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Home > Services > Optical Technical Support >Deploying and Troubleshooting a 6 Node SONET Ring

Deploying & Troubleshooting a 6 Node SONET Ring

SONET rings provide a fault tolerant, and flexible transmission architecture. Deploying SONET in a private network campus application provides the user with a great deal of flexibility. A ring provides self healing path redundancy and the flexible grooming of DS-1s and DS3s without the need for back to back terminals and patch panel cross connects.
SONET maps DS-1s into synchronous virtual tributaries, VT1.5s and DS-3s into synchronous transport signal of the first level, STS-1. VT1.5s are mapped into STS-1s for transport across the network.	The synchronous nature of the VT1.5 and STS-1 allows for direct access to the payload and facilitates efficient add drop multiplexing and grooming. We recently deployed a campus 6 node ring, see figure 1. The nodes in the network are given two letter designators. These are the initials of engineers, technicians, or other persons associated with the network. Our requirements were as follows: All traffic on the ring was to hub into node TW. Currently the traffic consists of 4 DS-1s per node. Future traffic may need to be routed between other nodes. Bandwidth for a future full motion video (DS-3) had to be reserved. The video had to be easily reconfigurable, that is, for several months a requirement might exist for video between node RS and GG and then at a later date video may be required between nodes RB and TW. A rich set of OAM&P functions should be provided for equipment and signal monitoring and trouble shooting. A fiber break should not drop traffic. The system should be upwardly compatible with new and emerging transport technologies as well as providing a migration path for larger bandwidth utilization. SONET technology would best fulfill these requirements. After exhaustive vendor evaluations it was decided to purchase AT&T, now Alcatel-Lucent DDM-2000 OC-3 multiplexers.

UNDERSTANDING THE CROSS CONNECTS

An OC-3 system can transport 84 DS-1s, 3 DS3s or any combination in-between. A simplified block diagram of the DDM-2000 OC-3 is shown in figure 2. DS-1s are interfaced to the SONET world by low speed modules. Each low speed module accepts 4 DS-1s and maps each DS-1 into a VT1.5. The 4 VT1.5s are then combined to form a virtual tributary group, VTG.

A maximum of eight low speed modules (7 service, 1 protect) will populate a low speed group in the DDM-2000 shelf, see figure 3. Seven VT groups are muxed into an STS-1 and passed to the optical line interface unit for muxing to the OC-3 line rate. The virtual tributary group to STS-1 multiplexing is performed by the MXRVO (Multiplexer-Virtual tributary -to Optical) circuit pack.

When transporting a DS-3 the MXRVO circuit pack is replaced by a DS-3 interface module which directly maps the DS-3 into an STS1. Looking at figure 3, the signal flow within the DDM-2000 is from right to left, low speed to function to main. Each step along the way taking you higher into the SONET hierarchy.

The OLIU performs two functions: multiplexing the STS-1s to the OC-3 line rate and time slot interchange, TSI. It is the TSI capability that allows the DDM-2000 to serve as an add drop multiplexer, ADM. In a point to point terminal multiplexer default mappings are typically used where traffic in low speed group A at the near end is mapped to low speed group A at the far end. Function group mapping likewise would be from A to A, B to B. and C to C. In a ring however this is not the case. The user needs to enter the cross connects, i.e. traffic must be assigned to time slots that are added, dropped, or passed through the nodes on the ring. Figure 4 generically shows this
.

As we are homing a total of 20 DS-1s to node TW, 4 DS-1s from each node all the traffic could fit within one STS-1. This also makes it very convenient to assign the traffic at JL to VTG-1, node MF traffic to VTG-2, and so on,
see figure 1.

Table I shows the cross connects and virtual tributary assignments for node TW. The first low speed (ls) channel to time slot entry in Table I is read as follows, low speed group "a", slot 1, DS-1 number 1, mapped to the main unit "m", STS-1

number 1, virtual tributary group number 1, DS-1 number 1. The last entry in Table I similarly is read, low speed "a", slot 5, DS-1 number 4, mapped to main "m", STS-1 number 1, virtual tributary group number 5, DS-1 number 4.

Every node needs to have all the traffic that appears on the ring defined as a cross connect at that node, regardless of whether or not traffic is dropped at that node.

CROSS CONNECT MAPPING: NODE TW ALL TRAFFIC DROPPED

1	1	a-1-1	m-1-1-1
2	1	a-1-2	m-1-1-2
3	1	a-1-3	m-1-1-3
4	1	a-1-4	m-1-1-4
5	2	a-2-1	m-1-2-1
6	2	a-2-2	m-1-2-2
7	2	a-2-3	m-1-2-3
8	2	a-2-4	m-1-2-4
9	3	a-3-1	m-1-3-1
10	3	a-3-2	m-1-3-2
11	3	a-3-3	m-1-3-3
12	3	a-3-4	m-1-3-4
13	4	a-4-1	m-1-4-1
14	4	a-4-2	m-1-4-2
15	4	a-4-3	m-1-4-3
16	4	a-4-4	m-1-4-4
17	5	a-5-1	m-1-5-1
18	5	a-5-2	m-1-5-2
19	5	a-5-3	m-1-5-3
20	5	a-5-4	m-1-5-4
DDM-2000 provisioning command - ent-crs-vt1:a-1-1,m-1-1-1 TABLE I

Table II shows the mapping at node MF. The first four entries show the dropped traffic.. The other 16 entries are the passed through traffic.. For the first DS-1 dropped at node MF the traffic is mapped, a-2-1 >>>>> m-1-2-1 >>>>> a-11 Node TW time slot Node MF. Only after all the cross connects at all the nodes are established will the ring support traffic.

CROSS CONNECT MAPPING: NODE MF VT GROUP 2 TRAFFIC DROPPED
LOCAL DS-1 #	VT GROUP	ls CHANNEL	TIME SLOT
1	2	a-1-1	m-1-2-1
2	2	a-1-2	m-1-2-2
3	2	a-1-3	m-1-2-3
4	2	a-1-4	m-1-2-4
		TIME SLOT
PASS THROUGHS	1	m-1-1-1	m-1-1-1
	1	m-1-1-2	m-1-1-2
	1	m-1-1-3	m-1-1-3
	1	m-1-1-4	m-1-1-4
	3	m-1-3-1	m-1-3-1
	3	m-1-3-2	m-1-3-2
	3	m-1-3-3	m-1-3-3
	3	m-1-3-4	m-1-3-4
	4	m-1-4-1	m-1-4-1
	4	m-1-4-2	m-1-4-2
	4	m-1-4-3	m-1-4-3
	4	m-1-4-4	m-1-4-4
	5	m-1-5-1	m-1-5-1
	5	m-1-5-2	m-1-5-2
	5	m-1-5-3	m-1-5-3
	5	m-1-5-4	m-1-5-4
DDM-2000 provisioning command - ent-crs-vt1:m-1-1-1,m-1-1-1 (pass through) TABLE II

The DDM-2000, when configured as a ring, utilizes unidirectional path protection switching. Therefore once the channel is assigned to a time slot, traffic traverses the ring in both a clock wise and counter clockwise direction.

Conceptually this is shown in figure 5. Here we have node A in communication with node C. When a subscriber at node A picks up the phone and says hello to his friend at node C, that "hello" travels clockwise around the ring, is cross connected through node B and exits the ring at node C.

When the subscriber at node C says hello back to his friend that "hello" enters the ring and again travels clockwise, is cross connected through node D, and is dropped at node A. This is denoted by the solid line in figure 5. Hence the term unidirectional in unidirectional path switched ring. Simultaneously the same traffic at nodes A and C is bridged onto a fiber and sent counter clockwise around the ring. This is the dashed line in figure 5.

The receiver at nodes A and C has access to both clockwise and counter clockwise traffic streams, the nodes decide which one to accept based on the received signal quality.

In figure 5 the outer ring is labeled "working traffic" and the inner ring "protection" traffic. A better nomenclature might be active and standby as all traffic is always present on both the inner and outer ring. Because of this there is no so called "normaled up" configuration. Power up sequence, fiber connection sequence, etc., will affect which path is associated with active traffic.

Figure 6 depicts what happens in the event of a fiber break. The path from A to C that was carrying traffic no longer exists so C switches to the protection path. Node A on the other hand is unaffected by the break and does not switch the receive path.

RECOVERING FROM A CABLE BREAK

Now that we have covered the background theory it would be instructive to see how actual equipment responds in the event of a cable break. Figure 7 shows the ring with a complete cable break between nodes RB and RS. In light of the previous discussion on path switching let's first check the stake of the paths on the ring. The DDM-2000 will generate the path state report shown in figure 8 when the "rtrvstate-path" command is entered at node TW.

At TW main 1 (ml) OLIU, receive, is associated with traffic that is flowing in the counter clockwise direction. After the cable break therefore ml at TW can no longer "see" nodes JL, MF, and RB along with their associated traffic VTG1, VTG2, and VTG3, respectively. Main2 (m2) on the other hand is cut off from nodes RS, GG and VTG4, VTG5 respectively.

This is exactly what figure 8 shows. There is a signal failure shown at ml-1-3-4 at node TW. This is OLIU Mains, STS-1 number 1, VT group number 3, DS-1 number 4. This is the fourth DS-1 at node RB. The signal failure at m2 is associated with VT groups 4 and 5, nodes RS and GG respectively. From this report alone you could infer a cable break between nodes RB and RS.

When bringing up the alarm list at TW figure 9 will appear. Every traffic carrying time slot has an incoming VT AIS (Alarm Indicator Signal). The service status is nsa, non service affecting. The ring thus works as advertised and no traffic is dropped.

On comparing figures 8 and 9 one can see that the incoming AIS's are associated with the ring signal failure condition of the path state report. The AIS's received by TW on ml are generated at node RS.

Recalling Table I when the cross connects were set up, VT groups 1,2,3, and 5 were cross connected as pass throughs at node RS. VT groups 1,2, and 3 never make it to node RS. The DDM-2000 at node RS then inserts AIS's which are terminated at TW on ml.

Likewise node RB inserts AIS's in VT groups 4 and 5, which are associated with nodes RS and GG respectively, i.e. the nodes cut off from node RB. The alarm list of figure 9 also indicates that there is alarm activity at all the other nodes as well.

R1-TW 70-01-01 03:11:41 DDM-2000 OC-3, R5.1.1
rtrv-state-path:m1-all COMPLD
*/ Path Protection State Report
----------Ring 1----------		----------Ring2----------

Address Act	APS Condition	Address Act	APS Condition

m1-1-1-1	signal failure	m2-1-1-1 Y
m1-1-1-2	signal failure	m2-1-1-2 Y
m1-1-1-3	signal failure	m2-1-1-3 Y
m1-1-1-4	signal failure	m2-1-1-4 Y
m1-1-2-1	signal failure	m2-1-2-1 Y
m1-1-2-2	signal failure	m2-1-2-2 Y
m1-1-2-3	signal failure	m2-1-2-3 Y
m1-1-2-4	signal failure	m2-1-2-4 Y
m1-1-3-1	signal failure	m2-1-3-1 Y
m1-1-3-2	signal failure	m2-1-3-2 Y
m1-1-3-3	signal failure	m2-1-3-3 Y
m1-1-3-4	signal failure	m2-1-3-4 Y
m1-1-4-1 Y		m2-1-4-1	signal failure
m1-1-4-2 Y		m2-1-4-2	signal failure
m1-1-4-3 Y		m2-1-4-3	signal failure
m1-1-4-4 Y		m2-1-4-4	signal failure
m1-1-5-1 Y		m2-1-5-1	signal failure
m1-1-5-2 Y		m2-1-5-2	signal failure
m1-1-5-3 Y		m2-1-5-3	signal failure
m1-1-5-4 Y		m2-1-5-4	signal failure
Figure 8

R1-TW 70-01-01 02:50:53 DDM-2000 OC-3 R5.1.1
rtrv-alm:all COMPLD
/* Active Alarms and Status Report

Alarm Level	Source Address	Date Time Detected	Srv	Description

MINOR	farend	01-01 02:50:41	-	R4-RB
MINOR	farend	01-01 02:43:07	-	R5-RS

ne-acty	farend	01-01 02:50:40	-	R3-MF
ne-acty	farend	01-01 02:43:10	-	R2-JL
ne-acty	farend	01-01 02:43:09	-	R6-GG
ne-acty	m2-1-4-1	01-01 02:43:07	nsa	inc. VT AIS
ne-acty	m2-1-5-1	01-01 02:43:07	nsa	inc. VT AIS
ne-acty	m2-1-4-2	01-01 02:43:07	nsa	inc. VT AIS
ne-acty	m2-1-5-2	01-01 02:43:07	nsa	inc. VT AIS
ne-acty	m2-1-4-3	01-01 02:43:07	nsa	inc. VT AIS
ne-acty	m2-1-5-3	01-01 02:43:07	nsa	inc. VT AIS
ne-acty	m2-1-4-4	01-01 02:43:07	nsa	inc. VT AIS
ne-acty	m2-1-5-4	01-01 02:43:07	nsa	inc. VT AIS
ne-acty	m1-1-1-1	01-01 02:43:06	nsa	inc. VT AIS
ne-acty	m1-1-2-1	01-01 02:43:06	nsa	inc. VT AIS
ne-acty	m1-1-3-1	01-01 02:43:06	nsa	inc. VT AIS
ne-acty	m1-1-1-2	01-01 02:43:06	nsa	inc. VT AIS
ne-acty	m1-1-2-2	01-01 02:43:06	nsa	inc. VT AIS
ne-acty	m1-1-3-2	01-01 02:43:06	nsa	inc. VT AIS
ne-acty	m1-1-1-3	01-01 02:43:06	nsa	inc. VT AIS
ne-acty	m1-1-2-3	01-01 02:43:06	nsa	inc. VT AIS
ne-acty	m1-1-3-3	01-01 02:43:06	nsa	inc. VT AIS
ne-acty	m1-1-1-4	01-01 02:43:06	nsa	inc. VT AIS
ne-acty	m1-1-2-4	01-01 02:43:06	nsa	inc. VT AIS
ne-acty	m1-1-3-4	01-01 02:43:06	nsa	inc. VT AIS

Figure 9

Figures 10 and 11 show the alarm lists from the two nodes that bracket the cable break, nodes RS and RB respectively. Both nodes have an incoming OC-3 LOS (loss of signal). This alarm is fairly indicative of a broken cable. Node RS also shows a holdover mode active alarm. This is because we initially configured the network to be ffrned off of the internal oscillator at TW (a BITS clock could be used as well), with all other nodes loop timed on the incoming OC-3 line to Main 1 OLIU.

R4-RB 70-01-01 03:02:34 DDM-2000 OC-3, R5.1.1
rtrv-alm:all COMPLD
/* Active Alarms and Status Report

Alarm Level	Source Address	Date Time Detected	Srv	Description

MINOR	farend	01-01 02:51:12	-	R5-RS
MINOR	main-2	01-01 02:43:51	-	inc. OC3 LOS

ne-acty	farend	01-01 02:56:15	-	R3-MF
ne-acty	farend	01-01 02:56:08	-	R6-GG
ne-acty	farend	01-01 02:56:06	-	R1-TW
ne-acty	farend	01-01 02:56:25	-	R2-JL
Figure 10

R5-RS 70-01-01 03:55:20 DDM-2000 OC-3, R5.1.1
rtrv-alm:all COMPLD
/* Active Alarms and Status Report

Alarm Level	Source Address	Date Time Detected	Srv	Description

MINOR	farend	01-01 02:52:24	-	R4-RB
MINOR	main-1	01-01 02:43:48	-	inc. OC3 LOS

ne-acty	farend	01-01 02:51:23	-	R3-MF
ne-acty	farend	01-01 02:43:52	-	R2-JL
ne-acty	farend	01-01 02:43:51	-	R6-GG
ne-acty	farend	01-01 02:43:51	-	R1-TW
ne-acty	-	01-01 02:43:48	-	holdover mode active
Figure 11

Lastly, let's look at the alarm list of node MF, figure 12. Here the only local alarms are incoming VT AIS's on the VT1.5s that are terminated at this node. Node RB expects to see traffic on VT group 2, originated at TW and destined for MF, node RB performs the pass through cross connect on this VT group. Because of the cable break there is no signal on Main 2 OLIU of node RB.- The DDM-2000 inserts an ADS into VT group 2 which is then terminated at node MF on m2 OLIU as shown on the alarm list. Nodes JL and GG also have incoming VT AIS's on VT groups that the nodes terminate respectively.

R3-MF 70-01-01 03:04:35 DDM-2000 OC-3, R5.1.1
rtrv-alm:all COMPLD
/* Active Alarms and Status Report

Alarm Level	Source Address	Date Time Detected	Srv	Description

MINOR	farend	01-01 02:51:26	-	R4-RB
MINOR	farend	01-01 02:43:48	-	R5-RS

ne-acty	farend	01-01 02:43:54	-	R2-JL
ne-acty	farend	01-01 02:43:53	-	R1-TW
ne-acty	farend	01-01 02:43:53	-	R6-GG
ne-acty	m2-1-2-1	01-01 02:43:52	nsa	inc. VT AIS
ne-acty	m2-1-2-2	01-01 02:43:52	nsa	inc. VT AIS
ne-acty	m2-1-2-3	01-01 02:43:52	nsa	inc. VT AIS
ne-acty	m2-1-2-4	01-01 02:43:52	nsa	inc. VT AIS
Figure 12

CONCLUSION

SONET rings inherently provide a high degree of reliability and survivability. In the event of a cable break the DDM-2000 protect traffic via the path switching ring architecture. The DDM-2000 has a rich set of OAM&P functions as evidenced by the presented alarm lists generated from a single cable break. In fact the user is sometimes overwhelmed by the barrage of alarms that can accompany a signal fault in a SONET ring system. However once the basics of SONET are understood the nodal alarms form a logical pattern which leads to fault isolation and repair.