802.3z CSMA/VCD proposal
Moti Weizman


motiw@nbase.com
818-773-0900 Ext. 244

Assumptions
        -One central repeater
        -Derived from network diameter issues
        -Bi-directional link - Half duplex data transmission
                Applicable for half duplex with a small change

The Solution
        Virtual Collision +
			Carrier extension +
						Frame batch

The Magic:Virtual Collision (CSMA/VCD)
        To avoid bandwidth loss due to collisions and eliminate capture effect. 
        -The hub broadcasts the first frame and discards frames from other
         stations, that collide
        -Mac always Transmit after at least 1 full slot time of IDLE's and retry
         after 0 slot time 

Carrier Extension
        To achieve 200 meter diameter. 
        -Extend to 256 bytes (Vs. 512 in CSMA/CD) all frames that are shorter
        than 256.
Frame batch
        To improve utilization for short frames
        -Each station should transmit all ready frames up to ~1500 bytes
        -The first frame should be extended if less than 256 bytes
        -Other frames within a batch are not extended. Carrier extension is
        inserted between frames

Changes To Standard
        -MAC transmit state diagram
        -Inter frame spacing is 1 slot time
        -Backoff always after zero slot times 
        -Carrier extension on first frame in a batch
        -Frame batch up to ~1500 bytes
        -Repeater state diagram
        -Remove JAM state

Summary
        -Simple and inexpensive (Simple analyses)
        -Network diameter 200 meter 
        -Deterministic bandwidth utilization, 60-80%
        -Good Side effects
                -Delay from frame ready to frame transmitted is bounded by 
                 (N-1)*(MaxBatch + EndToEndDelay) 
                -Fairness guaranteed
                -Priority ?
                -Comparison to Carrier Extension Only