IEEE 802.11aa: Improvements on video transmission over Wireless ...

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IEEE ICC 2012 - Ad-hoc and Sensor Networking Symposium

IEEE 802.11aa: Improvements on video transmission over Wireless LANs Kostas Maraslis

Periklis Chatzimisios

Anthony Boucouvalas

Department of Telecommunication Science and Technology University of Peloponnese Tripoli, Greece [email protected]

Department of Informatics Alexander TEI of Thessaloniki Thessaloniki, Greece [email protected]

Department of Telecommunication Science and Technology University of Peloponnese Tripoli, Greece [email protected]

section III, we study the individual problems that arise in video stream transport in wireless LANs and section IV concludes this paper.

Abstract—Transmitting multimedia streams over IEEE 802.11 Wireless Local Area Networks (WLANs) with high performance and reliability is a challenging task. In the current paper, we study the basic problems that video transmission encounters in wireless LANs and we present some of the proposed solutions. In particular, we will outline the solutions that were selected for the 802.11aa amendment to the IEEE 802.11 standard in order to provide reliable and robust transport of video streams in IEEE 802.11 wireless LANs.

II.

Keywords- IEEE 802.11aa, Video Transport Stream, Multicast, Overlapping Basic Service Set (OBSS)

I.

INTRODUCTION

x Improvement for the multicast/broadcast mechanism of IEEE 802.11 in order to offer better link reliability and low jitter characteristics. x A method for mitigating the effects of overlapping BSS environments to offer increased robustness, without the need for centralized management. x The ability to prioritize between different video transport streams that belong in the same EDCA Access Category. x To allow video streams to degrade in a graceful manner when the channel capacity is insufficient, by enabling packet discarding without any requirement for deep packet inspection. x Compatibility with the relevant mechanisms defined by IEEE 802.1AVB (802.1Qat, 802.1Qav, 802.1AS) for multimedia stream transport.

Video applications are rapidly becoming the main source of traffic in the Internet. With the presence of IEEE 802.11 networks constantly expanding, and the growing use of mobile devices with high computational power and display capabilities, users are adopting applications like video streaming and video conferencing that put a strain on the capabilities of IEEE 802.11 wireless networks. The first IEEE 802.11 wireless LAN standards were unable to handle efficient video transmission since the bandwidth was too small for the demands of video applications and they could only support a best-effort service which is unacceptable for the QoS restraints of video transmission. The improvements with IEEE 802.11n [1] brought enough bandwidth to accommodate the traffic needed for video transmission. Also, IEEE 802.11e [2] provided the necessary services to support differentiation between traffic that depends on Quality of Service restraints.

III.

However, there are still several issues that need to be addressed in order to provide a satisfactory reliable and highperformance experience to the end user whose expectations are defined by their experience with wired video media. The multicast mechanism provided by the legacy IEEE 802.11 is unreliable and cannot offer the necessary QoS for transmitting video streams. Also, the EDCA channel access function of newer standards needs improvements to support differentiation between video streams. Furthermore, overlapping LANs create problems especially for the desired Quality of Service. Finally, the 802.11 standard needs to be made compatible with the 802.1AVB standard for audio video streams in IEEE networks.

SELECTED PROBLEMS IN VIDEO TRANSPORT OVER WIRELESS NETWORKS

In this section we study in more detail the problems that affect video transport over wireless networks and the solutions that will be offered by the IEEE 802.11aa amendment. A. Group Addressed Transmission Service (GATS) Wireless networks today are dominated by unicast communication for the usual data and voice traffic. Nevertheless, in many situations where video transport is involved (as in broadcasting a movie to all the TV sets in a house or streaming a video to multiple users in a public hotspot), the use of multicasting offers many advantages. It actually allows for the conservation of bandwidth, making use of the inherently broadcast nature of the wireless medium.

The rest of the paper is organized as follows. In section II, we present the background of Task Group IEEE 802.11aa. In

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VIDEO TRANSPORT STREAM AMENDMENT

The IEEE 802.11aa Task Group is working on an amendment [3] to the 802.11 Medium Access Control (MAC) sublayer that specifies a set of enhancements to the standard. These enable transporting audio video streams with robustness and reliability while in the same time allow for the graceful and fair coexistence of other types of traffic. The main services the amendment will offer according to the proposal in [4] are:

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multicast frame. These protocols become inefficient as the number of recipients increases. The Batch Mode Multicast MAC (BMMM) proposed by M.T.Sun et al [8] is based on the BMW protocol but reduces the number of contention phases when the acknowledgements are sent to make it more efficient. Lyakhov et al. [9] propose the Enhanced Leader Based Protocol (ELBP) that further improves the BMMM protocol by using the BlockAck mechanism specified in IEEE 802.11e.

However, multicast communication in IEEE 802.11 wireless LANs suffers from many problems. As the wireless medium is inherently unreliable, in the IEEE 802.11 MAC protocol the reliable transmission of data is guaranteed by the feedback mechanism (i.e. every transmitted frame is acknowledged by the receiver). This mechanism is not used in multicasting since receiving acknowledgements from all the receivers would be inefficient, because it incurs a large overhead and raises issues about the scheduling and synchronization of receiving them. Additionally, the MAC layer of IEEE 802.11 networks specifies that multicast frames can only be transmitted using the Basic Access scheme. Therefore, it cannot use the RTS/CTS mechanism to protect broadcasting of the frame. This actually means that multicast transmission is unreliable, as the source does not know if the data was not received because of a collision or due to certain other problems that are common in wireless communications. Also there is no provision in the MAC layer for retransmissions of the data to ensure the reliable receipt. This must be implemented by higher layers, which introduces a large overhead. Thus, it would be much more efficient to support reliable multicast transmission in the MAC layer.

There are also some proposals based on dual tones on additional RF channels to protect multicast transmissions. Gupta et al. [10] propose a protocol based on Negative Acknowledgements (NAKs) and the use of busy tones. This kind of proposals requires additional transceivers in each device and additional spectrum bands for the busy tones making the implementation very difficult and inefficient. To provide for reliable multicast the IEEE 802.11aa amendment specifies the Group Addressed Transmission Service (GATS) that allows a station to request greater reliability for a group addressed stream. The service offers three mechanisms, in addition to the legacy No-Ack/No-Retry multicast. The utilized policy for a particular stream can be changed dynamically later. When setting up a stream to a multicast group with the AP, a station can request to use any of these three policies that are described as follows:

Another problem is that the IEEE 802.11 MAC specifies that multicast frames must be transmitted using one of the bit rates specified in the Basic Rate Set (BRS), which is a minimum set of bit rates that must be supported by all stations in a IEEE 802.11 wireless LAN to ensure that they can receive control frames. Although it is not necessary, most Access Points (APs) today use only the lowest bit-rate for transmitting multicast frames. This leads to lower throughput, which is also worsened by the performance anomaly problem of IEEE 802.11 wireless LANs. This phenomenon takes place where one station transmitting in low bit-rate causes a large degradation of the throughput for the whole network, as the low bit-rate transmission occupies the medium for a significant amount of time.

x 802.11v Directed Multicast Service This method was introduced in the IEEE 802.11v amendment for Wireless Network Management [11]. However, in IEEE 802.11aa this method can be used dynamically and switched with the other two policies. The DMS converts multicast traffic to unicast frames directed to each of the group recipients in a series. The transmission uses the normal acknowledgement policy and will be retransmitted until it is received correctly. This is obviously the most reliable scheme but it also has the greater overhead and does not scale well to multicast groups with a large number of members. x Groupcast with Retries (GCR) Unsolicited Retries This mechanism allows the transmission of the multicast frames to be repeated a number of times. The number of retransmissions is not specified in the standard and depends on the implementation. No acknowledgement mechanism is used. This method aims to increase the reliability of multicast transmission by increasing the probability of a station receiving the frame correctly. Thus, this method is less reliable but it offers better scalability and less overhead than other methods.

For all the above reasons, transmitting multicast frames in legacy IEEE 802.11 cannot support a reliable and high rate multicast stream. The problems with multicast in IEEE 802.11 networks have been studied in many previous works. Most of the proposed solutions can be divided in two categories. The first category is based on negative feedback. Kuri and Kasera [5] suggested the Leader Based Protocol (LBP) where one leader is responsible for sending CTS and ACK frames in response to RTS and data frames respectively in order to protect and acknowledge the transmission. The leader, as well as all the other receivers can send Negative CTS (NCTS) and Negative ACK (NAK) frames when a frame has not been received, to request a retransmission. The problem of finding a suitable leader is not solved in this paper. This protocol does not provide a high level of reliability since the only leader cannot provide information about the rest of the receivers. There is also the problem of CTS and ACK frames collisions.

x Groupcast with Retries (GCR) BlockAck This method extends the BlockAck mechanism specified in IEEE 802.11n for use in multicast transmissions to a group. The AP transmits a number of multicast frames and then requests from one or more of the recipients to acknowledge the receipt of the transmitted frames. Frames that have not been received correctly by one or more of the receivers can then be retransmitted. The choice of the stations from which an acknowledgement is requested is left to the implementation. Thus, this method can be used with many leader-based multicast methods and leader selection algorithms offering a high degree of reliability, scalability and performance, with the trade-off depending on the implementation.

The other category is based on positive feedback. Tang and Gerla in [6] proposed the Broadcast Support Multiple Access (BSMA) protocol that extends the RTS/CTS mechanism to multicast and the Broadcast Media Window (BMW) [7] method that receives an ACK from every receiver of the

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TABLE I.

Multicast Policy Legacy multicast DMS GCR Unsolicited Retries GCR BlockAck

CHARACTERISTICS OF GROUP ADDRESSED TRANSMISSION SERVICE POLICIES

Overhead None Large Small mplementation depended

Scalability High Low High mplementation depended

Complexity Low Medium Low High

Reliability Low High mplementation depended mplementation depended

Figure 1. Group Addressed Transmission Service with different policies

station to content for the medium, which are the minimum and maximum size of the Contention Window (CWmin and CWmax), the Arbitration Inter-Frame Space (AIFS) and the maximum size of the Transmission Opportunity (TXOP). Higher priority access categories have a smaller contention window and shorter inter-frame space, increasing the probability of the frame takes hold of the medium.

The legacy No-Ack/No-Retry multicast and the three alternative policies are illustrated in Fig. 1. Table I summarizes the characteristics of the different policies specified by the Group Addressed Transmission Service. Another proposal that was discussed by the IEEE 802.11aa Task Group but not included in the standard was by Miroll and Li [12] that suggested a leader-based protocol introducing to the IEEE 802.11 standard the idea of feedback cancellation. A leader is chosen to send the ACKs for every received frame. The rest of the recipients can send a NACK frame simultaneously with the leader’s ACK to cancel the ACK frame, when they have not received a frame or the frame is corrupt. This way they can force the retransmission of the corrupted frame. One problem that arises is that ACK frames are designed to be the more robust frames in the IEEE 802.11 protocol and because of the capture effect, the receivers may be able to receive an ACK frame correctly even if a NACK frame is sent at the same time. This proposal also included a FEC coding scheme where additional multicast frames are transmitted, to allow the receiver to decode the multicast stream even if some of the frames are corrupted.

The problem of the IEEE 802.11e EDCA function in the case of video transmission is that it offers only 4 access categories, with the highest priority access category reserved for voice traffic and the second one for video streams. However, there may be a need for simultaneous transmission of many video streams with different performance restrictions, for example video conferencing has a smaller tolerance for delay and jitter than streaming video. In the current EDCA scheme these streams should belong to the same access category and there is no way to give one stream higher priority over the other. Also, the smaller contention window for the video access category means that there is a higher collision probability in the presence of multiple streams that leads to retransmissions and throughput degradation. The IEEE 802.11aa amendment aims to increase the granularity of the EDCA access categories to allow for prioritization between different video streams. To achieve this, it specifies alternate queues for the two highest priority access categories, voice and video. As can be seen in Fig. 2, the packets for the primary and the alternate queue in each access category are kept in separate buffers. A scheduling function, which is implementation specific, schedules packets between the primary and the alternate queue before they are passed on to the internal collision resolution function of EDCA.

B. Stream Classification Service (SCS) The Stream Classification Service aims to cover two of the targets within the scope of the IEEE 802.11aa amendment; the need to differentiate between separate streams within the same access category and the need to allow for the graceful degradation of the stream in the case of bandwidth shortage. The EDCA function in IEEE 802.11e to provide Quality of Service for video and voice streams specifies Access Categories (AC) with different priorities. The differentiation is enforced by configuring the contention parameters used by the

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To further differentiate between video streams, the amendment specifies a Drop Eligibility Indicator (DEI) bit that can be set in the Traffic Specification (TSPEC) when a transport stream is set up. When a transport stream has the DEI bit activated, the retransmission procedure operates with a smaller maximum number for the short and long retry counter. Therefore, in the case of bandwidth shortage a stream that has the DEI bit activated is more likely to reach the maximum number of retransmissions and be discarded. Using the combination of the primary and alternate queue and the Drop Eligibility Bit, a transport stream that is set up between the station and the AP using the Stream Classification Service can achieve better prioritization between other streams. The enhancements in IEEE 802.11aa allow better mapping between the 8 user priorities which are defined in the IEEE 802.1D standard and the user priorities in the IEEE 802.11 standard, using the access categories of the EDCA function and the alternate queues.

Figure 2. Stream Classification Service (SCS)

A number of previous works studied the problem of Overlapping Basic Service Sets. Benveniste [14] first described the neighborhood capture problem with overlapping BSSs and suggested the use of a slotted CSMA/CA MAC protocol that allows the channel to be released globally at regular intervals, giving all APs equal chances to capture the medium. Mangold et al [15,16] showed how TXOP bursting increases the throughput in overlapping networks and, in combination with synchronizing the TXOPs between overlapping BSSs, improves the overall efficiency. Han et al [17] suggested a master AP that changes the EDCA contention parameters to give a group of “high priority” BSSs more priority over “low priority” BSSs. Fang et al [18] proposed stations listening to frames from overlapping BSSs to set their NAV to the highest value of the self BSS NAV or the overlapping BSS NAV.

C. Overlapping Basic Service Sets The problem of Overlapping Basic Service Set (BSS) is becoming more common as wireless networking devices are becoming more widespread. Simulations performed for the Task Group have shown that in a dense domestic environment there can be more than 30 overlapping networks [13]. Channel selection can only partially reduce the problem as, even in the 5 GHz band with 19 non-overlapping channels, we could have situations of 2 or 3 overlapping BSSs. When the BSSs are using the legacy channel access (Distributed Coordination Function) or the EDCA function of IEEE 802.11e, the traffic competes fairly between the networks resulting in reduced bandwidth in all of them. However, there is a possibility that one AP is in the middle of two other BSSs that cannot hear each other. In this case, the two networks monopolize the wireless medium and the AP in the middle is unable to get any traffic through (neighborhood capture effect) [14].

The approach that was selected by the IEEE 802.11aa Task Group to manage the Overlapping BSS situation is to provide a decentralized mechanism for neighboring APs to exchange information about the QoS depended traffic load in each BSS. This information can be used for more efficient channel selection and, if it is necessary for BSSs to share a channel, it allows APs to cooperate and work as one bigger QoS-aware network, sharing fairly the wireless medium.

The negative effect on the performance is more severe when the Quality of Service enhancements of IEEE 802.11e are used, as these enhancements break down and cannot support the performance demands of multimedia streams. When one of the BSSs uses EDCA with admission control, the Access Point controls the admission of new streams, based on the information it has about its own BSS, but it has no control and no information about the traffic in the other overlapping networks. Therefore, it cannot guarantee the desired protection to the admitted flows and ensure they are given the agreed Quality of Service. In the case where one of the networks uses the HCCA function, in which the AP schedules traffic during a Contention-Free Period and grants Transmit Opportunities (TXOP) to the stations, this network is able to protect the bandwidth in its own network, reducing the available bandwidth for the rest of the networks. However, when two or more networks use the HCCA function, the AP may not be able to allocate time when it needs to, as it has to obey the TXOPs of other networks. Again, this results in reduced bandwidth for the scheduled protected traffic and no real protection for QoS restraints.

The amendment defines the new QLoad information element which can be used by an Access Point to inform its neighboring APs about its QoS traffic load. This includes information about the number of BSSs the AP overlaps with, the QoS traffic load in the access point’s self BSS, and the total QoS traffic load of the overlapping BSSs. The AP monitors the admitted traffic flows in its own BSS and uses their traffic specifications to calculate a value for the self QoS traffic load. It also calculates the sum of the QoS traffic load of all the neighboring networks it is aware of. This information is useful to avoid the neighborhood capture problem. When an AP needs to select a channel to operate in, it can use the information from the QLoad elements it collects to make an optimal selection. The IEEE 802.11aa amendment includes an informative text with a recommendation for the channel selection procedure. First the AP attempts to find a channel that is free, then it attempts to find the channel that has the minimum number of overlapping BSSs, and finally the channel that has the minimum QoS traffic load. Two sharing

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schemes are then suggested for the APs admission control procedure, to ensure that they don’t allocate more resources that are available for the total network and that they share the wireless medium proportionally with the other APs.

[2]

The IEEE 802.11aa amendment also defines a set of control frames that can be used by APs using the HCCA function to inform its neighboring APs of their TXOP allocations schedule. The other APs can use this information to schedule their own TXOPs to avoid those already scheduled, and to ensure the necessary time is available when they are admitting a new traffic stream.

[3]

D. Interworking with 802.1AVB The IEEE 802.1 Audio/Video Bridging Task Group is working on a set of standards that will provide for high quality and low latency streaming of time-sensitive traffic through heterogeneous 802 networks. In particular, the IEEE 802.1Qat amendment specifies the Stream Reservation Protocol (SRP) [19] which is used to reserve network resources over the entire network path between the end stations, to guarantee the transmission and reception of a data stream across the network with a requested Quality of Service.

[4]

[5] [6]

[7]

SRP defines a set of signaling mechanisms that can be used by the source of a stream (called a Talker) to advertise a stream that it has available and define the resources that will be required, or the destination (called a Listener) to request a particular stream it wants to receive. The intermediate nodes check the availability of the required resources and propagate a positive or negative request to the next node. When a pair of positive requests for the same stream by a Talker and a Listener has been received by an intermediate node, it allocates the required resources and the stream transmission begins.

[8]

[9]

[10]

[11]

The IEEE 802.11aa Task Group worked closely with the AVB Task Group in order to make IEEE 802.11 networks compatible with SRP. This did not require major changes to the standard, but only the addition of some control frames to allow Access Points to perform the stream reservation procedures defined in IEEE 802.11Qat, and to function as end stations or intermediate nodes in a path between a Talker and Listener. Also, the mapping that was created between the user priorities of the IEEE 802.1D standard and the access categories offered by the IEEE 802.11 QoS service, as it has been extended by the IEEE 802.11aa amendment, is important to the compatibility of the two standards. IV.

[12]

[13]

[14]

[15]

CONCLUDING REMARKS

The IEEE 802.11aa amendment provides a foundation for video stream transport over IEEE 802.11 wireless LANs. It provides a set of mechanisms such as the multiple multicast policies, the Stream Classification Service and the QLoad information element, but the use of them is open to the implementation. The choice of the appropriate mechanism, depending on the situation and the setting of its parameters, is an issue of great interest for future research.

[17]

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[19]

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