SDH Document by Aman

7/27/2019 SDH Document by Aman

1/7

Synchronous Digital Hierarchy (SDH)

I. SDH multiplexing hierarchy

The multiplexing principles of SDH follow, using these terms and definitions:Mapping A process used when tributaries are adapted into VirtualContainers (VCs) by adding

justification bits and Path Overhead (POH) information.

Aligning This process takes place when a pointer is included in a Tributary

Unit (TU) or an Administrative Unit (AU), to allow the first byte of the Virtual

Container to be located.

Multiplexing This process is used when multiple lower-order path layer signals are adapted

into a higher-order path signal, or when the higher-order path signals are adapted into a

Multiplex Section.

Stuffing As the tributary signals are multiplexed and aligned, some spare capacity has been

designed into the SDH frame to provide enough space for

all the various tributary rates. Therefore, at certain points in the multiplexing hierarchy, this

space capacity is filled with fixed stuffing bits that carry no

information, but are required to fill up the particular frame.

At the lowest level, containers (C) are input to virtual containers (VC).The purpose of this

function is to create a uniform VC payload by using bit-stuffing to bring all inputs to a common

bit-rate ready for synchronous multiplexing. Various containers (ranging from VC-11 at 1.728

Mbit/s to VC-4 at 150.336 Mbit/s) are covered by the SDH hierarchy.Next, VCs are aligned into

tributary units (TUs), where pointer processing operations are implemented.

These initial functions allow the payload to be multiplexed into TU groups (TUGs). As Figure 12

illustrates, the xN label indicates the multiplexing integer used to multiplex the TUs to the

TUGs. The next step is the multiplexing of the TUGs to higher level VCs, and TUG-2 and TUG-3

are multiplexed into VC-3 (ANSI mappings) and VC-4. These VCs are multiplexed with fixed byte-

stuffing to form administration units (AUs) which are finally multiplexed into the AU group

(AUG). This payload then is multiplexed into the Synchronous Transport Module (STM).


2/7

Fig.1 multiplexing hierarchy

II. Frame structure of SONET/SDH

The information transmitted by SONET/SDH is organized into frames. These frames are

transmitted continuously one after the other. Each frame consists of a collection of overhead

fields and a payload. SONET/SDH equipment constructs these frames in the electrical domain

and then transmits them out optically. At the receiving end, the SONET/SDH equipment

receives the optical signal and converts it to the electrical domain in order to process the

frames. The electrical side of the SONET signal is known as the synchronous transport signal

(STS), and the electrical side of the SDH is known as the synchronous transport module (STM).

The optical side of a SONET/SDH signal is known as the optical carrier (OC).

Synchronous Transport Module Level 1 (STM-1)The Synchronous Digital Hierarchy (SDH) defines the Synchronous Transport Module Level 1

(STM-1) as multiplex signal of the lowest level. It has a transmission rate of 155.520 Mbit/s. The

STM-N bit rates of the standardized higher hierarchy level (N= 4 and 16) are always higher by

factor 4.the diagram below shows the structure of the byte-oriented STM-1 frame. The frame is

composed of 270 columns and 9 rows. The first 9 columns include the Section Overhead (SOH)

and the Pointer of the Administrative Unit (AU pointer). The remaining 261 columns are used

for transporting the payload. It consists of packed and multiplexed payload signals (tributaries)

and an accompanying Path Overhead (POH). The repetition frequency of the STM-1 frame is 8


3/7

kHz, i.e. one STM-1 frame has a length of 125 s. The transmission capacity of one byte in an

STM-N frame is thus 64 kbit/s. With STM-1, an Overhead capacity of 5184 kbit/s is transported

in addition to the traffic bit rate of 150,336 kbit/s.

Table 1. Bit rates in the STM-1 frameColumns x rows x 64 kbit/s Bit rate

STM-1 frame 270 x 9 x 64 kbit/s 155,520 kbit/s

Section-Overhead 9 x 9 x 64 kbit/s 5,184 kbit/s

Payload 261 x 9 x 64 kbit/s 150,336 kbit/s

The payload has no fixed phase relation to the STM-N frame. In order to be able to access the

payload, the Section Overhead block contains a pointer. It is located in the 4th row of the STM-

N frame.

The STM-1 base frame is structured with the following characteristics: Length : 2430 bytes Duration : 125 s i.e. 8000 frames/s Rate : 155.520 Mbit/s


4/7

Payload : 2340 bytes i. e. 149.760 Mbit/s;1byte i.e. 64 kbit/s (e.gspeech channel)

STM SynchronousTransportModuleMSOH MultiplexSectionOverhead

RSOH RegeneratorSectionOverheadSOH Section Overhead

STM-1 = AU-4 + SOHMultiplexing of base frame; STM-1 frames at 155.520 Mbit/s with each other to create a high-

rate frame of STM-4 at 622.080 M bit/s, STM-16 at 2488.320 M bit/s and STM-64 at 9953.280

M bit/s. In such a way that is a STM-N multiplex signal is formed by interleaving the individual

STM-1 frames

byte by byte.

III. PointerThe payload has no fixed phase relation to the frame. In order to be able to access the payload,

a pointer is transmitted in the overhead block. It permits the dynamic adaptation of the phase

of the Virtual Container to the frame. In this connection, dynamic means:

1. The phase of the Virtual Container can differ from that of the frame.

2. At different frequencies, the phase position can continuously vary without causing any loss of

information.

Pointer value modificationThere are two possibilities of modifying the pointer value:

a) Setting a new pointer valueb) Frequency matching.

a. Setting a new pointer valueIn case of modifications of the payload, it may be necessary to set a new pointer value. In order

to indicate this change, the so-called New Data Flag (NDF) is set. Then the new pointer value

is transmitted.

On the receive side, the NDF is evaluated. The new pointer value received indicates the new

position of the Virtual Container.

The NDF with the new pointer value is transmitted only once, i.e. in the first frame. There must

not be any further pointer operations within the next three frames.

b. Frequency matchingIf the frequency of the STM-N frame is not exactly identical with the one of the VC frame, the

pointer value is increased or decreased by 1 at regular intervals, while frame matching is

simultaneously performed by a positive or negative justification process.

After each pointer correction, at least three frames must be transmitted without pointer

modification.The frequency matching process for AU pointers is explained in the following

section. The process for the TU pointers is identical.


5/7

Positive justification

If the frame frequency of the VC is lower than that of the STM-N frame, stuffing bytes must be

inserted and the pointer value must be increased by 1 at regular intervals. The stuffing bytes

are inserted directly behind the last H3 byte. For an AU-3 one stuffing byte, for an AU-4 three

stuffing bytes are inserted. The new pointer (P+1) is then transmitted starting at the next

frame.

The next VC starts at the position indicated by the new pointer.

PTR(n): Pointer with value n

H3: 3 stuffing bytes used for negative justification

B: 3 stuffing bytes used for positive justification

Negative justification

If the frame frequency of the VC is higher than that of the STM-N frame, additional informationof the VC must be transmitted in the H3 bytes and the pointer value must be decreased by 1 at

regular intervals. The following three H3 bytes are filled with information. With AU-3, only the

H3 byte belonging to the VC to be stuffed is filled with information. The new pointer (P-1) is

transmitted starting at the next frame .


6/7

IV. Optical interface

Classification of optical interfaces

It is expected that optical fibres will be used in SDH-based systems for both inter-office

transport between stations and in intra-office applications for connecting equipment within a

single station. By appropriate combinations of transmitters and receivers, power budgets foroptical fibre line systems can be achieved which are optimized in terms of

attenuation/dispersion and cost with respect to the various applications. However, to simplify

the development of transverse compatible systems, it is desirable to limit the number of

application categories and corresponding sets of optical interface specifications for

standardization.

As shown in Table below, this Recommendation recognizes three broad application categories:

intra-office corresponding to interconnect distances less than approximately 2 km; short-haul inter-office corresponding to interconnect distances of approximately 15 km; long-haul inter-office corresponding to interconnect distances of approximately 40 km

in the1310 nm window and approximately 80 km in the 1550 nm window.Table 2

Within each category, it is possible to consider use of either nominal 1310 nm sources on

optical fibre complying with ITU-T Rec. G.652 or nominal 1550 nm sources on optical fibre

complying with ITU-T Recs G.652, G.653 or G.654. This Recommendation covers both

possibilities for the two inter-office applications and considers only nominal 1310 nm sources

on G.652 fibre for the intra-office application. Since the overall system characteristics andspecific values for the optical parameters generally depend on system bit rate, it is convenient

to classify the SDH optical interfaces based on applications considered in this Recommendation

using the set of application codes shown in Table 2. This application code is constructed in the

following way:

Application-STM level; Suffix number with the application designations being I (Intra-office), S

(Short-haul), or L (Long-haul), and the suffix number being one of the following: (blank) or 1

indicating nominal 1310 nm wavelength sources on G.652 fibre; 2 indicating nominal 1550 nm


7/7

wavelength sources on G.652 fibre for short-haul applications and either G.652 or G.654 fibre

for long-haul applications; 3 indicating nominal 1550 nm wavelength sources on G.653 fibre.

ITU-T Rec. G.957 (03/2006)

NOTE The use of the term intra-office is not meant to exclude any other applications

consistent with the set of optical parameters specified (e.g., B-ISDN user network interfaces

physical layer specifications defined in ITU-T Rec. I.432.2). The distances chosen for theapplication codes in Table 1 are based on parameter values that are achievable with present

technology and which are thought to suit network requirements. The intra-office and short-

haul inter-office application codes have been proposed as low-cost equipment

implementations. The long-haul application codes have been proposed to provide maximum

length repeater spans consistent with limits set by present technology and the objective of

transverse compatibility. The distances proposed may allow for the upgrading of present

systems by exploiting the 1550 nm region. The distances in Table 2 represent approximate

maximum repeater span distances.

SDH Document by Aman

Documents

Transcript of SDH Document by Aman