A Comparison between EPON and RPR Access Networks
Ricarda Carolina Rende
José Marcos Câmara Brito
National Agency for Telecommunications - Anatel
3073, Vergueiro Av. - 04101-300
São Paulo - SP - Brazil
ricarda@anatel.gov.br
National Institute of Telecommunications - Inatel
P.O. Box 05 - 37540-000
Santa Rita do Sapucaí - MG - Brazil
brito@inatel.br
Abstract— Ethernet Passive Optical Network (EPON) and
Resilient Packet Ring (RPR) are two important technologies used
as solutions to the last mile access network. EPON consists of an
OLT (Optical Line Terminal) and a set of ONU´s (Optical
Network Units) connected through star topology. RPR networks
use a dual ring topology to establish the communication between
end users and the central office. EPON networks are
characterized for their simplicity while RPR for robustness. This
paper we have compared, base on simulation, the performance of
these networks in an end user environment.
Index Terms—EPON, RPR, Traffic Analysis
I. INTRODUCTION
New telecommunications services demand high speeds in
telecommunications networks. The last mile access is usually
characterized as the bottleneck of these networks. In order to
overcome this problem, EPON (Ethernet Passive Optical
Networks) and RPR (Resilient Packet Ring) have been
proposed as options to increase the speed in the access
networks.
EPON networks consist of an optical broadcast
configuration connected through star topology. These
networks are composed of an optical line terminal (OLT) and a
set of optical network units (ONU´s) that transmit Ethernet
frames in upstream (ONU’s->OLT) and downstream (OLT>ONU’s) directions.
RPR (Resilient Packet Ring) networks have been defined as
data transfer networks among stations interconnected in a dualring topology. These networks are called resilient due to their
protection mechanisms in case of a single failure. The mainly
characteristics of these networks are: spatial bandwidth reuse
over different parts of the ring, transit queue to control the
buffer allocation of each node and fairness algorithm to ensure
proper traffic allocation over the ring.
EPON and RPR Performance Networks have been studied
separately. Regarding EPON networks, the authors of [1], [2],
[3] and [4] have proposed different mechanisms of bandwidth
allocation in order to better accommodate different traffic
types. On the other hand, at RPR Networks, the authors of [5]
and [6] have studied the transit queue behavior considering
different traffic demands.
The goal of this paper is to compare the performance of
these different networks used as solutions to last mile access
using optical fiber. As it can be observed, EPON and RPR
networks are quite different. EPON networks provide customer
access through star topology while RPR networks use ring
topology. Also it will be showed the access mode differences
of each network.
The remainder of the paper is organized as follows: Sections
II and III describe the characteristics of EPON and RPR
networks, respectively. Section IV presents the simulation
model that is used to analyze EPON and RPR systems
performance. Finally, conclusions are drawn in section V.
II. EPON NETWORKS
EPON networks are defined by the IEEE 802.3ah standard
[7]. EPON is a point to multipoint optical network with no
active elements between the source and the destination. All the
internal components used in an EPON network are passive,
such as optical fiber and splitters called ODN (Optical
Distribution Network). The Ethernet packets are transmitted
between an optical line terminal (OLT) located at the
provider’s central office and a set of optical network units
(ONU’s) located at the customer’s side. Due to the star
topology used in EPON networks, even the direct
communication between ONU’s has to pass through the OLT.
Ethernet packets in downstream direction are sent by
broadcast (OLT->ONU) to all ONU’s belonging to the
network. An ONU has the capability to accept only their
packets and discard the others. However, in upstream direction
(ONU->OLT), the ONU’s need to employ some mechanism to
avoid data collision. This is achieved by employing time-slotbased bandwidth allocation to different ONU´s, like in TDMA
(Time Division Multiple Access). This allocation can be static
(FBA – Fixed Bandwidth Allocation) or dynamic (Dynamic
Bandwidth Allocation).
Figure 1 illustrates the downstream channel (a) and the
upstream channel (b) related with an EPON Network.
Fig. 2 – RPR Network Topology
Fig. 1 – EPON Network a) downstream b) upstream direction
The FBA algorithm allocates a fixed time slot for each ONU
in every cycle. The advantage of this algorithm is the
simplicity. However, there is a risk of inefficient channel
allocation once the ONU will occupy the upstream channel for
its assigned time slot even if there is no frame to transmit. At
FBA the granted bandwidth for each ONUi can be given by
[3]:
Bi 
Tcycle  N  Tg   R  wi
8
(1)
RPR networks are resilient because they offer protection
mechanisms against single failure. The RPR technology is
considered the natural SDH (Synchronous Digital Hierarchy)
evolution, once the network operation and maintenance
characteristics are the same. The reason for the evolution is
because RPR is based on packet switch while SDH uses circuit
switch.
RPR networks can be distinguished from other networks due
to key features such as: spatial bandwidth reuse, access control
based on transit queue, fairness algorithm and protection
mechanisms.
A. Spatial Bandwidth Reuse
Spatial bandwidth reuse allows the concurrent transfer of
independent traffic on non-overlapping portions of a ringlet.
The ringlet is used only between the data transfer, from the
source to the destination, because the destination is responsible
for receiving and removing the data frame from the ring.
Figure 3 shows a spatial bandwidth reuse example; at the same
time, stations 2 and 7 are sending data frames to stations 4 and
9, respectively.
Where Tcycle is defined as the total time during which all
ONU’s receive permission to transmit data orderly to the OLT.
Tg is the guard time, R is the transmission rate and wi is the
weight for each ONU based on its SLA (Service Level
Agreements). If the SLA is not used (wi= w = 1/N for any i)
the expression (1) is simplified to:
Bi min 
Tcycle  N  Tg   R
8 N
(2)
It can be noticed that the Tcycle size is critical for the FBA
Algorithm. Making Tcycle too large will result in increased
delay for all Ethernet frames. On the other hand, making T cycle
too small will result in more bandwidth being wasted by guard
intervals, will increase the CPU processing load and might
cause problems with large packets.
Fig. 3 – Spatial Bandwidth Reuse
Fig. 3 – RPR Features a) Spatial Bandwidth Reuse b) Transit Queue
B. Transit Queue
III. RPR NETWORK
RPR network is a MAC protocol defined by the IEEE
802.17 standard [8]. The RPR topology is based on two
symmetric rings, ringlet 0 and ringlet 1, in such a way that they
work in opposite directions. Figure 2 shows the RPR network
topology.
This section presents the RPR access control method. While
Token Ring networks use a token to allow the station to
transmit, RPR networks employ transit queues in each station.
In this method, the data frames are temporarily stored in transit
queues following two rules: 1) the station must to prioritize the
data frames stored in transit queues, that is, the station can
transmit data only if the transit queue is empty; 2) A data
frame is temporarily stored in transit queues whenever such
data frame arrives at the time the station is already sending
other data frame. Figure 4 illustrates access control based on
transit queue.
the Wrapping mechanism that provides the alternative path as
S2>S3>S2>S1>S7>S6>S5>S4>S5>S6. Finally, Figure 4d
presents the passthrough mechanism used to bypass S5 station.
Fig. 4 – Transit Queue
It can be observed that RPR medium access control affects
the downstream stations, once the downstream stations only
have the permission to transmit if their transit queues are
empty. This deficiency is covered by using the Fairness
algorithm, which is going to create a traffic control mechanism
based on bandwidth request.
C. Fairness Algorithm
The Fairness algorithm is responsible for ensuring that each
station gets its fair share of the bandwidth. This algorithm
prevents one station from occupying a disproportionate share
of ringlet capacity with respect to other stations on the ringlet.
Different weight can be assigned to each station in order to
promote irregular bandwidth distribution.
The Fairness algorithm acts whenever there is ring
congestion. As mentioned before, upstream stations generate
the congestion. For this reason, the congested downstream
station sends fairness control frames on the opposing ringlet,
instead of sending fairness control frames to the next
downstream stations.
The congested station defines the fair rate based on the
traffic demand of each station and available bandwidth. This
fair rate value is sent to upstream stations and all upstream
stations adjust their transmission rates in order to eliminate the
network congestion.
D. Protection Mechanisms
The double ring topology allows two alternative paths
between the source and the destination in RPR networks. The
protection mechanisms observed in RPR networks are steering,
wrapping and optionally passthrough. Steering and Wrapping
mechanisms modify the original path in case of failure while
the passthrough performs the station bypass. Figure 5 shows
the differences between each mechanism. In the example, the
original path is S2>S3>S4>S5>S6. The single failure happens
between S3 and S4 stations. Using the Steering mechanism the
alternative path is S2>S1>S7>S6 (Figure 4b). Figure 4c shows
Fig. 5 – Protection Mechanisms a) Original Path, b) Alternative Path using
Steering, c) Alternative Path using Wrapping, d) Passthrough
IV. PERFORMANCE EVALUATION
Self-similar traffic models have permitted a more realistic
description of multi-service networks [9]. However the
analytic analysis is not a simple task. For that reason, a
simulation process was chosen to compare EPON-FBA and
RPR networks considering different traffic types.
Related with RPR networks the transit queue and protection
mechanism features were used during the simulation.
However, the spatial bandwidth reuse feature could not be
considered during the simulation due to the unique
communication between the users and the central office. The
simulation did not consider the direct communication between
end users. Also the Fairness Algorithm was not developed.
Three classes were used to distinguish the traffic types at
EPON and RPR networks: HI – high priority class, which is
delay-sensitive and requires bandwidth guarantees; ME –
medium priority class, which is not delay sensitive but requires
bandwidth guarantees; and LO – low priority class, which is
neither delay-sensitive nor bandwidth guaranteed. The stations
had independent buffering spaces for each class, which could
provide different packet treatment based on traffic type (delay
sensitive, bandwidth assured and best effort). Regarding the
transit queue feature in RPR networks, two transit queues were
used in each ONU, one for high priority traffic and the other
for medium and low priority traffic. Figure 6 shows the
difference between EPON and RPR user stations.
HI traffic was generated by Poisson distribution. Table 1
shows the Pareto parameters used for each traffic load.
TABLE I
A AND B PARETO PARAMETERS
Figure 6 – User Stations: a) EPON b) RPR
It was considered that each station had 5Mbps as the
maximum transmission rate. Once the usual transmission rate
for EPON network is 100Mbps and for RPR network is 155
Mbps, it was necessary to create two network topologies, as
showed in Figure 7. The first one was built with 20 ONU’s and
1 OLT (EPON Network) and the distance from each ONU to
the OLT was assumed to be 2 Km. The second topology (RPR
network) was designed with 31 ONU’s and 1 core, and the
distance was not a constant value. The maximum and
minimum distances were assumed as 3.5 Km and 0.5 Km, so
that the average distance was equal to 2 Km.
Figure 7 – Network Topologies a) EPON b) RPR
The ME and LO Ethernet frame length randomly varies
from 64 to 1518 bytes while HI frames were fixed at 80 bytes
[10]. The total traffic load of the entire network was changing
from 0.4 to 0.99. The HI traffic load for each ONU was fixed
at 5 voice channels (12 kbps each voice channel) and the
remainder load was divided almost equally between ME and
LO classes, 47% and 53% respectively. Self-similar traffic
(ME and LO traffic) was generated by Pareto distribution and
A simulation model was developed to both networks, using
the discrete event simulation tool called ARENA. The average
queue length and the average packet delay were used to
evaluate the performances of the networks. The number of
events was defined in order to achieve confidence interval
equal to 5% with 95% of confidence.
Figure 8 shows the queue length and average packet delay
as a function of the network traffic load. The queue length is
the average buffer occupation considering HI, ME and LO
traffics at the user station. On the other hand, the average
packet delay is defined as the average time between the packet
generation at the user station and the packet delivery at the
central office. It is considered the transmission, the
propagation, access and the queue delay times between the
source and the destination.
At RPR networks it was observed that the farthest user
station (if compared with the central office) had the worst
delay result for the HI and ME traffic types while the nearest
user station had the worst LO traffic type result. It happens
because the transmission and the propagation delay time are
more expressive for the farthest user station while that nearest
user station suffers the impact of the transit queue algorithm
deficiency already mentioned before. The results showed at
Figure 8 related with RPR networks represents this worst RPR
networks conditions, that is the choose of the farthest and the
nearest user stations for the HI/ME and LO traffic types,
respectively.
As it was expected at EPON networks the average packet
delay is strongly influenced by Tcycle. This parameter affects
directly the access delay time. In this way HI and ME traffic
delays for EPON networks is not much influenced by
transmission, propagation and queue delay times even for high
traffic load as is showed at Figure 8.
Analyzing the traffic types it was noted that the HI average
packet delay still the same for both networks even with high
traffic load. Concerning ME average packet delay it was
observed a smooth increase at both networks. Finally the LO
average packet delay had suffered the highest traffic load
impact in both networks, once it is the lowest priority type.
In general the results showed a better performance of the
RPR network compared to the EPON-FBA network for all
traffic types. The only exception happened during the traffic
overload for the RPR nearest user station (99% of traffic load).
This fact could be avoided through Fairness Algorithm.
V. CONCLUSION
This paper presented EPON and RPR networks used as
solutions to the last mile access. It was proved that both
networks are good access network solutions because both
could prioritize traffic. This aspect is very important to HI
traffic types that are delay sensitive. The results also showed
better response with RPR networks even for higher distances
between the end user and the central office. The weakness of
the EPON-FBA networks was observed due to the fixed
bandwidth allocation and the Tcycle parameter dependence.
Even EPON networks has presented worse response if
compared with RPR networks. It is expected that EPON
networks performance could be improved if dynamic
bandwidth allocation is used instead of fixed bandwidth
allocation. For this reason the next research will be the
comparison between RPR networks and EPON networks using
dynamic bandwidth algorithm.
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[1]
Figure 8 – Simulation Results a) Average Packet Delay b) Queue Length