Architecture for Control Networks

The common architecture specification defines a format of nested protocol data units (PDUs), rather similar to 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 UDP/IP.

Session Data Transport (SDT) is a reliable multicast transport protocol which operates over UDP/IP which can be used to group peers within a network into sessions and deliver messages to them individually or as a group. Message delivery is ordered and messages may be selectively sent 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.

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.

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.

Device Description Language (DDL) allows a machine parsable description of the interface and capabilities of any device to be defined.[1] 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.[2]

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 itself. 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.

Interoperability profiles (EPIs) are provided in ANSI E1.17 for initial service discovery in a system; for allocation of multicast addresses when used on UDP and IPv4; for 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).