Friday, 12 April 2013

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)
By doing manual configuration, circuit recovery achieved in seconds or
Allocation of capacity to preferred routes (according to certain time schedules)
Moving traffic from unfilled bearers onto fewer bearers to reduce waste trunk capacity
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 

Monday, 8 April 2013

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 :)

Saturday, 6 April 2013

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.

Thursday, 4 April 2013

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:
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.

Wednesday, 3 April 2013

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 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.