MTIM for IEEE 802.11 DCF Power Saving Mode Pengbo Si, Hong Ji, Guangxin Yue, Jingya Zhang Key Laboratory of Universal Wireless Communication, Ministry of Education Beijing University of Posts and Telecommunications Beijing, P.R. China E-mail:
[email protected] Abstract-In IEEE 802.11 wireless LAN, after the transmission of each beacon frame, there will be a large number of stations in power saving mode (PS mode, PSM) trying to send power saving poll (PS-Poll) frames to contend the right to receive the buffered results
downlink in
higher
packets. However, excessive contention collision
probability,
higher
power
consumption and lower throughput than usual. To solve this problem, we propose a downlink access scheme which modifies the traffic indication map (TIM) fields in beacon frames to enhance the control ability of access point (AP). This scheme which can be used in both the basic access scheme and the access scheme with RTS/CTS enables AP to constrain the number of stations in contention to be an optimal number. Furthermore,
the
proposed
MTIM
scheme
provides
a
convenient way for quality of service (QoS) differentiation by limiting the PS-Poll transmitting of stations with lower priority. Simulation
results
provided
in
this
paper
prove
the
performance improvement.
I.
INTRODUCTION
In recent years, wireless local area networks (Wireless LANs, WLANs) have become a very popular technology to provide high speed wireless access. IEEE 802.11 protocol [1] for Wireless LAN was approved by IEEE 802.11 task group in 1999. In the protocol, Medium Access Control (MAC) and Physical layer (PHY) specifications are included. Both infrastructure mode and Ad hoc mode are defined in 802.11 protocol. In an infrastructure Wireless LAN, access point (AP) and wireless stations compromise the entire network. IEEE 802. I I protocol also defined two mechanisms for media access control: distributed coordination function (DCF) and point coordination function (PCF). In this paper, only infrastructure WLAN with 802. I 1 DCF is considered, because it's simple and used most widely. Wireless stations in WLANs are generally battery-powered and thus energy limited. Consequently, power management is one of the key issues in Wireless LAN protocol design. IEEE 802. I I protocol allows wireless stations to work in power saving mode or active mode (AM). The transceivers of wireless stations in PS mode should be switched into awake state when they have packets to transmit and at every target beacon transmit time (TBTT), while they can be switched
into doze state when idle. A brief introduction to DCF and PSM will be provided in this paper. Power saving mode was provided by IEEE 802. I I protocol, and some other researchers are working towards the further reduction of the power consumption of wireless stations in Wireless LANs. The problem of collision in 802. I I WLANs is one of the most important reasons of wasting energy and time, because collisions result in retry actions or packet loss. To relieve the collision probability, laehyhuk Choi proposed a scheme by broadcasting notices of backoff time durations before data transmissions [2]. In [3], researchers believe that a number of continuous successful transmissions indicate the light traffic load, which should result in a smaller contention window. The authors of [4] have given a method for hidden terminals detection, and such a method provided a way to relieve the collision by hidden terminals. For stations in PSM, lung-Ryun Lee et al proposed a beacon-controlled scheme for congested WLANs [5]. In this scheme, AP calculates the maximum number of stations in PSM which should be allowed to contend the downlink transmissions. Beacon frames are used by AP to constrain the number of stations in contention. However, this maximum number is derived according to the transmission duration and the beacon interval, not the optimal number to minimize collision probability. Authors pointed out in [6] that by scheduling the AP's queues which buffer packets for stations in PSM, the overall performance will be improved. Some other schemes for power saving are also proposed. SPSM [7] gives up the fixed beacon interval by allowing wireless stations to wake up and fall asleep according to an "optimized" series of time points. The research products in [8] of Liang Chen, etc, considered the cross-layer optimization of both MAC protocol and TCP to save power. In this paper, to solve the problem of excessive contention which results in high collision probability, high power consumption and low throughput, we derive the optimal number of stations in contention through the analysis of the relationship between collision probability and the number of stations in contention. Furthermore, we propose a downlink access scheme which constrains the number of stations in contention to be the optimal number, by
This work was jointly supported by the National Natural Science Foundation of China (Grant No . 6 06721241F 010101) and Innovative Foundation Project for Post-Graduate Students, Beijing University of Posts and Telecommunications (No . 6, 2006).
1-4244-1474-1107/$25.00 ©2007 IEEE.
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modifYing the TIM fields in beacon frames. 802.11 PSM with Modified TIM (MTIM) not only performs better than standard IEEE 802.11 PSM protocol in power consumption, throughput and average delay, but also provides a convenient method for QoS differentiation by introducing a priority factor a which denotes how large the disparity in priority of services is. The proposed MTIM scheme suits to both the basic access scheme and the RTS/CTS access scheme, and the extension to multi-rate networks will be easily implemented. The overview of 802.11 DCF PSM is presented in section II. Section III describes the proposed MTIM scheme in detail. The simulation results are discussed in section IV and we will draw a conclusion in section V. II.
IEEE 802.11 DCF PSM
IEEE 802.ll protocol defined two MAC schemes: DCF which is used most widely, and point coordination function (PCF) which can only be used in infrastructure WLANs. DCF is based on carrier sense multiple access with collision avoidance (CSMAlCA) mechanism, allowing stations to access the channel randomly according to a distributed algorithm. Packets longer than a threshold are transmitted following RTS and CTS frames for resource reservation and collision probability reduction. This is called the access scheme with RTS/CTS. Transmission scheme without RTS/CTS is called the basic access scheme. Power management including PSM and AM was also defined in 802.11. The transceivers of wireless stations in AM are always active, while in PSM can be switched between "doze state" and "awake state". In infrastructure 802.11 WLANs, when a packet arrives at AP's MAC layer from the upper layer, if the destination address of the packet is a station in PSM, AP has to buffer it until the destination station wakes up and successfully transmits a power saving poll (PS-PolI). There's a TIM field in every beacon frame from AP to indicate the destination list of buffered packets. As long as a station in PSM receives a beacon frame in which the TIM field denotes that the station has packet(s) buffered in AP, it tries to transmit a PS-Poll frame according to DCF access scheme, i.e., after an idle DCF interframe space (OIFS) and backoff period, a PS-Poll frame will be sent. After successfully receiving a PS-Poll frame, AP should forward the data frame(s) after a short interframe space (SIFS) to the source station of the received PS-Poll, and an ACK frame following the data frame(s) is expected. III.
MTIM SCHEME
With standard IEEE 802.11 DCF PSM, after the successful transmission of each beacon frame, a large number of stations in PSM may keep awake to contend the downlink packets, because they know they have packets buffered in AP according to the TIM field in the beacon frame. However, excessive contention results in high collision probability, high power consumption and low throughput. Consequently, if the number of stations in
contention is limited, performance of be improved.
802.11 DCF PSM will
The modified TIM (MTIM) downlink access scheme for
802.11 PSM is designed to relieve the contention after each
TBTT. MTIM also provides the practical method for QoS differentiation and suits to both the basic access scheme and the access scheme with RTS/CTS.
The main idea of MTIM is to control the number of stations in contention by beacon frames from AP. In infrastructure 802.ll, AP acts as both a transceiver and a central control device. Consequently, enhancing AP's control ability enables the proposed MTIM scheme to be practical. AP evaluates the optimal number of stations in contention, and then modifies the TIM fields in beacon frames to constrain the stations' activities. The TIM fields are set according to the optimal number and the service type of each station. In MTIM, according to the size of contention window, the current traffic load, and the priorities of services, AP decides which stations have the right to contend the downlink packets buffered. With the modified TIM field which is set according to AP's decision, AP forwards beacon frames at each TBTT. Wireless stations need to add a new field into some of their frames. Nothing else should be changed compared to standard 802.11 protocol. A.
Estimate the Traffic Load
One of the duties of wireless stations with MTIM scheme is to estimate the traffic load and to report this information to AP. For the basic access scheme, an 8 bits paused time (PT) field is added into every PS-Poll and data frame to indicate how many times the transmission of this frame has been paused during the contention period because of other transmissions. Denote this number to be nn. For the access scheme with RTS/CTS, the same PT field with nn is added into each PS-Poll and RTS frame. The fields of only one byte length are added to some of the control frames, so the increase of MAC header can be ignored. nn counts from 0 to 255. nn 0 represents that the station gained the right to occupy the channel without being paused by any other transmission; 0 < npT < 255 represents that before occupying the channel, the contention period has been paused for nn times; nn 255 represents that the paused time equals to or larger than 255. AP collects this information as the estimation of the current traffic load of the network. =
=
During initialization, AP is expected to establish a PT table (PIT). After receiving a frame containing PT field, AP adds the number nn into PTT. Before every TBTT, AP computes the average value of the items in PTT and then clears PIT. Assume this average value to be NPT_mean. B.
Evaluate the Optimal Number of Contending Stations
To reduce the power consumption and the collision probability, and improve the throughput of WLAN, AP needs to calculate the optimal number of stations in contention.
351
Let Nrolai to be the total number of wireless stations in WLAN, NCom to be the total number of stations in contention, N.�llccess to be the number of wireless stations which successfully gain the right to occupy the channel, and New to be the number of timeslots in contention window (assuming no MAC layer retry). Ignoring the PRY bit error rate, for each station in contention, if and only if its selected number of backoff timeslots is different from the number selected by other stations, it will successfully gain the right to occupy the channel. Note that the slot access probability to be I1New, then the probability of station i selecting the kth backoff timeslots and gaining the right to occupy the channel is, ' pk
=
1 . P(NCI+ _ Chosen oF
*-
- eho."o
k,···,
Where N/:Il
-I k , .. . , N'ClI' _ Chosen
el",.,,,,
oF
k)
=
*-
k ' C'Chosen
=
I k' N'+ ell
)s 1 ",,"
( 1
1- -New New
--
(1)
represents the number of timeslots selected
.
v.
p
� =
� k=l
' pk
.
=
' N01' pk
=
v
( ell - l) U"' N
Set
TIM
Here we consider two situations:
1) There is only one service type in WLAN. 2) There are two or more service types in WLAN. For both of the situations, assume that after each TBTT, the number of stations which have packets buffered in AP is NBu[[ered l'SM Let the set of stations whose corresponding TIM bits are one to be S�td, if the standard IEEE 802.11 DCF PSM is used. In MTIM, if NrrM ;:0: NRuffered].IM, nothing should be changed; if NrrM < NRlljjered].IM, some of the stations in power saving mode should fall asleep after the beacon transmission.
Chosen
-I
by station j. Consequently, the probability of no collision between station i and other stations is, i
C.
-I
(2)
New
Assume that every station acts independently. The mean number of stations which transmit without collision is, (3) ModifYing the TIM field in beacon frame provides a method to control the number of stations in contention, for improving the probability of success. The derivative of NSllccessCNcom) in (3) is,
(4)
If there's only one service type in WLAN, randomly select NrrM_o NRllffered]SM - NTlM stations from S.�ld, and set the bits in TIM field corresponding to these selected Nl1M 0 stations to be zero. =
If there're n service types in WLAN, assume the set of stations with the highest priority is Spru, the set of stations with the second highest priority is SprU' , and the set of stations with the lowest priority is Spri n' Without loss of generality, we simplifY the situation of only two service types. We introduce a to be the priority factor, which denotes how large the margin is between the priority of the services with higher priority and the priority of the services with lower priority. a is defined in MTIM to be, if the number of stations with higher priority NHixher priority is larger than or equals to [Nnw x a ] , and the number of stations with lower priority NLower priority is larger than or equals to [NrrM x (a - I)]), then, • . •
in which NrrM_Higherpriorily denotes in MTIM, the number of higher priority stations whose corresponding bits in TIM field are set to be one, and NrrMJowerpriorily denotes the number of lower priority stations whose corresponding bits in TIM field are set to be one. According to the pre-defined a , Nl1M Hixher priority and Nl1M Lower priority are evaluated as:
Let (4) equals to zero and we can get the pole of (3): Npo," If
"nn =
In-1
[New/(New -1)] .
(5)
(4) is larger than zero; if NConl > NPo/e coni, (4) is less-than zero. So, NPo/e coni is the maximum point of (3). Let [.] represent the integral function, and NOpt cont [Nl'oie com] is the optimal number of wireless stations in contention. If the size of contention window is large, In(Newl Ncw- 1) tends to be 1 I New, so NOplJonl;:::; Ncw. NConl
