Mar 5, 2009

DPNSS Versus QSIG - Can They Coexist?

很多人聽過PBX之間交換的標準-QSIG,但是可能比較少人聽過DPNSS,事實上DPNSS也是另外一種跟QSIG類似的PBX交換標準,但是它們之間到底有那些不相同的地方及歷史上演進的結果為何?

請參閱下文Q&A:

http://www.pqmconsultants.com/coexist.htm

Q. What are DPNSS and QSIG?
DPNSS and QSIG are inter-exchange signalling protocols, primarily intended for the interconnection of nodes in a Corporate telecommunication Network (CN). The interconnection of PABXs using leased circuits is a typical application. Both DPNSS and QSIG are common channel signalling systems based on ISDN technology. They are open standards; that is, they permit signalling between equipment from different vendors.

Q. How did DPNSS come about?
The development of DPNSS commenced in 1981 with the decision by the UK telecomms industry (British Telecom, as was, and a number of PABX manufacturers) to develop a vendor independent private network signalling system. This work resulted in the protocol that is today widely used throughout the UK and elsewhere. The drivers behind DPNSS development are well known. They were the increasing demands of corporate communications, the advent of stored program control PABXs at the beginning of the 1980's, and the increased availability of digital transmission capacity.

Q. How did QSIG development start?
In the mid- to late 1980s work commenced at the European level, firstly in the Committee of European Post and Telecommunications (CEPT), and more recently in the European Telecommunications Standards Institute (ETSI) and the European Computer Manufacturers Association (ECMA). This work developed an architecture for private ISDNs and an inter-exchange signalling protocol based upon the ISDN concepts specified in ITU-T (formerly CCITT) Recommendations. The inter-exchange protocol, based on ITU-T Recommendation Q.931, has become known as QSIG.

Q. What caused this?
The UK telecomms industry pushed for it. It wasn't until later that other parts of the European industry recognised the need for an ISDN based vendor independent signalling system for PABX networks. Added impetus came in 1988 when the Commission of the European Communities added their political weight to the work.

Q. Why wasn't DPNSS used as the basis for QSIG?
In the mid-1980's, there was considerable opposition to DPNSS from outside the UK. DPNSS was seen, incorrectly, as a particular solution from BT. This generated so much resistance from other European countries. Commencing standards work at the international level based on DPNSS was a non-starter. Having said that, many of the companies and individuals active in QSIG standardisation are the same companies that participated in DPNSS development. They hold the "DPNSS compatibility issues" very much in the forefront of their minds.

DPNSS was only ever intended as an interim solution prior to an internationally agreed ISDN based solution becoming available. As things have turned out UK users have benefited from DPNSS for 15 years now.

Q. What is the status of QSIG development?
Today (Spring 1998) there are Standards for more than 30 QSIG supplementary services, including calling line identification, call transfer, and several forms of call diversion. Others are still being worked upon. QSIG has also achieved status as a worldwide standard in ISO/IEC.

Q. How does QSIG functionality compare with that of DPNSS?
It is equivalent; it is only a matter of time before QSIG is richer than DPNSS. However, adding supplementary services is not the whole story. QSIG will eventually be superior to DPNSS in many ways.

Q. Which manufacturers support QSIG?
Most of the major switching equipment manufacturers have developed products to support QSIG - although they don't necessarily market them actively in every country.

Q. When should an organisation consider migrating its network to QSIG?
It shouldn't necessarily. Whether or not an organisation chooses to use QSIG depends on the requirements. There are many factors influencing this decision, and they will be different for each organisation. For example:

Are there PABXs from more than one vendor in the network?
What are the vendors' product strategies?
Does the network have international links?
What functions must the network perform that are not provided now?
Will the network continue to be based on leased lines, or will these be replaced with switched ISDN connections?
Is Virtual Private Networking being considered?
Can DPNSS meet the future requirements?
There are no hard and fast rules.

Q. Is it possible to use QSIG and DPNSS in the same network?
Yes, networks containing a mixture of DPNSS and QSIG links are possible; the Channel Tunnel network is a real example. The DPNSS Working Party has specified, as long ago as 1991, the interworking between DPNSS and QSIG.

Q. Are multi-vendor networks a problem?
The issue of interworking (or interoperability) between equipment from different vendors is obviously important for any Standard that claims to be "open", as both DPNSS and QSIG do. In the DPNSS case, the early suppliers of equipment (Mitel, GEC, and Plessey) voluntarily participated in interworking trials. These days, standard interworking test schedules exist and a commercial testing service is available.

With QSIG the question of conformance testing versus interoperability testing is still open. Conformance testing aims to demonstrate that a product obeys the protocol rules whereas interoperability testing is concerned with demonstrating behaviour of the product in conjunction with another. The fact that a product demonstrates conformance with the protocol through conformance testing is no guarantee that it will interopearte with other products. Standardised interoperability tests have not been specified for QSIG yet.

Q. How long will it be before DPNSS is ousted by QSIG, and when will vendors stop offering DPNSS?
QSIG will generate security; it will endure because it has the backing of international standards bodies and enhancements are backwards compatible. In the long run DPNSS cannot endure. DPNSS is not being enhanced to support new applications and technologies. It's the same as asking how long any technology will endure - the answer is "for as long as there's market demand".

In our opinion DPNSS and QSIG will coexist alongside one another through the millenium.

However, protocol standards are not the whole story. There are many other standards affecting the construction of Corporate Networks. These, combined with political, regulatory, and commercial issues continue to make the construction and management of corporate telecommunication networks a complex subject.
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