CHAPTER 1 INTRODUCTION
1.1 Network Evolution
Over the past few years, the evolution of the Internet has proved that wavelength-division multiplexed (WDM) is the most flexible and robust solution for the present and future dynamic network traffic. As the Internet transitions from a best-effort network to a strategic global Internet protocol (IP) infrastructure, demands will not only be for higher bandwidth, but also for a wider range of integrated services demanding high reliability [1–5]. The broadband access network connects the service provider at a central office (CO) to the subscribers, which include businesses and homes. Recently, subscribers have been demanding high bandwidth services such as video-on-demand and high-speed internet for both downloading and uploading information. Current access technologies such as hybrid fiber-coax (HFC), digital subscriber line (DSL), and wireless networks have severe limitations in transporting symmetric traffic or high bandwidth information.
However, the nature of the traffic that is sent through the Internet is changing. New applications require higher bandwidths, support for constant-bit-rate (CBR) streaming media, symmetric data rates for peer-to-peer file transfer, low delay for interactive applications and security. The internet will need many architectural upgrades to accommodate these new demands. The common used methods are included:
1) IP — the Internet Protocol. It started life as protocol which was used for communication over the Internet. IP was optimized for data service and was packet
based, rather than circuit switched and also well suited to streamed audio and video broadcast.
2) ATM— Asynchronous Transfer Mode. It was probably the first serious contender for providing broadband multi-service network. ATM is a packet based system, using fixed length packets, and is designed to support a wide range of services such as voice, video and frame relay.
3) ADSL— Asymmetric Digital Subscriber Line. It was first developed to enable a broadband always-on connection to be provided over a copper pair. ADSL is asymmetric and supplies a greater downstream capacity than the upstream, typically of 1.5 Mbps downstream and 128 Kbps upstream bandwidth.
4) VDSL— Very high speed Digital Subscriber Line. An increased downstream of 50 Mbps can be achieved using VDSL, but this comes at the expense of shorter distance, typically less than 1500 ft (457.2 km).
5) PON — Passive Optical Network. It provides fiber communications without expensive electronics. PON are well studied to enhancing existing networks by replacing the copper between the local exchange and a flexibility point.
6) HFC— Hybrid-fiber Coax. HFC systems substitute most of co-axial cable between the cable TV head-end and the customer with fiber. It is used to transmit both broadcast video and high-speed data services.
Figure 1.1 shows the possible evolution skeleton for the network. On the access side, the first evolution will include the development of optical fiber in the feeder network that close to the customer by ADSL technology, allowing the last relatively short copper.
Similarly, the fiber in HFC networks will come closer to the customer, serving the small
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number of businesses and homes with the coax tree. This will support bit rate up to 10 Mbps downstream and meet the capacity requirement of small offices and homes. The copper based access, such as VDSL technology that support up to 50 Mbps downstream bandwidth, will gradually migrate to fiber access. The fiber link can be based on passive optical network technology (PON) and SuperPON using optical amplifiers. Ultimately, the bandwidth of 100 Mbps per subscriber will meet the demands of fiber-to-the-home (FTTH).
The transport network supports the accumulated traffic generated in the access network and the link capacities on the order of 1 Tbps will be necessary in the near future.
Currently, many networks are progressing to synchronous digital hierarchy (SDH) architecture, with a transport comprising a mesh of digital cross-connects (DXCs)
From: J. Aarnio, Nokia Research Center, 2002
Figure 1.1: Network evolution options.
Photonic Transport Layer Direct optical access
(SDH, ATM, WDM, IP, other)
WDM Direct optical access
(SDH, ATM, WDM, IP, other)
WDM open optical
interface open optical
interface
interconnecting rings. Fiber link in the long-haul networks is currently being upgraded to use WDM technique, which has similar functions to the electrical SDH network. The optical transport layer is a key evolutionary development because of its effectively transparent to bit rate and signal format. Furthermore, it also can support different types of access, such as WDM, IP and so on. Unlike HFC and DSL technologies, wireless networks such as wireless fidelity (WiFi) and world interoperability for microwave access (WiMax) do not require any optical fiber or cable for the transmission of information from the central office to the subscribers. Wireless networks allow users to be mobile but have limited bandwidth and security issues.
1.2 Motivation
Next generation broadband optical network will likely require a considerable increase in total spectral efficiency. The emergency of broadband communications has increased the need for bulk transport of high capacity signals and services. Optical WDM networks have been widely recognized as the dominant transport infrastructure for future Internet backbone networks with its potential of providing virtually unlimited bandwidth [5]. The reconfigurability of WDM networks has the following features: the network’s configuration can be increased by bypassing, adding or dropping the traffic and the capacity throughput enlarges when the traffic is multiplexed on the fiber by WDM.
However, in metro area networks (MAN) that encounter fiber shortages problem, bidirectional transmission is an appealing means of increasing the bandwidth utilization in a single optical fiber and, at the same time, reducing the operation and maintenance cost [6–8]. One of the core technologies in bidirectional transmission system is realizing
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of bidirectional amplification, which typically requires high gain, low noise and the elimination of Rayleigh backscattering (RB) [9]. One of the efficient methods in bidirectional transmission is using wavelength interleaving scheme. As an optical filter, an interleaver combines or separates a comb of dense wavelength-division multiplexed (DWDM) signals [10–12]. Although interleaver has been widely used in multiplexing and demultiplexing of DWDM optical signals, its applications in bidirectional transmissions have not been fully studied and verified. As a result, the motivation of this dissertation lies in the investigation of a reliable solution for bidirectional transmission in WDM network by using a new designed four-port interleaver.
Among several choices of modulation and demodulation formats in optical access networks, ON-OFF keying (OOK) format [13, 14] is the most popular for its simple generation. Recently, special attention has been given to differential phase-shift keying (DPSK), which was proving to be superior [15] relative to the traditional OOK in optical fiber communication system. This is due to its larger tolerance to fiber nonlinearity and noise from amplified spontaneous emission (ASE). The phenomenon of different modulation formats, such as OOK, return-to-zero DPSK (RZ-DPSK) and non-return-to-zero DPSK (NRZ-DPSK) with dissimilar amplification schemes, i.e. erbium-doped fiber amplifier (EDFA) and semiconductor optical amplifier (SOA) in bidirectional transmission systems would be investigated.
Since the downstream data is shared among several subscribers in HFC systems, the available bandwidth per user depends on the number of subscribers connected to the internet. Additionally, the upstream bandwidth is limited because of ingress noise generated from appliances at the subscribers end. Therefore, highly efficient, next
generation, broadband optical networks providing symmetric upstream and downstream information would be difficult to develop using HFC and DSL infrastructures. However, PON technology is a considered an ultimate solution. Network carriers have begun to deploy time-division-multiplexing passive optical networks (TDM-PONs) such as broadband PON (BPON), Ethernet PON (EPON) and Gigabit PON (GPON) and wavelength-division multiplexing passive optical networks (WDM-PON) in response to the current trend of data- and image-based services resulting from the rapid growth of all kinds of multimedia Internet applications. In addition, WDM-PON is a promising approach for gigabit optical access network [16].
How to reduce the cost is always the most important issue in WDM-PON system.
The first subject of WDM-PON is to design and implement a cost-effective scheme for bidirectional WDM-PON using the four-port interleaver. The second subject is to design a new method to provide select-cast services in WDM-PON system. The proposed WDM-PON has been implemented using optical carrier suppression and separation (OCSS) technology to generate a wavelength pair from a single laser source at the central office and deliver downstream signals in different modulation formats, i.e. OOK and DPSK. This method enables the co-location of both upstream and downstream WDM transmission in the central office. Additionally, the complexity, cost and maintenance of the optical network unit are reduced by enabling wavelength-independent operation.
Since radio/mobile access is continuously growing and placing increasing demands on the network. The third subject is to devise a simple and cost-effective configuration in WDM-PON to supply triple play service (TPS). Only one single-arm intensity modulator is needed in this proposed scheme to provide significant improvement on both power
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