New Timing Distribution Mechanism

1
New Timing Distribution Mechanism

• TICTOC WG, IETF 71th Philadelphia, USA
• draft-ji-tictoc-new-timing-distribution-mechanism-
  00.txt
• Kuiwen Ji (jikuiwen_at_huawei.com)

2
Agenda

• Background
• Route Technique using for timing distribution

3
Background

• Synchronization is typically distributed from one
  central office to another using the SONET/SDH
  signal for optical networks.
• Each node has two synchronization sources
• - a primary and secondary source.
• this provides a degree of protection for the
  synchronization network
• As a last line of defense, clock hold-over
  provides minimum service quality for a given time
  period.

4
Typical Master-Slave Synchronization Example
5
Todays Network

• Synchronization planning and distribution is
  administered manually base on SSM (G.781)
  usually.
• SONET/SDH networks are primarily implemented in
  linear and rings architectures.
• Now with the introduction of the network
  controlled by GMPLS and synchronous Ethernet, it
  is probable that the transport architecture will
  shift from linear/ring to mesh architecture.
• Mesh networks will provide more
  paths/combinations for synchronization
  distribution.

6
Using of SSM

• The SSM (G.781) has been used for a long time in
  Sync network.
• A synchronization coordinator usually determines
  how best to implement synchronization to each
  piece of equipment in the network and configure
  the priority of reference sources to each.
• We are careful to avoid timing-loops when
  planning there synchronization networks. Not
  every bi-directional link can be used even if
  they are available in the ring.

7
Limitation of SSM
Source 1
Source 1
Source 1
1
1
1
1
1
1
2
2
3
2
1
1
1
1
1
2
2
2
3
3
3
2
2
2
3
1
2
2
2
Source 2
Source 2
Source 2
C(v)
B(v)
A()
priority of reference sources

Nodes

Main timing tracing path
Backup timing tracing path
8
Another simple example
Source 1

• Clock source 1 is assumed to be a higher priority
  clock than clock source 2 for this example.
• Probably we can plan the synchronization like
  this.

1
1
2
1
1
1
2
2
Source 2
3
5
2
3
2
4
4
1
2
priority of reference sources

Nodes

Main timing tracing path
Backup timing tracing path
9
Multiple failures

• If the source 1 fails and a failure occurs
  between node 1 and 2, node 1 will go to holdover.
  
• Node 1 can get the synchronization from blue link
  but it cant use it now.
• We can change the priority of each node to make
  another configuration of course. But for
  preventing timing-loop we still cant use every
  link bi-direction even if it would be possible to
  use them.
• The point is that no one configuration is best
  for every type of possible failure condition.
  There is still limitation.

Source 1
X
1
Holdover
1
2
X
1
1
1
2
2
Source 2
3
5
2
3
2
4
4
1
2
priority of reference sources

Nodes

Main timing tracing path
Backup timing tracing path
10
Normal mesh network

• How could the synchronization be setup? Whats
  the best configuration?
• We need to be very careful to avoid timing-loop.
  Thus, we have to give up many of the
  bi-directional links.

Source 1
Source 2
Nodes
Source 3
Main timing tracing path
Backup timing tracing path
11
Agenda

• Background
• Route Technique using for timing distribution

12
Information distribution
Source 1

• With the GMPLS control plane, its possible to
  know the network topology and the state and
  condition of links.
• And the reference source attribution, like
  priority, quality can be distributed through
  route protocol OSPF.
• So all nodes know the network topology and which
  source output to be used and traced as the
  primary timing source.

1
4
2
3
5
7
8
6
9
Source 2
Node

13
Calculating the traceability paths
Source 1

• Each node calculates the timing tracing path to
  the master clock source based on the topology and
  the primary source.
• From the root of the primary reference, simple
  calculating algorithm like Dijkstra can be used
  to establish a shortest path tree.
• The synchronization distribution algorithm would
  be like a tree structure to prevent timing loop.

1
4
2
3
5
7
8
6
9
Source 2
Node

Timing tracing path
14
Building a timing tree
Source 1

• A ready message is sent when timing
  traceability path is setup and operational.
• Each node will not switch to a new
  synchronization source until it knows the new
  synchronization source is ready.
• After a node traces to a new timing source
  successfully, the node will send a message to the
  next to show it is ready.

1
M
4
M
M
2
3
5
7
8
6
9
Source 2
Node

Timing tracing path
15
Building a timing tree
Source 1

• A ready message is sent when timing
  traceability path is setup and operational.
• Each node will not switch to a new
  synchronization source until it knows the new
  synchronization source is ready.
• After a node traces to a new timing source
  successfully, the node will send a message to the
  next to show it is ready.

1
4
M
2
3
5
M
M
7
8
6
9
Source 2
Node

Timing tracing path
16
Building a timing tree
Source 1

• A ready message is sent when timing
  traceability path is setup and operational.
• Each node will not switch to a new
  synchronization source until it knows the new
  synchronization source is ready.
• After a node traces to a new timing source
  successfully, the node will send a message to the
  next to show it is ready.

1
4
2
3
5
M
7
8
M
6
9
Source 2
Node

Timing tracing path
17
Building a timing tree
Source 1

• A ready message is sent when timing
  traceability path is setup and operational.
• Each node will not switch to a new
  synchronization source until it knows the new
  synchronization source is ready.
• After a node traces to a new timing source
  successfully, the node will send a message to the
  next to show it is ready.

1
4
2
3
5
7
8
6
9
Source 2
Node

Timing tracing path
18
Failure occurs between nodes 3 and 7 disrupting
the sync path
Source 1
1
4
2
3
5
X
7
8
6
9
Node

Source 2
Timing tracing path
19
A second failure occurs between nodes 6 and 7
Source 1
1
4
2
3
5
X
7
X
8
6
9
Node

Source 2
Timing tracing path
20
Source 1 Fails
Source 1
X
1
4
2
3
5
7
8
6
9
Source 2
Node

Timing tracing path
21
Interworking with existing networks
BITS 1
BITS 4
SSMPRC
1
Source1
DNU
SSMPRC
1
PRC
1
2
BITS 3
PRC
1
PRC
1
2
SSMPRC
PRC
2
2
PRC
1
PRC
1
2
PRC
PRC
SSMSSU
2
BITS 2
Node which use automatic techniques

• All blue nodes could be viewed as one node
  which use traditional SSM at the boundary to
  interwork with others.

Node which doesnt use automatic techniques
Main timing tracing path
Backup timing tracing path
22
Benefits

• Can be used in future network like Synchronous
  Ethernet, 1588 or any network with GMPLS.
• Provide survivability (sync traceability) for
  multiple failures.
• Possibly ease requirements on clock holdover mode
  by providing traceability in event of multiple
  failures (i.e., maintain service quality).
• Easy planning and maintenance. People dont need
  to do complex work in Synchronization scheme and
  configuration.
• More

23
Next Step

• Timing distribution is very important for
  synchronization. Comments from the group are
  always appreciated
• Working with CCAMP with respect to the GMPLS
  extensions, which supports this feature.