DASS2
Encyclopedia
Digital Access Signaling System 2 (DASS2) is an obsolescent protocol defined by British Telecom for digital links to PSTN based on ISDN
. Although still available on request, it has been superseded by ETS 300 ("EuroISDN").
DASS2 is an improved version over DASS1
, based on experiences with DPNSS.
In the UK, the ISDN concept was first introduced to customers by BT with their
DASS2 connections. DASS2 (Digital Access Signalling System) is a BT designed
signalling standard. It was introduced before the Q.931
standard was finalised
by the International Community. British Telecom used the term ISDN when
describing their DASS2 lines.
DASS2 lines are provided to the customer on a 2Mbit/s link and can handle 30
simultaneous calls (64kbit/s each). DASS2 is still offered by BT and other UK
carriers. Q.931 is the name of the CCITT document that describes the agreed
signaling format for International ISDN. The CCITT used to be International
Telegraph and Telephone Consultative Committee. This is the organization that set out the internationally agreed standards for telecommunications. This organization has
subsequently evolved into the ITU. In the United Kingdom the Q.931 based
protocol is ETS 300 (also known as Euro ISDN). This is a very close
implementation of the original CCITT specification. This is a 2Mbit/s service as
with DASS2 but the feature capability is far greater and has negated the
problems associated with DASS2 including echo problems and circuit spikes. In
the UK both DASS 2 and Euro ISDN (ETS 300) lines are available to customers with EuroISDN as the preferred signaling type. Customers will normally choose
the desired signalling system, as this will be dictated by their CPE equipment,
usually a PABX. (CPE- Customer Premises equipment)
Most modern PABXs can handle many different types of signalling system, however
the trend seems to be away from the DASS2 which is no longer being developed by
BT and have been known to deny problems with their DASS2 circuits, and move
towards the internationally recognized Q.931 standard. The Q.931 standard is an
international standard utilized by many countries telephony service providers.
The CCITT specify the standards for the Layer 1,2 and layer 3 signalling
messages. The Layer 3 messages are the messages that actually control the call
setup, teardown, and routing.
The Layer 3 messages or Call Control messages are the minimum messages that must
be understood by the interfacing equipment. Individual service providers may
publish their own documentation that details further messages that will be
transported in addition to Q.931 messages. There are a number of additional
European documents that cover supplementary services. These cover features that
may be instigated by exchanges via the ISDN and require a higher degree of Layer 3 implementation.
Integrated Services Digital Network
Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...
. Although still available on request, it has been superseded by ETS 300 ("EuroISDN").
DASS2 is an improved version over DASS1
DASS1
Digital Access Signaling System 1 is a protocol defined by British Telecom for digital links to PSTN based on ISDN....
, based on experiences with DPNSS.
In the UK, the ISDN concept was first introduced to customers by BT with their
DASS2 connections. DASS2 (Digital Access Signalling System) is a BT designed
signalling standard. It was introduced before the Q.931
Q.931
ITU-T Recommendation Q.931 is ISDN's connection control protocol, roughly comparable to TCP in the Internet Protocol stack. Q.931 doesn't provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates...
standard was finalised
by the International Community. British Telecom used the term ISDN when
describing their DASS2 lines.
DASS2 lines are provided to the customer on a 2Mbit/s link and can handle 30
simultaneous calls (64kbit/s each). DASS2 is still offered by BT and other UK
carriers. Q.931 is the name of the CCITT document that describes the agreed
signaling format for International ISDN. The CCITT used to be International
Telegraph and Telephone Consultative Committee. This is the organization that set out the internationally agreed standards for telecommunications. This organization has
subsequently evolved into the ITU. In the United Kingdom the Q.931 based
protocol is ETS 300 (also known as Euro ISDN). This is a very close
implementation of the original CCITT specification. This is a 2Mbit/s service as
with DASS2 but the feature capability is far greater and has negated the
problems associated with DASS2 including echo problems and circuit spikes. In
the UK both DASS 2 and Euro ISDN (ETS 300) lines are available to customers with EuroISDN as the preferred signaling type. Customers will normally choose
the desired signalling system, as this will be dictated by their CPE equipment,
usually a PABX. (CPE- Customer Premises equipment)
Most modern PABXs can handle many different types of signalling system, however
the trend seems to be away from the DASS2 which is no longer being developed by
BT and have been known to deny problems with their DASS2 circuits, and move
towards the internationally recognized Q.931 standard. The Q.931 standard is an
international standard utilized by many countries telephony service providers.
The CCITT specify the standards for the Layer 1,2 and layer 3 signalling
messages. The Layer 3 messages are the messages that actually control the call
setup, teardown, and routing.
The Layer 3 messages or Call Control messages are the minimum messages that must
be understood by the interfacing equipment. Individual service providers may
publish their own documentation that details further messages that will be
transported in addition to Q.931 messages. There are a number of additional
European documents that cover supplementary services. These cover features that
may be instigated by exchanges via the ISDN and require a higher degree of Layer 3 implementation.
See also
- DSS1 (ETSI "EuroEuropeEurope is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
-ISDN", also used in many non-European countries) - DSS2 (Digital Subscriber Signalling System No. 2) - enhanced DSS1.