CSMA/CD is a reactive protocol, reacting to packet collisions. SMAC is a proactive protocol, preventing packet collisions.SMAC is not the next-generation CSMA/CD. If anything, the SMAC protocol is the next generation of TDMA (time division multiple access) – a protocol used in satellite communications. TDMA is also used in GSM (global system for mobile communications) digital cellular systems.Both the TDMA and SMAC protocols employ synchronous transmission management to allow multiple users to access a baseband channel simultaneously without interfering with each other.SMACtechnology,however,usesamoreprecisephase-synchronizationalgorithmto permitcloserspacingofpacketframesandtherebyeliminatesthewastedbuffer time TDMA needs to prevent packet collisions.
According to U.S. Patent 4,811,365, SMAC is a protocol "... wherein a plurality of nodes transmit over a common path on a time-sharing basis via a central hub."What happens is that one of the nodes on the network transmits a repeating, pre-established, pseudo-random, digital sequence at a pre-established bit rate. Once this sequence is established, this "reference node" begins to omit a portion of the sequence.Another node wishing to transmit in the shared medium synchronizes itself with the omitted portion of the sequence. Precise synchronization is achieved by adjusting the bit rate, phasing, framing, and sequence portion. Once these nodes are "in sync," both nodes can transmit without interfering with each other.As described in the patent: "Since the slave nodes are in precise synchronization with the reference node, and the length of each node's transmission is exact and pre-established, each node can precisely calculate the time at which to begin its transmission without need for delay between transmissions or the danger of overlap."
Shooting Silver Bullets
SMAC technology has an arsenal of "silver bullets" to rescue Gigabit Ethernet from obsolescence. The SMAC protocol:Can easily be applied over existing Ethernet environments and standards, including legacy systems.Is both compatible – and competitive – with Fibre Channel, SONET, ATM, and TDMA, among others.Is medium independent.Is platform independent.