1 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. Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 3 Presentation Outline • Revision of PDH technology • The SDH Hierarchy • Frame Formats • Traffic Management with SDH • Network architectures • SDH network design methodologies
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Introduction to SDH/SONET - Massey University · PDF file1 Introduction to SDH/SONET Professor Richard Harris Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 2 Objectives
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• 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.
• 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
• 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.
• 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.
• 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).
• 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.
• 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.
• 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.
• 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.
• 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)
• 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
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
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
• 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:
• 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".
• 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.)
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
• 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.
• 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.