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Ethernet Passive Optical Network (EPON) : Building a Next- Generation Optical Access Network


Properties of PONs PONs rely on light waves for data transfer. Only passive optical components are used such as optical fiber, splices and splitters. – PowerPoint PPT presentation

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Title: Ethernet Passive Optical Network (EPON) : Building a Next- Generation Optical Access Network

Ethernet Passive Optical Network(EPON)
Building a Next- Generation Optical Access Network
Authors Glen Kramer and Gerry
Pesavento(AllOptic Inc.).
  • COURSE CEG 790
  • Instructor Dr. Bin Wang
  • Presenter Ram Iyer

  • Introduction
  • What are Passive Optical Networks ?
  • Deployment Scenario of Next-Generation Access
  • Types of PON technologies
  • Different types of PON topologies
  • What are EPONs ?
  • How does an EPON work ?
  • Issues related to EPONs
  • Benefits of using EPONs
  • IEEE P803.3ah status
  • The market for EPONs
  • Conclusion

  • Internet has spawned genuine demand for broadband
    services, leading to unprecedented growth in
    Internet Protocol (IP) data traffic. This
    humongous data traffic is putting pressure on
    carriers to upgrade their networks.
  • An improvement over 56 kb/s is unable to provide
    enough bandwidth for emerging services such as
    the IP telephony, Video on Demand (VoD),
    interactive gaming, or two-way video

Per-user bandwidth requirements for new services
kept increasing as shown
A new technology is required which would be able
to handle the bandwidth hungry services.
What is a Passive Optical Network (PON) ?
  • Passive Optical Network (PON) is a high bandwidth
    Point-to-Multipoint (P2MP) optical fiber network
    based on the Asynchronous Transfer Mode protocol
    (ATM), Ethernet or TDM.
  • Components used in Passive Optical Network
  • PONs generally consist of an OLT (Optical Line
    Termination), which is connected to ONUs
    (Optical Network Units). OLT and ONUs are
    explained in the later slides of the presentation.

Properties of PONs
  • PONs rely on light waves for data transfer.
  • Only passive optical components are used such as
    optical fiber, splices and splitters.
  • PONs minimizes the fiber deployment in both the
    local exchange office and the local loop.
  • PONs provides higher bandwidth due to deeper
    fiber penetration, offering gigabit per second

Range of operation of PONs
PONs aim to break the First Mile (once called as
Last Mile) bandwidth bottleneck by targeting the
sweet spot between T1s and OC-3s that other
access network technologies do not adequately
address. PONs are capable of delivering high
volumes of upstream and downstream bandwidth (up
to 622 Mbps downstream and 155 Mbps upstream).
Deployment scenario of Next-Generation Access
  • A logical way to deploy optical fiber in the
    local access network is using a point-to-point
    (P2P) topology, with dedicated fiber which runs
    from the local access network to each end-user
    subscriber (Figure a)

Deployment of Next-Generation Access Network
  • Second method is to deploy a remote switch
    (concentrator) close to neighborhood since it
    reduces the fiber deployment as shown in (Figure
    b). The main downside of this curb switch
    architecture is it requires electrical power as
    well as the backup power at the curb unit and
    currently, one of the highest cost for local
    exchange carriers is providing and maintaining
    electrical power in the local loop.

Deployment of Next-Generation Access Network
  • In the third we can see that a PON actually
    minimizes the amount of optical transceivers,
    central office terminations, and the fiber
    deployment. As stated earlier a PON is a
    point-to-multipoint (P2MP) optical network with
    no active elements in the signals path from the
    source to destination. PONs basically use passive
    optical components, such as optical fiber,
    splices, and splitters. This is show in the
    Figure c.

Decrease in the number of Fibers and Transceivers
Point to point network - Number of Fiber
- 32
Number of Transceivers - 64
Curb-switched network - Number of Fibers
- 1
Number of Transceivers - 66
Passive Optical Network - Number of Fiber
- 1
Number of Transceivers - 33
Types of PON technologies
Asynchronous transfer mode PONs (APONs)
Ethernet PONs (EPONs)
Passive Optical Networks
  • APONs
  • Data is transmitted in fixed length 53-byte cells
    as specified by ATM protocol.
  • APONs dont deliver data, video and voice over a
    single platform.
  • APONs offer insufficient bandwidth
  • APONs are expensive
  • APONs do not provide broader service capabilities
  • EPONs
  • Data is transmitted in variable-length packets of
    up to 1,518 bytes according to IEEE 802.3
    protocol for Ethernet.
  • EPONs deliver data, video and voice over a single
  • EPONs offer higher bandwidth
  • EPONS are less expensive than APONs
  • EPONs provide broader service capabilities

Components used in PON topologies
All transmission in a PON are performed between
an optical line terminal (OLT) and optical
network units (ONUs).
What is Optical Line Terminal (OLT) ?
An OLT resides in the local exchange (central
office), connecting the optical access network to
the metro back-bone.
What are Optical Network Units (ONUs) ?
The ONU provides the interface between the
customers data, video, and telephony networks
and the PON. Its function is to receive
traffic in a optical format and convert it into
customers desired format (Ethernet, IP
multicast, T1, etc.)
Typical PON architecture
Different types of PON topologies
  • Tree topology
  • Bus topology
  • Ring topology
  • Tree with redundant trunk

PON topologies
Figure 3
Why do we require EPONs?
  • We require EPON technology since it has the
    following qualities
  • it is inexpensive,
  • simple, scalable and
  • capable of delivering bundled voice,
  • it provides data and video services to an
    end-user subscriber over a single network.

What are EPONs?
  • Ethernet passive optical networks (EPON) are an
    emerging access network technology that provides
    a low-cost method of deploying optical access
    lines between a carrier office (CO) and customer
  • We can say that, Ethernet Passive Optical
    Networks (EPONs) represents the convergence of
    low-cost Ethernet equipment and low-cost fiber
    infrastructure, to be the best candidate for the
    Next-Generation access network.

How does an EPON work ?
  • In a EPON the process of transmitting data
    downstream from the OLT to multiple ONUs is
    fundamentally different from transmitting data
    upstream multiple ONUs to the OLT.
  • The different techniques used to accomplish the
    downstream and upstream transmission in a EPON
    are shown in Figure 4 and Figure 5.

Downstream traffic in EPON
Downstream Traffic flow in an EPON
  • Consider the downstream traffic in EPON
  • In the Figure 4, the data broadcasted downstream
    from OLT to multiple ONUs in variable-length
    packets of up to 1,518 bytes, according to IEEE
    802.3 protocol. Each packet carries a header that
    uniquely identifies it as data intended for
    ONU-1, ONU-2 or ONU-3.At the splitter the traffic
    is divided into three separate signals, each
    carrying all of the ONU specific packets. When
    the data reaches the ONU, it accepts the packets
    that are intended for it and discards the packets
    that are intended for other ONUs. For example, in
    figure 4, ONU-1 receives packets 1, 2 and 3
    however only two packets are delivered to end
    user 1.

Downstream Frame Format in an EPON
Upstream traffic in EPON
Upstream Traffic flow in a EPON
Consider the downstream traffic in EPON
  • Figure 5 shows the upstream traffic is managed
    utilizing TDM technology, in which transmission
    time slots are dedicated to ONUs. The time slots
    are synchronized so that upstream packets from
    the ONUs do not interfere with each other one the
    data is couple onto the common fiber. For
    example, ONU-1 transmits packet 1 in the first
    time slot, ONU-2 transmits packet 2 in the second
    non-overlapping time slot, and ONU-3 transmits
    packet 3 in a third non-overlapping time slot.

Upstream Frame Format in an EPON
Transceiver Issues
  • There are number of issues which have surfaced
    by the use of transceivers (A transceiver is a
    device which is capable of transmitting and
    receiving signals)
  • Due to the unequal distances between the central
    office and ONUs, optical signal attenuation in
    the PON is not same for each ONU i.e. the power
    level received at the OLT will be different for
    each ONU (this is also called as near-far

Transceiver Issues Contd.
  • As shown in the Figure below, one ONUs signal
    strength is lower at the OLT, which is most
    likely due to the longer distance.

Approaches suggested to solve the attenuation
  • There are couple of approaches which are
    suggested in this paper but they have not been
    implemented since they have their own drawbacks
  • One of the approaches suggested is
  • To allow ONUs to adjust their transmitter power
    such that power levels received by the OLT from
    all the ONUs becomes the same.
  • Drawback of this approach
  • This method is not favored by the transceiver
    designers because it makes the ONU hardware more
    complicated, requires special signaling protocol
    for feedback from the OLT and ONU and most
    importantly degrades the performance of the all
    the ONUs to that of the of the most distant unit.

Is Encryption mechanism necessary in Passive
Optical Network ?
  • Encryption mechanism is necessary since a
    malicious ONU if placed in promiscuous mode would
    be able to read all the downstream packets.

On which layer of the OSI model the encryption
must be placed?
  • If the encryption is placed in the MAC layer then
    it will encrypt the MAC frame payload only, and
    leave the headers in plain text. This method
    prevents malicious ONUs from reading the payload,
    but they may still learn other ONUs MAC address.
  • Implementing the encryption scheme on the
    physical layer would encode the entire bit
    stream, including the frame headers and CRC. In
    this scheme no information is learned by a
    malicious ONU. But the difficulty is the physical
    layer is a connectionless layer. Requiring the
    Physical layer in a OLT to apply different keys
    for different ONUs will make it connection-aware.
  • So encryption in EPON still remains an open

Benefits of Ethernet PONs
  • Higher bandwidth up to 1.25 Gbps symmetric
    Ethernet bandwidth
  • Lower Costs lower up-front capital equipment and
    ongoing operational costs
  • More revenue broad range of flexible service
    offerings means higher revenues

Higher bandwidth
  • More subscribers per PON
  • More bandwidth per subscriber
  • Higher split counts
  • Video capabilities
  • Better QoS

Lower Costs
  • Cost reduction in the case of EPONs are
    achieved by simpler architecture, more efficient
    operations, and lower maintenance needs of an
    optical IP Ethernet network.
  • Eliminate complex and expensive ATM and SONET
    elements and dramatically simplify the network
  • Long-lived passive optical components reduce
    outside plant maintenance
  • Standard Ethernet interfaces eliminate the need
    for additional DSL or cable modems
  • No electronics in outside plant reduces need for
    costly powering and right-of-way space

More Revenue
Revenue opportunities from EPONs include
  • EPONs support for legacy TDM, ATM and SONET
  • Delivery of new Gigabit Ethernet, fast Ethernet,
    IP multicast and dedicated wavelength services.
  • Provisioning of bandwidth in scalable 64 Kbps
    increments up to 1 Gbps.
  • Tailoring of services to customer needs with
    guaranteed SLAs (Service License Agreement).
  • Quick response to customer needs with flexible
    provisioning and rapid service reconfiguration.

IEEE P802.3ah status
  • The standards work for Ethernet in the local
    subscriber access network is being done in the
    IEEE P802.3ah Ethernet in the First Mile (EFM)
    Task Force.
  • In order to evolve Ethernet for local
    subscriber networks, P802.3ah is focused on four
    primary standards definitions
  • Ethernet over copper
  • Ethernet over P2P fiber
  • Ethernet over P2MP fiber
  • Operation, administration, and maintenance (OAM)

  • IEEE 802.3ah has been approved. Materials
    concerning the
  • P802.3ah standards effort and the presentation
  • can be found at
  • http//
  • http//

The Market for EPONs
  • Analysts expect the optical access market to grow
  • CIBC (Canadian Imperial Bank of Commerce)
    forecasts the market for PON access system to
    reach 1 billion by 2004 from 23 million in
  • P2P optical Ethernet offer the best possibility
    of a turnaround in the telecom sector.

  • The future of broadband access network is likely
    to be a combination of point-to-point and
    point-to-multipoint Ethernet, optimized for
    transporting IP data, as well as time critical
    voice and video.

  • Topics in Lightwave Ethernet Passive Optical
    Network (EPON) Building the Next-Generation
    Optical Access Network
  • Glen Kramer and Gerry Pesavento, Alloptic,
  • http//