Multiple Access Techniques for ePON

Glen Kramer

glen.kramer@alloptic.com

IEEE 802.3 EFM Study Group Meeting, March 2001

3/9/2001 Alloptic, Inc. 2

“First Mile” ≠≠≠≠ LANHAVE

Non-cooperative (independent) usersNon-uniform traffic: most traffic flows between CO and customers, not much between customers

MUSTHave SLA with every customerNot punish all for one misbehaving customerGuarantee security (privacy)

3/9/2001 Alloptic, Inc. 3

PON – a natural step in access evolution

Point-to-point links

Too many fiber lines

2N transceivers

Concentration switch in the neighborhood

Power in the field

2N + 2 transceivers

PON – a distributed switch

Minimum fiber

N + 1 transceivers

Path transparency

CO

CO

CO

3/9/2001 Alloptic, Inc. 4

What is Ethernet PON (ePON)?

ePON MAC uses Ethernet framing and line coding.

Downstream channel uses true broadcast. Packets extracted by the MAC addresses.

Upstream transmission uses multiple access. Which multiple access scheme?

O N U 1O N U 2

O N U 3

O N U 4

O N U 5

O L T

O N U 1 O N U 2

O N U 3 O N U 4O N U 5

O L T

O N U 1

O N U 2

O N U 3

O N U 4

O N U 5

O L T

( a ) T r e e to p o lo g y

( b ) B u s to p o lo g y

( c ) R in g to p o lo g y

3/9/2001 Alloptic, Inc. 5

Why is it Ethernet PON?

Downstream channel uses true broadcast. Packets extracted by the MAC addresses.Not different from any shared-medium Ethernet LAN.

ONU

ONU

ONU

OLT

User

User

User2 1 32

21

32

21

32

2 1 32 2 2

1

3

PayloadHeader FCS

IEEE 802.3 frame

3/9/2001 Alloptic, Inc. 6

Multiple Access Schemes

Multiple Access

Spectrum Sharing(WDMA, FDMA)

- Many transceivers in OLT- High initial deployment cost- Fixed channel capacity- Scalability (Add/Remove user)

Time Sharing

+ One OLT transceiver for N ONUs+ Fractional lambda capacity- Higher speed electronics

Fixed TDMA

- Unused timeslots "eat" as much bandwidth as full timeslots- Self-similar traffic - highly bursty- ONUs synchronization

Statistical TDMA

Why?How?

Space Sharing(Just add fiber...)

Random Access(CSMA/CD, DQDB)

- Non-deterministic service time- Distance limitation (CSMA/CD)

Code Sharing (CDMA)

- Interchannel interference depends on # of ONUs- Much higher speed of components

3/9/2001 Alloptic, Inc. 7

Why statistical multiplexing is important?

Data/video traffic is self-similar, i.e., bursty at many timescales. Flow aggregation doesn’t help, i.e., no optimal fixed timeslot size exist.

Burst size distribution is Long-Range Dependant (heavy tailed): most bursts are small, but most bytes arrive in large bursts.

Network Traffic

0.0E+00

1.5E+04

3.0E+04

0 100 200 300 400 500

Timeslot occupancy

0.0E+00

1.5E+04

3.0E+04

0 100 200 300 400 500

Overutilized Underutilized

3/9/2001 Alloptic, Inc. 8

Statistical multiplexing schemes

Burst time and size are hard to predict => must use schemes with feedback (like polling).

Roll-call polling is nice, but requires ONUs to listen to each other, i.e., PON should be deployed as a broadcasting star or passive ring (too restrictive).

Hub polling could work, but walk times are very large.

Solution: Interleave polling routines in time.

3/9/2001 Alloptic, Inc. 9

Upstream transmission

ONU

ONU

ONU

OLT

User

User

User2 2

1

3 3 3

1 2 2 3 3 3

1

3 3 3

2 2

IEEE 802.3 frame

PayloadHeader FCS

ONU's timeslot

ONU sends Ethernet frames within assigned timeslot

Statistical multiplexing is achieved by adjusting timeslot size to the amount of availabledata

3/9/2001 Alloptic, Inc. 10

Interleaved polling scheme

- Grant messages

GOLT

ONU1

ONU2

ONU3

TxRx

TxRx

TxRx

TxRx

data

G

G

G

R

GR

RdataR dataRdata dataR

GR data

G

GdataR

G

R

data

- Request messages

- User's data (packet train)

3/9/2001 Alloptic, Inc. 11

SLA/Bandwidth provisioning

Maximum Transmission Window (MTW) determines the guaranteed bandwidth provisioned to ONU.MTW may be different for different ONUs.

Example:1 Gbps ePON; 16 ONUs with equal MTW

478 Mbps58.5 MbpsGuard Band = 8 µsMTW = 14625 bytes

600 Mbps60 MbpsGuard Band = 5 µsMTW = 15000 bytes

Best-case bandwidth (only one ONU has data to send)

Guaranteed bandwidth

3/9/2001 Alloptic, Inc. 12

Disconnected ONU

May happen often (especially in FTTH systems).Disconnected ONU should not use network resources.

Solution:New Request should arrive within RTT + ∆t after the GrantMissing Request (k Requsts) means ONU is disconnectedWait 1 minute before querying this ONU again.Disconnected ONU consumes only ~0.0005% of PON bandwidth.

Important:Make no assumptions about RTT of disconnected ONU.

3/9/2001 Alloptic, Inc. 13

Simulation results (uniform load)

Average Packet Delay

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

0.0 0.2 0.4 0.6 0.8 1.0Offered Load

Del

ay (s

)

TDM A

IPACT

Average Queue Size

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+08

0.0 0.2 0.4 0.6 0.8 1.0

Offered Load

Byte

sTDMA

IP ACT

3/9/2001 Alloptic, Inc. 14

Advantages of Interleaved Polling Scheme

Bandwidth utilization. If only one ONU is active, it can use up to 600 Mbps (with 5 µs guard band).

Lower delay. Delay is bounded by RTT, not frame time. Under maximum load behaves like TDMA system.

No ONU’s synchronization necessary. ONU sends data immediately on receiving (processing) the control message (Grant). No centralized framing necessary.

All “smarts” are in OLT. OLT may use various scheduling algorithms based on SLA, type of traffic, etc.

Fast detection of disconnected ONU. Disconnected ONU “consumes” only ~0.0005% of PON bandwidth.

3/9/2001 Alloptic, Inc. 15

SummaryePON-based “First Mile”...

Minimizes fiber deployment Uses minimum number of transceiversAllows for downstream video broadcasting (analog overlay?)Eliminates electronic equipment in the outside plantUtilizes bandwidth efficiently (with right protocol)

Need to studyGrant/Request scheduling approachesUpgrade scenariosSecurity/Privacy issues