Connection-Oriented Networks – Wissam FAWAZ1 Chapter 2: SONET/SDH and GFP TOPICS –T1/E1 –SONET/SDH - STS 1, STS -3 frames –SONET devices –Self-healing

Connection-Oriented Networks – Wissam FAWAZ

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


2

T1/E1

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

M U X

D E M U X

N input links

N output links

link

1

2

N

12

N


3

• 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.


4

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 sec.

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


5

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.


6

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).


7

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


8

The DS1 signal

• 24 8-bit time slots/frame– Each time slot carries 8 bits/ 125

sec, 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…

FTime slot 1

Time slot 2

Time slot 3

Time slot 24

. . .


9

• T1:– Total transmission rate: 24x8+1 = 193 bits per 125 sec, 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 sec, or 2.048 Mbps


10

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.


11

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.


12

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)


13

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).


14

The SONET/SDH hierarchy

Opticallevel

SONETlevel

(electrical)

SDH level

(electrical)

Data rate(Mbps)

Overheadrate

(Mbps)

Payloadrate

(Mbps)OC-1 STS-1 - 51.840 1.728 50.112

OC-3 STS-3 STM-1 155.520 5.184 150.336

OC-9 STS-9 STM-3 466.560 15.552 451.008

OC-12 STS-12 STM-4 622.080 20.736 601.344

OC-18 STS-18 STM-6 933.120 31.104 902.016

OC-24 STS-24 STM-8 1244.160 41.472 1202.688

Oc-36 STS-36 STM-12 1866.240 62.208 1804.932

OC-48 STS-48 STM-16 2488.320 82.944 2405.376

OC-96 STS-96 STM-32 4976.640 165.888 4810.752

OC-192 STS-192 STM-64 9953.280 331.776 9621.504

OC-768 STS-768 STM-256 39813.120 1327.104 38486.016

OC-N STS-N STM-N/3 N*51.840 N*1.728 N*50.112


15

• 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).


16

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


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• 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)


18

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

276275

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. .

.

. . .

. . .

. . .

. . .

. . .

. . .

. . .

. . .

1

2

3

4

5

6

7

8

9


19

The section, line, and path overheads

Section

Line

STS-1 STS-1

A B

regeneratorregeneratorSTS-1

A1

A12

STS-12. . .

STS-1

B1

B12

STS-12. . .

Section Section Section Section

LineLine

Path


20

• 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.


21

The SONET stack

Section

Line

Path

Photonic

Section

Line

Path

Photonic

Section

Line

Photonic

Section

Photonic

Section

Photonic

Section

Line

Photonic

AiA Regenerator Regenerator

BiB


22

STS-1: Section and line overheads

SOH

LOH

Column1 2 3

1 A1 A2 J0

2 B1 E1 F1

3 D1 D2 D3

4 H1 H2 H3

5 B2 K1 K2

6 D4 D5 D6

7 D7 D8 D9

8 D10 D11 D12

9 Z1 Z2 E2


23

• 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.


24

– 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

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


25

• 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.


26

The path overhead bytes

J1

B3

C2

G1

F2

H4

Z3

Z4

Z5

J1

B3

C2

G1

F2

H4 Z3Z4

Z5

Location of the POH The POH bytes


27

• 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.


28

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


29

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.


30

• 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.


31

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.


32

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

904

1

9

2

8

3

POH


33

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.

10 904

1

9

2

8

3

POH

7E 7E 7E

7E7E7E


34

•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


35

The STS-3 frame structure

Overhead section Payload section

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

. . .1st STS-1

1st STS-1 1st STS-1 1st STS-1 1st STS-12nd STS-1 2nd STS-1 2nd STS-1 2nd STS-1 2nd STS-13rd STS-1 3rd STS-1 3rd STS-1 3rd STS-1 3rd STS-1


36

• 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


37

• 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.


38

SONET/SDH devices

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


39

• 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):


40

• 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-NADM

The add/drop multiplexer (ADM)


41

SONET rings

ADM1

ADM2

ADM3

ADM4

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.


42

An example of a connection

A

B

TM1

TM2

ADM1

ADM2

ADM3

ADM4

DS1

OC12

DS1

OC12

OC12

OC12

OC3

OC3


43

• 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.


44

•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.


45

A digital cross connect (DCS)

Ring 1 Ring 2ADM

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.


46

SONET rings

ADM1

ADM2

ADM3

ADM4

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.


47

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)


48

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.


49

Protection schemes: point-to-point

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

ADM

Working

Protection

ADM


50

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.


51

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


52

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 3ADM 4

ADM 1 ADM 2

ADM 3ADM 4


53

• 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 3ADM 4


54

• 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 3ADM 4


55

Direction of transmission

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

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


56

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.


57

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)


58

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)


59

Two-fiber unidirectional path switched ring (2F-

UPSR)

ADM 1 ADM 2

ADM 3ADM 4

5

264 8

3

7

A

Protection ring

Working ring

1B


60

• 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


61

• 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.


62

Two-fiber bidirectional line switched ring (2F-

BLSR)ADM 1 ADM 2 ADM 3

ADM 4

7

396 12

5

11

A B

1

8

4

2

10

ADM 5ADM 6

C


63

• 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.


64

• 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.


65

Four-fiber bidirectional line switched ring (4F-

BLSR)

Working rings

ADM 1 ADM 2 ADM 3

ADM 4

A B

ADM 5ADM 6

Protection rings


66

• 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.


67

• 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


68

• 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 ringsProtection rings

ADM 1 ADM 2 ADM 3


69

B

ADM 1 ADM 2 ADM 3

ADM 4

A

ADM 5ADM 6

Working

Protection

ADM 1 ADM 2 ADM 3

ADM 4

A

B

ADM 5ADM 6

Working

Protection

B

Ring switching: Rerouting a connection:


70

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.


71

• 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


72

Privatelines

Ethernet ESCON FICONFiberChannel

Frame Relay POS

ATM

SONET/SDH

WDM/OTN

GFP

VoiceData (IP, MPLS,

IPX)SAN

DM

Video

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

HDLC


73

The GFP stack

GFP

GFP client-dependent aspects

GFP client-independent aspects

SONET/SDH G.709

Ethernet IP over PPP SAN data


74

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.


75

The GFP payload structure

Payload header

Payload

Payload FCS

Payload type

Payload type

Type HEC

Type HEC

0-60 bytesof

extension header

Payload FCS

Payload FCS

Payload FCS

Payload FCS

PTI

UPI

PFI EXI


76

GFP payload headervariable-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 bytesOf

Extension header

PTI

UPI

PFI EXI


77

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.


78

GFP-client independent functions

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


79

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

CorrectcHEC

2ndcHEC match

Non-correctablecore header error

No 2ndcHEC


80

• 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.


81

• 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.


82

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)


83

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


84

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


85

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.


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• 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


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• 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.


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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)


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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


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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)

GFP Services(600 Mbps)

…

…


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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.


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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.


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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..


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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


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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

Connection-Oriented Networks – Wissam FAWAZ1 Chapter 2: SONET/SDH and GFP TOPICS –T1/E1 –SONET/SDH - STS 1, STS -3 frames –SONET devices –Self-healing