Below is a white paper that provides an overview of SMDS.  SMDS is the   
network technology currently used by Bell Atlantic.  It is important that   
we understand the basics, advantages, and limitations of SMDS in   
evaluating the future network technology.

SMDS White Paper

SMDS basics:

SMDS is basically the high-speed digital equivalent of the switched phone   
system in that it has the following characteristics:

1.  The public phone company offers it.
2.  It is a Metropolitan Area Network (MAN) technology.
3.  It is regulated to handle traffic only within a LATA (This explains   
why they use SMDS)
4.  Traffic crossing LATA boundaries must be switched to circuits   
provided by a long-distance carrier
5.  It is switched, so that all subscribers use the same network.  Uses   
data packets
6.  Subscribers pay only for time actually used.
7.  Subscribers can purchase different levels of guaranteed availability   
and service.
8.  By conforming to specifications established by ANSI and IEEE, it is   
compatible with all similar systems.

SMDS advantages:

1.  Compatibility-SMDS is the same type of "guaranteed non-failure" that   
the phone companies mandated heritage of universal service has stood for.
2.  Simplicity-For the same reasons that ensure compatibility, connecting   
to a SMDS network is likely to be one of the easiest options for creating   
a MAN.
3.  Bandwidth on demand-SMDS delivers both the high bandwidth needed for   
multimedia files (not services) and fast data applications and the   
economy of charging only for time actually used.
4.  Familiar approach to network management-SMDS provides features   
similar to those found in LANs, including high throughput and low delay.
5.  Open access-Unlike a leased T1 or T3 line, which connects two   
specific sites, SMDS can open up communications with any other   
subscribers on a SMDS network.
6.  Security-The telco switching system screens frames to see if they are   
authorized to be transmitted to the destination or received from the   
source.  This security feature lets a subscriber build a virtual private   
network within a public network.
7.  Easy integration-Because of its similarity with LANs and   
compatibility with other emerging technologies, SMDS will evolve   
gracefully with the network.
8.  Little chance of network overload-The access classes benefit the   
subscriber because they limit the amount of traffic delivered to the   
equipment, which in turn reduces the potential for the network to   
overload the receiving equipment.

SMDS limitations:

1.  Cannot transport voice or video-Because it uses   
connectionless-oriented connectivity, SMDS cannot provide the isochronous   
transport needed for continuous signals.  This is a major problem if you   
want to have multimedia services.
2.  Regulated as an intra-LATA service-Though these regulations may soon   
be changing, currently, if sites you want connected to your network are   
in different LATAs, an additional level of complexity and expense comes   
into play.
3.  Dedicated access line-It is the subscriber's responsibility to get   
their signal to the telco's central office.  This means leasing a   
dedicated line, which is relatively expensive and tends to offset the   
"bandwidth on demand" advantage.
4.  Restricted access-The ugly side of the SMDS security advantage is   
that special arrangements must be made before you can communicate with   
another subscriber on the SMDS network.  You cannot just tell a new   
vendor to "send that CAD file over the SMDS net to us."  You must both   
first make arrangements with the telco to have your respective addresses   
programmed into the central office switch as numbers authorized to   
exchange traffic.

SMDS is primarily used by organizations that have numerous sites with in   
a metropolitan area.