Testing Timing & Synchronization in an Ethernet Backhaul
Driven by smartphones, mobile data traffic will grow twenty-six-fold globally between 2010 and 2015. This exponential growth in demand for mobile data will severely erode the profitability of service providers.
Today, most service providers employ a flat pricing model for mobile data. For these providers, controlling network infrastructure costs will help improve profitability. And replacing the legacy time division multiplexing (TDM) backhaul with Ethernet will go a long way toward controlling costs. However, providers will have to distribute timing to the base stations over the packet networks.
This whitepaper discusses the market trends that are causing service providers to upgrade the legacy TDM backhaul to an Ethernet backhaul. It also highlights the importance of testing new Ethernet backhaul deployments and presents the methodologies to test packet-based (IEEE 1588) and physical (SyncE) timing distribution.
TESTING TIMING AND SNYCHRONIZATION
IN AN ETHERNET MOBILE BACKHUL
October 2011
Rev. A 10/11
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Main Title Style
Subtitle Style
CONTENTS
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Market drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Network provider’s profitability curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Carrier-grade mobile backhaul over packet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Carrier-grade transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Carrier-grade timing and synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Synchronizing the nodes in an Ethernet backhaul . . . . . . . . . . . . . . . . . . . . . . . 4
Global Positioning System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Adaptive Clock Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
IEEE 1588v2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Synchronous Ethernet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Why is it important to test Ethernet backhaul deployments? . . . . . . . . . . . . . . . 8
Methodologies for testing MBH timing distribution . . . . . . . . . . . . . . . . . . . . . . 8
1588v2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1 . Functional and slave scale testing of PTP . . . . . . . . . . . . . . . . . . . . . . 8
2 . Time of Day accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3 . Packet delay variation of PTP packets . . . . . . . . . . . . . . . . . . . . . . . . . 10
SyncE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1 . Measuring output wander of SyncE clocks . . . . . . . . . . . . . . . . . . . . . . 11
2 . Measuring wander transfer and wander tolerance of SyncE clocks . . 12
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
SPIRENT WHITE PAPER • i
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
1 • SPIRENT WHITE PAPER
ABSTRACT
Driven by smartphones, global mobile data traffic will grow twenty-six-fold between 2010
and 2015. Today most service providers employ a flat pricing model for mobile data. The
exponential growth in demand for mobile data will severely erode the profitability of service
providers. Controlling network infrastructure costs will help improve profitability and replacing
the legacy time division multiplexing (TDM) backhaul with Ethernet will go a long way toward
controlling costs. However, providers will have to distribute timing to the base stations over the
packet networks.
Precision Time Protocol (PTP) and Synchronous Ethernet (SyncE) are two emerging and popular
methods of distributing timing to base stations. These technologies must be tested thoroughly
before deployment. Untested packet backhaul deployments will have serious ramifications—
including poor quality of experience (QoE) for end customers, customer churn and loss of revenues.
PTP boundary and slave clocks derive their timing from packets received from upstream PTP
neighbors. PTP clocks have to be constantly tested for accuracy in the presence of regular
traffic and under stressful traffic conditions. Providers must also ensure that the output packet
delay variation (PDV) of PTP packets originating from boundary and transparent clocks are
within limits.
SyncE clocks also derive timing from upstream SyncE neighbors via physical timing signals. The
output wander and wander tolerance of SyncE clocks must be constantly monitored and verified
to be within limits prescribed by ITU-T G.8262 standards.
Developing TDM-like reliability and timing distribution capabilities in an Ethernet backhaul will
accelerate the deployment of 4G/LTE services and will benefit both service providers and customers.
MARKET DRIVERS
Never before has the world been as connected and as mobile as it is now. Watch the missed
episode of your favorite TV show on your mobile device during your commute. Pay your bills
while waiting in your doctor’s office. Find the theater nearest to you while you eat dessert at
your favorite restaurant, and find out what’s showing and the show times before you calculate
the tip on your smartphone.
According to the Cisco Visual Networking Index study (2011), global mobile data traffic will
increase twenty-six-fold between 2010 and 2015. The study also predicts that traffic from
wireless devices will exceed traffic from wired devices by 2015.
As the volume of data, particularly video, grows, so do expectations. Mobile subscribers are
less tolerant of marginal voice quality than they were a decade ago. For now they may endure
buffering delays for some content, such as a YouTube video, but not for paid content, such as
a TV episode, a movie or a sports championship match. Video, already a large portion of fixed
and mobile traffic, is set to dominate the content-centric mobile network in the next few years.
For those who are ready to take advantage of these trends, this is good news.
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
SPIRENT WHITE PAPER • 2
NETWORK PROVIDER’S PROFITABILITY CURVE
Setting aside the social and interpersonal implications of an always-on world, the impact on the
broadband mobile network reaches beyond technical considerations such as congestion and
management to revenue models and questions of profitability.
Today, most service providers employ a flat rate pricing model for mobile data services. The
exponential growth in demand for mobile data in the next few years will severely erode the
profit margins of providers. In early 2011, Tellabs released an End of Profit study for service
providers in North America, Western Europe and Asia Pacific. The study projected that
profitability under the current revenue model drops to zero in two to four years.
The study concluded that carriers need to offer new revenue-generating services and better QoE
while simultaneously controlling network infrastructure costs. One of the biggest contributors
to network infrastructure costs is the mobile backhaul (MBH).
CARRIER-GRADE MOBILE BACKHAUL OVER PACKET
The proliferation of smartphones and bandwidth-intensive applications is leading mobile
providers to upgrade their wireless networks to 3G and LTE-based 4G, which allow downlinks of
up to 300 Mbits/sec. However, the weak link in this chain is the mobile backhaul. The backhaul
is the part of the network that connects the cell site to the mobile switching center. Legacy
backhaul does not have the capacity to keep up with the expected growth in data traffic.
Legacy mobile backhaul is TDM-based and expensive. TDM circuits, with their fixed bit rates,
do not scale easily in response to variations in demand and are six times as expensive to install
and maintain as packet-based connections. For these reasons, Ethernet is the inevitable choice
for next-generation backhaul deployments.
But it’s not that simple. Widespread adoption of Ethernet is dependent on enabling
technologies such as timing and synchronization and operations, administration and
management (OAM) capabilities. For carriers to be able to exploit the affordability of Ethernet in
the mobile backhaul network, it must support these capabilities.
Vendors and carriers have been working through the standards bodies to develop new protocols
to equip Ethernet with the capabilities required to support mobile backhaul applications.
Carrier-grade transport
MPLS Transport Profile (MPLS-TP), a joint effort of the ITU-T and the IETF, is being developed as
an extension to MPLS to enable static provisioning of connections, in-band OAM capabilities
and sub-50 ms failure switchover. In short, these carrier-grade transport capabilities enable the
packet-based Ethernet backhaul to offer TDM-like OAM and reliability.
For more information on MPLS-TP and how it brings carrier-grade OAM to the Ethernet backhaul
network, see the companion white paper “Testing MPLS-TP Deployments in the Mobile
Backhaul” at www.spirent.com.
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
3 • SPIRENT WHITE PAPER
Carrier-grade timing and synchronization
Mobile devices derive their frequencies from the over-the-air interface of base stations and
operate at these derived frequencies. In a mobile radio access network (RAN), base stations
must be synchronized to the same frequency, with an accuracy of ± 50 ppb (parts per billion).
The frequency and phase accuracy requirements of base stations for the different mobility
standards are shown in Table 1. If the frequencies differ by more than the specified amount,
a mobile device cannot latch onto the frequency of the adjacent base station during roaming.
Under such conditions, a call handoff attempt between two base stations will result in a
dropped call and the end user will suffer from a poor QoE.
Typically, a node residing at the edge of the mobile core derives timing from a primary reference
source (PRS) and acts as a grandmaster clock.
• In a TDM backhaul, all base stations are connected to the grandmaster clock through a
chain of TDM circuits (T1, SONET/SDH, etc.), which are responsible for distributing the
timing via physical signals. Every node in a TDM backhaul receives clocking information
on the TDM circuits connected to the Grandmaster and in turn, transmits the clocking
information on TDM circuits toward the base stations.
• In an Ethernet backhaul, in the absence of TDM circuits timing from the grandmaster has
to be distributed to the base stations by alternative means described in the next section.
TABLE 1 – SYNCHRONIZATION REQUIREMENTS FOR WIRELESS STANDARDS
MOBILE STANDARD FREQUENCY STANDARD PHASE/TIME OF DAY ACCURACY
GSM 50 ppb Not required
CDMA 2000 50 ppb 3 us
WCDMA 50 ppb Not required
TD-SCDMA 50 ppb 3 us
LTE 50 ppb 3 us
Figure 1 – Timing distribution from PRS to base stations in an Ethernet Backhaul
4G LTE
eNodeB
Cell site SGW/PGW
GPS
Cell site
3G eNodeB
Timing Flow
Ethernet
Backhaul
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
SPIRENT WHITE PAPER • 4
SYNCHRONIZING THE NODES IN AN ETHERNET BACKHAUL
There are four prevailing methods for achieving TDM-like synchronization in an Ethernet
backhaul: using a) Global Positioning Systems, b) Adaptive Clock Recovery techniques, c) 1588v2
for transporting timing over packet and d) Synchronous Ethernet. All these techniques differ
significantly in terms of cost, ease of deployment, accuracy and support from standards bodies.
Global Positioning System
In this method, all base stations must be equipped with GPS receivers. Receiving timing from
GPS ensures high frequency and phase/time-of-day (ToD) accuracy at the base stations.
However, the installation costs are high and the timing signals can be degraded or lost due to
absence of clear line of sight to the satellites such as in urban canyons. Widespread adoption
of LTE will increase the number of cell sites and in such a scenario, equipping each cell site
with a GPS receiver becomes cost-prohibitive. Timing distribution to cell sites via direct GPS
connections will not be the focus of this whitepaper.
Adaptive Clock Recovery
In adaptive clock recovery (ACR), timing is recovered based on the inter-arrival time of packets
or the fill level of the jitter buffer. The internal clock rate is changed as the jitter buffer level
increases or decreases. This method is primarily used to derive a synchronous clock from an
asynchronous packet stream. ACR implementations are proprietary and will not be the focus of
this whitepaper.
IEEE 1588v2
IEEE 1588v2, also known as Precision Time Protocol (PTP), uses packets transmitted multiple
times per second to maintain synchronization at nanosecond accuracy between a grandmaster
clock and slave clocks. PTP ensures automatic correction for propagation delays of timing
packets between master and slave clocks.
PTP uses the concept of timing domains, a logical grouping of synchronized clocks, to provide a
scope for PTP messages, data sets, state machines, and other PTP entities. A network or device
can be associated with multiple domains, but the established time for a domain is independent
of the time in any other domain.
Establishing timing for a domain happens in two steps:
a) Establish the master-slave hierarchy
b) Synchronize the clocks
All PTP devices in a domain discover each other and organize themselves into a timing hierarchy
using the Best Master Clock (BMC) algorithm. The highest quality clock in a domain becomes
the grandmaster and supplies timing to all other slave clocks in a domain, as shown below. The
BMC algorithm runs continuously to adapt to changes in the network. If the current grandmaster
is removed from the network or determined to no longer have the highest quality clock, the
algorithm determines a new grandmaster.
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
5 • SPIRENT WHITE PAPER
After the creation of the master-slave hierarchy, a series of event messages synchronizes
the master and slave clocks not only to the same frequency, but also to the same ToD. The
synchronization can be performed via a one-step clock (using a single event message) or a two-
step clock (using the combination of an event message and a subsequent general message). As
shown in the figure below, Sync, Follow_up, Delay, and Delay_resp messages are exchanged
between two-step master and slave clocks. The slave clock learns t1, t2, t3 and t4 from this
exchange and uses these values to compute the time offset and the one-way delay between
master and slave. The slave uses the computed offset to set its internal clock.
Figure 2 – PTP Master-slave hierarchy (from IEEE 1588v2 spec)
Figure 3 – PTP message exchange for offset computation (from IEEE 1588v2 spec)
Boundary Clock
S MM
Ordinary Clock-1
Grandmaster
M
Ordinary Clock-2
Slave
S
Ordinary Clock-3
Slave
S
Ordinary Clock-4
Slave
S
Boundary Clock
S MM
Master
Time
t1
t2
t1, t2
t1, t2, t3
t2
t3
t1, t2, t3, t4
t4
Slave
Time
Timestamps known
by Slave
Slave time offset =
(t2–t1) – (t 4–t3)
2
Sync
Follow-up
Delay
Delay-Resp
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
SPIRENT WHITE PAPER • 6
Boundary Clocks vs Transparent Clocks.
There are two alternatives for distributing timing from the grandmaster clock to the slave
clocks, both involving an intermediate switch between grandmaster and slave.
A boundary clock uses the time from the grandmaster to calibrate itself, and then serves as a
master to the clocks beneath it in the hierarchy. Although boundary clocks accumulate time
error if they are connected in a cascade, they are preferred if timing has to be distributed from
one master clock to hundreds of slaves.
Unlike a boundary clock, a transparent clock does not calibrate itself to track a master or
grandmaster clock. Instead, a transparent clock measures the delay it introduces to Sync and
Delay_Request messages and records that information (residence time) in the Follow_Up and
Delay_Response messages. The slave uses the information to adjust t1 and t4 before performing
its calculations. Transparent clocks measure residence times accurately.
Default profile vs Telecom profile
IEEE defined the original 1588 specification to support the timing requirements of industrial
automation. IEEE 1588v2 and the ITU-T G.8265 specification extended the protocol for use
in both industrial automation and telecom networks. G.8265 is also known as PTP Telecom
Profile.
Telecom profile specifications state that PTP clocks can be designated as slave-only or master-
only, without using the BMC algorithm for establishing the hierarchy. In addition, telecom
profile mandates support for unicast mode for message exchange, higher PTP message rates
(up to 128 per second), and the capability for PTP telecom slaves to negotiate the rates for
exchange of PTP messages with their master clocks.
A telecom network—especially a mobile access network—comprises of hundreds of access
nodes that lack the computing power of the more capable edge and core routers. With multicast
mode for the exchange of PTP messages, the slave access nodes are forced to examine and
discard Delay and Delay_Resp messages originated by or destined for all other PTP slaves
in that domain. These exchanges can negatively affect a PTP slave’s ability to process other
control plane or user data traffic. Higher PTP message exchange rates increase the probability
that PTP slaves will become accurately synched to their master clocks by allowing them to
discard PTP messages that arrive with a sizable PDV.
Synchronous Ethernet
Synchronous Ethernet is defined by the ITU-T as a means of using Ethernet to transfer timing
(frequency) via the Ethernet PHY layer. SyncE ports nominally operate within a frequency
tolerance range of ±4.6 ppm. Clocks that are used in Synchronous Ethernet are compatible with
clocks used in existing synchronization networks.
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
7 • SPIRENT WHITE PAPER
SyncE uses the same architecture as TDM synchronization and is defined in a set of ITU-T
recommendations.
• G.8261: Timing and Synchronization Aspects of Packet Networks
• G.8262: Timing Characteristics of Synchronous Ethernet Equipment Slave Clock (EEC) -
plus errata
• G.8264: Distribution of Timing Information Through Packet Networks
• G.781: Synchronization Layer Functions (Description of the SSM protocol)
In SyncE, a reference timing signal traceable to a Primary Reference Clock (PRC) is injected into
the SyncE network element using an external clock port. The SyncE interface is able to extract
the frequency of this reference timing signal and pass it to a system clock. The system clock
locks to this extracted frequency, which becomes the candidate frequency reference. This
reference frequency is, in turn, distributed to downstream SyncE nodes. Using this method, timing
originating from a PRS is distributed via the intermediate Ethernet equipment clocks (EEC), to the
base stations at the cell sites, which need accurate synchronization (show in figure).
SyncE defines the Ethernet Synchronization Messaging Channel (ESMC), which uses OAM PDUs
to pass synchronization status messages (SSM). These messages enable an EEC to select the
highest quality incoming reference signal from a set of synchronization references using a
quality level (QL) identifier.
Unlike IEEE 1588, which uses the packet layer to distribute ToD and frequency, SyncE uses
the physical layer and distributes only frequency. IEEE 1588 can be affected by network
impairments such as packet jitter, but SyncE is not affected by packet impairments. However,
timing can be distributed via SyncE only if all switches in the path are compliant with SyncE
standards. Operators may choose to employ a combination of 1588 and SyncE—using SyncE to
deliver frequency and using 1588 to deliver ToD end-to-end.
Figure 4 – SyncE timing distribution hierarchy
SyncE Slave
SyncE Slave SyncE Master
SyncE Slave
SSM Messages
S
S
M M
e
s
s
ag
e
s
Physical Timing Signal
S
S
M M
e
s
s
ag
e
s
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
SPIRENT WHITE PAPER • 8
WHY IS IT IMPORTANT TO TEST ETHERNET BACKHAUL DEPLOYMENTS?
For both equipment vendors and service providers, there are many obvious reasons to test new
Ethernet backhaul deployments.
• Loss of synchronization among base stations leads to dropped calls, customer churn
and loss of revenue. As Ethernet replaces TDM in the backhaul, it is important to test
timing distribution to base stations in the presence of high traffic and subscriber scale.
• A reliable Ethernet backhaul is necessary for the widespread deployment of 4G/LTE.
LTE will enable service providers to offer and monetize thousands of new value-added
services to mobile subscribers.
• Upgrading the backhaul means upgrading thousands of access and cell-site routers.
Even one malfunctioning router can break the entire synchronization chain and affect
thousands of end users.
METHODOLOGIES FOR TESTING MBH TIMING DISTRIBUTION
The primary concerns of service providers deploying an Ethernet backhaul are to verify the
accuracy of PTP and SyncE clocks and ensure that these clocks remain accurate even in the
presence of impairments and stressful traffic conditions. PTP and SyncE clocks differ in the
manner in which they distribute timing and therefore the testing methodologies and metrics for
success differ for these technologies.
1588v2
PTP clocks derive their timing from upstream PTP master clocks. DUT boundary clocks must
have the ability to form master-slave relationships and accurately supply timing (ToD) to
hundreds of slaves in different cell sites. In addition, the PDV of PTP packets originating from
boundary clocks, measured using the Packet TIE and min TDEV metrics. should remain within
limits. The tests required to validate these requirements are described below.
1. Functional and slave scale testing of PTP
With the increasing adoption of 4G/LTE, it is expected that the number of cell sites will greatly
increase, with each cell site serving a smaller area. In the future, PTP boundary clocks in the
radio access networks will each serve dozens to even hundreds of PTP slaves in different cell
sites. It is important to verify a boundary clock’s ability to exchange PTP messages and form
PTP relationships with hundreds of slave clocks.
In the picture below, the device under test (DUT) boundary clock receives timing from Test
Device 1, which serves as the grandmaster, and supplies timing to hundreds of slave clocks
emulated by Test Device 2. The BMC algorithm and PTP functional tests are verified on the DUT.
The following combinations should be tested while establishing PTP slave scale limits:
a) One-step and two-step clocks
b) Unicast and multicast mode for PTP message exchange
c) Low message rates (1 per second) and high message rates (up to 128 per second)
d) Statically-provisioned unicast slaves and slaves synchronized using unicast negotiation
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
9 • SPIRENT WHITE PAPER
Figure 5 – Slave scale testing of PTP boundry clock
2. Time of Day accuracy
The ToD accuracy of the DUT is essentially a measure of how accurately the DUT tracks its
Mastermaster, the Test Device 1. In the setup shown above, Test Devices 1 and 2 receive
their timing from a GPS source and are perfectly synchronized to each other. The time offset
measured by Test Device 2 is essentially a measure of the DUT’s offset from its Master, i.e. Test
Device 1, and is shown in Figure 6 below.
DUT’s offset from Master = ((t2-t1)-(t4-t3))/2 as measured by Test Device 2.
The ToD accuracy should be tracked under:
• normal Normal traffic conditions
• stressful Stressful conditions where the Test Device 2 emulates hundreds of slaves and
line-rate traffic is passing through the DUT interfaces.
Figure 6 – Establishing the accuracy of DUT boumdry clocks
Cell Site
Cell Site
Test Device 2
S SM
Test Device 1
3G/4G
Base Station
3G/4G
Base Station
DUT
Boundary Clock
Emulates multiple slaves Emulates 1588 Grand Master
Test Device 2
S SM M
Test Device 1
DUT
GPS
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
SPIRENT WHITE PAPER • 10
3. Packet delay variation of PTP packets
The packet delay variation of packets originating from the DUT will have a significant impact
on the accuracy of downstream PTP clocks. The biggest contributor to PDV is the unpredictable
queuing delay in the incoming and outgoing message buffers in switches and routers along
the PTP path. PDV in timing packets will cause downstream nodes to incorrectly program their
internal clocks—losing stability and accuracy.
The PDV exhibited by a DUT is measured by a downstream test device by calculating Packet
TIE and min TDEV—both are which are fast gaining acceptance as metrics for characterizing
PDV. A variation of this test is to introduce PDV into the PTP traffic stream entering the DUT and
measuring the PDV of the PTP traffic exiting the DUT. This variation measures the DUT clock’s
ability to filter out incoming PDV and yet maintain a stable output clock.
The methodology described above is depicted in Figure 7.
Figure 7 – Measuring the PDV of PTP packets
Impairment
Service
Measures PDV
(Packet TIE and Min TDEV)
P
TP & u
ser tr
affic
Test Device 1
DUT
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
11 • SPIRENT WHITE PAPER
SyncE
SyncE clocks also derive their timing from their upstream SyncE master clocks. Every SyncE clock
in a synchronization chain must remain accurate and stable, i.e. the wander of the output timing
signal must be within limits prescribed by ITU-T G.8262. A SyncE clock should also not amplify any
incoming noise by more than 0.2 db. These scenarios are described in greater detail below.
1. Measuring output wander of SyncE clocks
ITU-T G.8262 specifies the maximum output wander (noise) from a DUT when the DUT is
provided an ideal reference input signal. In the picture shown below, the impairment test device
is shown connected to a GPS timing source and provides an ideal reference input signal to the
DUT via SyncE.
The clock output signal from the DUT (10 MHz or Ethernet line) is fed into the impairment test
device. By comparing the reference input signal from GPS and the DUT’s output signal, the
impairment test device plots the MTIE and TDEV graphs and also provides pass/fail results for
EEC-Option 1 and EEC-Option 2. Such a test is crucial to determining the worthiness of the DUT
to be used as a SyncE EEC in a real-world Ethernet mobile backhaul.
Figure 8 – Ensuring output wander of SyncE DUT within limits
Impairment
Service
Reference Timing Input
Clock out
Ensure output
wander of DUT
within limitsDUT
GPS
MTIE/DEV
MTIE (ns)
Mask
MTIE
Time (ns)
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
SPIRENT WHITE PAPER • 12
2. Measuring wander transfer and wander tolerance of SyncE clocks
ITU-T G.8262 specifies the maximum amount of wander gain (limit < 0.2 dB) at a SyncE clock’s
output when subjected to input wander. This wander transfer requirement essentially ensures
that DUTs don’t amplify incoming noise. G.8262 also specifies the upper limit of the input
wander that the SyncE EEC (DUT) must tolerate without causing switchover, raising alarms, or
going into holdover mode.
• Wander transfer - The impairment test device is shown connected to a GPS timing
source but also uses an internal wander generator to impair the timing signal sent to the
DUT. The impairment device then compares the timing signal from the DUT’s clock-out
port and ensures that the phase gain is less than 0.2 dB (2.3%).
• Wander tolerance - The impairment device is also used to increase the input wander to
limits specified by G.8262 and the downstream Test Device 1 that is connected to the
DUT ensures that the DUT doesn’t raise alarms (LOS), switch to a new master, or go into
holdover (by inspecting the SSM QL).
Figure 9 – Wander transfer and wander tolerance testing of SyncE DUT
Impairment
Service
Varying Input Wander
Output Wander
MTIE/DEV
Clock out
Ensure phase
gain < 0.2 dB
DUT
GPS
Ensure no alarms
or switch over
MTIE (ns)
Mask
MTIE
Time (ns)
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
13 • SPIRENT WHITE PAPER
CONCLUSION
The exponential growth in demand for mobile data is threatening to erode the profitability
of service providers. Controlling network infrastructure costs will help improve profitability
and replacing the legacy TDM backhaul with Ethernet will go a long way in controlling costs.
However, providers will have to employ alternative means for distributing timing to the base
stations over the packet networks.
IEEE 1588v2 and SyncE have emerged as the standards for timing distribution over Ethernet.
Spirent’s industry-leading mobile backhaul testing solutions help service providers and
equipment vendors validate the performance of the most demanding applications on the mobile
Internet and ensure that mobile subscribers don’t suffer from dropped calls and poor QoE.
Spirent’s solutions validate packet timing protocols, such as IEEE 1588v2 and SyncE SSM, and
test physical timing characteristics by generating and measuring impairments, such as wander
and PDV.
TABLE 2
TIMING FUNCTIONALITY SPIRENT TESTCENTER SPIRENT ANUE 3500
1588v2
Boundary clock testing
Transparent clock testing
BMC & slave scale testing
Telecom profile testing
PDV - Measurement
PDV-insertion/playback
SyncE ESMC/SSM testing
Wander measurement
Wander transfer
Wander tolerance
To learn more about testing synchronization in next-generation networks, please contact
your local Spirent representative or visit our website at www.spirent.com/Networks-and-
Applications/Mobile_backhaul
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
SPIRENT WHITE PAPER • 14
Testing Timing and Synchronization in an Ethernet Mobile Backhaul
15 • SPIRENT WHITE PAPER
