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Planif 2000, a tool for planning SDH networks

Carlos S da Costa*, Joo Henrique Duarte Preto Paulo**, Pedro Azevedo***

*IT/ISCTE, Av. Rovisco Pais n 1, 1049-001 Lisboa, Portugal, Email: csc@lx.it.pt , Tel: +351-21 841 84 88

**MOTOROLA Portugal, Av. Jos Gomes Ferreira, Ed. Atlas I, 9, sala 22, 1495 Algs Portugal,Email: Joao.Paulo@motorola.com, Tel:+351 96 296 8256.

***SIEMENS Portugal, S.A., IC-OCN N-Network Management Rua Irmos Siemens, 1, 2720-093 Amadora, Portugal,

Email: Pedro.Azevedo@lis2.siemens.pt, Tel: +351 21 424 2509

Abstract

This paper describes the software tool "Planif 2000" that

implements a set of different algorithms to help a

Synchronous Digital Hierarchy (SDH) network planer in

choosing the final solution or a small set of solutions for

Synchronous Digital Hierarchy networks. A description ofthose algorithms is presented, and at the end, a relevant case

study is presented.

I. INTRODUCTION

SDH networks replaced old Plesiochronous Digital

Hierarchy (PDH) infrastructure of established operators and

is the solution for new operators. The SDH technology has

important advantages and possibilities for expansion and

combination with new technologies compared with the

previous technology, the PDH. SDH is a standard, supports

rates as high as 40Gbit/s, needs less equipment to extract

each tributary and each individual channel, exhibits high

configuration flexibility of services and lines. SDH allowsseveral new types of protection, which increase the system

availability, SDH technology supports a network

management entity (TMN). SDH allows the equipment

compatibility between different vendors.

When it is necessary to plan the SDH infrastructure to

cover a metropolitan region with several dozens of nodes

there is a multiplicity of solutions with different cost. An

exhaustive analysis of all the alternatives is time consuming

and inefficient so some heuristics are used to reduce the

number of solutions to a manageable number and even so the

number of solutions is large because many parameters can be

changed, e.g. number of nodes per ring, maximum capacity

of each link, etc.

The operator demands from the planner an efficient

solution, one that minimises the cost assuring a good quality

of service and protection. The inputs for such planning

process are the cartographic coordinates (or instead the

relative positions of the nodes), the traffic between nodes

and, optionally, some other restrictions. The output consists

of a set of SDH rings, a set of point-to-point connections a

list of equipment and the global cost.

"Planif 2000" is an auxiliary tool that implements a set of

different algorithms to help the planer in choosing the final

solution or a small set of solutions.

The "Planif 2000" converges towards a solution for a SDHcoverage of a region consisting of a central SDH ring of high

capacity that interconnects several secondary SDH rings, and

some point-to-point connections. "Planif 2000" assures that at

least two links end at each node so no node is isolated in case

of a single link failure.

"Planif 2000" allows the user to create/edit the network

through the user interface, and to visualise the net after each

algorithm allowing the planner to tune some parameters, likenumber of nodes per link, viewing each net structure by

itself, run each algorithm to check results or run all

algorithms in one go. In some steps, the user can choose from

more than one algorithm that can be applied usually carrying

to different results. At the end, the list of necessary SDH

equipment and its cost is presented to the user for the global

solution or for each structure.

II. SDH NETWORK PLANNING

When planning an SDH network the mechanism that

increases the reliability demands from the planner special

care due to the different protection mechanisms that he can

choose and the equipment requirements that each of them

demand.

The complexity of SDH networks due to its flexibility

versus PDH has the following items:

Grouping. While PDH cares about individual

multiplexing sections with the goal of reducing the

number of multiplexing units, SDH trails can be

optimised to convey the tributary units in containers

through chosen paths.

Equipment. The PDH equipment has a fixed attribution

between tributaries ports and output ports. SDH

flexibility is achieved by a switching matrix that allows

reconfiguration between tributaries and outputs asneeded.

Switching. PDH cross connection is carried manually

while SDH equipment performs cross connection

automatically.

The SDH planning process has to take in account:

The coexistence between high rate SDH and low rates

PDH nets.

The continuous increase of demand of

telecommunications services.

The market dynamic which makes difficult long-term

forecasts.

The rapid technological evolution that shortens theequipment life time.

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The SDH is pushed by two streams of evolution, the

telecommunication stream and the computer stream.

The existence of several telecommunication operators

instead of a single one.

The high reliability required from the equipment and

from the networks.

The application of SDH is carried at sub-network levels:

Central network, between transit central office

characterised by high capacity, high physical diversity,

with very special recover requirements and efficient

usage of the available bandwidth.

Regional network, between primary central offices

highly connected, with medium capacity, limited

physical diversity and specific recover and flexibility

requirements.

Access network, to connect access nodes with star or

tree topologies characterised by low capacity, low

physical diversity and specific flexibility requirements.

The planing process can be decomposed in several steps to

simplify the study:

The topological analysis, where the network connectivity

and flexibility are evaluated

The traffic analysis, with the analysis of the distribution

and general load of the network traffic.

The routing, where the most proper path for each

element in the traffic matrix is calculated. In this process

several politics can be used: reducing the number of

jumps, choosing the shortest path, chosen two disjoint

paths, traffic balancing. The ring is the most common

structure.

The architecture and management, The protection looks for traffic share, Multiplexing

Section Protection (MSP), path protection and specific

protections for ring configurations.

The grouping this process goal is to concentrate traffic

and to do so it uses the layer structure so it is possible to

concentrate between end-nodes, sub-connection and

connection

The equipment planning is carried out after the calculation

of the necessary network functions taking in account

equipment limitations and reducing the number of extracting

steps in high capacity nodes.

III. THE PLANIF2000 ALGORITHMS

Planif 2000 final goal is to study several ways of planning

to achieve a final solution for a telecommunications network

where criteria like lower cost, better capacity, less utilisation

of network elements and the grater or less number of rings

and point to point connections are used. This is carried by

Plannif2000 using several algorithms with different

complexity.

Topoinit Algorithm is used to initialise the traffic matrix.

The first approach is to consider a fully connected network.

This network has too many connections some of them not

cost effective. A simplification of this fully connected

network is needed to keep the computation time of the trafficrouting algorithm within acceptable values. In this procedure,

connections with no traffic are eliminated as well as long

distance connections usually with low traffic. Although there

is the danger of elimination a better solution that could be

find if an exhaustive approach was used. The limit of having

always a node with two connections was imposed to assure

redundancy connectivity. This connection matrixsimplification can be achieved using one of two algorithms:

The Crossing Algorithm analyses exhaustively all node

connections and considers the relative position and the

survivability of the network when removing

connections. This way, connections between nodes

working in parallel (or redundant) are eliminated [1].

The Average distance between connections algorithm

performs an exhaustive analysis of all network

connections. Connections are eliminated if the distance

exceeds a value corresponding to the average of the

distance of the connections of two nodes between the

two nodes in analyses. This value is weighted by a

factor enabling different solutions with variable

number of connections[1].Selection of Hubs . As a secondary analysis, it is possible to

identify nodes with higher traffic characteristics or higher

connectivity and the algorithm detects those nodes by an

exhaustive search. Those nodes with special properties will

usually concentrate traffic and will be used as gateway nodes

that switch traffic to a, to be designed, central ring. Two

algorithms were implemented:

One chooses hubs based on the number of connections

between nodes after the initialisation. The number of nodes

that are classified as hubs are in an interval of percentage

defined by the user.

The other algorithm works based on the quantity of traffic

that a node switches and is dependent on the initial traffic

matrix and on the total number of hubs chosen[1].Selection of point to point exclusive connections . The goal

of this algorithm is to remove from next optimisation steps

connections that could increase unnecessarily the future

rings capacity. The user defines from which hierarchy or

from how many connections of one hierarchy the connection

should be considered exclusive1point to point connection.

Ring generation . Rings are generated using the traffic

affinity algorithm. This algorithm starts by looking for the

node with more traffic terminated and that has not started anyring before. Then it chooses other nodes by traffic affinity

until it reaches the maximum number of nodes in a ring or the

defined percentage of traffic load of a ring.Simulated annealing is used to organise each ring generated

in the previous procedure. This algorithm finds the shortest

cyclic path between graph nodes.Central ring selection , It is crucial in a network that every

element of the traffic matrix is routed, hence the necessity of

having an element of network support where all rings and

1Exclusive is used because this traffic will be transported apart from

the rest and will not impact further the planning process.

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connections are linked together to assure the previous

requirement. Three methods were implemented to select a

central ring:

To select nodes with higher possibility of connecting

with more rings.

The selection based on nodes with have more rings incommon.

The selection based on nodes with more affinity of

traffic terminated.

The user has to select which method to use, and the number

of nodes of the central ring, or the percentage of occupancy

of the central ring.

Secondary rings selection . The goal of this stage is to find

the secondary rings to transport traffic between nodes not

belonging to the central ring. The rings must always have two

nodes that belong to the central ring due to redundancy and to

assure connectivity between all nodes. Two different

algorithms were implemented to select the secondary rings.

Secondary Rings-1, this algorithm uses the same principle

used in the generation of rings starting by the node with more

traffic terminated and choosing the others by traffic affinity.

This process forces that two nodes from the central ring are

chosen so there is redundancy to prevent faults. The

algorithm stops when all nodes belong to a ring

Secondary Rings-2, this algorithm starts by the node with

more traffic not belonging to the central ring. All the other

ring nodes are selected based on traffic affinity with the

restriction of having two nodes belonging to the central ring,

as previously.Routing and consolidation. This algorithm optimises the

network routing assuming that the net is only constituted bypoint to point connections, and the advantages of having

rings are not considered. This algorithm works over the

network connections resulting from the Topoinit algorithm

and the hubs already selected. It reroutes the 2Mbit/s traffic

through the higher capacity connections to be able to

eliminate some connections. The final goal is a network with

concentration of 2Mbit/s channels on STM-N channels with

the maximisation of N [1].

The Final solution selection algorithm is the combination of

several algorithms: the use of clusters for ring selection,

network routing with rings, ADM structures and cost

calculation. These steps are performed in sequence: The first step is initialisation and is necessary to obtain

the physical connections between nodes (uses the

Topoinit algorithm).

The second step is to find exclusive connection and the

traffic of those connections is temporarily removed

from the traffic matrix.

The third step consists of using the concept of cluster

to group nodes with higher traffic affinity.

The fourth step consists of finding a central ring.

The fifth step consists of generating secondary rings

with a number of nodes chosen by the user. The

algorithm used here is the one explained before but

considering the clusters.

The sixth step consists of selecting the secondary nodes

that best correspond to the clusters.

The seventh step consists of selecting point-to-point

connections to connect isolated nodes, as the previous

step to assure complete network connectivity. For

reliability reasons, the user can select redundancy forthese connections.

The eighth step consists of filling the nodes ADMs

with the traffic exchanged either with the PDH network

or with the other nodes. Depending on the type of

routing (the Dijkstra shortest path algorithm or

minimising the number of jumps between rings) there

are different values to ADM filling.

The ninth step consists of including the traffic of the

point to point exclusive connections found in step two.

The tenth step is the calculation of the cost of the final

solution based in a cost model using ADMs with

STM1,4,16,64 capacity.

IV. CASE STUDY

The case study consists of a metropolitan network with 21

nodes with the traffic represented in a 2Mbit/s circuit

requirement matrix. The network data was from a

Telecommunications Operator (Portugal Telecom) and

represents the traffic transportation needed for an urban area

with mixed services (data and voice).

The case study network showed a heavily loaded

connection between two central nodes where the exclusive

connection step was applied.

In addition, some of the nodes situated at the border of the

network area were chosen to carry traffic into the trafficmatrix, in order to handle external traffic requirements.

The traffic matrix used also considers the future growth of

the network so the 2Mbit/s circuit values are future

requirements.

When choosing the best results from each of the set of

results given by the algorithms the planer tried to follow

these rules:

Never chose network solutions where a lot of the

network traffic is carried by an only few

connections (choke points).

Never use rings with its nodes geographically very

distant or opposite on the network as the case study

is a metropolitan network. Do not overuse node to node connections as they do

not take advantage of the SDH ring benefits

(protection and management).

Good distribution of the equipment over the entire

network.

Even using these restrictions, the planner will get several

possible network solutions and he must choose the best fit for

his network.

Figure 1 shows the result of applying the selection of Hubs

algorithm. In the figure 1a), hubs were selected between all

nodes if they have 10 to 20% less traffic than the node with

more connections. This results in a relative low number ofhubs; four hubs marked in figure 1 a). The second case the

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percentage is between 30 and 40%. This result in increasing

the number of hubs. Seven hubs were found as can be seen in

figure 1b).

a)

b)

Figure 1 Hub selection a)10 to 20%b) 30 to 40%

The application of the Selection of point to point exclusive

connections to the metropolitan network results in the

connections marked in figure 2 a) where a percentage of

filling of 20% and at least one STM16 were the parameters

selected.

a)

b)

Figure 2 Selection of point to point exclusive connections a)

with 20% of filling STM16 b) with 80% of filling with STM4

Figure 2b) is the result of the same algorithm when a

percentage of 80% of filling and STM 4 circuits are selected

is. The jump to a higher hierarchy is carried out when there isthree circuits from the previous hierarchy.

The result of applying the ring generation algorithm with

crescent number of nodes per ring is the convergence to an

optimal ring; this can be seen comparing figure 3.

a)

b)

Figure 3 Ring generation with a percentage of filling of 80%.

a) with four nodes per ring b) with nine nodes per ring

Figure 4 is the result of secondary ring generation where

can be seen that all secondary nodes are connected by two

nodes to the central ring. In figure 4 a) secondary rings were

generated without following the topology limitation and in

figure 4 b) they were generated following the topology

limitation.

a)

b)

Figure 4 Secondary ring generation with eight nodes per ring

a) without following the topology limitation. b) following thetopology limitation.

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Comparing the initial topology with ten hubs, figure 5 and

the result of applying the consolidation algorithm, figure 6

can be seen a concentration of traffic in the connections

between hubs, solid lines.

Figure 5 Initial topology with ten hubs.

Figure 6 Result of the consolidation algorithm with weight

parameter equal to twenty.

a)

b)

Figure 7 Final solution for the case study a) standard solution

b) the best solution tested.

The final solution presented in figure 7 were obtained using

the following parameters: percentage of hierarchy occupancy:

80%; number of exclusive point-to-point connection: 1;

number of clusters: 3; max. number of nodes per cluster: 3;

min. number of nodes per cluster: 2; number of nodes in

central ring 5; max. number of nodes in secondary rings: 6;

min. number of nodes in secondary rings: 5; with topological

optimisation and USHR/SNCP protection.

The following results provided by Planif are used to chose

among some of the solutions:

Number of STM-X structures (where X is 1..64 and

structures are rings or connections) (see table below)STM

64STM

16STM

4STM

1ADM

64ADM

16ADM

4ADM

1

a) 1 4 3 2 5 20 6 4

b) 1 4 2 2 5 20 4 4

Number of ADMs of STM-X hierarchy used.

Percentage of rings versus point to point

connections. a) 0,66 b) 0,8

Central ring traffic vs. overall central ring capacity.

a) 0,64 b) 0,64

Cost of the solution, rings and connections (vendor

dependent)

To find the best optimised solution, Planifs default valueswere used to generate a network solution that was used to

compare all the other solutions too.

The main conclusion of this program when applied to the

Portugal Telecom metropolitan network is that the best

relation of capacity vs. cost is achieved when the secondary

rings have between 5 and 6 nodes. This number of nodes

assures that the hierarchy used is not too high, and there is no

ring overlapping.

IV. FINAL REMARKS

The design of a reliable, cost effective, upgradeable

telecommunication network is not an easy task for the

planner and has become more difficult due to the newrequirements and available technologies. With this tool, the

planner can easily try several solutions changing several

parameters, methods and algorithms.

Planif 2000 was developed to be a tool to aid the planner in

his task of choosing a network solution but will not by itself

generate the best solution. The network planner must study

the output it provides and decide on the best relation between

cost implementation and management of the final solution.

V. REFERENCES

[1]Joo Paulo, Pedro Azevedo, Planeamento de Redes

SDH, IST 1999.

[2] Sexton, M. e Reid, A., Broadband Networking, Artech

House, 1997.

[3] Sexton, M., SONET Interworking, Artech House, 1996.

[4] Geyer M., SDH Network Applications and related

Planning Issues, SIEMENS, NETPLAN 10, September 1996.

[5] Cardwell, R.H., Wu, T.H., Wasem, O.J., Survivable

SONET Networks Design Methodology, IEEE Journal on

selected areas in communications Vol. 12, n.1, January

1994.

[6] Sexton, M., Soto, O.G., Tardini, C., Wulf-Mathies, C.,

Planificacin y gestin de redes SDH, Comunicaciones

Elctricas Alcatel 4 trimestre 1993.