# Supplementary services under DMS

This section provides an overview of the supplementary services available with the DMS variant and how these services operate in the network architecture. It describes:

- DMS variant

- Using supplementary services

- Release link trunk (RLT) service

## DMS variant

DMS specifications describe a protocol designed to access network services from an intelligent user terminal. The protocol is not symmetric, and requires two distinct roles, the user side and the network side. NMS ISDN supports both sides for basic call control, but NMS ISDN supplementary services only support the user side.

## Using supplementary services

Under DMS, most supplementary services require subscription. Subscription services are optional services provided by the network operator on request. When you request network services from a provider, you can also request one or more subscription services. These services are not available unless the interface is provided with them.

## Release link trunk (RLT) service

The release link trunk (RLT) service allows an application at the user side to request the transfer of two independent calls. To use the service, the user should be subscribed to RLT.

The following table describes the call transfer operation that can be performed under DMS:

Operation | Description
Invoke two B channel transfer | The application sends a request to the network to transfer two independent calls.