Burst-mode Transceivers and Differentiated Classes of Service for PON. Dr. Monir Hossen ECE, KUET

Transcription

1 Burst-mode Transceivers and Differentiated Classes of Service for PON Dr. Monir Hossen ECE, KUET 1

2 Concept of Burst-mode Types of Burst-mode ONU Transmitter ~ ON/OFF with time slot fast ON/OFF is required OLT Receiver ~ different optical power for different ONU fast data recovery is required T1 T2 T3 ONU 1 User OLT IEEE frame Header payload Fcs ONU buffering 3 ONU User2 User3 Frame Check Sequence ONU sends Ethernet frames within assigned timeslot. 2

3 Requirements of BMTs - Guard Time Optical Power ONU burst transmit: laser turn-on/off time(t_on, T_off) OLT burst receiver: level recovery time(t_lr), CDR time(t_cdr) ONU i Guard Time ONU j Toff Ton Tlr + Tcdr EPON GPON Burst Timing T ON/OFF : 512 AGC+CDR: 400 T ON/OFF : 12.8 Guard:

4 Requirements of BM Transceivers - Power Wide Dynamic Range Magnitude of power varies with ONU: Different launching power Different fiber length Different connector loss 12 db dynamic range for EPON Solution Fast level recovery at the OLT receiver Remote ONU power control 4

5 Burst-Mode Transmitter Conventional Laser Driver Continuous mode Small rise/fall time Low jitter Conventional laser driver controls the average LD power Requirements of BM Laser Driver Quick Laser Turn On & Off Constant Optical Peak Output Power Peak-power detection 5

6 Support for Differentiated Classes of Service of PON (1/2) EPON is able to deliver tens to hundreds of MBPS to and from users. EPON is expected to support - Voice communications - Standard and high definition TV - Real time and near real time transactions - Video conferencing and - Data traffic Performance of EPON characterized by - Bandwidth - Latency - Throughput - Delay variation (Jitter) and - Packet loss ratio 6

7 Support for Differentiated Classes of Service of PON (2/2) To support these diverse application requirements networks should => Segregate all the traffics into a number of classes => Provide differentiated service for each class => Maintain classes of service (CoS) CoS includes intra-onu and inter-onu scheduling 7

8 Intra-ONU and Inter-ONU Scheduling Intra- ONU: A strict priority scheduling within the queue of an ONU Inter-ONU: A scheduling by the OLT among ONUs 8

9 Requirements to Support CoS EPON must classify the traffic into classes of service and provide differentiated treatment to each class. Each ONU is equipped with n queues serving n priority classes (P0, P1.. Pn). Here, P0 will get the highest and Pn will get the lowest priority. The limited W max is needed to guarantee a maximum interval between timeslots. Avoid bandwidth take over by a hungry ONU. 9

10 Traffic Modeling The architecture should support - Best-effort data - Variable-bit-rate video stream - Constant-bit-rate for Plain Old Telephone Service (POTS) lines and Private Branch Exchange (PBX) boxes The data classifies into three priority classes: P0 (Voice), P1 (Video), P2 (Data traffic) 10

11 Packet Delay Analysis Inter-ONU scheduler uses limited service discipline Intra-ONU scheduler uses strict priority queuing Here, offered load means load of an individual ONU and all ONUs have the uniform load 11

12 Light-load Penalty At the end of every timeslot each ONU generates a REPORT message containing the number of bytes in the queue. At light load, the requested queue size << W max Overhead to the effective packets ratio will be high. This is light-load penalty. 12

13 Optimization Schemes Two schemes are considered to overcome the light-load penalty => Limited service at OLT but tandem queues at the ONU (not strict priority queue) - This scheme eliminates the light-load penalty through modification to the intra-onu scheduling => Constant Bit Rate (CBR)-credit service at OLT (not Limited service) and priority queues at the ONU - This scheme eliminates the light-load penalty by changing the inter-onu scheduling 13

14 Tandem Queuing Stage 1 consists of multiple priority queue Stage 2 consists of FCFS First, transmit stage 2 s data then stage 1 s data Drawback is it increased delay for P0 (appr. 3 times) In limited/priority scheme overhead = 6.14% In limited/tandem scheme overhead = 2.13% So overhead is reduced in Tandem queuing Packet delay for Tandem scheme 14

15 CBR Credit Service Delay of P0 is less than Tandem scheme. It predicts the amount of P0 packets to arrive at the ONU during waiting time and adjust the granted timeslot size. The OLT predict and change the inter-onu scheduler but the intra-onu scheduler remains in default strict priority scheduler. It is called CBR scheme. 15

16 Calculation of Credit Interval To compute the credit size OLT determine the credit interval. The size of the next time slot should be increased due to the packets arrive during the credit interval. The credit interval is = t s Here, t R = timestamp in REPORT message, t s = start time of a granted timeslot, w = time slot size, R N = EPON line rate. But w depends on the number of additional CBR packets + w R N t R 16

17 Bandwidth Utilization(1/2) Bandwidth utilization determined by => The cycle time, guard time, and unused timeslot The average unused time slot in limited/priority and CBR- credit/ priority are same at higher load (above 0.6) To compute bandwidth utilization the following formula is used U = 1 N( GR N + R + W TR N REPORT ) Where, N = number of ONUs, G = guard time, R = average remainder, W REPORT = length of REPORT message, T = average cycle time, R N = EPON Transmission rate 17

18 Bandwidth Utilization(2/2) This figure shows that average cycle time for all schemes are approximately similar At light load, limited/tandem and CBR-credit/priority schemes have good bandwidth utilization than limited/priority scheme. But at high load all schemes are nearly same. 18