protection presentation

8/14/2019 Protection Presentation

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PROTECTION SWITCHING


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PROTECTION SWITCHINGFeature

Most Important Feature of SONET Standard is its Resiliency Nature i.e.Restoration of the Traffic in Case of Failure within a specified time duration.

Protection switch time

A protection switch refers to the action of re-routing the traffic to the protectionreserved bandwidth upon a failure on the working side. The SONET standardspecifies that a protection switch should take place within 60 msec. This timeinterval includes:

Failure detection time.

Switching time.

Frame resynchronization.

Propagation delay.

Typically, in the 60 msec time budget, 10 msec is reserved for fault detectionand 50 msec is reserved for traffic switching.

The main motivation behind this specification (60 msec switch over) is relatedto the requirement that the lower speed streams, such as DS1 and DS3, that aremultiplexed in the high-speed traffic stream, must not lose frame

synchronization at their receivers.


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Traffic Protection

Automatic Protection Switching (APS) is the capability of a transmissionsy

stem to detect a failure on a working facility, and to switch a standby(protection) facility to recover the traffic.

Bytes K1 and K2 in the Line Overhead of the first STS-1 signal are used tocarry the protocol that coordinates the protection switching. Therefore,protection switching in SONET is done at the line level by the Line Terminating

Equipment (LTE).

Protection switching occurs as a result of:

Signal failure (SF) LOS (Loss of Signal), LOF (Loss of Frame), AIS(Alarm Indication Signal) (AUTOMATIC)

Signal Degradation (SD) (BER Bit Error Rate 3: Excessive Error)

(AUTOMATIC)In response to certain user initiated commands. (MANUAL)


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Types Of Protection Switching

LINEAR PROTECTION

1+1 Linear Protection

1:1 Linear Protection

1:N Linear Protection

RING PROTECTION

UNIDIRECTIONAL

UPSR (Unidirectional Path Switched Ring)

BIDIRECTIONAL

2F-BLSR (Bidirectional Line Switched Ring)

4F-BLSR (Bidirectional Line Switched Ring)


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1 + 1 Linear Protection

Working

Protection

Head-end Tail-end

SelectorBridge

Working

Protection

Head-end Tail-end

SelectorBridge

One fiber is called the work ing fiber

and the other the protect ion fiber.

100 % Traffic is transmitted

simultaneously on both the Fibers.

Both fibers are usually diversely

routed.

In Normal conditionthe destination selects one of the two fibers based on thequality of the received signal, this being the Worker.

In Case of Failure in Worker, the Destination Switches to the Protection.1+1 is a fast restoration mechanism: since no APS signaling is required to

achieve a protection switch (assuming 1+1 unidirectional).

Doesnt support low priority traffic (extra traffic).


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61:1 Linear Protection

Working

Protection

Head-end Tail-end

SelectorBridge

Working

Protection

Head-end Tail-end

SelectorBridge

100% Traffic is transmitted only on

one fiber i.e. the Worker.

No traffic or Extra Traffic (Low

Priority Traffic) is Transmitted on

Protection Fiber.

Usually both the Fibers are diversely

routed

In Normal conditionthe TX and RX at Source and Destination are bridged onworking Fiber.

In Event of Failure in Worker, the TX and RX at Source and Destination switch to

Protection. The Extra Traffic Is lost during this Period.


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Node Node

Working Channel #1

Protection Channel

Working Channel #2

Working Channel #N

Node Node

Protection Channel

Working Channel #2

Working Channel #N

Working Channel #1

Traffic is provisioned only on working lines and NO or Extra Traffic is provisioned

on Protection Line..

N lines share one Protection Line.

During Failure, the protection Line takes care of only the Defective Line.

Economical Scheme.

If more than one working line becomes defective then , protection line will take

care of the working line with more priority

1:N Linear Protection


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8APS bytes for Linear Protection Switching

1 2 3 4 5 6 7 8

K1 Byte: Indicate the request

Channel #

1 2 3 4 5 6

Reverse

Channel #

Example: K-bytes definitions for linear protection as per Bellcore GR-253.(SONET)

K2 Byte: Indicate the bridging action

111 Line AIS110 Line RDI101 Bidirectional APS100 Unidirectional APS

0 1+1 Mode1 1:N Mode

Switch

PreemptionPriority

1111 Lockout of Protection1110 Forced Switch1101 Signal Fail - High (for 1:N only)1100 Signal Fail - Low1011 Signal Degrade - High (for 1:N only)1010 Signal Degrade - Low

10011000 Manual Switch01110110 Wait-to-Restore01010100 Exercisor00110010 Reverse Request (for bidirectional only)0001 Do Not Revert (for 1+1 non-revertive only)0000 No Request

7 8


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RING PROTECTIONS

UNIDIRECTIONAL

Information is only transmitted in one direction. A connection to the

neighboring NE may require to traverse an entire length of the ring to

complete the Connection.

Drawback :

Significant Propagation time variation can occur in both the TransmissionDirection.

BIDIRECTIONAL

Connections are Bidirectional i.e. both the Transmit and receive traverse the

same path and Distance in the Ring.

Shorter Path : Working Path Longer Path : Protection Path

We will look at the three most popular ring topologies;

Unidirectional Path Switched Ring (UPSR),

4 fibers Bidirectional Line Switched Ring (4F-BLSR) and

2 fibers Bidirectional Line Switched Ring (2F-BLSR).


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RING PROTECTIONS


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Unidirectional Path Switched Ring (UPSR)

In UPSR, one of the fibers is considered the working fiber and the other the protection

fiber.

Traffic is transmitted simultaneously on the working fiber in the clockwise direction andon the protection fiber in the counter-clockwise direction. I.e. B and E transmit in both

Directions simultaneously as in 1+1 protection.

In event of failure between C and D the receiver is E is switched over to Fiber 2(Red -

Protection) where it finds a connection immediately.

The advantages of UPSR relies in its simplicity and low cost.No knowledge of ring

configuration is required.

No Extra Traffic can be Provisioned In UPSR RING


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Working

Protection

In 4F- BLSR, two fibers are used as workingfibers and two are used for protection.

Working Traffic is routed to the shortest pathbetween two nodes in the ring. This maximizes

the amount of spatial reuse obtained.

Using 4F-BLSR 100% Protection can be

achieved.

Extra Traffic can be provisioned on the

Protection Fiber Pair

A BLSR ring can support up to 16 nodes(limited by the node identifier in the K-bytes).

Nortel OC-192 BLSR supports up to 24 nodes.

Maximum ring length is limited to 1200km (6

ms propagation delay).

Protection switch can be achieved in 60ms.

However, for longer rings (underseaapplications), the 60 ms restoration time has

been relaxed.

BLSR rings are widely deployed in long-haul

network, where the traffic pattern is more

distributed than in access network.

4-Fiber Bidirectional Line Switched Ring (4F-BLSR)


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134F-BLSR: Span switch

SPAN SWITCHING :

In a span switch, if a fiber is cut on

the working side, the traffic is routed

onto the protection fiber between the

two same nodes, as shown in thepicture.

No switching takes place on the

Other NEs in the ring.

A span switch is similar to a linear

1:1 protection switch. It is always

revertive and both directions are

always switched simultaneously

(similar to linear bidirectional switch).

Working

Protection

Span

Switch

A 4F-BLSR ring supports two types of protection mechanisms:

Span switching and Ring switching.


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4F-BLSR: Span switch


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154F-BLSR: Ring switch

RING SWITCHING :

If both the Worker and Protection

fiber get cut, service is restored by a

ring switch.

In a ring switch, the traffic on the

failed link is then rerouted around the

ring on the protection fibers.i.e. on the

Long Path

This requires coordination between

the two nodes that terminates thefailed fibers. APS protocol has to take

place around the long path of the ring.

During Ring Switch the Extra Traffic

Is lost.

Working

Protection


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4F-BLSR: Ring switch


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In 2F-BLSR, both the fibers are used

to carry 50% working traffic, and rest

50% of the capacity on each fiber is

reserved for protection purposes.

Therefore the total Capacity of the

Ring reduces to 50%.

Unlike a 4F-BLSR, span switch isnot possible in a 2F-BLSR. In the

event of a link failure, the traffic is

rerouted on the long path of the ring

using the protection capacity available

on the two fibers.

A 2F-BLSR ring supports only one

type of protection mechanism; Ring

switching.

2F-BLSR

Working

Protection


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2F-BLSR


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1 2 3 4 5 6 7 8

K1 Byte: Indicate the request

DestinationNode ID

1 2 3 4 5 6 7 8

K2 Byte: Indicate the bridging action

SourceNode ID

0 Short Path Request1 Long Path Request

SwitchPreemption

Priority

1111 Lockout of Protection [LP-s] orSignal Fail protection [SF-P]

1110 Forced Switch Span [FS-S]1101 Forced Switch Ring [FS-R]1100 Signal Fail Span [SF-S]1011 Signal Fail Ring [SF-R]1010 Signal Degrade Protection [SD-P]

1001 Signal Degrade

Span [SD-S]1000 Signal Degrade Ring [SD-R]0111 Manual Switch Span [MS-S]0110 Manual Switch Ring [MS-R]0101 Wait-to-Restore [WTR]0100 Exerciser Span [EXER-S]0011 Exerciser Ring [EXER-R]0010 Reverse Request Span [RR-S]0001 Reverse Request Ring [RR-R]0000 No Request

Source

Node ID 111 Line AIS110 Line RDI101 Reserved100 Reserved011 Extra Traffic010 Bridged & Switched001 Bridged000 Idle

APS K-Byte (K1, K2) Ring protection

Example: K-bytes definitions for BLSR protection as per GR-1230, issue 4.


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20Revertive mode and WTR (Wait to Restore)period Once protection switching has been employed, what happens when the original

working fiber cut has been repaired?

In theory, when using 1+1 protection scheme, no action is needed since therepaired fiber now becomes the protection fiber. This type of protection

switching is termed non-revert ive .

In 1:N protection scheme, however, it is desirable to have the traffic switchedback onto the original working fiber so that new failures can be handled and

low-priority traffic may continue to use the protection fiber. This type of

protection switching is termed revert ive.

1:N always uses revertive mode with a user-provisionable WTR period.

1+1 can use revertive (with WTR) or non-revertive mode.


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21protection Switching Priority


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protection switch requests can occur automatically by thesystem or by user initiated actions.

Automatic switch

User-initiated switches

Manual switch

Forced switch Lockout of Working

Lockout of Protection


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Manual Switch :- Initiates a switch from either a working line to the protection

line or vice versa. This command has the lowest priority

Forced Switch:-

Forces a switch from either a working line to protection line or

vice versa without regard to the state of either line.


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Wait to Restore (WTR)

Rapid protection switching and reversion can wreak havocon signal quality.

System must remain stable (good signal on working channel)

for the duration of WTR time before protection channels willrevert back to working channels.

WTR is 2nd lowest in Protection hierarchy (all user-initiatedand auto switches will override WTR).


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The Lockout of Working commands (LOW-span or LOW-ring) are not

signalled on the APS channel [the k-byte protocol] but are implemented

locally using the Data Communication Channels (DCC), while the

Lockout of Protection (span) is signaled on the APS with the K-byte

protocol.

LOW-span and LOW-ring are applied against the working line, whereas

LP-span is applied against the Protection line.

Lockout of Protection - span (LP-S) has the same priority as Lockout of

Working - Span (LOW-S), Lockout of Working - Ring (LOW-R), and

Lockout of Protection - all spans (LOP-all spans). Each of these

commands can coexist and must be cleared separately.

GeneralLockout: Prevent a working channel to use the protection channel, even in

case of line failure.

Lockout commands


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Lockout of Working -Span (must be applied atboth ends of the fiber span) prevents theaddressed working line from using its spanprotection partner for the purposes of spanswitching. If a working failure were to occur, aring switch would be allowed. The command has noimpact on the use of protection for any otherspan. It remains active until it is released.

In other words, LOW-span forces traffic totravel on the working line along the span.

Lockout of Working -Ring (must be applied at bothends of the fiber span) prevents the addressedworking line from using its ring protection partnerfor the purposes of ring switching, by disabling thenodes capability to request a ring switch. If a

working failure were to occur, a span switch to theprotection span would be allowed. It remains activeuntil it is released.

When active, no ring switch can occur along thespan.

When a Global lockout of working is applied to the ring (from PMEM), no ring switchcan occur anywhere around the ring.

W

P

W

P

Lockout of Working commands


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Lockout of Protection -Span is appliedagainst the protection line ; it prevents theuse of the span for any protection activityand prevents ring switching anywhere in thering. If a working failure were to occur on theaddressed span, neither a span switch (forthis particular span) nor a ring switch would

be allowed. Unlike lockout of working, lockoutof protection span is signaled via the APSchannel. It remains active until it is released.

When active, traffic is prevented fromtraveling on the protection line along thespan.

When a Global lockout of protection is applied to the ring (from PMEM), traffic cannottravel on the protection lines around the ring, preventing any span or ring switches.

W

Lockout of Protection

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