Introduction to SDH/SONET - Massey University · – Uni-directional Path Switched Ring (UPSR) – Bi-directional Path Switched Ring (BPSR) Semester 2 - 2005 Advanced Telecommunications

Introduction to SDH/SONET

Professor Richard Harris

Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 2

Objectives

• You will be able to:– Describe the basic frame format of SDH/SONET– Discuss architectural issues associated with networks

comprising SDH elements• SDH Ring structures and options• Dynamic reconfiguration methodologies

– Discuss mathematical models for SDH network design.


Presentation Outline

• Revision of PDH technology• The SDH Hierarchy• Frame Formats• Traffic Management with SDH• Network architectures• SDH network design methodologies


PDH - Revision

• Plesiochronous Digital Hierarchy - PDH• The ‘existing’ (‘old’) digital multiplexing/ transmission systems

are not properly bit synchronised since clocks in different parts of the network run at different rates.

• The differences in clock rates (hence exact bit rates at different locations) are allowed for by bit stuffing and/or data stream buffers.

• Since the differences in clock rates are tolerably small and are accounted for, these systems are said to be Plesiochronous rather than Asynchronous.


SDH - Synchronous Digital Hierarchy

• SONET - Synchronous Optical Network– Originally proposed by Bellcore– Later standardised by the ITU in recommendations G.707,

G.708 and G.709. It has become known as SDH.• SDH - Synchronous Digital Hierarchy• SONET was proposed to take advantage of high

speed digital transmission in optical fibres.• SONET and SDH are similar in many respects but

they are actually not quite identical.


Issues addressed by SONET / SDH

• Standardised multiplexing format• Optical standard for interconnection of optical equipment.• Administration, Operations and Maintenance (OAM) are all part

of the standard.• Interworking with existing signals.• Able to accommodate future applications including BISDN

broadband rates.


SONET Frame Structure

STSSTS--1 Frame Format1 Frame Format

Synchronous payload envelope (SPE) 87 octets

Path overhead 1 octet

810

90 * 9 = 810 Octets

Payload area

Section overhead 3 octets

Lineoverhead

6 octets

TransportOverhead3 octets


SDH Frame Structure

9 oc

tets

• General format of the STM-N frame structure270 x N octets

STM-N Payload Area261 x N octets

Section overheads 9 x N octets

Overhead& Pointers

Path overhead 1 octet2430 N


SDH/SONET Frame Structure

A note on interpretation of the diagrams

1 2 3

90

4 90

4

91

91

94 180

94 180

The example showsthe first 200 (approx.)

Octets of a STS-1Frame

1 2 3


SDH Signal Hierarchy - 1

• STS - Synchronous Transport Signal level– STS-1 = 51.84Mbps

• STM - Synchronous Transport Module– STM-1 = 155.52Mbps

• Why the discrepancy?– The lowest signal for ITU level 4 signal is 139.264Mbps

• STS signals can be multiplexed to produce the following signal levels STS-1, STS-3, STS-9, STS-12, STS-18, STS-24, STS-36, STS48. (The table in the next slide shows the equivalent ITU data rates.


SONET Designation

ITU Designation

Data Rate (MBPS)

Payload Rate

STS-1 51.84 50.112STS-3 STM-1 155.52 150.336STS-9 STM-3 466.56 451.008

STS-12 STM-4 622.08 601.344STS-18 STM-6 933.12 902.016STS-24 STM-8 1244.16 1202.688STS-36 STM-12 1866.24 1804.032STS-48 STM-16 2488.32 2405.376

SDH Signal Hierarchy - 2


SDH/SONET Frame Formats

• The basic SONETSONET building block is the STSSTS--11 frame, which has 810 octets transmitted once every 125µsec.– Check that this is equivalent to 51.84Mbps!!

• We can view the basic frame as a matrix of 9 rows with 90 octetseach 9 x 90 = 810 octets.

• Transmission is one row at a time from left to right and top to bottom

• The first 3 columns (3 x 9 = 27 octets) of the frame are assigned to overheads.– 18 octets for line overhead.– 9 octets for section overhead.


A Synchronous Broadband Network

• The main components of a synchronous broadband network are:– Terminal Multiplexer (TM)– Add/Drop Multiplexer (ADM)– Digital Cross Connect (DXC)– Network Management System (NMS)

• The network elements of SDH have primarily been designed for optical fibre transmission, but is equally applicable to digital microwave radio (DMR).


Add/Drop Multiplexers

TT ADMADM TTNode A Node CNode B

Customer Tributaries

Configurable

• The Add/Drop Multiplexers are used to add or drop traffic to the stream between nodes A and C.

• Within the ADM there is a small digital cross-connect facility which allows the traffic to be dropped or inserted, passed through or rearranged within the high speed stream. This is known as traffic grooming.

• Control may be local or remote through the network management system.


Digital Cross-Connect Switches

• Network protection is achieved with DXCs and a percentage of excess bandwidth in the transmission system between nodes. Note that the network management system decides which services should have priority and downloads the appropriate switch maps to the DXCs.

Fibre cut

Service Service

Service Service

Route A Route B Route A Route B


Link Rerouting

Old Path of DS1 DemandNew Path of DS1 Demand

• Replace Link B-C With B-A-D-CA

B C

D

DS3

W-DCS

W-DCSW-DCS

W-DCSDS3

DS3

DS3

(Where DS1 is USA term for 155Mb/s service and DS3 is the term for 45Mb/s service)


Path Rerouting

Old Path of DS1 DemandNew Path of DS1 Demand

• Reroute A-D DS1 Demand Over Spare FacilitiesA

B C

D

DS3

W-DCS

W-DCSW-DCS

W-DCSDS3

DS3

DS3


Advanced Network Architectures

ADMADM

ADMADM

ADMADM

ADMADM

DXCDXCDXCDXC

DXCDXC

HUBTERMTERM

TERMTERM ADMADM

MetropolitanNetwork

155Mb/s

622Mb/S

2Gb/s

622Mb/S

Distribution Network

IntercapitalNetwork


Ring Structures for SONET/SDH

• The ability of SONET/SDH to be deployed in ring architectures rather than as strictly point-to-point or point-multipoint architectures, has become the defining feature of SONET/SDH to date.

• The incentive for building SONET rings was to provide a means of standardizing the traditional 1:1 protection switching in a cost-effective manner.

• The self-healing ring, like 1:1 diverse protection structure, is totally automatic and provides 100% restoration capability for a single fibre cable cut and equipment failure.


Ring Structures (Continued)

• As technology advances and competition drives the prices for higher-rate systems towards those of lower-rate systems, SHRs tend to become even less costly to deploy than low-cost 1:N protection systems.

• Using these rings, thus improves network survivability and availability, while reducing cost.

• Hence, SONET self-healing rings are expected to form the major network infrastructure in future B-ISDN.


Distinguishing Attributes

• There are three main features that characterise all SONET rings,each with two alternatives.

• These basic distinguishing attributes are listed in the table below:

Line switchingPath switching

Level of protection switching

UnidirectionalBidirectional

Direction of the signal

2-fibre4-fibre

Number of fibres per link

ChoicesAttribute


Possible Ring Configurations

• Obviously, there are eight different SONET ring configurations arising from these attributes.

• To designate all these different types of ring architectures, various abbreviations are used. The abbreviations include:– Uni-directional Line Switched Ring (ULSR)– Bi-directional Line Switched Ring (BLSR)– Uni-directional Path Switched Ring (UPSR)– Bi-directional Path Switched Ring (BPSR)


Practical Rings

• In actual practice, however, only three of these eight types of rings have been built on a large scale, including:– fibre UPSR– fibre ULSR– fibre BPSR

• Most local exchange carriers have tended to favour 2-fibre rings of the unidirectional sort with either line or path switching.

• Inter-exchange carriers, on the other hand, have favoured 4-fibre BPSR


WorkingProtection

Key:

SONET/SDH Self Healing Rings

1 2

34

1 2

34

• Counter-rotating ring• Protection uses

separate fibre

• Point-to-point traffic arrangement

• Working and protection use the same fibre (reserve half bandwidth for protection)

(a) Unidirectional SHR (b) Bidirectional SHR with 2 fibres.


Sample Rings

(c) Bi-directional SHR with 4 fibres.

WorkingProtection

Key:

• Point-to-point traffic arrangement• Protection/Restoration uses separate fibres

1 2

34


SONET/SDH Rings Compared

Two-fibre Two-fibre Four-fibreUnidirectional Bidirectional BidirectionalRings Rings Rings

Regional andbeyond

SymmetricalDelays?

Multiple failures Usually a problem Usually a problem Usually not a problemBandwidth efficiency Medium Medium HighInitial cost Medium Medium High

Expansion costs Low Medium LowComplexity Low High Medium

No Yes Yes

Usually seen in: Cities Cities


Interconnected Rings

• Although, SHR’s are highly survivable the number of nodes on a ring, is limited by its capacity requirement and the number of hops between any two nodes.

• Hence, in order to utilise SONET self-healing ring technology in large networks, it is important to investigate efficient methods of interconnecting rings to overcome the problems of a large single ring.

• Desired features of an interconnected ring network include – preservation of survivability performance of single rings, – efficient routing, – simplified network control mechanism, and – appropriate control over problems, such as congestion.


Example Ring Network – SDH/SONET

Ring network

• What do you see as the advantages and disadvantages of a network arranged in a ring fashion?

E

E

E

E

E E

E

E E

E


Hierarchical self-healing network

Two-level single-homing hierarchical SHR network

Two-level dual-homing hierarchical SHR network


Interconnected Ring Network Design

• The major issue in designing survivable SONET self-healing ring networks is how best to utilize the unique characteristics of SHRs to meet different demand requirements in a cost-effective manner.

• For instance, the way rings are interconnected and the type of the rings used has an impact on the overall architecture, cost and survivability.


Designing HSHR Networks

• The problem of designing HSHR networks can be stated as follows:

• Based on the given information of a network, which includes – a set of nodes, – the geographical distance,– traffic demand between each pair of nodes, and – a cost function f(x,y) of a link with length x and capacity y,

• We need to find an optimal Hierarchical Self Healing Ring accommodating each node and minimising the total cost of the network.


Fibre-based Loop Network Design

• In the previous slide, we dealt with the design of interconnected ring networks, which can be used for designing large SONET survivable transport networks.

• Here we discuss the design of a fibre-based loop network i.e. an access network.

• Fibre facilities have been actively deployed in the feeder segment of local loop networks to reduce operating costs of present copper-based networks and to provide a fibre-optic infrastructure that will support new high bandwidth telecommunications services, such as broadband integrated switching services.



• The design problem for a loop network is – how to interconnect a set of customer locations through a

ring of end offices so as to minimize the total tariff cost and provide reliability.

• The input elements of the problem include – a set of end offices, – a set of digital hubs (switches), and – a set of customer locations that are geographically

distributed on a plane. • Each customer location is connected directly to its

own designated end office, which in turn, needs to be connected to exactly one selected hub.



• Then, the selected hubs are connected by a ring. • Each hub has a fixed cost for being chosen and each

link has a connection cost for being included in the solution.

• The objective is to design such a network at minimum cost. In other words, the aim is to connect all the end offices to at least one hub, in a most cost-effective way.


Problem Formulation

• Problem: Consider

• Subject to:

• where xij is binary variable equal to 1 if end office i is assigned to switch j.

• The first set of constraints guarantees that each end office is associated with a switch.

• The second set ensures that the switch capacity constraint is not violated.

,min ij ij

i j

z C X=∑

11

C

ijjX

=

=∑

1

T

i ijiW X W

=

≤∑


Problem Solution

• Greedy Heuristic with Tabu Search can be used for solving this network problem.

• This heuristic method, presented in [2], assumes that switches may be of different types and defines the capacity of a given switch as the number of OC-3 ports that may be used by the clients.

• The main objective of the Greedy Heuristic is to find a good solution quickly i.e. to design a minimum cost network subject to all the constraints described above.

• This method incorporates features of the well-known Steiner tree problem and the travelling salesman problem.


Conclusions

• Ring structures are the simplest method for ensuring the minimum level of protection for traffic flowing on high capacity links.

• Design methodologies can be complex and time consuming to implement and heuristics prevail due to the nature of the problem: Similarity to the travelling salesman problem, etc.


Clever ways to change topology!

• Network on the left is a star network.• Flip internal connections in central node and get a ……?


Models for Dynamic Reconfiguration

• There have been a number of different models proposed for dynamic reconfiguration in networks:– Harris (DSPN model). This is a different technology but the

model appears to be relevant to the SDH context.– Doverspike, Pack and Jha: Based on a stochastic model for

demand at the DS0 and DS1(1.5Mb/s) levels. The system uses state dependent routing of Krishnan and Ott.

– Herzberg: Simple LP model based around a simplification to the Gopal et al and the DSPN model and uses stochastic demand elements.

– Gopal, Kim, Weinrib: Model begins as an NLP to optimise a traffic weighted blocking formula. Heuristics used to solve problem.


Herzberg Model - 1

Define:Define:• g = Group capacity size (g=30 if 2Mb/s trunks)• Ci = Available bandwidth of link i=1,2,...L • Np= Number of OD pairs j=1,…,N(N-1)/2• Aj = Offered traffic to OD pair j• Pj = Number of chains for OD pair j• Xj,p = Amount of bandwidth assigned to OD

pair j through chain p


Herzberg Model - 2

• One possible objective function to use is to minimise the weighted traffic losses and this is done by Gopel et al. in their ITC paper.

• Equivalently, Herzberg maximises the carried traffic through the network as follows:

Subject to:

CapacityCapacityConstraintConstraint

BandwidthBandwidthconstraintsconstraintson OD pairon OD pair

max ( , )Y A Xj j jj

N p

=∑

1

1

1

1,2,

1,2,

0 and integer

p

j

Nipj jp i

j

Px nj j jp j p

p

jp

X C i L

M X X M j N

X

δ=

=

≤ =

= =

≥

∑

∑

…

…


Revised Herzberg Model - 1

• Gopal et al. developed an heuristic approach to the solution of this model. Herzberg exploited the nature of the Y functions and represented them as piece-wise linear functions using the coefficients given as Yjk

• where this represents the amount of traffic the k-th capacity unit assigned to OD pair j will carry, viz:

whereB(A,n) is the Erlang Loss Formula:

Yjk = Aj [B(Aj, (k-1)g) - B(Aj, kg)]


Revised Herzberg Model - 2

max Y X

X C

X M

X

X

jk jkpp

P

j

M

p

N

jkpi

jkp ip

P

k

M

j

N

jkp jp

P

k

M

jkpp

P

jkp

jjx

p

jjx

p

jjx

j

===

===

==

=

∑∑∑

∑∑∑

∑∑

∑

≤

=

≤

=

111

111

11

1

1

0 1

Subject to:

or

δ


Doverspike and Pack Model

• In their paper to Networks '92, Doverspike and Pack describe the SONET Switched Bandwidth Network or SSBN.

• The SSBN is a dynamic bandwidth strategy that aims to integrate – Dimensioning– Network operations– Customer control– Network restoration– Network planning.

• The SSBN aims to "automatically and quickly provision bandwidth, use intelligent network status based routing methods, and dynamically reconfigure the network to provide survivability andservice restoration features".

Today's Interoffice Network Design

Design Trunk NetworkPoint-Point

Load Forecast

Grade of

Service (GOS)

Design DS0 Network

Design DS1 Network

Design High Rate Network(565MB, 1.2GB, etc.)

Design Physical Network(Cable, Radio)

Originating

DS0 Forecast

Originating

DS1 Forecast(Unmultiplexed)

Originating

DS3 Forecast(Unmultiplexed)

DS0 Link Capacities

DS1 Link Capacities(Multiplexed)


High RateLink Capacities

DeterministicNetwork Design

StochasticNetwork Design

Tomorrow's Interoffice Network Design Process

Design Trunk NetworkPoint-Point

Load Forecast

Grade of

Service (GOS)

Design DS0 & DS1 Network

Design High Rate Network(565MB, 1.2GB, etc.)

Design Physical Network(Cable, Radio)

Originating

DS0 Forecast

Originating DS1

& Multiplexed DS1 Forecast

DS1 Link Capacities


High RateLink Capacities

DeterministicNetwork Design

StochasticNetwork Design

Level of

Performance


Demand Model

• At the DS1 level, the demand for SSBN can be characterised by DS1 requests for service.

• Their demand modelling is similar to circuit switched telephony in that it requires:– Arrival rate– Holding time distribution.

• However, it should be noted that arrivals are not Poissonian, service times are long (in the order of years perhaps!). Steady state conditions are unlikely to be achieved since the arrival rate changes before the end of a typical holding time!

• (The special service demand model is described in a separate paper which I have obtained from Doverspike.)


Routing Strategies

• The aim here is to select a path through the digital cross connect switches to service a DS1 demand request.

• Their system is based upon a modification of the Krishnan and Ott state dependent routing system. (Described shortly.)

Network SurvivabilityNetwork SurvivabilityNetwork SurvivabilityThe proposed SSBN method is designed to use path rerouting as

described earlier.(You will see that this is more efficient than link rerouting and I

used it in my DSPN model also.)


Overview of State Dependent Routing (Krishnan and Ott)

• For each link or trunk group k in the network determine a marginal cost fk(j) of adding a call to that trunk group when j of its trunks are already busy, for j=0,1,… skwhere sk is the number of trunks in the group.

• This cost represents the effect of the added call on the probable blocking of future calls on the group, and is defined to be the additional number of calls blocked on the group if the present call is accepted.Note that and

corresponds to the loss of a blocked call.0 1≤ ≤f jk ( ) f sk k( ) =1


Ideal SDR Method - 1

• Determine the cost for an arriving call in the current network state by considering each of the possible chains over which the call could be routed.

• If the minimum path cost for the call is < 1 then route the call on that minimum cost chain. Otherwise, reject the call.

V f xV f x f xV f x f x

AB

ACB

ADB

== += +

1 1

2 2 3 3

4 4 5 5

( )( ) ( )( ) ( )A B

C

D

12 3

4 5

xk = occupancy of trunk group k


Ideal SDR Method - 2

• In the original work by Krishnan and Ott the cost function was a ratio of Erlang Loss formulae, viz:

• yk is the load induced on link k by a nominal or reference routing scheme in which arriving calls are allocated amongst their admissible paths in a random fashion.

• It has been shown that these costs approximate a policy iteration method in a Markov decision process.

f j B s yB j yk

k k

k

( ) ( , )( , )

=


Revised SDR Method

• At the 12th ITC in Torino, Krishnan modified the cost function to take into account the specific OD pairs in the network, viz:

fik(j) = fk(j)gjk

• Where gjk was calculated from parameters of the nominal traffic allocation scheme mentioned earlier.

• Practical implementation of the SDR scheme involves obtaining network status information at 5 minute intervals and hence the scheme has become known locally as DR-5.

Documents

Introduction to SDH/SONET - Massey University · – Uni-directional Path Switched Ring (UPSR) – Bi-directional Path Switched Ring (BPSR) Semester 2 - 2005 Advanced Telecommunications