IEEE Wireless Communications • April 20104

IN DUSTRY PE R S P E C T I V E S

With the ubiquitous presence of 802.11 at homes, enter-prises, hotspots, and in the near future within automobiles,and the abundance of bandwidth offered by 802.11n, there isa strong momentum to extend the convenience of wirelessconnectivity to multimedia streaming applications. Userexpectations in terms of both ease of use and the quality ofthe rendered content are based on wired connections. Inorder for 802.11 to become a viable alternative to wired con-nections, user expectations have to be satisfied. Several chal-lenges need to be overcome to get there.

DEMAND ON THE NETWORKMultimedia applications have two requirements to achieveacceptable performance:• Timely delivery of payload, which is usually large in size• Sufficient medium time for transmission and/or retransmis-

sion of the payload to cope with the inherent unreliabilityof the medium

MULTIPLICITY OF SCENARIOSMultimedia content requires very large network bandwidth.For instance, 720p video at 60 frames/s using 10 b/colorrequires about 1.4 Gb/s. To stream the content over band-width-limited media like 802.11, the content (even real-timecontent) needs to be compressed. Future amendments to802.11 may relax this constraint. In the rest of this discussionwe focus on compressed multimedia content.

Multimedia streaming scenarios vary widely depending onthe content streamed, the capabilities of the source and sinkdevices, and the location of the devices. In some cases thecontents get delivered to different sinks (video to the displayand audio to one or more speakers, some wired and otherswireless).

As a result, multimedia streaming is challenging and muchmore complex than data and voice applications. The perfor-mance of a multimedia application depends on a lot of subsys-tems end-to-end; the 802.11 link is just one of them. Eachsubsystem should be optimized for performance in order forthe end-to-end performance to be acceptable.

802.11 suffered some negative perception that slowed itsadoption as the medium for multimedia streaming. This isdue to the poor performance observed when multimediaapplications designed for wired links were run as is over802.11 links that were optimized for data applications.Leveraging this negative perception, several proprietarysolutions were conceived and are still being pushed into themarket.

With 802.11n there is an opportunity to overcome thenegative perception and demonstrate the viability of 802.11as the medium of choice for multimedia streaming. Thedemand on wireless bandwidth for multimedia streaming isincreasing as the quality of streamed content evolves (e.g.,video could be standard definition, 720p, 1080p, or some-thing beyond). Enabling a solution that delivers exceptionalperformance for multimedia formats that are currently com-mon with the flexibility to evolve to support future formats iscritical for rapid adoption of 802.11-based multimediastreaming solutions.

There are several factors contributing to the overall multi-media performance. These factors can be partitioned into twosets.

MAC/PHY RELATED FACTORSWhile the QoS mechanisms specified in IEEE 802.11-2007provide a certain level of quality of service (QoS) for multi-media streams, additional mechanisms are required in orderto sustain the quality of a multimedia stream where there is atleast one 802.11 link. The additional mechanisms needed areaddressed in 802.11aa, the Robust Audio/Video StreamingTask Group, enabling a variety of use cases over 802.11.• Robust multicast: Multicast over 802.11 is not reliable since

there is no mechanism to ascertain that all members of themulticast group successfully receive the transmission.802.11aa specifies three multicast mechanisms: directedmulticast, unsolicited retries, and extended block acknowl-edgments (ACKs).

• Intra-AC prioritization: When there are multiple multime-dia streams within a base station system (BSS), a mecha-nism is needed to prioritize between them. The priorityinformation needs to be preserved end to end.

• Graceful degradation: In a video stream not all informationis of the same importance. Some information (e.g., datacorresponding to an I-frame) is more important (i.e., itsloss can have a higher impact on the quality of the ren-dered video) than others (e.g., data corresponding to a B-frame). A mechanism is needed to tag packets in a streamto identify their relative importance. The priority informa-tion needs to be preserved end to end.

• Overlapping BSS management: 802.11 provides two medi-um access mechanisms: enhanced distributed channelaccess (EDCA) and hybrid coordination function controlledchannel access (HCCA). Devices contend for mediumaccess with EDCA and are allocated contention-free medi-um access opportunities with HCCA. EDCA admissioncontrol is an extension of EDCA where 802.11 stations(STAs) advertise their load and, if enough resources areavailable, get corresponding medium time allocated by theAP. Both EDCA admission control and HCCA allocationsare local to a BSS and are not shared with neighboringBSSs. As a result, resource allocations within a BSS areinvalidated when one or more neighboring BSSs operate inthe same channel. Mechanisms to advertise BSS load andshare the available medium time across BSSs are required.

• Interworking with 802.1AVB: It is important to note that inmost multimedia streaming cases, the content would bestreamed over a wired/wireless network. 802.1AVB is a setof protocols (802.1Qat, 802.1Qav, 802.1AS and 802.1BA)for end-to-end multimedia streaming. Specifically, 802.1Qatdeals with reservation and maintenance of QoS for a multi-media flow. 802.1Qat deals with up to seven hops betweenthe source and the sink with up to two of the hops being802.11. 802.1Qat is a stream reservation protocol (SRP)that reserves resources in order for a stream to be streamedend to end. Some 802.11 mechanisms need to be extendedto meet the needs of 802.1Qat SRP. Once the reservation isset up, periodic QoS maintenance reports are monitored toensure that the required QoS is maintained.

HIGHER-LAYER FACTORSA large set of choices for higher-layer protocols exist for

streaming multimedia content over the Internet. Additionalwork is needed to fine tune parameters at the network layer inorder for these protocols to work optimally when 802.11 is the

MULTIMEDIA STREAMING OVER 802.11 LINKSGANESH VENKATESAN, INTEL CORPORATION

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underlying physical link. The following is a list of factors aboveand including the network layer that need to be understood inthe context of 802.11. In many instances the real reason forpoor or unsatisfactory performance is due to the lack of under-standing of these factors, while 802.11 is wrongly blamed.• Channel selection: As the number of devices using 802.11

increases, the chance of a BSS operating in the same chan-nel as another is increasing. Based on sensing the radioenvironment, a strategy to select the best band, channel,and channel width will improve multimedia performance.

• Coding strategies: Most 802.11 devices start off at the high-est modulation and coding scheme (MCS) and step downprogressively to cope with bit error rates (BERs). This set-tling time may be optimal for data/voice applications butdetrimental for video. In addition, enabling coding robust-ness mechanisms like low density parity coding (LDPC) canimprove multimedia performance.

• Layer 3 buffering: Lack of sufficient buffering in layer 3may cause packets to be dropped at the receiver despitehaving been delivered intact by the lower layers. While thebuffer sizes chosen for layer 3 may be appropriate fordata/voice applications, the same setting may be suboptimalfor multimedia applications. A mechanism is needed todetermine and set this parameter to best fit multimediaapplications.

• Application layer buffering: Additional buffering at theapplication layer can be used, especially in cases where thestreamed content is not real-time. With buffering artifactsdue to excessive delay and, in somecases, lost packets can be concealed.Mechanisms exist to determine thisparameter when operating over awired connection. These mechanismsneed to be adapted for operation over802.11 links.

• Error-resilient video decoders: Videodecoders can conceal a host of errorsincluding those due to lost packets.Error concealment techniques applyfor the case where maximum trans-mission unit (MTU) size is set to 1500bytes (up to seven 188-byte (MPEG-TS) multimedia packets). Aggregationin 802.11n is a key feature to reducechannel access overheads and improvechannel utilization. Error conceal-ment methods need to be revisited inthe context of aggregation and opti-mal value(s) of aggregation for multi-media applications should bedetermined.

• Error feedback to the encoder: Timelyfeedback from the decoder to throttlethe encoder improves effective qualityof the rendered content. This is criti-cal, especially in real-time streams.The encoder may be generating a cer-tain output stream based on someassumptions about the conditions atthe decoder. Feedback on changeswill help the encoder adapt to gener-ate the best output given the currentconditions at the decoder (in thereceiver).

• Application layer tagging: Some multi-media applications do not take advan-

tage of the medium access control (MAC) layer prioritiza-tion. Unless the 802.11 MAC performs some deep packetinspection to determine the type of a packet from a multi-media stream, appropriate priority is not assigned to it. Asa result, multimedia packets compete with time-insensitiveapplication packets resulting in poor multimedia perfor-mance.

• 802.11 implementation/configuration choices: A whole hostof implementation choices while implementing/configuringthe 802.11 MAC/PHY can result in suboptimal multimediaperformance. Thus far, most focus has been on achievingthe highest throughput under a given benchmark for any802.11 implementation. Achieving the highest throughputdoes not translate into the best multimedia performance.More work is needed to understand the optimal settings formultimedia performance with the least packet loss/errorrate and least end-to-end delay.

CONCLUSIONWhile 802.11 and specifically 802.11n can easily cater to thedemands of multimedia streaming applications and provide thesame experience as one would have with a wired connection,there are several factors that need to be understood and opti-mized to provide consistent performance. As the number ofdevices streaming multimedia content over 802.11 linksincrease, the impact of these parameters on performance is sig-nificant. Timely research on these topics is required in order tocope with the demand for multimedia streaming over 802.11.

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IN DUSTRY PE R S P E C T I V E S

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