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)

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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Line Rates and Hierarchy in SDH & SONET

The first hierarchy level for SDH is set at 155,520 kbit/s/s.

This is known as a Synchronous Transport Module 1 (STM-1).

Higher levels are simply multiples of the first level.

SDH allows for various PDH input rates to be mapped into containers as shown below:

• Container C11:      1544 kbit/s                  (1.5 Mbit/s) 
  Container C12:      2048 kbit/s                  (2 Mbit/s)
  Container C2:        6312 kbit/s                  (6 Mbit/s)
  Container C3:        49,536 kbit/s               (45 & 34 Mbit/s)
  Container C4:        139,264 kbit/s             (140 Mbit/s) 
  
  
  As can be seen from this chart, the only PDH rate that is not supported by SDH is 8 Mbit/s. 
  
  
  
  
  
  
  
  

Basic SDH Network Topology & Advantages of SDH

Let us read about the Basic SDH network topology. Detailed topology discussion will be done later.

Basic SDH Network Topology

    SDH networks are usually deployed in protected rings. This has the advantage of giving protection to the data, by providing an alternate route for it to travel over in the event of equipment or network failure.

Each side of the ring (known as A and B, or sometimes, East and West), consists of an individual transmit and receive fibre. These fibres will take diverse physical paths to the distant end equipment to minimise the risk of both routes failing at the same time.

The SDH equipment’s have the ability to detect the problem and will automatically switch to the alternate route.

SDH multiplexers transmit on both sides of the ring simultaneously, But to speed up switching times, they only receive on one side at any time. This means that only the receiving end needs to switch, thus reducing the impact of a fault on the customers' data.

 Features and Advantages of SDH

  In previous post we have seen the limitations of PDH. Now let us see the advantages of SDH.

·         SDH permits the mixing of the existing European and North American PDH bit rates.

·         All SDH equipment is based on the use of a single master reference clock source & hence SDH is synchronous.

·         Compatible with the majority of existing PDH bit rates

·     SDH provides for extraction/insertion, of a lower order bit rate from a higher order aggregate stream, without the need to de-multiplex in stages.

·       SDH allows for integrated management using a centralised network control.

·    SDH provides for a standard optical interface thus allowing the inter-working of different manufacturers equipment.

·         Increase in network reliability due to reduction of necessary equipment/jumpering.

Origin of SDH

             As seen from the previous post about PDH, PDH is a workable but flawed system.At the beginning it was the best available technology and was a giant leap forward in telecom transmission, As a result of growth in the field of silicon chips and integrated microprocessors, customer demand soon provided the need to introduce a new and better system.& it was expected  to solve the existing limitations of PDH.

        As a next step, Bellcore  introduced SYNTRAN (Synchronous Transmission) system. However this was only a development system. Soon it was replaced with SONET (Synchronous Optical Network).Initially SONET could only carry the ANSI (American National Standards Institute) bit rates i.e. 1.5, 6, 45 Mbit/s. Aim of the project was to provide easier international interconnection, Hence, SONET was modified to carry the European standard bit rates of 2, 8, 34 & 140 Mbit/s.

          In 1989 the ITU-T (International Telecommunications Union - Telecommunication's standardisation section), published recommendations which covered the standards for SDH. These were adopted in North America by ANSI (SONET is now thought of as a subset of SDH), making SDH a truly global standard