optical networks · optical networks ayşegül gençata, İtÜ, dept. computer engineering 3 the...

A. GençataİTÜ, Dept. Computer Engineering

2005

OPTICAL NETWORKS

Optical Access Networks

Optical NetworksAyşegül Gençata, İTÜ, Dept. Computer Engineering

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OutlineBroadband access network architectures employing Passive Optical Networks (PONs). The potential of PONs to deliver high bandwidths to users in access networks and their advantages over current access technologies have been widely recognized. PONs have made strong progress in terms of standardization and deployment over the past few years.The Ethernet PON (EPON).The technologies available for introducing wavelength-division multiplexing (WDM) in PONs.


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The “First Mile”The access network, also known as the “first-mile”network, connects the service provider central offices (COs) to businesses and residential subscribers.Also referred to in the literature as:

the “last-mile” network.the subscriber access network, or the local loop.

Subscribers demand first mile access solutions that havehigh bandwidth, offer media-rich Internet services, comparable in price with existing networks.

Similarly, corporate users demand broadband infrastructure through which they can connect their local-area networks to the Internet backbone.


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ChallengesBackbone network operators currently provide high-capacity OC-192 (10 Gbps) links. However, current-generation access network technologies such as Digital Subscriber Loop (DSL) provide 1.5 Mbps of downstream bandwidth and 128 kbps of upstream bandwidth at best. The access network is truly the bottleneck for providing broadband services such as video-on-demand, interactive games, video conference, etc., to end users.Broadband access solutions deployed today are the Digital Subscriber Line (DSL) and Community Antenna Television (CATV) (cable TV) based networks. These technologies have limitations because they were originallybuilt for voice and analog TV signals, respectively.Their retrofitted versions to carry data are not optimal.


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ChallengesDSL has a limitation on distance: Any DSL subscriber to a central office must be at a distance less than 6000 m because of signal distortions.Typically, DSL providers do not provide services to distances more than 4000 m.Variations of DSL such as very high bit-rate DSL (VDSL) can support up to 50 Mbps of downstream bandwidth. These technologies have much more severe distance limitations.The next wave of access networks promises to bring fiber closer to the home.

The FTTx model – Fiber-to-the-Home (FTTH), Fiber-to-the-Curb (FTTC), Fiber-to-the-Building (FTTB), etc.

FTTx aim at providing fiber directly to the home, or very near the home, from where technologies such as VDSL can take over.FTTx solutions are mainly based on the Passive Optical Network (PON).


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Comparison Between Access Technologies


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Next-generation Access NetworksOptical fiber is capable of delivering bandwidth-intensive, integrated, voice, data and video services at distances of 20 kilometers or beyond in the subscriber access network. A logical way to deploy optical fiber in the local access network is to use a point-to-point (PtP) topology.

Dedicated fiber runs from the CO to each end-user subscriber. A simple architecture. Requires significant outside fiber deployment as well as connector termination space in the Central Office (CO).N subscribers at an average distance L km from the central office requires 2N transceivers and NxL total fiber length (assuming that a single fiber is used for bi-directional transmission).


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FTTH Deployment Scenarios


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FTTH Deployment ScenariosTo reduce fiber deployment, it is possible to deploy a remote switch (concentrator) close to the neighborhood. This will reduce the fiber consumption to only L km (assuming negligible distance between the switch and customers). It will actually increase the number of transceivers to2N+2. The curb-switch requires electrical power as well as back-up power. Currently, one of the highest costs for Local Exchange Carriers (LECs) is providing and maintaining electrical power in the local loop.


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FTTH Deployment ScenariosIt is logical to replace the hardened (environmentally protected) active curb-side switch with an inexpensive passive optical splitter.Passive Optical Network (PON) is a technology viewed by many as an attractive solution to the first-mile problem.A PON minimizes the number of optical transceivers, CO terminations, and fiber deployment.A PON is a point-to-multipoint (PtMP) optical network with no active elements in the signal path from source to destination. The only interior elements used in a PON are passive optical components, such as optical fiber and splitters. An access network based on a single-fiber PON requires N + 1 transceivers and L km of fiber.


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PON TechnologiesOptical Splitters/Couplers

A PON employs a passive (not requiring any power) device to:split an optical signal (power) from one fiber into several fibers, andto combine optical signals from multiple fibers into one.

This device is an optical coupler. In its simplest form, an optical coupler consists of two fibers fused together. Signal power received on any input port is split between both output ports. The splitting ratio of a splitter is a constant parameter. N × N couplers are manufactured by staggering multiple 2x2 couplers.


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PON TopologiesMultiple topologies are suitable for access network, including tree, tree-and-branch, ring, or bus.Using 1:2 optical tap couplers and 1:N optical splitters, PONs can be flexibly deployed in any of these topologies. In addition, PONs can be deployed in redundant configurations such as double rings or double trees.Redundancy may be added to only a part of the PON, say the trunk of the tree.


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PON Topologies


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PON TopologiesAll transmissions in a PON are performed between an Optical Line Terminal (OLT) and Optical Network Units (ONUs). The OLT resides in the CO and connects the optical access network to the metropolitan area network (MAN) or wide-area network (WAN).The ONU is located either:

at the end-user location (FTTH and FTTB), or at the curb, resulting in fiber-to-the-curb (FTTC) architecture.


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Burst-mode TransceiversDue to unequal distances between the CO and the ONUs, optical signal attenuation in the PON may not be the same for each ONU. Thus, the power level received at the OLT may be different for different ONUs.If the receiver at the OLT is adjusted to receive high-power signal from a close ONU, it may mistakenly read ones as zeros in a weaksignal from a distant ONU. In the opposite case, if the receiver is trained on a weak signal, it may read zeros as ones when receiving a strong signal.To properly detect the incoming bit stream, the OLT receiver must be able to quickly adjust its zero-one threshold at the beginning of each received timeslot, i.e., it should operate in burst mode. A burst-mode receiver is necessary only in the OLT. The ONUs read a continuous bit stream (data or idle bits) sent by the OLT and do not need to re-adjust quickly.


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Ethernet PON (EPON) Access NetworksEthernet PON (EPON) carries data traffic encapsulated in Ethernet frames (defined in the IEEE 802.3 standard). Standard 8b/10b line coding

8 user bits are encoded as 10 line bits.Operates at standard Ethernet data rates.The first-generation PON standardized by ITU–T G.983 employed Asynchronous Transfer Mode (ATM) as the medium-access control (MAC) protocol.When its standardization effort was started in 1995, the telecom community believed that ATM would be the prevalent technology in backbone networks.However, since then, Ethernet has grown vastly popular.

Cheap line cardsWidely deployed in LANs today.


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EPONSince access networks are focused towards end users and LANs, ATM has turned out to be not the best choice to connect to Ethernet-based LANs. High-speed Gigabit Ethernet deployment is widely accelerating and 10-Gigabit Ethernet products are becoming available. Ethernet is a much more efficient MAC protocol to use compared to ATM.

Considerable amount of overhead introduced by ATM on variable-length Internet Protocol (IP) packets.Newly-adopted quality-of-service (QoS) techniques have made Ethernet networks capable of efficiently supporting voice, data,and video.


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EPON: Principle of OperationIn the downstream direction (OLT to ONUs):Ethernet frames transmitted by the OLT pass through a 1:N passive splitter and reach each ONU. Typical values of N are between 8 and 32. EPON operation is broadcast in the downstream direction.Packets are broadcast by the OLT and extracted by their destination ONU based on a Logical Link Identifier (LLID).LLID is assigned to the ONU when it registers with the network.


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EPON: Downstream Operation


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EPON: Principle of OperationIn the upstream direction, data frames from any ONU will only reach the OLT and will not reach any other ONU due to the directional properties of the passive optical combiner. In the upstream direction, the behavior of EPON is similar to that of a point-to-point architecture. However, data frames from different ONUs transmitted simultaneously may collide. Thus, in the upstream direction, the ONUs need to employ some arbitration mechanism to avoid data collisions. A contention-based media-access mechanism (similar to CSMA/CD)) is difficult to implement because ONUs cannot detect a collision in the fiber.An OLT could detect a collision and inform ONUs by sending a jam signal; however, the relatively large propagation delay in a PON (20 km) reduces the efficiency of such a scheme. To introduce determinism in frame delivery in the upstream direction, different non-contention schemes have been proposed.


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EPON: Upstream Operation


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Timeslot AssignmentAll ONUs are synchronized to a common time reference, and each ONU is allocated a timeslot in which to transmit. Each timeslot is capable of carrying several Ethernet frames. An ONU should buffer frames received from a subscriber until its timeslot arrives.When its timeslot arrives, the ONU bursts all stored frames at full channel speed. If there are no frames in the buffer to fill the entire timeslot, an idle pattern is transmitted.Thus timeslot assignment is a very crucial step.


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Timeslot AssignmentThe possible timeslot allocation schemes:

static allocation (fixed time-division multiple access (TDMA)) dynamically adapting scheme based on queue size in every ONU (statistical multiplexing scheme).

In the dynamic scheme, the OLT collects the queue sizes from the ONUs and issues timeslots.

Leads to significant signaling overhead between the OLT and the ONUs.The centralized intelligence may lead to more efficient use of bandwidth.

More advanced bandwidth-allocation schemes are also possible utilizing:

traffic priority, Quality-of-Service (QoS), Service-Level Agreements (SLAs), over-subscription ratios, etc.


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Multi-Point Control Protocol (MPCP)A supporting protocol to facilitate a dynamic timeslot allocation scheme. It has been standardized in the IEEE 802.3ah. Aims to define a signaling protocol between the OLT and the ONUs.Does not define any bandwidth provisioning scheme. MPCP consists of three functions.

Discovery Processing: An ONU is discovered and registered in the network while compensating for the round-trip time (RTT).Report Handling: ONUs generate REPORT messages through which bandwidth requirements are transmitted to the OLT. The OLT needs to process the REPORT messages to make bandwidth assignments.Gate Handling: Used by the OLT to grant a timeslot at which the ONU can start transmitting data. Timeslots are computed at the OLT while making bandwidth allocation.


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Discovery ProcessingDiscovery is the process in which newly-connected or offline ONUs register in the network. The steps:

1. OLT: The OLT periodically makes available a discovery time window during which the offline ONUs are given the oppurtunityto register themselves with the OLT. A DISCOVERY-GATE message is broadcast to all ONUscontaining the starting and the ending time of the discovery window.2. ONU: Any offline ONU, which wishes to register, waits for a random amount of time within the discovery window, and then transmits a REGISTER REQ message. The REGISTER REQ message contains the ONU’s MAC address. The random wait is required to reduce the probability of REGISTER REQ messages transmitted by multiple ONUs from colliding.


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Discovery Processing3. OLT: The OLT, after receiving a valid REGISTER REQ message register the ONU and allocates to it a Logical Link Identifier (LLID). The OLT transmits a REGISTER message to the newly-discovered ONU which contains the ONU’sLLID.4. OLT: The OLT transmits a standard GATE message, indicating a timeslot to transmit data.5. ONU: Receiving the GATE message, the ONU responds with a REGISTER ACK message in the assigned timeslot. Upon receipt of the REGISTER ACK, the discovery process is complete and now normal operation may start.


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Discovery Processing


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Report HandlingREPORT messages are sent by ONUs in their assigned transmission windows along with data frames. Typically, REPORT would contain the desired size of the next timeslot, based on ONU’s queue size. REPORT messages are generated periodically, even when no request for bandwidth is being made. This prevents the OLT from deregistering the ONU.Thus, for the proper operation of this mechanism, the OLT must grant the ONU a transmission window periodically. At the OLT, the REPORT is processed, and the data is used for the next round of bandwidth assignments.


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Gate HandlingThe transmitting window of an ONU is indicated in the GATE message from the OLT. The transmission start and transmission length times are specified. Upon receiving a GATE message matching the ONU’s LLID, the ONU will program its local registers with the transmission start and transmission length times.The ONU will also verify that the time the GATE message arrived is close to the timestamp value contained within the message.

If the difference in values exceeds some predefined threshold:The ONU will assume that it has lost its synchronization.It will switch itself into offline mode. The ONU will then attempt to register again using the next discovery process.

When the time at the local clock of the ONU reaches transmission start, the ONU starts transmitting data.


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Clock SynchronizationThe correct operation of MPCP depends on clock synchronization between the OLT and the ONU, which compensates for the RTT.Whenever the ONU receives a MPCP message, it sets its local time from the time-stamp of that message. When the OLT receives a MPCP message, it calculates the RTT as the difference between its local time and time-stamp of the message.


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Dynamic Bandwidth Allocation Algorithms in EPON

In the upstream direction (from ONU to OLT), the ONUs must share the channel capacity.Since the ONUs cannot communicate with one another, the OLT must assign timeslots in which the ONUs are allowed to transmit data. One method is to assign static timeslots for each ONU (TDMA).

cost-effective solution, since the OLT no longer has to poll the ONUs and schedule the timeslots.Therefore, avoids the need for REPORT messages in the MPCP protocol altogether. However, it lacks statistical multiplexing.


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DBA in EPONThe network traffic is bursty:

some timeslots overflow even under very light load. packets are delayed for several timeslot periods, while a large number of slots remain underutilized.

Hence, Dynamic Bandwidth Allocation (DBA) algorithms are needed.

OLT schedules the timeslots in which the ONUs may transmit. One of the first protocols proposed was the Interleaved Polling with Adapative Cycle Time (IPACT).


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IPACTThe OLT keeps track of the earliest scheduling time by a variable Tschedule. Thus, Tschedule is changed after each allocated timeslot.Whenever a REPORT message containing the requested timeslot from the ONU arrives at the OLT, the DBA agent at the OLT calculates the start time for the next transmission timeslot for that ONU.

To maintain high utilization in the upstream channel, the DBA agent allocates the next timeslot immediately adjacent to the already allocated timeslot with only a guard time interval separation.

Tstart = Tschedule + Tguard


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IPACTMaximum Scheduling Timeslot: If the OLT authorizes each ONU to send its entire buffer contents in one transmission, ONUs with high data volume could monopolize the entire bandwidth, and the average delay in the network could become very large. To avoid this situation, the OLT must limit the maximum transmission size. We define this as a limited-service scheme:

every ONU is allocated a timeslot to send as many bytes as it has requested, but no more than some upper limit which is defined as the Maximum Scheduling Timeslot.


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IPACTNext, the corresponding GATE message is transmitted by the OLT. Tschedule is modified as:

Tschedule = Tstart + length


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Service Disciplines in IPACTFixed service ignores the requested timeslot size and always grants a fixed timeslot, thus corresponding to synchronous TDMA. It has a constant cycle time.Limited service grants the requested timeslot size, but no more than the Maximum Scheduling Timeslot WMAX.Gated service does not impose the Maximum Scheduling Timeslot. Thus, the DBA agent allocates as much timeslot as is requested by the ONU.Constant-Credit service adds a constant credit to the requested timeslot size.Linear-Credit service uses a similar approach as the Constant-Credit service scheme. However, the size of the credit is proportional to the requested window.Elastic service: The maximum window is granted in such a way that the accumulated size of last N grants (including the one being granted) does not exceed N x WMAX bytes.


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Simulation ResultsRD Mbps the data rate of the access link from a user to an ONU.RU Mbps the rate of the upstream link from an ONU to the OLT.A system with N = 16RD = 100 MbpsRU = 1000 Mbps.Every ONU has a finite memory bufferSynthetic traffic traces that exhibit the properties of self-similarity and long-range dependence.


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Average packet delay for different service schemes


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Other Types of PONAPON/BPON

ATM PON (APON) is based on Asynchronous Transfer Mode (ATM) as the MAC layer protocol.The downstream frame consists of 56 ATM cells (53 bytes each) for the basic rate of 155 Mbps, scaling up to 224 cells for 622 Mbps.Initial work on ATM PONs was launched in the mid 1990s by the Full Service Access Network (FSAN) initiative which was started by service providers. Because the name APON led users to believe that only ATM-based services could be supported, the terminology was changed to Broadband PON (BPON). BPON has been standardized by the International Telecommunication Union (ITU) specification G.983.1.


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Other Types of PONGeneralized Framing Procedure PON (GFP-PON)

The GFP-PON is being standardized by the ITU in specification G.984.x.It proposes bit rates of up to 2.5 Gbps. It aims towards providing higher efficiency while carrying multiple services over the PON. It proposes a protocol using Generic Framing Procedure (GFP). Other functionalities such as dynamic bandwidth assignment, operation and maintenance, etc. are borrowed from APON.

Both APON and GFP-PON have the disadvantage of a complex protocol and implementation, relative to EPON.So, they have not gained much technical popularity amongst users and equipment vendors.


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WDM-PONAlthough the PON is a significant step towards providing broadband access to the end user, it is not very scalable. The basic form of PON employs only a single optical channel.The available bandwidth is limited to the maximum bit rate of an optical transceiver: under current technologies 1 Gbps. The attenuation due to splitting limits the maximum number of ONUs to 64. This limits the network’s scalability. The deployment cost of laying fiber in the access network is high, so, it is important to consider technologies which may help scale the PON capacity in future.


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WDM-PONThere will be a need for further increasing the bandwidth of the PON by employing Wavelength-Division Multiplexing (WDM).Multiple wavelengths may be supported in both upstream and downstream directions. Such a PON is known as a WDM-PON. WDM-PON is a point-to-point access network

Separate wavelength, between the OLT and each ONU.Each wavelength is routed by an Arrayed Waveguide Grating (AWG).Different ONUs can be supported at different bit rates. Each ONU can operate at the full bit rate of a wavelength channel.There is no sharing of the channel with any other ONU. The WDM-PON does not suffer power-splitting losses. Better privacy, less security concerns.


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Arrayed Waveguide Grating (AWG)The AWG is a passive device with a fixed routing matrix. It provides a fixed routing of an optical signal from a given input port to a given output port, based on the wavelength of the signal. Signals of different wavelengths coming into an input port will be routed to a different output port.


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Example: LARNETTDMA is used to share the upstream channel.

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