PMD Burst Mode Dynamic
Performance Requirement
List of Supporters:
Eyal Shraga
Frank Effenberger
Glen Koziuk
Hernando Valencia
Meir Bartur
Raanan Ivry
10/15/2001
Rob Carlisle
Ron Rundquist
Tony Anderson
Walt Soto
Wenjia Wang
Yusuke Ota
EFM Los Angeles October 17-19th
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Outline
• What is Burst Mode?
– Definition of relevant Burst Mode Terms
• Optical Extinction Ratio
• Optical “Off-State” Power
• Optical Dynamic Range
• Benefit of Burst Mode (BM)?
– Why do we need short Burst?
– Upstream Burst Efficiency Tradeoff
• Assumptions & Definitions
• Burst Efficiency as a function of transmission burst size
• PMD Upstream Dynamics for OLT Rx Path
– Upstream Guard Time Requirement
– Upstream Delimiter Requirement
– Upstream Overhead Byte Requirement
• PMD P2MP System CDR Synchronization
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PMD’s Burst Mode Analog ICs
• What is Burst Mode?
– P2MP PONs employ short bursts of data packets upstream
instead of continuous data packets used in Continuous Mode
(CM) P2P applications. Hence the term Burst Mode (BM).
Typical Headend
MDIO
BM
CDR
CM
LD
Laser
Diode
BM
LA
BM
TIA
WDM
Photo
Diode
OLT’s Burst Mode
Guard Time Constraint Dynamics
occurs here and affected by ONU/T’s
laser on/off performance.
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ONU/T Transceiver TxRx
Module
Bidi
Photo
Diode
CM
TIA
CM
LA
WDM
Laser
Diode
BM
LD
<20km
Point-to-Multipoint
w/ Passive Splitters enabling
<32 End Users
EFM Los Angeles October 17-19th
CM
CDR
PECL
OLT Transceiver TxRx
Module
Bidi
PECL
GMII
OLT
1.25Gbps
PON-PHY
Digital IC
Typical Client
ONU/T
1.25Gbps
PON-PHY
Digital IC
GMII
MDIO
Key:
CDR - Clock & Data Recovery
LA - Limiting Amp
TIA - Transimpedance Amp
LD - Laser Driver
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What is ER & “Off-State” Power?
• ER refers to the Extinction Ratio, which is the ratio of Optical
Power transmitted by ONU for logic one & zero levels during an
upstream packet burst. (ER > 10dB per ITU-T G.983.1)
• “Off-State” Power refers to how much the ONU is polluting
Upstream when Laser is powered off. (Off-State Power < -45dBm)
• ER & “Off-State” Power numbers as seen by OLT.
ER=10Log(PL1/PL0)
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Why do we care about ER &
“Off-State” Power?
• Helps to define performance for transition region between burst.
• Tradeoffs are captured within Guard Time bits fields.
• Upstream aggregated throughput performance impacted.
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What is the PMD’s Optical
System Dynamic Range?
• Dynamic Rage is related to ratio of Strongest to Weakest
Optical Signal seen by OLT.
10Log(Strong/Weak) = System Dynamic Range > 23dB example.
Strong
Weak
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PMD’s Burst Mode Benefits
• Why we need Burst Mode?
– Improves the efficiency of PON’s upstream data bit stream.
– Cost comparable with Continuous Mode (CM) ICs
– Saves ONU power by allowing ONU to power off TX path.
• Typical saving range between 50% to 90% compared to CM.
• What are the Dynamic relationships within BM ICs?
–
–
–
–
Clock synchronization and clock recovery.
Proper delineation of received data called Delimiter.
Time between data packets is called Guard time.
Guard time is strongly dependent on Power Ratio of back-toback data packet bursts.
– Photodiodes can be designed to minimize residual carrier
effects without adding cost
• Doping or masking the fringing field solves this problem
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Upstream Burst Efficiency
• Typical Bit-Stream Usage:
– Upstream (US) transmission burst is divided into slots,
– Each ONU/T is granted a group of consecutive slots for US burst of data,
– Each ONT transmission burst is lead by the Overhead Bytes.
• Small slot size enables high US grant granularity and is desirable for
overall US throughput efficiency.
• Most US bursts are relatively small due to nature of access networks
(Examples are small 64 byte US Ethernet packets ACK’s for DS data).
ONT K Transmission Burst (Burst_size)
Slot
n
Slot
n+1
Slot
n+2
Slot
n+3
Slot
n+4
Slot
n+5
ONT L Transmission Burst
Slot
m
Slot
m+1
Slot
m+2
Slot
m+3
OLT RX
Slot_size
Burst
Overhead
Control
Header
Upstream Payload
Overhead
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Upstream Burst Efficiency Equation
• Based on 3 variables: Slot_count, Slot_size & Overhead.
• Slot_count is data dependent, but the Overhead to Slot_size ratio R, can
be bounded to a small set of values. We cannot find the optimal
Overhead or Slot_size, but graphing Efficiency based on several
carefully chosen values for R should yield some insight.
US Burst Efficiency Equation
Efficiency =
Since Burst_size = Slot_count * Slot_size
Efficiency =
Burst_size - Overhead
Burst_size
Slot_count * Slot_size - Overhead
Slot_count * Slot_size
Slot_count Rearrange
Let SC = Slot_count and R =
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Overhead
Slot_size
Efficiency =
Efficiency =
SC
EFM Los Angeles October 17-19th
(2)
Overhead
Slot_size
Slot_count
SC - R
(1)
(3)
(4)
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From EFM’s July Meeting in Portland,
we can see the US Packet Distribution.
Downstream
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Upstream
EFM Los Angeles October 17-19th
~45% of US
Packets are
very small
(ACKs for
Ethernet DS
packets)
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Upstream Burst Efficiency Results
US Burst Utilization
1.10
> 90% efficiency
& sm all bursts
1.00
0.90
R=0.1
Efficiency
0.80
R=0.25
R=0.5
0.70
R=1
R=2
R=5
0.60
R=10
0.50
0.40
0.30
0.00
0.50
1.00
1.50
2.00
Log10(SC)
10/15/2001
2.50
3.00
3.50
• Plot of Burst Efficiency for several
reasonable R value (Overhead/Slot_Size)
Results
• Assumptions:
1. Upstream bursts tend to be small in
size with typical Slot_Count < 10.
2. > 90% efficiency desired.
• Graph suggests R ≤ 0.5
Scenario Example
If ~45% of US packets are ~64bytes,
then to obtain ~94% efficiency:
Burst_Size = 72 bytes,
Slot_Count = 9 & Slot_size = 8,
means Overhead ≤ 4 bytes
Conclusion
• Hence small Slot Sizes are desirable,
and Overhead should also be small.
EFM Los Angeles October 17-19th
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Guard Time Dynamics
Guard Time Constraints are:
• Strongly dependent on both
Optical Power P1 and P1/P2
• Strong to weak Optical Power
(>23dB dynamic range)
• Independent of the bit rate.
• When P1/P2 < 3db,
Guard Time ≅ 0(regardless of P1)
• P is the Optical Power of each Packet
• P1/P2 is the Ratio of the Optical Power
• Tmin is called the Guard Time between packets
and is the minimum packet spacing required to
properly recover the first bit in the second packet
Definition of Guard Time Dynamic Terms
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Example 1.25G Burst Mode Rx Path Trace Diagram
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Why do we need Delimiter or Preamble?
• Start of each Upstream Burst Packet must include known pattern to recover both
clock and data correctly, which are referred to as Delimiter & Preamble bits.
• Delimiter bits are used to delineate the start of a valid burst.
• Preamble bits are used for phase locking and may not be required.
• Bit values for Delimiter & Preamble bits vary per implementation and are
captured within ITU-T G.983.1 Overhead Bytes field.
>
Definition of Guard Time Dynamic Terms
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PMD Upstream Overhead Byte
Requirement
• Overhead Bytes are used at the
beginning of every packet burst.
• ITU-T G.983.1 Defines:
Overhead Bytes (OH) =
Guard Time + Preamble
+ Delimiter = 3 bytes
• 1.25G Prototypes show 3 Bytes
of Overhead is possible.
• RESET signal prior to packet
acquisition provided by OLT
PHY is implementation specific.
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Example Lab Trace for 1.25Gbps CDR
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System CDR Synchronization
• Clock and Data Recovery (CDR) performed
using bits within the Overhead Byte field.
– First n-bits may be lost in this process
• ONU CDR synchronizes frequency/phase and
bit alignment to the continuos downstream data
from OLT.
• OLT BM CDR reacquires lock, phase and bit
alignment between each upstream burst (i.e.
new phase acquired for each burst.).
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System CDR Synchronization
• Upstream transfer clock is derived from Downstream
transfer, which means PON system synchronized to
OLT transmitter.
OLT DATA
PON
IC
DATA
(MAC)
XTL
BM CDR
D
M
U
X
CMU
Headend
M
U
X
BM TIA
BM LA
PON splitter not shown
Client
CM LD
BM LD
CM TIA
CM LA
M
U
X
CMU
CM CDR
DATA
D
M
U DATA
X
ONU/T
PON
IC
(MAC)
Sync CLK
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Thank You
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EFM Los Angeles October 17-19th
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