If the address is disallowed, the data unit is not delivered. With address screening, a subscriber can establish a private virtual network that excludes unwanted traffic. This provides the subscriber with an initial security screen and promotes efficiency because devices attached to SMDS do not have to waste resources handling unwanted traffic. To accommodate a range of traffic requirements and equipment capabilities, SMDS supports a variety of access classes. Different access classes determine the various maximum sustained information transfer rates as well as the degree of burstiness allowed when sending packets into the SMDS network.
Credit is allocated on a periodic basis, up to some maximum.
Then, the credit balance is decremented as packets are sent to the network. The operation of the credit management scheme essentially constrains the customer's equipment to some sustained or average rate of data transfer. This average rate of transfer is less than the full information carrying bandwidth of the DS-3 access facility. The SIP is based on the DQDB protocol specified by the I EEE 802.6 MAN standard. As designed by IEEE 802.6, the DQDB standard can be used to construct private, fiber-based MANs supporting a variety of applications including data, voice, and video.