L2 Network Element, Topology

L-2 SDH NETWORK ELEMENT AND TOPOLOGY

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Basic SDH Network Elements •

SDH Regenerator

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Line Terminal Mux (LTM)

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Add Drop Mux (ADM)

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Synchronous Digital Cross Connect System (SDXC)

Having introduced you to the concept of an SDH Network, lets now take a look at the network “building blocks” and how they are configured. These network elements are now all defined in CCITT standards and provide multiplexing or switching functions. Line Terminal Multiplexers – can accept a number of tributary signals and multiplex them to the appropriate optical SDH at carrier, i.e. STM–1, STM–4 or STM–16. The input tributaries can either be existing PDH signals such as 2, 34 and 140 Mb/s or lower rate SDH signals. LTMs form the main gateway from the PDH network to the SDH. Add–drop Multiplexers – a particular type of multiplexer designed to operate in a through mode fashion. Within the ADM, it is possible to add channels to, or drop channels from the “through” signal. ADMs are generally available at the STM–1 and STM–4 interface rates and signals, i.e. 2, 34 or 134 Mb/s. The ADM function is one of the major advantages resulting from the SDH since the similar function within a PDH network, required banks of hardwired back–back terminals. Synchronous DXC – these devices will form the cornerstone of the new synchronous digital hierarchy. They can function as semi–permanent switches for transmission channels and can switch at any level from 64 kb/s up to STM–1. Generally, such devices have interfaces at STM–1 or STM–4. The DXC can be rapidly reconfigured under software control, to provide digital leased lines and other services of varying bandwidth.

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Regenerator – for SDH transmission over 50 km, regenerators are required with spacing dependent on the transmission technology (i.e. operating wavelength, receive, etc.). These are not just simple signal regenerators but have alarm reporting and performance monitoring capability. Since all network elements have alarm reporting capability, a fault can be isolated quickly to the individual transmission section with the problem.

Figure /G 958 Description of the regenerator timing functions An SDH regenerator shall not generate more than 0.01 UI rms jitter, with no jitter applied at the STM-N input 2.Regenerator Operation Figure illustrates the timing functions for regenerators.

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The regenerator timing generator (RTG) includes an internal oscillator. In normal operation, the SPI function recovers the timing from the incoming STM-N signal at reference point A and passes the data and timing to RST at reference point B, and passes the timing signal also to the RTG function at reference point T1. The RTG function provides the timing signal to the outgoing STM-N signal at reference point T0. The directionality of the timing signals is maintained. When transmitting MS-AIS, the RTG shall provide timing for the outgoing STM-N signal at reference point T0 using the internal oscillator. The long-term frequency stability of the internal oscillator in free-running mode shall be equal to or better than ± 20ppm. The RTG and SPI functions must accommodate timing from an incoming MS-AIS signal.

SDH Regenerator

Fig. •

The most basic element is the regenerator. You’ll find regenerators whenever transmission over 50 km is needed. They terminate and regenerate the optical signal. Spacing of regenerators depends on the wavelength being used, the power of the transmitted signal and the receiver’s sensitivity.

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Wavelengths of 1310 nm and 1550 nm are preferred because glass fibre is peculiarly transparent to light at these wavelengths. 1550 nm is preferred for long routes because even though the 1550 nm lasers cost

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more, the fibre is even more transparent at 1550 nm than 1310 nm and so lower regenerators are needed. •

The further the signal has to go, the greater the transmitted power and the more sensitive receivers have to be.

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That’s why fibre systems are described as short, intermediate and long reach systems. The standards define transmitted optical power and receives sensitivity for each type of system.

Line Terminal Mux:

Fig. •

The Line Terminal Mux will take a range of input tributaries, either 2, 34, 140 Mb/s or STM–1 and multiplex them onto a high rate optical carrier, i.e. STM–4 or STM–16.

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As an option, a Line Terminal Multiplexer may have a secondary terminal interface for internal (1+1) protection switching.

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Depending on the required regenerator spacing, optical interfaces of both 1310 nm and 1550 nm are generally available (1550 nm has lower attenuation characteristics and, therefore, supports greater regenerator spacing).

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Additional options on Line Terminal Multiplexer equipment provide for access to the orderwire channel (voice) and the Data Communication Channels (DCC).

Add Drop MUX:

Fig. The Add Drop Mux (ADM) is the basic SDH building block for local access to synchronous networks. It generally offers STM–1 interfaces (the next generation of ADMs will offer STM–4) and operates in a thru–mode fashion. A wide variety of plesiochornous tributary signals, such as 2 Mb/s can be added too or dropped from this “thru” STM signal. This capability is one of the key benefits provided by synchronous systems since ADM elements support a function that previously took banks to back–back equipment (i.e., a mux/demux chain). The ADM with its “thru–mode” capability adds a new dimension to network designs and can be formed into local access synchronous rings. Such network topologies will be discussed in more detail later.

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What is Add/Drop Mux ?

Fig. –

Add/Drop Mux is a Network Element which allows configurable add/drop of a subset of a payload (e.g. 2 Mbps traffic channels) tr from a higher rate data stream (e.g. 155 Mbps STM–1 traffic)

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In contrast with normal multiplexer, in which a high speed signal must be completely demultiplexed to some intermediate stage, at the minimum before access to a portion of signal can be achieved, on ADD/DROP Multiplexer allows access to the high speed signal directly and selects traffic channels.

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Will be terminating 64–2.048 Mbps or 3, 34.368 Mbps channels or a mix of them at TM.

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Access provided to 2.048 channels (any from 1 to 63) or 34.368 Mbps channels (any from 1 to 3) at ADM through software control.

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Add Drop MUX in a Network 1. In Tandem Configuration SITE A 2.048 Mbps 34.368 Mbps

TERMINAL MULTIPLEXER

SITE B 155 Mbps 155 Mbps

2.048/34.368 Mbps

ADD/DROP MULTIPLEXER

2.048/ 34.368 Mbps

2.048/ 34.368 Mbps

155 Mbps 155 Mbps

155 Mbps

ADM

155 Mbps

2.048/ 2.048/ 34.368 34.368 Mbps Mbps

2.048/ 34.368 Mbps

TM

2.048/ 34.368 Mbps

Fig.

Synchronous DXC •

The synchronous DXC functions as a semi–permanent switch for varying bandwidth transmission channels, i.e. 2 Mb/s  155 Mb/s (STM–1). Under software control, the cross–connect devices can pick out and reroute one or more lower order channels from the transmission signal without the need for demultiplexing. It is this capability which makes the digital cross connect such a powerful tool, allowing rapid configuration of the transport network to provide digital leased lines and other services.

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DXC devices are classified in terms of their line interface and switching level, i.e. a DXC 4/4 will have interfaces at STM–1 (or 140 Mb/s) and switch at the STM–1 (140 Mb/s) level, whereas a cross connect at the 64 kb/s channel level.

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Fig. •

The DXC 4/3/1 device will be used extensively to replace the digital distribution frames (DDF) which are used in present day digital exchanges. This will eliminate the network problems that result from faults in the wiring and rewiring of DDFs.

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Fig. Synchronous Cross–Connect

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2 x 2 DACS (Digital Access and Cross Connect Switch)

Fig. •

It can be seen from the diagram of 2x2 DACS that a 2 MB can be dropped from the STM–1 #1 east line and can be added on STM–1 #2 west line and vice–versa. This kind of functionality where a payload gets cross connected to other line is called DACS.

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One can visualise 2 x 2 DACS as two ADMs put in the form to crossconnect their payload at DDF. In DACS both ADMs are located in the same box. Note that 32 x 32 DACS can be seen as 32 ADMs arranged as shown.

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Supervision in Optical Domain Optical Supervisory Channel : •

Equivalent to ECC in SDH.

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1510 nm (1480, 1510 nm) are standarized.

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Terminated/regenerated at each station.

Optical Cross–Connect (OXC) •

Cross–connect with ‘N’ Inputs to ‘M’ Outputs.

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Each channel of OXC is transporting WDM channels.

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Functions : – – –

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Fibre Switching. Wavelength Switching. Wavelength Conversion.

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NETWORK TOPOLOGY •

Point–to–point link

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Bus Topology

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Ring Topology –

Collapsed ring

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Nested ring

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Hub Topology

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Star Topology

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Mesh Topology

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Mesh & Ring Topology

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Having identified and explained the current set of

network building blocks, we will now look at the various methods of constructing SDH networks in practice. •

Initially, SDH technology will be deployed in new

installations and then to replace or upgrade existing systems when they reach maximum capacity. At the simplest level, new point–to– point systems will use SDH Terminal muxes with the ability to expand to more complex SDH constructions later. We will now examine each possible topology in turn.

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Fig. Network Topology : Terminology

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Point to Point

Fig. •

SDH Line Systems are natural successors to the 140 Mb/s and 565 Mb/s line systems currently deployed in backbone networks. In new installations, these PDH capacities will commonly be replaced by STM–4 (622 Mb/s) line systems. Increasingly, STM–16 (2.4 Gb/s) line systems will be required to cater for the ever increasing bandwidth requirements of backbone networks.

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Since SDH systems will begin to appear in specific routes or overlay networks within the existing transmission network, co–existing with 140 Mb/s and 565 Mb/s systems, an issue of major importance will be the network management. This will have to cover the whole transmission network, including both the SDH and PDH parts.

Linear Network (BUS TOPOLOGY)

Fig.

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SDH NEs and be joined to form the Linear network as shown. The Network has LTM which marks the start of the SDH network and in between there can be add drop offices. The line protection can be given with the standby line for failure against fibre. The payload can be any of the PDH rate or the SDH line lower rate.

Rings Fig. •

The definition of the Add–Drop Multiplexer function makes SDH special because it allows operators to make rings of ADMs which can add and drop channels at any node. Rings are great because they give greater flexibility in the allocation of bandwidth to the different users and they allow rerouting of traffic should a link fail.

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Under normal operation, a 2 Mb/s tributary is sent round the ring in both the directions. The ADM assigned to drop the 2 Mb/s tributary monitors the two SDH signals for errors and delivers the one with better performance. This is known as path switching.

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When a catastrophic failure occurs, for example, when the fibre is cut by a road digger, the nodes either side of the failure loop the clockwise ring to the anticlockwise ring, allowing traffic to avoid the failed ring segment. This forms an extended ring which carries all the traffic to each node in the ring, allowing service to continue.

SDH Ring Topology – Highly survivable in nature. – Cost benefits Point to Point. Fibre Installation may be costlier. – Number of NEs will be less compared to Point–to–Point links. – Modified NEs are building blocks. STM1 Topology Ring Topology

Fig.

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Types of Ring Configurations – Single fibre rings. – Two fibre uni–directional rings. – Two fibre bi–directional rings. – Four fibre bi–directional rings. Single Fibre Rings – No protection possible in case of Link/Equipment failure. – Total traffic handling capability cannot exceed 63 for 2.048 Mbps or 3 for 34.368 Mbps. – Only uni–directional operation supported. Self Healing Ring (SHR) Ring – – Benefits –

Collection of nodes forming a closed loop. Each node is connected by duplex commn. facility.

Uses redundant bandwidth and/or equipment to restore disrupted services automatically.

Multiplexing devices used in the ring : ADMs

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SHR Architecture

Fig. USHR •

Working traffic is carried around the ring in one direction only.

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Ring capacity is sum of demands between nodes.

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Also called “Counter–Rotating–Ring”; traffic in prot. rotates opposite.

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1:1 (USHR/L); extended to 1:N, then not entirely self–healing.

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1+1 (USHR/P).

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Fig.

USHR/L •

Incoming and returning signal routed unidirectionally on working ring.

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On failure, adjacent nodes perform fold or looping function.

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Basic ADMs used (TSI not needed).

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Fig.

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USHR/P •

Based on concept of 1+1 protection.

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Traffic goes on a pair of fibres in opposite directions.

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Both receive signals monitored for alarms; only one used.

Mechanism –

Detection of LOS or line AIS

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Line AIS triggers Path AIS.

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Path AIS triggers prot switch.

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Detection of Path AIS on both side ? Multiple failure.

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Basic ADMs used : TSI not required.

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Form of channel switching; APS protocol (K1 and K2) not required.

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BSHR •

Working traffic travels in both direction between nodes.

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Two fibres required between the nodes.

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BSHR may use 4 or 2 fibres depending on spare capacity management.

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Can be in 1:1 or 1:N; (1:N is not entirely self–healing).

Fig.

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BSHR (2 Fibre) •

Working and prot. channels use same pair of fibres.

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Half of the bandwidth is reserved for protection.

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Traffic evenly split into outer and inner rings, filling half of the TS.

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On fibre break/equipment failure traffic switched to vacant TS.

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ATMs should have TSI capability.

Fig.

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BSHR (4 Fibre) • • • •

2F for normal and 2F for protection service. Prot. Swg. triggered by detection of failure at line level (using K1 and K2). Two basic ADMs required at each node, for Working and Prot. Schemes : – Loop back scheme : Prot against cable cut only; less conf complexity. – Loop back with span prot : Prot against fibre cut and equipment failure, more complex.

Fig.

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Mesh

Fig. •

As the SDH Network expands, the higher rate combination of Digital cross connect switches (DXC) and point–to–point optical interconnections wall form the “backbone” of future core networks.

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The SDH DXCs will connect in a “mesh” to give route diversity. The simplest arrangement will be 3DXC devices interconnected. If the direct links from one DXC to another fail, the alternative route via the third DXC will still be available and changes to circuit routing will be possible in milliseconds.

Mesh and Rings – The Ultimate Configuration

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When you add rings of ADMs to the “Mesh” structure of the network backbone, you have the ultimate flexibility of an SDH network. Route diversity will ensure network protection and survivability. Flexible software control of network elements will speed up new service provisioning and bandwidth management.

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In the future, we can envisage metropolitan SDH ring structures, around major towns and cities, for example, which provide the access network that connects corporate customers, cellular services and residential user multiplexers in the meshed network.

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In the future, local MAH (Metropolitan Area Network) and BISDN (Broadband ISDN) nodes will also interface to these SDH rings.

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At each “Network Node Interface”, the interworking of different vendors equipment should be assured if the equipment complies to the standards. However, there will likely be misinterpretations of the standards (particularly about overhead functions) which will require test equipment to resolve.

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The telecommunications network is becoming more and more software dependent. Just as happened with the AT&T networks in the US, when it failed due to an SS7 software malfunction, the reliability of the SDH management and control software will be paramount. Testing to eliminate software “bugs” will be essential to ensure network integrity.

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Such testing will be needed each time a new software revision is developed – potentially many times in the file to network element hardware.

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