sdh basics overview

SDH BASICS OVERVIEW

OverviewIntroduction to PDHTransport Network

Path and SectionVirtual Containers

Synchronous Transport ModuleProtection Rings and Capacity

SDH Multiplexing StructurePDH Mapping

SDH Bytes

The Transport Plane Offers a Transport Service between the service and access layers as well as between all of the physical locations on each of these planes Each of the telephone or data switches on the service plane relies on the transport plane to carry traffic between them as well as to and from end users. Hence the terminology TRANSPORT PLANE

Introduction to PDH

A Transport Network is used simply to Transport signals to their required destination and to deliver them in the same format as they were received

TRANSPORT PLANE

ServicePSTN

ATM

AccessSDH

PBX

Large businessTelephone

TRANSPORT PLANE

The Transport Network

The Transport Network provides an ability to carry Traffic between the points Just providing a set of links between Telephone Sites is not Sufficient. The following are the Key issues to be considered :

Resilience : The ability of the network to cope with a loss or failure of a link or node, alternative routes to be provided

Set up Cost (Investment) : Minimization is usually required

Services Supported : The network must be able to carry any services

Future Proof (Upgrade, growth) : The network must be able to evolve to satisfy new requirements, without disrupting existing services

Operation, Administration and Maintenance (OA & M): The system proposed must integrate into existing processes and contribute to minimizing operational costs

Connection Point Logical Communication Path

Multiplexing

In the early days of Telephony, each telephone circuit required it’s own set of Copper wires to connect two Telephones together

As demand increased, this technique became very expensive and impractical

Multiplexing is a method to combine more than one Telephone channel onto the same pair of wires

Telephone Telephone

Telephone Telephone

Telephone Telephone

Telephone Telephone

One line needed per connection

Before

Several Channels transmitted on one line

AfterUse one line(MULTIPLEXING)

European Standard30 Telephone lines multiplexed into a 2 Mbps signal

1

30

1

30

Multiplexed Signal

Several Channels transmitted on one line

Transmission systems that are designed to European standards combine 30 Telephone Channels to create a multiplexed signal

This allows vendors to make multiplexers that can interwork This signal is multiplexed using a technique called Time Division

Multiplexing TDM combines these signals which are in Digital form This conversion is done using PCM (Sampling, Quantizing,

Encoding) The transmission speed of this signal is 2.048 Mbps As far as Transport Networks are concerned the 2 Mbps signal is

the lowest signal that is carried and therefore, forms the building blocks for higher orders of Multiplexing

Several Channels transmitted on one line

Multiplexing Principles

The input signals to the Multiplexer are called Tributaries(Tribs) The output signal (multiplexed signal) is called the Aggregate For the tributaries to be multiplexed all tribs must be operating at

exactly the same speed The speed of the aggregate must be at least the sum of all the trib

speeds, this is because the multiplexer must transmit one byte from all tribs in the same length of time that it takes to receive one byte from one tributary

Multiplexing Principles

Input signals are called Tributaries(Tribs)

The output signal is called the aggregate

Mux

30 Channel PCM Frame

0 16 31

Time Slots (TS)

TS0 for frame alignment

TS16 for signaling

Channels for speech or data1-15

Channels for speech or data16 - 30

Although the frame contains 30 Telephone channels, a total of 32 Time slots are multiplexed.

TS 0 is used to transmit a set pattern of bits continuously so that the receiver can locate and lock on to the start of the frame, in this way all the channels can be demultiplexed and connected to the correct tributaries.

TS 16 is used to carry signaling information from the telephones to the Telephone Exchange and vice versa, such as off hook, on hook, dial tone and ring tone

Frame Size : 32 Bytes (256 Bits) Frame Duration : 125 us (1/8,000th of a second) Frame Rate : 32 x 64 Kbps = 2.048 Mbps

PDH Limitations

The problem in a plesiochronous network is illustrated by considering what a network operator has to be able to provide a business customer with a 2 Mbps leased line

If the high speed channel passes near to the customer, the operation of providing him a single 2 Mbps from within that channel is not simple

The use of justification bits at each levels in the PDH, means that identifying the exact location of the frame from a single 2Mbps line within a 140 Mbps channel is impossible

To access a single 2 Mbps line the 140 Mbps channel must be demultiplexed to it’s 64 constituent 2 Mbps lines via 34 and 8 Mbps

Once the required 2 Mbps has been identified and extracted , the channels must be remultiplexed back up to 140 Mbps for onward Transmission

As we know there are separate bit rate hierarchies in North America and Europe

This caused a lot of difficulty providing cross Atlantic communication due to incompatibility of the transmission speeds

Only the digital telephone channel itself operated at the same rate of 64 Kbps

Customer

140MLTE

140MLTE

140

34

34

8

8

2

14034

34

8

82

Tributary signals can only be accessed at the level from which they were multiplexed

Inflexible : Slow provision of services

Expensive : Lots of equipment required

Unreliable

Real Estate: A lot of space required for equipment

Spares ; More equipment means more spares to be held

Difficulty interworking between Europe and North America

North American Bit Rates

European Bit Rates

64 Kbps

X 24

1.5 Mbps

X 4

6 Mbps 45 Mbps 275 Mbps

2 Mbps 8 Mbps 34 Mbps

X 4X 4 X 4

X 7 X 6

X 32

140 Mbps

565 Mbps

X 4

Max Transmission rate limited to 565 Mbps

DS1DS2

DS3

First Order(E1)

Second Order(E2)

Third Order(E3)

Fourth Order(E4)

Not Specified by CCITT

Transport Network

The equipment used to implement a point to point connection is called a Terminal Multiplexer or line system

A terminal mux offers fixed connections between end user termination points

An SDH terminal takes in a number of electrical and optical signals (tributaries )and transmits them on the line as a single signal (aggregate)

The tributary signals may be 2,34,45 or 140 Mbps SDH terminals can also handle synchronous tributaries from other

SDH multiplexers All the traffic received from the aggregate is terminated by the

terminal mux (hence the name terminal)

Mux Mux

1

63 63

1

1 to 1

63 to 63

Fixed Network (Line System)

Terminal Multiplexers

Cross Connect

A simple point to point network does not allow changes to be made to the way the traffic is delivered between the points, and is therefore inflexible

A flexible network allows changes to be made to the way the points are connected and can respond to new connection requests faster than a fixed network

A cross connect, is a piece of equipment which provides flexible connections between it’s termination points

Point of flexibility (Cross Connect)

X

1

63

1

63 63

63

1

1

1 to 63

2 to 1

62 to 63

63 to 2

Add Drop Multiplexer

At each location the signal can get on or get off the Mux The key difference between a Terminal Mux and an ADM is that the

ADM is equipped with two aggregate units and sufficient cross connect functionality to cross connect the traffic from one aggregate port to the other or from an aggregate port to a tributary port

This also enables the signals to be transmitted from the aggregate in two directions (East & west)

A terminal mux can also be provided with an additional aggregate unit in order to provide an alternative route for resilience

Terminal Mux Terminal Mux

Add Drop Multiplexers

1 62 2 62

X X X

Flexible Network Add Drop Mux

1

63

63

Network Survivability

In the case of a point to point line system, resilience can be introduced by duplicating the link

This would require the Terminal Mux’s to be equipped with an additional aggregate unit

Maximum protection will be achieved if these two links are separately routed

In the case of a flexible network implemented with a cross connect, the cross connect is the major point of vulnerability

With a bus structure resilience can be achieved by adding an extra link

ADM ADM

Add Drop Multiplexers1 62 2 62

X X X

ADM Ring1

63

63

Alternate route for protection

X X

An ADM with protection ring

Ring Topology

ADM

ADM

ADM

ADM

ADM Ring

Line System

Optical Alternate Route

Terminal Mux Terminal Mux

Ring Topology

• Provides resilience to traffic• Ideal for maximizing capacity

Line System (Point to Point )

• Resilience can be provided by adding an additional fiber

Transport Plane

Transport Plane can be broken down into different distinct components Collector Function : At the edge of the network, there is a need to

collect traffic. Collector rings provide the network interface for all access applications including local exchanges, PABX, Access Multiplexers etc

Bandwidth Management Function : This tier provides routing, grooming and consolidation of the traffic between the collector rings and the high speed backbone.

High Speed Backbone: This tier provides a backbone function providing reliable high speed transport between geographical regions and locations

Path and Section

Path and Path overhead In transmission a path is defined as a circuit joining two nodes that

may pass through a number of intermediate nodes. In SDH, extra capacity is reserved to carry monitoring and

management information associated with the path Extra information associated with the path (generated at the

originating termination point and processed at the terminating termination point) is called the path overhead

This helps to know The quality of the overall end to end transmission The existence of the path between two terminating points Allows one end to communicate to the other end that there is

trouble in the signal received

Path and Path overhead

Path

A path is an end to end circuit

SDH allows information to be associated with the path (Path overhead)

Path overhead-Path trace-Error monitoring-Far end error

helps to

Section and Section Overhead

A section can be considered as one stage of an end to end path Defined as node to node transmission A path may be made up of a number of sections One section may also be the path The SDH also reserves some extra capacity within the defined bit

rates to carry information relating to the section This extra information associated with a section is called the section

overhead The section overhead allows control of node to node transmission

such as transmission quality The section overhead also provides extra communication channels,

one of them being the ECC (Embedded communication channel) used for network management Data Communication Network.

Section and Section overhead

Sections

SDH allows information to be associated with the Section (Section overhead)

Path overhead-Protection Switching-Error monitoringChannels for-Network Management-Maintenance Phone link

Section is node to node transmission

Virtual Containers SDH Multiplexers package the signals into small containers There are different sized containers for tributary signals with

different bit rates. Each container remains a defined size irrespective of any variations

in the bit rate of the PDH signal that it contains The POH is then added so that the signal and the quality of it’s

transmission are all traceable POH now stays with the VC through all intermediate multiplexers

until it reaches it’s destination SDH multiplexer where it is removed and processed

Any PDH signal which is packaged by an SDH multiplexer, is transported and delivered at the same bit rate as it entered the SDH network

If you put a 34 Mbps signal in, then you will get a 34 Mbps out and not just some of it’s constituent signals

Virtual Container = Signal + POH

Container is a defined. size and designed to carry a specific signal

Path Overhead -Path Trace

-Error Monitoring-Far End Error

-VC CompositionPath Overhead

Signal to send eg: 2 Mbps

VC

Information stays with the path (ie end to end circuit control)

Nesting of VCs

A VC can contain other VCs but only nested two deep Largest container is designed to carry all smaller containers Smaller containers cannot be put inside intermediate sized

containers prior to being put into the largest container

Path Overhead

Virtual Containers

VCs can be nested, but only 2 deep

VC Nesting

VC Numbering Scheme

There is no container for the 8 Mbps signal, therefore the VC-2 does not need a second digit ie VC – 21

The VC – 3 is designed to carry both signals at the third order, so once again does not require a second digit

The north american hierarchy does not have a fourth order and so the VC-4 also does not need a second digit

VC Numbering Scheme

1st Order 1 North America 1.5 Mbps VC - 11

  2 Europe 2 Mbps VC - 12

2nd Order 1 North America 6 Mbps VC -2

  2 Europe 8 Mbps No Container

3rd order 1 Europe 34 Mbps

VC – 3  2 North America 45 Mbps

4th Order 1 Europe 140 Mbps

VC – 4  2 North America N/A

VC packaging

One 2 Mbps signal is packaged into a VC 12 63 x 2 Mbps signals can go into a VC 4 (they are grouped in 3 sets

of 21) Alternatively a VC 4 can contain some VC 12 or VC 3 A 140 Mbps signal takes up a whole VC4

VC 12

VC 3

VC 4

2 Mbps

34 Mbps

140 Mbps

63 x VC 12(3 x 21)

3 x VC -3

Content Combinations

Largest Container

VC -4

VC - 4

VC 3 VC 321 VC 12

140 Mbps

VC 3 VC 3 VC 3

21 VC 12 21 VC 12 21 VC 12

VC Packaging

VCn and PDH Rates

The smallest container is for a 2 Mbps signal

There is no container for the standard 8 Mbps signal

The VC number reflects the PDH hierarchy level

Bit Rate PDH Europe SDH

  name container

140 M E4 VC 4

34 M E3 VC 3

8 M E2  

2 M E1 VC 12

64 K E0  

Synchronous Transport Module (STM)

Several VCs are placed in and carried by a STM

VC – 4

POH STM - n

VC – 4s are carried in a Transport Module

Smallest Transport Module : STM -1 = 155 Mbps(Including Payload)

Transport Module

The SOH is related to the STM in the same way as the POH is related to the VC

(VC – 4Or

n x VC -4 )Section Overhead

Only one transport module sent per section

Transport Module Contains

Higher Bit Rates

SDH offers higher bit rates at STM-4, STM-16 and STM-64

VC – 4 Still only one Section Overhead

VC – 4 VC – 4 VC – 4

Payload

STM -4 = 4 x STM-1

When higher bit rates are required, the STM-1 is multiplexed up to an STM-4.This is equivalent to 4 x STM-1 and operates at 622 Mbps. The payload areaIs increased to 4 x VC-4, but still there is only one STM this time it is STM-4

European Bit ratesSDH line Transmission rates are- 155 Mbps- 622 Mbps- 2.5 Gbps- 10 Gbps- You can see that the fastest standardized PDH rate is only 140 Mbps- SDH therefore offers much higher capacity systems than standard

PDH- Line signals can be electrical or optical- Electrical signals are used for short distances( eg between shelves in

a rack) for interconnection of equipment on the same site- Electrical signals are sent on copper wires, while optical signals used

for longer distances (from 40 Km) require optical fibers

European Bit Rates

Bit Rate PDH Europe SDH

  Name Container Transport

10 G     STM - 64

2.5 G     STM - 16

622 M     STM - 4

155 M     STM - 1

140 M E4 VC4  

34 M E3 VC3  

8 M E2    

2 M E1 VC 12  

64 K E0    

The SDH and SONET Comparison

Bit RatePDH North

America PDH Europe SDH SONET

  Name rate Name rate Container Transport Container Transport

10G           STM-64   STS/OC192

2.5G           STM-16   STS 48/OC 48

622M           STM-4   STS 12/OC 12

467M               STS 9/OC 9

155M           STM-1   STS 3/OC 3

140M     E4 140M VC4      

51M               STS 1/OC 1

45M DS3/T3 45M            

34M     E3 34M VC3      

8M     E2 8M        

6M DS2/T2 6M            

2M     E1 2M VC12      

1.5M DS1/T1 1.5M            

64K DS0/T0 64K E0 64K        

Protection Rings and Capacity

A rings capacity is the maximum number of paths that the ring can support

A ring’s maximum capacity occurs only when all of the traffic enters the ring at one node and exits the ring at an adjacent node

STM-1 ringVC 12

63

Ring Capacity = Max no of paths which can be set up

What is the maximum capacity of this STM-1 ring ?

Two main types of protection

Dedicated Protection Ring

A ring with Dedicated path Protection called a DP Ring is shown in figure

STM-1 ringVC 12

63VC 12

63

Traffic sent both ways

DP Ring

How many fully protected VC 12 paths can be set up?

63 Paths

Traffic is sent simultaneously on both sides of the ring (also called East and West), hence one path uses a complete lane on this 63 lane motorway

STM-16 Ring

How many fully protected VC4 paths can be set up in the STM-16 ring?

16 Paths

The traffic is sent both ways around the ring, and so in a sense it is duplicated

The receiver will now be receiving the same signal on two ports The decision has to be made as to which signal should be selected

to be passed to the tributary port Initially the decision to select is normally random, and carried out by

a automatic change over switch When to switch from one port to another, is based on fault reporting

and error detection functions of the multiplexer, and is switched automatically

Shared Protection Ring

Shared protection is achieved by reserving half the line capacity on each section for protection

ie 8 STM-1s are allocated for traffic and 8 STM-1s for protection

STM-16 ringVC 4/STM-1

1-8 for trafficVC 4/STM-1

9 -16 for protection

Traffic sent one way only

Half of each section capacity is for protection

How many fully protected VC-4 paths can be set up? 48 Paths

SP ring protects a path section around the ring Each section is responsible for ensuring that the traffic transported

gets to the other node Under normal operating conditions, the traffic is sent in one direction

(East or West) only using the 8 STM-1s for normal traffic Shared protection protects all traffic in the ring ( it is not possible to

protect only some paths, all paths are protected)

SP Ring protection

STM-16 ringVC 4/STM-1

1-8 for trafficVC 4/STM-1

9 -16 for protection

Traffic sent one way only

In case of failure, the signal is rerouted to the good side

Re routed Traffic

DP Ring Versus SP Ring

STM-16 Ring

6 Nodes

Max paths = 16

6 Nodes

Max paths = 48

8

8

8 8

8

8

An operator can choose a SP for the capacity advantages that it provides. This means the operator can obtain :

1. Better Network utilization

2. Capital investment saving

DP Ring – For hubbing traffic (all to the same point) and also the circuitry is simple, used in collector ring

SP Ring – For uniform Traffic and also the circuitry is complex, used in backbone ring

SDH Multiplexing Structure

C11VC11TU 11

C12VC12TU 12

C2VC2TU2

C3VC3AU3

TUG 2

TUG 3 VC3TU3

C4VC4AU4AUGSTM-n

1.5M

6M

2M

45M34M

140MX n

X 3

X 7

X 7

X 4

X 3

X 1

X 1X 3

SDH Multiplexing Structure The first level of the SDH is at 155.52 Mbps and is known as a

Synchronous Transport Module level 1(STM-1) Signal Higher rates are integer multiples of the first level bit rate and are

denoted by the corresponding multiplication factor of the first level rate.

At present , the following rates constitute the synchronous hierarchy

STM-1 = 155 MbpsSTM-4 = 622 MbpsSTM-16 = 2.5 GbpsSTM -64 = 10 Gbps The SDH allows for any of the current transmission rates except 8

Mbps to be mapped into containers, called VCs The containers can be combined into std formats in order to form

the payload of the STM-1 signal Different containers can be mixed, allowing for different rates to be

carried simultaneously within the same structure

SDH Multiplexing elements

Container(C-n), n = 1 to 4This is the basic element of the STM signal consisting of a group of

bytes allocated to carry the Transmission rates defined in ITU-T recommendation G.702 (i.e. 1.5 Mbps and 2 Mbps)

Virtual Container (VC-n), n = 1 to 4The lower order VC-ns (n = 1 or 2) are built up of the basic container

(C-n, n =1 or 2) plus additional capacity to carry POH informationThe higher order VC-ns (n = 3 or 4) are built up of either a single basic

container (C-n, n = 3 or 4) or an assembly of Tributary unit groups , together with the appropriate POH information

The POH information includes VC path performance monitoring, signals for maintenance purposes, and alarm status indicators

Tributary Unit (TU-n), n = 1 to 3

This element consists of a VC plus a Tributary unit pointer and provides adaption between the lower order path layer and the higher order path layer

The pointer value indicates the phase alignment of the VC wrt to the higher order VC it is added to.

The location of the pointer is fixed wrt to this higher order VC

The TU can be considered as the space within the higher order VC which is reserved for the lower order VC payload.

SDH Multiplexing elements

Tributary Unit Group (TUG-n), n = 2 or 3

This element is formed by a group of identical TUs or TUGs allowing mixed capacity payloads to be constructed

Administrative Unit (AU-n), n =3 or 4

This elements consists of a VC-n (n = 3 or 4) plus an AU pointer and provides adaption between the higher order paths and the multiplexer section layer .

The pointer value indicates the phase alignment of the VC-n with respect to the STM-1 frame.

The location of the pointer is fixed within the STM-1 frame structure

The AU can be considered as ‘the space’ within the STM frame which is reserved for the high order VC payload

SDH Multiplexing elements

Administrative Unit Group (AUG)

This element is formed by a group of byte interleaved AU s. The AUG has a fixed position in the STM payload

Synchronous Transport Module Level 1(STM-1)

This Is the basic element of the SDH and comprises a single AUG and the SOH information.

The STM-1 frame structure comprises an array of 270 columns by 9 rows of 8 bytes (2,430 Bytes)

SDH Multiplexing elements

PDH Mapping

Stuff

C12

34 Bytes

32 Bytes

2 Mbps

POH

VC12

35 Bytes

Pointer

36 Bytes

TU12

X 3

TUG - 2108 Bytes

X 7

TUG - 3 7 x 108 + 2 Columns of stuffing

774 BytesX 3

VC 4

2,349 Bytes

3 x 774 + 2 Columns of stuffing = 9 Bytes POH

+ 9 Bytes Pointer

2,358 Bytes

AU 4AUGX n

+ 72 Bytes SOH

2,430 Bytes

STM-n

Mapping

MultiplexingMultiplexing(SDH)

Aligning

STM-1 frame structure

Payload

2,349 BytesMSOH

AU Pointers9 Bytes

9 Bytes 261 Bytes

Frame Size = 2,430 Bytes

Frame Duration = 125 Us

RSOH

SOH – Carries control and management information Administrative pointers to cater for problems in timing variations The payload area for carrying subscriber data

VC 12 POH (V5 Byte)

SDH Bytes

1 2 3 4 5 6 7 8

BIP 2REI

RFI

Signal Label

RDI

BIP 2 – Bit interleaved parityUsed to provide an error monitoring function for the VC 12 path

REI – Remote error IndicationUsed to communicate detected BIP-2 errors back to the VC12 path originator

RFI – Remote failure indication

Signal Label – Used to indicate payload mapping and equipped status

RDI - Remote Defect Indication Used to indicate certain detected TU path alarms to the VC 12 path originator

VC-4 POH (HO POH)

VC4 Payload2,349 Bytes

VC4 Path overhead

J1 = Path TraceUsed to provide a fixed length string which is transmitted repetitivelySo that the receiving terminal can verify connection to the intended transmitter

B3 = Path BIPProvides an error monitoring function for the VC-4 path

C2 = Signal LabelUsed to indicate the composition of the VC4 payloads

G1 = Path StatusUsed to convey path terminating status and performance Information back to the VC-4 path originator

F2 = Path User ChannelUser communication purposes between path elements

H4= Multiframe indicatorProvides a generalized multiframe indicator for payloads

K3 = Spare

N1 = Network operator ByteMay be used for tandem connection maintenance

RSOH

A1,A2 = Framing ( These bytes are used for frame alignment purposes)

J0 RS Path Trace = Regenerator section path trace B1 BIP = Used to provide an error monitoring function for a

regenerator section E1 Order Wire = Used to provide an order wire channel which may

be accessed at regenerators and multiplexers F1 user channel = Reserved for user purposes D1 to D3 DCCr = The Data communication channel bytes provides

a 192 Kbps regenerator data channel.

MSOH B2

B2 BIP = Used to provide an error monitoring function for the multiplex section

K1, K2 APS Channel = These bytes are used for APS protocol E2 Order Wire = Used to provide an order wire channel which may

be accessed only at multiplexer section terminations. M1 MS REI Errors = Multiplex section REI for detected B2 errors D4 to D12 DCCm = The Data communication channel bytes

provides a 576 Kbps multiplex telemetry channel. S Sync = Synchronous status messaging Z1,Z2 Spare = Function not allowed

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