Scrambling SDH signal, why scrambler is used in SDH?

In SDH/SONET system,  receivers recover clock based on incoming signal. Insufficient number of 0-1 transitions causes degradation of clock performance. In order to avoid this problem and to guarantee sufficient transitions, SONET/SDH employ a scrambler.

All data except first row of section overhead is scrambled . Scrambler is 7 bit self-synchronizing   X⁷ + X⁶ + 1 . Scrambler is initialized with ones

This type of short scrambler is sufficient for voice data. But this is not sufficient  for data which may contain long stretches of zeros. So, while sending data an additional payload scrambler is used.

This modern standards use 43 bit   X⁴³ + 1.  It run continuously on ATM payload bytes (suspended for 5 bytes of cell tax) . Run continuously on HDLC payloads

Scrambler : 

Types of switching in SDH Rings.

SPAN SWITCHING & RING SWITCHING

Span switching :

This type of switching uses only protection fibers on the span where fault is detected.

Ring switching : 

In this type of switching, traffic is switched away from filed span to adjacent node via the protection fibers on the long path.

REVERTIVE & NON-REVERTIVE SWITCHING

We can implement two modes of protection switching in SDH networks, revertive or non-revertive.

In revertive switching, once the fault condition has cleared, the network enters a "wait to restore state. One the configured WTR time is elapsed, traffic will be switched back to main path. This will be useful, wjen main path is much shorter than protection path.

In non-revertive mode, even after clearance of fault condition, traffic will not switch back to main path automatically.

How Protection Switching is implemented in SDH?

      For protection switching, mainly K1, K2 bytes and B2 bytes in Multiplex Section Overhead of SDH frame are used. Normally K bytes carried in protection fiber are used to carry APS protocol. B2 bytes contain bit interleaved parity check of the previously transmitted MSOH plus the VC-n payload.

K1/K2 Byte strycture:

K1/K2 byte structure is as shown in above diagram. Upto maximum 16 nodes can be supported in a SDH ring with protection. This is because, only 4 bytes are used for source and destination ID. In 4F-rings, APS protocol is only active on the protection fibers. APS protocol is optimised for AU level of operation. Each node in the ring should be configured with a ring map. This ring map contains information about the channels that node handles. Also, Each node in the ring is given a unique Id number within the range 0 to 15. 

 At any point of time, each node will be knowing the current status of the ring ( normal or protected). When the protection switches are not active, each node sends K-bytes in each direction indicating "no bridge request".  At the time of failure in the ring between two adjacent nodes,  two paths may exist for communication. Short path is the one, which directly connects both the nodes. Longer path connects these two nodes via all other nodes on the ring. When a node receives a non-idle K-byte message containing a destination ID of another node on the ring,  that node will change to pass through mode. 

Let us read about types of ring protection in next post

BLSR,Bi-directional Line Switched ring

There are two types of BLSR deployed in various networks.
i. 2-fiber BLSR
ii. 4-fiber BLSR

2-fiber BLSR:

This system is also known as two fiber multiplex-section shared protection ring. Here, service traffic flows bi-directionally. Both the fibers carries service and protection channels.

When the protection channels are not required, they can be used to carry extra traffic, but at the time of protection switching, this extra traffic is dropped. Only ring switching is supported by this architecture. At the time of ring switching, those channels carrying service traffic are switched to the channels that carry the protection traffic in the opposite direction.

4-Fiber BLSR:

This system is also known as four-fiber multiplex-section shared protection ring. This is the most robust ring architecture. This is most expensive to implementbecause of the extra optical hardware required.

In this system, bi-directional pairs of fibers are used to connect each span in the ring. One bi-directional pair carries the working channels, while the other pair carries protection channels. 4F-BLSR supports both span switching and ring switching. ( but both not at the same time). Multiple span switches can coexist on the ring. This is because, only the protection channels along one span are used for each span switch.

What triggers a protection? 

Protection switching is triggered in following cases.

1. Signal Fail , detected as Loss of Signal (LOS) at receiver input. This may be due to faulty hardware in the upstream network equipment or due to broken fiber.

2.Signal degrade, this is monitored by monitoring B2 bytes.

In next post let us read about : How is protection switching implemented in SDH ( K1 & K2 Bytes)

Ring networks - G.841 - Interview notes for UPSR

       Further to Linear protection, let us read about ring protection. ITU-T recommendation covers several types of ring network architectures. Ring protection switching can be implemented at path level or at line level. Rings can  be uni-directional or bi-directional. & they may utilise 2-fiber or 4-fibers.

UPSR : Uni-directional Path Switched Ring : 

 In a uni-directional ring, service traffic flows in one direction. ( clockwise in below diagram). Protection traffic flows in opposite direction  (counter clockwise)

In this example, traffic from C to B travels in clockwise direction via A. Traffic from B to C travels directly in clockwise direction. This configuration is also known as multiplex section dedicated protection ring.  This is because, one fiber carries service traffic, while the other is dedicated to protect the main path.All traffic is added in both directions. Decision as to which to use at drop point (no signaling). Normally non-revertive, so effectively two diversity paths

Main advantage of this configurations are :

 single ended switching, simple to implement and does not require any protocol. Single ended switching is always faster while compared to dual ended switching. Chances of restoring traffic under multiple fail conditions is high. Also, implementation of this architecture is least expensive.

However this arcitecture is Inefficient for core networks. There is no spatial reuse. Node needs to continuously monitor every tributary to be dropped.

In next post let us read about BLSR - Bi-directional line switched ring

Traffic Protection on SDH Optical Networks, Interview notes for SDH protection

        Service survivability has become more important than ever. This is because telecommunication is used increasingly for vital transactions such as electronic fund transfer, order processing, inventory control & many other business activities ( e.g : e-mail, internet access). Users are willing to pay more to get guaranteed service.

   In SDH transmission system, Automatic Protection Switching ( APS) algorithms and performance/alarm monitoring are built in. This system allows the construction of linear point-to-point networks and synchronous ring topology networks  which are self- healing in the event of failure. Also, to minimize the disruption of traffic, the protection switching must be completed within the specified time limit  ( sub 50ms) recommended by ITU-T G.783 (linear networks) and ITU-T G.841 (ring networks).

      Upon detection of a failure (dLOS, dLOF, high BER),  the network must reroute traffic (protection switching) from working channel to protection channel. The Network Element that detects the failure (tail-end NE) initiates the protection switching. The head-end NE must change forwarding or to send duplicate traffic.  Protection switching may be revertive (automatically revert to working channel)

Key ITU-T recommendations :

                ITU-T recommendations define methods of protecting service traffic in SDH networks. Two important recommendations are :

1.Recommendation G.783 covers linear point to point networks.

2.Recommendation G.841 covers various configurations of multiplex section rings.

Linear ( point to point) protection :

 In a linear network, protection is achieved through an extra protection fibre.  It can protect the network from fiber or NE card failure. Different variants of linear protection are 1+1, 1:1 and 1:N.

How it works ?

Head-end and tail-end NEs have bridges (muxes). Head-end and tail-end NEs maintain bidirectional signaling channel. Signaling is contained in K1 and K2 bytes of protection channel. K1 – tail-end status and requests. K2 – head-end status .

Linear 1+1 protection :

This is simplest form of protection. Can be at OC-n level (different physical fibers) or at STM/VC level (called SubNetwork Connection Protection) or end-to-end path (called trail protection) Head-end bridge always sends data on both channels. Tail-end chooses channel to use based on BER, dLOS, etc. No need for signaling. For non-revertive cases, there is no distinction between. working and protection channels. BW utilization is 50%.

Linear 1:1 protection :

In this case, Head-end bridge usually sends data on working channel. When tail-end detects failure it signals (using K1) to head-end. Head-end then starts sending data over protection channel. When not in use, protection channel can be used for (discounted) extra traffic  (pre-emptible unprotected traffic).

Linear 1:N protection:

This is verymuch similar to 1:1 protection with a small difference. Here, in order to save BW we allocate 1 protection channel for every N working channels. Here, N limited to 14.

Let us read about ring networks in next post.

Tributary Unit (TU) Frames, Interview notes on Tributary Unit frames in SDH

     Different sizes of Tributary Unit frames are used in SDH & we have seen basic SDH multiplexing structure in earlier post.

Different TU-Sizes provided in SDH are TU-11, TU-12. TU-2 and TU-3 . 

1. TU-11 : A TU-11 frame consists of 27 bytes, structured as 3 columns of 9 bytes. These 27 bytes provide a transport capacity of 1.728 Mbps at a frame rate of 8000 Hz. It will accommodate the mapping of 1.544 Mbps DS1 signal. 84 TU-11's can be multiplexed into a STM-1 frame. Structure as as shown below.

2.TU-12 : A TU-12 frame consists of 36 bytes, structured as 4 columns by 9 bytes.These 36 bytes provide a transport capacity of 2.304 Mbps at a frame rate of 8000 Hz. It will accommodate the mapping of 2.048 Mbps E1 signal. 63 TU-12's can be multiplexed into a STM-1 frame. Structure as as shown below.

3.TU-2 : A TU-2 frame consists of 108 bytes, structured as 12 columns by 9 bytes.These 108 bytes provide a transport capacity of 6.912 Mbps at a frame rate of 8000 Hz. It will accommodate the mapping of DS2 signal. 21 TU-2's can be multiplexed into a STM-1 frame. Structure as as shown below.

4.TU-3 : A TU-3 frame consists of 774 bytes, structured as 86 columns by 9 bytes.These 36 bytes provide a transport capacity of 49.54 Mbps at a frame rate of 8000 Hz. It will accommodate the mapping of 34 Mbps E3 signal and North American DS3 signal. 3 TU-3's can be multiplexed into a STM-1 frame. Structure as as shown below.

Maintenance signals in SDH & abbreviations

Let us read about maintenance signals in SDH.

LOS     Drop of incomming optical power level causes BER of 10-3   or worse

OOF     A1, A2 incorrect for more than 625 us

LOF      If OOF persists of 3ms

B1 Error    Mismatch of the recovered and computed BIP-8

MS-AIS      K2 (bits 6,7,8) =111 for 3 or more frames

B2 Error   Mismatch of the recovered and computed BIP-24

MS-RDI     If MS-AIS or excessive errors are detected, K2(bits 6,7,8)=110

MS-REI     M1: Binary coded count of incorrect interleavedbit blocks

AU-AIS      All "1" in the entire AU including AU pointer

AU-LOP    8 to 10 NDF enable or 8 to 10 invalid pointers

HP-UNEQ  C2="0" for 5 or more frames

HP-TIM     J1: Trace identifier mismatch

HP-SLM    C2: Signal label mismatch

HP-LOM    H4 values (2 to 10 times) unequal to multiframesequence

B3 Error  Mismatch of the recovered and computed BIP-8

HP-RDI     G1 (bit 5)=1, if an invalid signal is received in VC-4/VC-3

HP-REI     G1 (bits 1,2,3,4) = binary coded B3 errors

TU-AIS      All "1" in the entire TU incl. TU pointer

TU-LOP     8 to 10 NDF enable or 8 to 10 invalid pointers

LP-UNEQ  VC-3: C2 = all "0" for >=frames;

  VC-12: V5 (bits 5,6,7) = 000 for >=5 frames

LP-TIM     VC-3: J1 mismatch; VC-12: J2 mismatch

LP-SLM    VC-3: C2 mismatch; VC-12: V5 (bits 5,6,7) mismatch

BIP-2 Err  Mismatch of the recovered and computed BIP-2 (V5)

LP-RDI    V5 (bit 8) = 1, if TU-2 path AIS or signal failure received

LP-REI    V5 (bit 3) = 1, if >=1 errors were detected by BIP-2

LP-RFI    V5 (bit 4) = 1, if a failure is declared

Abbreviations : 

AU       Administration unit

HP       High path 

LOF    Loss of frame

LOM   Loss of miltiframe

LOP    Loss of pointer

LOS    Loss of signal

LP       Low path

OOF   Out of frame

REI     Remote error indication (FEBE)

RDI     Remote defect indication (FERF)

RFI     Remote failure indication

SLM   Signal label mismatch

TIM    Trace identifier

TU      Tributary unit

UNEQ  Unequipped

VC      Virtual Container

C        container

Detailed study of multiplexing process in SDH - Interview notes - Part III

    Let us continue from previous post, where we studied about multiplexing of VC-12 into VC-4. In this post, you will read about VC-4 Path overhead and Mapping of VC-4 into STM-1 frame.

VC-4 Path Overhead:

     The VC-4 Path Overhead forms the start of the VC-4 payload area and consists of one whole column of nine bytes as shown below. The POH contains control and status messages (similar to the V5 byte) at the higher order.

  J1 - Higher Order Path trace. This byte is used to provide a fixed length user configurable string, which can be used to verify the connectivity of 140 Mbit/s connections. 

B3 - Bit Interleaved Parity Check (BIP-8). This byte provides an error monitoring function for the VC-4 payload.

G1 - Higher Order Path Status. This byte is used to transmit back to the distant end, the results of the BIP-8 check in the B3 byte

K3 -Automatic protection Switching (APS). K3 provides for automatic protection switching control with VC-4 payloads. Similar to the K4 bits in the 2 Mbit/s overheads

    Mapping of a VC-4 into an STM-1 frame.

   

           An AU pointer is added to the VC-4 to form an AU-4 or Administrative Unit -4.

The AU pointers are in a fixed position within the STM-1 frame and are used to show the location of the first byte of the VC-4 POH.

The AU-4 is then mapped directly into an AUG or Administrative Unit Group, which then has the Section Overheads or SOH, added to it. These section overheads provide STM-1 framing, section performance monitoring and other maintenance functions pertaining to the section path.

The VC-4 payload, plus AU pointers and Section Overheads, together form the complete STM-1 transport frame.

If you have any question , please write to me

Detailed study of multiplexing process in SDH - Interview notes - Part II

      Further to previous post, let us read about mapping VC-12 into TU-12, TU-12 to TUG-2, TUG-2 to TUG-3 & TUG-3 to VC4.

Mapping of a VC-12 into a TU-12 signal.

  In order to detect the start of the 2 Mbit/s signal and thereby the start of the customers data, The V5 byte must be seen be the distant end. This is achieved by adding four overhead bytes to the multiframe, which together form a calculated byte count to the start of V5. This is called a pointer value and is known as the TU Pointer.

There are four pointer bytes called V1, V2, V3 and V4, which are used to calculate the location of V5.

Multiplexing of TU-12 into a TUG-2:

Each VC-12 consists of 36 bytes of information and these 36 bytes fill up exactly 4 columns of the STM-1 frame. 

3 separate TU-12's are directly mapped together to form a TUG-2. 

The 3 TU-12's will fit exactly into 12 columns of the STM-1 frame .

Mapping of a TUG-2 into a TUG-3 signal:

The mapping of TUG-2's into TUG-3's uses fixed column interleaving and is shown below.

Mapping of a TUG-3 into a VC-4 signal:

  The three TUG-3's are column interleaved to form the VC-4 payload. At this point two columns of fixed stuffing are added and the 'VC-4 Path Overhead' is added to the start. 

These are the steps followed in multiplexing VC-12 into VC-4. In next post, we will read about VC-4 Path over head 

If you have any questions, you can add them in comments section. I will provide you the answer.

Detailed study of multiplexing process in SDH - Interview notes

       Let us study the overview of the process followed by a 2 Mbit/s PDH input signal until it becomes part of an STM-1 frame. You can compare the details of each individual stage to SDH multiplexing structure provided in previous post.

Mapping of a 2 Mbit/s PDH signal into a C-12:

      The 2 Mbit/s PDH input signal is mapped into a Container 12 (C-12). The input frame consists of 32 bytes of information and this fits directly into the C-12 as shown

Mapping of a C-12 into a VC-12:

 At the time of mapping a C-12 into a VC-12, we need to add four bytes of overhead control information. But we can add only one byte per frame of customers' data So, this process takes place over 4 consecutive frames & described below: -

Frame number One has two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. One byte of overhead control information added to the start. This byte of over head is called the V5 byte and is known as the VC-12 Path OverHead (POH).

Two more important features of the V5 byte are:

BIP-2 is Bit Interleaved Parity Check-2. This looks at the data in the C-12. It counts all of the binary one's that it sees in the odd bit positions (i.e. bits 1,3,5,7 etc) and then it counts all of the binary one's that it sees in the even bit positions (i.e. bits 2,4,6,8 etc). This BIP-2 is then recalculated at the distant end. If the count is different, then some bit corruption has occurred.

 FEBE is Far End Bit errors. This bit is set correspondingly to the result of the BIP-2 check. If errors are received at the distant end then there needs to be a mechanism for informing the sender of the problem.

Frame number Two has two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. It then has one byte of overhead control information added to the start. This control byte in frame 2 is the Lower Order Path Trace or J2 byte. J2 is used to check continuity of a 2 Mbit/s path.

Frame number Three has two bytes of fixed stuffing added to it. One byte is added at the start and one byte at the end. One byte of overhead control information added to the start. This control byte N2, in frame 3 is called the Network Operator or Tandem Control byte. N2 is used to transmit performance-monitoring information where the circuit spans differing vendors networks. 

Frame number Four has one byte of fixed stuffing added to the end. It also has one byte of variable stuffing added to the start. One byte of overhead control information added to the start. Control byte in frame 4 is called K4 and it is used for 2 Mbit/s Automatic Protection Switching or APS. APS is used to automatically switch a single 2 Mbit/s circuit to its alternate path if a fault condition occurs.

In the next post we will learn about : 

 Mapping of a VC-12 into a TU-12 signal.

SDH Concatenation, Interview notes on Contiguous concatenation

There are two types of concatenation in SDH. They are Contiguous concatenation and Virtual concatenation. In this article, let us learn about contiguous concatenation.

Contiguous Concatenation :

  The SDH frame can be thought of as transport lorry. The data to be transported is placed in the VC-4 'Container'. This is then hitched to the SOH 'Cab unit' that 'drives' the data to its destination.The maximum carrying capacity of the vehicle is determined by the size of the 'container'. Therefore although the SDH signal is 155 Mbit/s in size, the largest single circuit that can be transmitted at any one time by the customer is limited to the size of the VC-4 i.e. 140 Mbit/s.

When using higher rates of SDH (STM-4, STM-16 etc), multiple 'containers' and 'cabs' are added one after another, to form a bigger vehicle. The customer is still limited to a single circuit size of 140 Mbit/s however, because each individual 'container' is still the same size (140 Mbit/s). They can however transmit multiple 140 Mbit/s circuits simultaneously.

Standard STM-4 structure is given below

The limitation of 140 Mbit/s per individual circuit is not a efficient way of managing bandwidth. In order to overcome this limitation, a method of combining 'containers' together has been developed which is called 'Concatenation'.

 STM-4 concatenated structure (VC-4-4C) is as shown below

Concatenated paths are commonly defined as VC-4-xC circuits (where x is size of the concatenation), as shown below:

 STM-4 concatenation (written as VC-4-4c), provides a single circuit with a bit rate of approximately 600M (actually 599.04 Mbit/s). STM-16 concatenation (written as VC-4-16c), provides a single circuit with a bit rate of approximately 2.2G (actually 2.2396160 Gbit/s).

STM-1 Frame Structure & Section Over head

STM-1 Frame Structure

 STM-1 frame contains 2430 bytes of information. Each byte contains 8 data bits (i.e. a 64kbit/s channel). Duration of STM-1 transport frame is 125ms. The number of frames per second is 1 second / 125ms = 8000 Frames per second.

So, rate of STM-1 frame is calculated as follows: -

8 bits x 2430 bytes x 8000 per second = 155,520,000 bits/s or 155 Mbit/s.

 STM-1  frame chopped up into 9 segments, stacked on top of each other as shown in the diagram below. The bits start at the top left with byte number one and are read from left to right and top to bottom. They are arranged as 270 columns across and 9 rows down.

STM-1 Section Overheads

      The STM-1 Section Overhead (SOH) consists of nine columns by nine rows as shown below. It forms the start of the STM-1 frame.The SOH contains control and status messages at the optical fibre level.

First three rows are RSOH ( Regenerator Section Overhead), Fourth row is AU-4 pointer. Fifth to Ninth row are MSOH ( Multiplexer section Overhead).

A1 & A2 - STM-1 Frame Alignment. These 6 bytes are used for STM-1 frame alignment. They are the first bytes transmitted. Frame alignment takes place over three STM-1 frames.

J0 - STM-1 Section Path Trace. This byte is used to provide a fixed length user configurable string, which can be used to verify network topology connections.

B1 - Byte Interleaved Parity Check 8 (BIP-8). This byte provides an error monitoring function for the entire STM-1 frame after encoding.

B2 - Byte Interleaved Parity Check 24 (BIP-24). These 3 bytes provide an error monitoring function for the STM-1 frame before encoding.A comparison between the BIP-8 and BIP-24 checks reveal if there were any encoding errors.

D1 to D12 - Data Communications Channel (DCC). These bytes provide a data channel for the use of network management systems.

K1 - Automatic protection Switching (APS). This byte is used to perform automatic protection switching of the optical fibre.

X - Reserved. These bytes are reserved for national use.

All unmarked bytes are reserved for future international standardisation.

SDH Principles and Interview questions on SDH Multiplexing structure

Overview

     The SDH standard defines a number of 'Containers' each corresponding to an existing PDH input rate. Information from the incoming PDH signal is placed into the relevant container.Each container then has some control information known as the 'Path Overhead' (POH) and stuffing bits added to it. The path overhead bytes allow the system operator to achieve end to end monitoring of areas such as error indication, alarm indication and performance monitoring data. The container and the path overhead together form a 'Virtual Container' (VC).

    Due to clock phase differences, the start of the customers' PDH data may not coincide with the start of the SDH frame. Identification of the start of the PDH data is achieved by adding a 'Pointer'. The VC and its relevant pointer together form a 'Tributary Unit' (TU).

       Tributary units are then multiplexed together in stages (Tributary User Group 2 (TUG-2) - Tributary User Group 3 (TUG-3) - Virtual Container 4 (VC-4)), to form an Administrative Unit 4 (AU-4). Additional stuffing, pointers and overheads are added during this procedure.This AU-4 in effect contains 63 x 2 Mbit/s channels and all the control information that is required.

    Finally, Section Overheads (SOH) are added to the AU-4.These SOH's contain the control bytes for the STM-1 section comprising of framing, section performance monitoring, maintenance and operational control information.An AU-4 plus its SOH's together form an STM-1 transport frame.

 Graphical SDH Multiplexing Structure

Full SDH Multiplexing Structure :

Diagram below shows full SDH Multiplexing structure. PDH signals enter on the right into the relevant container and progress across to the left through the various processes to form the STM frame.

  2 Mbit/s Multiplexing Structure

Let us see the multiplexing stages of  2 Mbit/s circuit. The relative bit rate and process is shown for each stage.

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