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Victor S. FrostDan F. Servey Distinguished Professor

Electrical Engineering and Computer ScienceUniversity of Kansas2335 Irving Hill Dr.

Lawrence, Kansas 66045Phone: (785) 864-4833 FAX:(785) 864-7789

e-mail: frost@eecs.ku.eduhttp://www.ittc.ku.edu/

Specific Systems:

Passive Optical Networks (PONs)

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All material copyright 2006Victor S. Frost, All Rights Reserved

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Outline

• What is a PON?• Types of PON• PON Architecture• EPON

– How EPON works– EPON Protocol

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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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FTTx

• 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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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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PONAdvantages/Disadvantages

• Advantages– 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 components • Disadvantages

– Cost– Need to deploy new infrastructure

replace copper with fiber

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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

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Types of PONs

• 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

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Types of PONs

• EPON – Ethernet Based PONs– All data encapsulated in Ethernet frames (note customer

premises equipment dominated by Ethernet interfaces)– One 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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Types of PONs

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EPON Downstream flows

Modified from: Glen Kramer and Gerry Pesavento, “Ethernet Passive Optical Network(EPON): Building a Next-GenerationOptical Access Network”, IEEE Communications Magazine, February 2002

Passive Splitter Number of users typically limited to 64 splitting loss

Note ONU can be associated with multiple users

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EPON Upstream flows

Modified from: Glen Kramer and Gerry Pesavento, “Ethernet Passive OpticalNetwork(EPON): Building a Next-GenerationOptical Access Network”,

IEEE Communications Magazine, February 2002

•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)

ONU/OLT distances vary- Need power control- Guard times to overcome sync errors

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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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• From Glen Kramer, “Ethernet Passive Optical Networks,” McGraw-Hill, 2005

• P2PE = Point-to-Point Ethernet

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Multipoint Control Protocol (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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Example

From: IPACT: A Dynamic Protocol for an Ethernet PON (EPON)Glen Kramer and Biswanath Mukherjee, Gerry Pesavento, IEEE Communications Magazine ,February 2002

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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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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 up 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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MPCP-Normal Mode

• REPORT message– Time stamp– Bandwidth demands of the ONU– OLT receives the report and allocated

bandwidth

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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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MPCP-Normal Mode• Possible Policies

– 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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Bandwidth allocation in the OLTDBA Algorithm

• Transmission scheduling-Options– Round Robin– Descending order of reported queue

length largest queue first (LQF)– Dynamic allocation

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Dynamic Bandwidth Allocation

• Interleaved Polling with Adaptive Cycle Time (IPACT),

• Define-– n = cycle number– G(n) = Grant window– Q(n) ONU buffer size (backlog) that will be

served during G(n)– Q(n) generated at instant ONU generated

REPORT message– Q(n) is used to calculate the grant window size

in next cycle, G(n+1)

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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 Q(n) up to the limit of MTW

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• Interleaved Polling with Adaptive Cycle Time (IPACT)-allocation methods 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 per station• Accumulated size of last N grants < N*MTW• A backlogged ONU could get a grant of N*MTW

Dynamic Bandwidth Allocation

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Interleaved Polling with Adaptive Cycle Time (IPACT)

From: IPACT: A Dynamic Protocol for an Ethernet PON (EPON)Glen Kramer and Biswanath Mukherjee, Gerry Pesavento, IEEE Communications Magazine ,February 2002

W_max=15,000 BytesBW= 600 Mb/sMax transmission window = 2 msGuard time= 5 us

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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) – αD(n);

for stability 0<α<2• Want D(n) 0• Control loop has to be desinged to be stable and

avoid excessive oscillations

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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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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

# rows = Total # upstream bandwidth units, e.g. 100

OLT pools ONU in BW guaranteed list, if empty then polls the rows in the BE table

Best Effort Table

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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 aggageratestate of the ONU

• ONU schedules transmissions

– Strict priority– Non-strict priority

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References #14• Assi, C.M., et al., Dynamic bandwidth allocation for quality-of-service over Ethernet

PONs. Selected Areas in Communications, IEEE Journal on, 2003. 21(9): p. 1467-1477.• Hajduczenia, M., EPONs - revolution in access networks.• Haran, O., EPON vs. GPON: a practical comparison. 2005.• 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, 2002. 40(2): p. 66-73.

• Kunigonis, M., FTTH explained: delivering efficient customer bandwidth and enhanced services. 2005.

• McGarry, M.P., M. Maier, and M. Reisslein, Ethernet PONs: a survey of dynamic bandwidth allocation (DBA) algorithms. Communications Magazine, IEEE, 2004. 42(8): p. S8-15.

• Pesavento, G., Ethernet passive optical network (EPON) architecture for broadband access Optical Networks Magazine 2003.

• Upadhyay, P., Passive optical networks. 2005.• Zahr, S.A. and M. Gagnaire, An analytical model of the IEEE 802.3ah MAC protocol for

EPON-based access systems. 2006. p. 1-5.• Zheng, J. and H.T. Mouftah, Media access control for Ethernet passive optical

networks: an overview. Communications Magazine, IEEE, 2005. 43(2): p. 145-150.