l2 network element, topology

8/6/2019 L2 Network Element, Topology

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Ntw Elements & Topology

Basic SDH Network Elements

SDH Regenerator

Line Terminal Mux (LTM)

Add Drop Mux (ADM)

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. STM1, STM4 or

STM16. 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.

Adddrop 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 STM1 and STM4 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

backback terminals.

Synchronous DXC these devices will form the cornerstone of the new

synchronous digital hierarchy. They can function as semipermanent switches

for transmission channels and can switch at any level from 64 kb/s up to STM1.

Generally, such devices have interfaces at STM1 or STM4. The DXC can be

rapidly reconfigured under software control, to provide digital leased lines and

other services of varying bandwidth.

BRBRAITT, Jabalpur, 2


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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. Youll 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

receivers sensitivity.

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.

Thats 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 STM1 and multiplex them onto a high rate optical carrier,

i.e. STM4 or STM16.

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

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 STM1 interfaces (the next

generation of ADMs will offer STM4) and operates in a thrumode 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

backback equipment (i.e., a mux/demux chain). The ADM with its thrumode

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 STM1 traffic)

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.

Will be terminating 642.048 Mbps or 3, 34.368 Mbps channels or

a mix of them at TM.

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

Fig.

Synchronous DXC

The synchronous DXC functions as a semipermanent switch for varying

bandwidth transmission channels, i.e. 2 Mb/s 155 Mb/s (STM1).

Under software control, the crossconnect 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.

DXC devices are classified in terms of their line interface and switching

level, i.e. a DXC 4/4 will have interfaces at STM1 (or 140 Mb/s) and

switch at the STM1 (140 Mb/s) level, whereas a cross connect at the 64

kb/s channel level.


SITE A SITE B

TERMINALMULTIPLEXER

ADD/DROPMULTIPLEXER ADM TM

155 Mbps 155 Mbps 155 Mbps

155 Mbps155 Mbps155 Mbps

2.048/34.368 Mbps2.048/34.368Mbps

2.048/34.368Mbps

2.048/34.368Mbps

2.048/34.368Mbps

2.048/34.368Mbps

2.048/34.368Mbps

2.048 Mbps

34.368 Mbps


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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 CrossConnect



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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 STM1 #1 east line and can be added on STM1 #2 west line

and viceversa. This kind of functionality where a payload gets cross

connected to other line is called DACS.

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.

1510 nm (1480, 1510 nm) are standarized.

Terminated/regenerated at each station.

Optical CrossConnect (OXC)

Crossconnect with N Inputs to M Outputs.

Each channel of OXC is transporting WDM channels.

Functions :

Fibre Switching. Wavelength Switching. Wavelength Conversion.



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NETWORK TOPOLOGY

Pointtopoint link

Bus Topology

Ring Topology

Collapsed ring

Nested ring

Hub Topology

Star Topology

Mesh Topology

Mesh & Ring Topology

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 pointto

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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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.

RingsFig.

The definition of the AddDrop 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.

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 PointtoPoint links. Modified NEs are building blocks.

STM1 Topology

Ring Topology

Fig.



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Types of Ring Configurations Single fibre rings. Two fibre unidirectional rings. Two fibre bidirectional rings. Four fibre bidirectional 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 unidirectional operation supported.

Self Healing Ring (SHR)

Ring Collection of nodes forming a closed loop.

Each node is connected by duplex commn. facility.

Benefits 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.

Ring capacity is sum of demands between nodes.

Also called CounterRotatingRing; traffic in prot. rotates opposite.

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

1+1 (USHR/P).



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Fig.

USHR/L

Incoming and returning signal routed unidirectionally on working ring.

On failure, adjacent nodes perform fold or looping function.

Basic ADMs used (TSI not needed).



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Fig.



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USHR/P

Based on concept of 1+1 protection.

Traffic goes on a pair of fibres in opposite directions.

Both receive signals monitored for alarms; only one used.

Mechanism

Detection of LOS or line AIS

Line AIS triggers Path AIS.

Path AIS triggers prot switch.

Detection of Path AIS on both side ? Multiple failure.

Basic ADMs used : TSI not required.

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



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BSHR

Working traffic travels in both direction between nodes.

Two fibres required between the nodes.

BSHR may use 4 or 2 fibres depending on spare capacity management. Can be in 1:1 or 1:N; (1:N is not entirely selfhealing).

Fig.



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BSHR (2 Fibre)

Working and prot. channels use same pair of fibres.

Half of the bandwidth is reserved for protection.

Traffic evenly split into outer and inner rings, filling half of the TS. On fibre break/equipment failure traffic switched to vacant TS.

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 pointtopoint optical interconnections wall

form the backbone of future core networks.

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.

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.

In the future, local MAH (Metropolitan Area Network) and BISDN

(Broadband ISDN) nodes will also interface to these SDH rings.

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.

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.

Such testing will be needed each time a new software revision is

developed potentially many times in the file to network element

hardware.

l2 network element, topology

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