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.