TECHNOLOGY Ravenna – network neologism The founding of the first ancient lagoon settlement of Ravenna is difficult to attribute to one tribe but it prospered in the Roman Empire and survived all the vicissitudes after its fall. Today, Ravenna is twinned with six towns across Europe, including ROB JAMES’ local town of Chichester, and so perhaps it’s an apt name for a new open network audio standard that must encompass the many and various requirements of some very different bedfellows.

nnounced formally with some considerable bally-hoo at the 2010 IBC in and topology. All the protocols and mechanisms used in Ravenna are based on Amsterdam by ALC NetworX, whose brainchild it is under the direction well established and commonly used methods from the IT and audio industries of CEO Philipp Lawo, the Ravenna consortium already includes a and comply with various standards defined and maintained by the international number of partners – Genelec, Lawo, Merging Technologies, Innovason, standards bodies. DirectOut,A LSB/VSM Control, DSA Volgmann, and Schoeps. From a practical standpoint, Ravenna offers the tantalising prospect of real-time So, what is Ravenna and why is it relevant to audio professionals? In short, it distribution of audio and other media content in IP-based network environments represents a new take on the third generation form of audio interconnect, where with simple interoperability between equipment sourced from a variety of the first generation of interconnect is analogue point-to point copper, the second manufacturers. Although aimed principally at the broadcast market Ravenna has generation uses digital codes representing the analogue signal, conveyed point obvious applications in recording, live sound and the installation markets. to point over copper or fibre-optic cabling, and the third generation also employs The challenges with any audio transport based on IT are latency and accurate digital codes representing the analogue audio but transported as packets over synchronism. Ravenna is designed from the ground up to meet the strict network infrastructure. requirements of the pro audio market with low latency, full signal transparency Conventional real-time (and video) switches/routers are highly and a high level of reliability. It employs several ingenious strategies to these specialised and expensive devices designed and built specifically to distribute large ends and offers flexible configuration options to match performance to the specific numbers of signals with minimal delay with bit-for-bit veracity. Interest in Audio- requirements. While it is perfectly possible to use Ravenna over most existing over-IP is burgeoning among broadcasters and others principally for reasons of cost network infrastructures for non time critical applications, such as journalist and and flexibility. Network based audio and video distribution systems rely on off-the- news feeds, where sub-millisecond latency and absolute sync are the priority shelf IT technology designed originally to forward large numbers of data packets careful choice of network topology and using single sample per channel packets with no constraints on real-time delivery. For this reason standard IP technology offers the lowest possible latency. Adding samples/channels up to the maximum does have some disadvantages for audio transport. However, the massive scale of allowed packet size guarantees the maximum number of channels throughput. data networking equipment production means lower costs and the opportunity to A variety of stream formats with different sample rates can be used concurrently leverage existing infrastructure is too seductive to ignore, especially since it offers within the same network although format and sample rate selection may be limited the promise of a scaleable platform for future revenue earning developments such by the capabilities of specific nodes. as HD radio and data services. A Ravenna system requires a carefully configured IP network, a master clock Ravenna is very well suited to areas where complex audio routing/mixing device and any number of Ravenna enabled I-O nodes. The master clock can be systems are deployed. For example, in-house distribution either a dedicated device in broadcasting centres and WAN connections to satellite or any Ravenna node studios, OB vans, where hook up to venues with capable of serving as a the same infrastructure becomes simple, in venues grandmaster. The preferred themselves for local signal distribution and connection to time domain reference is just such OB vehicles when required. For live events and GPS. Simple streaming concerts it offers highly flexible temporary installation across a network can be possibilities and in theatres, opera houses and houses of achieved without any worship it can provide low-cost local signal distribution. synchronisation at all but Notwithstanding all of the above, Ravenna is also an in pro audio applications excellent candidate for relatively simple point to point tight synchronisation interconnects such as computer to audio interface. between all devices and For years the most ingenious minds in the audio streams is absolutely business have applied themselves to developing mandatory. While playback innovative connectivity solutions and there are already synchronisation in most a number of proprietary audio solutions that use applications requires networking hardware. Sadly most of this endeavour Ravenna overview block diagram showing all the basic components required for sample accuracy, one goal has ended up being used in proprietary products or a Ravenna network: an IP network, Ravenna I-O nodes and a time grandmaster. for Ravenna is to provide in a manufacturer’s licensing portfolio. superior performance Some of these developments have been by offering phase-accurate established as de facto standards and synchronisation as an option thus some, like MADI, for example, have rendering separate reference Word even managed to achieve standards body clock distribution throughout a facility status but mostly only by making the or venue redundant. technology freely available to everyone. A Ravenna node consists of a However, Ravenna, leaving aside the NIC (Network Interface Controller), a other advantages touted, is an open Media Clock which can slave to the standard based on the ubiquitous IP house clock or be a master in its own protocol. Specifically, protocol levels on or right, A-D and D-A convertors and above layer 3 of the OSI reference model. a packetiser and de-packetiser each Since Ravenna is purely based on layer 3 with its own sample buffer. A single protocols, it can operate in most existing node can support many audio I-O network environments. Unlike layer 1 or channels depending on the design. layer 2 solutions, it does not, in principle, Nodes can be configured remotely require its own network infrastructure. IP across the network and concurrent can therefore be transported on virtually unicast and multicast operation is Functional block diagram of a Ravenna node. any LAN and is used as the base layer for supported. communication across WAN connections (including the internet). Although in most Unicast (one-to-one) is used in application scenarios such as an individual cases Ethernet will be deployed as the underlying data link layer, IP is in general stream between two devices (e.g. a multichannel stream between a console and infrastructure-agnostic and can be used on virtually any network technology a recorder/DAW). This uses a point-to-point connection between the sender and

52 resolution November/December 2010 TECHNOLOGY receiver. Since each additional receiver adds its own links, any destination device will receive the full stream individual connection, network traffic increases with data on both network interfaces independently. If data every additional unicast stream. Multicast (one-to- from one link is corrupted, or one network link fails many) streaming is used in scenarios where a single completely, the uncorrupted data is still present on the source is to be distributed to many potential recipients other link. Changeover in the event of the failure of a (e.g. programme stream to journalists’ desktops). At network link is automatic. the sending end this only requires one connection The system design approach allows for operation per stream. Network switches are aware of which with or without centralised services for configuration/ participants (receivers) should receive any particular connection management. ALC NetworX recommends multicast and forward packets only to registered that basic device configuration (e.g. initial settings nodes. In multicast set-ups the network traffic only and setup of audio streams) should be executed via a increases on the last (closest to receiver node) web interface (http). However, other methods may be segment(s) of the network path. used in addition or as an alternative. Device discovery The network infrastructure must be able to transport is accomplished with DNS-SD (via an mDNS or DNS IP packets and must support a number of standard service). In small networks, without DHCP/DNS servers, operating protocols, e.g. RTP/RTPC for streaming the zeroconf mechanism — a fully automatic, self- since this is used widely and supports a wide variety configuring method — is used for auto-IP assignment of standard payload formats. Some of these formats and service advertisement and discovery. Streams will be mandatory for all Ravenna devices, others available on the network are represented by SDP will be optional. For example, this protocol offers records with extended information (i.e. a clock domain the possibility of standard media player applications identifier, RTP time stamp association, etc.) Clients can subscribing to Ravenna streams. connect to streams via RTSP Synchronisation across all nodes or SDP/http. is achieved via IEEE1588-2008 Ravenna is arguably (PTPv2 ). the most promising audio This is another standard protocol industry interconnect that can be used on IP. PTPv2 technology we have seen provides a means for synchronising for a long time since it offers local clocks to a precision defined in low cost of implementation AES-11. Accurate synchronisation to end users and equipment can even be achieved across WAN manufacturers while being connections when GPS is used as a able not only to replace common time domain. existing expensive and For the QoS () proprietary interface protocol DiffServ has been chosen technologies but also since it is widely supported by Com8313 piggyback module used in some to expand capability. most modern managed switches. of the technology prototypes shown at IBC Apart from the obvious Since other traffic can coexist with (Innovason’s Eclipse, Direct Out’s Producer. utility of being able to Ravenna on the same network, Com/R, Lawo’s Dallis and Crystal). send large numbers of Ravenna traffic must be channels between different on the fast track. Ravenna manufacturers kit, reliably, packets are assigned a high with low latency where priority classification to required and impeccable ensure expedited transport reproduction, Ravenna’s across the network, while IP nature makes it a other packets with lower natural for remote control priority are treated as of devices as diverse as best-effort traffic. Even digital microphones and within Ravenna there are loudspeakers as well as different priorities assigned the more obvious consoles, to different classes of DAWs and recorders. traffic. Synchronisation When implemented fully is assigned the highest at enterprise level Ravenna priority, immediately can satisfy a huge range followed by any real-time of needs from office loop media traffic, while control feeds to pretty much every and configuration traffic conceivable interconnect will be on a lower priority application in production level. Any non-Ravenna and recording. traffic would receive the Most importantly, lowest (standard) priority unlike most other existing and be treated as best- networking solutions, effort traffic. Performance A simplified OSI layer model highlighting IP Ravenna is an open protocol layer and RTP streaming layer. and capacity scale with the technology standard without capabilities of the underlying network architecture. a proprietary licensing policy. ALC NetworX is actively As you would expect for a system aimed at encouraging any interested parties to participate in broadcast, Ravenna supports redundancy. Although the enhancement and evolution of its capabilities. modern network infrastructures can be configured to ALC NetworX is also happy to assist manufacturers guarantee a high level of transport security and reliable interested in implementing Ravenna into their products, 24/7 operation, for added security there is the option since broad market acceptance will depend on take-up. of full network redundancy. Each Ravenna device can As a result of this enlightened attitude and Ravenna’s include two independent network interfaces, which many virtues I confidently expect to see many other can be connected to independent physical networks. manufacturers embrace the Ravenna philosophy in the By duplicating any outgoing stream to both network next months and years. n

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