'05 paper Design and implementation of an EPON DBA algorithm

Abstract

ETRI, 161 Gajeong-dong Yuseong-goo Daejeon 305-350 Korea 1. EPON and DBA Ethernet-PON(EPON) is the most promising access network due to its internet friendliness and low cost nature. EPON comprises an OLT in the central office and many ONU/ONTs in the residential area that are connected to the OLT in PON(passive optical network) topology using passive splitter(Figure.1). The OLT should arbitrate the upstream transmission of the ONUs and allocate upstream bandwidth resource to them. The upstream bandwidth usage request is delivered using report frames containing the amount of data waiting in the upstream queues of ONUs and the grant information is delivered to the ONUs using downstream gate frames containing grant in the form of (start time, length) pair. There are timers at OLT and ONUs and timestamp is used to synchronize this report/grant operation. In ETRI, we developed an EPON master, slave chipset and trial service was held in the city of Gwang-joo. The DBA(Dynamic Bandwidth Allocation) employs a simple cycle-based water-filling algorithm but it's very stable and provides high performance of over 92% upstream bandwidth usage. 2. Water-Filling DBA Algorithm The DBA scheme is cycle based and static short gates are generated for each registered ONUs to make sure reports are collected every cycle and the remaining time is dynamically allocated to the ONUs according to the reports. Allocating the remaining time to the ONUs is like water-filling in that unit length is subtracted from the available time and added to each ONU in a cyclic fashion until the requests are all satisfied or there is no available time left. CPU-generated gate frames are also considered because they will consume the same grant resource. This algorithm can also limit the maximum gate length and can guarantee minimum gate length set for each ONU. The water-filling is done for 2 phases of processing minimum guaranteed length and aggregate requested length. Figure 2 shows the example of the allocation result using the proposed and implemented dynamic gate allocation algorithm. It should be noted that by controlling gate length for each cycle, the bandwidth is also controlled. 3. Implementation of the DBA algorithm The core of the DBA gate generation logic is composed of 4 engines each processing 16 ONUs' requests and a combining block which gathers information from the engines and provides aggregate information needed for each engine's processing(Figure 3). Total 64 ONUs can be processed with the DBA processing logic. DBA water-filling starts after reports are read and max, min lengths are setup. Each engine processes 16 ONUs' requests at the same pace. They look at the common available gate length resource, subtracting and adding unit length to their ONUs. The 4 engines process with same ONU index and same phase(minimum guaranteed processing phase, or aggregate request processing phase). Figure 4 shows the flow chart of the engines. 4. Performance DBA performance depends on the number of ONUs, traffic type, and ONU queue size. Upstream delay is about 2 ms due to report and gate processing latency. The aggregate total upstream bandwidth is more affected by the overhead than DBA algorithm is always over 92% and can be deemed almost ideal. For 64 ONUs, the reports take 6.4 % of upstream bandwidth.

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BMW’05 paper
Design and implementation of an EPON DBA algorithm
Chan Kim, Tae Whan Yoo, Yool Kwon, Bong Tae Kim
ETRI, 161 Gajeong-dong Yuseong-goo Daejeon 305-350 Korea
1. EPON and DBA
Ethernet-PON(EPON) is the most promising access
network due to its internet friendliness and low cost
nature. EPON comprises an OLT in the central office
and many ONU/ONTs in the residential area that are
connected to the OLT in PON(passive optical
network) topology using passive splitter(Figure.1).
The OLT should arbitrate the upstream transmission
of the ONUs and allocate upstream bandwidth
resource to them. The upstream bandwidth usage
request is delivered using report frames containing
the amount of data waiting in the upstream queues of
ONUs and the grant information is delivered to the
ONUs using downstream gate frames containing
grant in the form of (start time, length) pair. There
are timers at OLT and ONUs and timestamp is used
to synchronize this report/grant operation. In ETRI,
we developed an EPON master, slave chipset and
trial service was held in the city of Gwang-joo. The
DBA(Dynamic Bandwidth Allocation) employs a
simple cycle-based water-filling algorithm but it’s
very stable and provides high performance of over
92% upstream bandwidth usage.
2. Water-Filling DBA Algorithm
The DBA scheme is cycle based and static short gates
are generated for each registered ONUs to make sure
reports are collected every cycle and the remaining
time is dynamically allocated to the ONUs according
to the reports. Allocating the remaining time to the
ONUs is like water-filling in that unit length is
subtracted from the available time and added to each
ONU in a cyclic fashion until the requests are all
satisfied or there is no available time left. CPU-
generated gate frames are also considered because
they will consume the same grant resource. This
algorithm can also limit the maximum gate length
and can guarantee minimum gate length set for each
ONU. The water-filling is done for 2 phases of
processing minimum guaranteed length and
aggregate requested length. Figure 2 shows the
example of the allocation result using the proposed
and implemented dynamic gate allocation algorithm.
It should be noted that by controlling gate length for
each cycle, the bandwidth is also controlled.
3. Implementation of the DBA algorithm
The core of the DBA gate generation logic is
composed of 4 engines each processing 16 ONUs’
requests and a combining block which gathers
information from the engines and provides aggregate
information needed for each engine’s
processing(Figure 3). Total 64 ONUs can be
processed with the DBA processing logic. DBA
water-filling starts after reports are read and max,
min lengths are setup. Each engine processes 16
ONUs’ requests at the same pace. They look at the
common available gate length resource, subtracting
and adding unit length to their ONUs. The 4
engines process with same ONU index and same
phase(minimum guaranteed processing phase, or
aggregate request processing phase). Figure 4
shows the flow chart of the engines.
4. Performance
DBA performance depends on the number of ONUs,
traffic type, and ONU queue size. Upstream delay is
about 2 ms due to report and gate processing
latency. The aggregate total upstream bandwidth is
more affected by the overhead than DBA algorithm is
always over 92% and can be deemed almost ideal.
For 64 ONUs, the reports take 6.4 % of upstream
bandwidth.

BMW’05 paper
For each ONU, upstream throughput is limited by the
ONU’s queue size. In steady state for constant rate,
when cycle is T and the traffic rate is R(bits/s), the
gate turn-on time is T*(R/RL) where RL is the line rate
of 1Gbps. During the gate off time, the buffer rises at
the rate of R for duration T(1-R/RL). The maximum
rate without loss can be calculated by setting the
maximum buffer level to ONU’s buffer size. That is,
RT(1-R/RL) = B. When B and T is known, we can
calculate the value of R. For example, when B =
128Kbits, the value of R is 146Mbps and this
matches with the experiments.
5. Conclusion
With simple cycle based water filling algorithm and
its implementation, stable DBA performance of over
92% upstream bandwidth for up to 64 ONUs was
achieved without the DSP or CPU. It is stable since
over-allocation is prevented by nature. This parallel,
sequential approach is quick enough with control
over maximum and minimum guaranteed bandwidth
for each ONU by controlling the gate length every
cycle.
OLT
ONU
ONU
ONU
Subscr iber
Network
Subscr iber
Subscr iber
Subscr iber
Subscr iber
Subscriber
Network
LLID=1LLID=2
LLID=1
LLID=2
LLID=1
LLID=2LLID=2 LLI D=1
LLID=1
LLID=2
LLID=1LLID=2
LLID used sect ion
Figure 1. Usage of LLID in EPON
a) Not all requests satisfied
b) All requests satisfied
Figure 2. Water-filling DBA algorithm
DBA
engine
DBA
engine
DBA
engine
DBA
engine
Combiner
Report
Reading
DBA
Gate
Frame
Gene.
Report Read
And C lear
Start
Min
Guaranteed
&
Requested
Length
Allocated
lengths
Status
Control
Gate Frame Generation Start
Total Available
DBA Gate
DBA cycle
Start Signal
DBA
Gate
Frames
Figure 3. DBA gate generator
For the ONU,
If M inimum Guar antee d is no t sat isfie d,
take from the available and add unit length
(inform the combiner)
If Minimum Guaranteed or
Aggregate is satisfied,
reset corresponding not-finished flag
Check whether to cont inue
processing for Minimum Guaranteed
( If available length remains and
minimum guaranteed is not
satisfied for any ONU)
Above con dition ?
Set not-finished flags
For Requests for
Minimum Guarant eed
and Aggregate
Stand-by
Start
Yes
No
Inc ONU index
Init ialize ONU in dex
Inc ONU index
For the ONU,
If Aggregate is not satisfied,
take from the available and add unit length
(inform the combiner)
If Aggregate is satisfied,
Reset corresponding not-finished flag
Check whether to c ontinue
Processing for Aggregate
(If availab le leng th re mains and
Aggregate is not
satisfied for any ONU)
Above cond ition ?
Yes
Inform the combiner
Of the completion
Inc ONU index
No
Figure 4. DBA engine’s flow chart