Optical Access and Metro Networks Module 2. Access and metro infrastructure

1. Place for logos of authors’ institutions Optical Access and Metro NetworksModule 2. Access and metro infrastructure Guido Maier, CoreCom, maier@corecom.it Achille Pattavina, Politecnico di Milano, pattavina@elet.polimi.it

2. Past access-network scenarioSeparated network systems Base station CO • Plain Service Telephone Networks (PSTNs) • Circuit-switched network • Wired solution: twisted-pair based • High reliability • Later, wireless solution: cell phone (GSM) • User bandwidth: 64 kbit/s (2 Mbit/s business) Twisted pair

3. Past access-network scenarioSeparated network systems RF Ampli Headend • Video service • Broadcast networks • Analogic signal • Unidirectional • Wireless solution: terrestrial / satellite radio-TV broadcast (Italy, Europe) • Wired solution: coaxial-cable CATV (USA, Germany, United Kingdom) • Bandwidth: hardly a GHz

4. Past access-network scenarioSeparated network systems CO Modem • Data service • Packet-switched networks • Packets over circuit • LAN: bandwidth up to 100s Mbit/s (10 Mbit/s to the user) • ATM: 155 / 622 Mbit/s (1.5 – 2 Mbit/s to the user) • ISDN or modem (physical integration with public PSTN): 64 – 2048 kbit/s

5. PSTN access-networkPhysical architecture • Primary network • High sharing • Cost minimization • Secondary network • Flexibility • Branching • Cables • Primary • 2400-2000 pairs • In duct or pipe • Secondary • 100-10 pairs • Trenched or aerial • Cascading more stages of cabinets is possible but rare Dedicated / leased line Distribution point (box) Distribution cabinet Primary cable Twisted pair Central office Secondary cable Distribution / secondary network Feeder / primary network Home network

6. PSTN access-networkPhysical architecture Dedicated / leased line Distriibution point (box) Distribution cabinet Primary cable Twisted pair Central office Secondary cable Distribution / secondary network Feeder / primary network Home network • Transmission systems • Generally bidirectional on a singletwisted-pair in the access network • Bidirectional on two pairs from the CO towards the transport network • Suitable one-pair/two-pair interfaceslocated in the CO • “hybrid” • 4-wires transmission requires echo control when over 4000 km (4 ms delay) • Residential subscribers • Each home reached by a single twisted-pair (also with ISDN) • Multiple in-house telephone plugs are daisy-chain connected • Business subscribers • May be reached by 2 pairs when connected with a direct digital circuit (4-wires ISDN)

7. PSTN access-networkPhysical architecture 10 p 10 p 10 p 10 p 10 p 40 p 40 p 10 p 20 p 30 p 10 p 10 p 10 p 10 p 10 p Dist. Cab 100 p • Distribution cabinet (Cab) • Originally simple passive cable-branching unit • From ’70s: digital demux (digital loop carrier), since digital multiplexing is used on the feeder network to reduce the actual number of pairs • 2.048 Mbit/s (Italy) or 1.5 Mbit/s (USA) systems for 30 channels (+ 1 for signaling) • 34.368 Mbit/s (Italy) for 480 channels (when fiber is used in the feeder network) • 1 + 1 analogic systems (for ISDN) • Allows an elastic distribution of capacity to adapt for new subscribers • Power loss • 0.4 mm diameter twisted pair: loss = 2 dB / km • In the past (analogic telephony) loss constraint was strictly dependent on distance and on the number of junctions • Today (digital telephony) the loss target is fixed to 8 – 12 dB, including device insertion losses • Typically, systems are designed for 6 – 7 dB loss from 300 to 3400 Hz Rigid branching Parallel derivations Flexible branching 100 p 100 p 100 p

8. PSTN access-networkNational-installation data • Italian PSTN access network • 23·106 subscribers, 11000 COs,  2100 subsc. / CO[France: 30·106 subs.; Germany: 35·106 subs., 6500 COs; United Kingdom: 26·106 subs., 7000 COs] • Average cable-length values • Primary: 1.1 km [Fr.: 1.9 km, Ge.: 1.7 km, UK: 1.3 km] • Secondary: 400 m [Fr.: 600 m, Ge.: 300 m, UK: 700 m] • 85 % buried • Occupation • Primary: 65 % [Fr.: >70%, UK: 75 %] • Secondary: 45 % [Fr.: 70%, UK: 65 %] • Muxed lines:  1000 @ 2 Mbit/s (30 channels) • USA • 100·106 subscribers • Average cable-length values • Primary: 2.7 km • Secondary: 600 m • 24 % above 5.5 km end-to-end • Muxed lines: several thousands @ 1.5 Mbit/s • World average • Average cable-length values • Primary: 1.8 km • Secondary: 520 m

9. PSTN access-networkPhysical-topology design • Central office locations are long-term designed in a specific plan. CO-areas (max 20000 – 30000 subscribers) are designed according to • Existing infrastructure; reliability; physical line parameters • A survey of the CO area identifies the subscriber distribution, keeping residential and business (dedicated lines) units into account • Feeder-cable duct (hosting feeder cables) are traced according to the territory, evenly distributing subscribers (barycentric design) • The CO area is partitioned in sectors (duct areas) with angles inversely proportional to the estimated cost of the cable-laying operation in the duct CO area boundary CO Duct area boundary Feeder duct

10. PSTN access-networkPhysical-topology design Cab area • The CO is partitioned into elementary areas, each one grouping max 100 pairs • Pairs inside the el. area are further grouped in n·10 groups, each group (roughly a building) assigned to a distribution point • 6 adjacent elem.areas make up a cab area • A cable area is formed by as many adjacent cab-areas as to exhaust a cable capacity, cables are laid down in the ducts accordingly • Design principles • A lot of redundancy is recommended to face subscriber-distribution changes (max 80 % utilization) • Each design step is carried out starting from the CO-area periphery and following the duct trace Primary cable Elementary area CO area boundary CO Duct area boundary Feeder duct Cable area boundary

11. Optical broadband access networkAdvantages of optical fiber and technology • Optics is a suitable technology for broadband access • Optical fiber advantages • Huge bandwidth per wavelength: up to 10 GHz and more • Wavelength Division Multiplexing (WDM): up to tens of channels per fiber • Very low loss (0.2 dB/km) • Mechanical strength and durability, small size, cables with 100s of fibers • Modulation transparency (analog and digital signals on the same fiber) • Low noise (typical Bit Error Rate 10-12) and immunity to external em. interf. • No need for maintenance (opposite to copper wires) • Fiber low cost (plastic fiber available for short-range transmission) • Optical components • Rapid technological evolution, increasing reliability, decreasing costs • Availability of passive components for splitting, combining and routing • Low cost and reliable fiber connectors • Sources (laser) and sensors (photodiodes) now cheap and well integrated within electronic interfaces

12. Optical broadband access networkDrawbacks of the adoption of optics • The major obstacle is the deployment cost • For new fiber plants: • Excavation and digging-authorizations usually in densely-populated urban areas • For existing infrastructure: • Replacement of copper with fiber (ducts and cabinets re-adaptation); • Legacy copper plant decommissioning • Indoor infrastructure • Difficulties for fiber in reaching houses using the same copper-wire cable-conduits (sharp bends are possible) • Environmental impact of the building sites • Few years ago an intensively-optical access solution seemed economically unfeasible • The presence of many optical-access network operators on the market today proves that the prediction was wrong • E.g.: Fastweb in Italy

13. Optical broadband access networkAlleviating fiber-deployment burdens • New regulatory system for metro-area cabling (e.g. Milano) • Aimed at fostering operator competition • Based on open requests for permissions to excavations • Cable ducts must be shared among different operators, so to reduce the impact on traffic and citizens’ life • Extensive presence of “dark fiber” • Optical fiber exceeding the present needs for capacity, deployed in view of future utilization • Not connected to any equipment • Typical of new buildings • The “dark fiber”: an attractive market • Fiber does not deteriorate nor it requires maintenance • Inexpensive • Digging after construction-sites are closed would be far more expensive

14. Optical broadband access networkAlleviating the burden: new plant technologies 30 cm 20 cm 10 cm bynder • Mini-trench • Urban and rural areas • Small milling machine • Rapid work execution • Suitable for multi-cable pipes • Does not interfere with other utilities due to a small depth • Micro-trench • Urban area; street and sidewalk • Quick and temporary deployment, easy to remove • Single cable pavement concrete multi-cable ITU-T Recommendations Series L.xx pavement rubber buffers 10 cm cable 1.5 cm

15. Optical broadband access networkAlleviating the burden: new plant technologies • Pipe reuse • Many cables in the same pipe • A partially-occupied cable can be reused • Facility-conduit utilization • Gas pipelines, sewer conduits, unused tunnels • Cables are drawn with the help of robots • Safety issues • Indoor cabling technology • Fiber “blowing” in specialized pipes • Fiber-pair (or UTP-5) insertion in existing cable-ducts (for twisted pairs)

16. Fiber in the loopFiber in the loop network architectures NIU CO Service feeder / primary network Downstream traffic Upstream traffic Distribution / secondary network • The term Fiber-in-The Loop (FITL) indicates any access network in which some or all the links are implemented with fiber • Simplified scheme • The access node is generically named Central Office (CO) • The user terminal is generically called Network Interface Unit (NIU) • Traffic • Downstream = towards the subscribers • Upstream = towards the central office • Terminology changes according to the specific implementation

17. Fiber in the loopFiber to the … (FTTx) classification • Several proposed solutions, resulting from the trade-off between the cost of substituting copper with fiber and the revenues coming from improved network performance • A section can remain in copper near the end users, where branching is the highest and cost-sharing among the subscribers is the lowest • Solutions named on the basis of the geographical location of the Electro-Optical (opto-electronic) Interface (EOI) • Classified cases • [Fiber To The Exchange (FTTE): EOI inside the CO • Actually indicates a full-copper access network; not a proper FITL solution] • Fiber To The Cab (FTTCab): EOI in the equivalent of the PSTN cabinet • Fiber To The Curb (FTTC) / Fiber To The Building (FTTB): EOI in the equivalent of the PSTN distribution point • Fiber To The Home (FTTH) / Fiber To The Office (FTTO): EOI in the NIU • Each operator tries to choose the configuration that minimizes extra costs and risks and maximize the exploitation of its legacy solution

18. Fiber in the loopTerminology according to ITU-T G.983 ONT FTTH/O ODN OLT ONU FTTB/C NT ODN Copper ONU NT FTTCab Copper ODN CO Home network Primarynetwork Secondarynetwork NIU Electronics • Optical Line Termination (OLT) • Access network terminal equipment in the CO • Optical Distribution Network (ODN) • Fiber section of the network • Optical Network Unit (ONU) • Interface between fiber and copper sections • Optical Network Termination (ONT) • NIU when the subscriber is reached by fiber • Network Termination (NT) • NIU when the subscriber is reached by copper From G.983 ITU-T Recommendation (1998)

19. Fiber in the loopHybrid Fiber and Coaxial (HFC) Regional Head-end Hub / Head-end RF Ampli Fiber Set-top computer / cable modem Fiber Node Coax • Typical of MSO / CATV operators • Max. 500 NTs per Fiber node (as the ONU is called) • << number of subscribers of a CATV network • Coaxial bandwidth: 850 MHz (limited by the RF amplifiers) • Downstream coaxial bandwidth utilization (Frequency Division Multiplexing) • 50 – 550 MHz: 80 6-MHz broadcast analogic TV channels • 600 – 750 MHz: 25 6-MHz channels carrying (via QAM) 38-Mbit/s digital signals, providing max. 6 Mbit/s (1.5 Mb/s average) downstream digital bandwidth per subscriber (Time Division Multiplexing)

20. Fiber in the loopHybrid Fiber and Coaxial (HFC) Regional Head-end Hub / Head-end RF Ampli Fiber Set-top computer / cable modem Fiber Node Coax • Upstream coaxial bandwidth utilization (Frequency Division Multiplexing) • 5 – 40 MHz: max. 160 kbit/s per subscribers • Cable modem • Downstream: 10 Mbit/s • Upstream: 33.6 – 128 kbit/s • Media Access Control (MAC) protocol required for upstream

21. Fiber in the loopHybrid Fiber and Coaxial (HFC) • Advantages • Exploitation of the existing CATV plants • Easy power-supply distribution by the coax cable itself • Major limitations • Small upstream bandwidth • Maintenance required for the copper section • Recent upgrades • High Density WDM in the ODN • Ring ODN (for survivability) • Mini fiber-nodes: a parallel secondary fiber-network carries upstream traffic exploiting the 750 – 1000 MHz bandwidth • Standard • Study Group IEEE 802.14 • ITU-T Recommendation L.47 (installation)

22. Fiber in the loopActive optical access network (metro rings) STB HUB Home network Optical ADM • Typical of new operators (green-field deployment) • Available in all FTTx versions (FTTCab, FTTC/B, FTTH/O) • Active switching equipment provides interconnection between different network sections • Optical Add Drop Multiplexers (OADMs) • SDH digital cross-connects / ADMs (Digital Loop Carrier – DLC) • ATM / Ethernet swishes • IP routers (Points of Presence – POP) CO Optical ADM VDSL modem VDSL m. Secondary fiber Primar (WDM) fiber ring Cab switch / POP CO switch / POP Secondary fiber ring SDH ADM Build. switch / POP

23. Fiber in the loopActive optical access network (metro rings) STB HUB Home network Optical ADM • WDM can be adopted in the primary fiber-loops to increase bandwidth • Copper section: • Twisted pair (with ADSL / VDSL) • UTP-5 (with 10/100 Fast Ethernet, especially for indoor plant in the FTTC/B solution) • Fully digital, bidirectional access, e.g. • SDH tributary (STM-1, E1/T1) or 1 Gbit/s Giga Ethernet for business subscribers • E.g. 10 Mbit/s Fast Ethernet for residential subscribers CO Optical ADM VDSL modem VDSL m. Secondary fiber Primar (WDM) fiber ring Cab switch / POP CO switch / POP Secondary fiber ring SDH ADM Build. switch / POP

24. Fiber in the loopActive optical access network (metro rings) • Advantages • High design flexibility • High reliability / survivability (ring topology) • Rely on well-established and widespread switching technologies and protocols (IP, Ethernet, ATM, SDH) • Control, administration and management by well-developed systems • Low cost subscriber equipment (e.g. Ethernet cards) • Disadvantages • High amount of active equipment and optoelectronic conversions • Not suitable for analogic signals • High deployment and network-equipment cost • Active-switches power supply

25. Fiber competitors in broadband accessDigital Subscriber Loop (xDSL) • Broadband access exploiting PSTN and twisted pairs • Requires installation of a modem-pair, one in the CO and the other at the users’ home • The best cost-saving solution for LECs and telcos that already own a widespread PSTN (e.g. Telecom Italia) • Recent regulation however oblige traditional telco to host in their COs xDSL of other operators • xDSL can integrate FITL networks (FTTCab and FTTC) when fiber deployment is not possibe for some subscriber in the short period • High-speed xDSL modems are still rather expensive

26. Fiber competitors in broadband accessDigital Subscriber Loop (xDSL) • The bandwidth efficiency (relevant information carried per Hz) is increased by • Advanced modulation techniques • Data compression (e.g. MPEG for video/audio) • xDSL technologies • Asymmetrical (ADSL) • Downstream: 6 Mbit/s; upstream: 1 Mbit/s; 10 km • High-speed (HDSL) • Downstream / upstream: 1.536 Mbit/s (T-1); 5 km • Very high-speed (VDSL) • Downstream / upstream: 51.84 Mbit/s (OC-1); 600 m • Rate Adaptive (RADSL) • Downstream / upstream: up to 51.84 Mbit/s (OC-1); 3 km • Performance strictly depends on the twisted-pair quality • It can not be guaranteed in a legacy copper plant (Fastweb case)

27. Fiber competitors in broadband accessWireless access technologies Optical • Recent options derived from cell-phones and personal mobility • Four main access solutions

28. Wireless broadband accessWireless LAN (IEEE 802.11) – Network architecture • Started in 1990, IEEE 802.11b in 1999, IEEE 802.11a in progress • Network-architecture options • Ad-hoc or point-to-point • Basic Service Set (BSS): infrastructure with a single access point • Extended Basic Service Set (EBSS): infrastructure with multiple access point

29. Wireless broadband accessWireless LAN (IEEE 802.11) – Physical layer • Infrared (IR) • Optical signals; wavelengths from 850 to 950 nm, diffused light or laser beam • Baseband transmission, 1 – 2 Mbit/s • Frequency Hopping Spread Spectrum (FHSS) • The signal modulates a narrow-band carrier with frequency varying (“hopping”) inside the total bandwidth • FSK (Gaussian FSK) modulation, 1 – 2 Mbit/s • Direct Sequence Spread Spectrum (DSSS) • The signal bits are time-domain modulated with a digital code (cipping code) • Differential Binary PSK, D Quadrature PSK, , 1 – 11 Mbit/s

30. Wireless broadband accessWireless LAN (IEEE 802.11) – MAC protocol • Variable length frames, similar to Ethernet • Each station cyclically is given an access period without contention (CFP) and a period with contention (CP) • A control sequence Beacon frame (B) is inserted at the beginning of each cycle • DCF: based on CSMA/CA similar to CSMA/CD (Ethernet IEEE 802.3) • Collision detection is notalways guaranteed (e.g. with long packets and far stations competing for a middle receiver – the hidden node problem) • Packet reception is ACKNOWLEDGED • Each transmitting station declares however a residual transmission time; each station tries to avoid collisions by taking the received timing info into account • A reservation-based mechanism is also provided • PCF based on polling

31. Wireless broadband accessGeneral Packet Radio Service (GPRS) - Architecture • The GSMnetwork (alias Public Land Mobile Network, PLMN) can not support data service • GPRS has been implemented by adding three functional units • Serving GPRS Support Node (SGSN) • Gateway GPRS Support Node (GGSN • Packet Control Unit (PCU).

32. Wireless broadband accessGeneral Packet Radio Service (GPRS) – Physical layer • Applying FDMA/TDMA an 8-slot frame is created to mux 8 Packet Data CHannels (PDCHs) with the following functions • Common control channels (PPCH, PRACH, PAGCH, PNCH) • Broadcast control channels (PBCCH) • Traffic channel (PDTCH) • Dedicated control channels (PACCH, PTCCH)

33. Wireless broadband accessGeneral Packet Radio Service (GPRS) – MAC protocol • A MAC protocol manages the assignments of the channels to the Mobile Stations (MSs). • The predefined PDCH is used to transmit transmit permits to the MSs in order to avoid collisions

34. Wireless broadband accessUniversal Mobile Telecom. Standard (UMTS) - Architecture • UMTS Subscriber Identity Module (USIM): identifies the user(the equivalent of the GSM SIM card) • Video-capable mobile terminal • The access network dynamically assigns communication resources • The core network carries: data, signaling, service • Max. reachable user rates • Macro-cells: 144 kb/s • Micro-cells: 384 kb/s • Pico-cells: 2 Mb/s

35. Wireless broadband accessUniversal Mobile Telecom. Standard (UMTS) - Physical l. • UMTS is based on the CDMA multiple access to the physical medium • Bits are encoded by time-domain modulation with carefully chosen “chip” sequences • Two access modes • FDDmode: CDMA + FDMA • TDDmode: CDMA + FDMA + TDMA

36. Wireless broadband accessLocal Multipoint Distribution Service (LMDS) - Architecture • The fiber-to-wireless conversion takes place into the base station • The access network is connected to the home network via a Network Interface Unit (NIU)

37. Wireless broadband accessLocal Multipoint Distribution Service (LMDS) – Physical l. • Downstream: TDM • Upstream: FDMA / TDMA

38. Wireless broadband accessLocal Multipoint Distribution Service (LMDS) – MAC protoc. • The termination point in the access node is called Air Interface Unit (AIU) • The upstream slots from each NIU are divided into three types • Polling: responses to AIU calls • Reserved: assigned to the NIU for data transmission • Contended: can be accessed by more NIUs (collisions possible)

39. Wireless broadband accessWireless access strength • Personal mobility • Low deployment costs (only for the base-stations) • Low-cost subscriber terminals • Rapid (immediate) deployment • Topology flexibility • Quick setup of temporary networks (e.g. conference Internet room) • Easy and immediate new-subscriber logon • Integration with the wide-spread cell-phone technology

40. Wireless broadband accessWireless access weakness • The RF spectrum is a public shared resource: limited availability of transmission bandwidth, regulated frequency assignment • Limited user bandwidth • High loss • Interferences, sensitivity to obstacles (multipath fading) and meteo conditions, typical BER without error-correction mechanisms 10-3) • Need for base stations with limited range • Network security, need for authentication and cryptography procedures • Limited transmission power of subscriber-terminals (battery power supply) • Electromagnetic pollution and environmental impact of the base stations

41. Wireless broadband accessNetwork architecture GPRS UMTS WLAN LMDS

42. Wireless broadband accessTransmission frequency allocation

43. Wireless broadband accessSubscriber’s bandwidth

44. Wireless broadband accessMultiple-access / multiplexing techniques

45. Wireless broadband accessSupported access services • GPRS: modem data rate; voice service provided by GSM • UMTS: innovative “location based” services

46. Wireless broadband accessNetwork protocols • GPRS MAC • Up to 8 time-slots assigned to mobile stations • Explicit signaling to start transmission when the first user has been selected • UMTS MAC • Similar to GRPS • WLAN MAC • PCF: controls terminal access without contention (real-time services) • DCF: controls terminal access with contention (asynchronous data services) • LMDS MAC • Transmission frequency and time-slots are allocated on user demand or by other dynamic techniques • Besides MAC, the network protocols also manage authentication, fragmentation/reassembly, cryptography, synchronization, error control/correction, power management