Chapter 11: Access Networks

Chapter 11: Access Networks TOPICS –  ADSL-based access networks –  Cable-based access networks –  ATM Passive Optical Networks (APON) Optic...
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Chapter 11: Access Networks

TOPICS

–  ADSL-based access networks

–  Cable-based access networks

–  ATM Passive Optical Networks (APON)

Optical Networks- Harry Perros

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Access network architectures

•  •  •  •  •  • 

xDSL (telephone lines)

Cable-based (DOCSIS)

Passive Optical Networks (PON)

Ethernet

Wireless

Satellites

Optical Networks- Harry Perros

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The ADSL-based access network

•  ADSL is one of the access technologies that can be used to convert the telephone line into a high-speed digital link.

•  It is part of a family of technologies called the x-type digital subscriber line (xDSL).

Optical Networks- Harry Perros

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xDSL data rates

•  •  •  •  •  • 

Name ADSL HDSL SDSL ISDL

VDSL

Meaning

Asymmetric DSL High data rate DSL Symmetric DSL ISDN DSL

Very high data rate DLS

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Data rate (Down/Up)

8 Mbps/ 800 Kbps



1.544 Mbps/2.048 Mbps

2.3 Mbps/2.3 Mbps

144 Kbps





52 Mbps/6 Mbps



Use





Data





T1/E1 replacement





Data





Data





Videødata



4

VDSL data rates

•  Data Rates

–  52 Mbps /6.4 Mbps up to 1,000 feet.

–  26 Mbps/26 Mbps up to 1,000 feet.

–  13 Mbps/1.6 Mbps up to 5,000

•  Services:

–  Video, Internet access and regular telephone services.

–  Delivered from a cabinet in the street which is connected to an APON.

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ADSL data rates

•  Downstream:

–  1.5 or 2 Mbps up to 18,000 ft

–  6.1 Mbps up to 12,000 ft

–  8 Mbps up to 9,000 ft

•  Upstream

–  64 Kbps to 800 Kbps

•  New ADSL standards:

–  ADSL2 and ADLS2+

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Deployment of ADSL  at the customer’s premise

Filter

Telephone

wires

Telephone wire

ATU-R

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PC

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The ADSL access multiplexer (DSLAM)

POTS

ATM switch

ATU-C

ATU-C

ADSL access multiplexer

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POTS

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The discrete multi-tone (DMT) technique

•  The twisted pair’s bandwidth extends to 1.1 MHz, and it is divided to 256 sub-channels, each occupying 4.3125 KHz, known as tones.

•  Sub-channels 1 through 6 are reserved for the voiceband region and it is used to provide basic telephone services.

•  The remaining sub-channels are used by ADSL

Optical Networks- Harry Perros

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In ADSL both the upstream and downstream data is sent over the same twisted pair.

•  This can be implemented using:

–  frequency division multiplexing (FDM) or

–  echo cancellation.

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•  Frequency division multiplexing

–  Up to 32 upstream sub-channels (i.e., from the user to the network) occupying the frequencies immediately above the voiceband region.

–  Also, there are up to 218 downstream subchannels (i.e., from the network to the user) occupying the frequencies above the upstream sub-channels.

Optical Networks- Harry Perros

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•  Echo cancellation:

–  An alternative solution is to let the upstream and downstream sub-channels use the same frequencies, and separate them using echo cancellation.

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The DMT symbol

•  Transmission is achieved by dividing time into fixed-sized intervals.

•  Within each interval, DMT transmits a data frame. Each data frame consists of a fixed number of bits.

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•  The bits in a data frame are divided into groups and each group is transmitted over a different subchannel using the quadrature amplitude modulation (QAM) technique

•  The number of bits sent over each sub-channel can be varied depending upon the signal and noise level in each sub-channel.

•  The signals from all the sub-channels are added and sent to the twisted pair. This signal resulting from each data frame is known as the DMT symbol.

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Bearer channels

•  The information transported by ADSL is organized into 7 independent logical bearer channels.

–  Four unidirectional simplex bearer channels, designated as AS0, AS1, AS2, and AS3.

–  3 bidirectional duplex bearer channels designated as LS0, LS1, LS2.

Optical Networks- Harry Perros

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Data rates for bearer channels (multiples of 32 Kbps)

• 

Bearer channel

Lowest multiple



Largest multiple



highest data rate





•  •  •  •  • 

AS0

AS1

AS2

AS3

LS0



1

1

1

1

1













192

144

96

48

20













6144 Kbps

4608 Kbps

3072 Kbps

1536 Kbps

640 Kbps

















•  • 

LS1

LS2



1

1







20

20







640 Kbps





640 Kbps



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•  STM traffic is mapped in bearer channels AS0 and LS0 in the downstream direction, and in LS0 in the upstream direction.

•  ATM traffic is mapped in the downstream direction in bearer channel AS0 and in LS0 in the upstream direction.

•  Other bearer channels can also be provisioned.

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Fast and interleaved paths

ASO

Fast path

AS1 AS2 AS3 LS0

CRC MUX/ Sync control

LS1 LS2 NTR

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CRC

Scram/ FEC

Scram/ Interleaved FEC buffer Interleaved path

D M T

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The super frame

Super frame (every 17 mec) 0

1

...

2

34

35

...

67

synch

Data frame fast byte

. . . fast

FEC byte byte

...

Fast data buffer

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FEC byte

Interleaved data bytes

interleaved data buffer

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The ADSL reference model architecture

NSPs

ATU-R

Context provider

ATU-C ATU-C

DSLAM

ISP

Corporate network

Regional public network

ATU-R

Access backbone Network access server (NAS)

ATU-R ATU-C ATU-C

ROC

DSLAM

Public backbone

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Access network

ATU-R

Customer premises

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Network access server (NAS)

•  It is used to provide connectivity between the ATU-Rs and the network service providers (NSP)

•  An ATU-R is connected to the NAS via a PVC. NAS maintains a single PVC to each NSP.

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PPP

•  ADSL users get IP connectivity to an NSP using PPP.

•  PPP functions:

–  Transport of different layer-3 protocols

–  Authentication,

–  IP address assignment

–  Domain name auto-configuration

–  Encryption,

–  Compression

–  Billing

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PPP over AAL 5

•  PPP frames are mapped into AAL 5 PDUs using

–  VC-multiplexed PPP

•  A PPP frame is directly carried in AAL 5 PDUs

–  LLC encapsulated PPP scheme

•  A PPP frame is encapsulated with a 2-byte LLC header and a 1-byte network layer protocol identifier, and then it is carried in an AAL PDU.

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 The L2TP access aggregation scheme



PPP PPP over ATM (PVC) IP

IP PPP L2TP

L2TP

PPP

PPP

UDP, FR, ATM, etc

UDP, FR, ATM, etc

AAL 5

AAL 5

PHY

PHY

L2TP network server (LNS)

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ATM

ATM

L2TP access concentrator (LAC)

PHY

ATM ADSL

DSLAM

ADSL Customer premise

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Layer 2 Tunneling protocol (L2TP)

•  A L2TP tunnel is not an actual connection, in the sense of an ATM connection.

•  Rather, it is a logical connection between two L2TP peers.

•  It can carry multiple PPP sessions

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•  L2TP utilizes two types of messages;

–  Control messages

•  Used to establish, maintain, and clear tunnels.

•  Messages are transported reliably. Errored or lost packets are recovered by retransmission.

–  Data messages

•  Used to carry PPP frames over the tunnel

•  Control and data L2TP messages are encapsulated using a common L2TP header.

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The PPP terminated aggregation scheme

IP IP

PPP over ATM (PVC)

IP

IP ATM FDDI Ethernet

FR, ATM, etc

PHY

PHY

Router

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IP PPP

PPP

FR, ATM, etc

AAL 5

AAL 5

PHY

PHY

ATM

ATM

Broadband access server (BAS)

PHY

ATM ADSL

DSLAM

ADSL Customer premises

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ADSL2

•  ADSL2 adds new features and functionality to ADSL

•  Standardized by ITU-T in 2002

•  ADSL2 achieves a maximum downstream data rate of 12 Mbps and a maximum upstream data rate of 1 Mbps.

•  It also extends the reach of ADSL by 600 feet.

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•  Features

–  Rate and reach improvement

–  Diagnostics

–  Low power/Sleep mode

–  Rate adaptation

–  Bonding for higher data rates

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Channelized Voice over DSL (CVoDSL).

POTS

. . .

Upstream

Downstream

. . .

. . .

KHz

64 Kbps DS0

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CVoDSL, VoATM, VoIP

Network

VoIP

VoATM

IP

IP

PPP

PPP

AAL2

AAL5

AAL5

ATM

AAL2

ATM

ADSL Physical layer

POTS

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Baseband POTS

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ADSL2+

•  ADSL2+ was standardized by ITU-T in 2003.

•  It doubles the downstream bandwidth of ADSL and ADSL 2.

•  For instance, it can achieve 26 Mbps at 1,000 feet, and 20 Mbps at 5,000 feet.

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POTS

Upstream

Downstream

ADSL2+

ADSL2

. . .

. . .

. . .

. . .

KHz

0.14 MHz

1.1 MHz

2.2

MHz

The ADSL2+ downstream bandwidth

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Cable-based access networks

Cable TV headend

Optical fiber

ONU

Coaxial cable



Due

to the combination of fiber optics and coaxial cables, this cable network architecture is known as the hybrid fiber coaxial (HFC) architecture.

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DOCSIS

•  High-speed access to the home is provided over an HFC plant using the data-over-cable service interface specification (DOCSIS).

•  This specification was developed by Cable Television Laboratories (CableLabs) for the cable industry in North America, Europe and other regions

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Basic features..

•  Bi-directional transfer of IP traffic between the cable’s headend and the home.

•  This is realized using a cable modem termination system (CMTS) at the headend, and a cable modem (CM) at each home.

•  The maximum distance between the CMTS and a CM is 100 miles, but it is typically limited to 10 to 15 miles

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•  The cable network is a shared-medium tree-like network with analog two-way transmission.

•  In the downstream direction, the cable network operates in the range of 50 to 864 MHz. The data is modulated onto a carrier and then multiplexed with the television and other signals.

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•  In the upstream direction, the cable network operates in the range of 5 to 42 MHz.

•  Each CM transmits IP packets towards the CMTS modulated on a carrier within the range of 5 to 42 MHz.

•  The DOCSIS MAC assures that there are no collisions in the upstream direction.

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The DOCSIS reference architecture

Video

Video

Data

MAN/WAN

network

CMTS

Data

CM

Tx

Fiber

ONU

Coax

Rx

CM

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CM

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The protocol stack on CMTS and CM

CMTS

CM stack

IP

IP

Bridge

Data

link

layer

PHY

layer

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802.2 LLC

802.2 LLC

Link

security

Link

security

DOCSIS MAC

DOCSIS MAC

(DS)

(US)

TC

Cable

Cable

PMD

PMD

(DS)

TC

Cable

network

Cable

PMD

(US)

Cable

PMD

Bridge

802.2

LLC

802.3 MAC

802.3

10Base-T

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The physical layer-Upstream

•  The upstream PMD sublayer uses the FDMA/ TDMA mode.

•  Access to the slots, referred to in DOCSIS as minislots, is controlled by the DOCSIS MAC protocol.

•  A mini-slot is a power-of-two multiple of 6.25 µs, i.e., it is equal to Tx6.25 µs, where T=2n and n=0,1,…,7. That is, T=1, 2, 4, 8, 16, 32, 64, 128.

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•  CDMA can also be used which permits multiple user to transmit at the same time in the same mini-slot.

•  Upstream speeds can be up to 10 Mbps, though it is typically limited in the range of 500 Kbps to 2.5 Mbps.

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The physical layer-Downstream

•  The downstream PMD uses a 6-MHz channel in the range of 91 to 857 MHz frequencies.

•  The transmission speed in the downstream direction, can reach 27 Mbps, though it is typically limited in the range of 1 to 3 Mbps

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Downstream TC sublayer

•  The downstream TC sub-layer was defined in order to provide a common receiving hardware at the CM for data and future video services.

•  This permits future video services to be offered in addition to the data services.

•  The TC sub-layer receives MAC frames from the DOCSIS MAC layer and produces a continuous bit-stream of 188-byte MPEG packets.

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The MPEG packet format

MPEG header

(4 bytes)

Pointer_field

(1 byte)

DOCSIS payload

(183 or 184 bytes)

•  MPEG header (four bytes): It comprises of various fields, such as a 1-bit payload_unit_start_indicator (PUSI) used to indicate the presence of a pointer_field in the first byte of the DOCSIS payload.

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MPEG header

(4 bytes)

Pointer_field

(1 byte)

DOCSIS payload

(183 or 184 bytes)

•  DOCSIS payload: This field carries DOCSIS MAC frames. It may or may not contain a pointer_field. If it contains a pointer_field, then the DOCSIS payload is 183 bytes, otherwise it is 184 bytes.

•  A DOCSIS MAC frame may begin anywhere within an MPEG packet payload and it may span several MPEG packets. Also, several DOCSIS MAC frames maybe contained within the same MPEG payload.

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The DOCSIS Mac frame format

•  The DOCSIS MAC protocol controls the upstream transmission of the CMs, and it provides quality-of-service and other features.

•  The DOCSIS MAC frame consists of header followed by an optional data PDU.

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The MAC header

FC

FC_TYPE

FC_PARM

MAC_PARM

LEN(SID)

EHDR

(Optional)

HCS

EHDR_ON

Frame control (FC)

–  FC_TYPE: This 2-bit field specifies one of the following four possible MAC frame formats: MAC header with packet PDU, MAC header with ATM cells, MAC header reserved for future PDU types, and MAC header used for specific control functions.

–  FC_PARM: A 5-bit field that contains parameters dependent on the value of the FC_TYPE.

–  EHDR_ON: A 1-bit field indicating whether an extended header is present or not.



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•  MAC-PARM: This 1-byte field contains parameters whose use depend on the value of the FC field.

•  LEN (SID): A 2-byte field that gives the length of the extended header (if present) plus the number of bytes that follow after the HCS field.

•  Extended header (EHDR): This is a variable size extended header, that is used optionally.

•  Header check sequence (HCS): The integrity of the MAC header is ensured by a CRC. The HCS is a 2-byte field that contains the FCS obtained using the pattern x16+x12+x5+1. The HCS cover the entire header, i.e. starting from the FC field and including the extended header.

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The MAC header with packet PDU is used to transport Ethernet packets. In this case, the MAC header is followed by a data PDU with the following fields:

–  Destination address (DA): A 48-bit field populated with the destination address.

–  Source address (SA): A 48-bit field populated with the source address.

–  Type/len: A 16-bit Ethernet type length field.

–  User data: A variable-length field that contains userdata of up to 1500 bytes.

–  CRC: This 32-bit field contains the FCS.

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MAC Headers

•  The following MAC headers are used for specific control functions:

–  MAC header for timing

–  MAC header for requesting bandwidth

–  MAC management header

–  MAC header for fragmentation

–  MAC header for concatenation

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The DOCSIS MAC protocol

•  The upstream channel is divided into mini-slots, the access of which is controlled by the CMTS through the MAP management message.

•  The CMTS continuously issues MAP messages to describe how groups of mini-slots are to be used.

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An example of MAP messages



MAP 1, 2, 3 describe how a set of contiguous mini-slots n1, n2, n3 are to be used by the CMs.

n1 mini-slots

Mini-slots mapped by MAP 1

CMTS

t1

t3

MAP 1

CMs

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t2

t6

t4

MAP 2

t5

n2 mini-slots

n3 mini-slots

MAP 2

MAP 3

t7

t9

MAP 3

t8

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Intervals:

•  The group of mini-slots mapped in a MAP message is divided into intervals of consecutive mini-slots. Each interval is designated by the CMTS for different type of use, such as:

–  Bandwidth requests

–  Data

–  Request to join the access network.

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•  In an interval:

–  CMs can start transmitting at any mini-slot, with the possibility of collisions.

–  Alternatively, each CM transmits over a number of consecutive mini-slots allocated to it by the CMTS, based on requests for bandwidth received from the CMs.

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Information Elements (IE)

•  The different intervals and how they can be accessed are described in the MAP message by different information elements (IE).

•  The following are some of the IEs defined:

–  The request IE

–  The request/data IE

–  The initial maintenance IE

–  The station maintenance IE

–  The short and long data grants IE

–  Data acknowledge IE

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An example of  the upstream transmission scheme

Slots mapped by first MAP

CMTS

t1

t3

t5

Request

MAP

CM

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t2

t6

t4

Second MAP

t11

t9

t7

MAP

t8

Data PDU

t10

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Quality of service

•  In the MAC protocol, a QoS scheme has been defined which provides different priorities for the transport of different flows of packets across the cable network.

•  The QoS scheme uses the concept of service flow, a unidirectional flow of packets from a CM to the CMTS, or from the CMTS to a CM

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ATM Passive Optical Networks (APONs)

• 

• 

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Components of APONs:

–  Optical line terminators (OLT) residing in a CO

–  The optical distribution network (ODN), and the

–  Optical network units (ONU).

The ONUs are connected to the OLT via the optical distribution network

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•  The optical distribution network consists of optical fibers in the form of a tree.

•  The signal is passively split between multiple fibers, each leading to a different ONU.

•  Passive splitters are made by twisting and heating several optical fibers until the power output is evenly distributed.

•  Low cost of passive splitters.

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Upstream/downstream  ATM data transfer

C B A

A

C B A

OLT

A B C

C B A

B

C B A

ONU

A

ONU

B

ONU

C

C

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Optical Line Terminator (OLT)

ATM

interfaces

ODN

interfaces

ATM switch

To the network

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To the home

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The G.983.1 network architecture

FTTH

ONT

Fiber

O L T

FTTB/C

Fiber

ONU

Copper/VDSL

ONT

FTTCab

Fiber

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ONU

Copper/VDSL

ONT

63

FTTH/B/C/Cab

•  Depending on the location of the ONU we have the following configurations:

–  Fiber to the home (FTTH) - ONU is in the home, and it is referred to as optical network terminator (ONT).

–  Fiber-to-the-basement/curb (FTTB/C) - ONU is in a building/curb. Distribution to the house is done over copper using ADSŁVDSL.

–  Fiber-to-the Cabinet (FTTCab) - ONU is in a cabinet. Distribution to the house is done over copper using ADSŁVDSL.

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Transmission characteristics

•  Downstream transmission: (1480-1580 nm band)

–  lamda1=1490 nm, for ATM data

–  lamda2=1559 nm, for video

•  Upstream transmission: (1260-1580 nm band)

–  lamda=1310 nm. Wavelength is shared by all the ONUs using a TDMA protocol for the multipoint-to-point shared medium connection.

•  G.983.1, the APON standard, permits also the use of two unidirectional fibers, operating at 1310 nm.

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•  Bit Rates:

–  Symmetric for FTTCab/C/B/H : 155.52 Mbps

–  Asymmetric for FTTCab/C/B: 155.52 Mbps upstream, and 622.08 Mbps downstream

•  Maximum fiber distance from an ONU to an OLT: 20 km.

•  Minimum supported split ratio with passive splitters is 1:16 or 1:32.

•  Minimum number of ONUs is 64.

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APON Services

•  The APON, as its name implies, supports the ATM architecture.

•  It can provide high-speed access for

–  Internet traffic,

–  voice over ATM, voice over IP, and

–  video services

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APON deployment

•  APON can be deployed in new neighborhoods and municipalities.

•  Municipalities have an interest in providing highspeed connectivity to their residents, and they can easily deploy APONs by passing the fiber through existing underground conduits that lead close to the homes.

•  Also, power companies can deploy the fiber using the existing poles that support the electrical cables!

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Frame structure-downstream

•  The downstream interface structure for both 155.52 Mbps and 622.08 Mbps consists of a continuous stream of time slots.

•  Each time slot contains either an ATM cell (53 bytes) or a 53-byte physical layer OAM (PLOAM) cell.

•  Every 28th time slot contains a PLOAM cell.

•  Groups of time slots are organized into frames.

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Frame for 155.52 Mbps :

56 time slots, 2 PLOAM cells, 54 ATM cells

PLOAM 1

Cell 1

. . .

Cell 27

PLOAM 2

Cell 28

. . .

Cell 54

56 time slots

Frame for 622 Mbps :

224 time slots, 8 PLOAMs, 216 ATM cells

PLOAM 1

Cell 1

. . .

Cell 27

. . .

PLOAM 8

Cell 190

. . .

Cell 216

224 time slots

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Frame structure-upstream

Cell 1

Cell 1

. . .

Cell 53

3 bytes

53 time slots

•  The upstream frame consists of 53 slots

•  Each slot consists of 56 bytes, of which the first 3 bytes are used for overheads and the remaining 53 bytes carry either an ATM cell or a PLOAM cell, or a divided-slots cell.

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•  Upstream overhead bytes:

–  Guard time: provides enough distance between two consecutive cells or mini-slots in a dividedslots cell to avoid collisions. Minimum guard time is 4 bits

–  Preamble

–  Delimiter: A unique pattern used to indicate the start of the ATM cell or a mini-slot in a dividedslots. It is used to acquire bit synchronization.

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Frame structure for  symmetric 155.52/155.52 Mbps PON

•  PLOAM 1

Cell 1

. . .

Cell 27

PLOAM 2

Cell 28

. . .

Cell 54

• 

56 time slots

Cell 1

. . .

Cell 1

53 time slots

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Cell 53

• 

Downstream and upstream frames as described previously.

The upstream and downstream frames are synchronized in the OLT.

Upstream cells are aligned to the frame using the ranging procedure.

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Frame structure for  asymmetric 622.08/155.52 Mbps PON

•  PLOAM 1

Cell 1

. . .

Cell 27

. . .

PLOAM 2

Cell 190

. . .

Cell 216

• 

224 time slots

•  Cell 1

Cell 1

. . .

Cell 53

The downstream rate is four times the upstream rate.

The upstream frame and the downstream frame are synchronized in the OLT.

Upstream cells are aligned to the frame using the ranging procedure.

53 time slots

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The PLOAM cell

•  It is used to convey physical layer OAM messages in the downstream and upstream direction.

•  In addition, the downstream PLOAM cells carry grants which are used by the ONUs for upstream transmission.

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Downstream structure of the PLOAM cell

Byte

Description

Byte

Description

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

IDENT SYNC 1 SYNC 2 GRANT 1 GRANT 2 GRANT 3 GRANT 4 GRANT 5 GRANT 6 GRANT 7 CRC GRANT 8 GRANT 9 GRANT 10 GRANT 11 GRANT 12 GRANT 13 GRANT 14 CRC GRANT 15 GRANT 16 GRANT 17 GRANT 18 GRANT 19

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

GRANT 20 GRANT 21 CRC GRANT 22 GRANT 23 GRANT 24 GRANT 25 GRANT 26 GRANT 27 CRC MESSAGE_PON_ID MESSAGE_ID MESSAGE_FIELD 1 MESSAGE_FIELD 2 MESSAGE_FIELD 3 MESSAGE_FIELD 4 MESSAGE_FIELD 5 MESSAGE_FIELD 6 MESSAGE_FIELD 7 MESSAGE_FIELD 8 MESSAGE_FIELD 9 MESSAGE_FIELD 10 CRC BIP

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•  IDENT byte:

–  Bits 1-7 are set to 0

–  Bit 8=1 for the first PLOAM cell of a downstream frame,

=0 for the other PLOAM cells in the frame.

•  SYNC 1, SYNC 2 bytes:

–  A 1 KHz reference signal provided by OLT is transported to the ONUS over these two bytes.

–  It is used by the ONUs to synchronize to the downstream frame.

•  GRANT fields:

–  Used by the ONUs for access on the upstream frame

•  MESSAGE fields:

–  Used to transport alarms and threshold-crossing alerts

•  BIP (bit interleaved parity):

–  Used for monitoring the BER on the downstream link.

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Grants

•  Each PLOAM cell contains 27 grants.

•  Per frame, only 53 grants are needed. These grants are mapped on the first two PLOAM cells of the downstream frame.

•  The first 53 grants are non-idle grants. The last one as well as all the grants in the remaining PLOAM cells for the asymmetric case are idle grants.

•  Groups of 7 grants are protected by CRC with a pattern: x8+x2+x+1. No error correction is done.

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Grant types

•  Data grant: Indicates an upstream ONU. The ONU can send an ATM cell or an idle cell if it has no data in the next frame.

•  PLOAM grant: Indicates an upstream ONU. The ONU sends a PLOAM cell in the next frame.

•  Divided-slots grant: It indicates a group of upstream ONUs. Each ONU in the group sends a mini-slot in the next frame.

•  Reserved grants:

•  Ranging grants: Used in the ranging protocol.

•  Unassigned grants: Indicates an unused upstream slot.

•  Idle grant: These grants are ignored.

OLT can address 32 (optionally 64) ONUs at the same time.

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Message field

•  MESSAGE_PON_ID:

–  It contains an ID number (PON_ID) which is associated with a particular ONU.

–  During the ranging protocol, the ONU is assigned a PON_ID number from 0 to 63.

–  The field can also be set to 0x40 for broadcasting to all ONUs.

•  MESSAGE_ID:

–  It indicates the type of message.

•  MESSAGE_field:

–  It contains the message.

•  CRC:

–  Same pattern. Protects the message fields. No error recovery

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Upstream structure of the PLOAM cell

Byte

Description

Byte

Description

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

IDENT MESSAGE_PON_ID MESSAGE_ID MESSAGE_FIELD 1 MESSAGE_FIELD 2 MESSAGE_FIELD 3 MESSAGE_FIELD 4 MESSAGE_FIELD 5 MESSAGE_FIELD 6 MESSAGE_FIELD 7 MESSAGE_FIELD 8 MESSAGE_FIELD 9 MESSAGE_FIELD 10 CRC LCF 1 LCF 2 LCF 3 LCF 4 LCF 5 LCF 6 LCF 7 LCF 8 LCF 9 LCF 10

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

LCF 11 LCF 12 LCF 13 LCF 14 LCF 15 LCF 16 LCF 17 RXCF 1 RXCF 2 RXCF 3 RXCF 4 RXCF 5 RXCF 6 RXCF 7 RXCF 8 RXCF 9 RXCF 10 RXCF 11 RXCF 12 RXCF 13 RXCF 14 RXCF 15 RXCF 16 BIP

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•  IDENT byte:

–  Bits 1-8 all set to 0

•  MESSAGE_PON_ID, MESSAGE_ID, MESSAGE_FIELD:

–  As defined previously

•  CRC:

–  As defined previously

•  BIP(bit interleaved parity):

–  As defined previously

•  LCF (laser control field) and RXCF (receiver control field) :

–  Used in physical layer

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Divided-slots cell (optional)

•  Upstream slot

ONUx

. . .

ONUy

• 

. . .

• 

Mini-slot payload 1 to 53 bytes

3-byte overhead

Optical Networks- Harry Perros

• 

It contains a number of mini-slots coming from a set of ONUs.

The OLT assigns one divided-slots grant per group of ONUs

The three-byte overhead is the same as defined previously

A mini-slot may contain information such as the queue-size which is used by the MAC protocol.

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Churning

•  Downstream cells are churned (scrambled) at the TC layer with a churning key sent upstream by the ONU.

•  The churning key is updated at the rate of at least 1 update/second per ONU.

•  A new churn key is sent to OLT by the ONU using the “new_churn_key” message.

•  Additional security can be provided using encryption at a higher layer.

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The MAC protocol

•  This is used by the OLT to allocate upstream bandwidth at the PON among the ONUs

•  Each ONU maps the upstream queue-length in its mini-slot in the divided-slots cell, requested by the OLT with a divided-slots cell grant.

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•  The MAC protocol has not been standardized

•  In the literature, several MAC protocols have been studied, where the bandwidth allocated to each transmitter depends on some performance measure such as the mean queue length, averaged over a short period of time.

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Ranging

•  In an APON, the ONUs may be at different distances from the OLT.

•  These distances may vary from 0 to 20 km.

•  Because of this, it is possible that in the upstream transmission, cells transmitted by different ONTs may overlap partially, and consequently collide.

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•  To avoid collisions, each ONU is placed at the same virtual distance from the OLT.

•  The process that measures the distance between the OLT and each ONU, and places each ONU in the same virtual distance is known as ranging.

–  Each ONU is given an equalization delay which is used by the ONU to adjust its time of transmission.

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APON

access networks

Local

broadcast

Satellite

Broadcast

video

Metro

WDM ring

ISP

VoATM

Internet

ONU

VoIP

Internet

NSP

Video

Regional

ATM net.

Access

G/W

OLT

ODN

Voice

G/W

DS-3

PSTN

Optical Networks- Harry Perros

Class 5

switch

VoATM

ONU

VoIP

Internet

Video

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