9 infrastructures 09 [modalità compatibilità] -...

Network Infrastructures – a.a. 2014-2015 Lecture 9 pag. 1

Network Infrastructures

A.A. 2014-2015Prof. Francesca Cuomo

Part of these slides are taken from:

Introduction to the Synchronous Digital Hierarchy (SDH), Caliptech

Farzad Safaei, ECTE 364/483,


3

The History of Digital Transmission

• ’70s - introduction of PCM into Telecom networks

• 32 PCM streams are Synchronously Multiplexed to 2.048 Mbit/s (E1)

• Multiplexing to higher rates via PDH

• 1985 Bellcore proposes SONET

• 1988 SDH standard introduced.

4

SDH goals

• Three major goals:– Avoid the problems of PDH– Achieve higher bit rates (Gbit/s) – Better means for Operation, Administration, and

Maintenance (OA&M)

• SDH is THE standard in telecommunication networks now

• It is designed to transport voice rather than data• It covers the lower 2-3 OSI layers• SONET/SDH defines only a point-to-point

connection in the network


5

Plesiochronous Multiplexing

• Before SDH transmission networks were based on the PDH hierarchy.

• Plesiochronous means nearly synchronous.• 2 Mbit/s service signals are multiplexed to 140 Mbit/s for

transmission over optical fiber or radio.• Multiplexing of 2 Mbit/s to 140 Mbit/s requires two

intermediate multiplexing stages of 8 Mbit/s and 34 Mbit/s.

• Multiplexing of 2 Mbit/s to 140 Mbit/s requires multiplex equipment known as 2, 3 and 4 DME.

• Alarm and performance management requires separate equipment in PDH.

6

PDH: Plesiochronous Digital Hierarchy

• Multiplex levels:» 2.048 Mbit/s

» 8.448 Mbit/s

» 34.368 Mbit/s

» 139.264 Mbit/s

• Uses Positive justification to adapt frequency differences


7

Pulse stuffing

• E1 lines are multiplexed synchronously

• E2 and above are not synchronous,– In general different input signals to this stage have independent

clocks.

– For this reason, it is called plesiochronous digital hierarchy

• Pulse stuffing is the practice of inserting extra dummy bits or pulses into each incoming signal until its rate is raised to that of a locally generated clock signal– With pulse stuffing, the outgoing rate of the multiplexer is higher

than the sum of maximum incoming rates

• Data are written in a temporary buffer

• Data are read from the buffer with a higher rate to transmit data to the (faster) transmission channel

• Every time the buffer is going to be empty, stuffing bit is transmitted instead of real data

• Stuffing MUST be signaled to the receiver, so that stuffing bits can be removed.

Pulse stuffing

8

ft0

ft0

ft0

ft0

fm0 ≥ Ntft0


9

Example of slip

1

2

3

4

5

6

7

8

9

10

11

124

7

5

6

time

IV

III

II

I

Memory

TS TL

Start of the slip (bit 4, 5, 6 e 7 are repeated)

TL < TS

• The output sequence is:

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

10

Minimizing the slip

1

2

3

4

5

6

7

8

9

10

11

12

time

IV

III

II

I

Memory

TS TL

Inibizione lettura

TL < TS

• Since the reading of the memory in position IV is performed before the writing the bit 4 is repeated two times

• The output sequence is:• 1-2-3-4-5-6-7-4-8-9-10-11-12


11

Pulse stuffing

Giustification bit

Input multiplexer

Output multiplexer

Tributary flow

Tributary flow

12

Problems of PDH

• The Plesiochronous Digital Hierarchy had a number of problems:– Each multiplexing section has to add overhead bits for

justification (higher rate -> more overhead)– Each part of the world has its own transmission hierarchy

(expensive interconnection equipment)– Justification (bit stuffing) spreads data over the frame

» add-drop-multiplexers are hard to build» extract a single voice call -> demultiplex all steps down» switching of bundles of calls (n * 64 kbit/s) is difficult (every switch

has to demultiplex down to DS0 level)– The management and monitoring functions were not sufficient in

PDH– PDH did not define a standard format on the transmission link

» Every vendor used its own line coding, optical interfaces etc.» Very hard to interoperate


13

PDH vs. SDH Hierarchy

• PDH transmission rates:• In US:

– 24 voice channels (64 k) gives a DS-1 (1544 kbps)

– 4 DS-1 gives a DS-2 (6312 kbps)– 7 DS-2 gives a DS-3 (44736 kbps)

• In Europe:– 30 voice channels (64 kbps) plus two

channels for framing and signalling gives an E-1 (2048 kbps)

– 4 x E-1 gives an E-2 (8448 kbps)– 4 x E-2 gives an E-3 (34368 kbps)– 4 x E-3 gives a E-4 (139264 kbps)

• SDH is designed to unify all transmission rates into a single Mapping hierarchy

14

PDH Multiplexing

• PDH Multiplexing of 2 Mbit/s to 140 Mbit/s requires 22 PDH multiplexers: – 16 x 2DME

– 4 x 3DME

– 1 x 4DME

• Also a total of 106 cables required.


15

PDH Add/Drop

• Because E1 lines are multiplexed synchronously, then each channel can be extracted by reading the appropriate timeslot

• E2 and above are not synchronous, the stuffing pulses make these plesiochronous

• Therefore, to add or drop a lower order stream (eg E1) to/from a higher order stream (e.g. E4) all the stages of multiplexing and demultiplexing must be performed

• If a small number of 2 Mbit/s streams passing through a site need to be dropped then in PDH this requires large amount of equipment to multiplex down to 2Mbit/s.

16

Extraction/Cross connection


17

What is SDH?

• The basis of Synchronous Digital Hierarchy (SDH) is synchronous multiplexing - data from multiple tributary sources is byte interleaved.

• In SDH the multiplexed channels are in fixed locations relative to the framing byte.

• Demultiplexing is achieved by gating out the required bytes from the digital stream.

• This allows a single channel to be ‘dropped’ from the data stream without demultiplexing intermediate rates as is required in PDH.

18

SDH Rates

• SDH is a transport hierarchy based on multiples of 155.52 Mbit/s

• The basic unit of SDH is STM-1: – STM-1 = 155.52 Mbit/s

– STM-4 = 622.08 Mbit/s

– STM-16 = 2488.32 Mbit/s

– STM-64 = 9953.28 Mbit/s

• Each rate is an exact multiple of the lower rate therefore the hierarchy is synchronous.


19

Carriage of PDH

• STM-1 carries 3 DS-3

• STM-1 carries 1 E-4



20

Transport of PDH payloads

• SDH is essentially a transport mechanism for carrying a large number of PDH payloads.

• A mechanism is required to map PDH rates into the STM frame.

• This function is performed by the container (C).• A PDH channel must be synchronised before it

can be mapped into a container.• The synchronizer adapts the rate of an incoming

PDH signal to SDH rate.


21

SDH and non-synchronous signals

• At the PDH/SDH boundary Bit stuffing is performed when the PDH signal is mapped into its container.

22

SDH Virtual Containers

• Once a container has been created, path overhead byte are added to create a virtual container (VC).

• Path overheads contain alarm, performance and other management information.

• A path through an SDH network exists from the point where a PDH signal is put into a container to where the signal is recovered from the container.

• The path overheads travel with the container over the path.


23

Numerical structures

• C: Container

• VC: Virtual Container

• TU: Tributary Unit

• TUG: Tributary Unit Group

• AU: Administrative Unit

• AUG: Administrative Unit Group

• STM: Synchronous Transport Module

24

Numerical structures

Virtual ContainerLow order

Container Tributary Unit Group

Payload (Virtual ContainerHigh order)

Virtual ContainerHigh order

Administrative Unit STM-1


25

SDH layers

circuit @ 64 kbit/s

Packet Leased lines

VC-11 VC-12 VC-2 VC-3

VC-3 VC-4

AUG + MSOH + RSOH (STM-N)

AUG + MSOH

bit di STM-N

Circuit

Path

Low Order

High order

Multiplexer section layer

Physical media layer

Regeneration section layer

26

Mapping Virtual Containers

• C-4 container being mapped into an STM frame via a VC-4 virtual container


27

SDH Hierarchy

• SDH defines a multiplexing hierarchy that allows all existing PDH rates to be transported synchronously.

• The following diagram shows these multiplexing paths:

28

Example: Multiplex path for the E1


29

Pointers

• SDH will still work if there are two different clocks in the network and the network becomes asynchronous

• The clock frequency of signals is adapted using positive/zero/negative stuffing

• Pointers are used adjust for the new frequency• Pointers allow tributary frame to float in the synchronous

transport module (STM) frame– The position of a lower order Virtual Container in a frame is

identified by two cascaded pointers

• Frame slips are accounted for by recalculating pointers– Eg, when the local clock frequency exceeds that of the clock in

transmitted signal’s node of origin, the negative justification byte is used to transmit additional rate, and the value of pointer is decreased by one unit

30

Pointers

• 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


31

Accommodating Jitter

Positive Stuff Negative Stuff

Negative justification bytes (3 data bytes)Positive justification bytes (3 dummy bytes)

32

Pointers

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


33

Extraction/Cross connection

• Possible to extract lower level tributaries from the high speed line signal without the need for bringing an entire payload down to the lower level signal.– In a typical PDH-based metropolitan network around

75% of high speed signals will undergo this mux/demux more than once.

• At each terminal only a fraction of aggregate traffic is to be dropped so this leads to cheaper nodes.

34

PCM frame structure

• The SDH frame rate is inherited from PCM.

• As with PCM, the SDH has 8 bits per time slot.

• As with PCM, the SDH frame rate in 125 s per frame.

• The following diagram shows the PCM frame:


35

Basic frame structure

36

SDH network elements

STM-n STM-n

Regenerator


37

SDH network sections

38

Terminal Multiplexer

• Tributaries may be electrical or optical

• Works as termination of the Path layer– Write POH (Path Overhead)


39

Add Drop multiplexer

• Terminate connections in the Line layer (Write MSO –Multiplexer Overhead)– Direct access to the SDH-frames in the bitstream

– Example: drop and add a STM-1 from/to a STM-4» Every STM-1 frame can be mapped in and out separately without

changing the remaining STM-N structure

40

Regenerator

• Perform 3-R regeneration of the signals (amplify, re-shape, re-clock)

• Terminates connections in the Section Layer– write SOH (Section Overhead)

» Regenerators perform opto/electro/optical conversion

• ADMs and DXCs have the functionality of regenerators, too


41

Digital Crossconnect System (DXC or DCS)

• Higher order DXC» HDXC only switch STM-1 frames» DXC 4|4 means “receive AU-4 and switch AU-4 granularity”

• Lower order DXC» LDXC mostly receive AU-4, but switch Tributary Units (TU)» DXC 4|3|1 receives STM-1, but may switch VC-12, VC-3 and VC-4

• DXC switches connections according of day-time-changes– Can be used to change the network topology

Automatic Protection Switching

• Point to point configurations


Multiplex Section Protection (MSP) 1+1 and 1:N

Selction

1+1

1:1

Selection

Switch

Switch

Switch

Switch

1

2

N

1

2

N

… …

1:N

Example of 1+1

Working line

Protection

Protection

TL

Tx

Tx

Tx

Tx

Rx

Rx

Rx

Rx

MU

X

DE

MU

XM

UX

DE

MU

X

Switch

TL

TL TL

PermanentBridge

PermanentBridge

Working line

Switch


TX

RX

TX

RX

TX RX

TX

TX

RX

TX

RX

RX

B1A

B2A

S1A

S2A

S1C

S2C

B1C

B2C

Working line

Protection

Lato

B

Lato

A

DEMUX

MUX

DEMUX

MUX

DEMUX

MUX

DEMUX

MUX III

III

IV

Example of 1:2

Working line

Activation sequence of Bridgesand Switches when the 2nd Demux side B detects the need for exchange(B = Bridge = Switching in Tx,S = Switch= Switching in Rx)

Network protection


47

Topologies

• Basically, SDH define point-to-point links– Different topologies can be configured using either

ADMs or DXCs

• Unidirectional Path-Switched Ring (UPSR)– Two counter-rotating fibers, one is

working, the other protection» Send simultaneously on both fibers» Receiver picks the stronger signal of both» No excessive signalling needed, fast and

easy

48

UPSR


49

Bidirectional Line-Switched Ring (BLSR/2)

Bandwidth allocation in BLSR


Bandwidth reuse

Protection in BLSR 2


53

Overall architecture of the transport network

54

SDH multiplexing/demultiplexing


55

SDH Digital Cross Connect

56

SDH path


57

SDH overheads

• The SDH overhead bits are used to control the appropriate section of the path– At the regenerator, multiplexer and path levels

• These control functions include– Error detection

– Alarm indication signals

– Data communication channels for maintenance staff

– Automatic Survivability

58

STM-N frame structure

• The three main areas of the STM-N frame are indicated:

» SOH;

» Administrative Unit pointer(s);

» Information payload.


59

Administrative Units in the STM-N

• The VC-4 is mapped into an STM frame via the administrative group (AU).

• The VC-4 associated with each AU-4 does not have a fixed phase with respect to the STM-N frame.

• The location of the first byte of the VC-n is indicated by the AU-n pointer.

60

Administrative Units in the STM-N


61

Insertion of VC-4 in AU-4

1 261

1

H1 Y Y H2 1 1 H3 H3 H3

J1B3C2G1F2H4F3K3N1 9

VC-4

AU-4

62

AU-4 structure

• 261 x 9 byte in 125 ms = 150336 kbit/s payload capacity

1 2611

9

0 - - 1 - - 2 - - --86--85

--88--87

--782--781

--89 --173--172

--696

H1 Y Y H2 1 1 H3 H3 H3


63

Multiplexing of N AUG in the STM-N

1 261

1 9

AUG

#1

AUG

STM-N

N x 9 N x 261

RSOH

MSOH

1 2 N 1 2 N

1 261

1 9

#N

12

N

12

N

12

N

64

Tributary Units in a VC-4

• The VC-4 can carry a container -4 (C-4). The C4 carries a 140 Mbit/s PDH signal.

• The VC-4 forms what is known as an high order path.• If lower speed PDH signals need to be transported these

are mapped into a tributary unit (TU).• The TUs are then multiplexed into a VC-4.• The VC-n associated with each TU-n does not have a

fixed phase relationship with respect to the start of the VC-4.

• The TU-n pointer is in a fixed location in the VC-4 and the location of the first byte of the VC-n is indicated by the TU-n pointer.


65

Tributary Units in a VC-4

66

Tributary units


67

STM-1 Section Overheads

• Nine rows of nine bytes at the front of the SDH frame form the section overhead.

• The first three rows are the regenerator section

• overhead.• The last six rows are the

multiplex section overhead.

68

VC-4 path overheads

• The first column of the VC-4 is the VC-4 path overhead.

• The overheads have been modified in the latest release of G.707


69

VC-12 overhead

• The first byte of the VC-12 is the VC-12 path overhead.

• The VC-12 frame is spread over four frames to form a VC-12 multiframe.

• Each of the four frames in the multiframe contains an overhead byte.

• The overheads have been modified in the latest release of G.707

70

V5 Byte

� The bits in the V5 byte are allocated as follows:


71

Framing bytes (A1 & A2)

• The framing byte locate the beginning of the STM frame

72

Synchronisation status marker byte (S1)

• The synchronisation status marker byte contains information about the quality of the embedded timing


73

Bit interleaved parity (B1, B2, B3)

• The BIP is calculated over the previous frame/multiframe

74

Bit interleaved parity (B1, B2, B3)


75

Orderwire (E1 & E2)

• The E byte carry the orderwire channels.

• The relief byte is used for ring protection» E1 – Regenerator section orderwire

» E2 – Multiplex section orderwire

76

DCC channels (D1 to D3 and D4 to D12)

• The DCC channels are used Element Management Software to pass management information between sites.


77

User channels (F1, F2, F3 & N2)

• The user channels appear at a front panel connector for use by the network operator

78

Section/Path trace bytes (J0, J1, J2)

• The section/path trace supports a string assigned to a path, this verifies continued connection to the intended transmitter


79

Remote error indication

• MS REI (M1) Indicates the count of the interleaved bit blocks (1 to N) that have detected an error.

80

High order path management


81

Low order path management

• The VC-12 path overhead signals are held in the V5 byte.

82

SDH layers

• The following diagram shows 2Mbit/s multiplexed to STM-1.

• The transmission path passes through five layers in this connection.


83

Layers - Example

84

Termination points

• Within a layer each path ends at a ‘termination point’

• A path in SDH can be visualized as a pipe, in the diagram the 140Mbit/s path passes unaltered through the multiplex section


85

Alarms and layers

• Each defect indication is associated with a layer

Network interconnection

Packet forwarding

Service differentiationTraffic engineering

Bandwidth provisioningProtection switching

High-speed multiplexing

Bandwidth reservation

High-speed transmission

Packet restoration

Current layering Optimization for Access/Edge/Metro

IP / MPLSIP / MPLS

SDHSDH

Optimization forBackbone

IP / MPLSIP / MPLS

OTN / ASONOTN / ASON

IPIP

ATMATM

SDHSDH

OTNOTN OTN / ASONOTN / ASON

OTN: Optical Transport NetworkASON: Automatically Switched Optical NetworkMPLS: Multiprotocol Label Switching

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