The Emerging WDM EPON by Mirjana Radivojević & Petar Matavulj

Author:Mirjana Radivojević & Petar Matavulj
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

4.4 Dynamic Bandwidth Allocation in EPON

In this section, we focus on the communication and data transmission in EPON. In order to do that, we have to define the term ‘transmission cycle’ in the first place. The transmission cycle is defined as a time interval between the moment when the foremost GATE message is completely received and the moment when the lattermost REPORT message is completely transmitted. In order to separate transmissions of different ONUs, a guard time is scheduled before each transmission window to perform the ranging for different distances between the OLT and ONUs and to provide transmitter switching time.

As previously explained, the DBA algorithm in EPON relies on two Ethernet control messages (GATE and REPORT) in its regular operation. In the one transmission cycle, every ONU must get the chance to transmit a portion of buffered frames and exchange the control messages with the OLT. Within each cycle and allocated window, the ONU sends REPORT message along with data in order to report bandwidth requirements for the upstream transmission of its traffic. A REPORT message can be either transmitted at the beginning of the timeslot, or at the end, depending on the bandwidth request approach implemented by the ONU. It contains the requested size of the next timeslot based on the buffer occupancy of the ONUs [47].

Each ONU uses a set of queues to store its frames and starts transmitting them as soon as its transmission window starts. The intra-ONU scheduler schedules the packet transmission for various traffic queues from local users, and the transmission window may comprise multiple Ethernet frames . An ONU can support up to 8 priority queues, as defined in 802.1Q [53]. The ONU should also account for additional overhead when requesting the next timeslot that includes an 8-byte frame preamble and a 12-byte inter-frame gap (IFG) between two consecutive frames. Between the allocated transmission windows of two ONUs, a certain guard time is needed to account for the laser on and off times, receiver recovery times, round-trip delay (which relates to the physical distance between the communicating ONUs), and other optic-related issues.

Upon receiving REPORT messages from all ONU s in the defined cycle, the OLT passes the messages to the DBA module which performs the bandwidth allocation computation. Apart from that, the DBA module will recalculate the RTT for the each ONU. The OLT assigns the transmission windows (TWs) via GATE messages, as shown in Fig. 4.15. The GATE message specifies the transmission start time and end time during which each ONU can transmit the queued customer traffic upstream to the OLT . The transmission could include multiple Ethernet frames , depending on the size of the allocated transmission window and the number of buffered packets at the ONU. Since packet fragmentation is allowed, in case the next frame does not fit the allocated timeslot, it will be deferred to the next timeslot [51–52].

Fig. 4.15 REPORT /GATE mechanisms and upstream transmission

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