Transcript GPONx

GPON (Gigabit Passive Optical
Network)
What is GPON?
Passive Optical Network (PON)
• Completely passive infrastructure
• Single fiber carries multiple wavelengths
• Example:
– 2.48 Gbps downstream (1490 nm)
– 1.24 Gbps upstream (1310 nm)
– Serve Remote Bldgs Up to 20Km
GPON (Gigabit Passive Optical
Network)
• EPON is Fast Ethernet (100 Mbps) PON, while
• GPON is Giga bits Ethernet (10 Gbps) PON.
– End-to-end GPON provides IEEE 802.3 standards based
Ethernet interfaces for both the network core, aggregation
and the user interface
• Ethernet PONs build on the ITU standard G.983 for
ATM PONs and seek to bring to life the dream of a full
services access network that delivers converged data,
video and voice over a single optical access system.
Emergence of Optical Networks
Core/Backbone/LongHaul
Optical
Line System
Mesh
Backbone
Network
OLS 40/80G
OLS 400G
800G/1.6T
Optical
Cross
Connect
Regional
Point
of
Presence
CO-1
Metro
Edge
Switch
Access
C/DWDM
Metro
Edge
Switch
CO-n
C/DWDM
Metro
Edge
Switch
Node
Local
Service
Node
Metro
DMX
Metro
DMX
C/DWDM
EPON
node
Regional/Metro
PON
Access/Enterprise
DSL,
FTTH
Categorizing Optical Networks
Who Uses
it?
Span
(km)
Bit Rate
(bps)
Multiplexing
Fiber
Laser
Receiver
Core/
LongHaul
Phone
Company,
Gov’t(s)
~103
~1011
(100’s of
Gbps)
DWDM/
TDM
SMF/
DCF
EML/
DFB
APD
Metro/
Regional
Phone
Company,
Big Business
~102
~1010
(10’s of
Gbps)
DWDM/
CWDM/T
DM
SMF/
LWPF
DFB
APD/ PIN
Access/
LocalLoop
Small
Business,
Consumer
~10
~109
(56kbps
- 1Gbps)
TDM/
SCM/
SMF/
MMF
DFB/ FP
PIN
DWDM:
CWDM:
TDM:
SCM:
SMF:
MMF:
LWPF:
DCF:
EML:
DFB:
FP:
APD:
PIN:
Dense Wavelength Division Multiplexing (<1nm spacing)
Coarse Wavelength Division Multiplexing (20nm spacing)
Time Division Multiplexing (e.g. car traffic)
Sub-Carrier Multiplexing (e.g. Radio/TV channels)
Single-Mode Fiber (core~9mm)
Multi-Mode Fiber (core~50mm)
Low-Water-Peak Fiber
Dispersion Compensating Fiber
Externally modulated (DFB) laser
Distributed Feedback Laser
Fabry-Perot Laser
Avalanche Photodiode
p-i-n Photodiode
Optical Transmission
electrical
signal
Optical
Fibre
Transmission
System
optical
signal
Optical
Fibre
Transmission
System
electrical
signal
Advantages of optical transmission:
1. Longer distance (noise resistance and less attenuation)
2. Higher data rate (more bandwidth)
3. Lower cost/bit
7
Optical Networks
• Passive Optical Network (PON)
– Fiber-to-the-home (FTTH)
– Fiber-to-the-curb (FTTC)
– Fiber-to-the-premise (FTTP)
• Metro Networks (SONET)
– Metro access networks
– Metro core networks
• Transport Networks (DWDM)
– Long-haul networks
8
Optical Network Architecture
DWDM
SONET
Long Haul
Network
Metro
Network
Metro
Network
transport network
PON
Access
Network
Access
Network
Access
Network
Access
Network
CPE (customer premise)
9
Passive Optical Network (PON)
• Standard: ITU-T G.983
• PON is used primarily in two markets: residential and
business for very high speed network access.
• Passive: no electricity to power or maintain the
transmission facility.
– PON is very active in sending and receiving optical signals
• The active parts are at both end points.
– Splitter could be used, but is passive
10
Passive Optical Network (PON)
OLT: Optical Line Terminal
ONT: Optical Network Terminal
Splitter
(1:32)
11
PON – many flavors
• ATM-based PON (APON) – The first Passive optical network
standard, primarily for business applications
• Broadband PON (BPON) – the original PON standard (1995). It
used ATM as the bearer protocol, and operated at 155Mbps. It
was later enhanced to 622Mbps.
– ITU-T G.983
• Ethernet PON (EPON) – standard from IEEE Ethernet for the
First Mile (EFM) group. It focuses on standardizing a 1.25 Gb/s
symmetrical system for Ethernet transport only
– IEEE 802.3ah (1.25G)
– IEEE 802.3av (10G EPON)
• Gigabit PON (GPON) – offer high bit rate while enabling
transport of multiple services, specifically data (IP/Ethernet)
and voice (TDM) in their native formats, at an extremely high
efficiency
– ITU-T G.984
12
xPON Comparison
BPON
EPON
GPON
Standard
ITU-T G.983
IEEE 803.2ah
ITU-T G.984
Bandwidth
Down: 622M
Up: 155M
Symmetric:
1.25G
Down: 2.5G
Up: 2.5G
Downstream λ
1490 &1550
1550
1490 & 1550
Upstream λ
1310
1310
1310
Transmission
ATM
Ethernet
ATM, TDM,
Ethernet
13
PON Case Study (BPON)
Optical Line Terminal (OLT)
(Central Office)
Packet Core
(IPoATM)
Optical Network Terminal (ONT)
(customer premise)
Two Ethernet ports
One T1/E1 port
Optical transport: 622M bps
T1/E1
802.3
CES
RFC2684
AAL1
AAL5
SAR/CS
ATM
TDM Core
(PSTN)
PON (G.983)
14
GPON
15
EPON Evolution
16
17
18
EPON Downstream
19
EPON Upstream
20
Optical Fiber Attributes
Attenuation:
Due to Rayleigh scattering and chemical absorptions,
the light intensity along a fiber decreases with
distance. This optical loss is a function of wavelength
(see plot).
Dispersion:
Different colors travel at different speeds down the
optical fiber. This causes the light pulses to spread
in time and limits data rates.
launch
receive

t
t
Chromatic Dispersion is caused mainly by the
wavelength dependence of the index of
refraction (dominant in SM fibers)
Modal Dispersion arises from the differences in
group velocity between the “modes” travelling
down the fiber (dominant in MM fibers)

t
Types of Dispersion
t
Non-Linear Effects in Fibers
Self-Phase Modulation:
When the optical power of a pulse is
very high, non-linear polarization terms
contribute and change the refractive
index, causing pulse spreading and delay.
Cross-Phase Modulation:
Same as SPM, except involving more than
one WDM channel, causing cross-talk
between channels as well.
Four-wave Mixing:
Stimulated Brillouin
Scattering:
Non-linearity of fiber can cause ‘mixing’
of nearby wavelengths causing
interference in WDM systems.
Acoustic Phonons create sidebands that
can cause interference.
Attenuation/Loss in Optical Fiber
3.0
First
Window
ATTENUATION (dB/km)
2.5
Second
Window
2.0
Third
Window
1.5
1.0
0.5
800
900
1000
1100
1200
1300
1400
1500
1600
1700
WAVELENGTH (nm)
1310nm
850nm
First window, second window, third
window correspond (roughly) to
first, second and third generation
optic network technology
•
•
•
1550nm
First Window @ 850nm
– High loss; First-gen. semiconductor diodes (GaAs)
Second Window @ 1310nm
– Lower Loss; good dispersion; second gen. InGaAsP
Third Window @ 1550nm
– Lowest Loss; Erbium Amplification possible
Dispersion Characteristics*
Third
Window
Second
Window
3.0
DISPERSION COEFF, D (ps/km-nm)
0
First
Window
-30
-60
-90
-120
800
900
1000
1100
1200
1300
1400
1500
1600
1700
WAVELENGTH (nm)
850nm
•
•
* Modal dispersion not
included
•
1310nm
1550nm
Standard SMF has zero dispersion at 1310nm
– Low Dispersion => Pulses don’t spread in time
Dispersion compensation needed at 1550nm
– Limits data transmission rate due to ISI (inter-symbol
interference)
Dispersion not so important at 850nm
– Loss usually dominates
Characterization of System Quality
Bit Error Rate:
Eye Diagram:
input known pattern of ‘1’s and ‘0’s and see how many
are correctly recongnized at output.
Measure ‘openness’ of transmitted 1/0 pattern using
scope triggered on each bit.
‘Eye opening’
Effect of Dispersion and Attenuation on Bit Rate
Attenuation limited
Distance (km)
30
20
Dispersion limited
1310nm
850nm
10
1550nm
Coaxial
cable
Twisted Pair
1
Cat 3
limit
x
0.1
1
Cat 5
limit
x
10
100
Bit rate (Mb/s)
Cat 7
limit
x
1000
10,000
• For short reaches (1-2 km), all optics are “Gigabit capable”
• For longer reaches (~10 km), only 1310/1550 nm optics are “Gigabit capable”
Technology Trends
850nm & 1310nm 
Preferred by high-volume,
moderate performance
data comm manufacturers
Reason? You need lots of them, they don’t need to go far,
and you’re not using enough fiber ($) to justify wavelength
division multiplexing (WDM), I.e. low-quality lasers are OK.
1310nm & 1550nm 
Preferred by high performance
but lower volume (today)
telecomm manufacturers
Reason? You don’t need lots, but they have to be good
enough to transmit over long distances… cost of fiber (and
TDM) justifies WDM… 1550nm is better for WDM
DFB vs. FP laser
Simple FP
DFB
+
+
gain
gain
mirror
-
cleave
l
mirror
-
AR coating
l
Etched
grating
FP:
• Multi-longitudinal Mode
operation
DFB:
• Single-longitudinal Mode
operation
• Large spectral width
• Narrow spectral width
• high output power
• lower output power
• Cheap
• expensive
Fiber Bragg Grating External Cavity
Laser for Access/Metro Networks
Typical FBG-ECL:
Lensed
tip
gain
FBG
T=25C
HR
T=85C
AR
Optical Power (dBm)
0
Dl (3dB)
T=20C typ<0.5nm
dl/dT ~ 0.01nm/oC
-20
-40
-60
<1nm grating
Bell Labs FBG-ECL:
XB region
gain
FBG
-80
1309.0 1309.5 1310.0 1310.5 1311.0 1311.5 1312.0
Wavelength (nm)
T=25,
85C
?
HR
•
•
AR
1-2nm grating
SHOW PLOTS OF FBG-ECL DATA
SHOW PICTURE OF XPONENT’S EXTENDED REACH FP
(from Xponent Photonics, Inc.)
Fiber Bragg Grating External Cavity Laser
FBG-ECL
output
-20
Typical
FP output
• Narrow FBG bandwith limits
output Dl~1nm for extended
reach or WDM applications.
Power (dB)
-30
-40
• Simple design (AR-coated FP,
XBR, butt-coupled FBG)
-50
• Mode-hop free operation over 070C
-60
-70
1305
1310
1315
wavelength (nm)
1320
1325
Wavelength Stability of FBG-ECL
DFB drift ~ 0.1nm/oC
FP drift ~ 0.3nm/oC
1311.0
lave dependence 0.008nm/C
Wavelength(nm)
1310.9
1310.8
1310.7
1310.6
CW, ~40mA bias
1310.5
1310.4
1310.3
20
30
40
50
60
o
Temperature ( C)
70
80
Filter bandwidths of
WDM Mux/Demux
0.8nm (100GHz)
DWDM:
• High channel count, narrow channel spacing
• Temp-stablized DFBs required
• Temp-stablized AWGs required (typically)
1480nm
>100 channels (C+L+S)
1610nm
20nm
CWDM:
• Low channel count, large channel spacing
• Uncooled DFBs can be used
• Filters can be made athermal
1260nm
18 channels (O,E,S,C,L)
1610nm
3.2nm (400GHz)
xWDM?:
• Moderate channel count, moderate channel spacing
• FBG-ECL or Temp-stablized DFBs required
• Filters can be made athermal
• suitable for athermal WDM PON!
1480nm
32-64 channels (C+L+S)
1610nm
Example 1: 10Gbps Coarse WDM
-Used currently in Metro systems (rings, linear, mesh)
-Spacing of CWDM ‘grid’ determined by DFB wavelength drift
-Current systems limited to 2.5Gbps due to cheaper optics
-Possible upgrade to 10Gbps?
CWDM Lasers
 16 uncooled, directly modulated CWDM lasers (DMLs)
 rated for 2.5 Gb/s direct modulation (cheap! - $350 a piece)
 NRZ-modulation at 10 Gb/s (careful laser mounting; no device selection)
2.5-Gb/s DML
50W line
47W chip resistor
CWDM System Improvement using
Electronic Dispersion Compensation
Example 2: Ethernet Passive Optical
Networks
Headend/CO
Outside Plant
PSTN
Internet
IP Video
Services
•
•
•
NO Active Elements in Outside Plant
Enable “triple-play” services
Simple & cheap
PON
Homes/Businesses
Choices of PONs
Architecture/Layout
Upstream Multiplexing
…
OLT
ONU
ONU
ONU
ONU
Linear Bus: lossy, fiber lean
TDM: simple, cheap
ONU
OLT
ONU
ONU
ONU
WDM:simple, expensive
Ring: lossy, protected
ONU
ONU
ONU
OLT
ONU
Simple or Cascaded Star: low loss
SCM: complex, expensive
OLT=Optical Line Termination (head-end)
ONU=Optical Network Unit (user-end)
EPON Access Platform
“premium access”
Management
Business
Data
optical
splitter
DFB
32 subscribers
Per EPON
.
.
.
EPON
BigIron 4000
NETWO RKS
1
2
3
4
Link
5
6
Link
7
8
Link
B 8G
8-port
Gigabit
Link
A ctivity
A ctivity
A ctivity
A ctivity
Base-FX
100
B24FX
Metro
Network
FOUNDRY
10G Ethernet
Or up to 6 1GbE
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
12 EPONS
Metro Edge
optical
splitter
Broadcast Video
VOD
Panther EPON OLT Chassis
Voice/IP
Services
1232  384 subscribers
Dynamic bandwidth
Guaranteed QOS
Note on Lasers:
-Use DFB at headend (shared)
-Use FP at Homes (not shared)
Residence
Lucent
EPON ONU
+ Gateway Video/IP Television
Voice/IP POTS service
High-speed data
FP
ONU Design
PON
1.25G BM
BiDi Xcvr
SERDES
(w/CDR)
GigE uplink
watchdog1
“CHILD”
BOARD
watchdog0
FPGA w/
Embedded
mProcessor
discovery
Periodic
Report
generator
Packet
memory
GMII
TX
Packet
Memory
Serial
Port
10/100bT
diagnostic
port
Mux
Demux
Memory
manager
Queue
manager
RX
Flash (CPU)
memory
CPU
TX
EPON MAC
EPON core
Report
Generator
“PARENT”
BOARD
FPGA
EPON driver
Timesta
CRC
mp
LLID
RX
Control
Parser
SERDE
S
&
Optics
OLT Design
ONU
PON
watchdog1
GigE uplink
SERDES
(w/CDR)
1.25G BM
BiDi Xcvr
watchdog0
discovery
Keepalive
scheduler
EPON driver MPCP driver
FPGA w/
Embedded
mProcessor
EPON core MPCP core
Grant
List
RX
GMII
TX
Packet
Memory
RTT table
Memory
manager
Queue
manager
RTT Processor
Report processor
Report
table
10/100bT
diagnostic
port
Flash (CPU)
memory
Serial
Port
Mux
Gate
Generator
TX
EPON MAC
Demux
Packet
memory
Timesta
CRC
mp
LLID
RX
SERDE
S
&
Optics
Control
Parser
FPGA
CPU
EPON downstream/upstream traffic
O
N
U
Control “Gates”
1
Edge Router
•
3
2
OLT
OLT
1
Upstream: Some form of TDMA
– ONU sends Ethernet Frames in timeslots
– Must avoid timeslot collisions
– Must operate in burst-mode
– BW allocation easily mapped to timeslots
2
3 3
O
N
U
O
N
U
Downstream: continuous, MAC addressed
– Uses Ethernet Framing and Line Coding
– Packets selected by MAC address
– QOS / Multicast support provided by Edge Router
Edge Router
•
2
1
2
2
3
Control “Reports”
2
O
N
U
O
N
U
1
O
N
U
2
2
3 3
ONU: Optical Network Unit
OLT: Optical Line Termination
PON TDMA  BURSTMODE OPTICS
• Because upstream transmissions must avoid collisions, each ONU must
transmit only during allowed timeslot
• Transmitting “0”s during quiet time is not allowed!
– Average “0” power ~ -10 to –5 dBm
– Summing over 16 ONUs would result in a ~1dBm noise floor
• Distinct from “Bursty” nature of Ethernet TRAFFIC
– Ethernet transmitters never stop transmitting (Idle characters)
– CDR circuit at receiver stays locked even when no data is transmitted
• Besides PONs, other systems use burstmode
– Wireless
– Shared buses/backplanes
– Optical burst switched (OBS) systems
BURSTMODE TRANSMITTERS
Data
Tx FIFO
Encoder
Serializer
Clock
Prebias
Optical
output
“1”
• Driving LD below
Threshold causes
Jitter
• Off-state ~ -40dBm
“0”
“off”
Ith
current
Modulation
current
Transmitter
Physical
Media
BURST-MODE RECEIVERS
Data
Rx FIFO
Decoder
Deserializer
Clock
Reset
•
•
PROBLEM OF FAST CDR LOCKING
GAIN LEVELING & DYNAMIC RANGE OF
OPTICAL RECEIVER
CDR
Limiting
Amp
Receiver
IMPACT ON EFFICIENCY
Upstream Bursts
Cascaded PON
ONU 2
ONU 1
OLT
1:8
1:4
ONU 1
ONU 2
..
.
guardband
Throughput Efficiency
1.05
Burst-mode transceivers
Utilisation
1
0.95
0.9
0.85
Our current situation
0.8
0.75
Standa
rd GE
transc
eivers
0.7
0
1000
2000
3000
Laser AGC
on settle
D
M
A
C
S
M
A
C
V
L
A
N
H
L
E
N
CDR
lock
Byte
sync
O P C
T L
T
S
I F R H
O E
T
I
D F O K
S N
L
P
ST T SM
AGC+CDR+LASER ON/OFF (ns)
ONU1 payload
(Ethernet Frames)
H F WC
U
D S D S A
L L S H
R
I P P E C
E A Z K
G
P T T Q K
N GS E SM
64 Bytes
Ethernet
IP
Laser
off
Data
~1460 Bytes
TCP
C
R
C
PON-FTTH Protocols
• ITU-T G.983
– APON (ATM ((Asynchronous Transfer Mode) Passive Optical
Network)
– BPON (Broadband PON)
• ITU-T G.984
– GPON(Gigabit-capable PON):
• IEEE 802.3ah
– EPON(Ethernet PON)
– GEPON
• IEEE P802.3av
– 10GEPON
ITU-T G.984 G-PON
• GPON (Gigabit Passive Optical Network) is the most widespread PON
protocol in FTTH networks in Europe and in United States. In 2003-2004
was approved by the ITU-T under the recommendations G.984.1, G.984.2,
G.984.3, G.984.4 and G.984.5.
• GPON Objectives
• This standard was created to cover new demands on the network:
– Multiservice transportation: TDM voice, synchronous SONET/SDH transport,
Ethernet (10/100 BaseT), ATM,...
– Multirate: Support multiple bitrate within the same protocol, including
symmetrical speeds of 622 Mb/s, 1.25 Gb/s, and asymmetric 2.5 Gb/s in the
downlink and 1.25 Gb/s in upstream.
– Maximum range of 20 km, although the standard has been prepared to reach
up to 60 km.
– OAM end to end.
– Protocol level security for the downlink due to the multicast nature of PON.
– The maximum number of users that can hang from a single fiber is 64 (the
system is ready to provide service to up to 128).
Features and Techniques
•
•
•
•
•
•
•
•
•
•
Data multiplexing: Both channels of information (downstream and upstream) travel in the same fiber using
different wavelengths. This scheme uses a WDM (Wavelength Division Multiplexing) multiplexing.
Downstream - TDM: TDM technology is used (Time Division Multiplexing). All data are transmitted to all the ONTs.
Each ONT filters the received data and is only able to access those data that are directed toward it. It is possible to
encrypt the traffic that is ongoing between OLT-ONT to be inaccessible to a second ONT modified to behave as a
spy.
Upstream - TDMA: TDMA technology is used (Time Division Multiple Access). The OLT monitors the upstream
channel, allocating transmission time windows to each ONT. It requires a media access control to avoid collisions
and to allocate bandwidth among users. The perfect synchronization of the packets upstream to the OLT is needed
in order to reconstruct the GPON frame. For this reason it is necessary for the OLT knows the distance to each
ONTs in order to take into account the delay experienced by the information from the user.
User ID: GPON provides a mechanism that allows the OLT to identify each ONT in the same fiber network. For this
reason, each ONT has a unique serial number which is known by the OLT.
Remote configuration of the ONTs: One of the main challenges that resolves GPON technology is the remote user
equipment. This ensures a significant cost savings resulting from the maintenance since it is not necessary to
intervene in the customer's home.
To do so, within the standard GPON it has developed a protocol called OMCI (ONT Management and Control
Interface). This protocol allows remote configuration of the ONTs. For each ONT provides a management channel
between OLT and ONT. Includes management, performance, alarm monitoring, fault and performance. The OMCI
protocol is one of the key aspects to ensure interoperability between manufacturers. There are various
mechanisms of OMCI information transmission..
Transport Protocols: The GPON standard provides two options regarding the transport protocols that can be used:
ATM is used by the UPON and BPON, which is a continuity solution.
GEM (GPON Encapsulation Method): This is a new protocol defined for use in the G.984s GPON.
Implementing Multicast: Multicast is a protocol used for television broadcasting. Not to be confused with the
video on demand service. This protocol, integrated in the ONT, OLT and decoder, enables the user to select the
television channel that receives at each moment.
GPON vs. GigE (Gigabit Ethernet)
Fiber To The Curb
Hybrid Fiber Coax and VDSL
• switch/transceiver/miniDSLAM located at curb or in basement
• need only 2 optical transceivers
but not pure optical solution
• lower BW from transceiver to end users
• need complex converter in constrained environment
core
N end users
feeder fiber
copper
50
access network
Fiber To The Premises
we can implement point-to-multipoint topology purely in optics
• but we need a fiber (pair) to each end user
• requires 2 N optical transceivers
• complex and costly to maintain
core
51
N end users
access network
An obvious solution
deploy intermediate switches
• (active) switch located at curb or in basement
• saves space at central office
• need 2 N + 2 optical transceivers
core
N end users
feeder fiber
fiber
52
access network
The PON solution
another alternative - implement point-to-multipoint topology purely in optics
• avoid costly optic-electronic conversions
• use passive splitters – no power needed, unlimited MTBF
• only N+1 optical transceivers (minimum possible) !
access network
1:2 passive splitter
core
N end users
typically N=32
feeder fiber
53
1:4 passive splitter
max defined 128
PON advantages
shared infrastructure translates to lower cost per customer
• minimal number of optical transceivers
• feeder fiber and transceiver costs divided by N customers
• greenfield per-customer cost similar to UTP
passive splitters translate to lower cost
• can be installed anywhere
• no power needed
• essentially unlimited MTBF
fiber data-rates can be upgraded as technology improves
• initially 155 Mbps
• then 622 Mbps
• GPON 1.25 Gbps UL
– 2.5 Gbps DL
54
Terminology
like every other field, PON technology has its own terminology
• the CO head-end is called an OLT
• ONUs are the CPE devices (sometimes called ONTs in ITU)
• the entire fiber tree (incl. feeder, splitters, distribution fibers) is an ODN
• all trees emanating from the same OLT form an OAN
• downstream is from OLT to ONU (upstream is the opposite direction)
NNI
downstream
upstrea
m
Optical Distribution Network
core
splitter
Optical Line Terminal
Optical Access Network
55
Optical Network Units
UNI
Terminal Equipment
PON types
many types of PONs have been defined
APON ATM PON
BPON Broadband PON
GPON Gigabit PON
EPON Ethernet PON
GEPON
Gigabit Ethernet PON
CPON CDMA PON
WPON WDM PON
in this course we will focus on GPON and EPON
(including GEPON)
with a touch of BPON thrown in for the flavor
56
Bibliography
• BPON is explained in ITU-T G.983.x
• GPON is explained in ITU-T G.984.x
• EPON is explained in IEEE 802.3-2005 clauses 64 and 65
– (but other 802.3 clauses are also needed)
Warning
do not believe white papers from vendors
especially not with respect to GPON/EPON comparisons
GPON
57
BPON
EPON
PON
principles
(almost) all PON types obey the same basic principles
OLT and ONU consist of
• Layer 2 (Ethernet MAC, ATM adapter, etc.)
• optical transceiver using different ls for transmit and receive
•
optionally: Wavelength Division Multiplexer
downstream transmission
• OLT broadcasts data downstream to all ONUs in ODN
• ONU captures data destined for its address, discards all other data
• encryption needed to ensure privacy
upstream transmission
• ONUs share bandwidth using Time Division Multiple Access
• OLT manages the ONU timeslots
• ranging is performed to determine ONU-OLT propagation time
additional functionality
• Physical Layer OAM
• Autodiscovery
• Dynamic Bandwidth Allocation
58
PON encapsulation
The majority of PON traffic is Ethernet
So EPON enthusiasts say
use EPON - it's just Ethernet
That's true by definition anything in 802.3 is Ethernet
and EPON is defined in clauses 64 and 65 of 802.3-2005
But don't be fooled - all PON methods encapsulate MAC frames
EPON and GPON differ in the contents of the header
EPON hides the new header inside the GbE preamble
GPON can also carry non-Ethernet payloads
PON header
59
DA
SA
T
data
FCS
BPON history
1995 : 7 operators (BT, FT, NTT, …) and a few vendors form
Full Service Access Network Initiative
to provide business customers with multiservice broadband offering
Obvious choices were ATM (multiservice) and PON (inexpensive)
which when merged became APON
1996 : name changed to BPON to avoid too close association with ATM
1997 : FSAN proposed BPON to ITU SG15
1998 : BPON became G.983
– G.982 : PON requirements and definitions
– G.983.1 : 155 Mbps BPON
– G.983.2 : management and control interface
– G.983.3 : WDM for additional services
– G.983.4 : DBA
– G.983.5 : enhanced survivability
– G.983.1 amd 1 : 622 Mbps rate
– G.983.1 amd 2 : 1244 Mbps rate
– …
60
EPON history
2001: IEEE 802 LMSC WG accepts
Ethernet in the First Mile Project Authorization Request
becomes EFM task force (largest 802 task force ever formed)
EFM task force had 4 tracks
•
•
•
•
DSL (now in clauses 61, 62, 63)
Ethernet OAM (now clause 57)
Optics (now in clauses 58, 59, 60, 65)
P2MP (now clause 64)
2002 : liaison activity with ITU to agree upon wavelength allocations
2003 : WG ballot
2004 : full standard
2005: new 802.3 version with EFM clauses
61
GPON history
2001 : FSAN initiated work on extension of BPON to > 1 Gbps
Although GPON is an extension of BPON technology
and reuses much of G.983 (e.g. linecode, rates, band-plan, OAM)
decision was not to be backward compatible with BPON
2001 : GFP developed (approved 2003)
2003 : GPON became G.984
–
–
–
–
62
G.984.1 : GPON general characteristics
G.984.2 : Physical Media Dependent layer
G.984.3 : Transmission Convergence layer
G.984.4 : management and control interface
Basic Configuration of PON
OLT = Optical Line Termination
ONU = Optical Network Unit
63
BMCDR = Burst Mode Clock Data Recovery
Typical PON Configuration and Optical Packets
64
l allocations - G.983.1
Upstream and downstream directions need about the same bandwidth
US serves N customers, so it needs N times the BW of each customer
but each customer can only transmit 1/N of the time
In APON and early BPON work it was decided that 100 nm was needed
Where should these bands be placed for best results?
In the second and third windows !
• Upstream
1260 - 1360 nm (1310 ± 50) second window
• Downstream 1480 - 1580 nm (1530 ± 50) third window
US
1200 nm 1300 nm
65
DS
1400 nm
1500 nm
1600 nm
l allocations - G.983.3
Afterwards it became clear that there was a need for additional DS bands
Pressing needs were broadcast video and data
Where could these new DS bands be placed ?
At about the same time G.694.2 defined 20 nm CWDM bands
these were made possible because of new inexpensive hardware
(uncooled Distributed Feedback Lasers)
One of the CWDM bands was 1490 ± 10 nm
same bottom l as the G.983.1 DS
1270
1490
So it was decided to use this band as the G.983.3 DS
and leave the US unchanged
guard
available
US
1200 nm 1300 nm
66
DS
1400 nm
1500 nm
1600 nm
1630
l allocations - final
US
1200 nm 1300 nm
DS
1400 nm
1500 nm
1600 nm
The G.983.3 band-plan was incorporated into GPON
and via liaison activity into EPON
and is now the universally accepted xPON band-plan
• US 1260-1360 nm (1310 ± 50)
• DS 1480-1500 nm (1490 ± 10)
• enhancement bands:
– video 1550 - 1560 nm (see ITU-T J.185/J.186)
– digital 1539-1565 nm
67
Data rates (for now …)
PON
BPON
Amd 1
Amd 2
GPON
EPON
10GEPON†
68
DS (Mbps)
155.52
622.08
622.08
1244.16
1244.16
1244.16
1244.16
1244.16
2488.32
2488.32
2488.32
2488.32
1250*
10312.5*
US (Mbps)
155.52
155.52
622.08
155.52
622.08
155.52
622.08
1244.16
155.52
622.08
1244.16
2488.32
1250*
10312.5*
* only 1G/10G usable due to linecode
† work in progress
Reach and splits
Reach and the number of ONUs supported are contradictory design goals
In addition to physical reach derived from optical budget
there is logical reach limited by protocol concerns (e.g. ranging protocol)
and differential reach (distance between nearest and farthest ONUs)
The number of ONUs supported depends not only on the number of splits
but also on the addressing scheme
BPON called for 20 km and 32-64 ONUs
GPON allows 64-128 splits and the reach is usually 20 km
but there is a low-cost 10 km mode (using Fabry-Perot laser diodes in ONUs)
and a long physical reach 60 km mode with 20 km differential reach
EPON allows 16-256 splits (originally designed for link budget of 24 dB, but now 30 dB)
and has 10 km and 20 km Physical Media Dependent sublayers
69
Line codes
BPON and GPON use a simple NRZ linecode (high is 1 and low is 0)
An I.432-style scrambling operation is applied to payload (not to PON overhead)
Preferable to conventional scrambler because no error propagation
– each standard and each direction use different LFSRs
– LFSR initialized with all ones
– LFSR sequence is XOR'ed with data before transmission
EPON uses the 802.3z (1000BASE-X) line code - 8B/10B
– Every 8 data bits are converted into 10 bits before transmission
– DC removal and timing recovery ensured by mapping
– Special function codes (e.g. idle, start_of_packet, end_of_packet, etc)
However, 1000 Mbps is expanded to 1250 Mbps
10GbE uses a different linecode - 64B/66B
70
FEC
G984.3 clause 13 and 802.3-2005 subclause 65.2.3
define an optional G.709-style Reed-Solomon code
Use (255,239,8) systematic RS code designed for submarine fiber (G.975)
to every 239 data bytes add 16 parity bytes to make 255 byte FEC block
Up to 8 byte errors can be corrected
Improves power budget by over 3 dB,
allowing increased reach or additional splits
Use of FEC is negotiated between OLT and ONU
Since code is systematic
can use in environment where some ONUs do not support FEC
In GPON FEC frames are aligned with PON frames
In EPON FEC frames are marked using K-codes
(and need 8B10B decode - FEC - 8B10B encode)
71
Gigabit Passive Optical Network
• One way of providing fiber to the home is through a Gigabit Passive
Optical Network, or GPON.
GPON is a point-to-multipoint access mechanism. Its main characteristic is
the use of passive splitters in the fiber distribution network, enabling one
single feeding fiber from the provider's central office to serve multiple
homes and small businesses.
• GPON has a downstream capacity of 2.488 Gb/s and an upstream capacity
of 1.244 Gbp/s that is shared among users.
– Encryption is used to keep each user's data secured and private from other
users.
– Although there are other technologies that could provide fiber to the home,
passive optical networks (PONs) like GPON are generally considered the
strongest candidate for widespread deployments.
Why choose GPON?
•
When planning a fiber-to-the-home (FTTH)
evolution for their access networks, service
providers can choose between three generic
FTTH architectures:
– point-to-point; active Ethernet; and passive optical
networking (PON) such as GPON.
Point-to-point
• "Point-to-point" is an Ethernet FTTH architecture
similar in structure to a twisted-pair cable phone
network; a separate, dedicated fiber for each home
exists in the service provider's hub location.
– The point-to-point architecture has merits for small-scale
deployments such as citynets, but is not suitable for largescale deployments due to its poor scalability in terms of
hub location space or the number of required hub
locations, power consumption and feeder fibers.
Active Ethernet
• An "active Ethernet" architecture is based on
the same deployment model as fiber to the
node (FTTN) with active street cabinets; it is
therefore feasible as a complement or
migration path towards FTTH for larger
deployments in very high-speed digital
subscriber line (VDSL)-dominated
environments.
GPON is a fully optical architecture
• GPON is a fully optical architecture option that offers the best of all
worlds.
• A GPON system consists of an optical line terminal (OLT) that connects
several optical network terminals (ONTs) together using a passive optical
distribution network (ODN).
• Like active Ethernet, it aggregates users in what is called the "outside
plant" or OSP, which means no mess of fibers in a central office
somewhere; like point-to-point, it avoids the need for active electronics in
the field by employing a passive OSP device (the optical splitter).
• Being a passive device, the GPON splitter requires no cooling or powering
and is therefore extremely stable; in fact, it virtually never fails.
How does GPON work?
•
•
GPON has been called "elegant" for its ability to share bandwidth dynamically on a
single optical fiber. Like any shared medium, GPON provides burst mode
transmission with statistical usage capabilities.
This enables dynamic control and sharing of upstream and downstream bandwidth
using committed and excess information rate (CIR and EIR) parameters. Users can
be assured of receiving their committed bandwidth under peak demand
conditions, and of receiving superior service when network utilization is low.
– While subscribers rarely require sustained rates of 100 Mb/s each, bursting beyond this to the
full line rate of a PON system (about 1.25 Gb/s upstream or 2.5 Gb/s downstream in the case
of GPON) is easily enabled using the right subscriber interface.
– This allows a GPON to be used for many years even if subscribers have a regular need to
transmit beyond an engineered guaranteed limit of 100 Mb/s.
GPON Components
• GPON OLT
• GPON ONT
• GPON Encapsulation Method (GEM)
GPON Components
• There are several primary components of a last-mile PON -- the OLT,
the fiber and splitters, and the ONU:
– OLT (Optical Line Terminal) - located at the CO, the OLT interfaces
with the metropolitan network. The main functionality of the OLT is to
adapt the incoming traffic (Voice/Data/Video) from the metropolitan
rings into the PON transport layer.
– ONU (Optical Network Unit) and ONT (Optical Network
Termination) - ONU and ONT are basically the same device – ONT is
located at the customer premise, and ONU is located outside the
home. ONU receives optical signal and converts it into an electrical
signal for use in the customer premises.
– PON Splitters - With a single PON splitter, 32 subscribers can be
served with two-way ATM. This way, it is not necessary to include a lot
of add/ drop multiplexers and install the dreaded OSP cabinet. The
PON splitters can be arranged in star, ring, or tree configurations to
increase reliability.
Optical Line Terminal (OLT)
• The Optical Line Terminal (OLT)
– Acts as the central aggregation element
– Located in the Core Data Center
– Replaces multiple L2 switches
– Can aggregate up to 8,000 end users
GPON Architecture
GPON Transmission Convergence (GTC)
Optical Line Terminal (OLT)
Ethernet Switching Unit (ESU)
OLT
• http://www.tellabs.com/products/tlab1100_g
pon-olt.pdf
Optical Network Terminal (ONT)
Typical ONT - Tellabs
Passive Optical Splitter
Fiber Distribution Hub (FDH)
GPON Standards
• GPON was developed with the support of the FSAN (Full Service Access
Network) Group and the ITU (International Telecommunication Union).
• These organizations bring the major stakeholders in the telecoms industry
together to define common specifications, ensuring full interworking
between OLTs and ONTs. The IEEE (Institute of Electrical and Electronics
Engineers) has also defined a PON standard, called Ethernet PON or EPON.
• The EPON standard was launched earlier than GPON and has been
deployed successfully. IEEE specs are however restricted to the lower
optical and media access layers of networks, and full interoperability for
EPON must therefore be managed in a specific case-by-case way at every
implementation.
• Additionally, EPON runs at only 1 Gb/s, upstream as well as downstream,
providing a lower bandwidth than GPON. These factors make EPON a less
attractive technology choice for providers making FTTH investment
decisions today.
GPON Standards - ITU-T
• GPON ITU-T G.984.1 GPON General
Characteristics
• GPON ITU-T G.984.2 Physical Media Dependent
(PMD)
• GPON ITU-T G.984.3 Transmission Convergence
• GPON ITU-T G.984.4 ONT Management and
Control Interface (OMCI)
• GPON ITU-T G.984.5 Enhancement Band
• GPON ITU-T G.984.6 Optical Reach Extension
(G.984.re)
GPON payloads
GTC payload potentially has 2 sections:
– ATM partition (Alen * 53 bytes in length)
– GEM partition (now preferred method)
PCBd
ATM cell
ATM cell
…
ATM cell
GEM frame
GEM frame
…
ATM partition
Alen (12 bits) is specified in the PCBd
Alen specifies the number of 53B cells in the ATM partition
if Alen=0 then no ATM partition
if Alen=payload length / 53 then no GEM partition
ATM cells are aligned to GTC frame
ONUs accept ATM cells based on VPI in ATM header
GEM partition
Unlike ATM cells, GEM delineated frames may have any length
Any number of GEM frames may be contained in the GEM partition
ONUs accept GEM frames based on 12b Port-ID in GEM header
101
GEM frame
GPON Encapsulation Mode
A common complaint against BPON was inefficiency due to ATM cell tax
GEM is similar to ATM
– constant-size HEC-protected header
– but avoids large overhead by allowing variable length frames
GEM is generic – any packet type (and even TDM) supported
GEM supports fragmentation and reassembly
GEM is based on GFP, and the header contains the following fields:
– Payload Length Indicator - payload length in Bytes
– Port ID - identifies the target ONU
– Payload Type Indicator (GEM OAM, congestion/fragmentation indication)
– Header Error Correction field (BCH(39,12,2) code+ 1b even parity)
The GEM header is XOR'ed with B6AB31E055 before transmission
PLI
(12b)
Port ID
(12b)
PTI
(3b)
102
5B
HEC
(13b)
payload fragment
(L Bytes)
Ethernet / TDM over GEM
When transporting Ethernet traffic over GEM:
– only MAC frame is encapsulated (no preamble, SFD, EFD)
– MAC frame may be fragmented (see next slide)
Ethernet over GEM
PLI
ID
PTI
HEC
DA
SA
T
data
FCS
When transporting TDM traffic over GEM:
– TDM input buffer polled every 125 msec.
– PLI bytes of TDM are inserted into payload field
– length of TDM fragment may vary by ± 1 Byte due to frequency offset
– round-trip latency bounded by 3 msec.
TDM over GEM
PLI
103
ID
PTI
HEC
PLI Bytes of TDM
GEM
fragmentation
GEM can fragment its payload
For example
unfragmented Ethernet
frame PLI ID PTI=001
HEC
DA
SA
T
fragmented Ethernet
frame PLI ID PTI=000
HEC
DA
SA
T
PLI
ID
PTI=001
HEC
data2
data
FCS
data1
GEM fragments payloads for either of two reasons:
– GEM frame may not straddle GTC frame
FCS
… GEM frag 1 PCBd ATM partition GEM frag 2 …
– GEM frame may be pre-empted for delay-sensitive data
PCBd
ATM partition
GEM frame
PCBd
ATM partition
urgent frame
104
…
large frag 1
PCBd
ATM partition
urgent frame
…
GEM frame
large frag 2
PCBd
We saw that the PCBd is
PSync (4B) Ident (4B) PLOAMd (13B)
BIP
PLend
PLend
US BW map
(1B)
(4B)
(4B)
(N*8B)
B6AB31E0
PSync - fixed pattern used by ONU to located start of GTC frame
Ident - MSB indicates if FEC is used, 30 LSBs are superframe counter
PLOAMd - carries OAM, ranging, alerts, activation messages, etc.
BIP - SONET/SDH-style Bit Interleaved Parity of all bytes since last BIP
PLend (transmitted twice for robustness) – Blen - 12 MSB are length of BW map in units of 8 Bytes
– Alen - Next 12 bits are length of ATM partition in cells
– CRC - final 8 bits are CRC over Blen and Alen
US BW map - array of Blen 8B structures granting BW to US flow
will discuss later (DBA)
105
GPON US considerations
GTC fames are still 125 msec long, but shared amongst ONUs
Each ONU transmits a burst of data
– using timing acquired by locking onto OLT signal
– according to time allocation sent by OLT in BWmap
• there may be multiple allocations to single ONU
• OLT computes DBA by monitoring traffic status (buffers)
of ONUs and knowing priorities
– at power level requested by OLT (3 levels)
• this enables OLT to use avalanche photodiodes which are sensitive to
high power bursts
– leaving a guard time from previous ONU's transmission
– prefixing a preamble to enable OLT to acquire power and phase
– identifying itself (ONU-ID) in addition to traffic IDs (VPI, Port-ID)
– scrambling data (but not preamble/delimiter)
106
US GPON format
4 different US overhead types:
• Physical Layer Overhead upstream
– always sent by ONU when taking over from another ONU
– contains preamble and delimiter (lengths set by OLT in PLOAMd)
BIP (1B), ONU-ID (1B), and Indication of real-time status (1B)
• PLOAM upstream (13B) - messaging with PLOAMd
• Power Levelling Sequence upstream (120B)
– used during power-set and power-change to help set ONU power so
that OLT sees similar power from all ONUs
• Dynamic Bandwidth Report upstream
– sends traffic status to OLT in order to enable DBA computation
if all OH types are present:
PLOu
107
PLOAMd
PLSu
DBRu
payload
US allocation example
DS frame
PCBd
BWmap
payload
Alloc-ID SStart SStop Alloc-ID SStart Sstop Alloc-ID SStart SStop
US frame
preamble
+
delimiter
guard
time
scrambled
BWmap sent by OLT to ONUs is a list of
• ONU allocation IDs
• flags (not shown above) tell if use FEC, which US OHs to use, etc.
• start and stop times (16b fields, in Bytes from beginning of US frame)
108
EPON format
EPON operation is based on the Ethernet MAC
and EPON frames are based on GbE frames
but extensions are needed
• clause 64 - MultiPoint Control Protocol PDUs
this is the control protocol implementing the required logic
• clause 65 - point-to-point emulation (reconciliation)
this makes the EPON look like a point-to-point link
and EPON MACs have some special constraints
• instead of CSMA/CD they transmit when granted
• time through MAC stack must be constant (± 16 bit durations)
• accurate local time must be maintained
109
EPON header
Standard Ethernet starts with an essentially content-free 8B preamble
• 7B of alternating ones and zeros 10101010
• 1B of SFD 10101011
In order to hide the new PON header
EPON overwrites some of the preamble bytes
10101010
10101010
10101010
10101010
10101010
10101010
10101010
10101011
10101010
10101010
10101011
10101010
10101010
LLID
LLID
CRC
LLID field contains
– MODE (1b)
• always 0 for ONU
• 0 for OLT unicast, 1 for OLT multicast/broadcast
– actual Logical Link ID (15b)
• Identifies registered ONUs
• 7FFF for broadcast
CRC protects from SLD (byte 3) through LLID (byte 7)
110
MPC PDU format
MultiPoint Control Protocol frames are untagged MAC frames
with the same format as PAUSE frames
DA
SA
L/T
Opcode
timestamp
data / RES / pad
Ethertype = 8808
Opcodes (2B) - presently defined:
GATE/REPORT/REGISTER_REQ/REGISTER/REGISTER_ACK
Timestamp is 32b, 16 ns resolution
conveys the sender's time at time of MPCPDU transmission
Data field is needed for some messages
111
FCS
Security
DS traffic is broadcast to all ONUs, so encryption is essential
easy for a malicious user to reprogram ONU to capture desired frames
US traffic not seen by other ONUs, so encryption is not needed
do not take fiber-tappers into account
EPON does not provide any standard encryption method
– can supplement with IPsec or MACsec
– many vendors have added proprietary AES-based mechanisms
– in China special China Telecom encryption algorithm
BPON used a mechanism called churning
Churning was a low cost hardware solution (24b key)
with several security flaws
– engine was linear - simple known-text attack
– 24b key turned out to be derivable in 512 tries
So G.983.3 added AES support - now used in GPON
112
GPON encryption
OLT encrypts using AES-128 in counter mode
Only payload is encrypted (not ATM or GEM headers)
Encryption blocks aligned to GTC frame
Counter is shared by OLT and all ONUs
– 46b = 16b intra-frame + 30 bits inter-frame
– intra-frame counter increments every 4 data bytes
• reset to zero at beginning of DS GTC frame
OLT and each ONU must agree on a unique symmetric key
OLT asks ONU for a password (in PLOAMd)
ONU sends password US in the clear (in PLOAMu)
– key sent 3 times for robustness
OLT informs ONU of precise time to start using new key
113
QoS - EPON
Many PON applications require high QoS (e.g. IPTV)
EPON leaves QoS to higher layers
– VLAN tags
– P bits or DiffServ DSCP
In addition, there is a crucial difference between LLID and Port-ID
– there is always 1 LLID per ONU
– there is 1 Port-ID per input port - there may be many per ONU
– this makes port-based QoS simple to implement at PON layer
RT
114
EF
BE
GPON
QoS - GPON
GPON treats QoS explicitly
– constant length frames facilitate QoS for time-sensitive applications
– 5 types of Transmission CONTainers
• type 1 - fixed BW
• type 2 - assured BW
• type 3 - allocated BW + non-assured BW
• type 4 - best effort
• type 5 - superset of all of the above
GEM adds several PON-layer QoS features
– fragmentation enables pre-emption of large low-priority frames
– PLI - explicit packet length can be used by queuing algorithms
– PTI bits carry congestion indications
115
PON control plane
116
Principles
GPON uses PLOAMd and PLOAMu as control channel
PLOAM are incorporated in regular (data-carrying) frames
Standard ITU control mechanism
EPON uses MPCP PDUs
Standard IEEE control mechanism
EPON control model - OLT is master, ONU is slave
– OLT sends GATE PDUs DS to ONU
– ONU sends REPORT PDUs US to OLT
117
Ranging
Upstream traffic is TDMA
Were all ONUs equidistant, and were all to have a common clock
then each would simply transmit in its assigned timeslot
But otherwise the signals will overlap
To eliminate overlap
• guard times left between timeslots
• each ONU transmits with the proper delay to avoid overlap
• delay computed during a ranging process
118
Ranging background
In order for the ONU to transmit at the correct time
the delay between ONU transmission and OLT reception
needs to be known (explicitly or implicitly)
Need to assign an equalization-delay
The more accurately it is known
the smaller the guard time that needs to be left
and thus the higher the efficiency
Assumptions behind the ranging methods used:
• can not assume US delay is equal to DS delay
• delays are not constant
– due to temperature changes and component aging
• GPON: ONUs not time synchronized accurately enough
• EPON: ONUs are accurately time synchronized (std contains jitter masks)
with time offset by OLT-ONU propagation time
119
GPON ranging method
Two types of ranging
– initial ranging
• only performed at ONU boot-up or upon ONU discovery
• must be performed before ONU transmits first time
– continuous ranging
performed continuously to compensate for delay changes
OLT initiates coarse ranging by stopping allocations to all other ONUs
– thus when new ONU transmits, it will be in the clear
OLT instructs the new ONU to transmit (via PLOAMd)
OLT measures phase of ONU burst in GTC frame
OLT sends equalization delay to ONU (in PLOAMd)
During normal operation OLT monitors ONU burst phase
If drift is detected OLT sends new equalization delay to ONU (in PLOAMd)
120
EPON ranging method
All ONUs are synchronized to absolute time (wall-clock)
When an ONU receives an MPCPDU from OLT
it sets its clock according to the OLT's timestamp
When the OLT receives an MPCPDU in response to its MPCPDU
it computes a "round-trip time" RTT (without handling times)
it informs the ONU of RTT, which is used to compute transmit delay
OLT sends MPCPDU
Timestamp = T0
ONU receives MPCPDU
Sets clock to T0
ONU sends MPCPDU
Timestamp = T1
OLT receives MPCPDU
RTT = T2 - T1
time
OLT time
ONU time
T0
T2
T0
T1
RTT = (T2-T0) - (T1-T0) = T2-T1
OLT compensates all grants by RTT before sending
Either ONU or OLT can detect that timestamp drift exceeds threshold
121
Autodiscovery
OLT needs to know with which ONUs it is communicating
This can be established via NMS
– but even then need to setup physical layer parameters
PONs employ autodiscovery mechanism to automate
–
–
–
–
–
–
discovery of existence of ONU
acquisition of identity
allocation of identifier
acquisition of ONU capabilities
measure physical layer parameters
agree on parameters (e.g. watchdog timers)
Autodiscovery procedures are complex (and uninteresting)
so we will only mention highlights
122
GPON autodiscovery
Every ONU has an 8B serial number (4B vendor code + 4B SN)
– SN of ONUs in OAN may be configured by NMS, or
– SN may be learnt from ONU in discovery phase
ONU activation may be triggered by
– Operator command
– Periodic polling by OLT
– OLT searching for previously operational ONU
G.984.3 differentiates between three cases:
– cold PON / cold ONU
– warm PON / cold ONU
– warm PON / warm ONU
Main steps in procedure:
– ONU sets power based on DS message
– OLT sends a Serial_Number request to all unregistered ONUs
– ONU responds
– OLT assigns 1B ONU-ID and sends to ONU
– ranging is performed
123 – ONU is operational
EPON autodiscovery
OLT periodically transmits DISCOVERY GATE messages
ONU waits for DISCOVERY GATE to be broadcast by OLT
DISCOVERY GATE message defines discovery window
• start time and duration
ONU transmits REGISTER_REQ PDU using random offset in window
OLT receives request
• registers ONU
• assigns LLID
• bonds MAC to LLID
• performs ranging computation
OLT sends REGISTER to ONU
OLT sends standard GATE to ONU
ONU responds with REGISTER_ACK
ONU goes into operational mode - waits for grants
124
Failure recovery
PONs must be able to handle various failure states
GPON
if ONU detects LOS or LOF it goes into POPUP state
• it stops sending traffic US
• OLT detects LOS for ONU
• if there is a pre-ranged backup fiber then switch-over
EPON
during normal operation ONU REPORTs reset OLT's watchdog timer
similarly, OLT must send GATES periodically (even if empty ones)
if OLT's watchdog timer for ONU times out
• ONU is deregistered
125
Dynamic Bandwidth Allocation
MANs and WANs have relatively stationary BW requirements
due to aggregation of large number of sources
But each ONU in a PON may serve only 1 or a small number of users
So BW required is highly variable
It would be inefficient to statically assign the same BW to each ONU
So PONs assign dynamically BW according to need
The need can be discovered
– by passively observing the traffic from the ONU
– by ONU sending reports as to state of its ingress queues
The goals of a Dynamic Bandwidth Allocation algorithm are
– maximum fiber BW utilization
– fairness and respect of priority
– minimum delay introduced
126
GPON DBA
DBA is at the T-CONT level, not port or VC/VP
GPON can use traffic monitoring (passive) or status reporting (active)
There are three different status reporting methods
• status in PLOu - one bit for each T-CONT type
• piggy-back reports in DBRu - 3 different formats:
– quantity of data waiting in buffers,
– separation of data with peak and sustained rate tokens
– nonlinear coding of data according to T-CONT type and tokens
• ONU report in DBA payload - select T-CONT states
OLT may use any DBA algorithm
OLT sends allocations in US BW map
127