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:
1. All traffic on the ring was to hub into node TW. Currently
the traffic consists of 4 DS-1s per node.
2. Future traffic may need to be routed between other nodes.
3. 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.
4. A rich set of OAM&P functions should be provided for
equipment and signal monitoring and trouble shooting.
5. A fiber break should not drop traffic.
6. 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 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 tahng 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.
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.
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DDM-2000 provisioning command - ent-crs-vt1:a-1-1,m-1-1-1
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DDM-2000 provisioning command - ent-crs-vt1:m-1-1-1,m-1-1-1 (pass through)
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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 | |||
| 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 | |
| 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
|
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 |
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.
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.
| 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
|
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 |
| 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
|
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 |
| 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
|
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 |
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.