five times as large as that of IEEE 802.11b/e.
W.-Y. Choi: A Centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs 309
Acknowledgement. The author would like to thank the anonymous reviewers for their constructive comments.
References [1] IEEE Std 802.11a: Wireless LAN medium access control (MAC) and physical layer specifications: High speed physical layer in the 5 GHz band. IEEE (1999). [2] IEEE Std 802.11b: Wireless LAN medium access control (MAC) and physical layer specifications: Higher speed physical layer (PHY) extension in the 2.4 GHz band. IEEE (1999). [3] IEEE Std 802.11g: Wireless LAN medium access control (MAC) and physical layer specifications: Further higher data rate extension in the 2.4 GHz band. IEEE (2003). [4] IEEE Std 802.11e/D4.3: Wireless lan medium access control (MAC) and physical layer specifications: Medium access control (MAC) enhancements for quality of service (QoS). IEEE (2003). [5] ETSI TS 101 761-1: Broadband radio access networks (BRAN); HIPERLAN type 2; data link control (DLC) layer; part1: Basic data transport functions. ETSI (2000).
[6] ETSI TS 101 761-2: Broadband radio access networks (BRAN); HIPERLAN type 2; data link control (DLC) layer; part 2: Radio link control (RLC) sublayer. ETSI (2001). [7] Andersin, M.; Rosberg, Z.; Zender, J.: Soft and safe admission control in cellular networks. IEEE/ACM Trans. on Net. 5 (1997), 255–265. [8] Nuaymi, L.; Godlewski, P.; Mihailescu, C.: Call admission control algorithm for cellular CDMA systems based on best achievable performance. Proc. IEEE VTC 2000-Spring (2000), 375–379. [9] Comaniciu, C.; Mandayam, N.: Delta modulation based prediction for access control in integrated voice/data CDMA systems. IEEE J.S.A.C. 18 (2000), 112–122. [10] Ciotti, C.; Borowski, J.: The AC006 MEDIAN projectoverview and state of the art. Proc. ACTS Summit 1996 (1996), 362–367. [11] Wu, G.; Hase, Y.; Inoue, M.: Broadband radio access integrated network (BRAIN) in MM-Wave band: Indoor wireless LAN prototype. Proc. PIMRC 1998 (1998), 23–27. [12] Zahedi, A.; Pahlavan, K.: Capacity of a wireless LAN with voice and data services. IEEE Trans. on Commun. 48 (2000), 1160–1170.
Letter A Centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs Woo-Yong Choi Abstract: The admission control algorithm that can be performed at the MAC (Medium Access Control) layer in a real-time is proposed for the decision for accepting or rejecting the requests for adding traffic streams to an IEEE 802.11e wireless LAN (Local Area Network). In numerical examples, we apply the proposed admission control algorithm to VOIP (Voice Over Internet Protocol) traffic streams, and obtain the maximum numbers of VOIP traffic streams that can be admitted to IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs for various delay requirements. Keywords: IEEE 802.11, MAC, Traffic Stream, Admission control, VOIP
1. Introduction The wireless LAN (Local Area Network) standards have been developed by IEEE 802.11 WG (Working Group) and ETSI (European Telecommunications Standards Institute) BRAN (Broadband Radio Access Networks). [1– 6] Using the ISM (Industrial, Science, Medical) band at 2.4 GHz, the IEEE 802.11b version and the IEEE 802.11g version, which is the physical layer extension of the IEEE 802.11b version, provide data rates of up to 11 Mbps and 54 Mbps at the wireless media, respectively. And, the IEEE 802.11a version can achieve data rates of up to 54 Mbps at the wireless media using the OFDM (Orthogonal Frequency Division Multiplexing) modulation technique in the unlicensed 5 GHz band. IEEE 802.11 basic MAC (Medium Access Control) protocol based on CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) was designed to support the best-effort services in wireless LAN environments. However, by the growing interest in wireless LANs supporting QoS (Quality of Service) services, IEEE 802.11 WG has continued the standardization activities for the QoS enhanced MAC protocol, which is called HCF (Hybrid Coordination Function) in the draft. [4] According to the HCF protocol in [4], traffic stream addition request and response messages for adding traffic streams to an IEEE 802.11e wireless LAN should be exchanged between QSTAs (QoS Stations) and a QAP (QoS
Received July 15, 2003. Revised December 22, 2003. Woo-Yong Choi, Mobile Telecommunication Research Lab., Electronics and Telecommunications Research Institute, 161 Gajeongdong, Yuseong-gu, Daejeon, 305-350 Korea. E-mail: [email protected]
Access Point) before the traffic stream services are actually activated. The QAP should notify QSTAs of the decision for accepting or rejecting the traffic stream addition requests from QSTAs by sending the response messages. The centralized admission control algorithm is necessary for the QAP to make the decision efficiently considering the service requirements of traffic streams and the currently available bandwidth. In the literature, the call admission control for mobile networks has been studied by many researchers. [7–9] However, the admission control algorithm for traffic streams in IEEE 802.11e wireless LANs has not been addressed in the literature known by us. In this paper, the admission control algorithm that can be performed at the MAC layer in a real-time is proposed for the decision for accepting or rejecting the requests for adding traffic streams to an IEEE 802.11e wireless LAN. The admission control algorithm derives the service rate for satisfying the delay requirement of each traffic stream. And, the decision for accepting or rejecting traffic streams is made based on the service rate derived for each traffic stream and the currently available service rate. In numerical examples, we apply the proposed admission control algorithm to VOIP (Voice Over Internet Protocol) traffic streams, and obtain the maximum numbers of VOIP traffic streams that can be admitted to IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs for various delay requirements.
2. Related works for QoS in wireless LANs Two access mechanisms, which are called EDCF (Enhanced Distributed Coordination Function) and HCF controlled channel access in the draft, were proposed for MAC QoS enhancements in IEEE 802.11 wireless LANs. [4] The former was devised for the relative QoS differentiation among four access categories and is the QoS enhanced version of the legacy DCF. The latter is the enhanced version of the legacy PCF (Point Coordination Function), and can be used to provide the absolute QoS to real-time services such as VOIP. HIPERLAN (High Performance Radio Local Area Network) Type 2 developed by ETSI BRAN can provide QoS services via the fixed capacity agreement mechanism. [5] This mechanism allows the fixed amounts of radio bandwidth to be periodically granted to the traffic streams, which are setup by pre-negotiations between AP and STAs. Two approaches, ACTS (Advanced Communication Technologies and Services) MEDIAN and BRAIN (Broadband Radio Access Integrated Network) systems, were proposed to realize the wireless ATM (Asynchronous Transfer Mode) concept in wireless LAN environments. [10, 11] 1434-8411/04/58/04-305 $ 30.00/0
306 W.-Y. Choi: A Centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs
3. Traffic stream admission control We want to model the input traffic of a traffic stream, TSi by the arrivals of bursts with the constant inter-arrival time, Di (seconds) as shown in Fig. 1. L i (bits) and Pi (seconds) denote the burst size and the length of the burst period, respectively. Many MPDUs (MAC Protocol Data Units) compose a burst. Generally, the burst size and the length of the burst period are variable, and the delay performance of the traffic stream is degraded as the burst size, L i and the length of the burst period, Pi are larger. We want the maximum delay that the traffic stream experiences with the maximum burst size, which is specified in the traffic stream addition request message, to be within the delay bound specified in the traffic stream addition request message for the traffic stream. (Actually, the burst size can be variable in the range of [0, maximum burst size]. But the maximum delay of the traffic stream will be within the delay bound.) For this purpose, the burst size, L i is set to the maximum burst size and the length of the burst period, Pi can be obtained as Pi =
Li PRi
(1)
where PRi is the peak data rate (bits/second) in the traffic stream addition request message for the traffic stream. From the mean data rate, MRi (bits/second) in the traffic stream addition request message for the traffic stream, the inter-arrival time, Di can be obtained as Di =
Li − Pi MRi
no data in the transmission queue, the service rate, Si allocated to the traffic stream can be temporarily used to transmit other traffic such as the best-effort traffic in the basic service set. And, when the traffic stream has data again to transmit, the service rate should be taken back to serve the traffic stream. Actually, the burst period of the traffic stream can be variable, and this phenomenon of switching the allocation of the service rate between the traffic stream and other traffic will happen in an irregular and unpredictable manner. For this reason, while the traffic stream has no data to transmit, it will be appropriate that the service rate is used to transmit the best-effort traffic demanding no guaranteed QoS. If the mean data rate, MRi ≤ Si < the peak data rate, PRi , the queue state, Bi for TSi , which is the number of bits in the transmission queue for TSi , will fluctuate as shown in Fig. 2. (If Si < MRi , the traffic stream, TSi will experience infinite mean delay. And, if Si ≥ PRi , TSi will experience no queueing delay.) The increasing and decreasing slopes of the queue state, Bi are PRi − Si and −Si , respectively. Therefore, the maximum queue state, Bˆi can be obtained as Bˆi = (PRi − Si )Pi
Using the maximum queue state, Bˆ i and (1), we can obtain the maximum delay, Ti that the traffic stream, TSi can experience as follows: Ti =
(2)
We want to provide the traffic stream with the constant service rate, Si (bits/second). While the traffic stream has
(3)
Bˆ i (PRi − Si )Pi (PRi − Si )L i = = Si Si PRi · Si
(4)
From the preceding equation, we can derive the minimum service rate, Si satisfying the delay bound, DBi , which is specified in the traffic stream addition request message for
Fig. 1. Arrival Pattern of Traffic Stream, TSi .
Fig. 2. Fluctuation of Transmission Queue State, Bi for Traffic Stream, TSi .
W.-Y. Choi: A Centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs 307
4. Numerical examples
the traffic stream, TSi , as follows: Si =
PRi · L i L i + DBi · PRi
(5)
If the mean data rate, MRi is larger than Si obtained by the preceding equation, the actual minimum service rate should be adjusted to be MRi . (In this paper, the MAC and physical layer processing delays are ignored because the processing delays are relatively very small compared with the delay bound requirement.) Let us denote by R (bits/second) the total available service rate that can be assigned for transmitting the MPDUs of the traffic streams of QSTAs served by a QAP. Then, we can devise the following admission control algorithm for the QAP to decide accepting or rejecting the requests for adding traffic streams. ALGORITHM STEP 0: Initialize the currently available service rate, A R : A R ← R. STEP 1: If the traffic stream addition request message with the peak data rate (PRi ), delay bound (DBi ), the maximum burst size (L i ), and the mean data rate (MRi ) is received, calculate the service rate, Si as follows: PRi · L i , MRi Si = Max (6) L i + DBi · PRi STEP 2: If A R > Si , accept the traffic stream addition request, and update A R : A R ← A R − Si . Otherwise, reject the traffic stream addition request. If the traffic stream deletion request message is received and the service rate, Sold was assigned to this traffic stream, update A R : A R ← A R + Sold . Go to STEP 1. In the preceding algorithm, STEP 1 calculates the service rate, Si for the traffic stream corresponding to the traffic stream addition request message. And, STEP 2 decides accepting or rejecting the traffic stream addition request and decreases the currently available service rate, A R by the service rate, Si if the request is accepted. If the traffic stream deletion request message is received, STEP 2 takes back the service rate that was assigned to the traffic stream, and increases the currently available service rate, A R by the service rate, Sold assigned to the traffic stream.
Fig. 3. SIFS, Physical Layer Header, and MPDU.
In this section, we will obtain the maximum numbers of VOIP (Voice Over Internet Protocol) traffic streams that can be admitted to IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs by the admission control in the previous section. (IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs are IEEE 802.11a, IEEE 802.11b and IEEE 802.11g wireless LANs with IEEE 802.11e MAC protocol, respectively.) The burst period length, Pi and the burst inter-arrival time, Di of each VOIP traffic stream are, respectively, 1.5 seconds and 1 second, which are from the voice traffic model in [9]. The user payload of each VOIP MPDU is 88 bits. [12] With the IMBE speech coder, the total number of MPDUs generated in the burst period by each VOIP traffic stream is 4.8 Kbps * 1.5 seconds/88 bits = 82 where 4.8 Kbps is the speech coding rate of the IMBE speech coder. [12] UDP (User Datagram Protocol), IP (Internet Protocol) and MAC layer headers are included in each MPDU as shown in Fig. 3. Therefore, the burst size, L i is 4.8 Kbps * 1.5 seconds + 82 * (UDP, IP and MAC header length) = 7,200 bits + 82 * (16 bits + 224 bits + 240 bits) = 46,560 bits where the lengths of UDP, IP and MAC headers were obtained from [4] and [12]. (For convenience of presentation, the CRC field was included in the MAC header.) The actual peak data rate, PRi is L i /Pi = 46,560 bits/1.5 seconds = 31.0 Kbps. The mean data rate, MRi is PRi * 1.5 seconds/(1.5 seconds + 1 second) = 18.6 Kbps. According to the HCF protocol in [4], we can omit ACK frame transmissions for real-time traffic streams like VOIP traffic streams. As shown in Fig. 3, a SIFS (Short InterFrame Spaces) and the transmission time for a physical layer header are necessary for each MPDU transmission. Considering the bandwidth consumption by the SIFS time and the physical layer header transmission time, the actual available service rate, R that can be assigned for transmitting the MPDUs of the traffic streams can be obtained as follows: L MPDU / Rˆ ˆ R= R (7) SIFS + TPHY + L MPDU / Rˆ where Rˆ is the raw transmission rate at the wireless media, TPHY the transmission time for a physical layer header, and L MPDU the MPDU length. According to [4, 12],
308 W.-Y. Choi: A Centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs
Fig. 4. Maximum Numbers of VOIP Traffic Streams for IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e Wireless LANs versus Delay Bound, DBi .
L MPDU = UDP, IP and MAC header length + payload length = 16 bits + 224 bits + 240 bits + 88 bits = 568 bits. Rˆ = 54 Mbps, SIFS = 16 µs and TPHY = 24 µs for IEEE 802.11a and IEEE 802.11g wireless LANs, and Rˆ = 11 Mbps, SIFS = 10 µs and TPHY = 192 µs for 802.11b wireless LANs. [1–3] Therefore, by (7), the actual available service rate, R that can be assigned for transmitting the MPDUs of the traffic streams in IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs is 11.34 Mbps, 2.20 Mbps and 11.34 Mbps, respectively. (R is far less than Rˆ because the VOIP MPDU size of L MPDU = 568 bits is too small, compared with the maximum allowable MPDU size of at least 18,672 bits. [4] In Fig. 3, we neglected the QoS polling frame transmissions because many MPDUs can be transmitted after a QoS polling frame, and the bandwidth consumption by the transmissions of QoS polling frames will be small. [4]) The number of STAs should be taken into account to analyze the DCF and EDCF protocols, which are the contention-based protocols, since the collisions among STAs occur more frequently and the MAC performance becomes worse as the number of STAs increases. But, according to the IEEE 802.11e HCF controlled channel access protocol that is considered in this paper, the access point transmits the separate polling frames to grant the transmission opportunities to the traffic streams that are even from the same STA, and the other STAs keep from sending data frames and no collision can be assumed to occur in the HCF controlled channel access protocol. In the polling frames, the traffic streams for which the poling frames are intended are specified. For this reason, we can treat the traffic streams like the traffic streams are from
the different STAs. Therefore, the number of STAs is not considered and only the number of traffic streams is considered in this paper. Now, we have the values of the traffic parameters of the VOIP traffic streams, (R = 11.34 Mbps (for IEEE 802.11a/e and IEEE 802.11g/e) or 2.20 Mbps (for IEEE 802.11b/e), PRi = 31.0 Kbps, L i = 46,560 bits, MRi = 18.6 Kbps), and are ready to apply the admission control algorithm in the previous section to VOIP traffic streams. For the various delay bounds, DBi = 10 ms, 50 ms, 100 ms, 150 ms, . . . , 600 ms, the maximum numbers of traffic streams that can be admitted to IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs by the admission control algorithm in the previous section are plotted in Fig. 4. Using computer simulations, the maximum numbers of traffic streams in IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs were obtained and are also plotted in Fig. 4. We considered the polling frame transmissions in computer simulations. (Note that in Fig. 3, the polling frames were neglected for the analytical approach.) From Fig. 4, we can see that the analytical and simulation results match very closely although the simulation result values are a little smaller than the analytical values due to the bandwidth consumption by the polling frame transmissions in the computer simulation analysis. The maximum numbers of simultaneous VOIP traffic streams that IEEE 802.11a/e, IEEE 802.11b/e and IEEE 802.11g/e wireless LANs can support increase almost linearly as the delay bound, DBi gets looser. The VOIP capacity of IEEE 802.11a/e is the same as that of IEEE 802.11g/e. The maximum number of VOIP traffic streams of IEEE 802.11a/e and IEEE 802.11g/e is about five times as large as that of IEEE 802.11b/e.
W.-Y. Choi: A Centralized MAC-Level Admission Control Algorithm for Traffic Stream Services in IEEE 802.11e Wireless LANs 309
Acknowledgement. The author would like to thank the anonymous reviewers for their constructive comments.
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