The Motion Picture Expert Group (MPEG) generates standards for digital video and audio compression. There are several MPEG codecs, with MPEG-4 being a very popular high-compression video codec standard. There are different implementations, profiles and options that are somewhat compatible. The standard does NOT dictate how the video stream is placed into packets, nor any of the watermarking and digital signatures applied to prevent evidence tampering; so in the public safety video world, open interoperability is between cooperating vendors. The latest codec, called Part 10, is the same as H.264, and provides a 20% to 40% bandwidth improvement over the MPEG-4 Simple or Advanced Simple Profiles. Each generation of codec uses improved compression techniques, while building on the prior generation’s standards.
Some of these internal codec details include:
• MJPEG—Motion JPEG where each image is compressed with JPEG and all images are transmitted;
• And MPEG-2—standard digital cable, DVD or DTV, which uses I frames (full image frame), P frames (predictive frame from prior frame) and B frames (interpolated from both prior and following frame). Each frame is processed as 8x8 blocks. Typical compression results using MPEG-2 are:
• Cable—352x240, about 4 Mbps;
• DVD—352x240, 352x420, 740x468, up to10 Mbps;
• MPEG 4—incorporates frequency domain prediction with a finer resolution of picture elements and motion vectors;
• And MPEG-4 10, H.264—includes spatial domain prediction, variable block size and a host of other enhancements.
The Wireless Network
There are a number of tools available to design a wireless network to reliably carry video traffic. The first is video codec implementation, where the settings provided for frame rate, resolution and target bit rate allow trade-offs to be made with video quality and the traffic applied to the network. The second results from the QoS (Quality of Service) provided by the WiMAX (Worldwide Interoperability for Microwave Access) protocol, which is utilized in Tyco Electronics’ M/A-COM VIDA (Voice, Interoperable, Data and Access) Broadband network. The bandwidth available via each wireless link can be dynamically allocated to the multiple traffic flows according to the QoS parameters configured for each. Thus, the capacity is assigned according to pre-configured priorities, rather than simply channel contention or congestion effects.
Different applications have different requirements regarding handling their traffic on the network, and QoS determines the ability of the network to meet a given traffic service level requirement in terms of throughput, latency and jitter. Requirements are defined in terms of these QoS parameters:
• Latency: How much time it takes an IP packet to be transmitted from Point A to Point B;
• Jitter: The variation in latency among packets;
• And bandwidth: The rate at which application traffic is carried by the network.
Guaranteed QoS is assured as all communication scheduled by the base station(s), is synched with contention slots that are provided for subscriber stations (clients) re: the requested bandwidth. This coordinated scheduling feature of the WiMAX protocol provides significant advantages by:
• Eliminating contention between registered clients;
• Maximizing channel utilization;
• Enabling guaranteed bandwidth services for critical multi-media applications; • And offering four QoS classes that enable customization for specific mission-critical applications.
802.16 WiMAX vs. Other Protocols
The foregoing discussion mentions some of the WiMAX capabilities that contrast to Wi-Fi. Learning from the Wi-Fi experience, WiMAX purposely includes strong security tools, such as X.509 digital certificate authentication and encryption. WiMAX is a scheduled protocol, so the contention DoS (Denial of Service) and heavy traffic issues are avoided, and true QoS can be guaranteed. These are tremendous benefits for wireless video. The drawback is that the network has to be designed with this functionality in mind. Additionally, the desired service flows must also be configured. As a result, the ad-hoc network formation is not as free form as Wi-Fi or mesh protocols. In reality, this is a minor point, as the ad-hoc feature only works well when the wireless network is very lightly loaded. The 1/n multihop capacity degradation along with the contention issues of Wi-Fi and mesh topologies force some upfront design and adding WAN backhaul points to reduce the number of hops and accommodate the traffic load. The bottom line is that the spectrum is not used as efficiently, and the ad-hoc benefit is counter to providing user QoS, which makes video possible.
Advanced video compression techniques such as MPEG4 and H.264 have dramatically reduced the bandwidth requirements for video, to the point that effective transport and delivery of remote video over wireless links are now a reality. Video traffic, however, is still a significant drag on the flow of network traffic and is still subject to real-world bandwidth limitations and propagation constraints. Thus, link impairments and bandwidth contention must be accounted for in the system design of the network. This trade-off encompasses RF path engineering, network topology and traffic engineering, QoS mechanisms and IP networking.
Comparison of 802.11 protocol-based networks and 802.16 protocol-based networks shows that while the 802.11 networks can transport video as an incidental “non-critical” service, they lack the intrinsic security and true QoS to provide reliable operation so important to public safety. The QoS service flows provided by the 802.16 protocol (WiMAX) are designed specifically to address this latter issue. Security is built in, with x.509 digital certificates for authentication and AES encryption.
In the end, it is necessary to understand both the configurable parameters the video applications provide, as well as the network QoS controls to ensure satisfactory operation. A wireless network providing true QoS, such as M/A-COM VIDA Broadband, brings the reality of public safety video one step closer to the image most often represented in Hollywood.
Thomas Brown is chief engineer, Network Products, M/A-COM, Tyco Electronics.