COMPARISON OF COLOURED SECTION RING WITH CLASSICAL SDH RING ARCHITECTURES
Laurent BLAIN(1), André HAMEL(2), Tivadar JAKAB(3), Alain SUTTER(4)
(1) France Telecom CNET LAB/SAR/SRI Technopole ANTICIPA 2 avenue Pierre Marzin F-22307 Lannion Cedex France
Phone: +33 96 05 28 14 Fax: +33 96 05 12 52 E-mail: blain@lannion.cnet.fr
(2) France Telecom CNET LAB/RIO/ARO Technopole ANTICIPA 2 avenue Pierre Marzin F-22307 Lannion Cedex France
Phone: +33 96 05 34 26 Fax: +33 96 05 32 26 E-mail: hamel@lannion.cnet.fr
(3) Technical University of Budapest Department of Telecommunications Sztoczek u. 2. H-1521 Budapest Hungary
Phone: +36 1 463 1010 Fax: +36 1 463 3266 E-mail: jakab@hit.bme.hu
(4`) France Telecom CNET PAA/ATR
38-40 rue du Général Leclerc F-92131 Issy les Moulineaux Cedex France
Phone: +33 1 4529 4188 Fax: +33 1 4529 6069 E-mail: sutter@issy.cnet.fr
Abstract
The Coloured Section Ring* architecture is based on wavelength division multiplexing technique. It is reutilising standard SDH equipment and the well known linear multiplex section protection automatic protocol and provides a full transparent 100%
protected two-fibre ring architecture for SDH applications.
The architecture gives the possibility to define a logical order of nodes in the ring different from the physical (cabling) one and to duplicate nodes in the same ring system. The efficiency of the new ring architecture is demonstrated in comparison with path protected and multiplex section shared protected ring architectures.
* Coloured Section Ring technique is covered by a patent owned by France Telecom.
1. INTRODUCTION AND MOTIVATION Transmission networks of the present and the near future are based on SDH (SONET) transmission technology. Many SDH based networks are in service or under installation all over the world.
The self-healing ring architectures are effective solutions for full protected transmission networks.
PNOs successfully operate path protected (PP) rings and multiplex section shared protected (MSSP) rings in their networks. However, the ring link capacity often limits the optimal utilisation of expensive add-drop multiplexer (ADM) capacities in two-fibre ring architectures, because the half of the ring capacity in MSSP rings or in many cases more than the half in PP rings are reserved for protection.
To overcome this limitation a wavelength division multiplexing based full transparent SDH architecture named Coloured Section Ring is proposed [1].
2. COLOURED SECTION RING 2.1. The Ring Architecture
The basic idea of the Coloured Section Ring (CS ring) architecture is to take advantage of the linear multiplex section protection (MSP) protocol available in standard SDH ADMs and the wavelength routing in a two-fibre bi- directional ring to increase the transmission capacity.
In the CS ring the SDH ADMs access the ring via optical add-drop multiplexers (OADM). The purpose of an OADM is to insert a transmit signal from an SDH ADM and extract a received signal to an SDH ADM at a particular wavelength (Figure 1). The OADM is transparent for other wavelengths not concerning the node. The transmit and receive signals can be at two different wavelengths, however there are technological limitations towards the total number of wavelengths in a wavelength multiplex.
In usual SDH rings the ADMs are connected to their two neighbours using multiplex sections. In Coloured Section Rings a particular wavelength is dedicated to each multiplex section (therefore a multiplex section is called a coloured section).
This solution gives the possibility to connect two
ADMs regardless of the physical order of nodes on the ring cabling infrastructure and a logical ring structure can be established (Figure 2).
The CS ring is a multihop optical architecture. But not an all optical one, because the routing functionality is realised in the electrical domain. The route between two logically non adjacent nodes are set up via the SDH ADMs of the intermediate nodes (Figure 2) through optical/electrical/optical conversions.
2.2. The Protection Mechanism
The protection against a fibre cut is provided using the well known standardised MSP protocol.
The transmitted signal is split and permanently bridged to both the working and protection systems. The decision on which signal to use is made by the receiver end analysing the signals at the receive terminal. The non-revertive single-ended protection switching is performed in the electrical domain. No transfer of extra information is required simplifying the procedure considerably.
In CS ring architecture the MSP implicates that every optical line interface is duplicated in the SDH ADMs (Figure 2). Working and protection signals are transmitted through the OADM in the opposite directions via the two fibres. Thus, a connection between two nodes uses two divers routed fibre pairs on the complementary arcs of the ring (Figure 3).
3. EXPERIMENTAL RESULTS
First CS ring experiment was carried out with a 3 node bi-directional STM-1 ring. A 8 nm channel spacing was adopted and multilayer OADMs are installed at each node for wavelength routing [3].
3-cavity Fabry-Perot filters exhibit 6.3 nm FWHM transmission spectrum and the in- band crosstalk is 20 dB in a 4.5 nm domain, adjacent cross-talk is better than 25 dB. For more details see [4].
4. NEW CAPABILITIES - NEW LEVELS OF OPTIMISATION
4.1. New Capabilities of Ring Architecture
Based on to the wavelength routing a logical order of nodes, different from the physical cabling infrastructure can be realised in CS
rings. This capability gives the possibility to reduce transit traffic via the intermediate nodes and links and improves the utilisation of SDH ADM capacities. Not only the connection order of nodes can be modified in CS rings, but nodes can be multiplied in the same ring as well. In case of mainly concentrated traffic patterns (e.g. in
hubbed network structures) the duplicated insertion of hub nodes can improve the utilisation of SDH ADM capacities considerably.
4.2. New Levels of Optimisation in Ring Dimensioning
Figure 1 Node architecture in coloured section ring
Figure 3 Linear MSP in coloured section ring Figure 2 Wavelength routing in coloured
section ring
In the basic SDH ring dimensioning problem a set of nodes (a network cluster) with fixed connection order and the transmission demands are given, the capacities of the ring links and the ADMs are specified. Generally, the target of the planning process is to fulfil the demands with minimum network cost (e. g. in a simplified representation with minimum number of ADMs).
As consequences of new capabilities of CS rings new dimensions of the planning problem can be identified. Since in the basic ring dimensioning problem the connection order of nodes is fixed according to the cabling infrastructure, dimensioning CS ring the order of nodes on the logical ring layer is target of optimisation.
Taking into account that a node can be duplicated in the same CS ring the number of nodes having a possible access to a given ring system is not fixed as well. Generally, the number of nodes to be multiplied strongly depends on the total amount of originated and terminated demands of the nodes and the demand structure itself.
These features make the CS ring dimensioning problem more complex than the classical SDH ring dimensioning.
5. COMPARISON CASE STUDIES
The aim of the presented comparison case studies is to provide basic information about the capabilities of Coloured Section Ring and to evaluate the architecture in comparison with classical PP and MSSP rings for real network cases.
The comparisons are performed on number of equipment basis and on cost basis as well. On equipment basis the efficiency of capacity utilisation and the impact of increased tributary capacities of SDH ADMs on CS ring dimensioning are studied. Cost studies are elaborated to compare the total network cost of PP, MSSP and CS rings. To evaluate the cost savings available for optical ADMs the cost gap per active node between classical and CS network costs are calculated.
The applied cost approach tries to keep the generality of the study. The cost model is simplified, only installation costs are concerned.
The total network cost is defined by the total cost of installed SDH ADMs, because the management and operational costs can strongly depend on the network environment. The fibre cost is not included in the total network cost.
Generally, the lifetime costs are more accurate for comparison of network architectures. However, in our case the simplification of the cost approach
does not have a significant impact on the final conclusions. The ignored management and operational costs of CS ring architecture are not higher than the same costs for classical rings. In the CS rings there are less active equipment to manage than in classical rings. Linear MSP is one of the simplest protection techniques, extra OADMs are simple and reliable equipment built up of passive optical elements. The CS rings need less fibre in all cases. It is fact on the other hand, that WDM solution needs more spare parts for optical interfaces because of the different wavelength of transmitters.
The SDH ADM costs are derived from the average market prices and are given in relative units. Since the configuration of SDH ADMs in classical and CS rings are different (there are duplicated optical interfaces in the latter case), the cost specifications are based on a functional equipment model. The elements of the model are:
- optical interfacing (shortly will be referred as optical costs)
- tributary interfacing, local cross-connecting and multiplexing (shortly will be referred as electrical costs).
An SDH ADM with two optical interfaces and with 16 STM-1s tributary capacity costs one relative unit. Extra costs are assigned to extra optical interfaces and to increased tributary capacities. It is difficult to specify the relative cost of an optical ADM (even in the used relative units), because the equipment is not commercialised yet. Instead of that the difference between PP or MSSP ring and CS ring cost is divided by the number of CS ring nodes on the logical ring layer and presented as a cost gap for OADMs in each active node. (If the cost gap is negative there are no savings on CS ring architecture comparing with PP or MSSP ring.) To dimension CS rings a simple heuristic was elaborated and implemented in frames of an integrated planning tool . The method tries to find proper order of nodes for each ring (if the cluster can be realised only with more than one ring system) and assign as much demand to the rings as possible in a greedy way taking into account node multiplication possibilities. The dimensioning of PP and MSSP rings was carried out by a complex software tool based on mixed integer-linear programming and simulated annealing [2].
Two scenarios are specified: Scenario 1 is to compare PP, MSSP and CS rings and Scenario 2 is to study the impact of different ADM tributary capacities on the CS ring dimensioning.
The comparison of classical SDH rings and CS ring are very sensitive for the ratio between the optical and electrical costs. Four sub-scenarios are specified with different optical-electrical cost ratios as 1:1, 1:2, 1:3 and 1:4.
The network cases are derived from realistic French and Hungarian networks.
The dimensioning is made under the following constraints and limitations:
- Maximum 8 wavelengths in a CS ring system are supposed. It limits the number of active nodes in a CS ring system in 8.
- No optical amplification is taken into account.
(Only metropolitan area networks are concerned.)
- A significant part of the demands relates to concentrated traffic, since the clusters are taken from dual-homing network structures.
- In Scenario 1 PP, MSSP and CS rings are STM-16 two-fibre ring architectures based on ADM-16s with 16 STM-1s tributary capacity.
- In Scenario 2 CS rings are realised with ADM-16s with 16, 24 or 32 STM-1s tributary capacity.
The summary of the network cases are given in Table 1 at the end of the paper.
7. ANALYSIS OF RESULTS
The good performance of CS ring architectures can be shown comparing the relative SDH ADM utilisation in different
cases. The theoretical lower bound of needed SDH ADMs is considered as 100%.
A simple lower bound can be calculated for the number of needed SDH ADMs from the total traffic of nodes and the ADM tributary capacity.
It can be depicted on Figure 5 that for ADMs with 16 STM-1 tributary capacity the lower bounds can be reached with CS rings in all studied cases, however, classical ring solutions exceed it with 10-100%.
Comparisons on equipment basis are informative concerning the flexibility and performance of Coloured Section Ring architecture, however the different equipment configuration in classical ring and CS ring makes difficult to draw general conclusions. Some cost comparisons are elaborated for that purpose.
Comparing the total network costs the CS ring is less expensive than the PP ring from cost ratio 1:2 in all studied cases. The savings on CS ring solutions are significant in all but one studied cases.
The comparison of MSSP ring and CS ring shows less differences. However, for the all but one studied network cases CS rings are less expensive from cost ratio 1:2. The savings are considerable in the two third of studied cases.
The cost of optical ADMs are not included in the network cost. To evaluate whether the savings are large enough to cover extra cost for OADMs the average differences between the PP or MSSP ring cost and CS rings cost per active node are
calculated.
In results of Figure 6 the SDH ADMs in the PP rings are with 16 STM-1s tributary capacity, in the CS rings are with 16, 24, and 32 STM-1s tributary capacity. The optical-electrical cost ratio is 1:2.
There is at least one CS ring solution for each network case, where the savings are far enough for OADM (OADM is supposed to cost 0.1 relative unit), and for the major part of network examples there are significant savings in the network costs.
In results of Figure 7 the SDH ADMs in the MSSP rings are with 16 STM-1s tributary capacity, in the CS rings are with 24 STM-1s tributary capacity. The studied optical-electrical cost ratios are 1:1, 1:2, 1:3, 1:4.
In that case the CS rings are less expensive for 7 networks (networks with 5 or 6 nodes). In network examples with 7 or 8 nodes the transit traffic is higher in the nodes, thus CS rings with 16 STM-1 SDH ADM tributary capacity provide economical solutions for that cases. For a network
0 20 40 60 80 100 120 140 160 180 200
1 2 3 4 5 6 7 8 9 10 11 12
Network Cases
Relatived ADM Capacity Utilisation [%] PP Ring [16]
MSSP Ring [16]
CS Ring [16]
Legend
Reference: the theoretical
minimum modular capacity
Figure 5 Needed SDH ADMs for different architectures
-0.20 -0.10 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
1 2 3 4 5 6 7 8 9 10 11 12
Network Cases Cost Gap for One Additional OADM
Trib.: 16 STM-1s Trib.: 24 STM-1s Trib.: 32 STM-1s Cost option: 1:2
Reference classical architecture:
PP i Legend
Figure 6 Difference between PP and CS ring costs
example with 4 nodes and large total traffic CS ring with 32 STM-1 SDH ADM tributary capacity gives the best solution. For detailed results see [5].
8. EVALUATION OF COLOURED SECTION ARCHITECTURE Based on the description of the architecture and the analysis of comparison case studies advantages and drawback of the Coloured Section Ring architecture can be summarised as follows:
8.1. Advantages
8.1.1. Increase of ring capacity
The use of wavelength routing allows an increase in ring capacity. By applying MSP for protection, no capacity is used inside the SDH STM-n frame for protection, thus leading to a full use of the capacity for working transmission. In addition to that, being able to build the logical ring according to the demand pattern reduces the transit traffic.
The choice of the node connections should be made in order to maximise the direct traffic between the neighbouring nodes and consequently to reduce the traffic transiting in nodes.
8.1.2. Existing ring upgrade
When the traffic increases and the installed networks are saturated, the main solution to provide additional transmission capacity are:
- To install new SDH rings
- To upgrade to more powerful TDM ring (e.g.
from an STM-4 ring to an STM-16 ring)
- To introduce WDM on the existing rings, while keeping the already installed equipment (only OADMs and MSP must be added).
The last solution, presented in this paper, has the benefit of reutilising the existing equipment. This is an asset for smooth network evolution.
8.1.3. Compatibility with SDH management The Coloured Section Ring is compatible with SDH ring management system. No new protection protocol is needed and therefore no upgrade of management software. Once the logical node order has been specified, the CS ring is considered by the management system as a standard SDH ring.
8.2. Drawbacks 8.2.1. WDM restraints
The use of a defined wavelength for each multiplex section on the ring implies the use of specific optical interfaces in the SDH ADMs with transmitters at a selected wavelength. Except in
case of wavelength conversion, this is the situation for every solution using WDM technology. The application of specific optical interfaces leads to operational constraints. The spare parts used for replacement of optical interfaces in case of failure must now be specific (with the same wavelength as the replaced optical interface). The multiplied number of spare parts bring additional cost to solutions using WDM technology.
8.2.2. Incomplete protection
The MSP only applies to protect coloured sections. Therefore, a total failure of a node (concerning the OADM or the SDH ADM) includes the loss of the traffic terminating in the node (that is unavoidable) but also the traffic transiting in the node. This problem does not exist with the classical SDH rings. The CS ring architecture does not preclude the use of path protection. The use of path protection in conjunction with the solution presented here solves the problem of transit traffic loss in case of a node failure. However, the application of path protection annihilates the major advantages of the original solution by using a spare in the frames for protection. Still, as the non-direct traffic can be limited by the proper choice of node connection order (logical ring), this solution can be attractive even in conjunction with path protection. Or in an other solution path protection can be reserved for very important traffic.
8.2.3. Optical power budget
The use of WDM includes additional constraints on the optical budget. OADMs add insertion losses that decrease the power budget. An important constraint is linked to the protection section propagating on the complementary arc which length is in the worst case equal to the ring perimeter minus the shortest distance between two neighbouring OADMs. This problem does not exist for small rings (e.g. rings in urban zones), but extra optical amplification is required for longer rings, causing extra costs.
-0.40 -0.20 0.00 0.20 0.40 0.60 0.80
1 2 3 4 5 6 7 8 9 10 11 12
Network Cases Cost Gap for One Additional OADM
Cost opt.: 1:1 Cost opt.: 1:2 Cost opt.: 1:3 Cost opt.: 1:4 SDH ADM trib. capac.:
24 STM-1s Reference classical arch.:
MSSP ring Legend
Figure 7 Difference between MSSP and CS ring costs
More detailed calculation can be found in [4].
9. CONCLUSIONS
Coloured Section Ring architecture provides a new method to increase self-healing SDH ring capacities. With the applied protection technique and the new levels of optimisation as logical order of nodes and multiplication of nodes the ring capacity is increased and a more effective utilisation of SDH ADM capacities can be achieved than in classical two-fibre SDH rings.
The cost of Coloured Section Ring is very sensitive for the cost of optical tributaries because of the applied protection technique. However, in a wide cost range the CS ring is less expensive than the classical path protected or multiplex section shared protected ring in many network applications. The presented cost comparison shows that CS rings can be realised with a lower cost in a metropolitan network area in most cases.
In case of large total originating and terminating demands of nodes the application of SDH ADMs with 24 STM-1s tributary capacity in coloured section rings is cost effective.
Because of the good compatibility with the standard SDH equipment the coloured section ring seems to be a good alternative in the near future to upgrade existing SDH rings or install new ones at a low cost.
ACKNOWLEDGEMENT
The contribution of Tivadar Jakab to the presented work was supported partly by France Telecom CNET partly by the COPERNICUS 1463 ATMIN project.
REFERENCES
[1] L. Blain, F. Chatter, A. Hamel, A. Sutter and V.
Tholey: " Increased capacity in a MSP protection ring using WDM techniques and Optical ADM: the coloured section ring "
accepted in Electronic Letters
[2] ALOA Algorithmes pour L'Optimisation d'Anneaux Description des fonctionnalites d'un prototype d'optimisation de reseaux SDH en Anneaux. Document d'Etude DE/ATR/ORI/95 France Telecom CNET
[3] A. Hamel et al: " Multilayer add-drop multiplexers in a self healing WDM ring network, OFC'95, pp 84-85.
[4] L. Blain et al: " WDM technique and MS protectionin a standard ring: the 'coloured section' ring", Proceedings of NOC'96, ICO Press 1996, Vol 1, pp 255-259.
[5] L. Blain, A. Hamel, T. Jakab and A. Sutter: "
Comparison of classical and WDM based ring architecture", Proceedings of NOC'96, ICO Press 1996, Vol 1, pp 260-268.
Table 1 Summary of network cases
Network case 1 2 3 4 5 6 7 8 9 10 11 12
Number of nodes 4 4 5 5 6 6 6 6 7 7 8 8
Total demand [STM-1] 35 65 37 68 41 45 70 75 37 69 31 65 Hubbed traffic [%] 89 89 92 95 83 91 87 92 70 65 62 60
Meshed traffic [%] 11 11 8 5 17 9 13 8 30 35 38 40
