1. Optical access networks
As part of the course: “TTM1” by Steinar Bjørnstad

2. Content Optical Access Networks
Motivation
Main characteristics
FTTC, FTTB, FTTH
WDM-PON
WDMA
Statistical Multiplexing
WDM light-sources for access networks
Systemarchitecture
Protocol-stack
“PON in adolescence, from TDMA to WDM-PON”

3. Expected characteristics of future access-networks
Need for real-time services
Evolve from text-based web to image and video-based web.
Convergence among broadcast services and Internet-sevices
Everything in one fiber
Symmetrical traffic-pattern?

4. Bandwidth and access-networks
Triple-play supports
HDTV broadcast
Standard definition TV channels (multitude)
Voice (over Internet Protocol VoIP)
Plain old telephony service (POTS)
Video on demand
Video conference
Red selection: is offerd in current Triple-Play networks

5. Expected bandwidth growth
Perhaps too optimistic?
Remember data-
compression!

6. Access-networks and cost
Costs have to be shared among several customers
Components cost is more important here than for metro and core networks
Laying fiber and digging ditches for the fiber may represent ~50 % of the total costs
Will represent a lower limit to the costs of FTTx installation
If copper wires are already laid in tubes in the ground, then digging and laying of the fiber is substantially cheaper
Equipment costs represent ~25 %
As the technology improves and the volume increases the costs are continously reduced

7. Access technologies properties: xDSL
Typically asymmetric, downlink 1/4-1/8 of uplink
Twisted pair copper cable, fundamental physical limit is close, Shannon theorem
Bandwidth/distance tradeoff 52
VDSL
Shannon 25 15
Capacity
Mbit/s
ADSL 6
ADSL/RealADSL2 1
1.5 3 6
Distance (Km)
VDSL required for high capacity triple play

8. ADSL plant CPE DSLAM typically 300m – 3 km optical fibre
DSLAM: DSL Access Multiplexer CPE: Customer Premises Equipment

9. Upgrading to VDSL CPE BAP optical fibre VDSL-26 Mbit asym: < 1km
DSLAM
CPE
BAP
DSLAM
optical fibre
VDSL-26 Mbit asym: < 1km
VDSL-52 Mbit asym: < 300m
BAP: Broadband Access Point

10. FTTx OLT Another access technology in the building
Fibre cable: Fiber to the Home (FTTH)
Fibre cable: Fiber to the Curb (FTTC)
Another access technology
in the building
Fibre cable: Fiber to the Building (FTTB)

11. Fiber to the Home (FttH) variants
Many Fibers =>
no external power
is needed
Consentrator =>
less fibers,
needs power
Passive =>
Higher power loss
Do not need power

12. Point-to-Point Optical Network
OLT
ONU
FttH architecture comparison
pros:
the ultimate performance
cons
use of many fibres
multi-fibre cable
several 10s of kms
Schematic of Physical Plant

13. Active (AON) versus passive (PON) Optical Network
OLT
ONU
several kms
Remote
Node (RN)
Active = needs power!
Passive = passive splitting
(No need for power)
multi-fibre cable
Schematic of Physical Plant

14. PON: SCMA, TDMA, WDMA Sub Carrier Multiple Access (SCMA)
Unique RF frekquency to each subscriber. Share wavelengths
Time Division Multiple Access (TDMA)
Collision avoidance with access protocols
ATM-PON (B-PON), Gigabit PON (G-PON), Ethernet-PON (E-PON), Gigabit Ethernet PON (GE-PON)
Wavelength Division Multiple Access (WDMA)
no collisions
higher capacity
more expensive

15. Passive Optical Network (TDMA)
Time-sharing of
fiber resources
ONU
OLT
downstream
passive splitter
Limitation on power budget
Burst mode transmission
Different power from each subscriber
Makes capacity upgrades difficult
up to 20km
OLT: Optical Line Terminal ONU: Optical Network Unit

16. Passive Optical Network (TDMA)
FttH architecture comparison
pros:
passive fibre plant
low OpEx
one connection at OLT
cons:
broadcast centric
less scalable
less upgradeable
complex customer differentiation
ONU
OLT
upstream
passive splitter
up to 20km
OLT: Optical Line Terminal ONU: Optical Network Unit

17. TDMA PON’s – Two variants
EPON – Ethernet PON
Japan and Korea
Low interface cost
Integrated with Ethernet OAM
GPON – Gigabit PON
Widely deployed in US and Europe
Higher bandwidth and bandwidth efficiency than EPON
Native support of legacy services
Longer reach

18. Downstream Ethernet-PON
ATM is expensive, Ethernet sells in high volume and is therefore cheap
QoS og VLAN
Fiber resources in E-PON is shared and Point-to-Point
Ethernet broadcast downstream (as in CSMA/CD)
All frames are received by all subcribers
Upstream the ONUs must share capacity and resources

19. Upstream and multiple access
Collisions must be avoided
Too long distances implies a too long collision domain
Time-sharing is therefore preferred, timeslots to each ONU
All ONUs are synchronized to a common time-reference
Buffer in ONU assembles packets and sends in time-slot
Allocation of resources is an issue

20. GPON/EPON characteristics

21. WDM PON for the future
GPON/EPON may not handle future requirements on bitrate
10GPON – 10 Gb/s
Power budget imposes severe limitations on distances and splitting ratio
WDM-PONs solves the limitations of TDMA-PON
Dedicated wavelength to each subscriber
May be combined with TDMA-PON in a hybrid, allowing 1:1000 splitting ratio.
Many variants of WDM-PON

22. WDM-PON (WDMA)
OLT
ONT
WDM, One wavelength to each subscriber

23. Basic WDM-PON architectures
B&S architecture
Passive splitter
Unique filter in ONU
Individual wavelength upstream
Broadcast security issues
AWG based
Low insertion loss, 5 dB
Universal Rx
Wavelength specific Tx
Periodic routing behavior
AWG + Identical ONU’s
Single shared wavelength upstream (TDMA)
Broadband LEDs and spectral slicing give poor power budget
Bidirectional OLT using a circulator

24. Colourless identical ONU’s
SOA broadband modulators + seed lasers: Laser adjust to Seed wavelength
Separate upstream and downstream fibre required
Reflective SOA
Re-use OLT Tx wavelength
Seed signal achieved using FSK downstream
FSK removed in RSOA and replaced by OOK upstream

25. Most Cost effective: CWDM-PON
16 CWDM wavelengths on SFW supports 8 ONU’s
1270 nm to 1610, ITU-T standard
High power budget but potential problems with old fibers (OH peak)
Employs standard low-cost pluggable SFP modules
Capex is low, Opex moderate (higher than colourless)
DWDM much more expensive than CWDM, why?

26. Power budget CWDM What is a power budget? What is it useful for?
What causes the greatest loss?
Why is the power budget higher for DWDM compared to CWDM

27. CAPEX Cost on different PON-solutions
CWDM most cost-effective, but lowest splitting ratio
Amplified TDMA highest splitting ratio

28. Unified infrastructure: core to access
PON not only to residentials
Mobile back-haul
ADSL back-haul
Enterprise networks
Combine with WDM Metro rings
Combine with ROADM nodes
Cost optimization
Common management and control plane required
Common protocols required (Not SDH and Ethernet and…)

29. Summary GPON and Point-to-point is presently being deployed
In Europe
GPON does not handle the future needs for bandwidth
WDM-PON and point-to-point scales
Hybrid GPON and WDM-PON allows a gradual migration towards WDM-PON
PON’s may be used for more than access to residentials
Business customers
Mobile base-station back-haul
DSL back-haul