1. Passive Optical Networks (PONs)
Specific Systems:
Passive Optical Networks
(PONs)
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Victor S. Frost
Dan F. Servey Distinguished Professor
Electrical Engineering and Computer Science
University of Kansas
2335 Irving Hill Dr.
Lawrence, Kansas 66045
Phone: (785) FAX:(785)
All material copyright 2006
Victor S. Frost, All Rights Reserved
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Aug. 2005

2. Outline What is a PON? Types of PON PON Architecture EPON
How EPON works
EPON Protocol
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3. What is a PON
A passive optical network is a point-to-multi-point architecture for delivering last-mile connectivity without any active components in the distribution network.
A Passive Optical Network consists of
an optical line terminator (OLT) located at the Central Office (CO)
set of associated optical network units-ONUs, (or optical network terminals-ONTs) located at the customer’s premise.
optical distribution network (ODN) comprised of fibers and passive splitters or couplers for connectivity
Information Flows
Downstream Point to multipoint (P2MP)
Upstream  Shared
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4. FTTx Fiber To The x (FTTx) x = H = Home*
Fiber To The x (FTTx)
x = H = Home
x = C = Curb or x = N = Neighborhood
x = P = Premises
x = B = Business or x = O = Office
*From: Gerd Keiser, “ FTTX, Concepts and Applications,” Wiley, 2006
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5. PON Topologies
From: Glen Kramer and Gerry Pesavento, “Ethernet Passive Optical Network
(EPON): Building a Next-Generation Optical Access Network”,
IEEE Communications Magazine, February 2002
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6. PON Advantages/Disadvantages
Increase bandwidth to Gb/s or higher in the future using multiple wavelengths
Increase reach between CO and customer
Reduce fiber cost, one fiber to OTL to optical splitter
Support downstream broadcast
Analog video
Digital Video
Video over IP
Reduce cost via use of passive
Disadvantages
Cost
Need to deploy new infrastructure  replace copper with fiber
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7. Types of PONs A/BPON – ATM Based PONs/Broadband based PONs
Based on ATM technology
Two downstream wavelengths (1550nm and 1490nm) and one upstream wavelength (1490nm). The 1550nm channel will be used for an RF or IP video overlay.
ITU standard G.983
GPON – Gigabit PONs
Shares up to 2.5Gbps shared bandwidth among 32 users;
Uses same wavelength plan of BPON.
ITU standard G.984
Main attribute supports multiple protocols
ATM,
Ethernet
TDM
EPON – Ethernet Based PONs
All data encapsulated in Ethernet frames (note customer premises equipment dominated by Ethernet interfaces)
Two downstream wavelengths (1550nm) and one upstream wavelength (1310nm).
Shares 1.25Gbps in shared bandwidth
GigaEthernet PON (GePON) increases shared bandwidth to 2.5Gbps.
IEEE 802.3ah
VLAN (IEEE 802.1Q)
Prioritization (IEEE 802.1p)
GPON and EPON are currently the major alternative technologies  focus here is on EPONs
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8. Types of PONs
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9. Note ONU can be associated
EPON Downstream flows
Note ONU can be associated
with multiple users
Passive Splitter
Number of users typically limited to 64  splitting loss
Modified from: Glen Kramer and Gerry Pesavento, “Ethernet Passive Optical Network
(EPON): Building a Next-GenerationOptical Access Network”,
IEEE Communications Magazine, February 2002
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10. EPON Upstream flows All ONUs are in time sync
ONU/OLT distances vary
- Need power control
- Guard times to overcome sync errors
All ONUs are in time sync
Time slots can carry multiple Ethernet Frames
Permission to send in a time slot done by the OLT using a Multipoint control protocol (MPCP)
Modified from: Glen Kramer and Gerry Pesavento, “Ethernet Passive Optical
Network(EPON): Building a Next-GenerationOptical Access Network”,
IEEE Communications Magazine, February 2002
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11. MAC Protocols Initialization-auto-discovery mode Ranging MAC address
Synchronization
Assign Logical Link identifier (LLID)
EPON is treated as a collection of logical point-to-point links by higher layers
Reserved
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12. From Glen Kramer, “Ethernet Passive Optical Networks,” McGraw-Hill, 2005
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13. MPCP-Normal Mode
OLT sends a GATE message to give ONU access to transmit
ONU sends a REPORT message to tell the OLT its state, e.g., buffer contents *
*From: Jun Zheng and Hussein T. Mouftah, “Media Access Control for
Ethernet Passive Optical Networks: An Overview,”
IEEE Communications Magazine, February 2005
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14. MPCP-Normal Mode
From: Michael P. McGarry, Martin Maier, and Martin Reisslein Ethernet PONs:
A Survey of Dynamic Bandwidth Allocation (DBA) Algorithms
IEEE Communications Magazine, Vol. 42, No. 8, pages S8-S15, August 2004
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15. MPCP-Normal Mode Uses a polling like mechanism, i.e., cycle based
Higher layer at OLT instructs the transmission of a GATE message to ONUi, containing
Time stamp
Granted start time
Granted transmission window
ONUi receives GATE message
Synchronizes clock (avoids collisions in the down stream)
Transmits at the start time
Continues to transmit up to the granted window
Transmission opportunity may send multiple Ethernet frames (variable length)
REPORT message included in the transmission window
No fragmentation is allowed
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16. MPCP-Normal Mode REPORT message Time stamp
Bandwidth demands of the ONU
OLT receives the report and allocated bandwidth
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17. MPCP-Normal Mode Each ONU is polled once in a cycle
Upper limit on the granted transmission window controls
Maximum bandwidth allocated to ONU
Insures all ONUs have an opportunity to transmit
There can be different polling policies
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18. MPCP-Normal Mode Poll and stop Interleaved Polling*
Poll and stop
Like master slave
Receive REPORT before generating next GATE
No ONU sync needed
Poor throughtput
Interleaved Polling
Send GATE before receiving the REPORT
OLT does not have upto date ONU state information
Interleaved polling with stop
Use all reports in previous cycle to determine allocations in next cycle
*From: Jun Zheng and Hussein T. Mouftah, “Media Access Control for
Ethernet Passive Optical Networks: An Overview,” IEEE Communications
Magazine, February 2005
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19. Bandwidth allocation in the OLT
Transmission scheduling
Round Robin
Descending order of reported queue length largest queue first (LQF)
Dynamic allocation
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20. Dynamic Bandwidth Allocation
Interleaved Polling with Adaptive Cycle Time (IPACT)
Let n = cycle number
G(n) = Grant window
Q(n) ONU buffer size (backlog) that will be during G(n)
Q(n) generated at instant ONU generated REPRT message
Q(n) is used to calculate the grant window size in next cycle, G(n+1)
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21. Dynamic Bandwidth Allocation
Interleaved Polling with Adaptive Cycle Time (IPACT)
If Q(n) = 0 OLT still generates GATE
Allocation methods
Fixed service: Always gives Maximum transmission window (MTW)
Limited service: Window size is set to max(MTW, Q(n))
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22. Dynamic Bandwidth Allocation
Interleaved Polling with Adaptive Cycle Time (IPACT)-continued
Constant Credit service: Window size is set to Q(n) + credit
Accounts for packets arriving to the ONU after REPORT sent
Credit too big  wasted BW
Credit too small  increase delay
Linear credit: similar to constant credit, but the size of credit is proportional to the requested window
Elastic Service
Maximum cycle time is enforced = N*MTW; N= #ONU
No limit on MTW
Accumulated size of last N grants < N*MTW
A backlogged ONU could get a grant of N*MTW
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23. Dynamic Bandwidth Allocation
Problem: Traffic arrives at ONU after REPORT sent but before GATE arrives, OLT dose not know about this traffic
Let A(n-1) = traffic arriving between request for n-1 cycle and the grant for the nth cycle
D(n) = G(n) –[Q(n-1) + A(n-1)]
Grant based on G(n+1) = G(n) – aD(n); for stability 0<a<2
Control loop has to be desinged to be stable and avoid excessive oscillations
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24. Bandwidth guaranteed polling: (BGP)
Divided ONUs into two disjoint sets:
bandwidth guaranteed
Non-bandwidth guaranteed (best effort)
Bandwidth guaranteed nodes are characterized by their service level agreement (SLA) with the network provider
Upstream capacity is divided into “equivalent bandwidth units”, e.g.,
Upstream capacity = 1 Gb/s
N = 64
equivalent bandwidth unit = 10 Mb/s
Then there are 100 equivalent bandwidth units
OLT has two tables
ONUs with guaranteed bandwidth
# rows = # bandwidth units
ONUs without guaranteed bandwidth
Not fixed
Rows in guaranteed bandwidth table can be used for best effort ONUs
OLT polls best effort ONUs in order they appear in their table and during time allocated for empty rows in guaranteed bandwidth table
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25. Bandwidth guaranteed polling: (BGP)
From: Michael P. McGarry, Martin Maier, and Martin Reisslein Ethernet PONs:
A Survey of Dynamic Bandwidth Allocation (DBA) Algorithms
IEEE Communications Magazine, Vol. 42, No. 8, pages S8-S15, August 2004
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26. Intra-ONU queueing and scheduling
Approaches
OTL can control
REPORT messages in status in each queue, MPCP supports up to 8 queues
GATE then responds for a specific queue
ONU can control
REPORT message relfects aggagerate state of the ONU
ONU schedules transmissions
Strict priority
Non-strict priority
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27. References #14
Assi, C.M., et al., Dynamic bandwidth allocation for quality-of-service over Ethernet PONs. Selected Areas in Communications, IEEE Journal on, (9): p
Hajduczenia, M., EPONs - revolution in access networks.
Haran, O., EPON vs. GPON: a practical comparison
Keiser, G., FTTX concepts and applications. Wiley series in telecommunications and signal processing. 2006, Hoboken, N.J.: John Wiley & Sons : IEEE. xvii, 293 p.
Kramer, G., Ethernet passive optical networks. Communications engineering series. 2005, New York: McGraw-Hill Professional. xvii, 307 p.
Kramer, G. and G. Pesavento, Ethernet passive optical network (EPON): building a next-generation optical access network. Communications Magazine, IEEE, (2): p
Kunigonis, M., FTTH explained: delivering efficient customer bandwidth and enhanced services
McGarry, M.P., M. Maier, and M. Reisslein, Ethernet PONs: a survey of dynamic bandwidth allocation (DBA) algorithms. Communications Magazine, IEEE, (8): p. S8-15.
Pesavento, G., Ethernet passive optical network (EPON) architecture for broadband access Optical Networks Magazine 2003.
Upadhyay, P., Passive optical networks
Zahr, S.A. and M. Gagnaire, An analytical model of the IEEE 802.3ah MAC protocol for EPON-based access systems p. 1-5.
Zheng, J. and H.T. Mouftah, Media access control for Ethernet passive optical networks: an overview. Communications Magazine, IEEE, (2): p
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28. References
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