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Digital Access Signalling System 1

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Digital Access Signaling System 1 (DASS1) is a protocol defined by British Telecom for digital links to PSTN based on ISDN.

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Template:Com-stub Overview of ISDN DPNSS Signaling overviews a typical ISDN network deployment. “User-side” systems (typically Private Branch Exhcnages (PBXs) interconnect to “Network-side” systems (typically Carrier Local Exchanges) using T1 or E1 ISDN Primary Rate Interface (PRI) facilities.


Typical ISDN PRI facilities comprise one Signaling Channel (or “D-Channel”) with remaining circuits allocated as Bearer Channels (or “B-Channels”) to carry Voice or Data. For T1 facilities, ISDN PRI have 1 x 64 Kb/s D-Channel + 23 x 64 Kb/s B-Channels. For E1 facilities, ISDN PRI have 1 x 64 Kb/s D-Channel + 30 x 64 Kb/s B-Channels (the remaining channel is used for clocking & synchronization). It is also possible to have PRI with no D-Channel at all if the PRI is a member of a group of PRI where another PRI has the D-Channel for the group. Instead of PRI, ISDN interfaces may also consist of Basic Rate Interface (BRI), or 1 x 16 Kb/s D-Channel + 2 x 64 Kb/s B-Channel. BRI are typically used for Residences or Very Small Offices. Owing to their small size, BRI will not usually be monitored directly by GeoProbe systems. Regardless of ISDN Interface type, the D-Channel carries signaling information used for Call Control between the User and Network systems. The application-level protocol used for Call Control is known as “Digital Subscriber System #1” (DPNSS). Figure 2 2 shows additional detail on how the D-Channel is used for DPNSS Call Control signaling. Each D-Channel carries multiple Virtual “Data Link Connections” (DLC) managed by a common “Link Access Control D-channel” (LAPD) transport layer. Each DLC is identified by a “Data Link Control Identifier” (DLCI) value carried in LAPD message headers. The DLCI value is unique within the context of each PRI, so the combination of a PRI “Interface Identifier” + DLCI value creates a DLC identifier that is unique network-wide.

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