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 

Understanding Synchronization Protection Basics in Transmission network

Let us consider a ring network. Normal synchronization works around a ring. In this case, Nodes B-F are line timed. Node A is timed to an external reference. When a sync source is failed,  new time source should be selected in a reasonable amount of time

If synchronization is not restored , BER  will be  increasing through time.

SPS Timing Loops (SPS = Synchronization Protection Switching):

During a ring failure, simple reference switching would result in timing loops as shown below.

Operations – Normal Flow :

Let us read about the operation. Following diagram shows the normal flow of operation. Synchronization messaging in normal operation.

S1 = Stratum 1 Traceable

DU = Don’t Use

HO = Holdover

Operations – Fiber Cut:

In the ring, if fiber cuts between B & C,  Node C goes into short term holdover as shown below.

Then,   Node F switches to timing from Node A as shown below.

Finally ring is reconfigured and all nodes are again synchronized to BITS as shown below.

Please let me know if any clarification required :)

Reference Clocks & alternative clock source in SDH network.

Reference Clocks:

Let us read about reference clocks. Precision of internal clock is classified into so called “Stratum” levels. Accuracy of reference clock is defined as the ratio of bit slip happening (causing a bit error)

Stratum 1 => 1 x 10^-11 (synchronization to atomic clock)

Stratum 2 => 1.6 x 10^-9

Stratum 3E => 1 x 10^-6

Stratum 3 => 4.6 x 10^-6

Stratum 4 => 32 x 10^-6 (typical for IP routers)

When we are distributing the clock in the network, accuracy level might decrease at each hop in clock distribution. Originally providing Stratum 1 clocks for each network element was far from being  economical, even providing this service at multiple locations was too much demanding. So clock distribution methods were developed to minimize the number of high accuracy clocks needed in the network.

Global Positioning System (GPS) includes Stratum 1 atomic clocks on the satellites. Cheap GPS receivers are available in the market and they make it possible to have a Stratum 1 time source at almost any place. This reduces the need  for time synchronization network (might even go away in the future…).

Clock Distribution Methods :

Various clock distribution methods are as described below.

When all equipment is at the same location, External clock input might be used. This is usually BITS = Building Integrated Timing Signal. It uses an empty T1 or E1 framing to embed clock signal. Might be provided as a dedicated bus reaching into each rack in a CO environment. BITS should be generated from a Stratum 1 clock. Typically it will be deployed with a hot spare alternative source for fail-over.

Network elements not close to a BITS source should recover clock from the line.  While distributing the clock, Clock distribution network should not have loops, so a tree distribution topology should be configured. Usually carrier network element will have  Stratum 3 accuracy when running free. By synchronization to the reference clock, this clock is running at the same rate as the reference clock (that is Stratum 1). Minimum requirement for any network element is 20 ppm (that is between Stratum 3 and Stratum 4).

Alternative Clock Sources:

If the trail to the reference clock source is lost, the network element still continues normal operation. However, alarm might be generated.  After some time the clock might drift away so much, that bit errors would occur. Some time is left for switching over to an alternative clock source. Then the network element gets into a holdover state. Requirement is to have less than 255 errors in 24 hours.

A hierarchy of potential clock sources should be configured at each network element to achieve a high-availability operation. Typically a maximum 3 alternative time reference sources might be configured.  This is meaningful only if there are different paths to the alternative time reference sources. If only one natural path exists to a single time reference source, then the path must be protected by automatic protection switching. This requires some extra signaling to do it properly, called SPS = Synchronization Protection Switching.

Synchronization requirements & modes of timing in SDH transmission networks

 For synchronization of a transmission network, Frequency variation of bits transmitted should be inside the limits determined by the next hop’s ability to transmit these bits further.  Stuffing allows for some limited tolerance. In order to guarantee a low level of BER Frequencies should be synchronized all over the network.  Usually  Synchronization is done by recovering the embedded clock signal from the input signal . Synchronization source should have a very precise clock (reference clock). Reference clock might be reached only by multiple hops, but number of hops should be minimized.

Synchronization modes for transmission networks:

In a transmission network, Each network element has to be configured for time synchronization. Time reference distribution should minimize delay.

Various timing alternatives available are:

–        External

–        Line

–        Loop

–        Through.

Let us see the details .

External timing:

In this mode, all signals transmitted from a node are synchronized to an external source received by that node; i.e. BITS timing source.

Line Timing : 

In this mode,  All transmitted signals from a node are synchronized to one received signal.

Loop timing:

In this mode, the transmit signal in a optical link, east or west, is synchronized to the received signal from the same optical link.

Through timing : 

In this mode, the transmit signal in one direction of transmission around the ring is synchronized to the received signal from that same direction of transmission.

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 : 

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