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AL-ITT/M10/CH3/1

A l l i a n c e T e l e c o mSDH: The Synchronous Digital HierarchyAL-ITT : Alliance Institute for Telecom Technology

For Training Purpose only

Co ri hts © 2006 All iance Telecom P Ltd.

www.alliancetelecom.net

The basis of Synchronous Digital Hierarchy (SDH) issynchronous multiplexing - data from multiple tributarysources is byte interleaved.

In SDH the multiplexed channels are in fixed locationsrelative to the framing byte.

De-multiplexing is achieved by gating out the requiredbytes from the digital stream.

This allows a single channel to be ‘dropped’ from thedata stream without de-multiplexing intermediate ratesas is required in PDH.

What is SDH?



AL-ITT/M10/CH3/2





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 = 2588.32 Mbit/s

– STM-64 = 9953.28 Mbit/s

Each rate is an exact multiple of the lower rate therefore the

hierarchy is synchronous.

SDH Rates



AL-ITT/M10/CH3/3





ATM: 149.760 kbit/s

E4: 139.264 kbit/s

DS3: 44.736 kbit/s

E3 : 34.368 kbit/s

AUG C-4

TUG-3 TU-3 VC-3

C-3AU-3

x1

x3

x7

x7

x3

x1

STM-N AU-4 VC-4

VC-3

TUG-2

x3

xN

x1

x4

DS1: 1.544 kbit/sTU-11 VC-11

C-11

E1: 2.048 kbit/sTU-12 VC-12 C-12

SDH ITU-T G.707

ATM: 48,384 kbit/s

DS2: 6.312 kbit/sTU-2 VC-2

C-2x1

STM-0

Pointer processing

Multiplexing

Aligning

Mapping

SDH ETSI

SDH defines a multiplexing hierarchy that allows all existing PDH rates to betransported synchronously. The following diagram shows these multiplexing paths:

SDH Hierarchy



AL-ITT/M10/CH3/4





Multiplex path for the E1



AL-ITT/M10/CH3/5





SDH is essentially a transport mechanism for

carrying a large number of PDH payloads.

A mechanism is required to map PDH rates intothe STM frame.

This function is performed by the container (C).

A PDH channel must be synchronized before it

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

PDH signal to SDH rate.

Transport of PDH payloads



AL-ITT/M10/CH3/6





At the PDH/SDH boundary Bit stuffing is performed

when the PDH signal is mapped into its container.

SDH and non-synchronous signals



AL-ITT/M10/CH3/7





Once a container has been created, path overheadbyte are added to create a virtual container.

Path overheads contain alarm, performance andother management information.

A path through an SDH network exists from thepoint 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.

SDH virtual Containers



AL-ITT/M10/CH3/8





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

Logical association

Physical association

NOTE – Unshaded areas are phase aligned. Phase alignment between the unshaded

and shaded areas is defined by the pointer (PTR) and is indicated by the arrow.

Mapping Virtual Containers



AL-ITT/M10/CH3/9





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 us per frame.

The following diagram shows the PCM30 frame:

2 Mbit/s PCM30 frame structure



AL-ITT/M10/CH3/10





STM-N frame structure is shown in the Figure below. The three main areas of the STM-N frame are indicated:

SOH

Administrative Unit pointer (s)

Information payload.

Basic SDH frame structure



AL-ITT/M10/CH3/11





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

network becomes asynchronous.

Pointers are used to adjust for the new frequency.

Different clock rates in SDH



AL-ITT/M10/CH3/12





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 withrespect to the STM-N frame.

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

1

3

5

9

4

270270 Columns (Bytes)

1 9

RSOHRSOH AU-4AU-4

MSOHMSOH

AU Pointer AU Pointer VC-4VC-4

VC-4PO

H

VC-4PO

H

C-4C-4

Administrative Units in the STM-N



AL-ITT/M10/CH3/13





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

Tributary Units in a VC-4



AL-ITT/M10/CH3/14





Tributary Units in a VC-4



AL-ITT/M10/CH3/15





2 Mbit/s are mapped asynchronously into aVC-12.

VC-12s are distributed over four frames knownas a VC multiframe.

The figure shows this over a period of 500 μs. V5 byte:

Tributary units



AL-ITT/M10/CH3/16





Nine rows of nine bytes at the

front of the SDH frame form thesection overhead.

The first three rows are the

regenerator section overhead.

The last six rows are the

multiplex section overhead.

STM Section Overheads



AL-ITT/M10/CH3/17





The Container (C)

Basic packaging unit for tributary signals (PDH)

Synchronous to the STM-1 Bitrate adaptation is done via a positive stuffing procedure

Adaptation of synchronous tributaries by fixed stuffing bits

Bit by bit stuffing

The Virtual Container (VC)

Formation of the Container by adding of a POH (Path Overhead)

Transport as a unit through the network (SDH)

A VC containing several VCs has also a pointer area

Functions and Characteristics of the

Individual Elements



AL-ITT/M10/CH3/18





A number of functions are defined in the overhead channels to ensure proper

transport of the payload.

The Section Overhead (SOH):

The overall capacity of the SOH is 4.608 Mbit/s (9x8x64kbit/s), of which 30

bytes(1.920 Mbit/s) have fixed definitions. The remaining 64kbit/s channels

are not specified. Six are reserved for national use. Although six bytes are

reserved for medium dependent functions (e.g. radio link systems). The

columns 1,4 and 7 corresponds also to the STS-1 frame.

Functions of the SOH:

–Contains maintenance, monitoring and operational functions

–Each byte refers to a 64kbit/s channel

–Splitted into RSOH and MSOH

–Protect the connection from point of STM-1 assembly to point of

disassembly.



AL-ITT/M10/CH3/19





VC-3/4 POH

J1

B3

C2

G1F2

H4

F3

K3

N1

V5

J2

N2

K4

VC-11/12/ 2 POH

Media dependent bytes

X Reserved for national use

SOH: Section overhead

POH: Path overhead

The overheads (SOH, POH) are used for maintenance

and supervision of the SDH transmission network.

RSOH

MSOHPayload

P

O

H

Pointer

AU - PTR

STM-1 SOH

A1 A1 A1 A2 A2 A2 J0 X X

D1 D2 D3

B2 B2 B2 K1 K2

D4 D5 D6D7 D8 D9D10 D11 D12S1 M 1 E2 X X

B1 E1 F1 X X

H1 Y Y H2 1 1 H3 H3 H3

Embedded Overhead Bytes



AL-ITT/M10/CH3/20





Parity check(B1 calculated by regenerator and multiplexers)

Data communication channels(D1...D3, F1 between regenerators)

Voice communication channels(E1 between regenerators)

Frame Alignment(A1, A2)

Section Trace(J0 Identification of regenerator source)

A1 A1 A1 A2 A2 A2 J0

B1 E1 F1

D1 D2 D3

B2 B2 B2 K1 K2

D4 D5 D6

D7 D8 D9D10 D11 D12

S1 M1 E2

AU - Pointer

Functions of Regenerator Section Overhead



AL-ITT/M10/CH3/21





Parity check (B2)

Alarm information (K2)

Remote error indication(M1,K2)

Automatic protection switching(K1, K2 Bytes)

Data communication channels(D4 to D12 between multiplexers)

Clock source information (S1)

Voice communications channels(E2 between multiplexers)

A A1 A1 A2 A2 A2 J0

B1 E1 F1

D1 D2 D3

B2 B2 B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12S1 M1 E2

AU - Pointer

Functions of Multiplexer Section Overhead



AL-ITT/M10/CH3/22





Parity checkB3, V5/ BIP-2 calculated by path terminating point

Alarm and performance information(V5, G1)

Structure of the VCSignal label C2

Multiframe indication for TUs (H4)

User communications channelbetween path elements (F2, F3)

Identification of the Path Source(Path Trace J1, J2)

The Path Overhead is evaluated at the end point of the

transmission system where the ‘unpacking’ takes place.

J1

B3

C2

G1

F2

H4F3

K4

N1

V5

J2

N2

K4

VC-3/4 POH

VC-11/12/2 POH

Functions of Path Overhead



AL-ITT/M10/CH3/23





SDH Network Elements



AL-ITT/M10/CH3/24





SDHADM : Add Drop Multiplexer

DXC : Digital Cross

Connect

TM : Terminal Multiplexer

Hybrid Networks Connect Old and New Technologies



AL-ITT/M10/CH3/25





Local Network

STM-4

STM-16

STM-1

S T M -

1Exchange

FlexMux

Subscriber

Access

Mux

64/2M

Local

Exchange

Trunk NetworkL 1

Trunk Network

L 2

SDHNetworkTopology

SDHNetworkTopology

Trunk

Network L 2



AL-ITT/M10/CH3/26


For Training Purpose onlywww.alliancetelecom.net

Simpler multiplexing(low SDH level can be directly identified from higher SDH level)

Simple D&I of traffic channels(direct access to lower level systems without synchronization)

Allows mixing of ANSI and ETSI PDH systems

SDH is open for new applications(It can carry PDH, ATM, HDTV, MAN,...)

SDH provides TMN (ECCs)(for centralized network control)

Advantages of SDH

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