Network Management Basics

Operational Tasks:

Following basic operational tasks are performed by network management system:

Protection :

Protection switching takes place within milliseconds ( sub 50 ms) & hence Circuit recovery in milliseconds ( failure should not be detected by voice customers)

Restoration:

By doing manual configuration, circuit recovery achieved in seconds or

Provisioning:

Allocation of capacity to preferred routes (according to certain time schedules)

Consolidation:

Moving traffic from unfilled bearers onto fewer bearers to reduce waste trunk capacity

Grooming:

Sorting of different traffic types from mixed payloads into separate destinations for each type of traffic.

OAM Functions and Layers

Level 1 - Regenerator Section: Loss of synchronization, signal quality degradation

Level 2 - Multiplex Section : Loss of frame synchronization, degraded error performance

Level 3 – Path : Assembly and disassembly, cell delineation control.

Data Communication Channel (DCC)

DCC is a in-band channel to facilitate communication between all Network Elements (NE) in a network. This facilitates remote login, alarms reporting, software download, provisioning 

Detailed operation of BLSR & Squelching.

Operation – Traffic flow :

Bi-directional traffic between two nodes is transported over a subset of the "ring sections" or "spans". In this configuration, Minimum capacity equals line rate. Capacity is in general expressed as number of AU4, or bandwidth. The bandwidth is provided by an integer number of AU4 payload.

Maximum bandwidth capacity :

 Here, each span has, in each direction, a capacity of up to half the number of AU4 in the STM-N (i.e. 8 AU4 for an STM-16 section). All traffic from a node goes to adjacent  nodes.

Max. capacity = 0.5 (line rate) x number of nodes.

Note: This Max is achieved only of the working traffic is transported only between two adjacent nodes.

Extra Traffic:

We can utilize shared protection bandwidth for Extra traffic. This extra traffic is not protected & it  could be lost when a failure of working traffic occurs

Operations – Fiber Cut :

Let us consider a scenario, where fiber cuts between A&B. We have a working traffic from A-C and C-A. This failure interrupts A-C and C-A traffic . Now Node A and Node B detect failure

Now node A and node B will switch the traffic to protection path. No dedicated protection bandwidth - only  used when protection required.  Only nodes next to the failure know about the protection switch.  No traffic lost.

Operations – Node Failure:

Let us consider that we have live traffic from D-F and F-D. If node B fails, Failure interrupts D-F and F-D traffic. Node A and C detect failure

Now Both  node A & C switch the traffic to protection channels. Only nodes next to the failure know about the protection switch. In this scenario, only   Traffic to/from  failed node lost.

Squelching Problem :

When a node fails, traffic terminating on those nodes cut off by failures could be misconnected to other nodes on the ring in case of using a local fail-over decision .

Consider a scenario, where we have active traffic from Node F-B , B-F and E-B, B-E. If Node B fails,

Squelching misconnection occur : Node F now talking to Node E instead of Node B

This can be avoided by path AIS Insertion. STM Path AIS is inserted instead of the looped STM-1#7. No mis-connections

Squelching Summary :

Squelching is in general used when extra traffic is used, it is used when normal traffic is switched to the protection entity and replaces the extra traffic. Squelching prevents that in case protection switch is active the normal traffic is output instead of the original extra traffic by outputting AU-AIS. You can also read clause 7.2.3.2 of ITU-T G.841

Squelching is required to assure that misconnections are not made. It is  required for bidirectional line switched rings only, since it is the only ring to provide a reuse capability of STM-1s around the ring. This is only required when nodes are cut off from the ring. Also this is only required for traffic terminating on the cut off nodes.

A ring map that includes all STM and VC Paths on the ring is available at every node on the ring. Squelching is also required for extra traffic since the extra traffic may be dropped when a protection switch is required.

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 : 

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.

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.

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).

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