Chapter 2: SONET/SDH and GFP

Connection-Oriented Networks - Harry Perros 1

Chapter 2: ��SONET/SDH and GFP

TOPICS – T1/E1 – SONET/SDH - STS 1, STS -3 frames – SONET devices – Self-healing rings – Generic frame protocol, and Data over SONET


T1/E1

•  Time division multiplexing allows a link to be utilized simultaneously by many users

MUX

DEMUX

N input links

N output links

link

12

N

12

N


•  The transmission is organized into frames. •  Each frame contains a fixed number of time slots. •  Each time slot is pre-assigned to a specific input

link. The duration of a time slot is either a bit or a byte.

•  If the buffer of an input link has no data, then its associated time slot is transmitted empty.

•  A time slot dedicated to an input link repeats continuously frame after frame, thus forming a channel or a trunk.


Pulse code modulation

•  TDM is used in telephony •  Voice analog signals are digitized at the end

office using Pulse Code Modulation. •  A voice signal is sampled 8000 times/sec, or

every 125 µsec. •  A 7-bit or 8-bit number is created every 125 µsec.


The Digital Signal (DS) and ��ITU-T standard

•  A North American standard that specifies how to multiplex several voice calls onto a single link.

•  The DS standard is a North American standard and it is not the same as the international hierarchy standardized by ITU-T.

•  Both standards are independent of the transmission.


T carrier / E carrier •  The DS signal is carried over a carrier system

known as the T carrier. –  T1 carries the DS1 signal, –  T2 carries the DS2 signal etc

•  The ITU-T signal is carried over a carrier system known as the E carrier.

•  The DS and ITU-T hierarchy is known as the plesiochronous digital hierarchy (PDH). (Plesion means “nearly the same”, and chronos means “time” in Greek).


Digital signal number Voice channels Data Rate (Mbps)DS0 1 0.064DS1 24 1.544

DS1C 48 3.152DS2 96 6.312DS3 672 44.736

DS3C 1344 91.053DS4 4032 274.176

Table 2.1: The North American Hierarchy

Level number Voice channels Data Rate (Mbps)0 1 0.0641 30 2.0482 120 8.4483 480 34.3684 1920 139.2645 7680 565.148

Table 2.2: The international (ITU-T) hierarchy


The DS1 signal

•  24 8-bit time slots/frame –  Each time slot carries 8 bits/ 125 µsec, or the channel

carries a 64 Kbps voice. –  Every 6th successive time slot (i.e, 6th, 12th, 18th,

24th, etc), the 8 bit is robbed and it is used for signaling.

•  F bit: Used for synchronization. It transmits the pattern: 10101010…

F Time slot 1

Time slot 2

Time slot 3

Time slot 24

. . .


•  T1: –  Total transmission rate: 24x8+1 = 193 bits per 125 µsec, or 1.544 Mbps

•  E1 –  30 voice time slots plus 2 time slots for synchronization

and control –  Total transmission rate: 32x8 = 256 bits per 125 µsec,

or 2.048 Mbps


Fractional T1/E1

•  Fractional T1 or E1 allows the use of only a fraction of the T1 or E1 capacity.

•  For example: if N=2, then only two time slots are used per frame, which corresponds to a channel with total bandwidth of 128 Kbps.


Unchannelized frame signal

•  The time slot boundaries are ignored by the sending and receiving equipment.

•  All 192 bits are used to transport data followed by the 193rd framing bit.

•  This approach permits more flexibility in transmitting at different rates.

•  This scheme is implemented using proprietary solutions.


The Synchronous Optical NETwork (SONET)

•  Proposed by Bellcore (Telecordia). –  It was designed to multiplex DS-n signals and

transmit them optically. •  ITU-T adopted the Synchronous Digital

Hierarchy (SDH), as the international standard. –  It enables the multiplexing of level 3 signals

(34.368 Mbps)


STS, STM, OC

•  The electrical side of the SONET signal is known as the synchronous transport signal (STS)

•  The electrical side of the SDH is known as the synchronous transport module (STM).

•  The optical side of a SONET/SDH signal is known as the optical carrier (OC).


The SONET/SDH hierarchy


•  SONET/SDH is channelized. – STS-3 consists of 3 STS-1 streams, and each

STS-1 consists of a number of DS-1 and E1 signals.

– STS-12 consists of 12 STS-1 streams •  Concatenated structures (OC-3c, OC-12c, etc)

– The frame of the STS-3 payload is filled with ATM cells or IP packets packed in PPP or HDLC frames.

– Concatenated SONET/SDH links are commonly used to interconnect ATM switches and IP routers (Packets over SONET).


The STS-1 frame structure 1 2 3 4 5 6 … 90

1 1 2 3 4 5 6 … 90

2 91 92 93 94 95 96 … 180

3 181 182 183 184 185 186 … 270

4 271 272 273 274 275 276 … 360

5 361 362 363 364 365 366 … 450

6 451 452 453 454 455 456 … 560

7 561 562 563 564 565 566 … 630

8 631 632 636 634 635 636 … 720

9 721 722 723 724 725 726 … 810


•  Main features – The frame is presented in matrix form and it is

transmitted row by row. – Each cell in the matrix corresponds to a byte – The first three columns contain overheads – The remaining 87 columns carry the

synchronous payload envelope (SPE), which consists of user data, and additional overheads referred to as the payload overhead (POH)


An SPE may straddle between ��two successive frames

Frame i

Frame i+1

1 2 3 4 5 6 . . . 90 1 2 3 4 5 6 7 8 9

276

276 275

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

1 2 3 4 5 6 7 8 9


The section, line, and path overheads

Section

Line

STS-1 STS-1

A B

regenerator regenerator STS-1

A1

A12

STS-12

. . .

STS-1

B1

B12

STS-12

. . .

Section Section Section Section

Line Line

Path


•  Section: a single link with a SONET device or a regenerator on either side of it.

•  Line: A link between two SONET devices, which may include regenerators

•  The section overhead in the SONET frame is associated with the transport of STS-1 frames over a section, and the line overhead is associated with the transport of SPEs over a line.


The SONET stack

Section

Line

Path

Photonic

Section

Line

Path

Photonic

Section

Line

Photonic

Section

Photonic

Section

Photonic

Section

Line

Photonic

Ai A Regenerator Regenerator Bi B


STS-1: Section and line overheads

SOH

LOH

Column1 2 3

1 A1 A2 J02 B1 E1 F13 D1 D2 D3

4 H1 H2 H35 B2 K1 K26 D4 D5 D67 D7 D8 D98 D10 D11 D12

9 Z1 Z2 E2


•  The following are some of the bytes in the section overhead (SOH) : – A1 and A2: These two bytes are called the

framing bytes and they are used for frame alignment. They are populated with the value 1111 0110 0010 1000 or 0xF628, which uniquely identifies the beginning of an STS-frame.

–  J0: This is called the section trace byte and it is used for to trace the STS-1 frame back to its originating equipment.


– B1: This byte is the bit interleaved parity byte and it is commonly referred to as BIP-8. It is used to perform an even-parity check on the previous STS-1 frame after the frame has been scrambled. The parity is inserted in the BIP-8 field of the current frame before it is scrambled. Detected errors are reported as maintenance information.

– E1: This byte provides a 64 Kbps channel can be used for voice communications by field engineers.


•  The following are some of the bytes in the line overhead (LOH) that have been defined: –  H1 and H2: These two bytes are known as the pointer

bytes, and they contain a pointer that points to the beginning of the SPE within the STS-1 frame. The pointer gives the offset in bytes between the H1 and H2 bytes and the beginning of the SPE.

–  B2: This is similar to the B1 byte in the section overhead and it is used to carry the BIP-8 parity check performed on the line overhead section and the payload section. That is, it is performed on the entire STS-1 frame except the section overhead bytes.


The path overhead bytes

J1

B3 C2 G1 F2

H4 Z3

Z4 Z5

J1 B3 C2 G1 F2 H4 Z3 Z4 Z5

Location of the POH The POH bytes


•  The following are some of the bytes that have been defined: –  B3: This byte is similar to B1 used in the section

overhead and B2 used in the line overhead. It is used to carry the BIP-8 parity check performed on the payload section. That is, it is performed on the entire STS-1 frame except the section and line overhead bytes.

–  C2: This byte is known as the path signal label and it indicates the type of user information carried in the SPE, such as, virtual tributaries (VT), asynchronous DS-3, ATM cells, HDLC-over-SONET, and PPP over SONET.


The STS-1 payload

•  The payload consists of user data and the path overhead.

•  User data: – Virtual tributaries: sub-rate synchronous data

streams, such as DS-0, DS-1, E1, and entire DS-3 frames

– ATM cells and IP packets


Virtual tributaries

•  The STS-1 payload is divided into seven virtual tributary groups (VTG).

•  Each VTG consists of 108 bytes (12 columns) •  Each VTG may carry a number of virtual

tributaries, i.e., sub-rate streams.


•  The following virtual tributaries have been defined: – VT1.5: This virtual tributary carries one DS-1

signal and it is contained in three columns, that take up 27 bytes. Four VT1.5’s can be transported in a single VTG.

– VT2: This virtual tributary carries an E1 signal of 2.048 Mbps. VT2 is contained in four columns, that is it takes up 36 bytes. Three VT2’s can be carried in a single VTG.


•  VT3: This virtual tributary transports the unchannelized DS-1 signal. A VT3 is contained in 6 columns that takes up 54 bytes. This means that a VTG can carry two VT3s.

• VT6: This virtual tributary transports a DS-2

signal, which carries 96 voice channels. VT6 is contained in 12 columns, that is it takes up 108 bytes. A VTG can carry exactly one VT2.


ATM cells

•  Mapped directly onto the SPE. An ATM cells may straddle two SPEs.

10 Cell 1 Cell 2

Cell 2 Cell 3

Cell 14 Cell 15

Cell 15

90 4 1

9

2

8

3

POH


IP packet over SONET

•  IP packets are first encapsulated in HDLC and the resulting frames are mapped into the SPE payload row by row as in the case above for ATM cels.

10 90 4 1

9

2

8

3 POH

7E 7E 7E

7E 7E 7E


•  IP packets can also be encapsulated in PPP instead of HDLC.

•  A frame may straddle over two adjacent SPEs, as in the case of ATM.

•  The interframe fill 7E is used to maintain a continuous bit stream


The STS-3 frame structure

Overhead section Payload section

1 2 3 4 5 6 7 8 9 10 11 12 270

. . .

1st S

TS-1

1st S

TS-1

1st S

TS-1

1st S

TS-1

1st S

TS-1

2nd S

TS-1

2nd S

TS-1

2nd S

TS-1

2nd S

TS-1

2nd S

TS-1

3rd S

TS-1

3rd S

TS-1

3rd S

TS-1

3rd S

TS-1

3rd S

TS-1


•  The channelized STS-3 frame is constructed by multiplexing byte-wise three channelized STS-1 frames. As a result: –  Byte 1, 4, 7, … , 268 of the STS-3 frame contains byte

1, 2, 3, … , 90 of the first STS-1 frame. –  Byte 2, 5, 8, …, 269 of the STS-3 frame contains byte

1, 2, 3, … , 90 of the second STS-1 frame –  Byte 3, 6, 9, …, 270 of the STS-3 frame contains byte

1, 2, 3, … , 90 of the third STS-1 frame. •  This byte-wise multiplexing, causes the columns

of the three STS-1 frames to be interleaved in the STS-3 frame


•  The first 9 columns of the STS-3 frame

contain the overhead part and the remaining columns contain the payload part.

•  Error checking and some overhead bytes are for the entire STS-3 frame, and they are only meaningful in the overhead bytes of the first STS-1 frame.


SONET/SDH devices

•  Several different equipment exist: –  Terminal multiplexer (TM) –  Add/drop multiplexer (ADM) –  Digital cross connect (DCS)


•  It multiplexes a number of DS-n or E1 signals into a single OC-N signal

•  It consists of a controller, low-speed interfaces for DS-n or E1 signals, an OC-N interface, and a time slot interchanger (TSI)

•  It works also as a demultiplexer

. . .

DS-n

OC-N

DS-n

TM

The terminal multiplexer (TM):


•  It is a more complex version of the TM •  It receives an OC-N signal from which it can

demultiplex and terminate (i.e., drop) any number of DS-n or OC-M signals, where M<N, while at the same time it can add new DS-n and OC-M signals into the OC-N signal.

. . .

DS-n. OC-M

OC-N OC-N ADM

The add/drop multiplexer (ADM)


SONET rings ADM

1 ADM

2

ADM 3

ADM 4

OC3

OC3

OC3

OC3

•  SONET/SDH ADM devices are typically connected to form a SONET/SDH ring.

•  SONET/SDH rings are self-healing, that is they can automatically recover from link failures.


An example of a connection

A

B

TM 1

TM 2

ADM 1

ADM 2

ADM 3

ADM 4

DS1

OC12

DS1

OC12

OC12

OC12

OC3

OC3


•  A transmits a DS-1 signal to TM 1 •  TM 1 transmits an OC-3 signal to ADM 1 •  ADM 1 adds the OC-3 signal into the

STS-12 payload and transmits it out to the next ADM.

•  At ADM 3, the DS-1 signal belonging to A is dropped from the payload and transmitted with other signals to TM 2.

•  TM 2 in turn, demultiplexes the signals and transmits A’s DS-1 signal to B.


•  Connection setup: – Using network management procedures

the SONET network is provisioned appropriately. This is an example of a permanent connection.

– It remains up for a long time. •  The connection is dedicated to user A

whether the user transmits or not.


A digital cross connect (DCS)

Ring 1

Ring 2

ADM

ADM

ADM

ADM

ADM

ADM

DCS

•  It is used to interconnect multiple SONET rings •  It is connected to multiple incoming and outgoing OC-N

interfaces. It can drop and add any number of DSn and/or OC-M signals, and it can switch DSn and/or OC-M signals from an incoming interface to any outgoing one.

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Munich

Dusseldorf

Berlin

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Vienna

Dublin

Brussels

Amsterdam

On-Net Market with Metro Fiber Network

On-Net Market

Multiple Tier 2 Metro Routes

Longhaul Network

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London

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Newark

Mobile

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Durham

DenverDayton

Boston

Austin

Albany

Wichita

Seattle

Roanoke

Raleigh

Phoenix

Orlando

Oakland

Norfolk

Jackson

Houston

Detroit

Chicago

Buffalo

AugustaAtlanta

Syracuse

Staunton

Stamford

Savannah

Sarasota

San Jose

Portland

Hartford

Danville

Columbus

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Pensacola

Milwaukee

Melbourne

Las Vegas

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Cleveland

VERMONT

Cincinnati

Baltimore

Wilmington

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Sacramento Pittsburgh

Greenville

Fort Worth

Fort Myers

Clearwater

Charleston

Boca Raton

Tallahassee

Spartanburg

San Antonio

Saint Louis

Rocky Mount

New Orleans

Minneapolis

Little Rock

Kansas City

GainesvilleBaton Rouge

Harrisonburg

Indianapolis

Orange County Oklahoma City

Daytona Beach

Salt Lake City

San Luis Obispo

Saint Petersburg

Colorado Springs Richmond

Rochester

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Winchester

Greensboro

Jacksonville

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Winston-Salem

San Francisco

West Palm Beach

Washington D.C.

Fort Lauderdale

Charlottesville

Troy

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Bryan

Boise

Toledo

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Eugene

Modesto

McAllen

Madison

Lincoln

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El Paso

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New Haven

Kalamazoo

Green Bay

South BendDes Moines

Birmingham

Bakersfield

Wichita Falls

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San Bernardino

Corpus Christi

Springfield

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Toronto

Montreal

Memphis

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Olympia

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Dallas

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Harrisburg

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The Level 3

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European network includes wavelength capacity.

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Budapest

Bratislava

Bremen

Hanover

Bucharest

Bilbao

Bordeaux

Barcelona

Bilbao

Bordeaux

Barcelona

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Network



Self-healing SONET/SDH rings

•  SONET/SDH rings have been specially architected so that they are available 99.999% of the time (6 minutes per year!)

•  Causes for ring failures: – Fiber link failure due to accidental cuts, and

transmitter/receiver failure – SONET/SDH device failure (rare)


Automatic protection switching (APS)

•  SONET/SDH rings are self-healing, that is, the ring’s services can be automatically restored following a link failure or degradation in the network signal.

•  This is done using the automatic protection switching (APS) protocol. The time to restore the services has to be less than 50 msec.


Protection schemes: point-to-point

•  Schemes for link protection –  dedicated 1+1 –  1:1 –  Shared 1:N

ADM

Working

Protection

ADM


Working/protection fibers •  The working and protection fibers have to

be diversely routed. That is, the two fibers use separate conduits and different physical routes.

•  Often, for economic reasons, the two fibers use different conduits, but they use the same physical path. In this case, we say that they are structurally diverse.


Classification of self-healing rings Various ring architectures have been developed based on the following three features: – Number of fibers

•  2 or 4 fibers – Direction of transmission

•  Unidirectional bidirectional – Line or path switching


Number of fibers: 2- or 4-fiber rings

Two-fiber ring: fibers 1, 2, 3, and 4 are used to form the working ring (clockwise), and fibers 5, 6, 7, and 8 are used to form the protection ring (counter-clockwise).

1

2

3

4

5

6

7

8

ADM 1 ADM 2

ADM 3 ADM 4

ADM 1 ADM 2

ADM 3 ADM 4


•  In another variation of the two-fiber ring, each set of fibers form a ring which can be both a working and a protection ring. The capacity of each fiber is divided into two equal parts, one for working traffic and the other for protection traffic.

1

2

3

4

5

6

7

8

ADM 1 ADM 2

ADM 3 ADM 4


•  In a four-fiber SONET/SDH ring there are two working rings and two protection rings, one per working ring.

ADM 1 ADM 2

ADM 3 ADM 4


Direction of transmission •  Unidirectional ring:

– signals are only transmitted in one direction of the ring.

•  Bidirectional ring: – signals are transmitted in both directions.


Line and path switching

•  Path switching: Restores the traffic on the paths affected by a link failure (a path is an end-to-end connection between the point where the SPE originates and the point where it terminates.)

•  Line switching: Restores all the traffic that passes through a failed link.


Based on these three features, we have the following 2-fiber or 4-fiber possible ring architectures: – Unidirectional Line Switched Ring (ULSR) – Bidirectional Line Switched Ring (BLSR) – Unidirectional Path Switched Ring (UPSR) – Bidirectional Path Switched Ring (BPSR)


Of these rings the following three are used: – Two-fiber unidirectional path switched ring

(2F-UPSR) – Two-fiber bidirectional line switched ring

(2F-BLSR) – Four-fiber bidirectional line switched ring

(4F-BLSR)


Two-fiber unidirectional ��path switched ring (2F-UPSR)

ADM 1 ADM 2

ADM 3 ADM 4

5

2 6 4 8

3

7

A

Protection ring

Working ring

1 B


•  Features: – Working ring consists of fibers 1, 2, 3 and 4,

and the protection ring of fibers 5, 6, 7, and 8. – Unidirectional transmission means that traffic is

transmitted in the same direction. A transmits to B over fiber 1 of the working ring, and B transmits over fibers 2, 3, and 4 of the working ring.

– Used as a metro edge ring to interconnect PBXs and access networks to a metro core ring


•  Self-healing mechanism: – Path level protection using the 1+1 scheme. The

signal transmitted by A is split into two. One copy is transmitted over the working fiber 1, and the other copy is transmitted over the protection fibers 8, 7, and 6.

– During normal operation, B receives two identical signals from A, and selects the one with the best quality. If fiber 1 fails, B will continue to receive A’s signal over the protection path. The same applies if there is a node failure.


Two-fiber bidirectional line switched ring (2F-BLSR) ADM 1 ADM 2 ADM 3

ADM 4

7

3 9 6 12

5

11

A B 1

8

4

2

10

ADM 5 ADM 6

C


•  Features: –  Used in metro core rings. –  Fibers 1, 2, 3, 4, 5, and 6 form a ring, call it ring 1, on

which transmission is clockwise. Fibers 7, 8, 9, 10, 11, and 12 form another ring, call it ring 2, on which transmission is counter-clockwise.

–  Both rings 1 and 2 carry working and protection traffic. This is done by dividing the capacity of each fiber on ring 1 and 2 to two parts. One part is used to carry working traffic and the other protection traffic.

–  A transmits to B over the working part of fibers 1 and 2 of ring 1, and B transmits to A over the working part of fibers 8 and 7 of ring 2.


•  Self-healing mechanism: – The ring provides line switching. If fiber 2 fails

then the traffic that goes over fiber 2 will be automatically switched to the protection part of ring 2.

– That is, all the traffic will be re-routed to ADM 3 over the protection part of ring 2 using fibers 7, 12, 11, 10, and 9. From there, the traffic for each connection will continue on following the original path of the connection.


Four-fiber bidirectional line switched ring (4F-BLSR)

Working rings

ADM 1 ADM 2 ADM 3

ADM 4

A B

ADM 5 ADM 6

Protection rings


•  Features – Two working rings and two protection rings.

The two working rings transmit in opposite directions, and each is protected by a protection ring which transmits in the same direction.

– The advantage of this four-fiber ring is that it can suffer multiple failures and still function. In view of this, it is deployed by long-distance telephone companies in regional and national rings.


•  Self-healing operation (span switching): –  If a working fiber fails, the working traffic will

be transferred over its protection ring. This is known as span switching.

ADM 1 ADM 2 ADM 3 ADM 1 ADM 2 ADM 3

Normal operation Span switching


•  Self-healing operation (ring switching): – Often, the working and protection fibers are

part of the same bundle of fibers. When the bundle is cut the traffic will be switched to the protection fibers. This is known as ring switching.

ADM 4

Working rings Protection rings

ADM 1 ADM 2 ADM 3


B

ADM 1 ADM 2 ADM 3

ADM 4

A

ADM 5 ADM 6

Working

Protection

ADM 1 ADM 2 ADM 3

ADM 4

A

B

ADM 5 ADM 6

Working

Protection

B

Ring switching: Rerouting a connection:


Generic Framing Procedure (GFP)

•  This is a light-weight adaptation scheme that permits the transmission of different types of traffic over SONET/SDH and in the future, over G.709.


•  GFP permits the transport of a) frame-oriented traffic, such as Ethernet, and b) block-coded data for delay-sensitive storage

area networks (SAN) transported by networks such as Fiber Channel, FICON, and ESCON

over SONET/SDH and G.709. •  GFP is a result of joint standardization effort

by ANSI committee T1X1.5 and ITU-T. •  It is described in ITU-T recommendation G.

7041


Private lines Ethernet ESCON FICON Fiber

Channel

Frame Relay POS

ATM

SONET/SDH

WDM/OTN

GFP

Voice Data (IP, MPLS, IPX) SAN

DM

Video

Existing and GFP-based transport options for end-user applications

HDLC


The GFP stack

GFP GFP client-dependent aspects

GFP client-independent aspects

SONET/SDH G.709

Ethernet IP over PPP SAN data


GFP frame structure

Payload

Core header

Payload length Payload length

Core HEC Core HEC

Payload header

Payload

Payload FCS

•  GFP core header –  Payload length indicator

(PLI) - 2 bytes. It gives the size of the payload.

–  Core HEC (cHEC) - 2 bytes. It protects the PLI field. Standard CRC-16 enables single bit error correction and multiple bit error detection.


The GFP payload structure

Payload header

Payload

Payload FCS

Payload type

Payload type

Type HEC

Type HEC

0-60 bytes of

extension header

Payload FCS

Payload FCS

Payload FCS

Payload FCS

PTI

UPI

PFI EXI


GFP payload header��variable-length area from 4 to 64 bytes.

•  Payload type - 2 bytes –  Payload type identifier (PTI) - 3 bits.

Identifies the type of frame: •  User data frames , Client mgmt frames

–  Payload FCS indicator (PFI) - 1 bit. Identifies if there is a payload FCS

–  Extension header identifier (EXI) - 4 bits. Identifies the type of extension header.

–  User payload identifier (UPI) - 8 bits. Identifies the type of payload

•  Frame-mapped Ethernet •  Frame-mapped PPP (IP, MPLS) •  Transparent-mapped Fiber Channel •  Transparent-mapped FICON •  Transparent-mapped ESCON •  Transparent-mapped GbE

•  Type HEC (tHEC) - 2 bytes. It protects the payload header. Standard CRC-16.

Payload type

Payload type

Type HEC

Type HEC

0-60 bytes Of

Extension header

PTI

UPI

PFI EXI


GFP payload trailer

Payload header

Payload

Payload FCS

Payload FCS

Payload FCS

Payload FCS

Payload FCS

•  Optional 4-byte FCS. –  CRC-32 –  Protects the contents of

the payload information field.


GFP-client independent functions

•  The client independent sublayer supports the following functions: – Frame delineation – Client/frame multiplexing – Payload scrambler – Client management


Frame delineation

•  The frame delineation mechanism is similar to the one used in ATM.

•  The cHEC is used to assure correct frame boundary identification

hunt

Presync

Sync

Correct cHEC

2nd cHEC match

Non-correctable core header error

No 2nd cHEC


•  Operation: – Under normal conditions, the GFP receiver

operates in the Sync state. The receiver examines the PLI field, validates the cHEC, and extracts the framed higher-level PD. It then moves on to the next GFP header.

–  When an uncorrectable error in the core header occurs (i.e., cHEC fails and more than one bit error is detected), the receiver enters the Hunt state.


•  Hunt state: – Using the cHEC it attempts to locate the

beginning of the next GFP PDU, moving one bit at a time (Same as in ATM - see Perros “An introduction to ATM networks, Wiley 2001.

– Once this is achieved it moves to the Pre-Sync state, where it verifies the beginning of the boundary of the next N GFP PDUs.

–  If successful, it moves to the Sync state, otherwise it moves back to the hunt state.


Frame multiplexing

•  Client data frames and client management frames are multiplexed, with client data frames having priority over client management frames.

•  Idle frames are inserted to maintain a continuous bit flow (rate coupling)


GFP client-specific functions

•  The client data can be carried in GFP frames using one of the two adaptation modes: – Frame-mapped GFP (GFP-F) applicable to

most packet data types – Transparent-mapped GFP (GFP-T) applicable

to 8B/10B coded signals


Frame-mapped GFP

•  Variable length frames such as: – Ethernet MAC frames, –  IP/PPP packets – HDLC-framed PDUs can be carried in the GFP payload.

•  One frame per GFP payload. •  Max. size: 65,535 bytes


Transparent-mapped GFP

•  Fiber Channel, ESCON, FICON, Gigabit Ethernet high-speed LANs use 8B/10B block-coding to transport client data and control information.

•  Rather than transporting data on a frame-by-frame basis, the GFP transparent-mapped mode, transports data as a stream of characters.


•  Specifically, the individual characters are de-mapped from their client 8B/10B block codes and then mapped into periodic fixed-length GFP frames using 64B/65B block coding.

•  This reduces the 25% overhead introduced by the 8B/10B block-coding.

•  Also, transparent mapping reduces latency, which is important for storage related applications


•  The first step, is to decode the 8B/10B codes. The 10 bit code is decoded into its original data or control codeword value.

•  The decoded characters are then mapped into 64B/65B codes. A bit in the 65-bit code indicates whether the 65-bit block contains only data or control characters are also included

•  8 consecutive 65-bit blocks are grouped together into a single superblock.

•  A GFP frame contains N such superblocks.


Data over SONET/SDH (DoS)

•  The DoS architecture provides an efficient mechanism to transport efficiently data (Ethernet, Fiber Channel, ESCON/FICON) and voice over SONET/SDH.

•  It relies on a combination of – GFP – Virtual concatenation, and – Link capacity adjustment scheme (LCAS)


Virtual concatenation Sub-rate streams: •  The bandwidth of a SONET link is divided into a

fixed number of sub-rate streams. (A SONET STS-48 link is divided into 48 sub-rate OC-1’s)

•  Each sub-rate stream or a group of sub-rate streams can be used independently by a user to carry data (GFP framed) or voice.

•  This provides more flexibility than the rigid SONET/SDH STS-N hierarchy


Example: bandwidth partitioning ��using sub-rate streams

STS-48/ 2.4 Gbps

…

STS-1

STS-1

36 STS-1/ 1.8 Gbps

VT1.5

VT1.5

VT1.5

VT1.5

TDM Services (600 Mbps)

TDM Services (600 Mbps)

…

…


Virtual concatenation: •  This scheme maps an incoming traffic

stream into a number of individual sub-rate payloads.

•  The sub-rate payloads are switched through the SONET/SDH network independently of each other. An intermediate node is not aware of the relation between these sub-rate streams

•  At the destination, they are used to reconstruct the original traffic stream.


Example •  A 1 GbE can be carried over SONET using

7 independent STS-3c (7x155,520 = 1,088).

•  If virtual concatenation was not available, it would have required an OC-48c (2.488 Gbps), since it cannot fit into an OC-12c.

•  This would have resulted to major waste of the capacity of the OC-48c.


Link capacity adjustment scheme (LCAS)

•  This scheme permits to dynamically adjust the number of sub-rate streams allocated to a specific input stream, whose transmission rate may vary over time.

•  This feature is useful in adjusting bandwidth requirements on a time-of-day basis.

•  LCAS can be also used to re-route traffic due to a link failure or maintenance..


Data over SONET Architecture •  GFP, virtual concatenation, and LCAS are the

building blocks of an integrated voice/data service over SONET/SDH (DoS) –  Bandwidth is allocated in increments of 50 Mbps

(OC-1bit rate minus overheads) –  Efficient framing with small overhead –  Coexistence of legacy services (voice) with data service

in a single SONET/SDH frame –  Dynamic bandwidth allocation –  Network management through SONET/SDH existing

network management system


Layer 1/2 hybrid network via DoS

•  This DoS scheme permits coexistence of TDM and data services (GFP). – TDM is handled at layer 1 – Data is handled using GFP which can be seen as

layer 2. •  Can be implemented on a SONET/SDH ring

to add/drop both TDM and data at each node.

Documents

Chapter 2: SONET/SDH and GFP