One of the major issues in the networking industry today is tremendous demand for more and more bandwidth. However, with the development of optical networks and the use of Dense Wavelength Division Multiplexing (DWDM) technology, a new and probably, a very crucial milestone is being reached in network evolution. The existing SONET/SDH network architecture is best suited for voice traffic rather than today's high-speed data traffic. To upgrade the system to handle this kind of traffic is very expensive and hence the need for the development of an intelligent all-optical network. Such a network will bring intelligence and scalability to the optical domain by combining the intelligence and functional capability of SONET/SDH, the tremendous bandwidth of DWDM and innovative networking software to spawn a variety of optical transport, switching and management related products. This paper deals with optical network architecture and explains virtual topology along with optical layer and higher layer interface.


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Introduction


The most used network architecture is the client-server architecture. In a client-server
architecture the server passively waits for a request, until the client actively sends a
request to the server. The server then executes the request and sends the reply back to
the client.
One of the rst computer networks were isolated local area networks (LANs), with
a client-server architecture. The clients were cheap terminals, attached to a screen and
a keyboard. At the time, the clients required low network bandwidth. The only data
transmitted was the keyboard activity sent to the server, and the screen updates sent
back to the client.
The terminals used in these networks are classied as thin clients. This is because
most of the processing is done at the server, while the client typically process keyboard
input and screen output.
Some advantages with the thin client approach are:
• A lower hardware costs, as there is usually no need for disk, a lot of memory, or
a powerful processor. This also creates a longer turnover time, because it takes
a longer period of time before the equipment becomes obsolete.
• A lower administration cost, as the clients are almost completely managed from
the server. All installations and upgrades are done on the servers, and not on
each client.
• A higher client reliability, as the client hardware has less points of failure.
• Increased security, as no sensitive data ever resides on the client. The local
environment is usually highly restricted, and the protection against malware is
centralized on the servers.
The need to connection to other networks or clients from the existing network, created
the next step for computer networks. The connection between the networks was
typically created by leased lines or by dial-up connections. The new networks were
called metropolitan area networks (MAN) or wide area networks (WAN) depending on
the range of the networks. With the creation of these new networks, terminals could
now connect to other servers in other networks, and process data in other computer
environments.

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Presented by:M.VARUN

P O N : Passive Optical Network

INTRODUCTION
The PON is an access network based on Optical Fibre. It is designed to provide virtually unlimited bandwidth to the subscriber. A passive Optical network is a single, shared optical fibre that uses a passive optical splitter to divide the signal towards individual subscribers.
A typical Passive Optical Network

PON is called passive because other than at the central office there are no active element within the access network. A PON enables an service provider to deliver a true triple play offering of voice, video and data, an important component of the data offering can be IPTV.PON are getting more widespread in rollout of Fibre To The Home(FTTH) infrastructure.
A passive optical network (PON) is a system that brings optical fiber cabling and signals all or most of the way to the end user. Depending on where the PON terminates, the system can be described as fiber-to-the-curb (FTTC), fiber-to-the-building (FTTB), or fiber-to-the-home (FTTH).
A PON consists of an Optical Line Termination (OLT) at the communication company's office and a number of Optical Network Units (ONUs) near end users. Typically, up to 32 ONUs can be connected to an OLT. The passive simply describes the fact that optical transmission has no power requirements or active electronic parts once the signal is going through the network.
All PON systems have essentially the same theoretical capacity at the optical level. The limits on upstream and downstream bandwidth are set by the electrical overlay, the protocol used to allocate the capacity and manage the connection. The first PON systems that achieved significant commercial deployment had an electrical layer built on Asynchronous Transfer Mode (ATM, or "cell switching") and were called "APON." These are still being used today, although the term "broadband PON" or BPON is now applied. APON/BPON systems typically have downstream capacity of 155 Mbps or 622 Mbps, with the latter now the most common. Upstream transmission is in the form of cell bursts at 155 Mbps.
Multiple users of a PON could be allocated portions of this bandwidth. A PON could also serve as a trunk between a larger system, such as a CATV system, and a neighborhood, building, or home Ethernet network on coaxial cable.
The successor to APON/BPON is GPON, which has a variety of speed options ranging from 622 Mbps symmetrical (the same upstream/downstream capacity) to 2.5 Gbps downstream and 1.25 Gbps upstream. GPON is also based on ATM transport. GPON is the type of PON most widely deployed in today's fiber-to-the-home (FTTH) networks in new installations and is generally considered suitable for consumer broadband services for the next five to 10 years. From GPON, the future could take two branches: 1) 10 GPON would increase the speed of a single electrical broadband feed to 10G; and 2) WDM-PON would use wavelength-division multiplexing (WDM) to split each signal into 32 branches.
A rival activity to GPON is Ethernet PON (EPON), which uses Ethernet packets instead of ATM cells. EPON should be cheaper to deploy, according to supporters, but it has not garnered the level of acceptance of GPON, so it is not clear how EPON will figure in the future of broadband access.

Motivation:
The growing popularity of the Internet, IPTV, Video On Demand, Video Conferencing, Gaming are the key factors behind the development of new access method which would meet the bandwidth requirement. Access network based on copper has distance and bandwidth limitation and will start running out of capacity in near future. The access methods based on the optical fibre are getting more and more attention as they offer the ultimate solution in delivering different services to the customer premises. Due to the lack of active units in the light path the architecture of PON is simple, cost effective and offered bandwidth that is not possible to achieve by other access methods.

PON Services: PON enable users with the following services:
Digital Entertainment:
(i) IPTV
(ii) Video on Demand
(iii) Video Telephony
(iv) Audio on Demand
(v) Gaming, etc.
Broadband Data services:
(i) High Speed Internet access with bandwidth 256 kbps-100 Mbps
(ii) VoIP ( Voice Over IP)Telephony
The PON, or passive optical network, is a network structure that carries optical fiber cabling and the resulting signaling to within a short distance of the point of termination. Rather than relying on a network composed of multiple switching interfaces to carry the signal, the passive optical network employs a structure that may require not more than a couple of switch points. There may be one switch to allow a sender to jump onto the network and another switch to allow the signal to jump off the network just prior to reaching an end user. A passive optical network may be structured in several different configurations, depending on the individual application and system limitations.
There are essentially three different passive optical network configurations in common use to day. One configuration is referred to as a fiber-to-the-curb or FTTC. This application takes the signaling to a facility that is literally located outside the business or home. A connection from the end-user is run to interact with the switch and thus allow the signal to complete the journey.
A fiber-to-the-building or FTTB moves one step further in the process of terminating the signal than the FTTC manages. With this application of a passive optical network, the signal remains on the network until it enters the building. It is at that point that the signal comes off the originating network and terminates on a local area network that is connected with one or more of the businesses physically located in the building.
A third type of passive optical network is the fiber-to-the-home or FTTH. This type of network connectivity carries the signal directly into a switch co-located with the home and requires nothing more for termination than a connection to a device that is capable of receiving the transmission.
The “passive” is a passive optical network calls attention to the fact that the signaling process does not require any additional power sources in order to keep moving to the point of termination. The signal is simply passing through the network and will follow a logical flow until it reaches the end user.
When employed in a stand-alone system, a passive optical network allows for easy transmission of date to various points along the network. Each end user is allocated a fixed amount of bandwidth for both sending and receiving data. An administrator can make adjustments in the allowed amount of bandwidth, based on the total capacity of the network and the individual needs of each end user. In addition, the administrator can configure the network to connect with an outside system, such as a cable hookup or traditional phone line in order to allow the flow of data in and out of the network from outside users.

PON Architecture:
The elements of a PON are
(i)Optical Line Terminal(OLT)
(ii) Passive Optical Splitter and
(iii) Optical Network Unit(ONU).

The Optical Line Terminal is the main element of the network and is usually placed in the Local Exchange. It is a network element with PON line card, basically a aggregation switch. It works as an interface between core network and PON network.

Optical Splitter is a passive device with single input and multiple output. Optical power at input is split evenly between outputs. Not only signal travels from input to the outputs, signal can also travel from the output to the input. Splitters can be placed anywhere in between CO and Subscriber premises. It is used to connect an optical port of OLT with multiple subscribers.

Optical Network units(ONUs) serve as an interface to the network and are deployed at customer premises. It provides several interfaces for accessing triple play services and in the upper side it connects with the OLT via optical spliter.

Although PONs can exist in three basic configuration(tree, bus and ring), the tree topology is favored due to smaller variation in the signal power from different end station. PON uses 1490 nm for the downstream wavelength and 1310 nm for the upstream wavelength. Signals are inserted or extracted from the fibre using a coarse wavelength division multiplexer (CWDM) filter at the CO and subscriber premises.

In the other direction, from ONU’s to the OLT, the signals from different ONUs arrive at inputs of the spliter. Although the signals can not reach different ONUs, as they traverse through the splitter they get mixed with each other and the superposition of all signals is received at the OLT. Hence in the upstream direction the TDMA method is used to avoid the interference of signals from different ONUs.
Early work on efficient fiber to the home architectures was done in the 1990s by the Full Service Access Network (FSAN) working group, formed by major telecommunications service providers and system vendors. The International Telecommunications Union (ITU) did further work, and has since standardized on two generations of PON. The older ITU-T G.983 standard is based on Asynchronous Transfer Mode (ATM), and has therefore been referred to as APON (ATM PON). Further improvements to the original APON standard – as well as the gradual falling out of favor of ATM as a protocol – led to the full, final version of ITU-T G.983 being referred to more often as broadband PON, or BPON. A typical APON/BPON provides 622 megabits per second (Mbit/s) (OC-12) of downstream bandwidth and 155 Mbit/s (OC-3) of upstream traffic, although the standard accommodates higher rates.
The ITU-T G.984 (GPON) standard represents a boost, compared to BPON, in both the total bandwidth and bandwidth efficiency through the use of larger, variable-length packets. Again, the standards permit several choices of bit rate, but the industry has converged on 2.488 gigabits per second (Gbit/s) of downstream bandwidth, and 1.244 Gbit/s of upstream bandwidth. GPON Encapsulation Method (GEM) allows very efficient packaging of user traffic with frame segmentation.
The IEEE 802.3 Ethernet PON (EPON or GEPON) standard was completed in 2004 (http://ieee8023/), as part of the Ethernet First Mile project. EPON uses standard 802.3 Ethernet frames with symmetric 1 gigabit per second upstream and downstream rates. EPON is applicable for data-centric networks, as well as full-service voice, data and video networks. 10Gbit/s EPON or 10G-EPON was ratified as an amendment IEEE 802.3av to IEEE 802.3. 10G-EPON supports 10Gbps/1Gbps. The downstream wavelength plan support simultaneous operation of 10Gbps on one wavelength and 1Gbps on a separate wavelength for operation of IEEE 802.3av and IEEE 802.3ah on the same PON concurrently. The upstream channel can support simultaneous operation of IEEE 802.3av and 1Gbps 802.3ah simultaneously on a single shared (1310nm) channel.
A PON takes advantage of wavelength division multiplexing (WDM), using one wavelength for downstream traffic and another for upstream traffic on a single [Non-dispersion shifted fiber] (ITU-T G.652). BPON, EPON, GEPON, and GPON have the same basic wavelength plan and use the 1490 nanometer (nm) wavelength for downstream traffic and 1310 nm wavelength for upstream traffic. 1550 nm is reserved for optional overlay services, typically RF (analog) video.
As with bit rate, the standards describe several optical budgets, most common is 28 dB of loss budget for both BPON and GPON, but products have been announced using less expensive optics as well. 28 dB corresponds to about 20 km with a 32-way split. Forward error correction (FEC) may provide another 2-3 dB of loss budget on GPON systems. As optics improve, the 28 dB budget will likely increase. Although both the GPON and EPON protocols permit large split ratios (up to 128 subscribers for GPON, up to 32,768 for EPON), in practice most PONs are deployed with a split ratio of 1x32 or smaller.
A PON consists of a central office node, called an optical line terminal (OLT), one or more user nodes, called optical network units (ONUs) or optical network terminals (ONTs), and the fibers and splitters between them, called the optical distribution network (ODN). ONT is an ITU-T term, whereas ONU is an IEEE term. In Multiple Tenant Units, the ONT may be bridged to a customer premise device within the individual dwelling unit using technologies such as Ethernet over twisted pair, G.hn (a high-speed ITU-T standard that can operate over any existing home wiring - power lines, phone lines and coaxial cables) or DSL. An ONT is a device that terminates the PON and presents customer service interfaces to the user. Some ONUs implement a separate subscriber unit to provide services such as telephony, Ethernet data, or video.

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Thank u

[attachment=13877]
OPTICAL NETWORK ARCHITECTURE
Abstract
One of the major issues in the networking industry today is tremendous demand for more and more bandwidth. However, with the development of optical networks and the use of Dense Wavelength Division Multiplexing (DWDM) technology, a new and probably, a very crucial milestone is being reached in network evolution. The existing SONET/SDH network architecture is best suited for voice traffic rather than today’s high-speed data traffic. To upgrade the system to handle this kind of traffic is very expensive and hence the need for the development of an intelligent all-optical network. Such a network will bring intelligence and scalability to the optical domain by combining the intelligence and functional capability of SONET/SDH, the tremendous bandwidth of DWDM and innovative networking software to spawn a variety of optical transport, switching and management related products. This paper deals with optical network architecture and explains virtual topology along with optical layer and higher layer interface.
1.Introduction
Just like every other layer defined in networking, a layer architecture has to be defined for the optical layer. A multi-wavelength mesh-connected optical network is used to define the architecture of the optic layer. A lightpath is defined as the path between two nodes and is equivalent to a wavelength on each page link on that path. Two aspects of the network topology have been described : physical topology and virtual topology. The physical topology has WDM cross-connect nodes interconnected by pairs of point-to-point fiber links in an arbitrary mesh topology as shown in the following figure.
Fig. A WDM network consisting of crossconnect nodes interconnected by pairs of point-to-point fiber optic links(i.e physcial topology)
2. Virtual Topology
As shown in the below figure, the virtual topology of a network is the set of all lightpaths. It is a logical topology and the direction of the arrows actually show the direction of the lightpaths.
Fig. The virtual topology of the WDM network of previous figure
An optical layer, also known sometimes as Layer 1, is the layer between the physical layer and the datalink layer or other higher layers for that matter. By using this additional layer in say an ATM network, we would be removing the need of having to convert to optical signal to electric signals and to cells before switching them through ATM switches, if we have wavelength switches in the network. ITU-T SG 15 has defined the optical layer itself as consisting of layers. The three sublayers of the optical layer are:
(i) Optical Channel(OCh) layer: This corresponds to light paths
(ii) Optical Multiplex Section(OMSn) layer: This corresponds to links
(iii) Optical Amplifier Section(OASn) layer : This corresponds to page link segments between optical amplifiers.
3. Optical Layer and Higher Layer Interface
Just like every other layer-layer communication, optical layer communicates with the higher layers, both above and below in the protocol stack by means of Service Data Units (SDUs). Besides these guarantees, SDUs also have to be defined to allow for proper exchange between higher layers and the optical layer (in both directions).There are certain services that this new layer must provide to the higher layers.
1 Addressing :
It is obvious that there must be some mechanism for the higher layers to ask for lightpaths from particular nodes in the network. This is done by having an addressing scheme to describe the nodes in the network.
(i) Multicast capability:
This is an optional capability depending on whether multicasting is a
feature enabled in the network.
(iii) Light tree:
A light tree is a point-to-multipoint version of a lightpath. Optical multicasting capability at routing nodes has been suggested to increase logical connectivity and thereby further reduce the hop distances that have to be traversed. Optical multicasting is better than electronic multicasting because it is easier to split an optical signal into many identical optical signals rather than copying a packet in an electronic buffer. Using optical splitters does this function of "splitting" an optical signal. An n-way optical splitter is a passive device that does the above defined "splitting" in such a way that at least one output signal has a power less than or equal to 1/n th times the input power. Optical amplifiers would be needed in the network. The suggested approach is to have a so-called splitter bank. This splitter bank will do the optical splitting and also the optical signal amplification. An interesting point is that this splitter bank could have more features such as wavelength conversion and signal regeneration for "multicast" as well as "unicast" signals in the network. Now this splitter bank is then used to construct a multicast-capable wavelength-routing switch(MWRS). The basic components of this MWRS are optical switches, splitter bank, multiplexors, demultiplexors. Information coming in through a fiber page link is first demultiplexed into separate signals (different wavelengths) and then switched by an optical switch. At this point depending on whether the signal is unicast or multicast, they are sent through different paths. The multicast signals are sent to the splitter bank and the amplified multiple identical signals are then switched by another optical switch. Finally all the signals that are to be sent out on one fiber page link are multiplexed together before being sent out.Mathematic formulation of the light-tree-based virtual topology design problem isthe next step. An optimization problem having any one of the following objective functions is possible:
(a)Minimization of the network-wide average packet hop distance.
(b)Minimization of the total number of opto-electronic components.
(iv) Uni or Bi-directional lightpaths:
When all wavelengths travel in the same direction within a fiber, those wavelengths are called unidirectional wavelengths(or lightpaths). The implication here is that another parallel fiber has to be there that supports the opposite direction lightpath When we have the whole channel split in such a way that for each lightpath in the forward direction there is another lightpath in the opposite direction within the same fiber, such lightpaths are known as bi-directional lightpaths. It is obvious that the transmission bandwidth is reduced. Which one of these lightpaths is chosen depends on the type of traffic. Since failure recovery is a very important aspect of any network, network control has been proposed as a decentralized function though for the early versions a centralized function is also acceptable. Network management criteria, interfacing between network control and network management has all been properlydefined.
4.Conclusion
In this paper we have discussed optical network was described. Virtual topology and Optical layer have been described in much detail. Issues such as Network Control and Network Management ware also discussed. Research work is also being done to try and achieve the difficult goal of a high-speed all-optical network. New concepts such as All-optical switching are coming up. 1 Tbps systems are expected in the market by early 2002/2003. Network providers will start leasing out wavelengths (or "lambdas") instead of leasing lines. Cost will be an important issue in widespread deployment of optical systems. A lot of implementation issues, the setting up of standards need to be addressed for an all-optical network to come out at a reasonable cost. How long or for that matter whether we will ever achieve an all-optical network is a moot question