# Access network

[toc]

:::info
- It is self-study course: five online PDFs and two quizs and two online tests.
- Exam will be held like oral exam.
- First three labs need to preview the material before the class.
- Practical classes will be held on 10/17, 10/31, 11/14, and 11/28.
- 9:15 a.m.
- 90 minutes
:::

---
## Reference
:::info
- [High-speed Internet Access (online + download)](https://techpedia.fel.cvut.cz/en/single/?objectId=129)
- [Optical systems and networks (online + download)](https://techpedia.fel.cvut.cz/en/single/?objectId=7)
- [Selected chapters from FTTH Handbook (PDF for download)](https://www.tmgtestequipment.com.au/assets/products/media/bkd0/guide/Fiber_to_the_home_Handbook_Guide.pdf)
:::
- Glossary
:::warning
:::

---
## High-speed Internet Access - textbook
:::info
- VDSL2 and G.fast connections are via symmetrical copper pairs and fiber optic
- VDSL2 follow ADSL technology
- nowadays VDSL2 connections and G.fast are latest evolutionary steps in xDSL technologies
- their implementation to access telecommunications networks are also part in the concepts of FTTx networks
:::
### Communications and telecommunications
:::success
- telecommunications is a discipline concerned with their communication man
- the modern development basically began in the nineteenth century
- today is an integral part of the field of ICT (Information and Communication Technology)
- it is not possible to interconnect direct links all telecommunications devices, for this reason, the telecommunications network to be logically structured in a certain way
- two parts of telecommunication network
- core(backbone)
- access
- core network: aims to interconnect various network nodes of telecommunications provider
- standardized transmission medium is single-mode optical fiber
- transmission speed range in the 10s Gbit/s
- operational distances are from 10s to 1000s kilometers
- ring topology is typical
- access network: last point between provider and subscriber, ISP(Internet Service Provider) is usually considered local exchange host and RSU(Remote Subscriber Unit)
- transmitting medium is symmetrical metallic pair (single-mode optical fibers are optional)
- transmission speed range from 1s up to 100s Mbit/s
- operational distance is 100s meters to 1s kilometers
- tree topology
- metallic access network
- the access telecommunications network are based on symmetrical copper pairs in most European countries, conception design of the network counted mainly with
- telephone services via analogue phone lines POTS (Plain Old Telephone Service) in low freq. range
- basic digital connections BRA-ISDN (Basic Rate Access - Integrated Services Digital Network) in the range up to 80 kHz
- cables using wires with copper cores with diameters of 0.4, 0.6 and 0.8 mm diameter insulation max. 1.7 mm and have an outer casing
- cables in the bearing are filled with a suitable embodiment the filling material (gel), which forms protection against water
- conductors have polyethylene foam isolation, cables in the self-supporting version (hanging cables) are not filled and full use polyethylene isolation
- hybrid access network
- nowadays metallic access network is approaching their limits
- physical parameters of twisted pair(attenuation, group velocity of propagation, crosstalks, etc) reduce transmission parameters
- solution to this situation is to replace the symmetrical pairs in the access network for optical fibers
- but this solution is too expensive. Therefore, the replacement of copper pairs for optical fibers takes place gradually
- main types of FTTx networks:
- FTTH: Fiber To The Home
- FTTO: Fiber To The Office (office or business premises)
- FTTB: Fiber To The Building
- FTTC, FTTCab: Fiber To The Curb, Fiber To The Cabinet (means to a local cabinet or sidewalk column)
- FTTN: Fiber To The Node (a local hotspot - street cabinet, column)
- FTTEx: Fiber To The Exchange (local digital exchanges)
- FTTdb: Fiber To The Distribution Point
- digital subscriber line
- xDSL (Digital Subscriber Line) are specially designed to allow better use of the potential of copper in the access network infrastructure, for relatively high transmission speed
- in purely matallic solutions of the access network, near the main distribution exchanges, can use ADSL, SHDSL, and on limited distance also second improved generation ADSL2+ and VDSL
- in hybrid optical and metallic access network solutions, provider facilities closer to the subscribers can take full advantage of connections ADSL2+ and VDSL2 and G.fast
:::
### Digital subscriber line VDSL2
:::danger
- VDSL2 is a second generation of VDSL connection, but the innovation is taken from the second-generation connection ADSL2
- particularly the grid encoding (known as Trellis Code) used to repair the receiver single bit errors and the ability to correct error bursts caused by the impulse interference using a Reed-Solomon code and data interleaving
- and also SRA (Seamless Rate Adaptation, means can control the transmission speed dynamically), controlling the transmission power to reduce crosstalks to neighboring pairs, and activate power saving mode (known as Sleep Mode)
- extend utilized frequency band has one big disadvantage
- the attenuation of line in specified frequency band is growing with increasing frequency bandwidth
:::
:::success
- layout of VDSL2 connection
- DSLAM - DSL subscriber multiplexer
- Splitter - frequency filter on the subscriber and provider side
- VTU-C – VDSL2 transceiver (modem) on the provider side
- VTU-R – VDSL2 transceiver (modem) on the subscriber side
- U-RV - physical interface terminating lines on the subscriber side
- U-CV - physical interface terminating lines on the provider side (located in exchange)
- U-RV2 - physical interface between the splitter and the modem on the subscriber side
- U-CV2 - physical interface between the splitter and the modem on the provider side
- T/S - physical interface between the VTU-R, followed by the network infrastructure on the subscriber side (LAN)
- Z/U0 - physical interface twisted pair running POTS or ISDN BRA with limited frequency band by HUB
- Layer model of VDSL2 connections
- two parts of device model
- physical data transmission medium (independent on the data transmissions), called PMD (Physical Media Dependent) or PMS-TC (Physical Media Specific - Transmission Convergence)
- TPS-TC (Transport Protocol Specific - Transmission Convergencer)
- function block PMD: actual transmission of signal, generating and recovery cycle, modulation and demodulation, echo cancellation, compensation of the negative line parameters and initialize the connection --> low PHY
- function block PMS-TC: associated with creating frames, frame synchronization, data security realized by FEC (Forward Error Correction), interleaving, scrambling and descrambling --> Link + high PHY
- function block TPS-TC: base on functionality of the terminal device or the type of carried data traffic, multiplexing, demultiplexing and synchronization, prioritization of of certain services
- Reference points labeled as S/T are interfaces in the direction to user terminals
- Data transmission modes of subscriber terminal
- STM (Synchronous Transfer Mode): work synchronously depending on the DSLAM, upstream and downstream are carried out on all terminals at the same point in the same time
- reduction of near end crosstalk noise
- ATM (Asynchronous Transfer Mode): method of transmission ensures backward compatibility with existing ADSL connections, using the cell constant length, using channel identifiers VPI (Virtual Path Identifier) and VCI (Virtual Channel Identifier) to know the path
- PTM (Packet Transfer Mode): for transmission of streams in which the subscriber data are encapsulated into packets or frames, e.g. Ethernet frame protocol frame Point-to-Point packets or MPLS(MultiProtocol Label Switching)
- VDSL2 connection supports multiple independent data channels (different QoS requirements) which are transmitted via two independent pathways
- QoS is how to evaluate the subscriber satisfaction with the service customer pays for
- data transfer capabilities
- PMS-TC is designed to adapt and secure the subscriber data into a format suitable for transmission over twisted pair
- transmission paths for VDSL2 connections
- without data interleaving (mode FAST): lower delay for real time transmission but higher interference
- with data interleaving (mode INTERLEAVE): higher delay in transmission for file transfer protocol
- interleaving: errors can be stretched to better detect and repair using Reed-Solomon coding
- for video broadcasts with MPEG encoding, the effect of impulse noise manifests itself as so-called "blocking effect"
- Transfer security
- scrambling
- data sequence is scrambled in PMS-TC block
- scrambling is intended to eliminate a periodic sequence of transmitted data stream
- randomization of transmission brings lower demands on the required frequency bandwidth of the transmission path
- error detection
- 8-bit word that is generated by calculating a classical cyclic code CRC (Cyclic Redundancy Check)
- error correction
- Reed-Solomon code, it is in conjunction with interleaving
- very efficient for its low redundancy (typically seven percent)
- need to do the trade-off between the degree of error protection and overall transmission delay
- Variants of VDSL2 connections (the parameters of the block PMD)
- three types of VDSL2 connections
- Annex A - VDSL2 connection to the access network in the North America
- Annex B - VDSL2 connection to the access network in the Europe
- Annex C - VDSL2 connection to the access network in the Japan
- there are different profiles, broadcast schedules and frequency masks of PSD(Power Spectral Density) are set for VDSL2 connection in various annexes
- Multicarrier modulation DMT (Discrete Multi-Tone) can be used for the realization of data transmission for all three types of connections above
- principle of this modulation is used by the division of the entire frequency band into a series of independent subchannels(tones or carriers)
- recall: increase utilized frequency band -> reduce the maximum length of local loop -> because of attenuation at higher frequency
- profiles of VDSL2 connections
- common parameters of VDSL2 connections profiles used in Europe
- frequency band cannot be used as a single unit, but it is necessary to split the freq. band into at least two sub bands for two directions of transmission
- first method: echo cancellation
- second method: freq. division
- VDSL2 connection can use FDD to eliminate NEXT(Near End Cross Talk)
- parameters for data rate
- parameter MBDC(Minimum Bidirectional Net Data Rate) represents the minimum value of the transmission rate (the sum of transmission speeds in both directions)
- Speed NDR (Net Data Rate): data and it doesn't include business info. and header bits, and actually useful speed will be even lower than the value of NDR (part is used e.g. for header of Ethernet frames or IP packets)
- division of the freq. band
- VDSL2 connection must be able to provide both asymmetric and symmetric data transfer, so it is necessary to frequency sub bands further divided into smaller parts
- Rotation method and width of the individual frequency sub bands determined by the so-called as frequency plan -> Frequency plans determine which frequencies can be used for downstream or upstream
- there are two main freq. plans for VDSL2 connection
- plan 998 is for asymmetric data transfer
- plan 997 is for symmetric data transfer
- Frequency plans(997) for symmetrical data transfers
- Usage of symmetrical data transfers is especially developed for small and mediumsized companies because employees need not only download but also quickly upload
- 997 - fundamental frequency plan
- 997E - frequency plan for the downstream and upstream extended to 17 MHz to 30 MHz, respectively
- HPE - special frequency plan
- Frequency plans(998) for asymmetrical data transfers
- in asymmetric transfer mode, we should use VDSL2 connection to achieve higher transmission instead of ADSL connection
- 998 - fundamental frequency plan
- 998E - frequency plan for the downstream and upstream extended to 17 MHz, respectively up to 30 MHz
- 998ADE - frequency plan designed for data transfer with significantly asymmetric speeds
- Masks of the power spectral density used in 997 plans
- definition of performance level is going through a mask of the PSD(Power Spectral Density)
- and the mask represents the peak power of the transmitted signal at a particular frequency
- and PSD masks correspond with a specific frequency plan
- Masks of the power spectral density used in 998 plans
- total of 17 masks are defined in plans 998 in recommendation for VDSL2 connection(too many so I don't list)
- the choice of these masks provides suitable properties in spectral compatibility with already installed ADSL/ ADSL2+ systems
- we can observe the similar figure as freq. plan 997
- transmission chain of VDSL2 connections
- divided into three parts
- CPN (Customer Premises Network)
- NAP (Network Access Provider)
- NSP (Network Service Provider)
- subscriber network: terminal equipment, terminals, local data networks and LAN
- provider NAP: operates the transmission of signals from subscribers to the telecommunications nodes. VDSL2 connections NAP is composed of copper symmetrical pairs (metallic conductions) that result from CPN and terminating in access multiplexers DSLAM. Multiplexers ISP connects to its own core network, or connection to the service provider
- Sources of data that is transmitted through the NAP, located in the network service provider NSP, typically a company operating an archive of TV programs
- protocol structure of VDSL2 connections
- we mainly use standard Ethernet, Point-to-Point Protocol and a family of protocols based on TCP/IP
- transmission of IP packets is handled by using of PPP (Point-to-Point Protocol) at VDSL2 connection in PTM mode
- traffics meet the broadband access server BRAS (Broadband Access Server) of access provider in the aggregation point
- from BRAS, the data is also transported the end user to the Internet or to individual service providers through a VPN (Virtual Private Network), the entire solution is referred to as variant PTA(PPP Terminated Aggregation)
:::
### High-speed subscriber lines according to G.fast standard
:::danger
- G.fast is called as the 4th generation xDSL system
- higher transfer rates
- 500 Mbit/s to 100 m
- 150 Mbit/s to 250 m
- also some scenario for up to 1 Gbit/s
- utilization of short lengths of subscriber lines, called ad FTTdb (Fiber To The distribution point) - the location of distribution points will be very close to subscribers (up to 250 meters) - equivalent to the concept of FTTB (Fiber To The Building)
- inverse power
- simplified deployment of xDSL connections(self-installation service with parameters of optical connections)
- targets of improvement
- increase throughput
- extension of freq. band (with mask of PSD and specific freq. band)
- cancellation of crosstalks: eliminate crosstalk type FEXT(Far End Cross Talk) using appropriate modulation type called as VDMT(Vectored Discrete Multi Tone)
- increase reliability
- inverse multiplex
- phantom circuits
- key features of a standard G.fast include the implementation of adaptive modulation DMT with required support VDMT
:::
:::success
- Utilization of phantom circuits
- previously used for long-distance telephone lines to increase transmission capacity
- but it will increase strong crosstalk
- but!!! now we can use VDMT to eliminate the crosstalk it generates
- we can see the performance is improved after applying crosstalk cancellation
- Power supply of optical node from the user modem
- G.fast is expected to reverse the flow of power
> for me, it means it only needs very low power
- the node (mini DSLAM) will be power supplied from the local modem in the point that is called as "Distribution point"
- to solve the problem that power equipment placed in the access network between the PBX (Private Branch eXchange) of telecommunications operator and the subscriber
- outcome of power supply is from the subscriber side, where it is necessary to summarize the power output from multiple partipants at once
- Vector Modulation – VDMT
- VDMT principle is to modify each currently transmitted DMT symbol on each symmetric pair with respect to the current parameters of the transmission environment
- DMT modulation is used by the xDSL to handle narrowband interference in freq. channel
- can further increase the transmission speeds achievable using so-called as vector modulation VDMT (Vectored DMT) -> improve the SNR to higher the achievable transmission rate
- the dominant component of the interference is crosstalk interference
- crosstalk is signal transmission capacitive and inductive relationships between pairs
- the process is called as crosstalk coordination, about the coordination of individual broadcast connections in a bundle copper cable in connection with modulation VDMT
- according to the site crosstalk measurement, we distinguish between NEXT(Near End Cross Talk) and FEXT(Far End Cross Talk)
- NEXT can be eliminated by FDD(FDD Zipper, using Cyclic Suffix), e.g. co-location of modems in DSLAM -> reduce the transmission rate
- FEXT can be eliminated by VDMT
- VDMT is an extension of DMT modulation in a multi-user environment -> solves the problem of MIMO type and FEXT
- in downstream
- in the DSLAM
- we have a single point of information on DMT symbols to be sent in the next moment to metallic lines (we vector values of DMT symbols - hence vectored modulation DMT)
- DSLAM also knows parameters of the various symmetrical pairs of crosstalk and the links between them (known is a process of establishing a connection)
- on the receiving side modem end user will then have the symbols theoretically ideal parameters suitable for error-free detection, since DSLAM can get lots of info.
- synchronization transmission is also easily manageable since it is the central element of the DSLAM
- in upstream
- in the upstream direction it is not possible to adjust the transmitted symbol in the terminal equipment of a subscriber
- in addition, the terminal equipment can never have information about the symbols that are broadcast by other connections in metallic cable -> in the DSLAM (the central element) using a special block crosstalk canceller
- Advantages and disadvantages of VDMT
- advantage: eliminate crosstalk interference and thus to achieve higher transmission speeds reachable by xDSL connections
- disadvantage:
- high computational complexity in coordinating of the broadcast
- Modulation VDMT has benefits only in a situation where the propagation medium is used by all systems with DMT modulation
:::

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## High-speed Internet Access - exam
- [Online exam](https://techpedia.fel.cvut.cz/en/browse/?fileId=1722&objectId=129&fileInfo=html-test)

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## Optical systems and networks - textbook
:::info
- outline
- Optical Access Networks
- FTTx technology
- Active/Passive Optical Networks
- optical systems using Wavelength Division Multiplexing
- DWDM(Dense Wavelength Division Multiplexing)
- CWDM(Coarse Wavelength Division Multiplexing)
- system maintenance issues
- OTDR
- splicing
- signal recovery
- EDFA
- SOA
- Raman amplifier
- Dispersion compensation
:::
### Current ITU-T recommendations for optical transmission systems
:::danger
- PON(Passive Optical Network): it uses passive components, where power level decreases gradually with the distance from the laser sources
- Recommendation for Reach Extended PON: Passive Optical Network can use optical amplifiers in certain cases to increase the reach without building up vast active components infrastructure
- Reach Extended PON (REPON), where using an Erbium Doped Fibre Amplifier (EDFA) is permitted
:::
:::success
- ITU (International Telecommunication Union) published recommendation for Passive Optical Networks (PON), including Reach Extended PON. There are a number of options in terms of attenuation classes, reach, wavelengths and bit rates, there is an example (also including IEEE, ITU)
:::
### Systems with Wavelength Division Multiplexing - CWDM and DWDM grid
:::danger
- General idea is to increase the bit rate and information capacity of a system
- DWDM
- requires using narrow spectrum of radiations, which are Distributed Feedback Lasers (DFB)
- central wavelength must be stable (laser wavelength will be influenced by temperature), so it is recommended to use cooled DFB where operating temperature is fixed between 20 and 30°C
- CWDM
- Metropolitan networks can operate at much lower bit rates, so it is suitable to use CWDM
- uses 4 to 16 channels, with huge spacing of 20 nm, which allows using broader spectrum
- Fabry-Perot lasers
- non-cooled DFB
- is less expensive
:::
:::success
- CWDM and DWDM grid
:::
### Types of optical networks, their architectures and basic parameters (OTH, FTTx)
:::danger
- Hybrid solutions
- in general
- optical fibres are combined with symmetric metallic lines for ADSL2+ VDSL2
- optical fibres are combined with coaxial cables for Cable TV (CATV)
- combine with wireless network
- types
- FTTEx (Fibre to the Exchange): terminated at the local telephone exchange, DSLAM multiplexor splits signal to existing metallic lines to provide xDSL (Digital Subscriber Line) -> it is the most common solution nowadays
- FTTCab (Fibre to the Cabinet): optical fibres are terminated in an outdoor splitter
- FTTC (Fibre to the Curb): optical fibre reaches the group of buildings
- FTTB (Fibre to the Building): optical fibres reach particular buildings
- All optical solutions
- FTTO(Fibre to the Office): optical fibres terminate at the office of important customers with huge demands
- FTTH(Fibre to the Home): optical fibres are terminated at the end user's socket
- Access networks
- OAN(Optical Access Networks):
- Simplex transmission with SDM(Space Division Multiplexing): for each direction of transmission there is one fibre
- Duplex transmission with WDM(Wavelength Division Multiplexing): signals are transmitted in one fibre
- Duplex transmission with FDM (Frequency Division Multiplexing): signals is transmitted in one fibre, directions of transmission are around one wavelength and they are separated by frequency spacing
> it has the difference, short haul or long haul
- AON(Active Optical Networks): it employs active network components to interconnect network elements, like amplifiers
- PON(Passive Optical Networks):
- BPON - Broadband PON: boardband so it can cover several services
- GPON - Gigabit PON: the speed achieves Gigabit(high speed)
- EPON - Ethernet PON: Type 1 – max. reach is 10 km, Type 2 – max. reach 20 km.
:::
:::success
- Optical Transport Hierarchy
- signals of optical hierarchy are known as OTM (Optical Transport Module)
- the simplest option (zero level): no wavelength multiplex
- optical transport modules are denoted as OTM-n.m
> n: the number of channels(operating wavelengths), m: type of signal
- STM is Synchronous Transport Module, a basic transport unit in SDH(Synchronous Digital Hierarchy). N stands for multiplication of the basic unit's capacity
:::
### Splicing of optical fibres and installation of fibre optic links
:::danger
- splicing of optical fibers is to create permanent connections
- within flexible solutions of connectors, splices offer lower losses in comparison, and they can be used to repair damaged fibres
- extra of fibre is necessary while installing fibres, if a section is damaged, the extra length can bridge the gap and a splice can be made
:::
:::success
- the equipments which are necessary for splicing
- the fibres themselves
- strippers to remove the coating layer
- a cleaver to cut the fibre ends cleanly
- a splicer to align the fibres correctly and join them
- isopropyl alcohol for cleaning the de-coated fibres, and tissue for removing excess alcohol
- procedure
- two lengths of around 100 mm should be roughly cut from a spool of fibre
- use the strippers, the primary and secondary coating layers of fibre were removed up to approximately 50mm from each end, leaving only the cladding and the core intact
- isopropyl alcohol on a tissue should be used to clean any remaining coating from the exposed cladding
- the exposed fibre core and cladding should be cut cleanly using the cleaver, to ensure a flat and non-angled surface
- the fibre should be placed in the v-groove of the cleaver such that the end of the fibre should be extended sufficiently to reach the opposite side of cutter so that the fibre would be cut rather than bent
- the fibre is then clamped in position using a clip
- after cutting, we can move onto the actual splicing of the fibres
- two fibre ends are placed in the v-grooves of the splicer
- fine-tune the positioning of the fibres by pressing the buttons
- then splicer with fibres positioned close to the electrodes in the centre
> if the positioning and cut quality is bad, splicer will display warning on the screen, and fibres can be removed and repositioned or recut
- the outcome will look like this
:::
### Testing optical paths by using Optical Time Domain Reflectometer
:::danger
- Optical Time-Domain Reflectometer (OTDR) is a measurement device for optical communication systems
- by recording the intensity of Rayleigh back-scattered light from a source in time domain
- OTDR can approximate the distance travelled by the light based on the known refractive index of the fibre
- it is able to identify
- the attenuation of different lengths of the fibre
- the position of network components, like splices or connectors
- main target of OTDR: identification and location of defects within the optical communication system
:::
:::success
- OTDR should be connected to the end of an optical fibre in the order of tens of kilometers in length
- OTDR can be configured to produce pulses
- there is trade-off between penetration depth and resolution of the measurement, which is determined by pulse duration
- shorter pulses have better spatial resolution and worse penetration depth
- each application has its own preferred pulse duration and depth data
- there may be some fibre having bad adequate clarity after trade-off
> simple solution of this problem is to conenct OTDR at the opposite end of the fibre
- OTDR manages to identify network components, connectors and splices based on the shape of attenuation
- splice indicates short, sharp drop
- connector indicates local peak, with a greater drop-off on right-hand side of the peak
:::
### Regeneration of signals in optical networks by using link EDFAs
:::danger
- Erbium Doped Fibre Amplifiers
- Doped fibres are often used as optical amplifiers in optical networks
> Erbium is most common dopant
- EDFA solution comprises of distributed feedback (DFB) laser source, a 980 nm pump module, a multiplexer, an EDFA and spectral analyser
- DFB laser source and pump module's outputs were combined using the multiplexer
- transmitted through the EDFA
:::
:::success
- advantages
- broad operating band
- huge gain
- relatively flat gain for transparent optical networks using WDM(Wavelength Division Multiplexing)
- low price
- application as in-line amplifier
- disadvantages
- significant ASE noise (Amplified Spontaneous Emission)
- cannot be used as a booster because of gain suppression (saturation)
- cannot be used as a preamplifier without special filters
:::
### Semiconductor amplifiers (SOA) and Raman amplifiers
:::danger
- Semiconductor optical amplifier (SOA) as a gain medium, and are electrically pumped
- in practical, the temperature must be held steady to provide consistent amplification
- Raman amplification uses the Stimulated Raman Scattering (SRS)
> it is not always desired, in DWDM systems, SRS can produce crosstalk between transmission channels
- LRA – Lumped Raman Amplifier: since SRS accumulates along the entire fibre, it is more optimal to place Raman pump at the opposite end of a fibre
- DRA – Distributed Raman Amplifier
:::
:::success
- spectral characteristic of SOA
- the classification of the linear and nonlinear regions of the SOA in order not to operate non-linear region
- different Raman gain in different types of optical fibres
- different types of amplifiers
:::
### Dispersion compensators for fibre optic paths
:::danger
- dispersion in optical fibres
- dispersion causes pulse extension or pulse compression(if dispersion is negative)
- the unit of dispersion is [ps/nm] (picoseconds/nanometre), but in fibre optics, the length of optical structures is a key parameter, dispersion is referred to the unit length and is expressed as [ps/nm/km]
- the dispersion of 1 ps/nm/km means that the delay between the slowest and the fastest frequency component of an optical pulse, its bandwidth is 1 nm, will be 1 ps after the distance of 1 km
- Chromatic dispersion
> solution: the use of Dispersion Compensating Fibres (DCF) or special fibre gratings
- two components
- material dispersion
- waveguide dispersion
- Material dispersion (DMat): is due to the bandwidth of a laser source, which is not endlessly narrow (because in practice, there is no ideal monochromatic light which is with endlessly narrow bandwidth)
- material dispersion is persent in both single mode and multimode fibres
- material dispersion can be negative or positive
- Waveguide dispersion (WD): the shape of the whole pulse which is associated with the waveguide geometry
- waveguide dispersion is always negative, so it can be used for the compensation of material dispersion
- Modal dispersion
> solution: to decrease the speed for short trajectories, to increase the speed for long trajectories
- each of beams passes through the fibre to its output along different trajectory
- modal dispersion is present only in multimode fibres (there is only one mode in single mode fibres)
- do remember they are guided at the same phase and group velocity
- in Multi-mode Graded Index (MM-GI) fibres, the index of refraction of a core is not constant
- closer to core centre, the index is higher, trajectory is shorter, the speed is slower, because the centre of a core is a high-index material
- the densest material is in the centre of a core, layer around it is less dense
:::
:::success
- Dispersion Compensation
- Dispersion Compensating Fibres
- are specific for low negative dispersion parameter
- the most mature DCFs are those based on Photonic Crystal Fibres (PCF)
- DCFs for DWDM systems have to be able to compensate dispersion in all channels a once
- for broadband DCF, let it have strong negative dispersion parameter, then dispersion copies the reverse slope of a standard fibre
- Fibre Bragg gratings
- FBG in the fibre along its core
- it operates at specific wavelength, so many wavelengths = many gratings
- PMD – Polarization Mode Dispersion
- because of different refractive index for different polarization
- PMD is usually small, but it can still be a problem for high-speed transmission systems
- PMD is a random process
- to avoid PMD, we use
- specialty electronic polarization corrector
- specialty birefringent fibres with strong asymmetry
- Compensation schemes
- to decide which compensation options will be used (like pre or post compensation), it is recommended to do numerical simulation of network first
- not to achieve exactly zero dispersion, even no pulse spreading but it leads to a nonlinear phenomenon known as FWM – Four Wave Mixing, because Zero dispersion is one of a few conditions to originate it
:::
### Convergence and upgrade of optical networks
:::danger
- there is demand for higher information capacity require upgrading existing optical systems or running more systems on one fibre
- coexistence of optical systems refers to CWDM and DWDM networks sharing the PON infrastructure
:::
:::success
- DWDM over CWDM
- DWDM can replace the 5th channel in CWDM
- Hybrid solutions
- with dual phase amplifiers and post-compensation
- then do channel interleaving
- combining with 7x40 Gbps system
- P-DPSK and RZ-DQPSK, dual modulation formats
- have Cross Phase Modulation (XPM) problem
- Hybrid DWDM 10G/40G with safety band
- spectral split of 10G and 40G by safety band can help reducing the cross phase modulation from the 10G system
:::
### Conclusion
:::info
- Passive Optical Networks (PON):
- cheap
- bit rates are lower (about 10 Gbps)
- and the reach of a fibre span can be tens of kilometres
- Active Optical Networks (AON):
- allows achieving high bit rates over 1 Tbps by employing DWDM systems
- do "network planning" to solve "topology, attenuation, dispersion and nonlinear issues"
- channel rate at least 100 Gbps
- potential coexistence or convergence of different systems with different specifications
:::

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## Optical systems and networks - exam
- [Online exam](https://techpedia.fel.cvut.cz/en/browse/?fileId=1717&objectId=7&fileInfo=html-test)

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## Fiber to the home - textbook
This is an overview, not a design guide
### Introduction
:::info
- Optical fibre will be the main building block for future high-capacity networks
- Advantages of fibre
- high bandwidth
- long distances
- future upgrade potential
- lower maintenance and operational costs
- Key functional requirements for a FTTH network
- provision of high bandwidth services and content to each customer
- flexible architecture design that can accommodate future innovations
- direct connection between each end subscriber and active equipment, to ensure maximum available capacity for future service demands
- minimize disruption during network deployment
:::
### FTTH Network Description
:::info
- FTTH network
- large number of end users to central point known as an access node or point of presence(POP)
- access networks may connect some of the following:
- FTTH network may be considered to be part of the wider area or access network
- type of FTTH site

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- FTTH network environment can be split into
- city
- open residential
- rural
- building type and density - single homes or multi-dwelling units(MDUs)
- type of sites (in deciding the most appropriate network design and architecture)
- Greenfield – totally new build where the new network will be introduced at the same time as the buildings
- Brownfield – where there are existing buildings and infrastructure but it is lower standard
- Overbuild – adding to the existing infrastructure
- main factors for infrastructure deployment methodology are:
- type of FTTH area
- size of the FTTH network
- initial deployment cost (CAPEX)
- on-going costs for network operation and maintenance (OPEX)
- network architecture, for example PON or Active Ethernet
- local conditions, for example, local labour costs, local authority restrictions(traffic control) and others
- two most widely used topologies of FTTH architecture
- point-to-multipoint: passive optical network (PON) architecture
- in order to be operated by one of standardized PON technologies (like GPON, EPON), using time sharing protocols to control the access of multiple subscribers to the shared feeder fibre (like Ethernet switch)
- mainly GPON(or EPON) in case of point-to-multipoint connectivity
- point-to-point: ethernet transmission technologies
- each subscriber directly connect with dedicated fibre
- some scenario need it
- can also include PON technologies by placing the passive optical splitters in the access node
- different fibre termination points
- Fibre to the home (FTTH): each device is connected by dedicated fibre to equipment in the POP, or to the passive optical splitter, shared feeder fibre to POP
- Fibre to the building (FTTB): each optical termination box in the building is connected by a dedicated fibre to the equipment in the POP, or the optical splitter, using shared feeder fibre to the POP. in some cases, building switches are not individually connected to POP but are interconnected in a chain or ring structure (to deploy in existing topologies and save fibres and ports in POP)
- Fibre to the curb (FTTC): each switch/DSLAM, typically in a street cabinet, is connected to the POP via a single fibre or a pair of fibres, carrying the aggregated traffic of the neighborhood via Gigabit Ethernet or 10 Gigabit Ethernet
> the architecture sometimes is also called "Active Ethernet" as it requires active network elements in the field
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### Active Equipment
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- the choice of equipment depends on many variables including demographics, geography, financial calculations, and more
- different FTTH network architectures
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- Passive optical network
- schematic of GPON network
- components
- optical line terminal(OLT) in the point of presence(POP) or central office
- one fibre to the passive optical splitter
- fan-out towards end-users, each having an optical network unit(ONU)
- advantages
- reduced fibre usage
- without active equipment between OLT and ONU
- dynamic bandwidth allocation capabilities, which can save operational cost
- trends for access technology tends to be towards more symmetrical bandwidth (more multi-media file sharing, peer-to-peer applications)
- bandwidth management
- downstream: incoming data will be filtered by port ID
- upstream: assign different time slot to each ONU
- PON active equipment
- optical line terminal (OLT): usually suituated at central office or connection point
- optical network unit (ONU)
- indoor application (I-series)
- outdoor application (O-series)
- business application (B-series)
- FTTB application
- deployment optimization
- the splitter can be situated closer to the customers in case of high end-user density
- distributing the splitters in more than one level, which reduces the length of required fibre and eases the config., but increase the location of the installed splitters
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- Ethernet point-to-point
- two possibilities
- one dedicated fibre per customer between ethernet switch and home -> easier to implement but higher fibre usage
- one fibre to an aggregation point and dedicated fibre from there onwards
- can sum up the number of active ports and try to minimize the number of unused active network elements in POP
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- the exmaple of fibre specification
- for requirements not considered in the standard, the market offers optical transceivers with non-standard characteristics, for example, some types can bridge significantly longer distances to suit deployment in rural areas
- RF-based video solutions
- IPTV-based video solutions provide superior features over simple broadcast solutions
- kind of like existing video set combining with new fibre network
- first approach
- add additional fibre per customer is deployed in a tree structure
- second approach
- video signal is inserted into every point-to-point fibre at 1550nm (dedicated wavelength for video)
- in both cases ONU/CPE comprise two distinct part
- media converter that converts 1550nm into an electrical signal by coax interface
- an optical Ethernet interface into an Ethernet switch or router
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### Customer Equipment
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- due to high variety of services, "digital home" has arrived
- two functions in home environment
- ONU: where the fibre is terminated
- customer premise equipment (CPE): which provides the necessary networking and service support
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### Future Technology Development
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- Evolution of ITU passive optical network standard (ratified in 2010)
- IEEE also has its own evolutional standard (ratified in 2009)
- WDM-PON promises to combine the best of both standards
- target: sharing feeder fibres while still providing dedicated point-to-point connectivity
- use wavelength filters instead of splitters
- there is a logical upgrade path from current TDM-PON deployments to WDM-PON
- network device which WDM-PON needs is quite expensive (need to consider cost reduction)
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### Cost Considerations
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- Capital costs
- civil works cost the most
- Operation costs
- marketing, subscriber management, etc, so many factors, and actually they will base on network design, simplicity, ease of trouble-shooting and speed to repair
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### Infrastructure Sharing
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- because of high costs of FTTH deployment, infrastructure sharing is very important topic
- for each of these business models, infrastructure must be shared
- Vertically integrated
- covering passive, active and service layers, who offers services directly to their customers
- conveys traffic on their networking equipment using their own passive infrastructure
- Passive sharing
- passive infracture provides passive access to other players who focus on active and service layers
- Active sharing
- focus on active and passive infrastructure, and opens it up to service providers
- Fully separated
- fully separated model, each network component has its own provider
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- four methods of infrastructure sharing
- Duct: share the use of a duct network
- Fibre: share the FTTH network
- Wavelength: share/compete the same wavelength layer interface
- Packet: share/compete/prioritize the same packet layer interface
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### Infrastructure Network Elements
- explanation of infrastructure elements
- the position of each network element
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- explanation of access node
- often referred to as the point of presence (POP)
- starting point for optical fibre path to the subscriber customer
- physical size of the access node is determined by the size and capacity of the FTTH area
- explanation of drop cabling
- the final external link to the subscriber and runs from the last FCP to the subscriber building
- distance is less than 500m
- often much less in high-density areas
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- different type of cables
- Direct install cables: Direct install cables are installed into ducts
- Direct buried cables:
- metal protection cable: higher protected
- non-metal protection: lower cost
- Aerial cables: very dependent on environmental conditions
- Facade cables:
- facade installation is a suitable installation method for buildings, like large blocks of flats or terraced properties
- useful in Brownfield deployments
- Internal cabling:
- for residential properties, might be terminated on the outside structure of the house, or pass through the wall and terminate inside the house
- different schemes for cabling a MDU
- many suppliers have special riser (vertical) and drop (horizontal) cable solutions
- illustration of internal cabling of a building
- flexibility point
- incorporate PON splitters and patching for point-to-point networks -> provides local flexibility
- reduces the cost of the most expensive part of an MDU build – the internal cabling
- vertical riser
- also called riser
- install cable into riser shaft or duct
- for time saving during riser cables installation
- pre-connectorised solution
- extractable fibre elements
- horizontal drop
- the portion of the network linking the riser cable to the premises with the required number of fibres
- examples of riser to horizontal solutions
- Optical Network Unit(ONU)
- inside the customer premises
- optical fibres are terminated with a connector inside a customer interface box
- a patch cord used to link to the ONU
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### Deployment Techniques
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- Conventional duct infrastructure
- installation of cables by pulling, blowing or floatation techniques
- large main duct contains smaller sub-ducts(for individual cable installation)
- progressively pulled sub-ducts, one over the other as the network grows
- Duct network
> extra space is allowed
- single duct
- maximizes the number of cables
- with sub-duct
- reduce the total number of cables that can be installed
- but old cables can be removed

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- different types of duct cable
- cable installation by pulling
- cable installation by air blowing
> recently common solution, kind of like air "pushing"
- quicker than pulling
- allow longer continuous lengths to be installed -> reduce the amount of cable jointing
- cable decoring
- the core of copper cables can be replaced fastly and effectively
- Access and jointing chambers
- suitable sized access chambers should be positioned at regular intervals along the duct route
- must be large enough to enable all duct cable installation operations and joint maintenance
- microduct solutions
- example of ducts
- Thick-walled microducts
- for where temperatures may vary significantly
- Tight-bundled microducts: offer a larger number of microducts pre-installed in a duct
- loose bundled microducts: high crush resitance and long distance can be blown
- direct buried cable
- offers a safe, protected and hidden environment for cables
- requires careful survey to avoid damaging other buried services
- Aerial cable
- are supported on poles or other tower infrastructure
- relatively quick and easy to install
- Pre-terminated network builds
- usually we install termination of fibre in LAN(e.g. fibre-optic connectors in the factory)
- enables factory testing and hence improved reliability
- reducing the time and the skills needed in the field
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### Fibre and Fibre Management
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- What is otical fibre?
- "light pipe" carrying pulses of light generated by lasers or other optical sources to a receiving sensor(detector)
- fibre construction
- pulses are launched into the core region
- surrounding cladding layer keeps the light travelling down the core and prevents light from leaking out
- outer coating is usually made of a polymer
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- choice of optical fibre for FTTH
- usually based on singlemode fibre, multimode fibre may also be used in specific situations based on
- Network architecture: affects the data rate that must be delivered by the fibre
- Size of the network: refer to the numbers of properties served by the network
- The existing network fibre type: new network must be compatible with the fibre in the existing network
- Expected lifetime: designed with a lifetime of at least 30 years
- Power budget: determine how far away the POP can be from the customer
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- illustration of singlemode fibre and multimode fibre
- singlemode fibre
- small core size
- supports only one mode(ray pattern) of light
- most of fibre systems are based on this type of fibre
- the lowest optical attenuation loss and highest bandwidth transmission
- higher equipment cost than multimode fibre systems
- multimode fibre
- larger core size
- supports only multiple modes(ray pattern) of light
- different propagation speed of individual modes (modal dispersion) can be minimised by adequate fibre design
- operate with cheaper light sources and connectors
- fibre itself is more expensive than singlemode
- used extensively in data centres
- lower bandwidth and restricted fibre length
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- optical distribution frames
- ODF is the interface between the outside plant cables and the active transmission equipment.
- ODFs are usually situated in the POP, bringing together several hundred to several thousand fibres
- secure area (need to do disaster proctection)
- uninterruptible power supply (UPS) in large scale access nodes
- air conditioner
- splicing of fibres, there are two common methods
- fusion
- cheaper
- more error
- mechanical
- cable joint closures
- since cables are not endless in length and need to be branched off at several locations, intermediate splice closures are needed
- the major difference from street cabinets
- street cabinets are active components for the distribution of optical signals to end-users
- cable joint closures are passive components that provide protection to spliced or jointed optical cables
- typical three functions of street cabinet
- duct management: compartment to connect, separate and store ducts and cables
- base management: where ducts, modular cables and fibre-optic cables can be fixed and managed, usually on mounting rail
- fibre management: where the fibres of the different cable types can be spliced
- optical splitter, there are two technologies
- fused biconic taper
- made by fusing two wrapped fibres
- planar waveguide splitters
- optical paths are buried inside the silica chip
- only symmetrical splitters available as standard devices
- better insertion loss and uniformity at higher wavelengths compared to FBT over all bands
- better for longer wavelength, broader spectrum
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### Operations and Maintenance
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- Network planning guidelines
- site control and installation operation planning
> like sideways or poles, need careful planning and many cases cause disruption to traffic
- general management considerations
> familarity to cabling system, work experience, careful planning of installation are essential for efficient and safe operation
- safety
> proper safety zones using marker cones and traffic signals should be organised, and also need to do environmental testing
- construction, equipment and planning
> like a full survey of the complete underground duct system
- cabling considerations
> think about which duct to be used, and also calculation of route length
- operation and maintenance guidelines
- for me, measurement, recording and planning are the most important keypoints
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### FTTH Test Guidelines
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- key parameters we need to consider
- and the contamination is super imortant to be careful, like dust particle
- inspection instructions
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- how to qualify FTTH networks during construction
- two tester
- Method #1: Use of optical loss test sets
- Method #2: Use of an OTDR
- in short, we still need equipment to test
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### FTTH Network Troubleshooting
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- troubleshooting on out-of-service network can be carried out easily with a power meter or OTDR
- for me, it is kind of like checking the
- for phone line issues, it values
- open & short
- wire networking
- for ethernet wiring, it values
- for High-speed data over FTTx, it values
- the capacity of increased data flow
- establishment of connection
- for IPTV, it values
- summary of optical testing tools
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