CONCLUSION
The optimum solutions for synchronizing the new ring and chain topologies should be based on the three main ingredients for synchronization distribution; (1) Timing Quality, (2) Traceability and (3) Fault Toler-ance. Distributed PRS improves the reliability of the synchronization network by providing network nodes with an alternate and diverse source of timing. This solution is available today. In addition, the sync network can use the intelligence of synchronization messaging when it becomes available to ensure network survivability and repair work that may still be required.
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Figure 12: UTC Time The ultimate goal is to limit the amount of repair work
that synchronization messaging must do. By using strategic deployment of PRS's complemented with sync messaging, optimum fault tolerance and reliabil-ity can be obtained. This dual approach provides guaranteed traceability as well as survivability.
UTC Time
Clearly as a source of timing, UTC is the most accurate and the most universally accepted. (Figure 12)
The use of GPS and/or LORAN-C technologies are currently the chosen mechanism for reception of UTC time. Traditional solutions in this area have only been used at large hub locations due to their high capital equipment and installation cost. In addition, interference factors such as Selective Availability have prevented their wide scale deployment.
Telecom Solutions has addressed the obstacles above with the new Local Primary Reference (DCD-LPR) product line. The DCD-LPR provides a techno-logical, affordable and practical solution for wide-spread deployment of LORAN-C and GPS. Many telephone companies are currently in the process of deploying LPR technology at all of their TNC sites. These sites typically contain a high quality clock based on rubidium technology The combination of
UTC TIME Central
Office
Messenger
(Source)
(Destination)
Figure 13: Sync Distribution Using LPR And TNC-E
the Telecom Solutions TNC-E Enhanced Rubidium clock with the DCD-LPR far surpasses the 1x10-11 frequency accuracy required by ITU/ETSI for Pri-mary Reference Clock (PRC) performance. Since TNC sites are often points of interface between carriers as well as critical hubs in the network, these sites make strategic sense for deployment of primary reference sources.
Deployment of the LPR along with the currently used TNC-E clock, can form a very robust timing architec-ture. Following is a diagram which shows deploy-ment of a SDH chain (this also holds true for rings) being timed by these elements.
TNC-E SDH SDH SDH SDH TNC-E LPR LPR Timing from PDH Timing from PDH
at this point. Further, since the timing feed coming from NE-3 going to NE-2 is already at a good status, NE-2 can immediately switch to this timing feed and network operation is fully restored.
It should also be noted that since NE-2 has two PRS references directly connected to it, there is no reason for NE-2 to revert back to its original timing feed from NE-1. Thus the total number of rearrangements required is 2 (as compared to 8 in the previous example). Again, since there were two good (“G”) references feeding NE-2 rearrangement time was reduced. Recall that rearrangements are to be avoided or at least minimized whenever possible.
One of the primary objectives for the development of sync messaging was to enable network providers to ensure the integrity of the existing interoffice hierar-chical synchronization distribution network.
Figure 11: Sync Messaging Enhanced With Distributed PRS
Sync messaging, fundamentally, is a tool for fixing a problem after it has already occurred. It is only after a failure has taken place that sync messaging goes into action in order to attempt to repair synchroniza-tion distribusynchroniza-tion.
Sync Messaging will take time to reach a universally accepted approach. In addition, the mixture of plesiochronous and synchronous networks will also slow down the implementation of sync messaging.
From a practical standpoint, ensuring integrity would be best obtained by building a network with the highest level of reliability. A key ingredient to creat-ing reliability, is link diversity. Distributed PRS is a solution that can help to provide additional timing feeds into the network. This translates into a reduc-tion of rearrangements which improves reliability.
AA
SDHA
SDH PRS G1 D1 G1 G1 G1 G1State
1
NE #3 NE #2A
A
SDH SDH PRS B2 D1 B2 G1 G1 G12
NE #3 NE #2AA
AA
A
A
SDH SDH PRS B2 G2 D3 G1 G1 G13
NE #3 NE #2 = HoldoverFigure 10: SDH Ring Network With Additional Timing Source
traditionally been built from the perspective that sync is as vital as “Power and Ground”. It needs to have 100% up time and is there when you need it (Power, Ground and Sync). The synchronization networks which support SDH rings and chains should be built with this same philosophy. Let’s take the previous example, and modify the sync architecture so that a PRS is added at NE-3. (Figure 10)
Example 2
1. The scenario starts off the same as before. NE-2, is initially transmitting G1.
2. NE-2 enters a holdover mode in which it uses its internal clock to generate timing. It must switch to its internal clock because it has to take time to make a decision about the other available reference sources.
3. Since NE-3 is being timed from a local PRS , it is not affected by the message that was sent to it from NE-2. The propagation of the timing disruption stops
PRS NE#1 NE#6 NE#2 NE#5 NE#4 PRS PRS NE#3 Example 1 Results
Looking back at the sequence of events in example 1 let us analyze the bottom line impact on the network. The process of going into holdover is considered a "rearrangement". Likewise, the process of switching references and coming out of holdover is also considered a “rearrangement”.
If we review the rearrangement activity in the previ-ous example, the number of rearrangements that were needed to restore timing was 4. If we then consider that the timing feed which was initially broken (connection between NE-1 and NE-2) should eventually revert back to its normal operating state, then an additional 4 rearrangements will be required (making the total 8) before normal operating condi-tions are fully restored.
Rearrangements and reconfigurations are activities that are contrary to the fundamental principles of sync distribution. Synchronization networks have
Since NE-4 is not receiving timing from NE-3, it does not respond to this message. However, it should be pointed out that NE-4 now only has one good source of timing (coming from NE-5 only; so in effect its reliability has been reduced since it has lost its secondary timing feed).
4. 3, recognizes that the timing coming from NE-4 is good, so it switches to receiving timing from this new direction. NE-3 then sends out a G2 message to NE-2 informing that the timing coming from itself is now good.
5. NE-2 recognizes the timing coming from NE-3 is good, so it switches to receiving timing from this new direction.
Figure 9: Sync Messaging Sequence Now let’s see what happens when a fiber failure
occurs between NE-1 and NE-2.
1. NE-2 is transmitting G1 towards NE-3.
2. Input to NE-2 lost. NE-2 enters a holdover mode in which it uses its internal clock to generate timing. It must switch to its internal clock because it has to take time to make a decision about the other avail-able reference sources. In addition, NE-2 changes the timing status being sent to NE-3 from G1 to B2.
3. NE-3 receives the B2 message from NE-2 and also enters a holdover mode. Additionally, it sends out a B2 message to NE-4.
A
A
A
A
SDH SDH G1 D1 G1 D1 G1 G1 State 1 NE #3 NE #2A
SDH SDH B2 D1 B2 D1 G1 G1 2 NE #3 NE #2 SDH SDH B2 D1 B2 D1 B2 G1 3 NE #3 NE #2 = HoldoverA
SDH SDH B2 D1 B2 G2 D3 G1 4 NE #3 NE #2A
SDHA
SDH B2 G2 D3 G2 D3 G1 5 NE #3 NE #2PRS NE#1 NE#6 NE#2 NE#3 NE#5 NE#4 PRS
elements have the ability to send messages which identify themselves and instruct other elements to take specific action. The messages tell the SDH element clocks to reconfigure so timing is taken from an alternate route.
Example 1
Let’s use an example of a SDH ring to take a closer look at the principle of how sync messaging could function. In the following figure, timing is transferred from two timing sources which are located at NE-1 and NE-5. In the counterclockwise direction, NE-2 receives its timing from NE-1, NE-3 receives timing from NE-2 and NE-6 receives its timing from NE-5. In the clockwise direction, NE-4 receives its timing from NE-5.
Sync messaging is comprised of a set of messages. At the present time these message sets are still not universally accepted. For the sake of simplicity this example employs only a three state messaging scheme; “G” for good sync, “B” for bad sync and “D” for don't use. The events played out are denoted by a G, B or D message distinguished by a number in the succession in which they are transmitted or received.
Figure 8: SDH Ring Network FAULT TOLERANCE
One of the main application benefits of the new SDH ring architecture is “survivability”. Although the implementation of self-healing and automatic
reconfigurations makes sense for traffic connections, it adds complexity and administrative problems to synchronization distribution. Traceability and timing integrity become especially vulnerable. The end result is that fault tolerance of the sync network is compromised.
Synchronization Status Messaging?
In order to maintain the hierarchical synchronization structure within the new SDH topologies, the ITU/ ETSI standards body is considering a messaging technique that will allow SDH terminal equipment to determine the quality level of the timing source.
Contained within the SDH overhead are bytes that have been reserved for transmitting Sync Status messages. In addition, these messages can be passed between elements using the spare bits in the E1 2Mbit/s frame.
The fundamental strategy behind synchronization messaging is that when a failure occurs, network
Figure 7: Distributed PRS (Flattened Topology)
The administration of timing feeds is more complex within a SDH network. Since the fibre optic facilities are now synchronous and node timing is derived from the STM-n line rate, sync feeds can no longer transparantly make their way through a node (contrary to the plesiochronous network). In Figure 6, the timing feed coming from “Node D” is no longer viable since it doesn’t pass through “Node C” untouched (timing integrity is there-fore lost; LNC should not feed TNC).
G81s TNC TNC
PRS
G81s
PRS PRS
Figure 6: Timing transmitted over an SDH network
In order to create better timing integrity and ease administration within the SDH ring and chain environment, a new synchronization architecture comprising distributed PRS is already being widely considered in North America for the SONET architecture. Bellcore has recently distributed a Generic Requirements document (GR-2830-CORE) for primary reference sources relevant to SONET. Distributed PRS can be thought of as “flattening” the synchronization distribution. With a flatter distribution, every node can continue to have two diverse reference feeds directly traceable to a higher order clock source. This improvement in synchronization access between a particular node and the highest order source available (the PRS) guarantees optimum network performance. See Figure 7.
TNC TNC LNC PRC Node B Node A Node C Node D From Upstream PRS Hierarchy broken, LNC feeding TNC SDH SDH SDH SDH
TIMING INTEGRITY
The traditional method for ensuring the integrity of the existing hierarchical synchronization network is to use the master-slave structure. This approach makes use of the established hierarchical levels of clocks to pass timing between points in the network. In this architecture a higher level clock location passes timing to an equal or lower level location. Figure 4 illustrates this structure.
Synchronization rules prohibit a lower level from feeding a higher level location (e.g. LNC should not feed TNC). Timing is passed in only one direction; from higher levels to equivalent or lower levels of clock performance. Separate facilities are used as much as possible for routing the primary and second-ary timing feeds to a particular location.
Figure 4. Hierarchical Timing Distribution Quite often this requires that a clock feed from a PRC
site to a TNC site be routed through (not retimed by) an office that is a LNC node at a lower level. In a plesiochronous network, since timing is not transmis-sion dependent, routing of diverse sync feeds can be accommodated.
In Figure 5, "Node A" is passing timing to "Node B" and "Node C". A diverse feed from "Node D" is also passing timing to "Node C" and Node "B". The hierarchy is maintained since the clock in Node C (LNC) does not retime the sync feed going from "Node D" to "Node B" (only plesiochronous equip-ment is used).
Figure 5: Timing transmitted over a plesiochronous network
TNC
Primary
Reference
Clock
TNC LNC LNC LNC TNC TNC LNC PRC From Upstream PRS Node B Node A Node C Node DFigure 2: Timing transmitted over an SDH network
within the SDH payload however have been disquali-fied for use as a synchronization signal for SDH. The main reason for the disqualification is the inability to guarantee the level of pointer activity. Excess pointer activity will have a direct impact on timing phase movement and PRC traceability.
There are two solutions being recommended by Standards bodies: (1) use the STM-n line rate for synchronization distribution or (2) receive timing from a trail that is external to the SDH network.
In a SDH network, the signal rates of the facilities are defined to be fixed rates and therefore the SDH
then retransmitted. SDH network elements use a 4.6 ppm accuracy clock. When these 4.6 ppm clocks are cascaded, timing characteristics can be expected to accumulate.
The result of this effect is that when many clocks are linked together, timing problems can occur. Due to the much higher bit rates and the smaller equipment buffers of SDH, even a small perturbation in timing can have a dramatic impact on network operation.
SDH deployment will continue to grow in support of additional service offerings. Consequently, more complex ring and chain topologies with their cumula-tive timing effects will also proliferate. To ensure the success of SDH, a new approach to sync distribution is being considered which involves a wider distribu-tion of Primary Reference Sources. This approach creates a "flatter" distribution and avoids the timing problems resulting from cascaded clocks. Figure 3 illustrates this approach.
terminals can use these rates as a distributor of timing. Likewise, the SDH element must be able to generate accurate timing for the outgoing signal rate and other facilities. Figure 2 illustrates timing being passed over an SDH chain. Each node in a SDH network represents a location where timing is re-ceived, derived from the incoming signal rate and
Figure 3: Distributed Sources minimizes cascaded clock effects
PRC
SDH SDH SDH SDH
TNC-E
SDH clock receives timing but doesn't pass timing Feed from PRS Feed from PDH PRC SDH SDH SDH SDH Intervening SDH clocks STM-n passes timing
Timing subject to cumulative effects from SDH clocks
Figure 1: Timing transmitted over a plesiochronous network KEY ATTRIBUTES OF A SYNC NETWORK
Traceability
Traceability is without a doubt the most misused and misunderstood term in describing synchronization. It is often assumed if a PRC exists somewhere in the network, and a particular node is indirectly tied to it, the node is “traceable to Primary Reference timing”. Although true from a theoretical and quiescent standpoint, service providers responsible for synchro-nization, will quickly point out the difficulty in maintain-ing traceable timmaintain-ing in a real life network. This
difficulty is exacerbated when the complexities of a variable path timing network via intelligent connec-tions are considered.
As transmission networks convert from
plesiochronous to synchronous via SDH, the trace-ability of timing becomes further complicated with the addition of “intervening clocks”. Now, since each SDH element contains a clock that retimes the transmission facilities, these clocks will inevitably cause changes in the stability and hence traceability of the timing path.
Let’s first look at synchronization within a
plesiochronous network. Fundamentally, multiple 2Mbit/s (E1) signals are multiplexed into higher order bit rates 8/34/140Mbps (E2,E3,E4) and transported at the appropriate line rate.
PRC
Timing passes transparently
Facility clock rate not part of timing distribution
Traceability is maintained
PDH PDH PDH PDH
The 2Mbit/s signal can be used to pass timing and travels transparently through each node.
The clock rate of the facility is not tied to the clock rate of the 2Mbit/s signal. In fact, the clock rate of the facility connections between any two nodes is effectively independent of the clock rate between any other two nodes. The reception and generation of the clock signal passes through the multiplexing equipment and facility terminal without being re-clocked.
Figure 1 shows the timing flow through a
plesiochronous network. In this case traceability to the PRC source is maintained because the timing signal passes through without change.
Let us now look at the impact and differences be-tween plesiochronous and synchronous networks from a timing perspective. Due to the fact that timing in a SDH system is derived from the facility rate, each SDH element will have a direct impact on the timing signal.
In SDH a 2Mbit/s signal is mapped into a virtual container (VC) within the synchronous transport module (STM-n) by the use of “pointers”. This technique allows the 2Mbit/s to “float” such that its position in time is not fixed. These 2Mbit/s signals
THE SYNCHRONIZATION NETWORK
The goal of network synchronization is to get the best possible source of timing to all nodes of the network. The two fundamental requirements that must be in place to ensure network synchronization are:
(1) an accurate source of timing and
(2) a "reliable messenger" that distributes timing to all of the nodes.
The architecture employed to date to achieve net-work synchronization has been based on hierarchical timing distribution.
Hierarchical timing distribution involves the establish-ment of Primary Reference Clock (PRC) locations which then feed subtending nodes of either Transit Node Clock (TNC) or Local Node Clock(LNC) quality. These PRC locations fill the first fundamental require-ment of the hierarchical network for an accurate source. These locations typically contain replicated caesium clocks manually calibrated to Universal Coordinated Time (UTC).
The second requirement of hierarchical timing distribution is filled by qualified landline connections that serve as reliable messengers for transporting timing from one node to the next. These landlines
are typically engineered and maintained to minimize the effects of network rearrangements on synchroni-zation distribution.
SDH
The deployment of SDH introduces timing complexi-ties as a result of Ring and Chain transmission architectures. Now, the network is no longer made up of simple point-to-point connections but has been replaced with a transport network of “intelligent” and "survivable" connections.
These intelligent connections result in paths that are subject to change for our reliable messenger. In addition to the changes, these routes are no longer through paths but are subject to perturbations introduced by serial SDH network elements.
The subsequent administration and maintenance of this variable path timing network must also be carefully planned in order to ensure network integ-rity.
As the SDH network continues to grow, the role of landline connections, as the reliable messenger for timing distribution, will become more difficult to manage. Let us take a more detailed look at these difficulties.
Synchronizing The Rings And Chains Of SDH
Abstract
SDH must be implemented successfully in order for the network to support the burgeon-ing era of new high speed digital services. This application note addresses some of the synchronization problems and issues in the network that are a result of the new SDH ring and chain topologies. An analysis of the synchronization network with special attention to distribution architectures and the effects of SDH is presented. This paper will then conclude with a discussion of synchronization architectures that will enable timing distribution to migrate to a higher level of reliability in support of the new topologies and services.
