What Is a Service Layer?

The service type field is used to indicate the desired quality of service. This is a set of abstract and general parameters provided by the network constituting the Internet for service selection. This service type is used by the router to select the actual transmission parameters for a particular network, the next hop network, and the next router that routes data between networks. ^[1]

Service type field with 3

Depending on the specific implementation of the service, services can be divided into two types: connection-oriented services and connectionless services. This is due to the different requirements of the upper layer on the service quality of the lower layer.

BACnet application layer services are mainly based on a "read / write" mode. Although this mode has a very simple access interface, it is sometimes too simple and inefficient to access. With the development of applications, in order to improve access efficiency and enhance flexibility, it is necessary to extend the existing service types.

Application service extensions can be divided into two types of extensions according to the application layer service extension method: the first type of extension is a dedicated extension provided by the standard service, and the second type of extension is the definition of a new standard service.

According to the application layer service message encoding, the BACnetCOnfimedServiceChoice enumeration variable and BACnetUncon "rmedServiceChoice enumeration variable that define the service type are not allowed to be extended by the user. Therefore, users cannot define new application service types to extend application services. The extended functions provided by the standard service type are used to extend application services. The former is used to extend the certified service and the latter is used to extend the non-certified service. Identification number), service number (service number) and service parameters (service parameters) are encoded as user data. Among them, service number is unsigned integer data, this parameter can be used as the type code of the dedicated extended service.

In order to make the practical extension service universal and improve the efficiency of the service, a second type of extension must be performed, that is, the type of application service is extended and standardized. It is also very easy to extend the type of application service, just construct a new service type according to the standard application service and use it as the standard content.

The BACnet standard has so far expanded multiple application-layer service types, such as ReadRange, LifeSafetyOperation, and SubscribeCOVProperty services. ^[4]