Informational structure oHs

Vary in by their functional intended use in a following way:

1. OHs that are performing the trunk and its parts monitoring

• SOH (sectional OH): RS, MS

• POH (path OH)

2. Dynamical distribution of the PL in the STM (PTA - pointer)

The OH kinds of the corresponding digital SOH structure

Overheads informational structures

OH

↓

Sectional OH ↔STM

↓ ↓

RS MS (P_er control, automatic reservation, data transmission and service communication)

(frame alignment FA, control and signalling CS)

AU pointer (PTR) ↔AU (TU)

(phase position of the VC frame in the AU)

Path OH (POH) ↔VC

(CS and the maintenance information)

There are new concepts in SDH:

Trunk (path, way) – is the generalized concept of the channel.

Trunk = channel + OAM (the procedure, providing the path monitoring, condition control, automatic shifting on the reserve path when it’s necessary).

T he quality and path monitoring is performed by the VC OH.

Subscriber A Subscriber B

Section

STM OH

Trunk

(frame extracting, OH determination, section changing)

The informational SDH structures are formed by the corresponding transforming procedures using. This procedures and procedures obtained as a result of informational structure are shown in the SDH multiplexing block diagram.

The block diagram shows the process but not the facilities.

Block diagram:

PDH European hierarchy DBS multiplexing into the synchronous transport module

Procedures:

Placement

Phasing

Multiplexing

1. The placement

VC is formed from the C container with the help of placement. The container DBS bytes are situated in the certain positions of the VC during this operation. The VC is obtained as a result, which consists of the path OH (POH) and PL.

VC = POH + PL

2. The phasing

- is the process of administrative or tributary unit (AU or TU) form the VC.

TU = TU-pointer + VC

AU = AU-pointer + VC

3. The multiplexing

- is the corresponding informational structures DBS multiplexing in the transmission path and demultiplexing in the receiving path.

The1st level SDH hierarchy STM-1 forming (assembly) process is performed in a following way correspondingly to the block diagram:

The STM-1 can transmit

The PDC number in STM-1is: 63*30=1890

Lets

observe the STM assembly process from the E1 tributary (PDB) in steps:

The 1^st step

Two package bytes to the E1 frame

Overhead Docking (concatenation)

OH or Pointer

C-12 frame

2 package 32 informational bytes

The 2^nd step

The path OH (1 byte)

The 3^rd step

PTR Pointer (1 byte)

The 4^th step

The 5^th step

The TUG-2 OH (18 bytes)

The 6^th step

The POH 27 bytes

The 7^th step

The AU-4 PTB pointer (9 bytes)

The 8^th step

The 9^th step

Their two-dimensional picture of the observed informational structures is more compact and demonstrative.

The two-dimensional picture of the SDH informational structures transmission frame that are forming the STM from the E4 tributary located in the C-4 container.

Pointer POH

It is seen form the given informational structures that the VC-4 is formed by adding the clock overhead.

It is comfortable to show the VC-4 as the rectangular grid. The lines (groups) number of this and all following informational structures is equal to 9.

One column is used for the path OH. There is 1 byte in the line and column intersection. So the POH size is 9 bytes. The AU-pointer OH is added to the VC-4 in the 4^th line (group) and consists of 9 bytes.

The 4^th level AU is formed by this pointer adding. This informational AU-4 structure is inputted into the STM-1.

9 bytes in each line are used for OH: 1^st three lines – regeneration section OH, 5-9 – multiplexing section OH; the formed in advance AU-pointer is situated in the 4 line.

The general number of the bytes in each line of STM-1 = 261 (from AU-4) + 9 (OHs) = 270 bytes

The bytes number for the STM-N is M = n*m, where n – lines number, m – columns number.

The frame duration of informational SDH structures T_fr = 125 μs, f_fr = 8 kHz; f_cl = f_fr * N_fr = 8*6*M = 64 M kHz where N_fr – the bit number in the transmission frame 8M.

Let’s observe the STM-1 sectional OH content:

SDH performs the synchronous multiplexing when higher level STM forming from the lower levels STMs. The plesiochronous multiplexing is performed in SDH when STM assembling from the plesiochronous hierarchy DTS tributaries.

• R S errors control (MS)

• RS data transfer channel

• MS data transfer channel

• RS (MS) service communication channel

• Alignment that is used for the automatic shift to the reserve

• Frame alignment

• STM identifier

• Exploitation uses channel

• The path status in synchronization system

• The byte margin for indefinite functions

• The national use bits

Kinds of STM-N

The synchronous multiplexing

The synchronous DBS multiplexing is easier. (B, f_cl – are equal).

Let’s observe the time diagrams of synchronous multiplexing of 2 DBS (A and B) into the group bitstream as an example.

F rame Alignment signal

It is necessary to phase DBS before multiplexing and, than it is possible to obtain the group bitstream C. (phase B relatively to the A)

The multiplexing bitstreams rates are different when asynchronous multiplexing.

Let we have 2 oscillations and .

– is very small value

Phase depends on time. Close, but not equal frequencies assumed to be coincident by frequency, but different by phase that is changing in time.

PDH-DTS Asynchronous multiplexing

The time bit positions of these streams from frame to frame are changing in the multiplexed DBS (C). T1 < T2 in our example. So the direct extraction of the A or B streams bits is not possible. It is necessary to find the frame alignment signals of these DBS(that is – to install the STG(secondary time grouping) FA search equipment) and only after that it is possible the determine the position of bits for extracting.

I t is necessary to phase DBS before multiplexing and, than it is possible to obtain the group bitstream C. (phase B relatively to the A)

In asynchronous DBS multiplexing (Mx) time positions of these streams bits are changing, this fact doesn’t allow to unite these bits. Staffing method is used for DBS rates equalizing. Staffing is used when plesiochronous DBS uniting in PDH and in SDH.

Two-sided Staffing is used in SDH.

The pointers method is used for each tributary signal extraction from the aggregate SDH signal in addition to the Staffing method.

Let’s consider time diagram of asynchronous SDH DBS tributary Mx.

Mx A and B into stream C.

In service bits of united frames in addition to FA signal, by which the cycle beginning is defined, pointers are introduced, which show at which time interval PDH DBS payload is situated inside of informational SDH structure.

In our example:

For stream A – F_a ; B – F_b .

In this pointers value of time intervals is written. ( T_A for stream A; T_B for stream B).

After uniting of these streams in aggregate stream C the information of pointers is saved, it allows to allocate the necessary tributary stream of payload.

Two methods (stuffing and pointers method) allow the multiplexing.

STM (synchronous transport module) – 1 – 4… are multiplexed synchronously in SDH DTS. There are 2 methods: the lowest order STM Mx into the highest order STM. (STM – N from STM – 1):

• Direct Mx

STM – N = (STM–1) x N

• Cascade Mx – this means that forming of STM is carried out by stages:

1. STM–4 = (STM–1) x 4

2. STM–16 = (STM–4) x 4

3. STM–64 = (STM–16) x 4

The alternation of bytes or groups of bytes is used in synchronous STM Mx.

Cascade method of forming

STM-4 is formed by synchronous multiplexing of 4 STM-1streams.

The aggregate signal is formed by alternation of bytes at the output of STM-4. For first STM-4: …

The aggregate signal is formed by groups of bytes multiplexing at the output of STM-16.

STM modules

There are 3 known types of STM modules:

• Synchronous Mx (SM)

• Cross-connector (DxC)

• Network control unit (network manager)

These modules differed by their functions and by hardware at first steps of SDH network development.

The same form factor could perform functions of these different modules at a later date.

Element base allows different modules functions integration, i.e. STM partially performs functions of cross-connector today.

There are 3 known types of synchronous Mx:

• Terminal SM (SMT) , it’s main function – tributary signals multiplexing in transmission path and demultiplexing in receiver path;

• Add-drop SM (SMA);

• SMR (degenerate type SM) – signals regeneration + OAM ((operation, administration and maintenance) MS and RS sections monitoring)

SMA- separation and input of tributary streams from (into) aggregate.

These modules at transmission line structure chart look like in a following way:

A TS B С

Functions of SM and descriptions of these functions:

1. Mx (multiplexing) of input DBS

   OF

2. Local cross connections connect tributaries with each other

3) Through cross-connection can be used on different levels (VC – 4)

3. Bypass.

The difference of bypass method form through cross-connection method is: conjunction of ports when bypassing takes place in optical level without conversion in electrical signal.

5) Concentrator (HUB)

6) Regenerator.

where W – West, E – East ports

7) Cross – connector.

DxC functions:

Opportunity of information structures crossing in DxC is much greater than in SM, bigger number of fibers can be connected to it.

This is more difficult and expensive equipment than SM. During the explanation of DxC operating structure we will use the next designation:

There are three types of cross-connections in cross-connector :

• - interconnections;

• - local connections;

• - straight-through connections;

Straight-through connections	Type: in – out; out - in
Interconnections	Type: in – trib; trib – in; trib – out; out – trib.
Local connections	Type: trib - trib

There are 6 functions of cross-connectors:

1. Routing function;

Changing of routing direction.

2. Multiplexing function:

(DxC can perform both routing and other functions)

3. Translation from point to multipoint;

4. Rearrangement;

5. Extracting and inputting;

This method is analogue to extraction (inputting) method in ATS without losses.

6. Branching (This method is analogue to the selection method in ATS with spectrum loss)

Usually it is used for signal transmission quality controlling.

7. Regeneration; ( the 1^st and 2^nd streams in (6) and (5))

Crossing can be performed by using of different informational structures, with it different conditional designations of cross – connector are used:

Basic architectural elements of SDH

Architectural elements show ways of SDH modules connections in network.

1. The simplest architectural connection : “ point-to-point”

2. Configuration : “Linear circuit”

3. “ Star”

4. One of the most important SDH elements is “ring”, when basic SDH elements are connected into ring.

“Flattened ring “ ( all fibers are in one cable)

Ring :- one-way

- bidirectional.

One-way ring : ( doubled)

Reserved trace is used when primary one is broken because of existing of noises, stationary equipment fault (Mx) , decreasing of optical fiber quality, damage of cable.

In one-way ring duplex communication is organized by passing of signal in one direction (clockwise or counterclockwise).

Forward direction A – C : ADC – counterclockwise.

Backward direction C – A : CBA – counterclockwise.

Twodirectional ring.

• Normal mode.

• Breakdown mode.

Usually different compositions are applied when using these main architectural elements during the branched network designing. Let’s consider them:

1. Ring-radial network.

2. Ring-cascade structure can be realized in different variations, when two or more rings are connected to each other.

3. Cell (grid) structure.

Different architectural compositions are used on different levels of PN. 3-leveled structure of PN is used in Ukraine and CIS countries. The highest level – trunk network, middle level – zonal network, lowest – local network.

Large NN are supplied by the DxC and connected by each-to-each principle.

3 planes correspond to PN levels

The ring structure is mostly used at the lowest level.

DxC is used at middle level.

Grid structure is used at high level.

The basic architectural SDH elements are rings. To understand peculiarities let consider SDH advantages and disadvantages of ring structures and compare them with grid structure that is used in PDH DTS.

DTS PDH

DTS SDH

Compare these variants:

1) By quantity and price of station equipment;

Q- price of TS

For approximate calculations lets consider that SDH DTS and PDH DTS TS are approximately equal.

Q₂ – total price of equipment for variant 2

Q₂=4Q; Q₁= (3Q)x4 Q₂<Q₁

_{Mx TS}

2) By price of line path

Lets consider that cost of 1 km line path (cable, intermediate stations) of FOTS line path on copper cable.

I.e. difference in cost of these variants is defined by length.

L₂ – total line path length of second variant

L₂ = l₁+l₂+l₃+l₄

L₁= l₅+L₂+l₆ L₂<L₁

Variant 2 is better (on base of SDH)

It is necessary to use higher level synchronous Mx when traffic increasing in ring structure. It leads to cost increasing of station equipment. Cross communication lines are used on most loaded sections of ring for traffic intensity decreasing, i.e. gradual transition to grid structures.

The basic advantage of ring structure – is the network reliability increasing due to reserved trace organization using.

Telecommunication networks reliability increasing methods on basis of SDH DTS.

There are 3 methods of increasing of reliability of a network

Important element of protection is reconfiguration of a network.

Advantages of 1.1: simplicity of such control network construction;

Disadvantages: criticality to center breaking

Advantages of 1.2: high reliability.

Disadvantages: high network complicity increases reconfiguration time

The best variant is 1.3

Disadvantages of 2.1: big amount of time is necessary for optimal network configuration defining

Disadvantages of 2.2: big amount of tables is necessary that increases complicity

The best variant is 2.3

Sectioning of SDH communication lines

- partition of communication lines on some sections; signal transferring on these sections is controlled and can be changed to reserved one (manually or automatically)

Monitoring and management is performed on each section. Quality control of signal transferring through section is performed by using the parity check (BIP).

Controlled bit stream is divided on groups and builds into matrices in this method. Each row of such group – 1 block. Quantity of bits in group is written in name of this control method.

Received matrix is read and sent by columns, quantity of unities is defined in it.

If quantity is odd – 1 ; even – 0.

This procedure is performed in transmitter and repeated in receiver. The results of such coding are written down in correspondent bytes of sectional overheads. Comparison of transmitted data and received data allows to detect errors and quality of transmission DBS.

B1 bytes are used for its transmission in regeneration section, method of controlling BIP-8.

In MS: bytes B2 , method of controlling BIP-24.

In POH overhead (virtual) controlling is performed in byte B3, but method of controlling BIP-8.

Part of streams that are processed by synchronous Mx can be allocated for protection organization.

Maximal quantity of streams in STM-1 is 63. Quantity of work streams equals to one half when using 100% of reservation is used for protection resources of streams, i.e 31 streams are working, 32 – reservation.

If path resource is used (OF) , then maximal quantity of streams in STM-1 – 63 can be used as working and 63 streams of another fiber – reservation.

The DBS protection methods using protection shifting and different resources for this shifting we will call in a following way

There are next combination variants when the protection method organization:

Let’s compare the variant of protection path shifting (PS) with variant of protection ring shifting (SR):

Let in normal mode the stream flows counterclockwise. For example, emergency mode is between 2^nd and 3^rd NNs. During the protection shifting of the ring automatic shifting on reserve (stream, fiber) occurs at the end of the damaged section, in this case bypassing of this section in opposite direction takes place.

During the operating by scheme 1+1 (PS) the connection between A and B is carried out simultaneously in main and reserve paths. Fiber or stream can be used as the reserve.

Nowadays bidirectional ring is used usually. In this duplex organization method the delay of the signal in forward and backward directions is equal. In unidirectional ring the delay of the signal in forward and backward directions is different. The greater sizes of the ring the greater difference of the delays are. It makes the operation of equipment with automatic shifting on reserve more complex.

In schemes 1+1 main and reserve signals comes simultaneously in the receiving point (A and B); this signals are compared and after this less damaged by noises signal is chosen.

In scheme 1:1 automatic shifting to reserve takes place when signal degradation (basic).

In second variant reserve can be used for non-priority users, which can be neglected at emergence case.

RS always corresponds to scheme 1:1.

Line-path of SDH DTS

Line-path includes interfaces of TS with cable, intermediate stations (regenerators), propagation medium.

All types of stationary equipment vary in: Mx , DxC.

Optical fiber on basis of single-mode fiber or special RRTS is used as propagation medium. STM-0, STM-1, STM-4 are used in RRTS.

Characteristics of SDH line-path are regulated by ITU. These recommendations are given in the table

* - nowadays length of regeneration section can be 100 and more km-s interfaces of this column refer to long length sections (>40 km)

Equipment of SDH hardware

Construction design , technical capabilities may differ. Lets consider as example synchronous Mx of Norten Telecom of TN-1x type (1st level). This Mx is assembled in a rack, it includes:

• tributary blocks with 2 Mbps ports

• 2 blocks (basic and reserved) of payload manager, in which Mx, local cross shifting, etc. are performed.

• 2 aggregate blocks of SMT-1 with 155 Mbps ports, providing optical and electric interfaces.

• 2 (basic and reserved) power-supply blocks.

• Controller and management panel.

The necessary cross – connections are performed by controller and manager of useful load (MUL)

Structure chart of SM

ACB – is a tributarian block, E1 in this case is a trubutarian signal. To the main block the tact and cyclic frequencies are supplied. There are four such access blocks -> 64 E1. For the further Mx the DATA datumn is supplied to the main block, and also the CLK(clock frequency) and WES(cycle frequency).

The SDH network needs 2 additional networks for its functioning.