Architecture for Control Networks: Difference between revisions

Revision as of 16:38, 2 January 2016

Revision as of 17:12, 13 January 2016

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==Protocol architecture==

ACN defines a common protocol architecture, two major network protocols (SDT, DMP), a device description language (DDL) and a number of ‘E1.17 Profiles for Interoperability’ (known as ''EPI''s or ''[[#Interoperability Profilesprofiles|interoperability profiles]]'') which define how elements of the ACN architecture must be used in a particular context to achieve interoperability. For example, by providing specific values or ranges for timing parameters to be used in a particular network environment.

The breakdown of ACN into sub-protocols, Interoperabilityinteroperability Profilesprofiles and other small pieces has been criticized{{by whom}} as making ACN hard to read and understand but it makes the architecture highly modular and cleanly layered and this has allowed many of the pieces to be operated in other contexts or replaced or revised without changing the other pieces. For example, DMP has been operated over TCP as well as over SDT as defined in the initial standard, DDL has been adapted with little change to describe devices accessed by DMX512 (ANSI E1.31/Streaming ACN), and several Interoprabilityinteroperability Profilesprofiles have seen major revision or replacement without disturbing the other parts of the standard.

===Common Architecture===

The common architecture specification defines a format of nested ''Protocol[[protocol Datadata Units''unit]]s (''PDU''sPDUs), rather similar to [[Type-length-value|TLV]] encoding, which are used in the main protocols. It then defines how a minimal Root Layer Protocol is used to splice the higher level protocols into a lower level transport and defines such a Root Layer Protocol using the PDU format for use on [[User Datagram Protocol|UDP/IP]].

===SDT (Session Data Transport)===

Session Data Transport (SDT) is a [[reliable multicast]] transport protocol which operates over [[User Datagram Protocol|UDP/IP]] which can be used to group peers within a network into ''sessions'' and deliver messages to them individually or as a group. Messages delivery is ordered and messages may be selectively sent [[Reliability (computer networking)|reliably or unreliably]] on a message-by-message basis (reliability is very important for some data while avoiding the time and resource overhead of the reliability mechanism is beneficial for others). The reliability mechanism also provides online status so a component will detect when a connection is broken. SDT provides a high degree of fine tuning over the trade-off between latency, reliability levels and resource requirements and availability of large numbers of concurrent sessions means they are a powerful tool for grouping and managing components whose functions are related or whose communication requirements are similar.

===DMP (Device Management Protocol)===

Device Management Protocol (DMP) represents any device as a set of addressable properties which represent its current or desired state. Monitoring or control by a controller is achieved by setting or examining the values of those properties. To avoid the inefficiencies of polling, in addition to simply reading property values (using a ''Get-Property'' message) DMP provides a subscription mechanism whereby a device will asynchronously send event messages to all subscribed controllers when the value of a property changes.

DMP expects that its connections can provide reliability so that ''Set-Property'' and ''Event'' messages which form a large part of the operational bandwidth in a show situation do not require explicit acknowledgement at the DMP level. In the E1.17 standard and the majority of systems SDT provides this reliability but DMP has also been operated using TCP to provide its reliable connections.

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The size in bits, representation, read/write accessibility and function of each property in a DMP device is not determined by the protocol which only defines the mechanism to read and/or write the property value. Instead, that information must either be provided externally by a device description written in DDL or in limited cases may be pre-programmed by fore-knowledge of specific device types.

===DDL (Device Description Language)===

Device Description Language (DDL) allows a machine parsable description of the interface and capabilities of any device to be defined.<ref name="EngArts">http://engarts.com/ddl/index.html</ref> This description can be interpreted by a controller which may then automatically configure itself for controlling that device. The description not only provides the address and property mapping information which is necessary for DMP to operate but it can also contain a huge amount of information on the functionality, capabilities and semantics of the device in an extensible format which allows a controller to extract the features it needs for its specific context while skipping over information which is not relevant to its needs.<ref>http://powers.media.mit.edu/wiki/upload/E1-17ACN2006DDL.pdf</ref>

DDL is an [[XML]] based language and descriptions are contained in a small number of [[XML]] documents. In normal ACN systems the description for a device may be downloaded from the device itelf. However, descriptions may also be distributed in other ways (such as internet download) and since a description is valid for all devices of the same type, controllers can typically maintain a cache of descriptions for devices they commonly encounter.

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===Interoperability profiles===

'''Interoperability Profiles'''profiles ('''EPI'''sEPIs) are provided in ANSI E1.17 for initial [[service discovery]] in a system; for allocation of [[multicast address]]es when used on UDP and [[IPv4]]; for [[Port (computer networking)|UDP port]] allocation when multicasting, for [[IP address]] assignment in conformant systems, for protocol timeouts in specific environments and so on. Other EPIs which conform to the ACN Architecture have been developed outside the ANSI E1.17 standard (see below).

==External Extensions==