A main aspect of the access networks in Greece is the widespread deployment of xDSL technologies and in particular ADSL variants with speeds ranging from 2-24 Mbps downstream and up to 1 Mbps upstream. The asymmetric nature of the access network was a financially viable path for the telecom operators to provide broadband access to (almost) all of their customers, since (a) telephone lines had already been installed to practically every household, and (b) other broadband technologies (wireless and optical) had not penetrated the Greek Market. Moreover, given that (a) ADSL is still a profitable market in Greece, and (b) bandwidth demanding double-play and triple-play applications have only recently been offered, it is safe to say that ADSL is here to stay for the following years.

In a broader view, however, current research on broadband access networks no longer addresses the aforementioned asymmetry, but assumes that the vast spectrum offered by optical technologies is adequate to provide equal bandwidth to the upstream and downstream. This is the trend of the future and in the long run symmetric access will be provided to the end user; still in the shorter term the telecom operators in Greek will have to address the issue of upstream/downstream bandwidth asymmetry while upgrading their access networks to match (a) increasing access speeds and (b) a geographically expanded access market. Therefore, from a practical perspective the real question is whether a fully symmetric optical access network is needed in the following years or the telecom operator needs can be met by an asymmetric optical access network that can be gradually converted to symmetric by the replacement of as few equipment as possible.


Within the context of the above discussion we propose a multi-gigabit optical access network that offers asymmetric bandwidth in the upstream and downstream direction. The access network is based on passive optical network (PON) technology for the implementation of the fiber to the curb access part, comprising the Central Office (CO) that generates the optical signals, Remote Nodes (RNs) that distribute the optical signals and Optical Network Units (ONUs) that interface the optical network with existing vendor equipment (i.e. DSLAMs). 

The architecture is further detailed in the following figure for the downstream and upstream directions.

  • Downstream Direction: Downstream traffic is transported on optical wavelengths generated at the CO. Each RN isolates a single wavelength (by means of a passive wavelength demultiplexer) and feed it to the ONUs that belong to its network tree. The ONUs demultiplex the optical signal and distribute the traffic to the connected DSLAMs. The ONU-DSLAM interface will be one of the research areas for this project.
  • Upstream Direction: Upstream traffic is transported on a wavelength that is common to all optical devices. The upstream is slotted (could be TDMA, Ethernet or ATM): the ONUs multiplex the DSLAM traffic and transmit it whenever they have available timeslots to the upstream wavelength. This means that an arbitration mechanism must be used to avoid collisions. No complex amplification schemes are incorporated in the upstream, thus a burst mode receiver is required at the CO to account for variations in the optical power between timeslots.

The scientific and technological objectives of this project relate to all aspects of the proposed access architecture. 

In more detail, the project will address topics in the following areas:

Network Architecture

  • Design an xDSL compatible optical access architecture with 10 a Gbps backbone capacity. 
  • Study the proposed architecture in terms of the maximum number of end users it can support.
  • Study the proposed architecture in terms of the geographic coverage it may achieve.
  • Study the suitability of the architecture for FTTH purposes.

Network Subsystems

  • Provide specifications for and design the ONU/DSLAM interface.
  • Perform laboratory demonstration of the ONU/DSLAM interface and a field-trial involving multimedia delivery.
  • Design and experimentally demonstrate an 10 Gbps all-optical TDMA demultiplexer (ONU receiver subsystem).
  • Design and experimentally demonstrate an 10 Gbps all-optical power equalizer (CO receiver subsystem).

Link Layer Protocols and Multiple Access Techniques

  • Determine suitable multiple access schemes (TDMA/WDM, FDMA/WDM) and evaluate their feasibility and cost-effectiveness.
  • Develop fair and efficient MAC protocols for the downstream and the upstream.
  • Develop traffic grooming and traffic prediction algorithms that impose minimal jitter in the upstream data.

Physical Layer

  • Determine suitable modulation formats and evaluate their performance.
  • Provide specifications for the optical subsystems of the access network (CO, ONUs).
  • Design the optical subsystems of the access network (CO, ONUs).