Maintaining QoS
Nov 1, 2008 12:00 PM, By David Glidden
Providing high-quality video over IP is challenging.
Table 1. The IP four-layer hierarchy as defi ned by the IETF in RFC 1122
Originally designed to be opportunistic rather than guaranteed data delivery channels, IP networks have been enhanced to deliver streaming or real-time video services with a high, reliable quality of service (QoS). With such enhancements in place, broadcasters can use IP services for many of their core transport requirements, including digital newsgathering, special event coverage and network distribution feeds.
Broadcasters seeking to improve the QoS of their video-over-IP delivery services should understand the differing approaches to provisioning video services within the IP-related protocols, potential network impairments and emerging methods of improving the QoS of video-over-IP services.
Video-over-IP provisions
In the IP stack established in Request for Comment (RFC) 1122, the Internet Engineering Task Force (IETF) defined a four-layer hierarchy. (See Table 1.) Protocols such as FTP and HTTP are in the application layer. The Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) exist in the transport layer. The IP sits in the Internet layer, and the physical Ethernet connection resides in the link layer.
IP is a connectionless or datagram networking service that, by itself, does not provide end-to-end delivery guarantees. IP is not concerned with whether IP datagrams arrive at their destination delayed, damaged, duplicated or at all. Delivery guarantees are the responsibility of the transport and application layers. The IP, however, does provide for addressing, type of service codes, fragmentation and reassembly of data as well as the provision of security information.
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Two major methods of QoS when provisioning video-over-IP services are integrated services (IntServ), which allows for specific requests for priority service, and differentiated services (DiffServ), which classifies services by general type.
DiffServ facilitates QoS by providing instructions to routers using the information in the type of service (ToS) field in the IP datagram header. Information in this field, for example, can specify voice-over-IP services, streaming video or non-real-time video, as well as provide differing priorities to each.
Alternatively, the video-over-IP network can use the IntServ architecture to provide a specific QoS for each service flow. IntServ-based networks use the Resource ReSerVation Protocol (RSVP) to ensure that the required network resources are available for the video service.
Multi-Protocol Label Switching (MPLS) facilitates QoS in a packet-switched IP environment by adding a header that includes three bytes for the QoS. MPLS supports both the IntServ and DiffServ architectures.
At the transport layer, either UDP or TCP can be used. UDP is a unidirectional transport protocol that doesn't receive feedback about whether the packets have been successfully received and decoded. However, it can time packet transmission against an external clock, making it useful for real-time audio and video streaming applications. TCP requires feedback in the form of acknowledgment messages, so it enables the reliable transmission of video packets.
Classes of service
When contracting for video-over-IP service, broadcasters typically enter into service-level agreements with third-party providers. These agreements specify the minimum or guaranteed levels of network performance, including availability, packet delivery ratio, packet loss ratio, network delay, delay variation, service response time and the time to repair a faulty link.
The broadcaster will also be concerned about the video (or packet) transmission rate and the class of service. Desirable transmission rates, of course, vary depending on the type of video content being transported and the encoding scheme, with HD MPEG-2 content for network distribution requiring significantly higher transmission rates than SD H.264 content.
IP network providers use classes of service or priority queues to manage the flow of multiple services across their networks. Real-time video traffic should be given a high priority class of service. The IEEE 802.1p standard established eight priority levels for service. (See Table 2.)
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