A MAN data transport mechanism is the intended use of a Distributed Queue
Dual Bus (DQDB), and DQDB MANs are meant to function as connectionless
networks that link multi-megabit LANs together. Because of this, a
Macintosh isn't a likely first candidate end-node device for design and
manufacture of DQDB interface cards.
It's more likely that DQDB networks will connect to devices such as routers
and bridges. Routers and bridges (interconnect devices) will be the first
to have DQDB interfaces, and it may be a year or more before those come
into real use. There have been some SMDS/DQDB trials, and Bell Atlantic is
presently offering SMDS service to a limited area. Wide use of this
technology is still several years away.
If you need to put your Macintosh computers on a MAN today, you can use a
router to attach to the MAN and then a LAN (of your choice) to attach to
the Macintosh end nodes. There may eventually be an SMDS/DQDB interface
for the Macintosh when the service becomes a real commercial product.
Here's some background information on DQDB to help understand what it is.
DQDB Background Information
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Metropolitan area networks (MANs) could offer answers to the demand for
communications with anyone, anywhere, cheaply and reliably. The ideal
answer is a system that can support high speeds and geographical distances
of hundreds of kilometers and be available for everyone.
FDDI is a popular backbone technology capable of high-speed communications
and allows extended distances between networked sites. Presently FDDI is
considered suitable for high-speed workstation networking.
The telephone industry supports a proposed MAN implementation called Queued
Packet Synchronous eXchange (QPSX). The IEEE 802.6 committee changed the
name to Distributed Queue Dual Bus (DQDB) in order to avoid confusion with
an Australian company. MANs have implications that apply to corporate use
and to the entertainment industry for advanced video and voice services.
The Distributed Queue Dual Bus (DQDB) standard was adopted as IEEE 802.6 in
late 1990. DQDB describes a physical layer and Medium Access Control (MAC)
sublayer to support packet and isochronous (time-sensitive) data transport
over a metropolitan area. DQDB describes a transport network, not a MAN
service.
Bellcore's description of a MAN service is SMDS. As currently specified,
SMDS will provide the MAN service using DQDB as the underlying transport
network. The first in a series of TAs defining the requirements for SMDS
was released in October 1989. These documents include specifications for
the user interface, service structures, network components, billing
elements, and network management. SMDS TRs started to appear this year,
and SMDS demonstrations, trials, industry groups, and related standards
have also started to appear. SMDS will be very important in the RBOCs'
future deployment plans for MANS, particularly as an initial step to
offering Broadband ISDN (B-ISDN) services.
AT&T's competition comes from QPSX Systems of Perth, Australia, which backs
Distributed Queue Dual Bus (DQDB). QPSX is a joint venture of Telecom
Australia and the University of Western Australia, the developer of DQDB.
Communications industry analysts said there's little difference between
AT&T's SMDS and QPSX Systems' DQDB beyond the initials. "SMDS is AT&T's
implementation of DQDB," said Stuart Gavurin, a network analyst at Ernst &
Young.
In 1987, IEEE decided to use DQDB as the basis for MANs, and a year later
QPSX announced an agreement to allow AT&T to manufacture DQDB-compliant
products. Shortly after that, however, AT&T decided to pursue its own
implementation of MAN.
Gavurin said that the point for potential users to note is SMDS and DQDB
adhere to the 802.6 standard and are compatible with each other. Users can
buy products based on one implementation and know the products will work
with those based on the other implementation.