Link Utilization and Comparison of EPON and GPON Access Network ...

This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE GLOBECOM 2005 proceedings.

Link Utilization and Comparison of EPON and GPON Access Network Cost Sami Lallukka and Pertti Raatikainen VTT Information Technology Espoo, Finland {sami.lallukka, pertti.raatikainen}@vtt.fi Utilization of the link capacity affects the segmentation need of an optical network and hence has an effect on the overall access network cost. When a new optical access network is being built and the size of the population in the covered area is known, it is crucial to decide how many network segments are needed to offer adequate transport capacity to each connected user. As the take-rate increases, it is equally important to know when to divide the access network into a larger number of segments. This paper studies the EPON and GPON access network solutions in relation to the transport link utilization, which is further used to estimate the need to segment the network as penetration of the broadband access increases and finally to compare the overall network costs. Chapter 2 introduces the basics of the EPON and GPON concepts, chapter 3 derives the relevant measures to compare the concepts and chapter 4 presents the comparison results. Chapter 5 gives the concluding remarks.

Abstract— Optical transmission is getting more popular in the access network due to the increasing demand for bandwidth. The most advertised transport solutions for the optical access are Ethernet based PON (EPON) and Gigabit-capable PON (GPON), and the problem is which one to choose when establishing a new access network. This paper studies the network cost of EPON and GPON and bases the comparison on the link utilization.

I.

INTRODUCTION

Due to the continuing demand for larger bandwidth, the optical transport is gradually becoming general also in the access network. The conventional point-to-point transport solutions, such as SDH/SONET and Ethernet, are expensive to use in residential access, and therefore more economical solutions are being developed. The Passive Optical Network (PON) solutions are considered good candidates when throughput and cost are the major decision-making criteria. Several competing PON concepts have been developed, the most advertised ones of which are the Ethernet based PON (EPON) [1] [2] and Gigabit-capable PON (GPON) [3]. EPON is commonly assumed to be a more economical alternative, and GPON in turn is a more versatile concept offering transport service for a large set of higher layer transport concepts. The assumption of the cost-effectiveness of EPON comes from the fact that EPON has looser specifications, such as accuracy of timing and accuracy of physical signal levels, for the various network devices and functions. When considering the possible choice between EPON and GPON, it should be examined whether EPON really is more economical than GPON and in what circumstances. In evaluating this, one should consider issues such as compatibility with other transport systems, transport link utilization, cost to build a PON network, network segmentation need when the number of connected users increases and granularity of the offered transport service. As regards to the compatibility, GPON can be considered a better choice, because it adapts to various other transport concepts while EPON supports only Ethernet. On the granularity point of view, both concepts provide connection rates from very small to very large fragments of the transport link’s capacity and in this respect they can be considered equally good choices.

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II.

TECHNICAL CONSIDERATIONS

The basic idea of a PON is to connect a number of users to a central office (CO) via non-powered passive optical network components. A single fiber that is stretching out from the CO is split (closer to the end-users) into separate string, which each connect a customer site. Optical signals are transmitted and received by an Optical Line Termination (OLT) unit in the CO and by Optical Network Units (ONUs) at customer sites. The OLT transmits data to all ONUs by broadcasting the data frames in a common light wave channel and each ONU receives only those frames that are addressed to it. In order for an ONU to recognize which frames to receive, each frame carries an ONU identification tag (ONU ID), the size and form of which depends on the used transport concept. Transmission of data frames from the ONUs to the OLT is a much more complicate task, since all ONUs share a common upstream light wave channel. A mechanism is needed to allocate transport time slots for each ONU and additional mechanisms are needed to adjust signal levels and propagation delays, due to unequal ONU-to-OLT distances. The time slots are reserved, both in EPON and GPON, by the OLT using a system specific Dynamic Bandwidth Allocation (DBA) mechanism. The ONUs request transport capacity by sending their queue length reports, and based on that information the OLT assigns time slots for the ONUs. If the time

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Map). The maximum size of a time slot, allocated for an ONU, is 125 µs. The upstream Physical Layer Overhead (PLOu) depends on the upstream bit rate and is 15 bytes for the link rate of 1.24416 Gbit/s [5]. ONUs can report their traffic status to the OLT in three different ways: using status indication bits in the PLOu field, sending piggy-backed reports in the upstream Dynamic Bandwidth Report (DBRu) field or including an optional ONU report in the DBA payload [6].

slots are assigned for the ONUs in a regular fashion (by turns), then each ONU gets a transmission turn in a cyclic manner. The used DBA scheme and the chosen length of the cycle time have a significant impact on the introduced transport delay and thus on the efficiency of the network. When an ONU gets powered it first has to register as one of the active ONUs. The registration phase includes several initializing procedures of which at least ranging and registration has to be performed. The ranging procedure measures the round-trip delay between an ONU and the OLT, whereas the registration procedure selects an ONU ID for the activating ONU. When an ONU is registering itself, the other ONUs must stay silent. The registration period for ONUs, being powered up, is presented at programmable intervals and the length of the period depends on the longest ONU-to-OLT distance. For the 20-km distance the period should be at least 200 µs. The registration period is not required when all ONUs are active and is therefore not considered in comparison. These general operation principles are obeyed both in the EPON and GPON concepts. The main difference is that EPON uses standard Ethernet frames, whereas GPON uses synchronized frame-based transport. There are also some differences in the Medium Access Control (MAC) protocols and in line coding methods.

EPON frame Inter Frame Gap

12

Inter Frame Gap

GEM frame

EPON Preamble

8

Preamble

Control field

DA

6

DA

SA

6

SA

VLAN type VLAN ID Type/Length

2 2 2

VLAN type VLAN ID Type/Length

MAC client data

46 – 1500

MAC client data

FCS

4

FCS

5

GEM payload

Figure 1. EPON and Ethernet frame structures and GPON encapsulation for Ethernet traffic. GPON DOWSTREAM FRAME

A. Ethernet Passive Optical Network EPON is defined in IEEE standard 802.3ah [4]. Its nominal bit rate is 1 Gbit/s for the upstream and downstream directions and it exploits 8B10B block coding. The EPON frame is a standard Ethernet frame with some modifications in the preamble (see Fig. 1). The preamble is equipped with Logical Link ID (LLID), which is used for ONU identification. In the downstream direction, the OLT transmits EPON frames in a continuous stream. In the upstream direction, each ONU transmits its data frames during time slots assigned for it, and the multiplexed frames form a continuous stream of EPON frames. The EPON concept does not allow frame fragmentation and, thus, the time slot lengths should match the packet lengths to fully utilize the transport capacity of the allocated time slots. Each ONU requests time slots by sending separate messages to the OLT to indicate the ONU’s queue length. The OLT responds by sending separate grand messages to allocate time slots for each ONU. The maximum time slot length is 1 ms.

Frame interval = 125 ms PCBd

Control field 30 bytes

Payload

US BW Map Nx8 bytes

“Pure” ATM cells Section Nx53 cells

TDM & Data Fragments over GEM Section

GPON UPSTREAM FRAME ONU upstream time slot PLOu

DBRu

Payload

ONU upstream time slot

...

PLOu

DBRu

Payload

Figure 2. GPON downstream and upstream frame formats.

C. Upstream Physical Layer Overhead Successful transmission of data in the upstream direction entails support of processes like clock recovery and start of burst delimitation [7]. The laser on-off times and timing drift tolerance should also be considered. All these contribute to the EPON guard band and GPON’s upstream physical layer overhead. The EPON guard band consist of the laser on time (tEON), Automatic Gain Control (AGC) time (tAGC), Clock and Data Recovery (CDR) time (tCDR) and an additional dead zone (tDZ) is needed to allow timing variability (see Fig. 3). The GPON upstream physical layer overhead consist of the preamble time (tP), delimiter time (tD) and guard time (tG), which is the laser on time (tGON) added with timing uncertainty (tTU). GPON also employs power level adjustment functionality to fine-tune the power levels of its transmitters. The EPON guard band depends on the quality of the used components. Four levels have been specified for tAGC and tCDR. The laser onoff time is negotiable up to 512 ns.

B. Gigabit-capable Passive Optical Network GPON, which is defined in ITU-T standard series G.984, supports several bit rates, up to 2.5 Gbit/s, for both transfer directions [5]. GPON basis on the 125 µs long synchronous frames (see Fig. 2), similarly as the Public Switched Telephone Network (PSTN). Data are carried in the frames either using ATM cells or GPON Encapsulation Method (GEM) [6]. GEM supports transparent transfer of the Ethernet frames (see Fig. 1), in which case the Inter Frame Gap (IFG) and Preamble fields are extracted in the transmitting end and added automatically in the receiving end. The control overhead of the downstream frames is 30 bytes and each upstream time slot allocation is carried in an 8-byte record in the upstream bandwidth-mapping field (US BW

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Ethernet frame

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where tGB is the physical layer overhead (i.e. guard band). Utilization (ρGd) of a GPON downstream channel is given by

tG

GPON PHY

tGON

tTU

Data from ONU i Laser OFF

Laser ON tEON

tDZ

tP

tD

SYNC

FIND Burst

tAGC

tCDR

Data from ONU i+1

l Etp

tGB

EPON PHY

tDZ = 128 ns tEON = 512 ns (max) tAGC = 96, 192, 288 or 400 ns tCDR = 96, 192, 288 or 400 ns

tP = 35 ns (suggested) tD = 16 ns (suggested)

   N ONU × t f  l plou + l dbru  t f × RGnl −   t ct l GEMfo + l Etp     (4) ρ Gu = t f × RGnl where lplou is the length of the physical layer overhead (incl. PLOAMu field) and ldbru is the average number of DBRu l Etp

D. Dynamic Bandwidth Allocation The EPON and GPON standards do not explicitly specify any DBA algorithm to be used for the upstream time slot allocation. Only a framework is defined for the whole DBA process and its implementation is left for the designers. Several DBA algorithms have been developed for EPON [8][9][10] and algorithms suitable for GPON should follow the principles outlined in [3] and [11]. In general, the same principles apply for EPON and GPON DBA. An efficient DBA solution allocates bandwidth in a fair manner also for the prioritized traffic and it uses the ONU queue reports to fully exploit the upstream time slots. It should be capable of adapting fast to traffic changes and keeping the cycle time and polling interval in reasonable limits to guarantee low access delay. ITU-T specifies the maximum access delay to be 1.5 ms, which can be guaranteed by adopting maximum polling interval of 500 µs [3].

l dbru

MEASURES FOR COMPARISON

ρ Ed =

[t ct × R Enl − N ONU × l cm ] t ct × R Enl

Rk = αρCk.

(1)

ρ Eu =

l Efo + l Ep

[t ct × R Enl

− N ONU (l cm + t GB × R Enl )]

t ct × R Enl

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(5)

(6)

The total bit rate of segment Sk is the sum of the traffic from all ONUs, connected to the segment, i.e.

where lEfo is the EPON frame overhead, lEp the length of EPON payload, REnl the nominal bit rate of an EPON link, NONU the number of ONUs in a network segment, lcm the control message length and tct the cycle time. Utilization (ρEu) of an EPON upstream channel is given by l Ep

 t ct × RGnl  − l plou   N ONU  =  l dbru + l GEM

B. Network Segmentation When building a PON-based access network, segmentation is the way to guarantee adequate transport capacity per user as illustrated in Fig. 4. Provided that the size of the population, penetration of the optical access, percentage of users accessing the network during a busy hour and the minimum transport capacity per user are known, it is possible to calculate the number of needed network segments. Assume that the total transport capacity of segment Sk is Ck, line coding efficiency is α and utilization of the transport channel capacity is ρ, then the total bit rate Rk available for user data in segment Sk is

Utilization (ρEd) of an EPON downstream channel is given

l Efo + l Ep

)

where lGEM = lGEMfo+ lEtp.

A. Utilization of Channel Capacity

l Ep

(

fields in an upstream GPON frame. Since an ONU can send several GEM frames during its time slot and only the first of them carries the PLOu field and all frames carry the DBRu field, ldbru is approximated by

The target is to compare the network cost of an EPON and GPON system based on the utilization of the optical link’s transport capacity. The utilization affects directly the segmentation need in an optical network and thus has a clear effect on the total network cost.

by

(3)

where lGEMfo is the GEM framing overhead for Ethernet payload, lEtp the length of Ethernet payload, tf duration of the GPON downstream frame, RGnl the nominal bit rate of GPON link, lGfo the length of GPON downstream frame overhead and la is the length of the upstream allocation overhead. Utilization (ρEu) of GPON upstream channel is given by

tG = 26 ns (min) tGON = 13 ns (max)

Figure 3. Upstream physical layer overhead.

III.

l GEMfo + l Etp

ρ Gd =

  N ONU × t f   t f × RGnl − l Gfo − l a ×   t ct    t f × RGnl

Mk

(7)

Rk = ∑ rk ,i i=1

and the aggregate bit rate RT of all the K segments is K

RT = ∑ Rk

(8)

k =1

(2)

The number of users in segment Sk is Nk, which is the sum of users (nk,i) connected to all ONUs in segment Sk , i.e.

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Mk

Equation (14) can directly be used in evaluating the number of required segments. It includes several parameters and, by varying one parameter at a time and keeping the others fixed, we can study how K develops as a function of the selected parameter. The network costs are compared by dividing KE of EPON by KG of GPON to get the relative cost (η) between the two networks, i.e. η = KE/KG. A comparable picture of the network costs is obtained by setting the parameters (β, γ, N, and ro) that describe the network environment to be equal in both approaches. Inserting the EPON and GPON specific parameter into (14) and solving η we get

(9)

N k = ∑ nk ,i i=1

and the aggregate number NT of users connected to all the K segments is K

(10)

NT = ∑ N k k =1

If we assume that one end-user should have transport capacity of ro then segment Sk is able to support up to mk =

Rk ro

(11)

η=

simultaneous users and all the K segments support up to K

k =1

simultaneous users. Let’s suppose that in a target area all segments offer the same transport capacity C, the size of the population is N and the “broadband access” take-rate is γ (0