A virtual device driver (VxD) is a software device driver that emulates hardware and other devices so that multiple applications running in protected mode can access hardware interrupt channels, hardware resources and memory without causing conflicts. The Vxd was superseded by the Windows Driver Model (WDM) and is now obsolete.
A delta channel (D channel) is a signaling channel in an integrated services digital network (ISDN). D channels take care of call set up, control and maintenance. The basic ISDN channel holds two bearer channels (B channels) and one D channel. These contain data that relates to signaling errors, framing and other management signals. The speed of a D channel is 16 Kbps for basic rate interface and 64 Kbps for primary rate interface.
The D channel's technical capabilities include information on the terminal equipment that is originating and receiving calls. This includes required signaling type, and the terminal's ability to handle special services and features.
D channels carry signaling between a customer’s terminal device and a carrier’s end-switching office. Signaling information with end-to-end significance travels between the carrier switching office on the carrier's common channel-signaling network and the destination terminal through the receiving user’s D channel.
D channels operate based on a well-defined pair of layered protocols. D channel Layer 2 protocol is Q.921 for DSS1 signaling and is called linked access procedures, D channel (LAPD). It resides at the data link layer. The Q.931 protocol operates at upper layers - Layer 3 and above.
LAPD protocol operates between the terminal equipment and network termination over the D channel of the ISDN interface. The fields within LAPD include address, control, command/response bit, information and frame check sequence.
Q.931 D channel signaling protocol performs integration, carrying signaling information about the nature of the ISDN service required for specific calls between the end user terminal equipment and the ISDN carrier’s end office. The protocol conveys information such as service information, terminal capabilities, handshaking, etc. Service information includes information on the nature of the service requested for calls such as D channel packet-switched data, B channel packet-switched data, circuit-switched data, video and fax.
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