Internet DRAFT - draft-hjxl-ssn-ps
draft-hjxl-ssn-ps
Internet Working Group L. Han
China Mobile
Internet Draft Y. Jiang
J. Xu
Intended status: Informational X. Liu
Huawei
Expires: April 2016 October 20, 2015
Problem Statements of Scalable Synchronization Networks
draft-hjxl-ssn-ps-00.txt
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Abstract
With the wide deployment of 4G and beyond mobile networks, a great
number of cells need high precision frequency and/or time
synchronization for their normal operation. It is crucial to manage
the synchronization network in a scalable way and simplify the
monitoring and operation for synchronization networks. This document
analyzes the use cases and requirements in synchronization networks,
and provides a problem statement for scalable synchronization
networks.
Table of Contents
1. Introduction .............................................. 2
1.1. Conventions used in this document ...................... 4
1.2. Terminology ............................................ 4
2. Use cases for scalable synchronization network ............ 4
2.1. Synchronization configuration .......................... 4
2.2. Synchronization OAM .................................... 5
2.3. Synchronization network protection and recovery ........ 6
2.4. Multi-layer/Multi-domain synchronization network ....... 7
3. Synchronization Requirements .............................. 7
4. Security Considerations ................................... 8
5. IANA Considerations ....................................... 8
6. References ................................................ 8
6.1. Normative References ................................... 8
6.2. Informative References ................................. 8
7. Acknowledgments ........................................... 9
1. Introduction
In modern communication networks, most telecommunication services
require that the frequency or phase difference between the whole
network equipments should be kept within the reasonable range.
Especially for mobile networks, there is a requirement for high
precision network clock synchronization, including frequency
synchronization and phase synchronization.
For packet switching networks, SyncE and IEEE 1588v2 protocols are
widely deployed for frequency and time synchronization respectively
in mobile network. Synchronization path planning and provisioning are
very complex as so many parameters (e.g., quality level, priority,
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synchronization enable/disable, hop limit, holdover timeout, and etc)
need to be configured. Furthermore, configuration of SyncE must not
introduce any loops in the synchronization paths. Hence, deployment
of synchronization network requires professional skills in
synchronization protocols and also the engineering capability in
analyzing and planning the network topology.
With the deployment of 4G network, the density of cells is
explosively growing, as a result, the size of mobile networks and its
backhaul network has greatly increased (it may consist of tens of
thousands of network equipments in a single metro city). This
scalability requirement will pose a great challenge to realize
synchronization, and the management and monitoring of the
synchronization network becomes dramatically more complex for service
providers.
In the past, management and monitoring of synchronization networks
are mainly resorted to manual configuration and manual diagnosis,
which are complex, error-prone and very time-consuming. Thus it is
hard to avoid synchronization loops, erroneous configuration and
other mistakes. Therefore, it is important to provide some tools to
improve the efficiency of fault monitoring and detection in
synchronization networks.
As the synchronization is critical for the mobile services, it will
beneficial to provide path protection for synchronization networks,
so that single point of synchronization failure can be avoided (or
even provide multipoint protection as much as possible, i.e., even
when the working path and a protection synchronization path are both
lost, the network can figure out a new synchronization path so that
frequency source is still available. This may require that a third
synchronization port be configured as a recovery port).
Furthermore, as the mobile network size increases dramatically, the
synchronization performance is hard to be satisfied, e.g., care must
be taken to guarantee that a certain hop limit (e.g. 20 hops) of
time-distribution from the timing source to a cell site is not
exceeded.
This document provides some use cases and requirements on
configuration and management of a large synchronization network and
provides problem statements for scalable synchronization networks.
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1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.2. Terminology
OAM: Operation Administration and Maintenance
BMCA: best master clock algorithm
T-GM: Telecom Grandmaster, a device consisting of a Boundary Clock as
defined in [IEEE-1588], with additional performance characteristics
defined in [G.8273.2].
2. Use cases for scalable synchronization network
Following are some use cases of scalable synchronization networks
from a management and operation viewpoint.
2.1. Synchronization configuration
In a huge mobile backhaul network with more than 10,000 nodes, manual
planning and provisioning of synchronization network are very onerous.
For example, manual planning and configuration for a simple network
may need more than several weeks; furthermore, it is error-prone. And
the planning can't eliminate the risk of introducing loops to a
synchronization network.
To facilitate synchronization configuration, a central controller may
be introduced. The controller shall automatically compute, plan and
provision the synchronization paths based on the overall physical
network topology, thus it can eliminate the risks associated with
manual planning.
A typical controller for synchronization network can compute and
provision a synchronization network with tens of thousands of nodes
in just a few minutes, and it is guaranteed that no synchronization
loop will be introduced if the algorithm is correctly implemented.
Synchronization configuration via a centralized controller requires
that the controller be highly efficient, agile and reliable.
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To accommodate for different types of equipment implementations, a
common interface is needed for synchronization network configuration
and management, it can further provide the ability to retrieve the
network's synchronization configuration and states of a protocol
engine in a device. For example, whether the device is locked or not,
what is the port state of PTP port (i.e., master, slave or passive),
the current port ID associated with a frequency source in syncE, and
etc... This capability is essential for the management and
maintenance of synchronization networks.
2.2. Synchronization OAM
In the maintenance of a huge synchronization network, an operator may
encounter various synchronization problems. The traditional manual
trouble shooting hop by hop is very onerous. Even if the malfunction
equipments are located in a single operator network, the fault
detection procedure is very tedious, let alone in the case of network
interworking with a third party.
Traditionally, synchronization fault detection is done by checking
synchronization devices on a path one by one manually. I.e., an
operator must login to the device (i.e. the device is adjacent to the
fault base station or the device nearest to the base station among
the devices with the clock alarm), read the configuration information,
status and clock alarms information. After analyzing all the
information, if the operator still can't locate the source for the
fault, the operator must find the upstream device according to the
synchronization status information (i.e. the port state of 1588v2 and
the current tracing clock port ID of syncE). The operator must login
to each upstream device and check the synchronization information one
by one, until the source device of the synchronization fault is found.
If the operator cannot locate the fault by the current limited
information from the equipments, the operator may have to test the
synchronization performance manually by instrument.
This procedure requires that the operator must have a deep
understanding of the synchronization protocols and principle of
synchronization engineering. And it also is very time-consuming, and
sometimes, detecting a single clock fault may even cost up to ten
days.
Sometimes the clock synchronization performance of base station
degraded but no clock alarm is raised. Through synchronization fault
detection an operator cannot locate the true reason of service
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disruption. In that case, synchronization performance monitoring may
solve the problem by dynamically monitoring the synchronization
performance of all devices in the clock synchronization path for a
base station in problem.
Therefore, the functions of synchronization OAM shall include
synchronization fault detection and synchronization performance
monitoring, both are vital in the diagnosis of a synchronization
network.
2.3. Synchronization network protection and recovery
If a synchronization path is broken or degraded, it will seriously
influence the clock performance of the synchronization network, and
further affect the other services of the mobile network. Thus
protection and recovery of the synchronization network are very
necessary.
In general, if allowed by the network topology, the equipment should
be provisioned with a working and a protection synchronization path
for SyncE in a mobile network. Thus, the equipments in the mobile
network can realize synchronization protection with both the working
and backup clock ports.
Even when neither the clock signal on the working port nor on the
backup port is available (i.e. loss of signal or degrade of SNR
(Signal to Noise Ratio)), the equipment shall not lose the timing
source if there is connectivity to it. Ideally, the equipment should
select a third port with normal clock signal as a recovery port. And
the clock signal of the recovery port mustn't be from the equipment
itself (otherwise, a loop will be formed). When the clock signal of
the working port or backup port returns to normal, the device may
restore to the working or backup port.
In the time synchronization with the IEEE 1588v2, multiple time
synchronization ports of the device should be enabled. Through the
BMCA automatically selecting the time source can realize the
protection and recovery of the time source.
Central controller can also be a solution choice for this use case,
for example, provisioning and configuration of the recovery port in
advance or dynamic computation and configuration of the recovery port
on the fly.
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2.4. Multi-layer/Multi-domain synchronization network
In general, to guarantee the time synchronization accuracy, the
suggested hop restriction value from the frequency source to the end
equipment is 20 in the synchronization network. And the suggested hop
restriction value from the time source to the end equipment is 30.
The values may be defined differently for different operators.
As tens of thousands of equipments needs to be supported in the same
synchronization network, the planning, maintenance and performance of
synchronization network face new challenges, for example, the end
equipments may hardly satisfy the hop restriction in synchronization.
Hierarchical division of a huge synchronization network into multi-
layers and/or multi-domains may improve the scalability. For example,
the whole synchronization network can be divided into several domains
according to their locations.
The operators may also face new challenges after introducing the
multi-layer/multi-domain synchronization network, for example, the
synchronization OAM for the inter-domain synchronization network is
more complex. In the deployment of syncE, the clock fault or
performance degradation of edge devices in one domain may even
influence the devices of other adjacent domains.
3. Synchronization Requirements
In order to facilitate the provision and management of a large
synchronization network, the following requirements need to be
addressed:
a) The synchronization network should support a generic, vendor-independent and
protocol-neutral information model for synchronization to support
heterogeneous networks;
b) The synchronization network should support automatic configuration of
frequency and time synchronization parameters based on the generic
information model, which may requires a generic configuration interface;
c) The synchronization network should provide high reliability and resiliency,
which requires that each synchronization device should maintain at least two
useable timing source and switch to an alternate timing source automatically
when faults occur in the network; furthermore, a device should restore to
the working path when the working path is recovered.
d) The synchronization network should provide high scalability, which may
require a network supports to be divided into multiple logical domains
defining the scope of synchronization distribution, or require a
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synchronization protocol to maintain high precision timing signal along a
long synchronization path. From the management viewpoint, the network is
required to support provision and management by a central controller(even
for multi-layer/multi-domain case), or each synchronization device should
adjust its timing source automatically when the network adds or removes
devices;
e) The synchronization network should provide distributed signaling and
centralized signaling to support the traditional network architecture and
the innovative SDN architecture;
f) The synchronization network should provide flexible OAM (Operation
Administration and Maintenance) functions for synchronization, such as
troubleshooting and synchronization performance monitoring, which can be
called on demand if the requested timing performance is not met.
4. Security Considerations
It will be considered in a future revision.
5. IANA Considerations
There are no IANA actions required by this document.
6. References
6.1. Normative References
[IEEE-1588]IEEE 1588, Precision Clock Synchronization Protocol for
Networked Measurement and Control Systems, 2008
6.2. Informative References
[G.8261] ITU-T, Timing and synchronization aspects in packet networks,
August, 2013
[G.8275] ITU-T, Architecture and requirements for packet-based time
and phase distribution, November, 2013
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[ptp-mib] Shankarkumar, V., Montini, L., Frost, T., and Dowd, G.,
Precision Time Protocol Version 2 (PTPv2) Management
Information Base, draft-ietf-tictoc-ptp-mib-06, work in
progress
7. Acknowledgments
TBD
Authors' Addresses
Liuyan Han
China Mobile
Xuanwumenxi Ave, Xuanwu District
Beijing 100053, China
Email: hanliuyan@chinamobile.com
Yuanlong Jiang
Huawei Technologies Co., Ltd.
Bantian, Longgang district
Shenzhen 518129, China
Email: jiangyuanlong@huawei.com
Jinchun Xu
Huawei Technologies Co., Ltd.
Bantian, Longgang district
Shenzhen 518129, China
Email: xujinchun@huawei.com
Xian Liu
Huawei Technologies Co., Ltd.
Bantian, Longgang district
Shenzhen 518129, China
Email: lene.liuxian@huawei.com
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