07/03/2013Bahman R. Alyaei1 Chapter 8 Digital Transmission Systems Part 2.

07/03/2013 Bahman R. Alyaei 1

Chapter 8Chapter 8

Digital Transmission SystemsDigital Transmission SystemsPart 2Part 2


10 SDH and SONET10 SDH and SONET• SDH is an acronym for Synchronous Digital

Hierarchy. It is an European development.

• SDH: is a hierarchical set of digital transport structures (Overhead), standardized for the transport of suitably adapted Payloads over physical transmission networks.

• SONET is an acronym for Synchronous Optical Network . It is a North American development.

• The two (SDH and SONET) are very similar.


Continue…Continue…


Continue…Continue…• Either one can accommodate the standard E1

family (i.e., 2.048 Mbps, etc.) and DS1 family (i.e., 1.544 Mbps, etc.) of line rates.

• SDH/SONET is replacing PDH systems in the Transport Network.

• By Transport Network we mean the flexible high-capacity transmission network that is used to carry all types of information.

• By Flexible we mean that telecommunications operators are able to easily modify the structure of the transport network from the centralized management system.


10.1 Advantages of SDH/SONET10.1 Advantages of SDH/SONET

• SDH/SONET is based on the principal of direct synchronous multiplexing, where separate, slower signals can be multiplexed directly onto higher speed SDH/SONET signals without intermediate stages of multiplexing.

• SDH/SONET is more flexible and reliable than PDH and provides advanced network management and maintenance features.


Continue…Continue…• Can be used in Long-Haul Networks,

Local Networks and Loop Carriers, and it can also be used to carry CATV video traffic, ATM, and ISDN.

• In SDH/SONET format, only those channels that are required at a particular point are demultiplexed, thereby eliminating the need for back-to-back multiplexing. In other words, SDH/SONET makes individual channels “visible” and they can easily be added and dropped.


Continue…Continue…• The data rates for optical transmission are

standardized (i.e., vendor independent).

• Different systems are included in standards, for example, Terminal, Add/Drop, and Cross-Connection Systems.

• These systems make SDH/SONET networks more flexible than PDH systems, which include only terminal multiplexer functionality.


10.2 Why SDH/SONET10.2 Why SDH/SONET

• Originally, all communications in the telephone network was analog.

• Analog lines or analog microwave links were used to connect to switching offices.


Continue…Continue…• In about 1962, the network providers

began using digital communications between switching centers.

• This was PDH system (DS-Carrier in US and E-Carrier in Europe).



• As communications needs grew, many DS-Carrier or E-Carrier lines were needed between switching centers.

• In the late 1970’s optical communications began to be used to interconnect switching offices.E1 or DS1 E1 or DS1



• Prior to standardization, every manufacturer of optical communications used their own framing.

• The ANSI and the ITU began work in 1986 to define standards for optical communications.

• Both bodies finalized their first set of standards in 1988.



• They defined the following:1. Optical and Cupper interfaces

(wavelength, frequency, power, etc.).2. Rates, frame formats, and network

elements (Layers).3. Operations, Administration, and

Maintenance (OAM) functions including monitoring for valid signal, defect reporting, and alarms due to abundant overhead bits.


10.3 Basic SDH/SONET 10.3 Basic SDH/SONET Transmission Rates (Hierarchy)Transmission Rates (Hierarchy)

• SONET and SDH converge at SDH’s 155 Mbps base level, defined as STM-1 (Synchronous Transport Module-1).

• The base level for SONET is STS-1 (Synchronous Transport Signal-1) or OC-1 (Optical Carrier-1) and is equivalent to 51.84 Mbps.

• Thus, SDH’s STM-1 is equivalent to SONET’s STS-3 (3 x 51.84 Mbps = 155.52 Mbps).

• Higher SDH rates of STM-4 (622 Mbps), STM-16 (2.4 Gbps), and STM-64 (10 Gbps) have also been defined.


SDH Bit Rate SDH Capacity

STM-0 51.84 Mbps 21 E1

STM-1 155.52 Mbps 63 E1 or 1 E4

STM-4 622.08 Mbps 252 E1 or 4 E4

STM-16 2488.32 Mbps 1008 E1 or 16 E4

STM-64 9953.28 Mbps 4032 E1 or 64 E4

STM-256 39812.12 Mbit/s 16128 E1 or 256 E4

SONET Bit Rate SONET Capacity

STS-1, OC-1 51.84 Mbps 28 DS1 or 1 DS3





STS-768, OC-768 39812.12 Mbps 21504 DS1s or 768 DS3



Data Rates of SONET and Corresponding SDH Data Streams


Continue…Continue…• Multiplexing is accomplished by combining or

interleaving multiple lower-order signals (1.5 Mbps DS1 carrier, 2 Mbps E1 carrier, etc.) into higher-speed circuits (51 Mbps STS-1, 155 Mbps STM-1, etc.).

• By changing the SONET standard from Bit- Interleaving to Byte-Interleaving, it became possible for SDH to accommodate both transmission hierarchies.

• This modification allows an STM-1 signal to carry multiple 1.5 Mbps or 2 Mbps signals and multiple STM signals to be aggregated to carry higher orders of SONET or SDH tributaries.


11 SDH STM Signal11 SDH STM Signal

• SDH multiplexing combines low-speed digital signals such as 2, 34, and 140 Mbps signals with required Overhead to form a frame called STM-1.

• SDH is a Byte-Interleaving multiplexing system.

• An STM is the information structure used to support Section Layer Connections in the SDH.



• It consists of information Payload and Overhead (OH) information fields organized in a block frame structure which repeats every 125 μS.

• The information is suitably conditioned for serial transmission on the selected media at a rate which is synchronized to the network.

• STM-1 is the base level of SDH.


Continue…Continue…• The STM-1 frame, is created by 9 segments

of 270 bytes each (1-byte = 8-bits)• The first 9 bytes of each segment carry

Overhead (OH) information.• The remaining 261 bytes carry Payload. • When visualized as a block, the STM-1 frame

appears as 9 rows by 270 columns of bytes.• The STM-1 frame is transmitted row-by-row.• Row #1 first, with the most significant bit

(MSB) of each byte transmitted first, then the Row #2 and so on, up to Row #9.



• The STM-1 frame lasts for 125 μS, in other words, the 9 row segments will be transmitted in a total time equal to 125 μS.

• This will permit SDH to easily integrate existing digital services into its hierarchy.

• Therefore, there are 8000 frames per second.


The STM-1 frame

Segment no. 7

270 Bytes in (125/9) μS


STM-1 frame visualized as a block, and the direction of transmission



07/03/2013 Bahman R. Alyaei 24STM-1 frame visualized as a block


Continue…Continue…• Hence, the STM-1 frame rate Rf is

Rf = 8000 frames per second• The bit rate Rb of STM-1 frame is calculated

as follow:

Rb = Rf x Cf , where

Rf is the frame rate (frames/second).

Cf is the frame capacity (bits/frame).

• The frame capacity of a signal is the number of bits contained within a single frame.


Continue…Continue…• We know that the frame rate isRf = 8000 frames/second.

• Cf is calculated as followCf = 270 bytes/row x 9 rows/frame x 8 bits/byte = 19,440 bits/frame

• Then, the bit rate Rb of the STM-1 signal is calculated as follows:Rb = 8000 frames/second x 19,440 bits/frame

= 155.52 Mbps


Continue…Continue…• The multiplexing of multiple data

stream, plays an important role in SDH.• Byte Interleaving scheme is used to

multiplex multiple data stream. • The higher transmission levels

(Multiplex) such as STM-4 and STM-16 of the SDH Hierarchy are generated from integer multiples of STM-1 signal.

• In general, STM-N signal is generated by Byte Interleaving N STM-1 signal.



Multiplexing of STM-1 to generate STM-N



STM-N signal frame structure



• Example: • An STM-4 signal will be created by Byte

Interleaving four STM-1 signals.• The basic frame rate remains 8,000

frames per second, but the capacity is quadrupled, resulting in a bit rate of 4 x 155.52 Mbps or 622.08 Mbps.



Multiplexing of STM-1 to generate STM-4


11.1 SDH STM-1 Frame Structure11.1 SDH STM-1 Frame Structure

• As we know that, the SDH frame STM-1 consists of two parts:

1. The First Nine Columns comprise the Overhead (OH), occurs at a rate 9 x 9 x 8 x 8000 = 5.184 Mbps.

2. While the remainder is called the Payload, which is also called Virtual Container (VC), occurs at a rate 9 x 261 x 8 x 8000 = 150.336 Mbps.


Continue…Continue…• The OH is further divided into:

1. Section Overhead (SOH).

2. Administrative Unit Pointer (AU-PTR).• The Payload or Virtual Container (VC) is

further divided into:

1. Path Overhead (POH): One column.

2. Container (C): 260 columns and data rate given by 9 x 260 x 8 x 8000 = 149.76 Mbps.

07/03/2013 Bahman R. Alyaei 34SDH frame STM-1 structure

SOH

SOH

AU-PTR

ContainerPO

H

1 9 10 11 2701

34

9

9 columns

Overhead (OH)

261 columns

Virtual Container (VC)

125 μS

9 rows



• The SOH dedicates

1. Three Rows for the Regenerator Section Overhead (RSOH) and

2. Six Rows for the Multiplexer Section Overhead (MSOH).

• Rate of RSOH and MSOH is given by

RSOH = 3 x 9 x 8 x 8000 = 1.728 Mbps.

MSOH = 6 x 9 x 8 x 8000 = 3.456 Mbps.

07/03/2013 Bahman R. Alyaei 36SDH frame STM-1 structure

RSOH

MSOH

AU-PTR

ContainerPO

H

1 9 10 11 2701

34

9

9 columns

Overhead (OH)

271 columns

Virtual Container (VC)

125 μS

9 rows


11.2 The Truck Analogy11.2 The Truck Analogy

• SDH frame functions as a transport truck which has a tractor and a container type trailer.

• It packs the signals of different hierarchies into packages of different sizes like packing cargoes and then loads them into the truck.



• The contents carrier in the container are real goods.

• These are analogous to customer traffic, being carried in the Payload area of SDH frame.

SDH frame


11.3 The Function of OHs11.3 The Function of OHs

• The OH within the SDH signal supports network management at both the Path and Section levels.

• To realize layered monitoring, the OH is classified into SOH and POH.

• SOH and which includes RSOH and MSOH, is responsible for the section layer OAM.


Continue…• SOH functions are:1. Frame alignment pattern.2. Parity check.3. STM-1 identification.4. Alarm information.5. Automatic protection switching.6. Data communication channel.7. Voice communication channel.8. User channel.



• The POH is responsible for the Path layer OAM functions.


11.4 Function of The Pointer11.4 Function of The Pointer• SDH network is intended to be synchronous

network.• However, there will always be slight timing

differences because different clocks are being used or the same clock is being distributed over long distances.

• SDH Pointers allow this limited asynchronous operation within the synchronous network.

• It points the location of the VC in the STM frame.


12 SDH Signal Hierarchy12 SDH Signal Hierarchy

Typical SDH Communication Network



• There are three Sections in the SDH signal hierarchy:

1. Path.2. Multiplex Section. 3. Regenerator Section.• The Overheads (OHs) are always

generated at the beginning of a section and only evaluated at the end of a section.



The SDH Layer Model


12.1 SDH Network Elements12.1 SDH Network Elements• The SDH signal is layered to divide responsibility

for transporting the Payload through the network.• Each SDH Network Element (NE) is responsible

for

1. Interpreting and generating its overhead layer,

2. Communicating control and status information to the same layer in other equipment,

3. Terminating its overhead layer.



• As the Payload travels through the SDH network, each layer is terminated by one of a general class of NEs named

1. Regenerator Section Terminating Equipment (RSTE),

2. Multiplexer Section Terminating Equipment (MSTE),

3. Path Terminating Equipment (PTE).


12.1.1 Path Terminating 12.1.1 Path Terminating Equipment (PTE)Equipment (PTE)

• PTE is an entry-level path-terminating terminal multiplexer, acts as a concentrator of E1s as well as other tributary signals.

Terminal multiplexer example



• PTE is a terminating multiplexer.

• It is responsible for adding first order POH, MSOH, and RSOH to the data Container (C).

• Its simplest deployment would involve two terminal multiplexers linked by fiber with or without a regenerator in the link.

• This implementation represents the simplest SDH link (Regenerator Section, Multiplex Section, and Path, all in one link).


11.1.2 Regenerator11.1.2 Regenerator

• A regenerator is needed when, due to the long distance between multiplexers, the signal level in the fiber becomes too low.

Regenerator.



• The regenerator recovers timing from the received signal and replaces the existing Regenerator Section overhead (RSOH) bytes of the received STM signal before retransmitting the signal; the Multiplex Section Overhead (MSOH), Path Overhead (POH), and Container (C) are not altered.


11.1.3 ADM11.1.3 ADM• It is responsible for adding higher order

RSOH, and MSOH, to the received STM signal. It is also responsible for evaluating RSOH, MSOH, and POH.

• A single-stage Multiplexer/Demultiplexer can multiplex various inputs into an STM-N signal.

• At an Add/Drop site, only those signals that need to be accessed are dropped or inserted.


Continue…Continue…• The remaining traffic continues through

the network element without requiring special pass through units or other signal processing.

• In rural applications, an Add/Drop Multiplexer (ADM) can be deployed at a terminal site or any intermediate location for consolidating traffic from widely separated locations.



STM-N

Add/Drop multiplexer example.



A Synchronous Add–Drop Multiplexer (ADM)


11.1.4 DXC11.1.4 DXC• An SDH Digital Cross-Connect (DXC)

accepts various E-carrier and STM rates, accesses the STM-1 signals, and switches at this level.

• It is responsible for adding RGSO, and MSOH without altering the POH.

• One major difference between a DXC and an ADM is that a DXC may be used to interconnect a much larger number of STM-1s.



Digital Cross-Connect (DXC)



• It is ideally used at an SDH Hub Network.

• The DXC can be used for grooming (consolidating or segregating) of STM-1s or for broadband traffic management.

• For example, it may be used to segregate high-bandwidth from low-bandwidth traffic and send them separately to the high-bandwidth (for example video) switch and a low-bandwidth (voice) switch.


13 SDH Network13 SDH Network• SDH Core Transmission Stations (SDH-

CTSs) are usually located at each of the trunk and international exchanges and many of the larger local exchanges.

• Figure in the next slide illustrates the concept with an example of five CTSs (A to E), which are supporting the core transmission between a set of trunk telephone switching units, data nodes and private circuit nodes.


Core Transmission Network Configuration


Continue…Continue…• An SDH-CTS uses a combination of ADMs

and DXC equipment to provide the necessary transmission flexibility.

• The configuration of the ADM and the DXC are managed through a computer-based controller, which may be co-sited or remotely located.

• The SDH configuration controller allows the network operator to manage the configuration of the CTS flexibility points, through planning and assignment processes, as well as reconfigurations in real time to compensate for transmission link breakdowns.



• An SDH-CTS comprises a DXC on which the high speed transmission links terminate.

• For the example of a 2 Mbps block extraction from the incoming 155 Mbps link the DXC needs to be able to identify and manipulate the appropriate 2 Mbps tributary from the incoming SDH link and pass it to the outgoing link.

• The DXC is divided into a higher-order DXC switch-block handling the SDH transmission rates and a lower-order DXC switch-block handling the 2 Mbps and other PDH rates.



SDH Core Transmission Network Station – DXCs



• At smaller transmission nodes ADMs only are used to extract digital transmission blocks (at the PDH rates of 2, 8 and 34 Mbps) for the co-sited telephone switching unit, private circuit, and data units within the exchange building associated with the CTS.

• In order to maximize their (DXCs and ADMs) management capability, SDH networks are usually structured in a set of hierarchical levels.


SDH Core Transmission Network Station – ADMs


SDH Transmission Network Structure


Continue…Continue…• Figure in S#54 illustrates a typical SDH

network structure using DXCs and ADMs.

• At the top level (Tier 1) of the national network is a mesh of high-capacity SDH transmission links between flexibility nodes (CTS) of DXCs.

• This forms the inner portion, or backbone of the Core Transmission Network and links the major trunk exchanges, as well as private circuit and data nodes.



• Hanging off this level at Tier 2 is a set of SDH rings linking ADMs within a region of the country, serving smaller trunk exchanges, local exchanges and other nodes.

• Above the Tier 1 of the national network is the international portion of the Core Transmission Network, the Tier 0, which links to the network’s transmission gateways to the transmission networks of other countries – via submarine cable landing stations, microwave radio stations or satellite Earth stations.



A typical suite consisting of several racks of SDH transmission equipment.



• The yellow leads are optical patch cords that connect from Optical Distribution Frames (ODFs), where Backhaul Network based Optical Fiber Cables are terminated (see next slide).

• The white cables running across the upper parts of these racks feed to a Digital Distribution Frame (DDF) that then connects to Main Telephony switches, Internet and Routers/Switches, Television and Radio Program distribution networks.


Optical Distribution Frames (ODFs)


13.1 SDH Network 13.1 SDH Network ConfigurationsConfigurations

• There are four major configurations:

1. Point-to-Point.

2. Point-to-Multipoint.

3. Hub Architecture .

4. Ring Architecture .


13.1.1 Point-to-Point13.1.1 Point-to-Point

• Is the simplest network configuration.• It involves two terminal multiplexers linked

by fiber with or without a regenerator in the link.

• In this configuration, the SDH path and the Service path (for example, E1 or E4 links end-to-end) are identical.

• This synchronous island can exist within an asynchronous network world.



Point-to-Point Network Configuration

STM-1

E1

E3

STM-1

E1

E3


13.1.2 Point-to-Multipoint

• Also called Linear Add/Drop architecture.• It includes adding and dropping circuits along

the way (link) to facilitate adding and dropping tributary channels at intermediate points in the network.

• The SDH ADM is a unique network element specifically designed for this task. It avoids the current cumbersome network architecture of demultiplexing, cross-connecting, adding and dropping channels, and then remultiplexing.



Point-to-Multipoint Network Configuration


13.1.3 Hub Architecture13.1.3 Hub Architecture

• It accommodates unexpected growth and change more easily than simple point-to-point networks.

• It concentrates traffic at a central site using two or more ADMs, and a DXC switch, and allows easy re-provisioning of the circuits.



• There are two possible implementations of this type of network function:

1.Cross-connection at higher-order path levels, for example, using three E3 and E4 tributary in the switching matrix.

2.Cross-connection at lower-order path levels, for example, using 63 E1 tributary in the switching matrix.



Hub Network Architecture


13.1.4 Ring Architecture13.1.4 Ring Architecture

• The SDH building block for a ring architecture is the ADM.

• Multiple ADMs can be put into a ring configuration for either Bidirectional or Unidirectional traffic.

• The main advantage of the ring topology is its survivability; if a fiber cable is cut, for example, the multiplexers have the local intelligence to send the services affected via an alternate path through the ring without a lengthy interruption.



• The demand for survivable services, diverse routing of fiber facilities, flexibility to rearrange services to alternate serving nodes, as well as automatic restoration within seconds, have made rings a popular SDH topology.



Ring Architecture


13.2 Hybrid Network13.2 Hybrid Network

• The mixture of different applications is typical of the data transported by SDH.

• Synchronous networks must be able to transmit Plesiochronous signals and at the same time be capable of handling future services such as ATM.


Schematic Diagram of Hybrid Communication Network


14 Types of SDH Multiplexing14 Types of SDH Multiplexing

• SDH multiplexing combines low-speed digital signals such as 2, 34, and 140 Mbps signals with required Overhead to form a frame called STM-1.

• It also multiplexes ATM and ISDN signals into SDH frame.

• SDH is a Byte-Interleaving multiplexing system.



• SDH multiplexing includes two types:

1. Multiplexing lower-order SDH signals into higher-order signals.

2. Multiplexing low-rate tributary signals into SDH signal. The goods of different size is

analogous to different data rates such as 140 Mbps, 34 Mbps, and 2 Mbps.


14.1 Multiplexing 140 Mbps 14.1 Multiplexing 140 Mbps Signal To STM-1 SDH FrameSignal To STM-1 SDH Frame

• First, the 140 Mbps PDH signal (E4) is adapted via bit rate justification into Container level 4 (C-4).

• The C-4 has 9 x 260 = 2340 bytes.

• The frame rate of C-4 is 8000 frames/Sec, every 125 μS.

• The rate of E4 signal after adaptation is 9 x 260 x 8 x 8000 = 149.760 Mbps



• A column of POH is added in front of every C-4 block in order to implement real-time monitoring over the 140 Mbps path signals.

• The resulting block is called Virtual Container level 4 (VC-4) with a a rate of 9 x 261 x 8 x 8000 = 150.336 Mbps


C-4

260

9 139.264 Mbps149.760 Mbps

149.760 Mbps150.336 Mbps C-4PO

H

9

261

PO

H9

1


150.336 Mbps 149.760 MbpsVC-49

261

C-4PO

H

9

261

VC-49

261

=

07/03/2013Bahman R. Alyaei1 Chapter 8 Digital Transmission Systems Part 2.

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