Synchronization of multiple access points in the IEEE 802.11 Point coordination Function Dimitrios D. Vergados1,2 and Dimitrios J. Vergados1,2 1
Department of Information and Communication Systems Engineering University of the Aegean GR-83200 Karlovassi, Samos, Greece email:
[email protected] 2
National Technical University of Athens School of Electrical and Computer Engineering GR-15773 Zografou, Athens, Greece Abstract— Guaranteed QoS in WLANs can enable the support of new and demanding services. The IEEE 802.11 WLAN is the dominant solution for local area wireless networking, as it can provide low cost and easy to deploy high performance wireless connectivity. IEEE has defined two access methods for the 802.11 WLAN: DCF and PCF. Even though the DCF access mode can provide high throughput and low delay for a limited number of users, only the PCF access mode can deliver guaranteed quality of service when the number of users increases. The PCF access mode, as defined by IEEE, cannot operate efficiently when a number of access points with overlapping coverage areas communicate over the same channel. In this paper a new intra access point synchronization scheme is introduced, for allowing multiple access points to cooperate, providing guaranteed quality of service under the PCF operation Keywords- Mulriple aceess; 802.11; WLAN; PCF
I.
INTRODUCTION
The IEEE 802.11 WLAN [1] is the dominant solution for local area wireless networking, as it can provide low cost and easy to deploy high performance wireless connectivity. The basic concept behind the 802.11 WLAN is the CSMA/CA algorithm, for multiple access over a common channel. IEEE has defined two access methods: DCF (Distributed Coordination Function) and PCF (Point Coordination Function). The DCF access mode can provide high throughput, as well as low delays, when the number of active users is limited. However, as the number of users increases, the QoS provided to each user is reduces rapidly. Also, IEEE has introduced the PCF access method for providing QoS guarantees in 802.11 WLANs. This access mode operates on top of the DCF access method, and is used only for high priority traffic, after explicit bandwidth (and perhaps delay and jitter) reservations. The PCF access mode, as defined by the IEEE standard, cannot be efficiently applied in networks with more than one access points. The DCF access mode operates in a distributed manner, and can work in ad-hoc environments, and in multi-access point environments. On the contrary, the PCF access mode operates in a centralized manner. The 802.11 protocol specifies
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how the PCF access mode is implemented by one access point, but does not specify how multiple access points can cooperate in order to provide PCF service for a large coverage area. In this paper we propose a new scheme, the Super Point Coordination, that intends to allow the effective cooperation between a number of access points operating in overlapping coverage areas, so that the PCF service can be provided, in order to enabple hard QoS to wireless users. The Super Point Coordination scheme is a method for synchronizing polls by multiple access points, so that wireless stations can transmit without collision. In section II there is a brief description of the 802.11 protocol, in section III the problem of access point coordination and the proposed solution are described, and in section IV there are simulation results which on the one hand illustrate the advantages of the PCF access mode, and on the other hand validate the proposed scheme. II.
THE 802.11 WLAN
In this section there is a short overview of the MAC protocol of the 802.11 WLAN. The 802.11 WLAN supports two basic access modes. The Distributed Coordination Function (DCF) and the Point Coordination Function (PCF). A. Distributed Coordination Function The DCF access mode [3] is a decentralized method for controlling transmissions by wireless stations. The DCF access mode is based in the CSMA/CA algorithm. When the DCF method is used, every wireless node listens to channel before transmitting. If the channel is idle for a defined duration of time (DIFS), then it transmits its packet. When a background transmission is sensed, then the transmission is deferred, a backoff algorithm is applied and after that the packet is transmitted. In case of collision the packet is retransmitted with a larger backoff time. Even though a number of techniques have been proposed for differentiating the QoS provided to DCF traffic [2], [4], the distributed manner of the access algorithm in condition to the lack of centralized admission control and resource reservation,
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leads to extensive service degradation when the amount of incoming traffic increases. Furthermore, the DCF access algorithm resolves collisions in a random fashion, preventing guaranteed quality of service, even when the incoming traffic is well known. The distributed fashion of the DCF access mode makes it suitable for ad-hoc networks. B. Point Coordination Function The PCF access mode operates on top of the DCF access method, and is used only for high priority traffic, after explicit bandwidth reservations. The point coordinator, an entity located in the access point, can admit or reject the reservations according to the available resources, the desired QoS etc. Based on these reservations, the point coordinator polls the wireless high priority stations. Each station can transmit PCF traffic only after being polled, with no contention. Therefore, an upper bound can be set for PCF traffic delay, and a lower bound can be set for PCF traffic bandwidth. Due to the absence of contention and collisions, guaranteed quality of service can be set for PCF traffic. DCF traffic can be transmitted when the channel is sensed to be idle for a specified duration of time (DIFS), so DCF traffic can utilize the channel when no PCF traffic has to be transmitted.
where R is the transmission bitrate. The duration of the contention free period is
L tCFP = ∑ PIFS + PCF ,i R i
,
(3)
where LPCF,i is the duration the PCF transmission i. Therefore the bandwidth allocated to wireless PCF station i is equal to
Bi =
Li tdelay
,
(4)
where Li is the number of bits the station may transmit after being polled, and t delay is the interval between polls (note that Li < LDCF (MAX)). Thus, the PCF access mode can provide hard QoS, for both bandwidth and delay, and therefore it is a promising solution for QoS provisioning in WLANs, in order to support new and demanding services. III.
MULTI ACCESS POINT COORDINATION
The PCF polling scheme is designed to operate in a network where all station are in range from each other. All stations can receive any transmission made by any station, and only one access point is needed and therefore only one point coordinator. The capabilities of this network topology are obviously very limited. Figure 1. PCF Access mode
As illustrated in Figure 1. the point coordinator polls the wireless station during the Contention Free Period (CFP). During this period the wireless stations can only transmit if they have been polled. During the Contention Period (CP), the DCF access mode is used. The access delay for PCF traffic is at least equal to the time between CFPs. Therefore,
t delay = t CFP + tCP + t access ,
(1)
where tCFP is the duration of the contention free period, tCP is the duration of the contention period. taceess is the time needed for the point coordinator to gain control of the channel after a contention period. For this to happed, the channel must be idle for at least PIFS (a constant of the 802.11 standard). Also, if the largest duration of a DCF transmission (including RTS/CTS, ACKs etc) is LDCF(MAX) bits, then
t access MAX =
LDCF ( MAX ) R
+ PIFS ,
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(2)
In typical wireless local area network topologies, there are multiple access-points installed in “random” positions (Cell planning in in-door environment is very difficult). More than one access points can cover a large proportion of the service area, and two or more access points can even be in range one another. Independent polling by multiple access points can lead to collisions of PCF transmissions, witch would cause a violation of the required and reserved quality of service. On the other hand, in this network some stations may transmit simultaneously without collision, but others cannot. The point coordinators must determine which stations can transmit simultaneously, and schedule the transmissions in a way that ensures the absence of collisions, and also does not waste bandwidth. Suppose the access points where configured to operate in the PCF mode. Then each point coordinator in every access point would operate independently. If a station located in the overlapping coverage area of two access points was polled to transmit by one access point, then this transmission could collide with a polled station by the other access point. In order to allow all point coordinators to cooperate, we introduce a new entity called the super point coordinator. This entity is located somewhere in the network (not necessarily in an access point), and is responsible for controlling the operation of all the point coordinators in the access points. The super point coordinator
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could also be responsible for keeping track of the available wireless resources, in order to admit or reject high priority PCF allocations. This paper introduces a new scheme, the super point coordination, for coordinating the operation of multiple point coordinators, in order to enable the PCF operation in a multiaccess point WLAN. A. Simultaneus Transmission It is obvious that a multiple access scheme designed for a multi-access point must encourage as many stations as possible to transmit simultaneously, so that the available bandwidth is used efficiently. In the PCF access mode, the interval between consecutive polls is not fixed; on the contrary, the wireless station determines it. The reception of a small PCF frame by the point coordinator is immediately followed by the transition of the next poll. In the proposed scheme, the duration between polls is predefined by the super point coordinator. More than one poll can be transmitted simultaneously. A CON TIMER
POLL TIMER
POLL TIMER
POLL TIMER
POLL TIMER
POLL TIMER CFP
A C K
PO LL
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PO LL
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According to the proposed scheme, we assume that the point coordinators may have knowledge of all PCF stations in their coverage area (even the ones that are associated with different access points), but they do not have knowledge of the location of each station. Each point coordinator maintains a list of all stations that are in its coverage area and wish to be polled. A station will be registered in more than one list in different point coordinators, if it is in the coverage area of more than one access points. Station j ∈ APk if and only if station j is in the coverage area of access point k and j has register to transmit PCF traffic. In order to avoid collisions in the uplink, each access point must receive only one transmission at a time. There are some sets of stations that can transmit simultaneously without collision. We call these sets “collision free sets”. Let Sk be a set of stations. Sk is a collision free set if
∀i, j : i, j ∈ S k , (∃n : i ∈ APn ∧ j ∈ APn ) .
DATA
A C K
PO LL
A C K
PO LL
DATA
access points. The simplest method is the sequential point coordination, which is a sequential polling of all PCF stations, independently of their location. This method could only be used in cases where only a small proportion of the connections have real-time requirements, or in small networks. Another improved method for coordination is the simultaneous point coordination. In this method the contention free periods of every access point in the network are initiated simultaneously.
(5)
AP
DATA
A C K
PO LL
BEA CON
A C K
PO LL
DATA
CF End
AP
0
DATA
A C K
PO LL
CF End
AP
BS0
DATA
6
1
3
Figure 2. Synchronized PCF
4 BS1
Even though the idle periods can be larger than DIFS, DCF stations are prevented from transmitting by using the virtual carrier sensing mechanism of 802.11. Note that these modifications to the 802.11 protocol are limited only two the point coordinators and the pollable stations. The problem can be simplified if uplink (from the stations to the access points) and downlink (from the access points to the stations) traffic is transmitted in different times. Suppose there is only uplink traffic. Two stations can transmit simultaneously if the destination access point of the first station is not in range of the second station, and the destination access point of the second station is not in range of the first station. If not, a collision is possible. For downlink traffic, an access point can transmit to a station, only if all other access points in range of this station are not transmitting. B. Super Point Coordination A number of different synchronization methods could be applied for scheduling PCF transmissions among multiple
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BS2
5
2
13
7 11 10
8
BS4
12
BS3
9
Figure 3. Sample Topology
C. Scheduling Policy The main problems in the super point coordination scheme are: 1.
To determine the collision
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2.
To determine which of the collision free sets are going to be used for transmissions.
3.
To minimize the number of transmissions needed.
One approach for solving this problem would be to write down all the possible collision free sets, then to write down all the possible sets of collision free sets that would serve all the wireless station, and then choosing the set that leads to the least transmissions. This method would determine the optimal scheduling policy, but turns out to be computationally complicated. We propose a simpler algorithm for setting the scheduling sequence. Each column in the following table corresponds to the coverage area of an access point, and each row corresponds to a wireless station. Note that the rows of the table are sorted. Two wireless stations can transmit without possibility of collision if they do not have an ace in the same column. It is clear than node4 and node10 can transmit without collision, so they are the first two to transmit.
and used a modification of the PCF extension. The propagation module was the “Propagation/SimpleDistance” module of the ns2. B. Single Cell Topology The first series of simulation intends to illustrate the advantages of the 802.11 PCF access mode, when compared to the DCF access mode. 1) Distributed Coordination Function A series of simulations were carried out, whit a variable number of users. In Figure 4. there is a plot of the total throughput of the sustem (blue line), and the Max and min throughput per user (yellow and pink line). Furthermore, there are plots of the mean access delay for the system (light blue line), the Max mean delay per user (brown), min mean delay per user (purple line) and the over jitter (green line). DCF ACCESS MODE 0,40000
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TABLE I.
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Figure 4. One access point, DCF
Then the two rows are removed from the table, and the algorism is performed again to determine the next stations to transmit. So the produced collision free sets are {{4,10}, {5,0,8}, {7,6}, {3,11,9}, {1,12,13,2}}. All transmissions are competed after 5 steps, where as sequential polling would require 14 steps.
It is clear that the bandwidth is not shared in a fair manner, and that there are considerable variations in the mean delay of each user. The jitter introduced by the DCF access mode is considerable. 2) Point Coordination Function PCF ACCESS MODE
IV.
PERFORMANCE EVALUATION
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We performed a series of simulations in order to illustrate the advantages of the PCF access mode, and to verify the operation of the proposed super poll coordination scheme.
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A. Network Model All simulations have been implemented in the “Network Simulator” (ns2) [7]. The network consists of a number of wireless stations than send data to a wired node, through access points.
Throughput
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A CBR traffic generator was attached to each wireless station. The MAC protocol was the 802.11 module of the “Network Simulator” for the DCF case, and an extension of the 802.11 module was used for implementing the PCF protocol. The super point coordinator was implemented in a tcl script,
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Figure 5. One access point, PCF
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Total Throughput Kbps Min Throughput per User Kbps Max Throughput per User Kbps Mean Delay Sec Min Mean Delay Sec Max Mean Delay Sec Jitter Sec
The same series of simulations was carried out for the PCF access mode, and Figure 5. was produced. It is clear that PCF leads to fair bandwidth sharing, predictable average delay, and reduced jitter. Therefore an extension of PCF for multi-access point scenarios seems very desirable. C. Multi cell Topology The proposed algorithm was implemented in the “Network Simulator”. This algorithm was verified through numerous simulations, which consisted of a number of nodes communicating through some access points. Figure 6. was produced by a scenario with 10 access points and a variable number of wireless nodes. The locations of the access points and the wireless nodes are uniformly distributed.
PCF ACCESS MODE
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CONCLUSIONS
One of the most important issues in the WLAN environment is the QoS provision. In this paper we addressed the problem of providing hard- QoS to a multi-access point WLAN. Delivering hard QoS guarantees in the wireless domain is rather difficult since assumptions made in providing QoS guarantees in wired networks do not always hold in their wireless extension due to large-scale mobility requirements, limited radio channel resources and fluctuating network conditions. In this context, the Super Point Coordination scheme, a new extension of the 802.11 protocol, can be helpful for enabling the QoS-capable PCF access mode to operate in a multi-cell environment. This scheme is used for synchronizing the polls of a number of access points, in a way that can ensure that no collisions occur during the CFP, while attempting to maximize spatial reuse in the wireless spectrum. This scheme can be used to provide QoS service in large area wireless hop spots, so that new services can be supported.
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As it was shown in the paper, the PCF access mode is superior to the DCF access mode in terms of jitter and delay, and our new extension can provide the advantages of the PCF access mode for topologies where the traditional PCF cannot be implemented. Therefore, the proposed scheme has demonstrated exceptional behavior in quality of service provisioning for real-time applications in wireless local area networks.
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REFERENCES [1]
Figure 6. Multiple access points. PCF
[2]
V.
FUTURE WORK
The previous section shows that the PCF access mode can provide much better QoS than the DCF access mode, because it can provide low and predictable delay times and low jitter, and it can also lead to fare bandwidth sharing. Even though the super point coordination scheme for extending the PCF access mode for multi-access point environments appears promising, it still has a number of limitations. This scheme, in it’s current form, operates in a totally centralized manner, and therefore scalability issues may arise. Furthermore, some problems may occur, if two wireless domains are close to each other. At last, this scheme would be allot more useful if it could be also used for providing hard QoS to ad-hoc WLANs.
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[3]
[4]
[5]
[6]
[7]
IEEE std. 802.11, “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications“, 1999. A. Veres, A. Campbell, M. Barry, L. Sun, "Supporting service differentiation in wireless packet networks using distributed control", IEEE Journal on Selected Areas in Communications, no. 10, October 2001 pp. 2081-2093 G. Bianchi, "Performance analysis of the IEEE 802.11 distributed coordination function", IEEE Journal on Selected Areas in Communications, no. 3, March 2000 pp. 535-547 S. Mangold, S. Choi, G. Hiertz, O. Klein, B. Walke, "Analysis of IEEE 802.11e for QoS Support in Wireless LANs", IEEE Wireless Communications, no. 6, December 2003 H. Fattah, C. Leung, "An overview of scheduling algorithms in wireless multimedia networks", IEEE Wireless Communications, no. 5, October 2002 pp. 76-83. J. Ju, V. Li, "TDMA Scheduling Design of Multihop Packet Radio Networks Based on Latin Squares", IEEE Journal on Selected Areas in Communications, no. 8, August 1999 pp. 1345-1352 “The Network Simulator 2”, www.isi.edu/nsnam/ns
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