Optical Networks Access Networks · Access Network Evolution 8/31/12 10 ! Fiber Access is the future-proof technology offering ultra-high bandwidth and long reach. ! Passive optical

Optical Networks Access Networks

31 August 2012, Belem, Para, Brazil

Dr. Cicek Cavdar, [email protected]

Optical Networks Lab (ONLab) Royal Institute of Technology, Stockholm, Sweden

Special thanks to Biswanath Mukherjee from UC-Davis, Aysegul Yayimli from ITU, Jiajia Chen and Lena Wosinska from KTH for the class material.

Optical Networks, Access Networks, Cicek Cavdar, KTH, Sweden 2

Outline n  Broadband access network architectures employing

Passive Optical Networks (PONs). n  The potential of PONs to deliver high bandwidths to

users in access networks and their advantages over current access technologies have been widely recognized.

n  PONs have made strong progress in terms of standardization and deployment over the past few years.

n  We will examine: ¨  The Ethernet PON (EPON). ¨  The technologies available for introducing wavelength-division

multiplexing (WDM) in PONs.


The “First Mile” n  The access network, also known as the “first-mile”

network, connects the service provider central offices (COs) to businesses and residential subscribers.

n  Also referred to in the literature as: ¨  the “last-mile” network. ¨  the subscriber access network, or ¨  the local loop.

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

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

Hierarchy of Optical Networks

4

Core (Wide-area) - 1000s of km - Mesh

Metro - 100s of km - Ring

Access - a few km - Tree, ring, bus

CO

CO

RN

CO CO

Access network

Next Generation (NG) Fibre Access Network

}  NG fiber access networks will target: }  High bandwidth

}  >1Gbps per customer }  >1000 customers per fiber feed

}  Long reach }  >100km transmission distance

5

Evolution of access bandwidth

}  Efficient and fair resource allocation for open access

}  Cost efficient fault monitoring and network recovery strategies/mechanisms

Change access networks towards open

Service Delivery Platform



Zugangsebene

MobilfunkFestnetz NextGe

neration S

ervic

e & Sy

stem

Manageme

nt (O

SS)

Transport

Aggregation

Control plane (IMS)

Application

Fixed networkVoice

Fixed network Data

Mobilradio …

... to an open, standardisedmulti-‐layerarchitectureFrom"Stovepipes" per service ...

Access

MobileFixed




Zugangsebene

MobilfunkFestnetz

Zugangsebene

MobilfunkFestnetz NextGe

neration S

ervic

e & Sy

stem

Manageme

nt (O

SS)

Transport

Aggregation

Control plane (IMS)

Application

Fixed networkVoice

Fixed network Data

Mobilradio …

Fixed networkVoice

Fixed network Data

Mobilradio …

... to an open, standardisedmulti-‐layerarchitectureFrom"Stovepipes" per service ...

Access

MobileFixed

Access

MobileFixed

}  Merge backhaul and access into a single transport solution

}  Site reduction

Intended network consolidation in UK and Germany

Traffic Demand Drivers n  User behavior

¨ Always on (reachability, upgrades and downloads) ¨ File sharing

n  Services ¨ High-Speed Internet ¨ VoIP ¨  IPTV ¨ Gaming ¨ Telemedicine ¨ E-Goverment ¨ ……

8/31/12 6

Traffic increases

very rapidly!!

“Triple play”

8/31/12 7

Triple-play Capacity Requirements

"   Basic: design for basic triple play today "   Extended: design for the near future demand

Triple-play (3P)

Telephony TV Internet access Basic 3P

& Extended 3P

Basic 3P

Extended 3P Basic 3P

Extended 3P

Number of channels

1 2 4 1

Capacity 80 kb/s symmetric

2 x 9 Mb/s asymmetric

4 x 15 Mb/s asymmetric

10 Mb/s symmetric

30 Mb/s symmetric

Broadband Access Technologies n  Digital subscriber loop DSL (from telcos)

¨ High-speed digital access to the Internet ¨ Based on existing twisted pair

n  Cable (from cable TV companies) n  Wireless access: WIMAX, WIFI, 3G/4G (LTE) n  Fiber access

¨ Fiber to the cabinet (FTTCab) ¨ Fiber to the curb (FTTC) or Fiber to the Building

(FTTB) ¨ Fiber to the home(FTTH)

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Broadband Access Technologies n  Limitations

8/31/12 9

Technologies Capacity/user Max Reach ADSL 2 Mb/s (typical) 5.5 km

VDSL 20 Mb/s (typical) 1 km

Coax 2 Mb/s * 0.5 km

Wi-Fi 54 Mb/s (max) 0.1 km

WiMax 28 Mb/s (max) 15 km

EPON/GPON 30 Mb/s ~ 1Gb/s * 20 km

WDM PON 1Gb/s >20km

* Bandwidth depends on the number of users

FTTX

Access Network Evolution

8/31/12 10

}  Fiber Access is the future-proof technology offering ultra-high bandwidth and long reach.

}  Passive optical network (PON) is considered as the most promising solutions due to the relatively low cost and resource efficiency.

CATV

ADSL

FTTX-PtP

FTTX G/E-PON

FTTX WDM-PON

FTTX

WDM&TDM-PON

COST

xDSL

TIME

FTTX ultra-dense WDM-PON

NGPON

CAPACITY POTs

FTTX AON

Growth Prediction

8/31/12 11

•  IDATE predicts a drop in active ADSL in Europe from 2010 (in Japan happened in 2006)

•  In 2016 FTTX users are estimated to grow to 30% of the total broadband lines.

(Ref: IDATE February 2007)

Forecasts for Europe (subscribers in thousands)

8/31/12 12

Example of Power Consumption

Core node

Access node

Electronic

switching

Optical switchi

ng

Fiber-to-the-X (FTTX)

8/31/12 13

Frame/Cell relay

FTTH/O

FTTB/C

FTTCab O

LT

ON

U

ON

U

NT

ON

U

NT

ODN

Internet

ODN

Copper

Copper

CO

Leased Line

Telephone

ODN

Electronics

NIU

Interactive Video

Home network

OLT Optical line terminal ONU Optical network unit NT Network termination

Reach Extension

8/31/12 14

Passive & active optical distribution network (ODN)

COold

Remote node COnew

Home / Building

ONU

ONU

Edge Node

OLT

Flexibility points

From ONU to OLT (Reach>100km)

OLT

ONU

ONU

OLT

CO

CO

CO OLT

ONU

ONU

SME

ONU

Point-to-Point P2P

8/31/12 15

CO

N Users

L km

}  Standardized and mature technology }  N fiber lines }  2N transceivers }  Significant outside fiber plant deployment

Active Optical Network (AON)

8/31/12 16

}  Standardized and mature technology }  1 fiber line }  Curb Switch → power in the field }  2N+2 transceivers

CO

N subscribers

L km

Curb Switch

Passive Optical Network (PON)

8/31/12 17

}  Simple, under standardization technology }  1 fiber line }  N+1 transceivers }  Passive devices (splitters). No active elements

in the signal’s path from source to destination

CO

N subscribers

L km

Passive Optical Splitter


Passive Optical Network (PON) n  All transmissions in a PON are performed

between an Optical Line Terminal (OLT) and Optical Network Units (ONUs).

n  The OLT resides in the CO and connects the optical access network to the metropolitan area network (MAN) or wide-area network (WAN).

n  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.

Time vs. Spectrum Sharing

8/31/12 19

}  Downstream → point-to-multipoint network }  The OLT manages the whole bandwidth

}  Upstream → multipoint-to-point network }  ONUs transmit only towards the OLT }  ONUs cannot detect other ONUs transmissions }  Collisions may occur è Need of a channel separation

mechanism for resource sharing

TDMA Time Division Multiple Access

WDMA Wavelength Division Multiple Access

Time vs. Spectrum Sharing

8/31/12 20

}  TDM PONs: Current generation PON }  Standardized }  Use few wavelengths (typically 2) }  Low cost and mature devices (splitters, lasers, etc.) }  Limited power budget

}  Maximum distances ≤ 20km, Split ratios ≤ 64 }  Traffic distribution

}  Broadcast scheme downstream }  TDMA techniques upstream

}  Examples: APON/BPON, EPON & GPON

}  WDM PONs: Next generation PON (long term) }  Proposed in literature and/or demonstrated }  Introduce WDM technology and devices (AWG) }  Long-reach and high bandwidth

}  Hybrid WDM/TDM PONs: Next generation PON (short term)

TDM PON

8/31/12 21

•  Downstream traffic is broadcasted to all ONUs •  ONUs filter data (frames) by destination address

•  Upstream traffic is time division multiplexed •  an arbitration mechanism is required so that only a

single ONU is allowed to transmit data at a given point in time

WDM PON

8/31/12 22

WRPON: Wavelength Routed PONs (incl. AWG)

WPON: WDM broadcast and select PONs

Hybrid WDM/TDM PON

8/31/12 23


Next-generation Access Networks n  Optical 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.

n  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).


PON Technologies n  Optical Splitters/Couplers

¨  A PON employs a passive (not requiring any power) device to: n  split an optical signal (power) from one fiber into several

fibers, and n  to 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.


PON Topologies n  Multiple topologies are suitable for access

network, including tree, tree-and-branch, ring, or bus.

n  Using 1:2 optical tap couplers and 1:N optical splitters, PONs can be flexibly deployed in any of these topologies.

n  In addition, PONs can be deployed in redundant configurations such as double rings or double trees.

n  Redundancy may be added to only a part of the PON, say the trunk of the tree.


PON Topologies


Burst-mode Transceivers n  Due to unequal distances between the CO and the ONUs, optical

signal attenuation in the PON may not be the same for each ONU. n  Thus, the power level received at the OLT may be different for

different ONUs. n  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 weak signal from a distant ONU.

n  In the opposite case, if the receiver is trained on a weak signal, it may read zeros as ones when receiving a strong signal.

n  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.

n  A burst-mode receiver is necessary only in the OLT. n  The ONUs read a continuous bit stream (data or idle bits) sent by

the OLT and do not need to re-adjust quickly.


Ethernet PON (EPON) Access Networks n  Ethernet PON (EPON) carries data traffic encapsulated

in Ethernet frames (defined in the IEEE 802.3 standard). n  Standard 8b/10b line coding

¨  8 user bits are encoded as 10 line bits. n  Operates at standard Ethernet data rates. n  The first-generation PON standardized by ITU–T G.983

employed Asynchronous Transfer Mode (ATM) as the medium-access control (MAC) protocol.

n  When its standardization effort was started in 1995, the telecom community believed that ATM would be the prevalent technology in backbone networks.

n  However, since then, Ethernet has grown vastly popular. ¨  Cheap line cards ¨  Widely deployed in LANs today.


EPON n  Since 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.

n  High-speed Gigabit Ethernet deployment is widely accelerating and 10-Gigabit Ethernet products are becoming available.

n  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.


EPON: Principle of Operation n  In the downstream direction (OLT to ONUs): n  Ethernet frames transmitted by the OLT pass through a

1:N passive splitter and reach each ONU. n  Typical values of N are between 8 and 32. n  EPON operation is broadcast in the downstream

direction. n  Packets are broadcast by the OLT and extracted by their

destination ONU based on a Logical Link Identifier (LLID).

n  LLID is assigned to the ONU when it registers with the network.


EPON: Downstream Operation


EPON: Principle of Operation n  In 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.

n  In the upstream direction, the behavior of EPON is similar to that of a point-to-point architecture.

n  However, data frames from different ONUs transmitted simultaneously may collide.

n  Thus, in the upstream direction, the ONUs need to employ some arbitration mechanism to avoid data collisions.

n  A contention-based media-access mechanism (similar to CSMA/CD) is difficult to implement because ONUs cannot detect a collision in the fiber.

n  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.

n  To introduce determinism in frame delivery in the upstream direction, different non-contention schemes have been proposed.


EPON: Upstream Operation


Timeslot Assignment n  All ONUs are synchronized to a common time reference,

and each ONU is allocated a timeslot in which to transmit.

n  Each timeslot is capable of carrying several Ethernet frames.

n  An ONU should buffer frames received from a subscriber until its timeslot arrives.

n  When its timeslot arrives, the ONU bursts all stored frames at full channel speed.

n  If there are no frames in the buffer to fill the entire timeslot, an idle pattern is transmitted.

n  Thus timeslot assignment is a very crucial step.


Timeslot Assignment n  The 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). n  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. ¨  However, the centralized intelligence may lead to more efficient

use of bandwidth. n  More advanced bandwidth-allocation schemes are also

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


Multi-Point Control Protocol (MPCP) n  A supporting protocol to facilitate a dynamic timeslot

allocation scheme. n  It has been standardized in the IEEE 802.3ah. n  Aims to define a signaling protocol between the OLT and

the ONUs. n  Does not define any bandwidth provisioning scheme. n  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.


Discovery Processing n  Discovery 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 oppurtunity to register themselves with the OLT. A DISCOVERY-GATE message is broadcast to all ONUs containing 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.


Discovery Processing ¨ 3. 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’s LLID.

¨ 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.


Discovery Processing


Report Handling n  REPORT messages are sent by ONUs in their assigned

transmission windows along with data frames. n  Typically, REPORT would contain the desired size of the

next timeslot, based on ONU’s queue size. n  REPORT messages are generated periodically, even

when no request for bandwidth is being made. n  This prevents the OLT from deregistering the ONU. n  Thus, for the proper operation of this mechanism, the

OLT must grant the ONU a transmission window periodically.

n  At the OLT, the REPORT is processed, and the data is used for the next round of bandwidth assignments.


Gate Handling n  The transmitting window of an ONU is indicated in the GATE

message from the OLT. n  The transmission start and transmission length times are specified. n  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.

n  The ONU will also verify that the local time the GATE message arrived is close to the timestamp value contained within the message. ¨  If the difference in values exceeds some predefined threshold:

n  The ONU will assume that it has lost its synchronization. n  It will switch itself into offline mode. n  The ONU will then attempt to register again using the next discovery

process. n  When the time at the local clock of the ONU reaches transmission

start, the ONU starts transmitting data.


Clock Synchronization n  The correct operation of

MPCP depends on clock synchronization between the OLT and the ONU, which compensates for the RTT.

n  Whenever the ONU receives a MPCP message, it sets its local time from the time-stamp of that message.

n  When the OLT receives a MPCP message, it calculates the RTT as the difference between its local time and time-stamp of the message.


Dynamic Bandwidth Allocation Algorithms in EPON n  In the upstream direction (from ONU to OLT),

the ONUs must share the channel capacity. n  Since the ONUs cannot communicate with one

another, the OLT must assign timeslots in which the ONUs are allowed to transmit data.

n  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.


DBA in EPON n  The 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. n  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 Adaptive Cycle Time (IPACT).


IPACT n  The OLT keeps track of the earliest scheduling

time by a variable Tschedule. Thus, Tschedule is changed after each allocated timeslot.

n  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


IPACT n  Maximum 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.

n  To avoid this situation, the OLT must limit the maximum transmission size.

n  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.


IPACT n  Next, the corresponding GATE message is transmitted by the OLT.

Tschedule is modified as: Tschedule = Tstart + length


Service Disciplines in IPACT n  Fixed service ignores the requested timeslot size and always grants

a fixed timeslot, thus corresponding to synchronous TDMA. It has a constant cycle time.

n  Limited service grants the requested timeslot size, but no more than the Maximum Scheduling Timeslot WMAX.

n  Gated service does not impose the Maximum Scheduling Timeslot. Thus, the DBA agent allocates as much timeslot as is requested by the ONU.

n  Constant-Credit service adds a constant credit to the requested timeslot size.

n  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.

n  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 (maximum cycle time).


Simulation Results n  RD Mbps: Data rate of the access link from a

user to an ONU. n  RU Mbps: The rate of the upstream link from an

ONU to the OLT. n  A system with N = 16 n  RD = 100 Mbps n  RU = 1000 Mbps. n  Every ONU has a finite memory buffer n  Synthetic traffic traces that exhibit the properties

of self-similarity and long-range dependence.


Average packet delay for different service schemes


Other Types of PON n  APON/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.


Other Types of PON n  Generalized 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.

n  Both APON and GFP-PON have the disadvantage of a complex protocol and implementation, relative to EPON.

n  So, they have not gained much technical popularity amongst users and equipment vendors.


WDM-PON n  Although the PON is a significant step towards providing

broadband access to the end user, it is not very scalable. n  The basic form of PON employs only a single optical

channel. n  The available bandwidth is limited to the maximum bit

rate of an optical transceiver: Under current technologies 1 Gbps.

n  The attenuation due to splitting limits the maximum number of ONUs to 64.

n  This limits the network’s scalability. n  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.


WDM-PON n  There will be a need for further increasing the bandwidth of

the PON by employing Wavelength-Division Multiplexing (WDM).

n  Multiple wavelengths may be supported in both upstream and downstream directions.

n  Such a PON is known as a WDM-PON. n  WDM-PON is a point-to-point access network

¨  Separate wavelength, between the OLT and each ONU. ¨  Each wavelength is routed by a passive 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.


Arrayed Waveguide Grating (AWG) n  The AWG is a passive device with a fixed routing matrix. n  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.

n  Signals of different wavelengths coming into an input port will be routed to a different output port.


Example: LARNET §  TDMA is used to share the upstream channel.

？

For questions, please send e-mail: [email protected]

Note: In the presentation, most material are cited from related sources. Since some material cited here may be confidential, or not be allowed to be circulated, please directly contact their own sources if you will use them.

Optical Networks, Access Networks, Cicek Cavdar, KTH, Sweden

Documents

Optical Networks Access Networks · Access Network Evolution 8/31/12 10 ! Fiber Access is the future-proof technology offering ultra-high bandwidth and long reach. ! Passive optical