What is Add Drop Multiplexer?

This is the most common element to built up SDH rings.Dependent on the port configuration it gives access to all containers embedded in the STM-1 signals. These containers can be dropped and inserted in the SDH hierarchy.

Each tributary can also be inserted into each container of the STM-1 streams.

An ADM155 for example is a multiplexer, which handles 126 x 2Mbps tributaries
within 2 independent STM-1 datastreams (doublering).

Also the exchange of the containers between the STM-1 signals is possible.

The ADMs are controlled via TMN commands in embedded channels or the via
the Q-interface.

It has a built in switching unit, which supports Automatic Protection Switching
(APS) , if one line has interruptet.

What is Synchronous Cross Connect?

Digital Cross Connects(DXCs) handle signals of PDH and/or SDH technologies.They offer several inputs for those bitrates which are present as port cards in a specific hardware configuration of this network element.Today its possible to apply signals up to STM-4, in the future 2.5Gbit/s will bepossible.A cross connect extracts containers from all incoming signals. It can rout all incoming signals (or parts of them) to each outgoing signal, from one hierarcye to another, from SDH to PDH and vice versa.

Normally switching takes place at the basic container levels (VC12, VC3, VC4). Some DXCs offer access down to 64kbps by the help of byte-synchronous mapping.DXCs are located at such places where several different signals are coming together, with different bitrates. It also interconnects local levels to long distance levels, e.g. ADM rings on SLX or PDH on SLX. Its controlled via workstations and TMN and handles also all alarms and status information of different hierarchies.

A DXC requires fully structured signals (PDH 140/34/8/2Mbps) of the corresponding bitrate, otherwise it will generate alarms and measurements are more difficult or impossible.

DXCs are very complex machines - therefore the goal of most operators is to minimize the amount of DXCs and to replace them by the more cheeper Add Drop multiplexers (ADMs) wherever it's possible.

What is Digital Cross Connects?

Digital cross connectors (DXCs)

In the area of the subscriber network nodes the users are connected to the exchanges
(DSC) via the user network interface (UNI). Instead of this central switching
points local cross connects (DXC) should be used.

Cross connect systems are the heart and as such they determine the structure of the trunk network. The physical specifications for the NNI are contained in Re. G.703 (electrical) and G.957(optical).

Jitter and Wander Measurements

• Network output jitter (G.825)
• Network element output jitter (G.783, G.813)
• Jitter transfer function (G.958)
• Jitter and Wander tolerance (G.825, G.813)

How to Measure Jitter?

A jitter meter test set is basically made up from the following items:

• Pattern clock converter
• reference clock generator
• phase meter
• weighting filters
• peak value detector

What is the Source of Jitter?

•Interference signals
Impulsive noise or cross talk may cause phase variations (non systematic jitter). Normally high frequency jitter.
•Pattern dependent jitter
Distortion of the signal lead to so-called inter-symbol interference, which is pulse cross talk that varies with time (Pattern dependent jitter.
•Phase noise
The clock regenerators in SDH systems are generally synchronized to a reference clock. Some phase variations remain, due to thermal noise or drift in the oscillator used.
•Delay variation
Changes in the signal delay times in the transmission path lead to corresponding phase variations. These variations are generally slow (Wander). (e.g. Temperature changes in optical fibers).
•Stuffing and wait time jitter
During removing of stuffing bits gaps have to be compensated out by a smoothed clock.
•Mapping jitter
•Pointer jitter
During incrementing or decrementing of the pointer value. This shifts the payload by 8 or 24 bits corresponding to a phase hit of 8 or 24 UI.

What is Jitter?

Jitter

Periodic or random changes in the phase of the transmission clock referred to the master or reference clock. In other words, the edges of a digital signal are advanced or retarded in time when compared with the reference clock or an absolutely regular time framework. Jitter generally referes to deviations of more than 10Hz.

What is STM-1 frame structure ?

As indicated in the figure, the STM – n signal is multiples of frames consisting of 9 rows with 270 bytes in each row.
The order of transmission of information is first from left to right and then from top to bottom.
The first 9 bytes in each row are for information and used by the SDH system itself.This area is divided into 3 parts.

Regenerator Section Overhead(RSOH)
Multiplex Section Overhead(MSOH)
Pointers

What are Disadvantage of PDH?

• Inflexible and Expensive for Telecommunication Networking
• Inefficient use of transmission capacity (typically 75% fill per stage =>0.75e3=42% fill at 140 Mbps
• Extremely limited network management and maintenance support capabilities
• Higher rates are proprietary

what is SDH Transport?

• SDH was initially optimized for voice transport
• The huge demand of bandwidth for data and storage networks created new requirements
• Transport of Ethernet, Fibre Channel and Escon More efficient use of bandwidth for new services (finer granularity):

Example: A Gigabit Ethernet consumes a full STM-16 (i.e. 2.4 Gbps),
resulting in a 42% utilization

The answers

• Virtual Concatenation (VC): Allows the bandwidth to be adapted to the service requirements
• Link Capacity Access Scheme (LCAS): A signalling protocol to adjust the transport bandwidth dynamically
• Generic Framing Procedure (GFP): Adapts a frame or cell oriented service to a byte stream service.

What are SDH Standards?

Relevant Standards:
• ITU-T G.703 Physical/electrical Characteristics of Digital Interfaces
Plesiochronous Digital Hierarchy (PDH)
• ITU-T G.732/G.744/G.751
Synchronous Digital Hierarchy (SDH)
• ITU-T G.707 Network Node Interface for the Synchronous Digital Hierarchy
• ITU-T G.783 Characteristics of SDH Equipment Functional Blocks
• ITU-T G.784 SDH Management
• ITU-T G.957 Optical Interfaces for Equipment ans Systems relating to SDH
• ETS 300 147 Synchronous Digital Hierarchy Multiplexing Structure
• ETS 300 417 Generic functional requirements for SDH equipment

What are SDH virtual Containers?

• Once a container has been created, path overhead byte are added to create a virtual container.
• Path overheads contain alarm, performance and other management information.
• A path through an SDH network exists from the point where a PDH signal is put into a container to where the signal is recovered from the container.
• The path overheads travel with the container over the path.

What are Possbile SDH Rates?

SDH is a transport hierarchy based on multiples of 155.52 Mbit/s

The basic unit of SDH is STM-1:
STM-1 = 155.52 Mbit/s
STM-4 = 622.08 Mbit/s
STM-16 = 2588.32 Mbit/s
STM-64 = 9953.28 Mbit/s

Each rate is an exact multiple of the lower rate therefore
the hierarchy is synchronous.

PDH vs. SDH Hierarchy.

PDH transmission rates:
 SDH is designed to unify all transmission rates into a single Mapping hierarchy

What is PDH: Plesiochronous Digital Hierarchy?

Plesiochronous Multiplexing

• Before SDH transmission networks were based on the PDH hierarchy.Plesiochronous means nearly synchronous.
• 2 Mbit/s service signals are multiplexed to 140 Mbit/s for transmission over optical fiber or radio.
• Multiplexing of 2 Mbit/s to 140 Mbit/s requires two intermediate multiplexing stages of 8 Mbit/s and 34 Mbit/s.
• Multiplexing of 2 Mbit/s to 140 Mbit/s requires multiplex equipment known as 2, 3 and 4 DME.
• Alarm and performance management requires separate equipment in PDH.

What is Add/Drop Multiplexer?

• used in rings
• two STM-N line interfaces (East/West)
• large number of tributary interfaces STM-1, 140 / 34 / 2 Mbit/s(equipped up to full ring capacity)

What is Synchronous Cross-Connect?

Synchronous Cross Connect in SDH Network Elements Perform following action.

• used in meshed networks
• mainly STM-N interfaces
• large switching matrix (1024 STM-1)
• Wideband SXC: VC-12 granularity
• Broadband SXC: AU-4 granularity

AU-4 Pointer

The SDH pointer mechanism is a very elegant way of multiplexing multiple data
containers without the need to align the containers to a common frame start. Thus the framebuffers in SDH multiplexing equipment can be kept small and the transmission delay due to buffering is minimized.

The VC-4 container is allowed to float freely within the AU-4. The H1 and H2 pointer
bytes form a word with a range of 0 to 782 which indicates the offset, in three byte
increments, between the pointer and the first byte of the VC-4.

Administrative Unit (AU-4) and Virtual Container (VC-4)

The actual payload carried in an STM-1 frame is encapsulated in an administrative unit
(AU-4). The AU-4 consists of a VC-4 virtual container comprising 261 columns plus a 9
octet wide AU-4 pointer that points to the first octet of the VC-4 payload container.

SDH Section Overhead (SOH)

A relatively large number of 72 octets in an STM-1 frame have been reserved for various management and monitoring purposes. This so-called section overhead (SOH) is further divided into a regenerator section overhead (RSOH) and multiplex section overhead(MSOH).

SDH Frame Structure

A single SDH frame is called a Synchronous Transmission Module (STM-1). Transmitted
over a duration of 125 µs, the frame consists of 2430 octets organized as 9 rows of 270 octets each. A single octet in an SDH frame represents a 64 kbps channel (8 bits every 125µs), several octets can be aggregated to form containers for larger data rates.

What are the Advantages of SDH?

Advantages of SDH:-

• Worldwide standard
• First standard for bitrates higher than 45/140 Mbps
• Standardised optical interfaces
  simple coding of the electrical signal into the line signal (scrambling)
  Integration of multiplexer and line equipment
• Huge overhead capacity for error monitoring and for management purposes (TMN)
• Direct access to tributary channels
• Only small buffers required --> small group delay figures
•  Provides concept for clock/synchronization, protection switching etc.
•  Suitable as the physical layer of ATM networks

What is SDH?

Synchronous Digital Hierarchy is a communication protocol based on hierarchic levels and designed for high speed digital data links operating on optical fibre transmission bearers.

The protocol has been developed in view of the need for eliminating the limits set by the asynchronous hierarchy so as to meet the growing demands for improved flexibility and efficiency in line utilization.Initially the principal object was to develop a synchronous network with optical interfaces,designed and developed by different manufacturers, but all mutually compatible.

Subsequently other aspects were also taken into consideration and a synchronous system wasdefined which offered the following advantages compared with former methods:

• simplified multiplexing/demultiplexing techniques;
• direct extraction/insertion of lower speed tributaries without having to
• multiplex/demultiplex the entire frame;
• exchange operations, administrative and maintenance capability.