The Brothers WISP – GPON vs Active Ethernet

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2 responses to “The Brothers WISP – GPON vs Active Ethernet”

1. 

   Dave Taht

   2023-02-27

   This was really good, but I still don’t really understand how gpon multiplexes things. Is there a request/grant cycle, or just timeslots? how much space between timeslots?

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   1. 

      Tommy Croghan

      2023-07-08

      I am not an expert in GPON, but I do know it is a TDMA protocol. (Different frequencies are used for the OLT transmissions vs the ONT transmissions)

      Looking into ITU-T G.984.1 it seems that it’s a ATM protocol, but I do not know in reality what kinds of latencies are incurred in various situations. Although most operators try to keep the number on ONT’s on a single OLT port below 64 (my company maxes out on 32)

      8.3.5 Transport-specific TC functions
      8.3.5.1 Frame structure
      The downstream interface structure for both 155.52 Mbit/s, 622.08 Mbit/s, and 1244.16 Mbit/s
      consists of a continuous stream of timeslots, each timeslot containing 53 octets of an ATM cell or a
      PLOAM cell.
      Every 28 time slots, a PLOAM cell is inserted. A downstream frame contains two such PLOAM
      cells and is 56 slots long for the 155-Mbit/s downstream case. For the 622-Mbit/s case, it contains
      eight PLOAM cells and is 224 slots long. For the 1244-Mbit/s case, it contains sixteen PLOAM
      cells and is 448 slots long.
      In the upstream direction, the frame contains 53 time slots of 56 bytes for the 155-Mbit/s upstream
      case, and for the 622.08-Mbit/s case, it contains 212 time slots. The OLT request an ONU to
      transmit an ATM cell via grants conveyed in downstream PLOAM cells. At a programmable rate,
      the OLT requests an ONU to transmit a PLOAM cell or a minislot. The upstream PLOAM rate
      depends on the required functionality contained in these PLOAM cells. The minimum PLOAM rate
      per ONU is one PLOAM cell every 100 ms. The OLT defines the bandwidth allocated to the
      upstream minislots.

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