Efficiency optimization in WiFi Networks with Enhanced QoS Boost the utilized network capacity
Onur OGUZ Roxana OJEDA Zhipeng ZHAO Bruno ROUVIO
White Paper Series
WHITE PAPERS WP/QOSWF2014
iFi
Efficiency optimization in WiFi Networks with Enhanced QoS: Boost the utilized network capacity E XECUTIVE S UMMARY
terms of experienced QoS, number of supported services and energy consumption. QoS WiFi powered networks enjoy collision free data traffic and can exploit up to 90% of the network capacity. These networks can accommodate significantly more stations/services and also they cope better with the mixed services networks with respect to the standard counterparts. Along with its seamless integration in the WiFi standard, QoS WiFi becomes a simple yet powerful mechanism toward efficient digital homes and big data networks.
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With the increased number of smart devices trying to access and communicate over the same wireless channel, more packet collisions occur leading to increased number of retransmissions. This phenomena drastically impairs the network efficiency. Unfortunately the current WiFi MAC protocol fails to address this issue properly and presents significant bottlenecks; the network resources are poorly managed and the utilized network capacity is far from being optimal. As a result, the wireless network suffers from Packet Drops, Energy Waste as well as Capacity Waste.
Comsis’ QoS WiFi is fully compatible with the WiFi standard and has been tested extensively by Orange to certify its benefits. It is our pleasure to partner with ”Digital Home” and ”Big Data” device providers to build the efficient networks addressing the requirements of today and tomorrow.
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A better MAC mechanism is the key to overcome the issues plaguing wireless networks. Comsis, with its proven R&D background and in depth knowledge of WiFi networks, has developed and implemented a fresh MAC mechanism, namely QoS WiFi, which can boost the wireless network efficiency in
Contents Revision History . . . . . . . . . . . . . . . . . . . . . . .
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Case Study: A Mixed Service Network . . . . . . . . . .
Congested Wireless Space . . . . . . . . . . . . . . . . .
1
QoS WiFi in Questions . . . . . . . . . . . . . . . . . . . 11
Collision Avoidance in IEEE 802.11 . . . . . . . . . . . .
3
About Comsis . . . . . . . . . . . . . . . . . . . . . . . . 12
Service Awareness for QoS: EDCA . . . . . . . . . . . .
5
Notice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
QoS WiFi - A Fresh Approach to Medium Access Control
7
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . 14
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CONTENTS
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R EVISION H ISTORY V ERSION 0.0 alpha beta 1.0
E XPLANATION - Created - First Draft - Final Draft - Release
AIFS AP CSMA CTS CW CWF DCF DIFS EDCA IP MAC PCF QoS RTS SIFS SOC TCF TXOP WLAN
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D ATE 2014/01 2014/02 2014/03 2014/03
Arbitration inter-frame space Access Point Carrier sense multiple access Clear to send Contention window contention window function Distributed coordination function DCF Inter-frame Space Enhanced Distributed Channel Access Intellectual property Medium access control Point coordination function Quality of Service Request to send Short Inter-frame Space System on Chip Tournament Contention Function Transmission Opportunity Wireless local area network
Throughput
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Figure 2. Throughput and average successful frame delays versus number of smart devices in the network.
of stations (smart devices) and networks, sharing the channel, it becomes more and more difficult to answer the questions such as ”which device uses the channel?”, ”when it uses the channel?” and ”for how long does it uses the channel?”, and allocate network resources accordingly in a distributed fashion.
Co ms is
There are many different examples of connected indoor services, such as entertainment, security, office management, energy management, smart homes etc. These services are handled over a distributed array of networks, and inevitably, the huge portion of data traffic generated by these networks is communicated over the air (i.e. wireless networks). Given its ability to support high data rates, its low operational cost (license free ISM band) and its ubiquity, the WiFi networks will be the work horses of the indoor data highways.
100%
AveragenFramenDelayn[ms]
The omnipresence of high speed data access and involvement of numerous smart devices in our daily lives is having a remarkable effect on indoor (home or office) networking technologies. With the internet of things concept, practically every appliance and piece of consumer electronics is moving towards acquiring a digital idientifier, making it addressable and connected on the net. Consumers today already own multiple types of connected devices and this number will continue to increase [1]. According to ABI Research, connected devices are potentially the ”Next Big Thing” in the consumer electronics industry and could reach a global market value of US $10 billion in 2014. The figure will balloon with growth in sensor and control technologies, mobile applications, network traffic, big data management, analytics and cloud computing.
UtilizednNetworknCapacity
Congested Wireless Space
These challenges are addressed by the medium access control (MAC) mechanisms that are usually described by the standard of the wireless network technology at hand. UnfortuThe widest adopted WiFi standard (IEEE802.11n) can achieve nately, even for one of the most advanced standard i.e. IEEE up to 600 Mbps data rate [2, Table 20-37] and with the forth802.11 WiFi family, the MAC mechanism is far from being opcoming standards (e.g., 11ac) the available rates will soar. On timal. the other hand, wireless network throughput can reach these data rates only under very specific conditions. An important Two main shortcomings of the most widely used WiFi MAC bottleneck of the wireless networks is the efficient utilization of mechanism is visualized in Figure 2 for an example network. the wireless medium i.e. channel. With the increasing number In this illustrative network each smart device joining the network generates a frame traffic (hence requires a throughput) around 3% of the network capacity, in order to communicate its information. It is evident from the figure that, even at its peak, throughput (successfully communicated frames) can not exceed 60% of the network capacity. Furthermore, over a certain number of smart devices in the network, the throughput decreases drastically due to the collisions in the wireless network.
Wireless Communication Medium
The second issue illustrated in Figure 2 is the latency of successfully transmitted frames within the wireless network. It can be seen that after a certain point the successful frame latency becomes saturated even when the throughput reduces. Unlike our toy network, a common network would be composed of different devices offering different services where the data rate requirements and the latency tolerances differ for each type of service. And when delay sensitive services are concerned (e.g., HD video) the frame latency plays a significant role to ensure quality of service (QoS) experienced by the service user.
Figure 1. Smart devices in a wireless network sharing a common channel, compete for the channel use.
In a nutshell, with the increased number of smart devices trying to access and communicate over the same wireless channel, more packet collisions occur leading to increased number of retransmissions. This phenomena drastically impairs the
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CONGESTED WIRELESS SPACE
2
network efficiency. Unfortunately the current WiFi MAC protocol fails to address this issue properly and presents significant bottlenecks; the network resources are poorly managed and the utilized network capacity is far from being optimal.
Packet Delays/Drops For delay sensitive services such as Video Call, it may happen that a collided packet doesn’t get transmitted within a certain duration and becomes invalid. In such a case, the end user would suffer from distorted QoS. Energy Waste An energy bill is attached to each retransmission of a given packet. In other words, avoiding collisions, hence retransmissions, is a key to an energy efficient network. Capacity Waste As a result of a deficient medium access control (MAC) mechanism, only a portion of the network’s data capacity gets utilized. In fact, if the number of stations in a network increases beyond certain point, the utilized capacity (network throughput) starts to suffer drastically.
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iFi
A better MAC mechanism is the key to overcome the issues plaguing wireless networks. In this white paper, we present a fresh MAC mechanism, namely QoS WiFi, which can boost the wireless network efficiency in terms of number of supported services and energy efficiency. In Comsis we have developed QoS WiFi in a way that it fits seamlessly in the current and future WiFi standards.
Inefficient MAC plagues networks
Collision Avoidance in IEEE 802.11
IEEE 802.11 standard defines two MAC methods, namely Point coordination function (PCF) for the star shaped networks and Distributed Coordination Function (DCF) for the mesh networks, in order to manage the shared channel access among stations.
AIFSi
Backoff Slots –
Backoff Slots – decrease counter
Busy Medium
AIFSk
Contention Window DIFS SIFS
Data Frame
Backoff Slots – decrease counter Slot time
Defer Access
Immediate access when counter reaches zero
Figure 3. IEEE 802.11 channel access mechanisms: DCF and EDCA.
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PCF resides in an Access Point (AP) to coordinate the communication within the star topology network. In this mode channel access coordination is centralized, the AP sends a poll frame to the PCF capable station to permit it to transmit a frame. Due to the priority of PCF over DCF, stations that only use DCF might not gain access to the medium. As the channel is allocated centrally, there is no need for contention. Furthermore, if the AP is located in the geographical center of the network, hidden node problem is avoided by PCF method. On the other hand, in the star topology networks, devices must talk to each other via the AP which requires two transmissions per frame i.e. source STA to AP and AP to destination STA. This hampers the network capacity utilization significantly and hence star topology (or PCF) seems to be supported only in very few hardware devices as it is not part of the WiFi Alliance’s inter-operability standard.
IDLE MEDIUM
Busy Medium
The main challenge that the wireless networks are facing is the organization of the traffic over the wireless medium. The MAC mechanism used throughout the network determines the efficiency of the network. Briefly, the MAC mechanism aims to avoid the frame collisions in the wireless network.
DCF is the fundamental MAC technique of the IEEE 802.11 based WLAN standard. DCF employs a CSMA/CA with binary exponential backoff algorithm to allow decentralized access. Besides its basic implementation, IEEE has described some optional improvements to remedy issues such as hidden node problem, excess frame latency problem for sensitive services and time/energy waste due to frame collisions/retransmissions.
Frame Collisions - The networks suffer from 2 types of collisions; Header collisions and Hidden node collisions. Header collisions occur at the beginning of the frame when more than one stations try to access the channel simultaneously, while the hidden node collision is caused by a ”hidden node” which can not detect the busy channel and starts transmitting its frames. - Collisions cause retransmissions which translate to packet delays, excess channel occupancy, energy waste etc.
transmission attempt of a given data frame, CW is set equal to the minimum contention window size (CWmin ). The station decrements its backoff interval counter by one for each idle ”slot duration” observed after DCF Inter-frame Space (DIFS). The countdown is paused when activity is detected on the channel, and resumed when the channel is sensed idle for more than a DIFS again. The station tries to transmit its frame when the backoff counter reaches zero. (cf. Figure 3). If the transmission is not successful (i.e. collision or error), the station doubles the bound value CW until the maximum contention window size (i.e. CWmax ) is reached. Note that, larger contention windows slow down the transmission of packets and reduce the probability of header collisions. In case of a successful (i.e. collision-free) transmission, the station resets its contention window bound to CWmin . Common MAC parameters used in DCF over different 802.11 based systems are listed in Table 1. Table 1. Common MAC Parameters used in WiFi Slot Time tSIFS tDIFS CWmin CWmax
802.11a/n/ac 9µs 16µs 34µs 15 1023
802.11b 20µs 10µs 50µs 31 1023
RTS/CTS O PTION D ISTRIBUTED C OORDINATION F UNCTION When more than one node need to access the channel, it is necessary to implement a coordinated channel access mechanism to increase the chance of collision free transmissions. The DCF MAC mechanism is illustrated in Figure 3. Briefly, any station aiming to access the channel selects a random backoff interval, an integer that is in the range (0, CW). Here CW is the contention window upper bound. For the first
The stations in the wireless network do not use collision detection function as they cannot detect collisions by listening their own transmissions. Instead a handshaking method is employed, making use of a positive acknowledgment (ACK) frame (14 bytes) from the destination. Depending on the use of the optional mode, additional control signals RTS and CTS are employed during the handshaking. This process is illustrated in Figure 4. When the destination station successfully receives a frame (or RTS), it will transmit an acknowledgment frame (ACK or
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COLLISION AVOIDANCE IN IEEE 802.11
Another advantage of the RTS/CTS mechanism is to limit the time lost due to header collision. This follows the fact that the RTS frame is much shorter than the data bearing frame. Comparison provided in Figure 5 shows that, even without the hidden nodes in the network, the gained time thanks to RTS/CTS mechanism helps reducing the average packet delay as well as improving the system efficiency.
Data
Source SIFS
Destination
SIFS CTS
SIFS
DIFS ACK
Figure 4. IEEE 802.11 CSMA/CA Protocol handshaking
CTS) after a short inter-frame space (SIFS). If the sender does not receive the response control frame within a specified timeout, or it detects the transmission of a different frame on the channel, it reschedules the frame transmission according to the previous backoff rules.
employing/Rts/CTS without/RTS/CTS
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Figure 5. Comparison of throughput and average successful frame delays with and without RTS/CTS mechanism.
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When the optional access method is used, an RTS frame (20 bytes) is transmitted by the source and the destination should accept the data transmission by sending a CTS frame (14 bytes) prior to the transmission of the actual data packet. Note that the stations residing in the senders range and hearing the RTS packet should defer their transmissions for the duration specified by the RTS. Nodes that are hidden from the transmitter but residing in the range of the receiver can overhear the CTS packet and hence refrain from transmitting. In this way, transmission can proceed without interference from the hidden nodes.
Generated/Traffic
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Note that the collisions do not always occur at the beginning of a transmission (header collisions). Consider the situation where a number of transmitting stations that are hidden from each other, trying to communicate with a common receiver. In this case they would continue with their backoff countdown process even if the channel is busy, as they are outside of the actual transmitter’s range and hence assume the medium to be idle. Eventually, as its counter reaches zero, a hidden station would transmit its data frame causing collision at the receiver. To overcome this issue, optional RTS/CTS mechanism is added to the transmission.
100u
Average Frame Delay [ms]
If a failed transmission is encountered after the retransmission limit, the frame will be dropped.
On the other hand, as illustrated in Figure 4, the optional RTS/CTS overhead introduces additional time (i.e. 2× tSIF S + tRT S + tCT S ) and hence may not be utilized for short data frames.
iFi
RTS
Basic access method
Utilized Network Capacity
Optional RTS/CTS overhead
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Service Awareness for QoS: EDCA Devices sharing a wireless medium create different types of contents/services. These services usually have different performance requirements including availability (uptime), bandwidth (throughput), latency (delay), and error rate. The goal of QoS is to guarantee the ability of a network to deliver predictable results for given services. In IEEE 802.11 WLAN standards, the QoS concerning the latency is targeted through discrimination of services and prioritization of network traffic accordingly. This approach is described in Section 9.2.4.2 of the standard [2] as an enhanced distributed channel access (EDCA) mechanism. E NHANCED D ISTRIBUTED C HANNEL A CCESS : EDCA
Service Voice Call Video Call Digital TV Music Surveillance Web Surfing
Size bytes 48 20000 20000 20 7000 1500
Briefly, high priority service classes uses that use smaller CWmin and CWmax thus benefit from reduced backoff period. This leads to lower latency for these frames and hence higher QoS. In addition, AIFS replaces DIFS in EDCA approach. According to this, each AC uses different AIFS period before resuming their contention process. Stations that use lower AIFS encounter fewer collisions and count down the backoff counter faster than the other stations; also leading to a better QoS. These modifications are illustrated previously in Figure 3. Finally, EDCA allows stations to transmit multiple frames of AC2 and AC3 without contending again, known as contention free bursting (CFB). CFB is limited by the TXOP limit specified for each service class. Longer limit means that the service class can transmit more frames; hence, it receives better QoS.
EDCA capable stations, implements a queue for each AC such that each queue has its own QoS parameters and backoff counter. This may result in a collision within the station (i.e. virtual collision) and this case is handled virtually simply by selecting and transmitting the highest priority queue involved in the collision. Table 2. Default EDCA Parameters for each AC AC Type 0-Background 1-Best Effort 2-Video 3-Voice DCF
CWmin 15 15 7 3 15
CWmax 1023 1023 15 7 1023
AIFS 7 3 2 2 2
St.
AC
4 4 2 1 4 1
AC3 AC3 AC2 AC2 AC1 AC0
Packets/ St. 50K 30K 75K 50K 30K 10K
In order to visualize the QoS improvements introduced by EDCA, a mixed AC network with 16 stations is analyzed. The composition of the network is given in Table 3, and the average frame delay per service is illustrated in Figure 6 for three MAC mechanisms: DCF without RTS/CTS, DCF (with RTS/CTS) and EDCA (with RTS/CTS).
[ms]
30
DCFhwithouthRTS/CTS
DCF
EDCA
Frame Latency
For each AC, an enhanced variant of the DCF contends for transmission opportunity (TXOP) using a set of EDCA parameters i.e. contention window size limits, Arbitration interframe space (AIFS), and TXOP limit. Common parameters are listed in Table 2.
Freq. per s. 50 30 75 50 30 -
EDCA I MPROVEMENTS
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EDCA discriminates between four service types by attributing to it one of the four access categories (AC): AC0 (Background), AC1 (Best Effort), AC2 (Video) and AC3 (Voice). Here the lowest priority service is assigned to AC0 while the highest priority service is assigned to AC3.
Table 3. A mixed service network
25
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VoicehCall VideohCall AC3 AC3
DigitalhTV AC2
Music AC2
Surveillance AC1
Web AC0
Figure 6. Frame latencies in a mixed network. It is clear from the average frame latencies that EDCA prioritizes AC3 services and reduces the frame latencies by 25% with respect to the DCF, and by more than 50% for the case without RTS/CTS. Concerning the frame latencies of AC2 services, the performance of DCF and EDCA are comparable. On the contrary, EDCA severely penalizes AC1 and AC0 services, for whom the frame latencies with EDCA exceeds even that of DCF without RTS/CTS. Nevertheless, this increase in latency usually is not noticed by the service user and hence it has no impact on the experienced QoS. S HORTCOMINGS OF EDCA
TXOPmax 0 0 3.008ms 1.504ms 0
The QoS provided by EDCA is not without a price. As explained above, the latency for high priority access categories is reduced by using smaller maximum contention window size, which leads to a reduced backoff period. This approach introduces a bottleneck for the network with respect to the number of devices that can be supported which is bounded by the CW size.
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SERVICE AWARENESS FOR QOS: EDCA
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AC1 80% AC2
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Figure 7. Throughput and average number of head collisions per frame associated with different ACs in EDCA. Collisions translate to energy consumption due to frame retransmissions.
Another result of the small CW is the increased number of collisions for priority AC services, and this causes excessive frame retransmissions hence energy consumption as well as inefficient channel use.
It is evident from Figure 7 that, the network can accommodate only 11 AC3 stations in total without having the throughput deviated considerably from the required rate. And it can attain only slightly above 30% of the channel capacity. As seen from the figure, the throughput varies for different ACs and remains short of 80% channel utilization at its best for AC1 services which has almost identical parametrization with DCF as given in Table 2, and doesn’t oversee frame latency. On the other hand, it is important to note that, the number of collisions per frame increases drastically for priority services, or in other words, with decreasing CW size. For the given network, the AC3 services experience more than four collisions (leading to five retransmissions) per frame in case of the arrangement of 11 stations. On the contrary, lower priority services do not suffer from collisions as severely, thanks to their large CWs.
iFi
Generated Traffic
AC0
90%
Number of collisions per frame
Utilized Network Capacity
100%
EDCA drawbacks:
Soared collision rates for high priority services Can not accommodate many high AC service stations Penalization of low priority services Energy use is much higher than DCF due to collisions.
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As an example, consider a network composed of identical AC stations, where each station generates data traffic around 3.0% of the channel capacity. For this scenario, the total throughput of the network, and the average number of head collisions per frame is depicted in Figure 7 as a function of the number of stations in the network and as a function of AC.
QoS WiFi - A Fresh Approach to Medium Access Control
Several alternative MAC mechanisms have been introduced in the literature to remedy the shortcomings of CWF based protocols. It is reported that a significant improvement can be achieved by so called Tournament Contention Function (TCF) based protocols [3].
Station C Key = 101010 (hidden from A)
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DCF and EDCA protocols used in IEEE 802.11 utilize a contention window function (CWF) to coordinate the medium access where the size of the contention window is doubled when a collision occurs. However, as explained previously, these protocols exhibit rapidly increased collision rates with the number of stations in the network.
E1
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S1
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Figure 8. TCF used in QoS WiFi. The origin of received signals are indicated by associated colors.
In a nutshell, TCF protocols employ a bounded contention phase, during which all the stations with a packet to transmit, compete in a tournament for the next transmission right. The rare case where several stations win the tournament results in a collision. A TCF base protocol has been also proposed by the Hiperlan [4], a twin standard of IEEE 802.11a developed in the same period.
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station with key value ”0” hears any other node’s transmission during the first slot of the round, it looses the tournament and acts as a repeater (echo) in the remainder of the tournament. The echo is sent during the second slot of the round and they ensure hidden node participation in TCF. In Figure 8 the stations A and C are disqualified at rounds 1 and 4 respectively. Based on TCF, Comsis, with Orange , has developed and prototyped a solution that maximizes the efficiency of the tourAfter N rounds, the terminal which has only particinament protocol providing equity between users and a bounded pated at the tournament in the transmission mode or access time as required by multimedia services. It is referred has not heard at least one station when it was in the to as QoS WiFi and to our knowledge, this is the first WiFi proreception mode, wins the tournament and therefore, it totype taking advantage of a TCF. can send a frame immediately during the first available transmission interval. Thanks to our optimized key generation algorithm, it is highly probable that there will T OURNAMENT C ONTENTION F UNCTION be only one winner. TCF based MAC mechanism provides the authorization or the Completion The winner of a tournament must know when it prohibition to send a standard frame during the next available can send the next frame. For this purpose, stations need transmission interval for the participant stations. The decision to be coordinated with the legacy frame exchange mechis based on a set of exchanges i.e. tournament, between the anisms. Thus, the station that has initialized the tournacontending stations. For this purpose, each station generates a ment will send a new synchronization signal i.e. tournakey, that is an N -digit binary sequence. ment final (TF), and each terminal hearing the synchronization will repeat this TF to help the hidden nodes to The QoS WiFi tournament is played in three phases, namely synchronize. Initialization, Development and Completion. An illustration of the tournament between three stations (A, B and C) is proNote that the QoS stations (including the creator) may also vided in Figure 8. participate in tournaments without the intention of winning if they do not need to use the channel. These stations utilize a Initialization This phase is divided into three slots. During zero key to generate echoes to avoid that a hidden node initithe first slot, a terminal creates tournament by transmit- ates an exchange before the end of the tournament. ting a tournament start (TS) signal. All other stations which detect the TS signal synchronize with the tournaR OUND R OBIN A CCESS ment creator. In Figure 8 station A is the creator. During the second slot, a tournament echo (TE) signal is transmitted by all the stations that have received TS. This aims to permit the hidden stations (cf. Station C in Figure 8) that have not received TE to participate in the tournament. The hidden stations receiving TE send a hidden tournament (TH) signal during the third slot to inform their participation in the tournament. Development This phase is divided into N rounds each of two slots, where the stations act with respect to the corresponding digit of their key. The value ”1” will allow the transmission of a signal ”S1” and the value ”0” will indicate that the station must listen to the channel. If a
In theory, an optimal key generation algorithm can reduce the collision probability down to 5% for a tournament of 6 rounds between up to 50 contending stations [3]. However, to further reduce the variance of access times, we introduced an optional Round-Robin key generation algorithm where it is insured that all the participant stations win the tournament once before a station wins it for the second time. This is illustrated in Figure 9. Briefly, at the beginning of the procedure, each station picks a random key and the tournament takes place. Evidently, the terminal with the highest key wins the tournament and the
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011011
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Figure 10. With QoS WiFi, throughput in excess of 90% network capacity can be attained
Figure 9. In Round Robin mode, different station wins each successive tournament.
right to send its data after the completion phase of the tournament.
The QoS WiFi network owe its superior efficiency to the prevention of frame collisions via an optimized TCF. In Figure 11, the collisions per frame for different contention mechanisms are shown. To be fair, each mechanism is analyzed where the network size is limited to achieve a throughput satisfying 99.9% of the generated traffic, e.g., 11 stations for EDCA AC3, 25 stations for DCF, or 29 stations for QoS WiFi. As seen from this comparison, at their optimum load, DCF based networks experience more than one collisions per frame while EDCA access may experience as much as 4.3 collisions per frame on average. This leads to at least five (re)transmissions of each data packet!
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The Round-Robin scheduling takes place in the following tournament with the procedure for updating the keys. As all the stations know in principle the winning key, they can generate a new key as a function of the one that won the previous tournament. Basically, instead of selecting a new random key, the stations just shift their keys by an amount determined by the winning key.
date much more stations and/or services than DCF or EDCA counterparts.
iFi
101010
Succesive Tournaments
101101
Utilized Network Capacity
QOS WIFI - A FRESH APPROACH TO MEDIUM ACCESS CONTROL
If more than one stations win the tournament (i.e. collision) then these stations each pick a new key at random.
The situation is improved drastically for the case of QoS WiFi contention; despite the fact that QoS WiFi powered networks accommodate much more stations/services, they experience 0.13 collisions per frame when operating in the ”Random” key mode and 4.13 collisions per 100K frames when operating in ”Round Robin” mode. This means that, in practice QoS WiFi powered networks transmit each frame only once.
In Round Robin mode, collisions can only occur when new stations arrive in the system. When a collision is detected, the colliding stations re-initialize their key generation algorithm (random behavior) which has a fair chance to avoid next collision. As a result, the stations converge rather quickly to a collision-free key states where all stations persistently transmit.
Collisions / Frames 1.51 / 1
1.30 / 1
4.30 / 1
QoS WiFi
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We can view the Round Robin procedure as a cyclic selection of the stations, which operates in a distributed context. The system nicely adapts to various numbers of stations and has a fair chance to avoid collisions from the first tournament. Then, thanks to our optimized key update algorithm, the stations will keep their key spacings corresponding to the state without collision.
3.44 / 1
1.40 / 1
Q O S W I F I B OOSTS N ETWORK E FFICIENCY 1.3 / 10
As explained above, QoS WiFi prevent collisions even in a crowded network, thanks to its optimized contention mechanism. Obviously, any reduction in the packet collisions would increase the network efficiency as new frames from additional stations and/or services may be handled by the network, instead of retransmitting collided frames. The improvement in the network throughput is illustrated in Figure 10 where the utilized network capacity with increasing number of stations is compared for different MAC mechanisms. This comparison underlines that for the given network configuration, QoS WiFi can exploit in excess of 90% of the network capacity; this is 10% more than the closest alternative. More importantly, QoS WiFi powered networks can accommo-
25 stations
22 stations
24 stations
19 stations
11 stations
29 stations
29 stations 4.13 / 100K
DCF
AC0
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AC2
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Round Robin
Figure 11. Average number of collisions for different MAC mechanisms operating within the largest supported near-lossless network.
Case Study: A Mixed Service Network 1.0E+01
Collisions per Frame
Here we will analyze the quality of service provided by different MAC mechanisms in a mixed network which accommodates ”Voice Call”, ”Video Call”, ”Digital TV”, ”Music”, ”Surveillance” and ”Web” following services simultaneously. Such a network with 16 stations has been described previously in Table 3. The traffic generated by each station is also given in Table 3 and amounts to a total of 50.1248 Mbps.
DCF
EDCA
QoS Random
QoS RR
1.0E+00 1.0E-01 1.0E-02
1.0E-03 1.0E-04
In the analysis we will make following assumptions in order to single out different MAC mechanisms’ impact on the QoS:
1.0E-05
Voice Call AC3
Video Call AC3
Digital TV AC2
Music AC2
Surveillance AC1
Web AC0
Overall
The excess traffic and hence energy consumption caused by the collisions can be observed by comparing the channel occupation (i.e. active and idle periods) of the network. In Figure 13 channel occupation for a total load of 650K frames is depicted for different contention mechanisms. It is clear from the figure that QoS based services occupy the channel only 84.25% of the time which indicates a traffic just above the 50.1248 Mbps generated/required by the network. On the other hand, the standard MAC mechanisms DCF and EDCA cause around 90% channel occupation indicating a traffic of 58.5 Mbps, that is an additional 8.37 Mbps (16.7%) consuming network resources just for frame retransmissions due to header collisions. Another consequence of the collisions is the frame drops due to the excess latency. As illustrated in Figure 14, for the simulated network, especially AC2 and AC3 services suffer from frame losses (up to 6% loss) with standard contention
DCF
100R
Frame Success Rate
First of all, let’s look at the frame collisions in this 16-station mixed service network. In Figure 12 the average collisions per frame is identified with respect to the service (or AC) as well as the overall collision rate. In alignment with our previous discussions, EDCA turns out to be the only mechanism that experiences more than one collision per frame. Especially for the AC3 services the two collisions per frame are observed when EDCA is utilized. The collisions in DCF and QoS networks are distributed rather evenly as these do not discriminate between services. On the other hand, while DCF experiences one collision per two frames on average, QoS WiFi experiences only 5 collisions per 100 frames in the random mode, and 2.2 collisions per 10K frames in the round robin mode.
EDCA
84.25%
QoSWRandom
15.77%
84.23%
15.75%
8.27%
91.73%
89.76%
Channel Occ
10.24%
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Only Header Collisions: We assume that the RTS/CTS op- Figure 12. Frame collision rate for different services in a mixed network employing different MAC tion in the standard, or the TCF of QoS WiFi completely mechanisms. solves the hidden node problem, therefore we assume only header collisions take place in the network. Error Free Frames: In the analysis, we are not interested in 100% Busy Idle error correction capabilities of the network, therefore 95% we consider a collision free transmission is a successful transmission i.e. throughput is equal to goodput. 90% IEEE 802.11n PHY The stations employ high throughput 802.11n85% PHY with a 20 Mhz bandwidth and MCS-7 which limits 80% the theoretical network throughput to 65 Mbps. RTS/CTS This option is used only for the data frames where 75% DCF the data packet size is greater then 5000 bytes. This EDCA QoS6R andom means, in the network at hand RTS/CTS mechanism is QoS6R ound6R obin employed only for the ”Video Call”, ”Digital TV” and ”Surveillance” services. Figure 13. Channel occupation statistics QoSWRR
98R
96R
94R
92R
VoiceWCall AC3
VideoWCall AC3
DigitalWTV AC2
Music AC2
Surveillance AC1
Web AC0
Overall
Figure 14. The frame success rate for different services in a mixed network
mechanisms while QoS based contentions reduce the frame drop rate considerably. Note that the frame drop phenomena affects AC2 and AC3 services severely as the losses deteriorates the experienced QoS. This can be seen by comparing two screen captures i.e. employing QoS WiFi in Figure 15 and with EDCA in Figure 16. The simulation results clearly illustrate the superiority of QoS WiFi over the standard MAC mechanisms. QoS WiFi powered networks enjoy virtually collision free data traffic which translates into numerous improvements for the network such as QoS enhancement, increased number of services, energy efficiency, etc. Along with its seamless integration in the WiFi standard, QoS WiFi becomes a simple yet powerful mechanism toward efficient digital homes and big data networks.
COMSIS S.A.S— 3 Rue Broussais, 75014, Paris, France. Tel: +33.1.45.89.17.00 — http://www.comsis.fr
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CASE STUDY: A MIXED SERVICE NETWORK
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Figure 15. Screen capture from ”Big Buck Bunny” at a receiver station of a QoS WiFi powered network.
Figure 16. Screen capture from ”Big Buck Bunny” at a receiver station of a EDCA powered network.
QoS WiFi in Questions W HAT ARE THE SIGNIFICANT BENEFITS OF Q O S W I F I ? Among the numerous benefits of QoS WiFi, most significant ones are: • • • • • •
Compliance with traditional WiFi systems Implicit hidden node avoidance Collision free network traffic Energy saving Service fairness Bounded access time providing better QoS for multimedia services
I S Q O S W I F I IEEE 802.11 WLAN COMPLIANT ?
You don’t need to modify the standard 802.11n internal functionality of the 802.11n chip as the QoS technology is complaint with the standard. However, additional I/Os are required a standard 802.11n chip to allow data exchange. A RE THE Comsis IP S PATENTED ? Yes, Comsis has 11 patents, ten of which are specific to MIMO architecture and implementation of MIMO algorithms and one is specific to QoS technology. Our IPs are the result of many years of fundamental research and a unique combination of know-how and applied mathematics.
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QoS WiFi is compliant with the IEEE 802.11 WLAN standards including 11n and 11ac. When the QoS functionality is enabled, QoS appliances generate specific frames which are only useful for the QoS appliances but that are exchanged (Tx and Rx) using the standard’s mechanisms. It can be integrated on any WiFi standard and regardless of the number of receiver antennas.
W HAT KIND OF MODIFICATIONS MUST BE IMPLEMENTED ON A STANDARD 802.11 N IP TO TAKE ADVANTAGE OF THE Q O S FUNCTIONALITY PROVIDED BY AN EXTERNAL COMPONENT ?
C AN WE ADD Q O S W I F I FEATURE TO ON THE SELF DEVICES ?
Yes, QoS WiFi feature can be enabled for on the shelf devices through QoS powered USB WiFi adapters. C AN Q O S W I F I TECHNOLOGY BE INTEGRATED ON EXISTING 11 N / AC SOC S ?
Comsis has spent 5 years working on the the QoS WiFi technology. During this time we developed a complete Baseband and Radio board, the entire IEEE 802.11n protocol stack and validated QoS WiFi on test bench. Given these efforts, we can extract and provide the QoS IP technology to be implemented on a SOC. W HAT IS THE INCREASE IN THE DIE SIZE WHEN IMPLEMENTED ON A EXISTING 802.11 N S O C? Adding QoS functionality does not increase the die more than 5% due to baseband operations. C OULD Q O S W I F I BE INTEGRATED AS A STANDALONE CHIP ON AN 802.11 N ARCHITECTURE ? Yes, the QoS functionality can be integrated as a standalone chip (violet) on the architecture of a standard 802.11n circuit. The functionality needs to exchange control, status and configuration signals with a standard 802.11n MAC IP and shares the analog devices with the 802.11n PHY IP. Therefore, the 802.11n IP must be able to provide information to the QoS MAC layer and to receive essentially timing values from it. The Analog Management module allows both of the PHY modules to share the same analog modules by switching between them.
D O UWB OR PLC SYSTEMS COMPETE WITH 802.11 N / AC / AD
Ultra-wide band (UWB) is a complementary peer-to-peer technology which is dedicated to high data rate transmission over a limited distance. Power-line communication (PLC) systems have several drawbacks concerning attainable data rates and network flexibility as each station must be directly connected to a power outlet. Neither of these systems compete with IEEE 802.11n/ac/ad.
I S THERE ANY DEGRADATION IN Q O S PERFORMANCE WHEN TRANSMITTING VIDEO AND SMALL VOICE FRAMES ?
The test scenarios include simultaneous data pack and (small) voice frames with no degradation of these services. I S IT POSSIBLE TO IMPLEMENT Q O S FUNCTIONALITY AND THE W I F I SYSTEM ON TWO INDEPENDENT CHIPSETS ?
The QoS functionality can be implemented in a standalone circuit which can be interfaced with the WiFi chip. D OES IT MAKE SENSE TO INCLUDE THE Q O S FUNCTIONALITY IN A 3×3 MIMO CIRCUIT WHICH ALREADY PROVIDES A VERY HIGH THEORETICAL RATE ? QoS targets a collision free network traffic, therefore regardless of the number of antennas, the QoS functionality will improve the effective data throughput. W HAT IS THE BEST CHOICE BETWEEN A 2×2 MIMO CIRCUIT AND A SISO Q O S CIRCUIT ? As the number of users in a network increases, the effective rate of a 2×2 MIMO circuit can attain half of the theoretical value. With the QoS functionality, you approach the theoretical limit regardless of the number of users. As a consequence, the 2×2 MIMO performs as an expensive circuit with a SISO equivalent data throughput. It should be noted that the QoS functionality also reduces the channel access time which is a requirement for quality video service.
COMSIS S.A.S— 3 Rue Broussais, 75014, Paris, France. Tel: +33.1.45.89.17.00 — http://www.comsis.fr
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About Comsis Comsis is a fabless semiconductor company headquartered in Paris. In Comsis, we provide IEEE 802.11n Wireless Local Area Network (Wlan) broadband communication intellectual properties (IP), system-on-chips (SoC) and products. Comsis-enabled systems allow users to access voice, data and streaming video throughout a home/enterprise environment or outdoors with outstanding quality. The combination of superior technology and low cost establishes Comsis as a worldwide leader of IEEE 802.11n and derivative technologies.
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B ACKGROUND Founded in 1995, basically as an outsourcing company for R&D projects, Comsis’ initial focus was on bespoke components and systems (GSM/IS95, UMTS, and DVB).
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In 2003, AF1 a private investment company focused on hi-tech start-ups, took a majority stake in Comsis which allowed us to complete our initial FPGA WiFi MIMO developments to address fast prototyping of WiFi services. In 2005 Comsis demonstrated real time video streams with guaranteed QoS via wireless broadband technology on MIMO Wi-Fi. Between 2006 and 2007, Comsis has developed its first FPGA kit incorporating again its own IEEE802.11n IP. This kit has been demonstrated to the R&D teams of France Telecom (Orange), Sagemcom, Thomson and Alcatel. During this period thanks to increasing collaborative R&D projects with these companies Comsis maintained its competitive advantages. In the meantime addition Comsis has also been involved in R&D projects for the Defence sector (customized secured communication systems) based on its proprietary MIMO solution. As a fruit of its efforts Comsis is awarded the ”Best innovative French Company” on 2007.
With 2008, Comsis made available its first FPGA development kit as an ideal platform for WLAN-capable SoC prototyping. After 2010 Comsis developed firmer R&D partnerships with Orange, Sagem, STm, Valeo, ISMT, Thales, Adant, TUT, Inria, Renault on different technological focuses. This helped Comsis to widen its IP portfolio and its competency on areas such as beam forming, network control, quality of service, V2V and V2I communications, low power consumption, reduction of interferers, video transmission, etc. Finally since 2013 Comsis has focused on products addressing specific market needs. As a consequence it has introduced:
4G/WiFi Gateway targeting coverage extension for white space areas Smart WiFi jammer targeting public security and spectrum management QoS WiFi platform 2nd generation video over WiFi platform for digital home and big data markets Mobile WiFi platform enabling vehicle-to-vehicle, vehicle-toinfrastructure and infrastructure-to-vehicle communications for traffic security services, parking managements and smart city applications.
Contact us Sales inquiries Technical support General contact
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More information about Comsis products and services can be found on our website. COMSIS S.A.S— 3 Rue Broussais, 75014, Paris, France. Tel: +33.1.45.89.17.00 — http://www.comsis.fr
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Notice ALL INFORMATION PROVIDED IN THIS WHITE PAPER, INCLUDING COMMENTARY, OPINION, DESIGN SPECIFICATIONS, REFERENCE BOARDS, FILES, DRAWINGS, DIAGNOSTICS, LISTS, AND OTHER DOCUMENTS (TOGETHER AND SEPARATELY, MATERIALS) ARE BEING PROVIDED AS IS. COMSIS S.A.S. MAKES NO WARRANTIES, EXPRESSED, IMPLIED, STATUTORY, OR OTHERWISE WITH RESPECT TO MATERIALS, AND EXPRESSLY DISCLAIMS ALL IMPLIED WARRANTIES OF NONINFRINGEMENT, MERCHANTABILITY, AND FITNESS FOR A PARTICULAR PURPOSE.
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Information furnished is believed to be accurate and reliable. However, COMSIS S.A.S. assumes no responsibility for the consequences of use of such information or for any infringement of patents or other rights of third parties that may result from its use. No license is granted by implication or otherwise under any patent or patent rights of COMSIS S.A.S. Specifications mentioned in this publication are subject to change without notice.
Copyright: ©2014 COMSIS S.A.S. All rights reserved.
COMSIS S.A.S— 3 Rue Broussais, 75014, Paris, France. Tel: +33.1.45.89.17.00 — http://www.comsis.fr
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Bibliography [1] Alan Young and Manoj Barara. Connected homes: Enabling a digital lifestyle. Technical report, Wipro Council for Industry Research, 2012.
ˆ [3] J´erome Galtier. Tournament methods for wlan: Analysis and efficiency. In Arie Koster and Xavier Muoz, editors, Graphs and Algorithms in Communication Networks, Texts in Theoretical Computer Science. An EATCS Series, pages 379–400. Springer Berlin Heidelberg, 2010.
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[4] Broadband Radio Access Networks (BRAN): HIgh PErformance Radio Local Area Network (HIPERLAN) Type 1: Functional specification, 1998.
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[2] IEEE Computer Society. IEEE 802.11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications - Revision of IEEE Std 802.11-2007, March 2012.
COMSIS S.A.S— 3 Rue Broussais, 75014, Paris, France. Tel: +33.1.45.89.17.00 — http://www.comsis.fr
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