Emerging Architectures for Optical Broadband Access Networks Rong Zheng and Daryoush Habibi The School of Engineering and Mathematics Edith Cowan University Perth, WA 6026 Email:
[email protected],
[email protected] Abstract— The range and bandwidth of the services which must be offered by access networks have increased significantly over recent years. Furthermore, there is a fair degree of uncertainty about the trend of services which might be demanded by users in the future. Therefore, the architecture of access network becomes an important development area in order to adapt to these trends and make it scalable for future growth. In this paper we provide an overview of emerging architectures for broadband optical access networks. We also discuss challenges that component manufacturers might address to keep in pace with the market’s rapid growth.
I. I NTRODUCTION The access network is the link between end user and the rest of network. It is known as either “the last mile” or “the first mile” problem which is the bottleneck of communication system that limits communication speed in any Internet and broadband access schemes. Broadband technologies, which include all evolving high speed digital technologies that provide consumers integrated access to voice, high speed data, video, video-on-demand, and interactive delivery services, are fundamental components of the communication revolution[1], [2]. Over recent years, there are a variety of different and competing access technologies developed. These include optical transmission techniques such as fiber-to-the-home (FTTH) and passive optical network, transmission over the classical links (digital subscribe line, DSL), transmission over coaxial cable modem; the broadband satellites, broadband fixed wireless and wireless local area networks (WLAN) or mobile systems. Those technologies promise to break the existing bandwidth bottleneck and deliver broadband services to both business and individual users. Presently, the leading technologies in the access network are DSL [1] and cable modem which make use of the existing infrastructure. DSL uses the existing copper plant to provide data over voice, independent voice and data transmission, and plain old telephone service (POTS). Cable modem technology is offered by cable companies to provide broadband services to subscribers. Both systems are limited to rates per subscriber in the order of several Mbps downstream and a few hundred kbps upstream due to propagation, congestion and crosstalk considerations. Thus, at best they can offer no more than one to two orders of magnitude improvement over voice grade modems. There still remains some four more orders of magnitude of bandwidth starvation compared to the usable bandwidth of backbone network. Furthermore, the high degree
of bit rate asymmetry of both DSL and cable modem are hardly counter to the trend toward more peer-symmetric traffic loads for some important new applications. Therefore, while they are interesting and sophisticated, sustaining the success of DSL and even cable modems is going to be difficult. A most promising future proof access technology will be FTTx(fiber-to-the-home, to business, curb, block, etc.)passive optical network. Fiber optic cables can support an extraordinary high bandwidth – possibly reaching 1000 Mbps, compared to 1.5 Mbps for typical cable and DSL offerings. A FTTx network will remove the bottlenecks of other access technologies and upstream bandwidth can be significantly increased. Thus, it will be a long term preferred alternative to other technologies for full services (two-way voice, video, and high-speed data) broadband access network, especially in areas when deploying new access network. In this paper we will review several architectures for optical broadband access network and address the related photonic technologies within PON sceneries II. B ROADBAND ACCESS WITH PASSIVE O PTICAL N ETWORK Broadband passive optical network is the most promising approach to establish a cost-effective access network. It achieves excellent economy because multiple end users share optical fiber and central office equipments. Its specifications were originally discussed and determined in Full Service Access Network (FSAN) [2], [4] which is an international group formed by network operators in 1995. Since then, a series of ITU standards G983 have been developed. As recommended in ITU-T G.983.1, the basic architecture of an access passive optical network [3] is shown in Fig. 1. An optical line terminator (OLT) usually resides at central office (CO) and provides the interface to access network. From there a single feeder fiber is connected to a passive power splitter. The outputs of passive splitter are connected to subscribers directly or to further splitters via distribution fibers. Since all subscribers share optical power from OLT, this network can be called power splitting PON (PS-PON). Coarse wavelength multiplexing technology (CWDM) is used at OLT and optical network unit (ONU) to separate the downstream (from OLT to ONUs) and upstream (ONUs to OLT) traffic. The downstream uses 1550nm wavelength while the upstream uses 1360nm. With this structure, it can deliver limited digital video as basic band signals, time-multiplexed with voice and data signals.
Basic ONUn PS - OLT
Splitters Photodetector (1550nm)
LD(1550nm)
CWDM CWDM
LD(1360nm)
services. By using one infrastructure of PON, the broadcast channels can be distributed simultaneously with basic band traffic, this is called WDM overlay PON. In the following sections, we will discuss this approach in details.
Photodetector (1360nm)
III. A RCHITECTURES FOR ENHANCEMENT SERVICES Fig. 1.
Fundamental PON architecture
Gordon et al. [5] proposed a single fiber FTTx architecture, called “Fiber Vista” for delivering analog and digital video along with data using cable modem technology. Their system is similar to hybrid fiber coaxial system where the fiber is taken to home. There were also others [6] who explored simultaneous delivery of all broadcast and switched video channels digitally to every subscriber in the passband on the same fiber that is used for the 155 Mbps baseband signals. BellSouth [7] firstly offered the services to customers which included video and high-speed data. Its implementation is a broadband overbuilds, with telephone service continuing to be provided over existing copper wires while two types of PONs are used in their system. The first one provides high speed data services for users. The second is a video PON which provides analog video in 50-550MHz radio frequency (RF) band and digital video in 550-750MHz RF band. The second PON feeds an analog optical network terminal (ONT) to provide the entertainment video service set. Those two PONs use the same physical layout as defined in ITU-T G983.1. Usually a broadcast analog video channel may require 3 to 6 Mbps bandwidth depending on the content, while a HDTV channel needs about 20-30 Mbps bandwidth. Since 155 Mbps basic bandwidth is shared among up to 32 ONUs, the number of video channels that can be delivered simultaneously to users as part of basic band signals is limited. This problem will become more severe when PON is extended to residential customers where all 32 users on an OLT may like to view and record different video programs on multiple devices. These limitations will be relaxed partially if the bit rate is increased to 622 Mbps. However, it will require an upgrade of central office equipments and all ONUs. The future access network will use a “one for all” architecture, merging the traditional CATV and telephony networks into one. The distribution of CATV signals is an essential part of the service bundle for FTTH. There are two possible technical approaches for delivering this extra downstream signal next to the bi-directional data signals. Frequency division multiplexing (FDM) approach uses a single ONT receiver for both band signals and requires the CATV and data signals to be well separated in the electrical spectrum. This either limits the possibility increasing of downstream bit rate, or requires an anticipative shifting of the carriers to high frequencies, that seriously impacts the design of CATV receiver. Moreover, the FDM approach is incompatible with the high requirements for CATV reception. The second approach use a second wavelength to overlay on the existing PON to carry the extra
Followed the publication of ITU-T recommendation G983.3 in 2001, an additional optical spectrum was allocated to increase the network’s greater service capability. The wavelength range of 1480-1500nm is dedicated to support one basic downstream channel and 1539-1565nm band is allocated to support a variety of usage scenarios for the enhancement services which can be of distributive nature, like broadcast community access television (CATV), or dedicated connections, such as leased wavelength for particular users. The enhancement band provides the extra services without loss of basic downstream band signals. It can also be used for future network’s upgrading to coarse wavelength division multiplexing (CWDM) and dense wavelength division multiplexing (DWDM). This will accelerate the installation of fiber in access network. A. Architecture of WDM video overlays with the existing PON As discussed previously, there are several limitations within the PS-PON. WDM technology will dramatically improve the capabilities of access network and enhance the range and quality of services that can be delivered to customers [6], [8]. The huge bandwidth afforded by WDM is critically needed by the access network. With WDM, it is possible to construct wavelength based point-to-point virtual links on a shared PON fiber infrastructure. WDM technology also provides a high degree of configurability, therefore enabling efficient sharing of resources. The high flexibility, capacity and transparency of WDM also makes it an attractive upgrade solution offering additional degrees of freedom due to insertion of various wavelengths to support both entertainment services and more demanding business applications. Enhancement ONU Splitters Laptop computer
Enhancement OLT PS-OLT
LD(1550nm)
Telephone WDM
Fax
Modulator RF Mixer TV
Local TV
Fig. 2.
Cable TV
Architecture for WDM video overlay transition stage
With the adoption of ITU-T Recommendation G.983.3, the passive optical network can be upgraded with additional services through the overlay of enhancement band within existing architectures [9]. In these systems, the OLT generates both PON signals in basic band and video channels in enhancement band. The two band signals are multiplexed in CO and are sent to the feeder section through a CWDM multiplexer. Fig. 2 shows this architecture where the user can choose either a basic service or an enhancement service. Analog or
digital TV signals are first multiplexed using FDM in RF domain. Then the RF signals are used to modulate laser transmitter with the enhancement band. The two band optical signals are multiplexed by a CWDM multiplexer at the CO. Through the feeder fiber, they are transmitted to the outside plant where they are separated to two band signals again. The passive optical splitters further split the two band signals individually. Then according to different end user’s requests, enhancement service can be distributed through recombination of the two band signals with a further WDM combiner, as shown in the second branch in Fig. 2, or, the user can choose basic service with the basic connection similar to PS-PON subscribers as shown in the first branch in Fig. 1. By applying this architecture, the users can choose either basic band service or enhancement band service by selecting appropriate connections. This can be implemented as the transitional network’s upgrade architecture. Splitter Enhancement OLT
Basic ONU
1490nm 1310nm
WDM filter PS-OLT
WDM
Enhancement ONU
LD(1550nm)
Modulator
1539nm~1560nm Fax
Variable Splitter RF Mixer
TV
Cable TV Local TV
Fig. 3.
PON with enhancement band
With the decreasing price of the receiver at the enhancement ONU, the network will finally provide the enhancement service to all end users. The upgrade architecture of PON can be simplified as shown in Fig. 3 where only passive optical splitters are uses as branching devices at outside plant. In this architecture, all users can access the enhancement band service. OLT Passive Remote Node Laser Array
ONUs Filter1
Mux
λ1 , λ 2 ... λ16
1.3 µm
1 x16 Power splitter Router
1.3 µm
Filter16
Fig. 4.
PD λ16
1.3 µ m
ONUs Passive Branching Device
λ1 λ17
λ1, λ2,… λ 16
Receiver Source
1×16 Router
WDM Receiver
λ17, λ18,… λ32 λ16 λ 32
Receiver Source
Receiver Array OpticalDemux
Fig. 5. WDM PON with different upstream and downstream wavelengths for each of the ONU
All architectures discussed in this section provide end users with improved downstream bandwidth and more services. For upstream traffic, the bandwidth can be enhanced by applying TDM and dynamic bandwidth assignment (DBA) protocol [11]. However, due to passive optical splitters in these architectures, there are some drawbacks which can be categorized as follows: • For downstream signal, the optical power intended for one ONU is split between all ONUs, leading to a N-fold power budget penalty. • Since the PON bandwidth is shared between all subscribers, it can allocate bandwidth in a flexible and dynamic way. However the price for flexibility is that electronic and optoelectronic components in OLT and all ONUs must work on aggregate bit rate. • Since downstream information is broadcast to all ONUs, these ONUs can receive all payloads of PON and the privacy and security become serious issues. • Network diagnostics and fault identification of the outside plant in PON are difficult. There are also additional losses introduced by filters or CWDM multiplexer. B. Architectures of wavelength routing WDM-PON
Source
λ1 , λ 2 ... λ16
PD
OLT Multi-Wavelength Source
PD λ1
λ1 , λ 2 ... λ16
used by applying a reflective modulator at ONU, but allow transmission in only one direction at a time. This arrangement is referred to as ping pong or time compression multiplexing (TCM) protocol [10].
Source
The layout the broadcast and select WDM-PON
More virtual point-to-point links through this basic PSPON can be established to meet some individuals’ extreme bandwidth requirements. This can be realized through the architecture shown in Fig. 4. In this architecture, more wavelengths can be used and multiplexed at OLT to carry the traffic targeted at different users. the receiver at ONU should be equipped with proper filters to select the specific wavelength. In this way, the end user’s extreme bandwidth demands can be met. For upstream traffic, the same wavelength can be
To overcome the limitation of the previous architectures, more complicated WDM-PON architecture can be designed through wavelength routing. In this category, the network is built with wavelength router as branching device. The bundled wavelengths from CO are transmitted through feeder fiber to branching site where the multiple wavelengths can be sorted out to output ports. Fig. 5 shows the basic layout of this architecture. In this architecture, wavelength based point-topoint connections are established using wavelength router such as arrayed waveguide grating (AWG) [12], [13], [14]. The dedicated optical transmissions in both downstream and upstream direction are bit rate and transmission format transparent. This architecture offers huge bandwidth and high flexibility. It improves the network integrity and privacy significantly. At present, the photonics components used in this architecture such as AWG and multi-wavelength lasers [15], are
quite expensive, making it hard to upgrade the network to the full WDM architecture. However, there are a number of transitional upgrades that can be realized cost effectively.
• •
ONUs
λ1
Passive Branching Device
OLT
•
Receiver F-P Laser
1.3µm Laser Array
λ1, λ2…λ16 Router/ splitter PD Array
1.3µm
λ16
Receiver
1.3µm
F-P Laser
Fig. 6. WDM upgrading in the downstream direction for point to point connection
The architecture provides a mixed connection for the basic PON and enhancement PON (WDM overlay). Both optical power splitter and WDM wavelength router can be used in the architecture to provide end users either basic PON connection or the dedicated WDM connection. The architecture uses power splitters and WDM filters. Therefore, all signals from basic PON and WDM channels can be broadcast to all ONUs. The advantage of this configuration is that it avoids the need to replace power splitters, thus lowering the upgrade cost. There are two possible ONU configurations which allow end users to accept: (1) both the basic PON and the WDM signals, as shown in the second branch in Fig. 7. (2) only one of the services, as the first and last branch in Fig. 7. IV. I SSUES TO BE ADDRESSED FOR THE EMERGING
Fig. 6 is one of such transitional architectures [16]. The downstream traffic uses dense WDM in the 1.5µm wavelength window and is routed at outside plant by the wavelength router to different users. The upstream traffic uses power-combining in the 1.3µm window, therefore, much cheaper Febry-Perot Lasers can be used and all end users share the same upstream wavelength. C. Mixed broadcast and wavelength routing WDM-PON In the previous two sections, two categories of WDMPON architectures were discussed. In this section, we discuss the architecture which uses both passive optical splitter and wavelength router [17], [12] as branching devices. The new architecture can meet end users different requirements and will be the ultimate architecture solution for optical access network. This is shown in Fig. 7. In this architecture, all services which are carried by different wavelengths are multiplexed at the central office(CO) through a WDM multiplexer, and then transmitted through the feeder fiber. At the outside plant, the signals can be processed in different ways either with passive power splitters or wavelength routers. End users can choose basic PON, enhancement PON or WDM-PON network. The architecture has the following features:
WDM ONU WDM OLT WDM OLT
Broadcast
WDM DWDM Filter
Modulator
WDM Rx
WDM Tx
A W G WDM Coupler WDM Filter
PS-OLT
PS-PON and Enhancement band
Basic ONU
WDM ONU
Variable Splitter Enhancement ONU
WDM Filter
Fig. 7.
PS-PON
Passive Splitter
WDM Architecture for Optical Access Network
Basic ONU
ARCHITECTURES
Whether in WDM overlay PON or fully WDM-PON upgrade architectures, there are some photonics technical challenges that the network operators must tackle. One challenge comes from the WDM laser sources which are used in OLT and ONUs. These laser sources should work without cooler and for each ONU, it needs a different laser source. These increase the cost a lot. With the laser diode technology advances, we can expect that the price of WDM lasers will decrease, and new technologies will be developed to deal with the laser sources for ONUs. The second challenge is from branching devices which include WDM multiplexer and de-multiplexer. This will be a cost issue for the future DWDM upgrade. However, this may be addressed with the arrayed waveguide grating (AWG) technology, because the wavelengths used in optical access network are not as dense as in backbone DWDM systems. The third challenge is that network operators must design a common multi-wavelength receiver for individual users. For the application in WDM overlay PON architecture, the receiver at ONU should be able to receive two band signals and at the same time transmit upstream signal at a wavelength outside these bands. There are a number of alternatives for realizing the receiver. Fig. 8 shows two configurations for the receiver. In Fig. 8(a), the downstream signals are first separated by the first filter and received by photo-detector 1 (PD1) which is used for enhancement band at 1550nm. The signal is then processed by the second filter and received by photo-detector 2 (PD2) which is used for the basic band at 1490nm. These two filters are transparent to the upstream signals. In the receiver of Fig. 8 (b) the first filter transmit both downstream signals while reflecting the upstream signal to the OLT. The second filter can separate the two downstream bands. These filters can be realized using two dichromic mirrors in the system. However, this will increase the cost of the receiver as discrete optical components are used. Another disadvantage of this receiver is that with the upgrade to WDM, the dichromatic mirror needs to be designed at different wavelengths for different users, adding to the cost of ONU and the complexity of network. In order to maintain the upgradability of network, a new module
of the receiver is expected to be designed which can receive different wavelengths for different users. Basic Band Receiver PD2
ƒ¸ 2:1490nm Fiber Pigtail
LD ƒ¸ 1:1310nm
ƒ¸ 1
ƒ¸ 2, ƒ¸ 3
Enhanced Band Receiver
ƒ¸3:1550nm PD1
(a) Basic Band Receiver PD2
ƒ¸ 2:1490nm Fiber Pigtail
PD1 ƒ¸3:1550nm
ƒ¸ 1
ƒ¸ 2, ƒ¸ 3
Enhanced Band Receiver
ƒ¸ 1:1310nm
LD
(b)
Fig. 8.
The structures of the receiver
V. C ONCLUSION A number of alternative architectures for upgrading the passive optical network to either WDM overlay PON or WDM-PON architectures were reviewed in this paper. The major solutions discussed were application of the enhancement band and implementation of WDM-PON. These architectures use broadcast and select, wavelength routing or the mixture of both topologies. With these new architectures, the end users can share the network resources and network operators will be able to supervise the failure of fiber or terminal equipment in real time. These architectures will also eliminate the need for time-multiplexing and ranging protocols as in PON and can provide virtual point-to-point links with data transparency and a high degree of data security and independence. Although WDM-PON architectures are designed for gast growing networks of the future, at present due to some technological issues need to be addressed by component manufacturers, it may be some time before they can be implemented in optical access network. The PS-PON architecture will therefore remain dominant in the market for some time while WDM overlay on PON can be gradually deployed as the transitional stage architecture. With the advances in photonics technologies, WDM-PON will eventually be realized and enable the network operators to provide customers with an extensive range of broadband services.
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