SDH-Synchronous Digital Hierarchy

SDH: Synchronous Digital Hierarchy.

SDH is an international optical interface standard to transmit different types of signals on OFC. It accepts various native TDM signal formats and multiplexes-adds- or demultiplexes-drops- these signals without conversion. It provides improved network management- in band- and direct access to tributaries.

Plesiochronous Digital Hierarchy-PDH-includes all ITU TDM hierarchies including T-carrier and E-carrier systems. Signals are considered to be plesiochronous if their rates are nominally same, with any variations constrained to specified limits. PDH is defined by ITU G.702 and includes NADH-North American Digital Hierarchy-, EDH-European Digital Hierarchy- and JDH-Japanese Digital Hierarchy. All these systems share the basic rate of 64kbps that correspond to a PCM voice channel.

Synchronization

1.PRS / Master clock / Stratum1 GPS or Atomic clock.
2.Line timing. Derived from incoming SDH signal from a high-speed interface.
3.Loop timing. Similar to Line timing but incoming SDH is taken from a CPE or TM-Terminal Mux- than an ADM.
4.Stratum3. Free running internal oscillator.

Although synchronous networks display accurate timing, some variations occur between different network devices or between networks. This difference is known as phase variations. Short-term phase variations above 10 Hz are called Jitter and long tem phase variations below 10 Hz are called Wander. In digital networks, jitter and wander are handled by buffers found in the interfaces within different network devices. One example is a slip buffer, used to handle frequency difference between read and write operations.
Here bit stuffing is used and it is called controlled slip. The timing accuracy requirements increase as the stratum hierarchy increases as shown below.


Stratum clock Hierarchy
Mini. Free run Wander Bit slips controlled frame
Accuracy (0.12 hsec Increment) Slips

Stratum 1 +/- 1*10^-11 3.3hr 18hrs 20.6 wks
Stratum2 +/- 1.6*10^-8 7.5 sec 41sec 2.17hrs
Stratum3 +/- 4.6*10^-6 26ms 140ms 27sec
Stratum4 +/- 32*10^-6 4ms 20ms 3.9sec











SDH Layers

SDH has four optical interface layers as follows.
1.Path layer
2.Multiplex Section –MS- layer
3.Regenerator Section –RS- layer
4.Photonic layer.

Path layer multiplexes or demultiplexes the lower level VC-n payloads. POH is added or stripped at this layer and the alarm information contained in this layer represents the end-to-end status. Lower order multiplexers, subscriber loop access systems are examples of Path Terminating Equipments.
Signal at STM-N level is transported between NE’s in this layer. MSOH is added or stripped at this layer. The MSOH is used for communication between major nodes and for monitoring errors. High order mux, Optical Line Terminator are examples of MSTE-MS Terminating Equipments.
This layer is there between a TM and regenerator or between two regenerators.
RSOH is added or stripped at this layer. It is used for long haul transport.
Deals with transport of bits across physical fiber medium i.e. EàO and OàE conversion.

SDH Multiplexing

SDH Multiplexing follows a rigid hierarchy as follows.



1.Low level PDH signal:: Mapped by adding Justification bits à Container ( C11 for T1, C12 for E1, C2 for DS2, C3 for E3 and DS3, C4 for E4)
2.:: Mapped by adding POH à Virtual Circuits (VC11,VC12,VC2,VC3,VC4)
3.:: Aligned with TUPointers à Tributary Units ( TU11,TU12 and TU2)
4.::Multiplexed à TUG –Tributary Unit Group-( TUG2, TUG3)
5.:: Multiplexed à Higher Order VC’s.( VC3,VC4)
6:: Aligned with fixed byte stuffing and AUPointersà Administrative Unit (AU3,AU4)
7.:: Muxed à AUG(AUG1,AUG4,AUG16,AUG64,AUG256)
8.:: Muxed with MSOH and RSOHà STM-N


Higher level of the Synchronous hierarchy is formed by byte interleaving of the payload from a number N of STM-1 signals and then adding TOH of size N times that of an STM-1 and filling it with new management data and pointer values as appropriate. Before transmission the STM-N signal is scrambled to randomize the bit sequence for better transmission performance. A few bytes of overhead are left unscrambled to simplify subsequent de-multiplexing.

Telecom

Hi,
Let me put some notes on TDM, WDM and later SDH, here in this blog

Time Division Multiplexing.

To share a medium by many signals multiplexing techniques like TDM, FDM, CDMA, and WDM are used.

TDM along with digital encoding techniques such as Pulse Code Modulation-PCM- forms the base of telecommunication systems.

Analog voice signal-4000Hz- is sampled at a frequency of 8000 Hz and quantized (non linear) -to reduce non-linear quantization error, i.e. percentage error more at lower amplitude- to get a 64Kbit PCM signal (DS0) .

PCM encoding is done as follows.

The analog signal level 0-4096 mV is divided into 8 (000-111) segments-in A-law - .

Voltage (mV) Segments (ABC)
0-31 0 0 0
32-63 0 0 1
64-127 0 1 0
128-255 0 1 1
256-511 1 0 0
512-1023 1 0 1
1024-2047 1 1 0
2048-4095 1 1 1

Quantization level is determined by dividing the range by 16, to get WXYZ

E.g. If voltage is 102 mV ABC= 010, WXYZ= 1001 (since 64/16 = 4, then 102 comes in the 9th level)
So the corresponding PCM signal is P A B C W X Y Z = 1 0 1 0 1 0 0 1
P=1 for +ve and 0 for -Ve

Decoding of PCM

E.g. 1 0 1 0 1 0 0 1

It will be expanded to 13 bits.
D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13
Determine the segment value ‘n’ of the incoming signal… Here n=2.
Write W in D10-n position followed by X Y Z….. Here put W at D8 X at D9,Y at D10 and Z at D11.
W X Y Z
3. Always put 1 after Z…….Here put 1 at D12.

1 0 0 1 1
4. Always put φ before W(φ=0 for segment 0 and 1 for other segments)..Here D7=1.
1 1 0 0 1 1
5. Where ever blank put 0 …
0 0 0 0 0 0 1 1 0 0 1 1 0
6. Convert the binary to decimal……..Here 102mV.
There could be slight variation due to quantization error.
CAS-Channel Associated Signaling (in-band) and CCS-Common Channel Signaling (out of band) are two signaling schemes used in TDM. CAS forms are Ear and Mouth, Loop start, and ground start signaling. ISDN uses CCS.

E-Carrier

The basic unit of the E-Carrier system is 64kbps DS0, which is multiplexed to from transmission formats with higher speeds.
A E1 frame is formed by 32 time slots. TS0-TS31.
A E1 multi frame is formed by 16 frames. F0-F15. And 2 Sub Multi Frames -SMF- of 8 frames each.

The TS0 of even numbered frames is FAW-Frame Alignment Word- and TS0 of odd numbered frames is Alarm and Supervisory bits
TS16 is for signaling information (CAS).TS16 of F0 is MFAW-Multi FAW- of F1 is CAS of 1st and 15+1st channels. TS 16 of F15, first 4 bits are CAS of 15th channel and next 4 bits that of 30th.
CRC of 4 bits is calculated for a SMF and sent in the first bit position of 4 odd frames in the next SMF.

E1 Errors and Alarms.

SES-Severely Errored Seconds- Error rate of 10^-3

UAS-Un Available Seconds- Unavailable time begins at the onset of 10 consecutive SES. It also begins at a LOS-Loss Of Signal or Loss Of Frame.

Bit Errors-Are bits that are in error. Bit errors are not counted during unavailable time.

ISDN-Integrated Services Digital Network

ISDN allows voice and data to be transmitted simultaneously using end-to-end digital connectivity.
All devices that are ISDN ready (e.g. ISDN phone or feature phone) are designated TE-1 –Terminal Equipment 1- All other equipments that are not ISDN capable but have RS-232 or RJ-11 interface , like analog phone or modem, are called TE-2.
The U interface from exchange is terminated in NT-1 at the customer premises to give the customer a S/T interface.

Wavelength Division Multiplexing

Is a process of multiplexing different wavelengths (frequencies c=f*λ) on a single fiber. It can be considered as a form of FDM coupled with TDM.
Frequency is standardized rather than wavelength since it is independent of the transmission medium.
Wavelength is measured in nm and frequency in GHz.

High band width of OFC can be understood from the fact that visible light have a wavelength of 400-750 nm. So freq. is (c= f* λ) 400THz-750 THz (Tera = 10^12). So BW= 350 THz. So if 1 Hz can carry two bits of information, the light have an information carrying capacity of 700 T Bits approximately.



The optical frequency bands used with various WDM systems are as follows:
O-band (Original band) : A range from 1260nm to 1360nm.
E-band (Extended) : 1360 - 1460
S-band (Short wavelength) 1460 - 1530
C-band (Conventional) 1530 - 1565
L-band (Long Wavelength) 1565 - 1625
U-band (Ultra long) 1625 - 1675 nm


Channel Spacing
The minimum wavelength separation between two different channels multiplexed on a fiber is known as channel spacing. Channel spacing ensures that neighboring channels do not overlap. It is a function of the precision of the LASER. (Light Amplification by Stimulated Emission of Radiation, generates EM Signal in IR region i.e. λ > 750 nm).The more precise the tuning, the lower the channel spacing required, but the LASER will be costlier.

In theory, a fiber should be able to carry 150 wavelengths with 100GHz (0.8nm) spacing between wavelengths, 300 wavelengths with 50 GHz (0.4nm) spacing, 600 with 25GHz spacing and 1200 λs with 12.5 GHz (0.1nm) spacing. Assuming that each wavelength operates at 40 Gbps, it provides a theoretical maximum of 6 Tbps with 100 GHz spacing and 48 Tbps with 12.5 GHz spacing.

ITU-T has published a wavelength grid for interoperable standards, for DWDM, anchored to 193.1 THz. Or 1552.52 nm.

CWDM. Coarse Wavelength Division Multiplexing
Uses Lasers that have a bit rate of up to 2.5 Gbps (STM-16) and can multiplex up to 18 λs providing a maximum of 45Gbps over a single fiber.

CWDM systems are characterized by a channel spacing of 20nm or 2500GHz. CWDM grid is made up of 18 wavelengths defined within the range of 1270 nm to 1610 nm.






DWDM: Dense Wavelength Division Multiplexing
Uses Lasers that have a bit rate of up to 10Gbps (STM-64) and can multiplex up to 240 λs. i.e. 2.4 Tbps on a single fiber. It uses EDFA (Erbium Doped Fiber Amplifier)
ITU-T DWDM grid channel spacing is 50GHz or 100GHz to be operated in the C or L band window.(Where as legacy fiber is optimized for O-band).