The Second International Conference on Innovations in Information Technology (IIT’05)

A DISTRIBUTED CONTROL PLANE ARCHITECTURE FOR EPON: SIMULATION STUDY AND FEASIBILITY EXPERIMENT

A.S.M. Delowar Hossain, H. Erkan, M. A. Ali

Department of Electrical Engineering, Graduate School of The City University of New York New York, NY 10016-4309, USA

ASMDHossain@gc.cuny.edu

ABSTRACT Ethernet Passive Optical Network (EPON), an access network technology, has drawn considerable research attention in recent years. EPON can be implemented in centralized or decentralized architecture in the access network. Most of the researches were focused on the centralized architecture, where bandwidth allocation decisions for Optical Network Units (ONUs) are made by distant Optical Line Terminal (OLT). In centralized approach, ONU reports have to travel to distant OLT to inform the OLT about their bandwidth needs so ONUs may be granted upstream bandwidth. Any software failure in the OLT will halt the ONU upstream transmission. In this paper we introduce two works: a simulation study of a distributed Dynamic Bandwidth Allocation (DBA) scheme over a decentralized architecture and an experimental study to explore the control plane feasibility of such an architecture. Firstly, we observe the performance of the decentralized DBA scheme associated with the Ethernet over Star Coupler-based Passive Optical Network (PON) architecture, where OLT is excluded from the implementation of the upstream time slot assignment. To implement this distributed control plane, the architecture facilitates inter-ONU connectivity to exchange control information among ONUs. Secondly, we conduct a physical layer experiment to assess the feasibility of implementing such a control plane architecture. In addition to the added flexibility and reliability of a distributed architecture, the proposed DBA performance over the distributed EPON is shown to be more efficient than its centralized counterparts. Furthermore, the control plane of such an architecture is also proven to be feasible by physical layer experiment.

Keywords: EPON, DBA, Bit Error Rate (BER), SNR.

1. INTRODUCTION

Due to the shortcomings of Cable Modems and DSL, it became a necessity to come up with new technologies to increase the bandwidth needs of end-users in access network so that they can get Broad-band services at bandwidths far exceeding current access technologies. EPON is a PON proposed to overcome the bandwidth bottleneck at the Access Networks [1-4]. EPON is a PON that carries Ethernet traffic. A PON is a point-to-multipoint fiber optical network with no active elements in the signal’s path. It consists of a single shared optical fiber connecting a service provider’s central office to a passive star coupler (SC), which connects to number of ONUs located near residential customers. Each ONU receives a dedicated short optical fiber from the SC but shares the long distribution trunk fiber. All transmissions in a PON are performed between an OLT and ONUs. Traffic from an OLT to an ONU is called ‘downstream’ (point-to-multipoint) and traffic from an ONU to the OLT is called ‘upstream’ (multipoint-to-point). Two wavelengths are used: typically 1310 nm (λup) for the upstream transmission and 1550 nm (λd) for the downstream transmission. In the downstream direction, an EPON operates as a broadcast and select network. The OLT has the entire bandwidth of the channel to broadcast standard formatted 802.3 Ethernet frames to all ONUs. Each ONU extracts those frames that contain the ONUs unique Media Access Control (MAC) address. In the upstream direction, multiple ONUs share the transmission channel. Thus, the ONUs need to employ some arbitration mechanism to avoid collisions. Many bandwidth allocation schemes emerged based upon Multi-Point Control Protocol (MPCP) is developed by IEEE 802.3ah task force [1] to support an upstream bandwidth allocation. Most of these upstream bandwidth arbitration schemes are centralized at OLT. In general, the OLT arbitrates the upstream transmissions by allocating an appropriate timeslot to each ONU to transmit upstream traffic and an ONU is only allowed to transmit during that timeslot. ONUs use REPORT messages in each cycle (transmitted along with data timeslot) to

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inform the OLT about their bandwidth requirements. Upon receiving a REPORT, the OLT passes the message to a Bandwidth Allocation module responsible for bandwidth allocation decision. Then OLT assigns the timeslots with GATE messages. Several bandwidth allocation schemes have recently been reported in the literature ranging from a static allocation to a dynamically adapting scheme based on instantaneous queue size of ONUs [5-6]. The simplest is the static TDMA scheme in which every ONU gets a fixed timeslot [5]. While this scheme is very simple, it results in inefficient upstream channel utilization since statistical multiplexing among the ONUs is not possible. A Dynamic Bandwidth Allocation scheme called Interleaved Polling with Adaptive Cycle Time (IPACT) based on Grant (GATE) and Request (REPORT) messages has been presented in [5]. This scheme uses an interleaved polling approach where the next ONU is polled before the transmission from the previous one has arrived. This scheme provides statistical multiplexing for ONUs and results in efficient upstream channel utilization. Generally, in any centralized scheme the network is completely dependent on OLT and for every decision ONU report has to travel to distant OLT. Thus, any software failure in OLT DBA module will cripple the upstream transmission. To eliminate the inefficiencies of centralized schemes, we previously proposed a distributed control plane for a decentralized architecture[6], where the OLT is excluded from the implementation of the time slot assignment. To implement that distributed control plane, the architecture facilitates inter-ONU connectivity to exchange control information concerning their transmission needs amongst themselves. The distributed DBA in [6] performed similar to the centralized IPACT, but brought flexibility, reliability and optimization. Even though that DBA has advantages, it has some inefficiencies which require attention. We propose an advanced DBA to address those inefficiencies (discussed later).

In this paper we introduce two works: a simulation study of an advanced distributed DBA over a decentralized architecture and an experimental study to explore the control plane feasibility of such an architecture. Firstly, we demonstrate a distributed DBA which outperforms IPACT [5] and previous distributed DBA [6]. To implement this distributed control plane, the architecture must facilitate physical layer inter-ONU connectivity to exchange control information among ONUs (to decide upstream bandwidth allocation). That leads to the second part of our work. We conduct a physical layer experiment to assess the feasibility of implementing such control plane architecture. In addition to the added flexibility and reliability of a distributed architecture, the proposed DBA associated with the distributed EPON is shown to be more efficient than its centralized counterparts and previous distributed DBA. Also, the control plane of such an architecture is proven feasible through physical layer experiment. The proposed architecture, beside reducing the OLT complexity by internetworking among connected users (ONUs), offers efficient shared LAN capability[7].

The rest of the paper is organized as follows: Section 2 provides an overview of the distributed EPON architecture, proposed DBA and simulation result analysis. Section 3 presents the physical layer feasibility experiment and results. Finally some concluding remarks are presented.

2. DISTRIBUTED ARCHITECTURE AND BANDWIDTH ASSIGNMENT

2.1 Architecture

Fig. 1 shows the general architecture [6] of this approach. As can be seen from Fig. 1, a portion of the optical signal power transmitted by an ONU upstream transmitter toward the OLT will be redirected back and broadcasted to all ONUs; this broadcast is the foundation of inter-ONU connectivity. It will be used to exchange ONU control information to initiate DBA calculation at ONUs (decentralized DBA). This can be achieved by connecting two ports of a 3xN SC with each other through an optical isolator as shown in Fig.1. Note that in addition to the conventional transceiver maintained at each ONU (a λup transmitter and a λd receiver), this approach requires an extra receiver tuned at λup. A base band direct detection circuit is needed to detect the redirected control channel (λup) in order to recover the control update information. Basically the control signal which is generally sent to OLT in a centralized architecture will be redirected to all the ONUs in decentralized architecture. Thus, there is no additional signaling required in the

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decentralized architecture. Even though the decentralized architecture increases the complexity/ cost of the ONU, we will see later in this work that the added flexibility and reliability of such architecture and efficient network performance of associated DBA can justify the extra cost. This architecture assumes a cycle-based upstream link, where a cycle is defined as the time that elapses between two updating executions of the scheduling algorithm. The cycle length varies to accommodate the dynamic upstream traffic conditions. We present a short review of existing distributed DBA [6]. The cycle is divided into three periods; a static update period (control plane), a fixed waiting period when no upstream traffic is allowed (processing control messages and running the algorithm) and a dynamic transmission period (data plane). The ONUs exchange signaling and control information (control plane) concerning their queue status and their transmission needs amongst themselves. Then, the ONUs simultaneously and independently run instances of the same DBA algorithm (waiting period), while no upstream traffic is allowed, outputting identical bandwidth allocation results. Once the algorithm is run, the ONUs sequentially and orderly transmit their data (data plane) without any collisions, eliminating the OLT's centralized task of processing requests and generating grants for bandwidth assignment. Details of control plane operation and upstream bandwidth assignment is shown in [6]. Unlike centralized DBA in [5], the decentralized DBA in [6] takes all ONU requests in consideration at DBA calculation time. Thus, it is globally optimized. Along with flexibility and reliability of distributed approach, the DBA in [6] is at least as efficient as the centralized IPACT. But it has some inefficiencies which require attention. In each cycle, bandwidth is wasted because it requires two sets of inter-ONU guard band (one for data from ONUs to OLT and another one for all inter-ONU exchanged control frames) in the place of one[5]. Moreover, it also wastes bandwidth due to idle period when DBA is calculated. Therefore, we propose an advanced decentralized DBA which will eliminate these drawbacks and enhance network performance and will outperform the DBAs in [5] and [6].

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Figure 1: Distributed EPON Architecture.

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2.2 Proposed DBA

The proposed DBA is a decentralized scheme, where ONUs decide bandwidth allocation among themselves (OLT is excluded from it) based on the exchange of inter-ONU control information. It has an advanced mechanism than the DBAs in [5] and [6] to improve network performance. In a given time slot, an ONU first transmits its control report (queue information) to all the ONUs (redirected signal). Then that ONU transmits its data to the OLT. By the end of cycle, all ONU reports are exchanged among themselves through redirected upstream control signal. Then simultaneous DBA algorithm will be executed at each ONU yielding a unique set of ONU upstream bandwidth assignments identically produced in each ONU for next cycle. Contrary to the DBA in [6], where control and data have separate time slot in a cycle requiring separate set of guard bands for each case, in the proposed DBA the control and data are sent in same time slot of an ONU. That saves the proposed DBA one set of guard band than the DBA in [6]. Since the DBA scheduling takes into consideration all the ONU requests (unlike IPACT), it is globally optimized. Also, the DBA in [6] requires idle cycle time for DBA calculation, when no upstream traffic is allowed. Proposed DBA achieves better performance by eliminating idle cycle time.

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Figure 2: Proposed Decentralized DBA Scheduling

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The DBA module only need the report frames to do DBA calculation at the ONUs. As mentioned earlier, an ONU transmits report frame before the data frames. Therefore, once the last ONU of cycle N broadcasted its report to other ONUs and the data is still being transmitted to OLT, ONUs will complete DBA calculation for cycle N+1 (see Fig. 2). The data transmission time of last ONU of cycle N will compensate for DBA calculation for cycle N+1. Under low ONU load, the last ONU of cycle N may not have long enough transmission time to compensate for DBA calculation for cycle N+1. So, we will add additional mechanism to the DBA. Since ONU requests (for next cycle) up to a pre-fixed amount (Bmax) will be granted all the time, DBA will allow ONUs requesting up to Bmax in cycle N to transmit (in sequence of their request) at the beginning of cycle N+1 without waiting for DBA calculation. This will compensate for DBA calculation time which may extend from cycle N to cycle N+1. If there is no busy ONU at all, then all the ONUs will receive grants up to Bmax in advance; eliminating the need of an end of cycle DBA calculation. Thus, for all conditions the DBA calculation time for ONUs reporting > Bmax is compensated for. In another words, the globally optimized DBA calculation is done without any idle cycle time. The sequence of transmission for ONUs reported> Bmax is based on the shortest report first (SRF); and if there is a tie, then the decision is based on the sequence of report.

There are two other inefficiencies of the DBAs in [5] and [6] are to be addressed in the proposed DBA. First is the issue of left over cycle bandwidth[8] and the second is the frame boundary mismatch with grant (ONU slot remainder)[9]. The above mentioned DBAs assign the requested amount of bandwidth to an ONU as long as the request ≤Bmax and a fixed maximum bandwidth (Bmax) when the ONU request > Bmax. The assignment of maximum of Bmax (when report > Bmax ) to an ONU may not match with a frame boundary of the ONU frames; thus portion of slot bandwidth will be unused/wasted (fragmentation of Ethernet frame to fit the remaining slot is not allowed). Also, there will be left over cycle bandwidth due to some ONUs requesting below maximum allowable bandwidth. This left over bandwidth could be distributed to needy ONUs to improve utilization. The proposed DBA will assign bandwidth considering ONU frame boundary and distribute the left over cycle bandwidth to needy ONUs (discussed next). An ONU reports its entire queue size to all other ONUs as long as it is not more than Bmax. Otherwise, ONU reports 8 MAC frame boundaries from its queue starting with the boundary just below Bmax ; the next 6 boundaries are at least 1KB apart and the 8th boundary reports the entire queue length. The DBA will calculate the total unclaimed bandwidth for all the ONUs reported <Bmax in this cycle. Then it will find the total excess demand of all the ONUs requested >Bmax. Now DBA has to take the unclaimed bandwidth from low demand ONUs and distribute it to high-demand (needy) ONUs. To do that, first it will calculate each high-demand ONU's share of this unclaimed bandwidth (in addition to Bmax) according to the ratio of their demands exceeding Bmax [8]. Once the grant calculation for the high-demand ONU is completed, the DBA will generate grant that matches with the highest MAC frame boundary (close to the calculated grant value) of ONU data to eliminate ONU slot remainder [9]. Then the total left over bandwidth and total over demand of ONUs will be recalculated. This process continues untill all the high-demand ONUs are assigned their additional share of bandwidth matching their frame boundaries. Thus the ONU slot remainder and unclaimed cycle bandwidth is reused. This dynamic calculation of bandwidth assignment transfers all unused bandwidth from low demand ONUs to high demand ONUs in a fair manner without costing any DBA calculation time. Slot remainder elimination and dynamic reuse of all unused bandwidth, save frames from waiting in queue until the next cycle and make room for new packets to arrive, resulting in minimization of delay and higher utilization.

Note that all ONUs are synchronized to a common reference clock extracted from the OLT downstream traffic. Clocking information, in the form of a synchronization marker, is included at the beginning of each downstream frame cycle. The synchronization marker is a 1-B code that is transmitted every 2 ms to synchronize the ONUs with the OLT.

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2.3 Simulation Results

To compare the performance results of the proposed decentralized DBA with that of the centralized IPACT[5] and the decentralized DBA in [6], we used a system with 16 ONUs generating self-similar traffic [10]. The access link data rate from users to an ONU is 100 Mbps and upstream link data rate (from an ONU to the OLT) is 1 Gbps. Maximum cycle time is 2ms and inter-ONU transmission guard band is 5 microsecond. Bmax is calculated as 15KB. Fig.3 presents the mean frame queuing delay and channel bandwidth utilization for all three DBAs as a function of total network load. The proposed decentralized DBA demonstrates better queuing delay (Fig. 3a) and utilization (Fig. 3b), especially at higher load when it is very crucial. We see while the total network load is around 0.7, the delay performance of proposed DBA is about 1.5

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Figure 3: Proposed Decentralized DBA vs other DBAs

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ms better than its counterparts. When the load is close to 0.9, the proposed DBA outperforms others by 300-400 ms. This advantage is crucial for time sensitive traffic, specially when network is saturated. The advantage surfaces around total load of 0.8 (≈ONU load 0.5), because at that load ONU traffic arrival equals to its departure and from there on ONU demands start to exceed Bmax. Note that when the ONU demands start to exceed Bmax, then the advantage of ONU slot remainder elimination and reuse of left over cycle bandwidth becomes effective. Due to the same reasons, the utilization figure also demonstrates better performance at higher ONU load. In summary, the improvement of proposed DBA is due to reuse of unclaimed cycle bandwidth and elimination of the followings: ONU slot remainder, DBA idle time and additional inter-ONU guard band [6]. Furthermore, the DBA calculation in ONUs is based on all ONU reports, which allows globally optimized distribution of bandwidth.

3. FEASIBILITY EXPERIMENT AND DISCUSSION

3.1 Experimental Setup

The experimental work presented in this section focuses only on the control plane implementation of the proposed decentralized EPON architecture in [6]. Fig 4 depicts the experimental setup. Each ONU has two sections; a transmitter and a receiver. The upstream transmitter section consists of an HP Data Generator that generates programmable 26-1 PRBS NRZ signal at 625 MHz. The NRZ signal directly drives a 1310 nm FP laser that has a Rise/Fall time of 0.3 ns. Since control messages are fixed in size, the relative timing of the upstream transmitters is programmed to emulate a fixed TDMA assignment. The Receiver section consists of an Agilent photodetector front end and an HP Data Analyzer. Since we only consider control plane signal, there are no data used in this experiment (we only have control time slot one after the other). Each ONU transmits its update control message in its assigned fixed time slot. These messages are then combined at the SC and a multiplexed control signal is created at output port #1 of the 4x2 Bi-directional Coupler (BDC). Thus, the control signal output power drops down to 1/N of the initial power, where N is the number of ONUs. This signal is then fed back into the second output port #2, where it is broadcasted to all ONUs. Consequently, the main objective of this experiment is to explore whether the SNR of the redirected control signal is still adequate for the ONU receiver to recover the control messages when BDC size increased to support more ONUs.

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Figure 4: Experimental Setup of Decentralized EPON Control Plane

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3.2 Result Analysis

First, we analyze the eye diagram (Fig. 5) of the redirected control signal. The eye is open with a measured extinction ratio of 26 dB. Second, we discuss the experimental performance of the proposed decentralized control plane architecture with 4 ONUs to estimate the maximum number of ONUs supportable in this architecture. While the transmitter power is 2 dBm, the initial received power with 0 Bit-Error-Rate (BER) is read to be -13.7 dBm for 4 ONUs setup. In another words, the power budget for 4 ONUs is 15.7 dB which is same as the itemized loss budget of Table1. In real life application (complete setup with data plane), each ONU requires two receivers; one to receive downstream OLT signals and other one to receive redirected upstream control signal. So, to facilitate two receivers with multiplexed signals, we need to include a multiplexer (CWDM) before the two receiver inducing a loss of 0.8dB. Note that the experiment is for control plane and the additional CWDM is needed for complete operational setup where data plane is included. Only for realistic calculation purpose we added the CWDM in our final calculation and that supposed to increase the measured loss (power budget) for 4 ONUs to 16.5 dB. Fig. 6 shows received power versus Q for the control plane setup for this architecture. As can be seen from the figure, Q value of 16.4 (equivalent to BER of 10E-12) is maintained till input

Figure 5: Eye Diagram of Received Signal in Decentralized Control Plane

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power to the receiver is around -37dBm. The largest number of ONU’s that can be attached to a single PON is limited by the optical power budget. When scaling this architecture to support higher number of ONUs, the only thing needed to be changed is BDC size. Since 4 ONUs power budget is 16.5 dB and we can sustain 0 BER till -37dBm, we have adequate power (over 20dBm) to support more ONUs (compensate for increased splitting loss). Each time we double the size of BDC (i.e 4x2 to 8x2), we induce additional 6dB loss in the redirected (2 pass) control signal. So, we can expand the BDC size 20/6 ≈3 fold (23=8 times) of the initial size (4 ONUs). Thus, total supportable number of ONUs are 4x8=32 ONUs. Note that we found this result by estimating on the basis of experimental results of 4 ONUs, where the total power budget for 32 ONUs will be 16.5 +18=34.5dB. It is same as the calculated value found in Table 1 for 32 ONUs (34.5dB). So, the experimental estimation is similar to the calculated value. This number could be further increased with higher transmitter power and better receiver sensitivity.

Table 1. Control Plane Power Budget Analysis (see Fig. 4 setup) Losses Measured

4 ONUs (control plane setup)

Calculated 32-ONUs

(complete setup) SMF Fiber Loss between ONUs and BDC (2km max for Round Trip ) 0.8 dB 0.8 dB ONU Circulator Insertion Loss (2 pass, includes splices losses) 1.5 dB 1.5 dB Coupler/Splitter (BDC) Loss (2-pass, includes 1.4 dB BDC excess loss for any number of ONUs) 13.4 dB 31.4 dB CWDM Insertion and excess loss (used to multiplex downstream OLT signals and reflected upstream control signal for ONU receivers) N/A 0.8 dB Total Loss 15.7 dB 34.5 dB

CONCLUSION

The proposed enhanced distributed DBA for decentralized architecture is globally optimized and outperforms the centralized IPACT and previous decentralized DBA. We have demonstrated the feasibility of implementing this distributed EPON architecture with associated DBA. Furthermore, it has been shown that the SNR of the redirected control signal is still adequate for 32 ONUs to recover the control messages at their receivers. In this architecture, the control signal which is generally sent to OLT in a centralized architecture will be redirected to ONUs. Thus, there is no additional signaling required. While the proposed distributed architecture increases the complexity of the ONU, however, the added flexibility and reliability of such architecture and efficient performance of the associated DBA justify the extra cost. The future work will extend to a test bed to observe MAC layer performance of the data plane.

REFERENCES

[1] IEEE 802.3 Ethernet in the First Mile Study Group, http://www.ieee802.org/3/efm/public/index.html [2] G. Kramer and G. Pesavento, “Ethernet Passive Optical Network (EPON): Building a Next-Generation

Optical Access Network,” IEEE Com. Mag., pp. 66-73, Feb. 2002. [3] Alloptic, “Ethernet Passive Optical Networks,” The International Engineering Consortium,

http://www.iec.org . [4] G. Kramer, et al. “Ethernet PON: Design and Analysis of an Optical Access Network,” Photonic

Network Communications Journal, vol. 3, No. 3, July 2001 [5] G. Kramer, B. Mukherjee, and G. Pesavento, “IPACT: A Dynamic Protocol for an Ethernet PON

(EPON),” IEEE Comm. Mag., pp. 74-80, Feb. 2002.

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[6] A. Hadjiantonis, T. Rahman, A. Khalil, G. Ellinas, M. F. Arend and M. A. Ali, “A Novel Decentralized Ethernet-Based PON Architecture” Proc. IEEE ICC’04, Paris, France, June 2004.

[7] N. Nadarajah et al., "LAN Emulation on Passive Optical Networks using RF Subcarrier Multiplexing", in Proc. LEOS '04, Rio Grande, Puerto Rico, Nov. 2004

[8] C. Assi, Y. Ye, S. Dixit, and M. A. Ali, “Dynamic bandwidth allocation for quality of service over Ethernet PONs,” IEEE J. Select. Areas Commun., Dec. 2003.

[9] G. Cramer et al., “On supporting differentiated classes of service in EPON-based access network,” J. Opt. Networks, 2002.

[10] M. S. Taqqu, W. Willinger, and R. Sherman, “Proof of a fundamental result in self-similar traffic modeling,” ACM/SIGCOMM Comput. Commun. Rev., vol. 27, pp. 5–23, 1997.