Geoffrey M. Garner

IEEE 802.1 AVB and Its Application in Carrier-Grade Ethernet [Standards Topics]

Ethernet is increasingly being used in carrier networks to transport real-time traffic, including wireless backhaul network traffic, time-sensitive audio/video applications in access networks, and circuit emulation for legacy services. With the replacement of traditional circuit-switched networks with Ethernet-based packet networks, it must be ensured that the application timing and QoS requirements are met. The IEEE 802.1 Audio/Video Bridging Task Group is developing a comprehensive set of standards to enable high quality, low-latency streaming of time-sensitive applications. These standards specify a means of providing time synchronization (IEEE 802.IAS), a resource reservation protocol (IEEE 802.1Qat), and a set of forwarding and queuing rules that bound the variability of delay in an AVB network (IEEE 802.1Qav). These standards are described, including their potential application to carrier-grade Ethernet networks.

Overview and timing performance of IEEE 802.1AS

IEEE 802.1AS is being developed in the 802.1 working group as part of a set of standards for audio/video bridging (AVB). AVB networks will carry time-sensitive, high-quality, audio/video traffic, and IEEE 802.1AS will provide synchronization for these networks and ensure that the jitter, wander, and synchronization requirements for the time-sensitive traffic can be met. IEEE 802.1AS includes an IEEE 802-specific layer 2 profile of IEEE Std 1588TM-2008, plus additional requirements needed to ensure performance for both full-duplex 802.3 (Ethernet) and 802.11 (WiFi) networks. The standard provides for very few options, both to simplify the configuration and operation by the user and to result in low cost. At present, the major features and requirements of 802.1AS are decided, with planned completion by 1Q2009. This paper gives an overview of the 802.1AS standard, describing which features of IEEE Std 1588TM-2008 it uses and what additional requirements it contains. The paper then describes performance testing that is in progress using early implementations of IEEE 802.1AS bridges and end stations.

Synchronization of audio/video bridging networks using IEEE 802.1AS

The Audio/Video Bridging project in the IEEE 802.1 working group is focused on the transport of time-sensitive traffic over IEEE 802 bridged networks. Current bridged networks do not have mechanisms that enable meeting these requirements under general traffic conditions. IEEE 802.1AS is the AVB standard that will specify requirements to allow for transport of precise timing and synchronization in AVB networks. It is based on IEEE 1588-2008, includes a PTP profile that is applicable to full-duplex IEEE 802.3 transport, and adds specifications for timing transport over IEEE 802.11, IEEE 802.3 EPON, and CSN media. This article provides a tutorial on IEEE 802.1AS that updates earlier descriptions, and new simulation results for timing performance.

New simulation and test results for IEEE 802.1AS timing performance

IEEE 802.1AS is a standard being developed in the 802.1 Working Group to provide precise timing and synchronization for bridged networks. It is one of the 802.1 Audio/Video Bridging (AVB) standards, and a major use of it will be to ensure that jitter, wander, and time synchronization requirements for time-sensitive applications in a residential AVB network are met. However, 802.1AS (and the other AVB standards) can be used in other applications as well. Each application will be specified by an ‘AVB profile’, and each profile will provide for very few options to simplify the configuration and operation by the user and result in low cost. IEEE 802.1AS includes an IEEE 802-specific layer-2 profile of IEEE Std. 1588TM - 2008; plus additional specifications to allow transport over 802.11 (WiFi), Ethernet PONs, and coordinated shared networks such as MoCA; plus additional requirements needed to ensure timing performance. IEEE 802.1AS is nearing completion, and is expected to be balloted by 4Q2009. This paper gives a brief overview of IEEE 802.1AS, focusing on recent developments. It then describes new simulation modeling and results for jitter, wander, and synchronization performance. Finally, it describes performance test results using 802.1AS bridges and end-stations.

Provision of Precise Timing via IEEE 1588 Application Interfaces

The protocol specified in IEEE 1588, together with a profile, define a timing system that may be used to supply precise timing to applications. However, IEEE 1588 does not say anything about the interface to the applications. In designing this interface, the application performance requirements (e.g., jitter, wander, time synchronization) must be considered. For example, an application that requires microsecond or better time synchronization needs a hardware or firmware interface; a software interface can result in exceeding the synchronization requirement by a factor of 1000 or more. This paper describes the performance requirements of example applications. It then describes a general application interface in abstract terms. Finally, it describes realizations of the interface that can meet the performance requirements for selected applications.

Using an IEEE 802.1AS network as a distributed IEEE 1588 boundary, ordinary, or transparent clock

IEEE 802.1AS includes a very specific profile of IEEE 1588 that only runs at layer 2 over networks that follow the IEEE 802 architecture. It has some significant performance and scalability advantages, but at the cost of not allowing non-PTP-aware devices. This paper describes how a network having a common source of time can act as a distributed IEEE 1588 boundary, ordinary, or transparent clock, allowing the network to transport synchronization between portions of an IEEE 1588 network domain, and do this for any number of domains simultaneously. The network that acts as a distributed clock can be a PTP network supporting a profile that is different from that of the domains whose timing it is transporting. As one example, an IEEE 802.1AS network can act as a distributed IEEE 1588 boundary, ordinary, or transparent clock. As part of the discussion, the paper also shows that an IEEE 1588 boundary clock and peer-to-peer transparent clock are functionally equivalent in the manner in which they transport synchronization, and that the principal difference between the two is that the former invokes a best master clock algorithm (either default or alternate) and implements the full PTP state machine, while the latter does not. The concepts of distributed BC, TC, and OC, and the equivalence of the BC and peer-to-peer TC may be considered a new way of looking at the transport of synchronization in a network based on IEEE 1588.

Timing and synchronization for audio/video applications in a converged residential ethernet network

Future residential networks will be converged, i.e., will carry multimedia, data, and voice applications in a single network infrastructure. These networks will have several features to ensure acceptable Quality of Service (QoS) and minimal administration by users. The features include bandwidth reservation, admission control, and network synchronization. The latter is needed mainly to ensure acceptable jitter, wander, and time synchronization performance for the multimedia applications (these are part of the QoS requirements). This paper describes the jitter, wander, and synchronization requirements for time-sensitive audio and video applications. A general scheme for providing network synchronization, which is being considered for Residential Ethernet, is then described, along with several specific approaches.