-------------------------------------------------------------------------------- HSSG CSMA/VCD proposal Moti Weizman NBase Communication motiw@nbase.com 818-773-0900 Ext 244 WHY Inexpensive hub with reasonable performance to push shared and backbone gigabit CSMA/CD Disadvantages When many stations are active, bandwidth utilization peak at ~50% and drops after that. Fairness not guaranteed Huge variable delay Network diameter limitation Options Scaling existing MAC and repeater Adv. Known technology Simple Disadv. Network diameter too small (~30 meter) Add carrier extension up to 512 bytes Adv. 200 meter Network diameter Simple Disadv. Bandwidth drop for short frames Bandwidth drop due to propagation delay Higher collision probability Add frame burst Adv. Compensate for bandwidth lost when short frames Disadv. Still higher collision probability Relatively complex implementation Retransmission of whole batch after a collision Discard whole batch when collision Mechanism to keep carrier within a batch Manageability 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 CSMA/CD Collision Scenario The Magic:Virtual Collision (CSMA/VCD) Collision is no longer a "Dirty-Word" The hub broadcasts the first frame and discards frames from other stations, that collide Mac always retry after 0 slot time Virtual Collision (CSMA/VCD) Adv. No loss of bandwidth when collision, ~85 percent channel utilization. Network diameter is almost twice the existing CSMA/CD (before applying carrier extension) Disadv. No fairness Close stations have advantage over farther stations Stations that capture the channel will have advantage in the next frame Why Double Network Diameter Worst case time from A start of transmit to A recognize a collision CSMA/VCD Fairness Solution Default IFG is Round-Trip-Delay + 12 Bytes from last activity on the channel After a collision IFG is 12 Bytes time CSMA/VCD Fairness Solution (Cont.) Adv. Simple Disadv. Bandwidth loss when no collision due to larger IFG 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 Next frames within a batch transmitted without extension, with carrier extension between frames (instead of IFG) Adv. No need to retransmit the whole batch, If the first frame does not collide all other frames will be successful due to the carrier extension between the frames All frame received are good Simple Utilization Vs. Frame Length Existing Vs. Proposal Changes To Standard MAC transmit state diagram IFG after a successful batch transmission is round-trip + minimum IFG IFG after collision and between frames in a batch is minimum IFG Carrier extension on first frame in a batch and between frames Frame batch up to ~1500 bytes Repeater state diagram No need for JAM state Summary Simple and inexpensive Network diameter 200 meter Bandwidth utilization 60-80% Side effects Delay from frame ready to transmit to frame transmitted is bounded to (N-1) * (MaxBatch + EndToEndDelay) Fairness guaranteed Priority ?