Quality Of Multimedia Data Transmission - MULTIMEDIA

Quality of Service (QoS)

Quality of Service (QoS) for multimedia data transmission depends on many parameters. Some of the most important are:

  • Data Rate. A measure of transmission speed, often in kilobits per second (kbps) or megabits per second (Mbps)

  • Latency (maximum frame/packet delay). Maximum time needed from transmission to reception, often measured in milliseconds (msec). In voice communication, for example, when the round - trip delay exceeds 50 msec, echo becomes a noticeable problem; when the one - way delay is longer than 250 msec, talker overlap will occur, since each caller will talk without knowing the other is also talking.

  • Packet loss or error. A measure (in percentage) of error rate of the packetized data transmission. Packets get lost or garbled, such as over the Internet. They may also be delivered late or in the wrong order. Since retransmission is often undesirable, a simple error - recovery method for real - time multimedia is to replay the last packet, hoping the error is not noticeable.

  • FIGURE 16.1: Jitters in frame playback: (a) high jitter; (b) low jitter

    Jitters in frame playback: (a) high jitter; (b) low jitter
    In general, for uncompressed audio / video, the desirable packet loss is < 10 - 2 (lose every hundredth packet, on average). When it approaches 10%, it becomes intolerable. For compressed multimedia and ordinary data, the desirable packet loss is less than 10 - 7 to 10 - 8.

  • Jitter (or delay jitter). A measure of smoothness of the audio / video playback. Technically, jitter is related to the variance of frame / packet delays. A large buffer (jitter buffer) can to hold enough frames to allow the frame with the longest delay to arrive, to reduce playback jitter. However, this increases the latency and may not be desirable in real - time and interactive applications. The above figure illustrates examples of high and low jitters in frame playbacks.

  • Sync skew. A measure of multimedia data synchronization, often measured in milliseconds (msec). For a good lip synchronization, the limit of sync skew is ±80 msec between audio and video. In general, ±200 msec is still acceptable. For a video with speaker and voice the limit of sync skew is 120 msec if video precedes voice and 20 msec if voice precedes video. (The discrepancy is probably because we are used to have sound lagging image at a distance.)

Multimedia Service ClassesBased on the above measures, multimedia applications can be classified into the following types:

  • Real - Time (also Conversational). Two - way traffic, low latency and jitter, possibly with prioritized delivery, such as voice telephony and video telephony

Table Requirement on network bandwidth / bitrate

Table Requirement on network bandwidth / bitrate

  1. Priority data. Two - way traffic, low loss and low latency, with prioritized delivery, such as e-commerce applications

  2. Silver. Moderate latency and jitter, strict ordering and sync. One - way traffic, such as streaming video; or two - way traffic (also Interactive), such as web surfing and internet games

  3. Best Effort (also Background). No real - time requirement, such as downloading or transferring large files (movies)

  4. Bronze. No guarantees for transmission

Perceived QoS. Although QoS is commonly measured by the above technical parameters, QoS itself is a "collective effect of service performances that determine the degree of satisfaction of the user of that service," as defined by the International Telecommunications Union. In other words, it has everything to do with how the user perceives it.

In real - time multimedia, regularity is more important than latency (i.e., jitter and quality fluctuation are more annoying than slightly longer waiting); temporal correctness is more important than the sound and picture quality (i.e., ordering and synchronization of audio and video are of primary importance); and humans tend to focus on one subject at a time.

User focus is usually at the center of the screen, and it takes time to refocus, especially after a scene change. Together with the perceptual nonuniformity we have studied in previous chapters, many issues of perception can be exploited in achieving the best perceived QoS in networked multimedia.

TableTolerance of latency and jitter in digital audio and video

Tolerance of latency and jitter in digital audio and video

QoS for IP Protocols

QoS policies and technologies enable key metrics discussed in the previous section such as latency, packet loss, and jitter to be controlled by offering different levels of service to different packet streams or applications.

Frame relay routing protocol and ATM provide some levels of QoS, but currently most Internet applications are built on IP. IP is a "best - effort" communications technology and does not differentiate among different IP applications. Therefore it is hard to provide QoS over IP by current routing methods.

Abundant bandwidth improves QoS, but in complex networks, abundant bandwidth is unlikely to be available everywhere (in practice, many IP networks routinely use oversubscription). In particular, it is unlikely to be available in all the access links. Even if it is available everywhere, bandwidth alone can't resolve problems due to sudden peaks in traffic.

Differentiated Service (DiffSety) uses DiffServ code [Type of Service (TOS) octet in IPv4 packet and Traffic Class octet in IPv6 packet] to classify packets to enable their differentiated treatment, It is becoming more widely deployed in intradomain networks and enterprise networks, as it is simpler and scales well, although it is also applicable to end - to - end networks. DiffServ, in conjunction with other QoS techniques, is emerging as the de facto QoS technology. See IETF Request for Comments (RFC) 2998 for more information.

Multiple Protocol Label Switching (MPLS) facilitates the marriage of IP to OSI layer 2 technologies, such as ATM, by overlaying a protocol on top of IP. It introduces a 32 - bit label and inserts one or more shim labels into the header of an IP packet in a backbone IP network. It thus creates tunnels, called Label Switched Paths (LSP). By doing so, the backbone IP network becomes connection - oriented.

The two main advantages of MPLS are to support Traffic Engineering (TE), which is used essentially to control traffic flow, and Virtual Private Networks (VPN). Both TE and VPN help delivery of QoS for multimedia data. MPLS supports eight service classes. For more detail refer to RFC 3031.

DiffServ and MPLS can be used together to allow better control of both QoS performance per class and provision of bandwidth, retaining advantages of both MPLS and DiffServ.

Prioritized Delivery

When a high packet loss or error rate is detected in the event of network congestion, prioritized delivery of multimedia data can be used to alleviate the perceived deterioration.

Prioritization for types of media. Transmission algorithms can provide prioritized delivery to different media — for example, giving higher priority to audio than to video — since loss of content in audio is often more noticeable than in video.

Prioritization for uncompressed audio. PCM audio bitstreams can be broken into groups of every nth sample — prioritize and send k of the total of n groups (k ≤ n) and ask the receiver to interpolate the lost groups if so desired. For example, if two out of four groups are lost, the effective sampling rate is 22.05 kHz instead of 44.1 kHz. Loss is perceived as change in sampling rate, not dropouts.

  • Prioritization for JPEG image. The different scans in Progressive JPEG and dif­ferent resolutions of the image in Hierarchical JPEG can be given different priorities, for example, highest priority for the scan with the DC and first few AC coefficients, and higher priority to lower - re solution components of the Hierarchical JPEG image.

  • Prioritization for compressed video. Video prioritization algorithms can set priorities to minimize playback delay and jitter by giving the highest priority to reception of I - frames and the lowest priority to B - frames. In scalable video (such as MPEG - 2 and 4) using layered coding, the base layer can be given higher priority than the enhancement layers.

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