L2_slides

Transmission techniques and multiplexing hierarchies

Switching Technology

S38.165

http://www.netlab.hut.fi/opetus/s38165

© P. Raatikainen Switching Technology / 2003

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Transmission techniques and multiplexing hierarchies

• Transmission of data signals

• Timing and synchronization

• Transmission techniques and multiplexing

– PDH

– ATM

– IP/Ethernet

– SDH/SONET

– OTN

© P. Raatikainen Switching Technology / 2003

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1

Transmission of data signals

• Encapsulation of user data into layered protocol structure

• Physical and link layers implement functionality that have relevance to switching

– multiplexing of transport signals (channels/connections)

– medium access and flow control

– error indication and recovery

– bit, octet and frame level timing/synchronization

– line coding (for spectrum manipulation and timing extraction)

© P. Raatikainen Switching Technology / 2003

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Transmission of data signals (cont.)

PLH

LLH

NLH

User data

TLH Transport layer payload

Network layer payload

Link layer payload

Physical layer

© P. Raatikainen Switching Technology / 2003

• error coding/indication

• octet & frame synchronization

• addressing

• medium access & flow control

• line coding

• bit level timing

• physical signal generation/ recovery

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2

Synchronization of transmitted data

• Successful transmission of data requires bit, octet, frame and packet level synchronism

• Synchronous systems (e.g. PDH and SDH) transfer additional information (embedded into transmitted line signal) for accurate recovery of clock signals

• Asynchronous systems (e.g. Ethernet) transfer additional bit patterns to synchronize receiver logic

© P. Raatikainen Switching Technology / 2003

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Timing accuracy

• Inaccuracy of frequency classified in telecom networks to

– jitter (short term changes in frequency > 10 Hz)

– wander (< 10 Hz fluctuation)

– long term frequency shift (drift or skew)

• To maintain required timing accuracy network nodes are connected to a hierarchical synchronization network

– Universal Time Coordinated (UTC): error in the order of 10

-13

– Error of Primary Reference Clock (PRC) of the telecom network

10

-11

© P. Raatikainen Switching Technology / 2003

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3

Timing accuracy (cont.)

• Inaccuracy of clock frequency causes

– degraded quality of received signal

– bit errors in regeneration

– slips: in PDH networks a frame needs to be duplicated or is lost due to timing difference between the sender and receiver

• Based on applied synchronization method networks are divided into

– fully synchronous networks (e.g. SDH)

– plesiochronous networks (e.g. PDH), sub-networks have nominally the same clock frequency but are not synchronized to each other

– mixed networks

© P. Raatikainen Switching Technology / 2003

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Methods for bit level timing

• To obtain bit level synchronism receiver clocks must be synchronized to incoming signal

• Incoming signal must include transitions to keep receiver’s clock recovery circuitry in synchronism

• Methods to introduce line signal transitions

– Line coding

– Block coding

– Scrambling

© P. Raatikainen Switching Technology / 2003

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4

Line coding

Uncoded

+V

1 1 0 1 0 0 0 0 1 1 0 0 0 0 0 0 1 1 0 1

+V

ADI

ADI RZ

+V

AMI RZ

+V

-V

© P. Raatikainen

ADI - Alternate Digit Inversion

ADI RZ - Alternate Digit Inversion Return to Zero

AMI RZ - Alternate Mark Inversion Return to Zero

Switching Technology / 2003

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Line coding (cont.)

• ADI, ADI RZ and codes alike introduce DC balance shift

=> clock recovery becomes difficult

• AMI and AMI RZ introduces DC balance, but lacks effective ability to introduce signal transitions

• HDB3 code, used in PDH systems, guarantees a signal transition at least every fourth bit

HDB3

+V

-V

1 1 0 1 0 0 0 0 1 1 0 0 0 0 0 0 1 1 0 1 v

B v

HDB3 - High Density Bipolar 3

© P. Raatikainen Switching Technology / 2003

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5

Line coding (cont.)

• When bit rates increase (> 100 Mbit/s) jitter requirements become tighter and signal transitions should occur more frequently than in

HDB3 coding

• CMI coding was introduced for electronic differential links and for optical links

• CMI doubles bit rate on transmission link -> higher bit rate implies larger bandwidth and shortened transmission distance

1 1 0 1 0 0 0 0 1 1 0 0 0 0 0 0 1 1 0 1

+V

CMI

CMI

-V

- Coded Mark Inversion

© P. Raatikainen Switching Technology / 2003

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Block coding

• Entire blocks of n bits are replaced by other blocks of m bits (m > n)

nBmB block codes are usually applied on optical links by using on-off keying

• Block coding adds variety of “1”s and “0”s to obtain better clock synchronism and reduced jitter

• Redundancy in block codes (in the form of extra combinations) enables error recovery to a certain extent

• When m>n the coded line signal requires larger bandwidth than the original signal

• Examples: 4B5B (FDDI), 5B6B (E3 optical links) and 8B10B (GbE)

© P. Raatikainen Switching Technology / 2003

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Coding examples

4B5B coding

Input word

1 0 0 0

1 0 0 1

1 0 1 0

1 0 1 1

1 1 0 0

1 1 0 1

1 1 1 0

1 1 1 1

0 0 0 0

0 0 0 1

0 0 1 0

0 0 1 1

0 1 0 0

0 1 0 1

0 1 1 0

0 1 1 1

Output word

1 1 1 1 0

0 1 0 0 1

1 0 1 0 0

1 0 1 0 1

0 1 0 1 0

0 1 0 1 1

0 1 1 1 0

0 1 1 1 1

1 0 0 1 0

1 0 0 1 1

1 0 1 1 0

1 0 1 1 1

1 1 0 1 0

1 1 0 1 1

1 1 1 0 0

1 1 1 0 1

Other output words

0 0 0 0 0 Quiet line symbol

1 1 1 1 1 Idle symbol

0 0 1 0 0 Halt line symbol

1 1 0 0 0 Start symbol

1 0 0 0 1 Start symbol

0 1 1 0 1 End symbol

0 0 1 1 1 Reset symbol

1 1 0 0 1 Set Symbol

0 0 0 0 1 Invalid

0 0 0 1 0 Invalid

0 0 0 1 1 Invalid

0 0 1 0 1 Invalid

0 0 1 1 0 Invalid

0 1 0 0 0 Invalid

0 1 1 0 0 Invalid

1 0 0 0 0 Invalid

© P. Raatikainen Switching Technology / 2003

5B6B coding

Input word

0 0 0 0 0

0 0 0 0 1

0 0 0 1 0

0 0 0 1 1

...

1 1 1 0 0

1 1 1 0 1

1 1 1 1 0

1 1 1 1 1

Output word

1 0 1 0 1 1

1 0 1 0 1 0

1 0 1 0 0 1

1 1 1 0 0 0

...

0 1 0 0 1 1

0 1 0 1 1 1

0 1 1 0 1 1

0 1 1 1 0 0

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Scrambling

• Data signal is changed bit by bit according to a separate repetitive sequence (to avoid long sequences of “1”s or “0”s)

• Steps of the sequence give information on how to handle bits in the signal being coded

• A scrambler consists of a feedback shift register described by a polynomial (x

N

+ … + x m

+ … + x k

+ … + x + 1)

• Polynomial specifies from where in the shift register feedback is taken

• Output bit rate is the same as the input bit rate

• Scrambling is not as effective as line coding

© P. Raatikainen Switching Technology / 2003

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Scrambler example

SDH/STM-1 uses x 7 +x 6 +1 polynomial

Preset x 0

D x 1

D x 2

D x 3

D x 4

D x 5

D x 6

D

+ x 7

D

Data in

+

Data out

© P. Raatikainen Switching Technology / 2003

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Methods for octet and frame level timing

• Frame alignment bit pattern

• Start of frame signal

• Use of frame check sequence

© P. Raatikainen Switching Technology / 2003

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Frame alignment sequence

• Data frames carry special frame alignment bit patterns to obtain octet and frame level synchronism

• Data bits scrambled to avoid misalignment

• Used in networks that utilize synchronous transmission, e.g. in PDH, SDH and OTN

• Examples

– PDH E1 frames carry bit sequence 0011011 in every other frame (even frames)

– SDH and OTN frames carry a six octet alignment sequence

(hexadecimal form: F6 F6 F6 28 28 28) in every frame

© P. Raatikainen Switching Technology / 2003

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Start of frame signal

• Data frames carry special bit patterns to synchronize receiver logic

• False synchronism avoided for example by inserting additional bits into data streams

• Used in synchronous and asynchronous networks, e.g., Ethernet and HDLC

• Examples

– Ethernet frames are preceded by a 7-octet preamble field

(10101010) followed by a start-of-frame delimiter octet

(10101011)

– HDLC frames are preceded by a flag byte (0111 1110)

© P. Raatikainen Switching Technology / 2003

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Frame check sequence

• Data frames carry no special bit patterns for synchronization

• Synchronization is based on the use of error indication and correction fields

– CRC (Cyclic Redundancy Check) calculation

• Used in bit synchronous networks such as ATM and

GFP (Generic Framing Procedures)

• Example

– ATM cells streams can be synchronized to HEC (Header Error

Control) field, which is calculated across ATM cell header

© P. Raatikainen Switching Technology / 2003

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Plesiochronous Digital Hierarchy

(PDH)

• Transmission technology of digitized telecom network

• Basic channel capacity 64 kbit/s

• Voice information PCM coded

• 8 bits per sample

• A or

µµ

law

• sample rate 8 kHz (125

µµ

s)

• Channel associated signaling

(SS7)

• Higher order frames obtained by multiplexing four lower order frames bit by bit and adding some synchr. and management info

• The most common information switching and transmission format in the telecommunication network is PCM 30 (E1)

E0

E1

E2

E3

1920 channels

34.368 Mbit/s x 4

480 channels

8.448 Mbit/s x 4

120 channels

2.048 Mbit/s

...

x 32

30 channels

64 kbit/s

E4

139.264 Mbit/s

1 channel x 4

© P. Raatikainen Switching Technology / 2003

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PDH E1-frame structure

(even frames)

Multi- frame

F0 F1 . . . F14 F15

Voice channels 1 - 15

T0 T1 T2 T0

. . .

T15 T16 T17

Voice channels 16 - 30

. . .

T28 T29 T30 T31

Frame alignment time-slot

C 0 0 1 1 0 1 1

Frame alignment signal (FAS)

Error indicator bit (CRC-4)

© P. Raatikainen

Signaling time-slot

0 0 0 0 1 A 1 1

Multi-frame alignment bit sequence in F0 Multi-frame alarm

Switching Technology / 2003

Voice channel 28

B1 B2 B3 B4 B5 B6 B7 B8

Polarity

Voice sample amplitude

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PDH E1-frame structure

(odd frames)

Multi- frame

F0 F1 . . . F14 F15

Voice channels 1 - 15

T0 T1 T2 T0 . . .

T15 T16 T17

Voice channels 16 - 30

. . .

T28 T29 T30 T31

Frame alignment time-slot

C 1 A D D D D D

Error indicator bit (CRC-4)

Data bits for management

Far end alarm indication

© P. Raatikainen

Signaling time-slot a b c d a b c c

Channel 1 signaling bits

Channel 16 signaling bits

Switching Technology / 2003

Nowadays, time slot 1 used for signaling and time slot 16 for voice

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11

CRC-4 multi-frame format

CRC-4 calculation ensures, that the frame alignment function can not lock into a user signal of

(x0011011)

PDH standards/ITU-T

- G.703

- G.704

- G.706

- G.711

Submulti-frame

SMF #1

Time-slot 0

Frame Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7

6

7

4

5

2

3

0

1 c1

0 c2

0 c3

1 c4

0

0

1

0

1

0

1

0

1

0

A

0

A

0

A

0

A

1

Sa4

1

Sa4

1

Sa4

1

Sa4

1

Sa5

1

Sa5

1

Sa5

1

Sa5

0

Sa6

0

Sa6

0

Sa6

0

Sa6

1

Sa7

1

Sa7

1

Sa7

1

Sa7

SMF #2

8

9

10

11 c1

1 c2

1

0

1

0

1

12

13 c3

E

0

1

14 c4 0 0

15 E 1

Sub-frame #1 (SMF #1):

Sa: Spare bit reserved for national use

A: Remote Alarm (FAS) indication

Frame alignment signal: 0011011

CRC-4 Frame Alignment Signal: 001011

A

CRC multi-frame not aligned with MFAS TS16

0

A

0

A

0

A

1

Sa4

1

Sa4

1

Sa5

1

Sa5

0

Sa6

0

Sa6

1

Sa7

1

Sa7

1

Sa8

1

Sa8

1

Sa4

1

Sa5

0

Sa6

1

Sa7

1

Sa8

1 1 0 1 1

Sa4 Sa5 Sa6 Sa7

Sub-frame #2 (SMF #2):

E: CRC-4 error indication used by receiving equipment

C1, C2, C3, C4: CRC-4 bits

Sa8

Bit 8

1

Sa8

1

Sa8

1

Sa8

1

Sa8

© P. Raatikainen Switching Technology / 2003

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Multiplexing/PDH

• Tributaries have the same nominal bit rate, but with a specified, permitted deviation (100 bit/s for 2.048

Mbit/s)

• Plesiochronous = tributaries have almost the same bit rate

• Justification and control bits are used in multiplexed flow

• First order is octet-interleaved, but higher orders are bit-interleaved

© P. Raatikainen Switching Technology / 2003

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PDH network elements

concentrator

n channels are multiplexed to a higher capacity link carrying m channels (n > m)

multiplexer

n channels are multiplexed to a higher capacity link carrying n channels

cross-connect

– static multiplexing/switching of user channels

switch

– switches incoming TDM/SDM channels to outgoing ones

© P. Raatikainen Switching Technology / 2003

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Example PDH network elements

Concentrator

n > m

m output channels

Cross-connect

DXC

Multiplexer

4

4

4

3

3

3

2

2

2

1

1

1

n = m

m output channels

© P. Raatikainen Switching Technology / 2003

Switch

4

4

4

3

3

3

2

2

2

1

1

1

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13

SDH reference model

MPX

STM-n

DXC

STM-n

R

STM-n

R

STM-n

MPX

Multiplexing section

Regeneration section

Regeneration section

Regeneration section

Multiplexing section

Path layer connection

- DXC Digital gross-connect

- MPX Multiplexer

- R Repeater

© P. Raatikainen Switching Technology / 2003

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Synchronous digital hierarchy

Major ITU-T SDH standards:

- G.707

- G.783

STM-256

40 Gbit/s x 4

STM-64

10 Gbit/s x 4

STM-16

2.48 Gbit/s x 4

STM-4

622 Mbit/s x 4

STM-1

155 Mbit/s

© P. Raatikainen Switching Technology / 2003

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14

Multiplexing/SDH

• Multiplexing hierarchy for plesiochronous and synchronous tributaries (e.g. E1 and E3)

• Octet-interleaving, no justification bits - tributaries visible and available in the multiplexed SDH flow

• SDH hierarchy divided into two groups:

– multiplexing level (virtual containers, VCs)

– line signal level (synchronous transport level, STM)

• Tributaries from E1 (2.048 Mbit/s) to E4 (139.264 Mbit/s) are synchronized (using justification bits if needed) and packed in containers of standardized size

• Control and supervisory information (POH, path overhead) added to containers => virtual container (VC)

© P. Raatikainen Switching Technology / 2003

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Multiplexing/SDH (cont.)

Different sized VCs for different tributaries (VC-12/E1, VC-3/E3,

VC-4/E4

Smaller VCs can be packed into a larger VC (+ new POH)

Section overhead (SOH) added to larger VC

=> transport module

• Transport module corresponds to line signal (bit flow transferred on the medium)

– bit rate is 155.52 Mbit/s or multiples of it

– transport modules called STM-N (N = 1, 4, 16, 64, ...)

– bit rate of STM-N is Nx155.52 Mbit/s

– duration of a module is 125

µ s (= duration of a PDH frame)

© P. Raatikainen Switching Technology / 2003

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SDH network elements

• regenerator (intermediate repeater, IR)

– regenerates line signal and may send or receive data via communication channels in RSOH header fields

• multiplexer

– terminal multiplexer multiplexes/demultiplexes PDH and SDH tributaries to/from a common STM-n

– add-drop multiplexer adds or drops tributaries to/from a common

STM-n

• digital cross-connect

– used for rearrangement of connections to meet variations of capacity or for protection switching

– connections set up and released by operator

© P. Raatikainen Switching Technology / 2003

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Example SDH network elements

STM-n

STM-n

STM-n

Cross-connect

DXC

Add-drop multiplexer

STM-n

STM-n

STM-n

ADM

STM-n

Terminal multiplexer

ADM

STM-n

STM-n

2 - 140 Mbit/s 2 - 140 Mbit/s

© P. Raatikainen Switching Technology / 2003

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16

Generation of STM-1 frame

PDH/E1

Justification

VC-12

+ POH

MUX

+ POH

VC-4

+ SOH

STM-1

© P. Raatikainen Switching Technology / 2003

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STM-n frame

Three main fields:

Regeneration and multiplexer section overhead (RSOH and MSOH)

Payload and path overhead (POH)

AU (administrative) pointer specifies where payload (VC-4 or VC-3) starts

nx9 octets nx261 octets

3

1

5

RSOH

AU-4 PTR

P

O

H

MSOH

© P. Raatikainen Switching Technology / 2003

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Synchronization of payload

Position of each octet in a STM frame (or VC frame) has a number

AU pointer contains position number of the octet in which VC starts

• lower order VC included as part of a higher order VC (e.g. VC-12 as part of VC-4)

STM-1 no. k

STM-1 no. k+1

RSOH

AU-4 PTR

MSOH

RSOH

AU-4 PTR

MSOH

VC-4 no. 0

VC-4 no. 1

VC-4 no. 2

© P. Raatikainen Switching Technology / 2003

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ATM concept in summary

• cell

– 53 octets

• routing/switching

– based on VPI and VCI

• adaptation

– processing of user data into ATM cells

• error control

– cell header checking and discarding

• flow control

– no flow control

– input rate control

• congestion control

– cell discarded (two priorities)

© P. Raatikainen Switching Technology / 2003

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18

ATM protocol reference model

© P. Raatikainen

AAL

ATM

Convergence sublayer (CS)

Segmentation and reassembly (SAR)

Generic flow control

VPI/VCI translation

Multiplexing and demultiplexing of cells

Phys

Cell rate decoupling

HEC header sequence generation/verification

Cell delineation

Transmission frame adaptation

Transmission frame generation/recovery

Timing

Physical medium

Switching Technology / 2003

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Reference interfaces

ATM network

NNI

EX

UNI

TE

© P. Raatikainen Switching Technology / 2003

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ATM cell structure

5 octets

ATM header

48 octets

Cell payload

GFC

VPI

VCI

ATM header for UNI

VPI

VCI

VCI

PTI

HEC

VPI

ATM header for NNI

VPI

VCI

VCI

VCI PTI

HEC

CPL

CPL

© P. Raatikainen

UNI - User Network Interface

NNI - Network-to-Network Interface

VPI - Virtual Path Identifier

VCI - Virtual Channel Identifier

GFC - Generic Flow Control

PTI - Payload Type Identifier

CPL - Cell Loss Priority

HEC - Header Error Control

HEC = 8 x (header octets 1 to 4) / (x

8

+ x

2

+ x + 1)

Switching Technology / 2003

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ATM connection types

VCI 1

VCI 2

VPI 1

VCI 1

VCI 2

VPI 2

Physical channel

VPI 1

VPI 2

VCI 1

VCI 2

VCI 1

VCI 2

© P. Raatikainen Switching Technology / 2003

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20

Physical layers for ATM

• SDH

(Synchronous Digital Hierarchy)

– STM-1 155 Mbit/s

– STM-4 622 Mbit/s

– STM-16 2.4 Gbit/s

• PDH

(Plesiochronous Digital Hierarchy)

– E1 2 Mbit/s

– E3 34 Mbit/s

– E4 140 Mbit/s

• TAXI 100 Mbit/s and IBM 25 Mbit/s

• Cell based interface

– uses standard bit rates and phys. level interfaces

(e.g. E1, STM-1 or STM-4)

– HEC used for framing

© P. Raatikainen Switching Technology / 2003

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Transport of data in ATM cells

Network layer

ATM adaptation layer (AAL)

ATM layer

IP packet

≤≤

65 535

AAL 5 payload

Pad 0 - 47 octets

(1+1+ 2) octets

4 octets

P

UU/

CPI/

LEN

CRC

5

H

48

Cell payload H Cell payload H Cell payload H

Physical layer

© P. Raatikainen

P

UU

- Padding octets

- AAL layer user-to-user identifier

CPI - Common part identifier

LEN - Length indicator

Switching Technology / 2003

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21

9 octets

3

1

SOH

AU-4 PTR

SOH

5

ATM cell encapsulation / SDH

261 octets

...

STM-1 frame

VC-4 frame

© P. Raatikainen

F2

H4

Z3

Z4

Z5

J1

B3

C2

G1

...

...

VC-4 POH

Switching Technology / 2003

...

ATM cell

2 - 43

ATM cell encapsulation / PDH (E1)

TS0

TS0

TS0

TS0

TS0

Header

Header

32 octets

TS16

TS16

TS16

TS16

TS16

Header

Head.

TS0

• frame alignment

• F3 OAM functions

• loss of frame alignment

• performance monitoring

• transmission of FERF and LOC

• performance reporting

© P. Raatikainen Switching Technology / 2003

TS16

• reserved for signaling

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22

Cell based interface

Frame structure for cell base interfaces:

P

L

27

IDLE or

PL-OAM

1 2

H ATM layer H ATM layer

...

26

H ATM layer

P

L

27

IDLE or

PL-OAM

• PL cells processed on physical layer (not on ATM layer)

• IDLE cell for cell rate adaptation

• PL-OAM cells carry physical level OAM information

(regenerator (F1) and transmission path (F3) level messages)

• PL cell identified by a pre-defined header

• 00000000 00000000 0000000 00000001 (IDLE cell)

• 00000000 00000000 0000000 00001001 (phys. layer OAM)

• xxxx0000 00000000 0000000 0000xxxx (reserved for phys. layer)

H = ATM cell Header, PL = Physical Layer, OAM = Operation Administration and Maintenance

© P. Raatikainen Switching Technology / 2003

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ATM network elements

• Gross-connect

– switching of virtual paths (VPs)

– VP paths are statically connected

Switch

– switching of virtual channel (VCs)

– VC paths are dynamically or statically connected

DSLAM (Digital Subscriber Line Access Multiplexer)

– concentrates a larger number of sub-scriber lines to a common higher capacity link

– aggregated capacity of subscriber lines surpasses that of the common link

© P. Raatikainen Switching Technology / 2003

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Ethernet

• Originally a link layer protocol for LANs

(10 and 100 MbE)

• Upgrade of link speeds

=> optical versions 1GbE and 10 GbE

=> suggested for long haul transmission

• No connections - each data terminal (DTE) sends data when ready - MAC is based on CSMA/CD

• Synchronization

– line coding, preamble pattern and start-of-frame delimiter

– Manchester code for 10 MbE, 8B6T for 100 MbE,

8B9B for GbE

© P. Raatikainen Switching Technology / 2003

2 - 47

Ethernet frame

64 - 1518 octets

Preamble

7

S

F

D

1

DA

6

SA T/L

6 2

Payload

46 - 1500

CRC

4

Preamble - AA AA AA AA AA AA AA (Hex)

SFD - Start of Frame Delimiter AB (Hex)

DA - Destination Address

SA - Source Address

T/L - Type (RFC894, Ethernet) or Length (RFC1042, IEEE 802.3) indicator

CRC - Cyclic Redundance Check

Inter-frame gap 12 octets (9,6

µµ

s /10 MbE)

© P. Raatikainen Switching Technology / 2003

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24

1GbE frame

512 - 1518 octets

Preamble

7

S

F

D

1

DA

6

SA

6

L

2

Payload

48 - 1500

Preamble - AA AA AA AA AA AA AA (Hex)

SFD - Start of Frame Delimiter AB (Hex)

DA - Destination Address

SA - Source Address

T/L - Type (RFC894, Ethernet) or Length (RFC1042, IEEE 802.3) indicator

CRC - Cyclic Redundancy Check

Inter-frame gap 12 octets (96 ns /1 GbE)

Extension - for padding short frames to be 512 octets long

© P. Raatikainen Switching Technology / 2003

CRC Extension

4

2 - 49

Ethernet network elements

Repeater

– interconnects LAN segments on physical layer

– regenerates all signals received from one segment and forwards them onto the next

Bridge

– interconnects LAN segments on link layer (MAC)

– all received frames are buffered and error free ones are forwarded to another segment (if they are addressed to it)

Hub and switch

– hub connects DTEs with two twisted pair links in a star topology and repeats received signal from any input to all output links

– switch is an intelligent hub, which learns MAC addresses of DTEs and is capable of directing received frames only to addressed ports

© P. Raatikainen Switching Technology / 2003

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25

Optical transport network

• Optical Transport Network (OTN) being developed by

ITU-T (G.709) specifies interfaces for optical networks

• Goal to gather for the transmission needs of today’s wide range of digital services and to assist network evolution to higher bandwidths and improved network performance

• OTN builds on SDH and introduces some refinements:

– management of optical channels in optical domain

– FEC to improve error performance and allow longer link spans

– provides means to manage optical channels end-to-end in optical domain (i.e. no O/E/O conversions)

– interconnections scale from a single wavelength to multiple ones

© P. Raatikainen Switching Technology / 2003

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OTN reference model

OMPX

OA OA

OTS OTS

OMS

OCh

- OCh

- OA

- OMS

Optical Channel

Optical Amplifier

Optical Multiplexing Section

- OMPX Optical Multiplexer

- OTS Optical Transport Section

© P. Raatikainen Switching Technology / 2003

OTS

OMPX

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26

OTN layers and OCh sub-layers

SONET/

SDH

ATM Ethernet IP

Optical channel

Optical multiplexing section

(OMSn)

Optical transport section

(OTSn)

© P. Raatikainen Switching Technology / 2003

OPU

Optical channel payload unit

ODU

Optical channel data unit

OTU

Optical channel transport unit

2 - 53

OTN frame structure

Three main fields

Optical channel overhead

Payload

Forward error indication field

GbE IP

ATM/FR

SONET/SDH

DWDM

FR SONET/SDH ATM GbE IP

© P. Raatikainen

Och Payload

Client

Digital wrapper

Switching Technology / 2003

FEC

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27

OTN frame structure

1 ..... 16

4080 bytes

17 ................................... 3824 3825 ... 4080

Och overhead

Payload FEC

3

4

1

2

1 ..... 7 8 ..... 14

Frame alignmt. OTU overhead

15 ... 16

ODU overhead

OPU overh.

OTU - Optical transport unit

ODU - Optical data unit

OPU - Optical payload unit

FEC - Forward error correction

© P. Raatikainen

• Frame size remains the same (4x4080) regardless of line rate

=> frame rate increases as line rate increases

• Three line rates defined:

• OTU1 2.666 Gbit/s

• OTU2 10.709 Gbit/s

• OTU3 43.014 Gbit/s

Switching Technology / 2003

2 - 55

Generation of OTN frame and signal

OTN frame generation

Client signal

OPU ODU OTU

+ OPU-OH + OTU-OH

+ FEC

Client signal

OCh

Client signal

OCh

OTN signal generation

OMUX OMS

© P. Raatikainen Switching Technology / 2003

OTS

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28

OTN network elements

optical amplifier

– amplifies optical line signal

optical multiplexer

– multiplexes optical wavelengths to OMS signal

– add-drop multiplexer adds or drops wavelengths to/from a common

OMS

optical cross-connect

– used to direct optical wavelengths (channels) from an OMS to another

– connections set up and released by operator

optical switches ?

– when technology becomes available optical switches will be used for switching of data packets in optical domain

© P. Raatikainen Switching Technology / 2003

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29

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