Network Synchronization - ericsson.net

Network Synchronization A key component of Ericsson’s Evolved IP Network solution Technology paper

This document outlines the need for effective network synchronization, describes technology options, and presents consistent and comprehensive synchronization solutions for mobile and converged network operators. The document describes synchronization at a holistic level, taking into account the needs from access networks including radio, and IP/MPLS transport networks. Information is presented from the perspective of Ericsson as a mobile broadband infrastructure provider, an end-to-end solution integrator and long-term standards body contributor.

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Introduction Synchronization is an essential prerequisite for all mobile networks to operate. It’s fundamental to data integrity, and without it data will suffer errors and networks can suffer outages. ‘Loss of synchronization’ problems can be difficult to diagnose and resolve quickly and add to operational costs. Poor synchronization affects customer satisfaction and is therefore revenue affecting too. Radio base stations rely on having access to reliable and accurate reference timing signals in order to generate radio signals and maintain frame alignment. Effective synchronization also permits hitless handover of subscriber connections between adjacent radio base stations. Routers and switches in the transport network may therefore be required to provide synchronization to the radio base stations in order for them to handle and transport data properly. Synchronization affects and underpins network applications and configurations such as Evolved Multimedia Broadcast Multicast Services (EMBMS). ‘Time of Day’ synchronization is also a prerequisite for functions such as operations and maintenance and accurate network timing is an essential part of clear and accurate charging of network services. The mobile network evolution to LTE and future planning for 5G networks and services has generated an increased and pressing need for the delivery of accurate phase synchronization. Apart from the need for these networks to provide everincreasing data rates and lower network latencies, more sophisticated synchronization schemes are needed to support new features. Examples include the coordination between base stations when delivering broadcast video, and the avoidance of interference between macro and small cell base stations. Different radio technologies and features have different synchronization requirements. These can be categorized into two main types - frequency synchronization and phase synchronization. Many different options exist to provide frequency synchronization, although fewer exist that can reliably deliver the required accuracy and stability for phase synchronization. Synchronization is a strategic component of Ericsson’s ‘Evolved IP Network’ (EIN) solution which provides comprehensive IP transport infrastructure for mobile and converged operator networks. EIN takes a holistic approach to network design and integrates synchronization into all parts of the operator’s network avoiding a piecemeal and potentially ineffective solution which would result in poor network performance. EIN is regularly updated with new synchronization features in order to match the pace of change of open international standards.

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Ericsson’s Role in Synchronization Synchronization is fundamental to Ericsson’s business, having been a mobile infrastructure vendor through all generations of mobile systems. As technology and standards have evolved then new solutions are required and Ericsson has developed and deployed new synchronization solutions to support hundreds of Ericsson network operator customers around the world.

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Ericsson plays a leading role in synchronization standardization, primarily but not exclusively through the International Telecommunication Union whose Study Group 15 has responsibility for ‘Network synchronization and time distribution performance’. Ericsson is also a member of the International Telecom Sync Forum (ITSF) and Workshop on Synchronization in Telecommunication Systems (WSTS). Naturally, Ericsson products and solutions comply with the appropriate standards, and multivendor interoperability is frequently tested.

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Technology Primer

3.1

Types of Synchronization: The following types of synchronization describe the alignment between two clock signals drawn in light and dark blue in the examples below. On the left side the two clocks are drawn independently and on the right hand side they are shown superimposed on each other.

Frequency synchronization: Two clocks that are aligned in terms of their repeating interval (the ‘frequency’), but not in terms of phase or time.

Phase synchronization: Two clocks that are aligned in terms of their repeating interval (the ‘frequency’), and also ‘phase’ (a one-second interval), but without a common time origin (or ‘epoch’).

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Time synchronization: Two clocks that are aligned in terms of their repeating interval (the ‘frequency’), their ‘phase’ (a one-second interval), and share a common time origin (or ‘epoch’).

No synchronization: Two clocks that have no alignment in terms of frequency, phase or time.

Radio Base Stations (RBS) are the main drivers for synchronization requirements today. Although the RBS only requires frequency (and in some circumstances phase) synchronization on the air interface, the phase and time synchronization terms are often used inter-changeably. This is because phase synchronization is most commonly achieved by distributing a common time synchronization reference. Whereas the diagrams above show frequency, phase and time synchronization, the requirements from 3GPP are for frequency and phase synchronization only.

3.2

Accuracy and Stability The quality of a synchronization signal is defined in terms of accuracy and stability, in comparison with the nominal value. The diagrams below show an analogous example of shots fired at targets to depict the difference between accuracy and stability. Accuracy is concerned with the ‘closeness’ of a measurement to the actual (true) value. Stability is concerned with the repeatability of that measurement if it were taken several times without changing any conditions which may otherwise affect the result. In networking terms, both accuracy and stability characterize the deviation in the local clock compared with a reference clock elsewhere in the network.

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i) Not accurate, not stable

iii) Not accurate, but stable

ii) Accurate, not stable

iv) Accurate and stable

It is essential that a network synchronization solution be accurate and stable. When considering whether a clock will meet the requirements to ‘lock’ and provide an adequate timing source, both accuracy and stability should be considered together. Ordinarily, cases (i) and (iii) would never lock, case (iv) would always lock, and target (ii) could potentially lock if the algorithm used is able to filter the deviation (jitter) of the measurements. This is because the maximum error is a significantly smaller value than those in cases (i) and (iii). It’s important to note that while open standards exist (such as ‘PTP’ – see later section), the clock recovery algorithm that governs the ability to deal with jitter is proprietary. This greatly influences the resulting overall clock quality and the ability to lock to a given clock source. This is especially so when compared to the alternative approach of frequency synchronization delivered over the physical layer that makes use of a phase-locked loop (PLL).

3.3

Technology Trends Historically frequency synchronization has been provided either by a Global Navigation Satellite System (GNSS) or derived from the transport network to which the network device requiring synchronization was connected. In North America CDMA networks use the space-based Global Positioning System (GPS) for synchronization. Elsewhere in the world GSM networks used (and in some cases, continue to use) timing information from the PDH and SONET/SDH transport networks for synchronization purposes. The migration from TDM-based transport to all-packet transport has dramatically changed network synchronization schemes. With packet-based transport, it is no longer possible for a radio base station to recover timing from the PDH transportdelivered ‘northbound’ connection as this has been replaced by Ethernet, which traditionally does not provide the level of accuracy required by the RBS. Newer 3GPP features may also require phase synchronization that TDM-based networks cannot provide, which will force operators still using such networks to look for alternate solutions.

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GNSS provides an accurate and stable synchronization source, but the financial cost to equip every site in a network with a GNSS-derived synchronization source may be prohibitive because of the requirement to install and manage additional equipment. Cost concerns for GNSS synchronization are more prevalent for small cell sites where the number of sites is increased compared with macro sites, and an indoor small cell will require a cable run to somewhere else in the building where an external line of sight can be made to a satellite. This has been a significant driver in favor of PTPbased synchronization compared with GNSS for indoor deployments. Where the cost of deploying GNSS,-based synchronization have been prohibitive, with both small cell and macro installations, then other synchronization schemes have frequently been used.

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Global Navigation Satellite System A number of GNSS systems are operational including the Global Positioning System (GPS) operated from the USA, and ‘GLONASS’ operated from Russia. GPS was first launched in 1978 and achieved full operational status in 1995, whereas GLONASS became fully operational with global coverage in 2011. In addition to these two systems there is the Galileo project, created by the European Union and the European Space Agency, and the BeiDou Navigation Satellite System (BDS), commissioned by China. Neither of these is fully operational at the time of writing, but both plan to be by 2020. The main advantage of a GNSS is full independency from the transport network, providing a reliable synchronization signal so long as the station remains locked to a minimum number of satellites. Concerns have been raised however, that a GNSS may be at risk of technical interference (jamming), as well as political interference (as each is funded at a governmental or political region level). Therefore some operators have a policy to either require a backup independent of the chosen GNSS, or not to use GNSS at all in their network. For those operators happy operating with GNSS today there is one further consideration. While GNSS may work well for macro radio base stations, each of which normally has a clear line of sight to the sky, small cells are less likely to have this visibility. Combined with the relatively high cost of a GNSS-based solution, the conclusion is that an alternative needs to be found to deliver synchronization to these small cell radio base stations.

3.5

IP RAN Synchronization Network synchronization has always been a key system requirement for Ericsson as operator customers can demand complete end-to-end network deployments with Ericsson providing the ‘prime integrator’ role or ‘turnkey supplier’ role. The solution portfolio therefore includes various IP RAN synchronization options under the ‘Evolved IP Network’ (EIN) solution offering. Frequency and Phase-based synchronization are both supported, historically using Network Time Protocol and, more recently, the Precision Time Protocol.

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3.5.1

Packet-based frequency synchronization with NTP With the transition of WCDMA networks from ATM to IP, Ericsson identified a need for a new approach to synchronization. At the time, work was starting up in standardization bodies to consider packet-based solutions, but the urgency was such that a proprietary approach was initially taken. The Network Time Protocol (NTP) has been available for many years, and is used in core networks to distribute time of day to an accuracy of hundreds of milliseconds to those applications that need it, such as billing. Radio Base Stations have a much stricter accuracy requirement. Therefore Ericsson defined a specific clock recovery algorithm to be able to deliver such accuracy, when using NTP over a packet-based backhaul network, without placing any requirements on the backhaul beyond an appropriate Quality of Service (QoS) design. Developed initially for WCDMA, Ericsson’s well-proven NTP solution also provides support for LTE FDD, without requiring support from the backhaul network. The specific synchronization solution used for an particular RBS site depends on whether an SIU or TCU is used at the site. This is because they are equipped with a very high quality oscillator that makes it possible to relax the requirements on the IP network, compared to those RBS sites with only a digital unit or baseband unit. When an SIU or TCU is used, round-trip delay variation for at least 50% of synchronization packets must be less than 100ms. Without the SIU or TCU, the roundtrip delay variation must be less than 10ms for 99% of the synchronization packets.

This NTP-based synchronization solution supports all backhaul technologies such as ethernet over fiber, ethernet over microwave, ethernet over SDH, VDSL, ADSL and PON. As such it provides a highly versatile and cost effective solution for network operators migrating to packet-based transport.

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3.5.2

Packet-based frequency synchronization with PTP This is a comprehensive PTP solution compliant with telecom profile G.8265.1 for a standardized solution and features a stand-alone PTP Grandmaster synchronization source. It does not require any support from the backhaul network and has the same delay variation requirements as the NTP solution (described above) with respect to the use of SIU or TCU. As with the NTP-based solution, all the listed backhaul options are supported.

3.5.3

Packet-based phase synchronization with PTP This third solution provides full timing support for phase synchronization and follows the principles set by the ITU-T for the first telecom profile for phase/time synchronization. The solution provides end-to-end support with the IEEE 1588 default profile, or with the G.8275.1 profile. The diagram below shows a network scenario with Synchronous Ethernet (SyncE) and PTP working together to synchronize a radio access and transport network.

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Standalone PTP server

PTP packets

2G Router

3G

SYNC

Router

Packet Network

LTE

MINILINK

BSC

RNC

MINILINK

2G

3G

MINILINK

MINILINK

Synchronous Ethernet and IEEE 1588 (G.8275.1) Synchronous Microwave

S-GW

LTE

There is a maximum delay variation requirement of 100 microseconds between PTP packets for the digital unit or baseband unit to achieve successful phase synchronization lock in TDD-LTE networks.

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Products and Use Cases

4.1

Supporting new LTE Features The introduction of LTE Advanced and an array of new features and potential applications is driving new requirements on network synchronization. In turn, these new synchronization requirements are driving new product requirements. Some examples of LTE features requiring synchronization support from network products are described below. LTE Time Division Duplex (‘TDD’) places a synchronization requirement of 5µs to 10µs within a cell radius greater than 3km, and just 1.5µs within a cell radius less than 3km. LTE Broadcast requires synchronization within a maximum of 1.5µs to 5µs. Inter eNodeB co-ordination requires synchronization within 1.5µs in order to support features such as coordinated multipoint (‘CoMP’), Carrier Aggregation and Further Enhanced Inter-cell Interference Coordination (‘FeICIC’, an advanced interference management technique). Positioning based on observed time difference of arrival (OTDOA) requires synchronization within 100ns. A GNSS solution is the only viable option today which is able to deliver this degree of precision.

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4.2

Synchronization Support in the Ericsson Portfolio Ericsson’s wide product portfolio in radio systems and IP transport provides support for both frequency-based and phase/time-based synchronization telecom profiles. Across the Ericsson product portfolio, present and future synchronization requirements are key inputs to product development and among the most fundamental features to be verified in operator deployments. The portfolio includes the following product families:

Ericsson Radio System family including RBS 6000

• Modular system architecture is designed to evolve smoothly to 5G with multistandard, multi-band and multi-layer technology. It reduces site acquisition issues, with dramatic gains in capacity density and energy efficiency. The system will address growing mobile data needs, expected to reach 25 exabytes per month by 2020, when 5G is expected to be commercialized. • Comprises a broad range of new products including macro and small cells, antenna systems, IP transport, microwave nodes, rails and other site equipment for indoor and outdoor applications. • Packet-based frequency synchronization with both NTP and PTP (G.8265.1), supporting a packet slave clock. • Packet-based phase synchronization (G.8275.1), supporting a T-TSC (Telecom Time Slave Clock).

Ericsson Router 6000 family • Works with Ericsson Radio System to deliver unprecedented routing capacity, reduced latency and QoS capabilities, and effectively couples radio and IP transport for the 5G future. The Router 6000 series is part of a comprehensive suite of router platforms running one network operating system (IPOS), spanning from cell-site routers to edge, core and data centers. • Router 6000 offers high capacity radio-integrated IP transport for mobile backhaul and metro access applications. It also combines with Ericsson Network Manager to provide unified management and control of a network operator’s radio and transport network. The routers offer support for LTE Advanced, 5G and M2M applications. • Packet-based frequency synchronization (G.8265.1) with packet slave clock • Packed-based phase synchronization (G.8275.1), with T-BC (Telecom Boundary Clock).

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MINI-LINK family • Microwave transport solution, suitable for new mobile backhaul networks or evolving existing mobile networks, or fixed broadband over microwave applications. The portfolio spans all transport technologies (IP, MPLS, Ethernet and TDM), all frequency bands (from 4 to 80 GHz) and is suitable for fronthaul and backhaul applications. • All-outdoor, all-indoor and split-mount solutions scale from single hop applications to large aggregation nodes. The portfolio supports all migration scenarios from TDM via Ethernet to IP and MPLS. It also supports both Line-ofSight (LOS) and Non-Line-of-Sight (NLOS) configurations for efficient outdoor small cell deployments • Packet-based frequency synchronization (G.8265.1) supporting a packet slave clock • Packed-based phase synchronization (G.8275.1), supporting a T-BC and TTSC

Ericsson SP family • Powerful packet aggregation nodes optimized for packet-only networks using fiber or microwave as transport. The SP family offers switching and routing functionality with a capacity range from 16 Gbps to 120 Gbps. • Interface and technology flexibility, low power consumption, Advanced Service OAM features, and MPLS support. SP 400 series uses the same Ericsson IP OS as Ericsson router series in order to provide aligned transport solutions. • Packet-based frequency synchronization (G.8265.1) supporting a packet slave clock • Packet-based phase synchronization (IEEE 1588-2008), supporting a Boundary Clock and Ordinary Clock.

Ericsson SPO 1400 family • Family of multi-service and optical packet transport solutions for metro networks. An integrated platform that supports networking layers from WDM to TDM and Packet and provides the simplest and most cost-effective path for modernizing metro optical networks. With metro convergence, all services flow over one network and one platform with one management system. • Provides high capacity and low power consumption, featuring SONET/SDH and PDH support, L2 connection-oriented Ethernet switching and aggregation, and WDM technology. • Packet-based frequency synchronization (G.8265.1) supporting a packet slave clock • Packed-based phase synchronization (G.8275.1), supporting a T-BC (Telecom Boundary Clock) and T-TSC (Telecom Time Slave Clock).

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Ericsson SSR 8000 family

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The Ericsson SSR 8000 family of Smart Services Routers provides operators with a highly scalable, consolidated platform that offers services for both fixed and mobile network infrastructure.

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Packet-based phase synchronization (IEEE 1588-2008), supporting a Boundary Clock.

Summary Network synchronization is fundamental to providing high quality services, and synchronization planning is essential when deploying new network technologies or introducing new services. Without sufficient planning and expertise, network synchronization will be problematic and services will suffer. Ericsson is an industry leader in deploying effective synchronization schemes for new technologies, as a supplier of end-to-end networks and global services. When new technologies are brought to market that change synchronization requirements, this becomes an important aspect of mobile backhaul network design. Careful product selection can be essential to deliver the necessary synchronization to the RAN. With proven solutions based upon GNSS, Synchronous Ethernet, NTP and IEEE 1588 (PTP), Ericsson is a safe choice for backhaul network design, supply and integration. These solutions have been independently tested and verified by the European Advanced Networking Test Center (EANTC) and reports are publicly available for download. http://www.eantc.de/public-reports/

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Network Synchronization - ericsson.net

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