Outline. Designing and measuring PON Conclusions. WDM-PON advantages & challenges AWG router transmitter sources - wavelength specific & colorless ONU

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2 Outline Introduction history (ISDN, xdsl, cable modem) economic considerations (bandwidth, cost) PON Architecture FTTx TDM-PON OLT & ONU, burst mode, DBA, ranging, security APON, EPON, GPON, SuperPON WDM-PON advantages & challenges AWG router transmitter sources - wavelength specific & colorless ONU Designing and measuring PON Conclusions

3 Introduction Access networks have been traditionally called last-mile networks as they comprise the last segment connection from service providers central office (CO) to end users Surge in bandwidth demand driven by fast-growing video on-demand (VOD) services (YouTube started in 2005.) emerging applications such as network gaming, peerto-peer downloading etc.

4 Introduction Access networks twisted copper pairs coaxial cable Wi-Max optical fibers Optical fibers have been widely used in backbone networks huge available bandwidth and very low loss Traditional telecommunication networks were developed for analog voice services The Internet is the driving force for digital local loops (xdsl)

5 ISDN Integrated services digital network (ISDN) First broadband DSL standard developed in the 1980s by CCITT (predecessor of ITU-T) Offers the so-called 2B+1D encoding scheme on a single twisted pair two 64-kbps (2B) channels for voice and data one optional 16-kbps (1D) digital channel Total of 144 kbps data rate in both directions ISDN services were never popular high cost lack of killer applications

6 xdsl Various flavors of DSL technologies (xdsl) have been invented for broadband data delivery on twisted copper pairs DSL services make use of the higher frequency range on twisted pairs for data transmission DSL data rates and transmission distances are limited by signal impairments inside twisted copper pairs copper wire quality, bridge taps on twisted pairs, and cross talk between neighboring twisted pairs all degrade the signal quality ADSL frequency spectrum

7 xdsl

8 xdsl

9 Cable modem Traditional CATV networks were one-way broadcast systems Coaxial cable is a very good broadband medium (compared to twisted pairs) usable frequency range up to 1 GHz Hybrid fiber coax (HFC) systems Tree-and-branch coaxial cable plant forms a shared medium among the cable modem users customer data multiplexed using TDM scheme bandwidth shared similarities to the most commonly seen power-splitting PON architecture although completely different media for transmission.

10 Killer applications The field of telecommunications took a significant dive in the beginning of this century because of the exuberance in capacity deployment After some years of stagnancy, demands for bandwidths are growing again Killer applications of the Internet First killer application - Second killer application - World Wide Web Third killer application? peer-to-peer networking network gaming voice-over IP video on-demand (VOD)

11 Economic considerations How much bandwidth will be enough for an end user? 640K ought to be enough for anybody. Bill Gates Very hard to predict

12 Economic considerations Point-to-Point (PtP) access system Point-to-MultiPoint (PtMP) active access system

13 Economic considerations Point-to-MultiPoint (PtMP) passive access network

14 Economic considerations PON has been invented for over 20 years, it has not been commercially successful until recently In order for PON to become commercially viable, the cost of running a fiber local loop (labor + capital) needs to be in par with that of running a twisted pair loop An access system connects end users to COs through local loops in two ways run a separate pair of from each end user to the assigned CO local loops first connect end users to a remote terminal (RT) which multiplexes the individual signals on a feeder line

15 PON Architecture The general structure of a modern telecommunication network consists of three main portions backbone (or core) network metro/regional network access network

16 FTTx? Fiber Copper FTTN (Node) FTTC (Curb) FTTB (Building) FTTH (Home)

17 Access methods in PON TDMA WDMA SCMA CDMA

18 TDM-PON TDM-PON uses a passive power splitter as the remote terminal The same signal from the OLT is broadcast to different ONUs by power splitter signals for different ONUs are multiplexed in time domain ONUs recognize their own data through the address labels embedded in the signal Most of the commercial PONs (including BPON, GPON, and EPON) fall into this category

19 TDM-PON Critical component (with TDM-PON) is the power splitter. Losses due to power splitter can be approximated by: Note: the power splitter is a reciprocal component the same losses are encountered in upstream direction Most common power splitter - Planar Lightwave Circuit (PLC)

20 TDM-PON Typical measured transmittivity characteristic of a 1:32 power splitter (10 log N = 15 db)

21 TDM-PON The wavelength division duplex method separates upstream and downstream transmission signals using different wavelengths Coarse 1,31/1,49-μm wavelength duplexing scheme easy wavelength control, no temperature control is needed Optionally TV signal at 1,55 μm

22 Analog CATV Signal Overlay TV services on a TDM-PON - directly broadcasting analog TV signal to end users on the 1,55-μm wavelength using a wavelength coupler 1,55-μm wavelength can be amplified by an EDFA Problems: 1. Increased cost 2. Additional management complexities 3. Analog TV broadcast is being replaced by digital services

23 Optical Line Terminal (OLT) OLT may contain multiple MAC (medium acces control) and PMD (physical medium dependand) layers it may be connected to multiple PON systems cross-connect at the OLT provides the interconnection and switching among different PON systems, ONUs, and the backbone network

24 Optical Network Unit (ONU) ONU provides the connection to the OLT in the PON section through the ONU MAC and PMD service adaptation layer in the ONU provides the translation between the signal format required for client equipment connection and the PON signal format

25 Burst mode operation Downstream direction the OLT interleaves frames destined for different ONUs as a continuous stream and broadcasts to all ONUs each ONU extracts its own frame based on the header address Upstream direction each ONU has its own optical transmitter to communicate with the OLT only one optical receiver at the OLT ONUs take turns to send their data to the OLT (when ONU is not sending upstream data, it has to turn off its transmitter to avoid interference) in a TDM-PON system, burst mode transmission is used in the upstream direction

26 Burst mode operation Every time an ONU transmits a signal burst to the OLT, it first sends a preamble sequence to the OLT preamble is used as a training sequence to adjust OLT decision threshold and perform synchronization guard time reserved between bursts from different ONUs (OLT receiver needs to recover to its initial state) adds overhead ONUs are at different distances from the OLT signals from different ONUs will experience different fiber delays before reaching the OLT timing reference needed (ranging process) - round-trip time (RTT) Ranging is usually done at the time an ONU joins a PON if multiple ONUs attempt to join the PON at the same time, collision may occur. Collisions in discovery are resolved by ONUs backing off with a random delay

27 Dynamic Bandwidth Allocation DBA allocates bandwidth to each ONU according to upstream traffic demand and requirements Requirements on DBA: Fairness - Allocates the bandwidth between the users fairly Low delay - Can achieve the maximum delay time below the designated delay value and minimize the latency as much as possible High efficiency - Can increase the efficiency of the bandwidth and increase the peak rate as much as possible DBA priority levels Best Effort dynamically allocated when available Non-Assured dynamically allocated in proportion to assured bandwidth, when available Assured guaranteed to be available, can be dynamically reallocated if not used Fixed guaranteed reserved whether used or not

28 Dynamic Bandwidth Allocation

29 Security concern Broadcast nature (in PS-PON) in downstream direction easy to eavesdrop downstream communication signals ONU cannot listen to the upstream transmission from another ONU (due to directional nature of the coupler at the RN) The biggest security exposure is in the ranging process when the OLT broadcasts the serial number and ID of the ranged ONU - malicious user can use this information for spoofing problem can be avoided through an authentication process (ONU is verified by a password known only to the OLT) To improve security ITU-T G standard defines a churning procedure scrambling the data for downstream connections with a key established between the ONU and OLT (some level of security in the physical layer) when security is important, encryption at the application layer

30 APON/BPON APON (ATM-PON), BPON (Broadband PON) Different aliases of the TDM-PON architecture based on the ITU G.983 standards ATM frames are used for transport The downstream time slots are 53-octet (byte) long 1 x 16/32 splitting ratio Bit-rates:

31 APON/BPON Wavelength allocation in ITU-TG Upstream: low-cost uncooled directly modulated Fabry-Perot (FP) laser Downstream: cooled directly modulated, narrow-spectrum DFB laser Analog overlaid video: cooled, externally modulated DFB laser

32 EPON New addition to the Ethernet family - IEEE 802.3ah Transmits data as native Ethernet frames (variable size: payload octets) Downstream and upstream bit-rate is equal to 1 Gbit/s (1.25 Gbit/s, 8B/10B encoding for easier clock recovery) FEC is optional in EPON (typically RS(255, 239) block codes) Data throughput is decreased when FEC is used EPON standard does not specify the implementation details of DBA 1 x 16/32* splitting ratio, * with FEC

33 GPON GPON - ITU-T G.984 series standards for PONs with gigabit capabilities higher data rates, greater distances and higher split ratios Downstream: / Mbps Upstream: /622.08/ / Mbps G-PON receivers need to handle higher receiver overload powers (larger dynamic ranges) power-leveling mechanism: OLT tries to balance the power it received from different ONUs by instructing ONUs to increase or decrease the launched power NRZ + Scrambling Optional FEC capability using RS(255, 239), data throughput decreased Transmitts data as ATM or GEM (GPON encapsulation method encapsulates different types of data) 1 x 16/32/64 splitting ratio

34 Comparison Standard BPON GPON EPON EPON ITU-T Rec. ITU-T Rec. IEEE 802.3ah G.983.x G.984.x 1000 BASE- PX BASE- PX20 Max. log. reach (km) Max. bit rate (Mbps) (1000 nominal) Protocol ATM ATM, GEM Ethernet Tx laser DN DFB DFB DFB DFB Tx laser UP FP FP FP DFB Rx detector DN PIN PIN PIN APD Rx detector UP PIN PIN PIN PIN Max. split ratio 1x32 1x64 1x16 (1x32) FEC NO G.975 G.975

35 Forward error correction (FEC) Specified in ITU-T Rec. G.975 Used in: EPON 1000BASE-PX10 : BCH (Bose-Chaudhuri-Hocquenghem) 1000BASE-PX20 : RS(255,239)) GPON RS(255,239) Used to increase: optical link budget fibre distance RS (255,239) encodes 239 information symbols adds 16 partity bits the information bits are not disturbed in the encoder decoder can detect and correct transmission errors BER INPUT = > BER OUTPUT = 10-12

36 Super PON Proposed to achieve better economy increasing the reach of PON systems beyond 20 km supporting higher splitting ratios of 1:64 or even 1:128 Challenges: minimum energy per bit is required to keep the BER, by increasing the speed the power required is increased. Higher splitting ratios further accentuate the problem amplifiers at 1,31 & 1,49 μm are not economical dispersion at 1,55 μm is non-negliable increased dispersion penalty RTT increases higher overhead Share group size increases better security required

37 WDM-PON An alternative to expand PON capabilities besides Super PON WDM coupler replaces the power splitter at the remote node Advantages RN is still passive (same low maintenance and high-reliability as PS- PON) each user has its own wavelength (excellent privacy) no P2MP media access control required no distance limitations imposed by ranging and DBA each wavelength can run at a different speed as well as with a different protocol

38 WDM-PON Challenges higher costs of WDM components temperature control - WDM components wavelengths tend to drift with environmental temperatures (athermal WDM components) colorless ONU operation in a WDM-PON, each ONU needs a different wavelength for upstream connection wavelength specific ONU introduces significant challenges in managing production lines, inventory stocks, sparing, and maintenance lot of the solutions have been invented to realize colorless ONUs in the last 20 years

39 AWG router The AWG router is a key element in many WDM-PON architectures Conventional N-wavelength WDM coupler is a 1xN device

40 AWG router A general AWG router consists of two star couplers joined together with arms of waveguides of unequal lengths Each arm is related to the adjacent arm by a constant length difference. These waveguides function as an optical grating to disperse signals of different wavelengths. Cyclical wavelength routing - demultiplexing property repeats over free spectral ranges (FSR)

41 AWG router Operating principle: input signal is divided equally into each fiber as each fiber is of different length, phase of each light component is different at the end of the fibers. The phase difference depends also on working wavelength λ i (frequency ν i ). If the difference in length of two adjacent fibers is ΔL, then the phase difference can be written as: exit from the array of fibers acts as an antenna array. The direction of the main beam depends on wavelength (frequency):

42 AWG router Typical parameters: Number of channels: up to 80 Channel spacing: 0.8 or 0.4 nm (100 GHz or 50 GHz) Insertion loss: 3 to 8 db Adjacent channel crosstalk: -25 db Optical return loss: -40 db Maximum wavelength deviation over temperature: ± 50 pm

43 Wavelength-specific sources One of the key requirements of transmitter sources in a WDM system is that the emission wavelengths remain stable and fixed to the WDM grid. Distributed Feedback (DFB) Lasers high speed, direct modulation, require active temperature control (~ 0.1 nm/ C), high power consumption, system complexity, cost Distributed Bragg Reflector (DBR) Lasers high speed, direct modulation, require active temperature control (~ 0.1 nm/ C), high power consumption, system complexity, cost Vertical-Cavity Surface-Emitting Laser (VCSEL) lower manufacturing cost (compared to DFB/DBR), lower power consumption (compared to DFB/DBR), direct modulation, wavelength stabilization necessary, lower optical output power

44 Colorless ONUs The emission wavelength of this category of ONUs is nonspecific selectively determined by external factors (filtering of AWG in the RN or the wavelength of an injection/seeding light into the ONU) this flexibility enables the exact same colorless ONU to be deployed across the network, mass production ONUs Based on Spectral-Slicing Techniques LED, SLED, EDFA, FP-LD Injection-Locked and Wavelength-Seeded ONUs FP-LD or RSOA + BLSs (broadband light source) Self-Injection-Locked FP-LDs and Self-seeding Reflective SOAs FP-LD or RSOA Source-Free ONUs Based on λ Reuse Schemes

45 Tunable laser scheme ONU with tuneable laser ultimate in terms of optical performance and flexibility the number of ONUs supported are determined by channel spacing of the AWGs and the tuning range of the laser Much more sophisticated laser is required compared to conventional EPON and GPON systems tunable lasers usually require internal wavelength lockers or an external network wavelength reference to ensure they operate at the correct wavelength channel to maintain a stable laser operating regime, external modulation rather than simple direct laser modulation is the norm

46 Sliced broadband source Each ONU contains a broad optical spectrum source (such as a superliminescent light emitting diode (SLED)) The broad spectral output of the ONU is spectrally sliced by AWG - the remaining power is wasted All customers ONUs have identical SLEDs When a simple LED is used as the transmitter it is only practical to have a few upstream data channels of 155 Mbit/s from this scheme

47 Reflective schemes The reflective SOA (RSOA) is in some ways related to both the asymmetric FP laser and the SLED. However, it also has important differences by reducing the front facet reflectivity close to zero it is possible to suppress all of the natural cavity modes unlike the SLED it is necessary to also ensure that there is an efficient reflection from the back facet The seeded RSOA approach has an advantage over the un-seeded SLED approach the optical power will be higher in the selected spectral slice due to the optical gain of the RSOA (~ db) gain saturation can be used to squeeze the excess noise produced by slicing higher figures of merit ( channels X bit rate) can be achieved (in practice 32 channels of GbE over 20 km) ASE of the RSOA at different temperatures and drive currents

48 Hybrid WDM/TDM-PON Another way to increase PON-system scalability besides the brute-force super-pon approach is to use a hybrid WDM/TDM-PON architecture Using a 16-wavelength AWG with 1x8 power splitter, operation of 128 ONUs was demonstrated WDM-Ethernet PON (WE-PON) in Korea EPON and WDM-PON technologies together using 32 wavelengths and 1x32 power splitters ~ 1000 users per WE-PON is possible.

49 Designing passive optical networks Most important power budget Total optical system/link loss is the sum of the following: OLT connector WWDM coupler Splices Fiber attenuation Splitter ONT connector Losses from dispersion, non-linear effects

50 Designing passive optical networks Link attenuation: BPON & GPON: A Class: 5 20 db B Class: db C Class: db EPON: PX db : downstream 5 20 db : upstream PX db : downstream db : upstream

51 Designing passive optical networks Optical return loss (ORL) Made up of: Rayleigh scattering (from fiber core) reflectance from all the interfaces found along the link Particularly critical for analog transmission 1550 nm CATV video signal used in PON ITU-T Rec. G983 & G984 allow minimum link ORL of 32 db IEEE 802.3ah allows between 15 to 20 db Effects of ORL fluctuations in the laser output power, interference at the receiver, lower carrier-to-noise in analog systems, higher BER in digital systems, potential permanent damage to the laser

52 Designing passive optical networks Test instruments: ORL test meter (optical continuous-wave reflectometer) Optical loss test set (OLTS) Visual fault locator (VFL) Live fiber detector (LFD) Optical time-domain reflectometer (OTDR) PON wavelength-isolating power meter (capable of measuring burst optical power of the ATM or Ethernet traffic)

53 Error detection in PON

54 Error detection in PON Measurement unit OTDR Control unit PC Software Pre-measurement management Post-measurement management Serial interface connection (RS232)

55 Error detection in PON Based on comparison of reference and test measurements Reference measurements performed after the installation of the OCS Measurement points defined at characteristic events on the OTDR trace (splices, fiber end, connector reflections)

56 Error detection in PON Error : the difference of the power level between test and reference measurements, at the measurement points, exceeds a predefined threshold level Test measurements automatically performed in time variable intervals

57 Management software Three modules Main module Comparison module Automatic test module

58 Conclusions BPON GPON EPON Motivation Low cost Flexible Major advantage Major drawback Application Future long term ATM Soon BW limited Small community Limited Any traffic, highest BW Splitting loss limited All Needs new splitting technology Widespread low cost technology Low cost, Internet application Fixed BW limited Apartment building 10 Gb EPON Others / future Splitter loss issue "unlimited" BW (WDM) High cost muxing All?

59 Conclusions Research activities at FER concerning PON Measuring and monitoring PON Development of low-cost colorless WDM-PON

60 Questions? Thank you on your attention!