A Distributed Scheduling Approach for Ethernet-based Passive Optical Networks

Marilet De Andrade, Lluis Gutiérrez, Sebastià Sallent Telematics Engineering Department Universitat Politécnica de Catalunya

Castelldefels, Barcelona, Spain [marilet, lluis.gutierrez, sallent]@entel.upc.edu

Abstract— Ethernet-based Passive Optical Networks (EPON) are being considered as the best candidates for the next generation broadband access networks. Several algorithms for dynamic bandwidth allocation in EPON have been proposed, as this is an open issue that was left out of the scope of the standard IEEE 802.3ah. Very few of those works address the possibilities for an on-the-fly (immediate) distributed scheduling method. In this paper we propose and simulate a distributed scheduling method where the user devices perform most of the dynamic distribution process. The results show an important improvement in terms of delay and queue size for high network loads.

Ethernet Passive Optical Networks (EPON), dynamic bandwidth allocation, distributed scheduling, access networks

I. INTRODUCTION The current requirements and challenges for the next

generation optical access networks can be achieved with the PON technologies. The Passive Optical Network (PON) is a subscriber access network technology that provides high bandwidth capacity over fiber. A PON which provides transport of Ethernet frames is called Ethernet PON (EPON).

Mainly the EPON is a point-to-multipoint access network with three basic elements: The OLT (Optical Line Terminal) located at the network side working as head-end, the ONU (Optical Network Unit) located at the user side, and a passive optical splitter. OLT controls ONU’s transmissions such that they do not collide, because access is TDM-based. The ONUs send Report messages requesting bandwidth (or a transmission timeslot), and OLT grants the corresponding bandwidth to the ONUs by means of a Gate message.

An open issue at the standard IEEE 802.3ah is Dynamic Bandwidth Allocation (DBA) algorithm for the EPON (for the upstream channel, from ONUs to OLT). DBA schemes have been considered as a way of handling bandwidth and Quality-of-Service (QoS) requirements in EPON. Most of the works are based in a centralized algorithm that runs at the OLT and allocates transmission windows to every active ONU. Many of the already proposed schemes try to emphasize on improving in terms of efficiency, delay performance, and support of quality of service, respectively. In this paper we propose a distributed scheduling mechanism in order to allocate bandwidth fairly and efficiently in an EPON. The control of the channel is centralized because it is done by the OLT, but the scheduling

process is distributed among all the active ONUs over the PON.

II. DISTRIBUTED SCHEDULING APPROACH FOR EPON The scheduling algorithm proposed here is performed

mainly by the ONUs in the EPON network. Some extra information must be sent to the ONUs from the OLT, which increases ONU’s complexity in some level. Specifically the OLT has to send a vector that will allow the ONU to make a calculation concerning its instantaneous transmission window size. The operation is simple and the information needed by the ONU can be sent through the Gate message frame in the reserved fields.

For simplicity we will study a case where there is just one queue in each ONU, but it can be easily extended to several queues.

Considering a PON system consisting of N ONUs, each ONU i has a fixed weight Φi that, accordingly with its guaranteed bandwidth agreement, corresponds to the following transmission window size (in bytes):

MAXN

j j

ii WW

å =F

F=

1

(1)

where WMAX is the maximum cycle size in bytes. The cycle is the period during which all actives ONUs transmits their traffic inside their granted timeslot. So, the OLT guarantees the instantaneous window size ΦiWMAX if:

11

=Få =

N

j j (2)

The proposed algorithm is explained as follows:

1. When the OLT receives a Report message containing the ONUi’s transmission window size requirement Ri for the next cycle, it updates a register (vector) of weights for cycle (n):

ONU1 ONU2 ONU3 …… ONUN Φ1(n) Φ2(n) Φ3(n) …… ΦN(n)

The OLT receives the new weight Φi(n), compares it with the fixed Φi , and then it updates the smallest value in the vector of the previous table.

Work supported by the Spanish Ministry of Education and Science, and FEDER under the projects TSI2005-06092 and TSI2006-12507-C03-03.

32nd IEEE Conference on Local Computer Networks

0742-1303/07 $25.00 © 2007 IEEEDOI 10.1109/LCN.2007.121

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If the EPON system has QoS classes, the gate and report messages would transmit the (NxM) matrix:

÷÷÷

ø

ö

ççç

è

æ

F×F×××

F×F=F

)()(

)()()(

1

111

nn

nnn

NMN

M (3)

being N the number of ONUs and M the number of QoS classes. In this description we consider M=1.

2. When the ONU receives the Gate message, it transmits the data up to the granted window Ri(n). Then the ONU updates the vector, and calculates the maximum window that such ONU can take on cycle n+1:

MAXN

j j

ii W

nnW

å =F

F=+

1)(

)1( (4)

and the required transmission window size:

( )iii QnWMINnR ),1()1( +=+ (5)

where Qi is the queue size in ONU i at that moment. Finally the ONU sends to the OLT (inside the Report message) Ri(n+1), and the new weight for the next cycle:

MAX

N

j jii W

nnRn

å =F+

=+F 1)()1(

)1( (6)

The process is repeated for every ONU.

III. SIMULATION RESULTS In order to evaluate the proposed mechanism, some event-

driven simulations in C++ have been performed. The scenario of the system is a tree topology with 16 ONUs. We chose a maximum cycle size of 2ms. The sources are set with a transmission rate of 62,5Mbps, and the channel capacity is 1Gbps. The traffic was set self-similar with the Hurst parameter set to 0.8. The ONU’s buffer was selected very large in order to avoid packet drops (100MB). In the simulations we compare the performance of the proposed algorithm with IPACT algorithm [1].

In Figure 1, IPACT experiences an abrupt step when the network load is bigger than 0.8 arriving to an average delay of 0.6s. On the other hand, for the distributed-DBA (proposed approach) the behaviour is very smooth and shows significant lower average delay for higher loads compared with IPACT.

In Figure 2 the queue keeps lower levels with the distributed approach than with IPACT. That means that the system with IPACT algorithm would require a bigger queue in order to avoid packet dropping. The proposed scheme exhibits more control over the transmission window size distribution, offering a better performance in terms of average queue size and delay if compared with IPACT at high loads.

IV. CONCLUSIONS We have proposed a new dynamic bandwidth allocation

algorithm for Ethernet-based Passive Optical Networks. It is based on a distributed mechanism for scheduling the allocation of bandwidth according to the guaranteed bandwidth agreed with each user. Using a simple algorithm, the ONU (user device) is able to proportionally schedule its transmission window based on its current queue requirements and the previous requirements of the rest of the ONUs.

Figure 1. Average and maximum packet delay vs. network load

Figure 2. Average queue size vs. network load

The proposed mechanism is distributed because the scheduling process is performed in the ONUs with the help of the OLT (head-end device) which sends an extra-parameter for the calculations individually. It is also dynamic because it really adapts the transmission window size of every ONU every cycle taking into account the overall fluctuating traffic behaviour. The simulations results showed a good performance in terms of delay and average queue size, especially at high network loads when it is significantly lower than IPACT. This is a preliminary study of the proposed mechanism that will be evaluated on different scenarios, and will be compared with other schemes.

REFERENCES [1] G. Kramer, B. Mukherjee and G. Pesavento, “Interleaved Polling

Distribution Scheme in an Optical Access Network”, in Journal Photonic Network Communications, vol. 4, no. 1, pp. 89-107, 2002.

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