CONVERGENT TRANSPORT&IP NETWORK GROUPTRANSMISSION TEAMSynchronous Digital HierarchyHoria Stefanescu

Topics

ResponsabilitiesSDH vs. PDHSDH OverviewProtection SchemesExample of synchronizationManagement of SDH equipmentsEthernet over SDHOptical fiber

Responsibilities of the SDH TRANS DepartmentProvisioningReconfiguration and OptimizationsProactive network design architecture to support traffic demandsSynchronizationManagementResilienceQoSInteraction with other departments (NOC, FSF, BSS, IP, NSS etc)

SDH vs PDHSDH:SDH offers higher capacity used for backhaul and backboneDirect extraction of tributary flowsEnd to end provisioning possible (no site intervention required for intermediate hops)Protection schemes available (ports & path)Interconnection with other equipments (e.g. MSC)

PDH:Used for the local loopNo synchronization planning neededNo additional drop equipments needed

SDH OverviewThe bit rate of the lowest level SDH signal is 155.520 Mb/s STM-1 (Synchronous Transport Module -1)

To obtain high-order signals it is necessary to multiplex lower level signal, the technique used is Byte interleaved MultiplexingSDH is a synchronous technology digital signals in all the network elements in a SDH network are controlled by a master clock

SDH provides universal standards for both electrical and optical interfaces, including standards on digital signal rate levels, frame structure, multiplexing method, monitoring and management => SDH equipment from different vendors can be easily interconnected.

In SDH low rate signals are directly dropped/inserted from high-rate signals.STM-NSTM-N Abundant overhead bits for OAM function are arranged in the frame structure of SDH signal => supervision on SDH possible.

STM-1 frame has 9 rows and 270 columns, every intersection is a byte.Base SDH rate =9 rows x 270 columns x 8 bits x 8000frames/s=155.52 Mb/sSDH networks are subdivided into various layers: physical layer (transmission medium), regenerator section (between regenerators or between a regenerator and a transport node), multiplex section.

Frame structure

The RSOH (regenerator section overhead) and the MSOH (Multiplex Section Overhead) monitor the corresponding section and layers.POH (Path Overhead ) is used to monitor the tributary signals on a real-time basis. The POH together with the payload form the Virtual Container (VC).Payload transports the tributary information.AU-PTR indicates the start location of the payload. This is needed because SDH uses an elegant way of multiplexing - based on pointers, without the need to align the containers to a common frame start. Thus the frame buffers in SDH multiplexing equipment can be kept small and the transmission delay due to buffering is minimized.

SDH ANALOGY

SDH Multiplexing Hierarchy

Example of Multiplexing Method140Mb/s -> STM-1

34 Mb/s ->STM-1

Network topologyStar

Major disadvantage: Single Point Failure, dont offer protectionAdvantage: Short transmission pathRingAdvantage: Offers protection (2 paths)Disadvantage:dependencesMeshAdvantage: Offers protection multiple pathsDisadvantage:Dependences, expensive to implementChainDisadvantage: no protections, dependences

PDH/SDH as transport for BSS

Protection schemesSNCP Subnetwork Connection Protection (LO & HO)This is the most used scheme of protection in ORO network which is mainly structured as a ring network.

Other schemes of protection are: MSP, MS-SPRING. These are for High Order.

When using SNCP the input signal is simultaneously fed into both the working and the protection fiber (double capacity needed). The receiving node selects the signal from the working fiber.

Example: Craiova local ring

Alias: CRA_5/1-MFS_CR1_GPRSTransport Level:E1Operational Status:Active

Example of SDH Equipment Alcatel 1660SM

Example of synchronization on a SDH nodeFollows a master slave architectureGeneral rule: A slave clock by a certain quality must never be synchronized from a lower quality signalAccording to this rule the node will emit a DNU (Do Not Use) message from the port on which it receives synchronizationIt is very important to avoid synchronization loops

Every SDH node contains a SDH Equipment Clock (SEC)T0 signal for internal clockT1 input synchronization signal extracted from STM-n signals;T2 input synchronization signal extracted from 2048 kbps signalsT3 input synchronization signal extracted from 2048 kHz signals;T4 output synchronization signalOSC internal oscillator;SETG Synchronous Equipment Timing Source;SQUELCH T4 signal is cut if the quality is below a certain threshold

The selection between the synchronization sources available at a node is made considering first the quality and then the prioritySynchronization status messaging (SSM) is a SONET and SDH protocol that communicates information about the quality of the timing source. SSM messages are transported as follows: If SSM is carried over an optical line, for both SONET and SDH the SSM is transported in the S1 byte. If SSM is carried over an electrical line: for SDH, the SSM is transported in the Sa bit of E1. for SONET, the SSM is transported in the outband loop code.The SSM messages enable SONET and SDH devices to select the highest quality timing reference automatically and to avoid timing loops.

Management of SDH equipmentsThe SDH equipments can be managed In-Band or Out-of-BandIn-Band management uses the D1-D12 bytes from SOHOut of Band management uses a dedicated SDH Data Communication NetworkEvery node is identified using an Network Service Access Point addressThe routing protocol used is OSI IS-IS: the SDH elements represent Level-1 nodes and the routers represent Level-1-2 nodes.Level 1 Intermediate Systems (IS) deliver and receive NPDUs from other systems, and relay NPDUs from other source systems to other destination systems. They route directly to systems within their own area, and route towards a Level 2 IS when the destination system is in a different area.

Level 2 Intermediate Systems (IS) routes between different areas.Level1-2 Intermediate Systems (IS) provides the transit from Level-1 IS to Level-2 IS.

SDH DCN

Ethernet over SDHSDH Virtual Ports The SDH path can be formed by concatenating VCs of the same typeThe advantage of using Ethernet is that on the same SDH pipe more customers can share the bandwidth among them by using traffic descriptors and traffic classifiers.Another advantage is that most customers use an Ethernet port rather than a TDM portAs for the SDH circuits, for EoS circuits also exists a provisioning tool

Types of architectures

ExampleThe different services are classified at the edge points via L2 processing (VLAN-Id and/or P-Bits) and identified and segregated in the transport network via a stacked Service VLAN (S-VLAN). It is also possible to handle Class of Services (Guaranteed, Best Effort and Regulated traffic) and to filter out undesired C-VLAN values. The fine bandwidth assignment is controlled at L2 via Traffic Descriptors. A shared SDH is used for trunking and L1 protection. The total bandwidth available is determined by the capacity of the SDH trunk. Topology: Point-to-Point

Transmission on fiber

Optical fibers are made of extremely pure optical glass. The glass in an optical fiber has far fewer impurities than window-pane glass. Fiber attenuation is caused by scattering, absorption and bending.Scattering reflection of small amounts of light in all directions as it travels down the fiber. Some of this light escapes out of the core, while some travels back toward the source. Some scattering is caused by miniscule variations in the composition and density of the optical glass material itself. Also, fibers with increased dopant concentration exhibit more scattering and greater attenuation than fibers with less dopant in the core. That is why multimode fibers, with their higher level of dopant in the core, have higher attenuation than single-mode fibers.

Absorption occurs when impurities, such as metal particles or moisture, are trapped in the glass.These cause attenuation at specific wavelengths by absorbing the light at that wavelength and dissipating it in the form of heat energy. Moisture occurs more naturally in fiber, and accounts for the rise in attenuation at the "water peak found near 1385 nm. This is why fibers were traditionally not used in this wavelength region.

Bending occurs in two forms - microbending and macrobending. Microbends are microscopic distortions along the length of a fiber, typically caused by pinching or squeezing the fiber. Microbends deform the fiber's core slightly, causing light to escape at these deflections. In single-mode fiber, microbending is wavelength-dependant, with microbend sensitivity increasing as you move to higher wavelengths like 1550 nm, and especially 1625 nm. In multimode fiber, microbend sensitivity is relatively constant across the wavelength spectrum. Macrobending occurs when a fiber is bent in a tight radius. The bend curvature creates an angle that is too sharp for the light to be reflected back into the core, and some of it escapes through the fiber cladding, causing attenuation.

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