1. technology and computing

APPLICATION WHITE PAPER
LTE-Capable Mobile Backhaul
Author:
Michael Ritter
ADVA Optical Networking
ADVA Optical Networking © All rights reserved.
With rising demand for mobile broadband services, operators are seeing a sharp
increase in bandwidth requirements. To keep pace with demand, operators must
evolve to new packet backhaul networks that offer increased capacity at lower
cost while providing the necessary service reliability and quality of experience
that users expect. This white paper focuses on the challenges operators face
when migrating to LTE and LTE Advanced radio access and the solutions they
need to profitably benefit from packet backhaul.
Introduction
The rising tide of data traffic experienced in mobile networks is putting the
backhaul infrastructure under more pressure than ever before. Data intensive
applications on powerful smartphone and tablet devices are popular with
many users and the arrival of LTE and LTE Advanced will only accelerate
this process. Infonetics Research reports that the number of mobile broadband
subscribers passed fixed broadband subscriptions in 2010 and is estimated to
reach 2.1 billion by 2015.
Source: Infonetics Research 2011
Figure 1: Mobile broadband subscriber growth
The introduction of LTE and LTE Advanced – also referred to as 4G radio access
technology – promises a whole new mobile broadband experience for private
and business users, with short latency and data rates beyond 100 Mbit/s. At
the same time, service differentiation and multiple quality-of-service profiles
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LTE-Capable Mobile Backhaul
will enable mobile network operators to efficiently use available spectrum while
offering differentiated services with a superior quality of experience to their
customers.
However, this new, fixed network-like performance can only be experienced
when supported by the backhaul network. There is general consensus in the
industry that only packet-based Carrier Ethernet backhaul will be able to meet
the challenges. Carrier Ethernet networks provide the bandwidth and flexibility
required to dynamically adapt to capacity and connectivity demand originating
from mobile services at cost points attractive to network operators.
Reliability of the mobile backhaul network is essential for efficient network
operations and providing a superior
user experience.
While efficiency and reduced cost per bit are important metrics, reliability of
the mobile backhaul network is essential for efficient network operations and
providing a superior user experience. With the introduction of LTE and LTE
Advanced, the architecture of the backhaul network becomes more diverse
and has many more dimensions. Connectivity between the mobile core and
the base stations is no longer strictly hub-and-spoke as with 2G and 3G
radio access technology. Base stations now communicate directly with each
other, exchanging signaling and user data without involving the mobile core.
They also use different anchoring points for signaling and data traffic in the
mobile core. Data plane and signaling plane are now completely separated.
Furthermore, the concept of small cells introduces another level of complexity.
Small cells are an important component of LTE to provide substantially increased
access capacity to a large number of users and enable a more efficient utilization
of the available spectrum.
Backhaul Fundamentals
Mobile networks are growing. In many countries, radio access network
installations have evolved from 2G to 3G and are now evolving to 4G while
maintaining a large portion of the legacy radio equipment. The diversity of radio
equipment installed at cell sites poses a challenge especially to the backhaul
network. While the IP-based architecture of the Evolved Packet Core (EPC) is
designed to replace former 2G and 3G core networks, this migration is a slow
process for many operators. Seamless handovers for both voice and data to
cell towers with older network technology such as GSM, UMTS and CDMA2000
Figure 2: Mobile backhaul in the context of the EPC
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therefore requires a careful design of the backhaul network in addition to the
mobile infrastructure itself. Transmission delays have to be kept at a minimum
across the entire backhaul infrastructure while legacy TDM and packet-based
traffic must be transported simultaneously. In addition, the backhaul network
needs to provide the flexibility to migrate to the anticipated long-term solution.
A mobile backhaul network based on physical fiber infrastructure clearly is the
ideal solution from a capacity, reliability and operational perspective. However,
many cell sites will be microwave- and copper-fed for years to come. While larger
cell sites and those acting as aggregation hubs can only provide the required
user experience when connected over fiber, there are many sites – especially
in rural areas – where new fiber deployment is not justified from a commercial
standpoint. Migration to packet-based microwave and Ethernet-over-Copper is
the alternative solution. For many of the small cells that are expected to be
deployed in metro areas during the coming years, microwave and the physical
infrastructure already in place will play a dominant role when designing the
backhaul network. Nevertheless, the share of fiber-fed cell sites is expected
to grow with copper losing its attractiveness due to bandwidth constraints and
microwave remaining at a stable share, cf. Figure 3.
Source: Infonetics Research 2011
Figure 3: Installed backhaul connections by physical medium
The architecture of packet-based mobile backhaul networks is not consistent
for all network operators. There are topological and operational differences
depending on whether the backhaul network is operated by the mobile service
provider or leased from a fixed-line network operator. While a single-operator
environment provides advantages in terms of simplicity and efficiency, the
multi-operator environment illustrated below is the typical case for fiber- and
copper-based backhaul. Fixed-line mobile backhaul services are often provided
by a third-party operator or a separate organization within the same operator.
In a multi-operator environment, mobile backhaul services are typically offered
over a converged, multi-service backhaul and aggregation infrastructure.
Network resources are then shared with other traffic originating, for example,
from DSL services and business Ethernet connections for enterprises.
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LTE-Capable Mobile Backhaul
These different scenarios result in a number of different challenges and
implementations when it comes to delivering mobile backhaul services in realworld deployments. In a single-operator environment, the complete network
infrastructure including radio access, backhaul and mobile core network is
controlled by one organization. The backhaul network can therefore be designed
and optimized according to the requirements of the mobile network. Service
Level Agreements (SLA) are typically not defined explicitly at intermediate
nodes.
Figure 4: Multi-operator mobile backhaul environment
In a multi-operator environment, the backhaul network operator provides an
independent service interconnecting the radio access network with the mobile
core. Quality of Service (QoS) is defined at the User Network Interface (UNI)
and must be met and reported by the backhaul network operator according to
the SLA agreed between both parties. Accurate SLA measurement, assurance
and reporting play a critical role in this context.
Challenges in Mobile Backhaul for LTE
The first challenge is providing differentiated QoS while keeping the transmission latency at a minimum. QoS differentiation enables mobile operators
to manage the performance of different streams of traffic. Even though Carrier
Ethernet backhaul provides significantly more capacity compared to legacy
TDM, dimensioning at peak rates is not practical and cost prohibitive. Backhaul
networks will therefore be oversubscribed in many cases, making sophisticated
QoS management a necessity and a powerful tool for managing user experience
and cost.
The optimum solution will essentially balance user satisfaction with economical
and technical feasibility. In this context, transmission latency becomes a
critical design factor, especially for delay-sensitive applications such as packetbased voice and online-gaming. Also, seamless call handover between cell
sites requires keeping transmission latency at a minimum. With LTE and LTE
Advanced, seamless handover can only be achieved when guaranteeing lowest
latency on the X2 interface, which directly interconnects base stations with each
other. Typical latency requirements in LTE are summarized in Table 1.
The importance of QoS management across packet-based mobile backhaul
networks mandate powerful tools for service assurance and simplified network
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operations. The QoS provided by the backhaul network must be constantly
measured and reported. Parameters such as packet delay, delay variation
and packet loss are important characteristics ultimately defining user
experience. The individual performance requirements must be met for each
The QoS provided by the backhaul
traffic stream and immediate measures need to be taken when the network
network must be constantly measured
can no longer assure the anticipated QoS. The potentially large amount of
and reported.
traffic streams transported over Carrier Ethernet mobile backhaul networks
additionally requires efficient procedures for performance verification testing
at service turn-up.
LTE Interface
Delay Budget
S1 – User Plane
50 – 300 ms
S1 – Control Plane
10 ms
X2 – User Plane
1 ms (recommended)
X2 – Control Plane
10 ms
Table 1: Delay budgets in LTE
As operators replace their TDM-based backhaul with Carrier Ethernet backhaul,
they face a major challenge: how to provide precise timing reference or
synchronization for base station clocks and do so in a cost-effective way.
Mobile services are dependent on timing and base stations need a stable
frequency reference to support mobility. Actually, operators are confronted
Mobile services are dependent on
with a broader, two-part challenge. Firstly, they must replace their TDMtiming and base stations need a stable
based clock function with a suitable packet clock. Secondly, as they deploy
reference to support mobility.
advanced LTE technologies incorporating Time Division Duplex (TDD)
multiplexing, they must eventually expand that packet-clock capability so
that it distributes not just the frequency reference but also phase and time-ofday information. The timing requirement for different LTE air interface standards
is summarized in Table 2.
Air Interface
Frequency
Time/Phase
LTE (FDD)
50 ppb
-
LTE (TDD)
50 ppb
3 µs
LTE MBMS
50 ppb
5 µs
Table 2: Air interface stability needs
Solutions for QoS and Latency Management
QoS differentiation helps to manage and allocate network resources during
times of congestion, adapted to the actual need of applications. It is a tool
that guarantees that traffic generated by certain applications – e.g., voice and
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control plane signaling – is prioritized over traffic from applications that are
less sensitive to delay or loss performance. Carrier Ethernet allows prioritizing
services by assigning to each service a specific QoS class, which is based on a
number of parameters. These parameters include packet delay, delay variation
and packet loss and are specified for the service across the entire backhaul
network.
The QoS defined for the LTE radio
interface has to be aligned with the
QoS experienced across the backhaul
network.
QoS must be managed consistently end-to-end. The QoS defined for the LTE
radio interface has to be aligned with the QoS experienced across the
backhaul network. Classification and tagging is therefore carried out by the
base stations and the gateways in the mobile core, based on the information
collected from policy servers. The 3GPP collaboration has defined a number
of QoS Class Identifiers (QCI) for LTE, each referring to a certain type of
application. The identifier is used as a reference for controlling packet
forwarding and treatment across the radio access network and is translated
into a packet priority marking to control packet forwarding across the Carrier
Ethernet backhaul network.
To meet the required QoS levels and simultaneously maintain cost efficiency,
Carrier Ethernet supports sophisticated traffic management capabilities. QoS
management in Carrier Ethernet networks enables better service to certain
selected flows, therefore significantly reducing overall bandwidth requirements
while still maintaining the QoS required for each individual flow. Figure 5
illustrates the architecture and the main building blocks of a generic traffic
management implementation that is compliant to Metro Ethernet Forum (MEF)
recommendations. The main functionalities include traffic classification, policing,
queuing and scheduling.
Figure 5: Carrier Ethernet traffic management architecture
In order to meet the level of scalability and flexibility imposed on the backhaul
network by LTE and LTE Advanced in particular, the Carrier Ethernet mobile
backhaul must support QoS management for a large number of traffic streams
– also called Ethernet Virtual Connections (EVC) – in a hierarchical queuing
architecture. The combination of multiple QoS profiles and the potentially
large number of connections between individual base stations – referred to as
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X2 interface – and between base stations and the ECP – referred to as S1
interface – creates a challenging environment. Only a solid traffic management
implementation with enhanced classification capabilities can assure efficient
usage of network resources while meeting strict QoS requirements.
Due to the increased autonomy of base stations and the improved user experience
aimed for, LTE networks are in general more sensitive to latency accumulated
across the backhaul network in comparison to 2G and 3G mobile networks. The
availability of strict priority queuing in the traffic management architecture is
therefore a must to meet the challenging latency limitations for signaling and
control plane traffic as well as for time-critical applications.
Operational Simplicity and Service Assurance
Continuous and standards-compliant
performance monitoring and automatic fault resolution are the foundation for SLA assurance and accurate
reporting.
The capability to cost-effectively provision intelligent services with differentiated
QoS metrics across the mobile backhaul network makes the availability of
powerful tools for installation, commissioning, performance management and
SLA reporting inevitable. Manual configuration and test procedures do not
provide operational efficiency and therefore limit scalability. Furthermore,
continuous and standards-compliant performance monitoring and automatic
fault resolution per traffic flow are the foundation for SLA assurance and
accurate reporting.
With Y.1564, the Telecommunication Standardization Sector of the
International Telecommunication Union (ITU-T) has defined a standard for
turn-up, installation and troubleshooting of services across Carrier Ethernet
networks. The test methodology allows for fast and complete validation of
Ethernet SLAs in a single test and with the highest level of accuracy. Services
that will run across the network are simulated during the turn-up phase and all
important SLA parameters are qualified simultaneously. Y.1564-compliant testing
also validates the QoS mechanisms provisioned in the network to prioritize the
different service types. It results in more accurate validation and much faster
deployment and troubleshooting compared to manual procedures.
To keep mobile networks alive and maintain the quality of experience users
expect, mobile operators are particularly interested in continuously understanding
the status of their packet backhaul services so they can localize faults and
trigger corrective actions from remote locations. The Ethernet Operations,
Administration and Maintenance (OAM) standards 802.1ag and Y.1731 defined
by the Institute of Electrical and Electronics Engineers (IEEE) and the ITU-T,
respectively, provide mechanisms for connection monitoring and performance
measurement on an end-to-end service level. Based on the hierarchical concept
shown in Figure 6, 802.1ag defines the following OAM tools:
• Connectivity Check
• Loopback
• Link Trace.
These tools make use of specific Ethernet frames that are following the same
path as the frames belonging to the monitored service. This has the additional
advantage that no explicit interworking with service protection and restoration
procedures or other dynamic network changes are required for compatibility.
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Figure 6: 802.1ag and Y.1731 connection monitoring
Y.1731 builds on 802.1ag to add in performance monitoring features on an endto-end service basis. Fault management and indication are supported by alarm
indication signaling and remote defect indication. The mechanisms defined in
Y.1731 enable backhaul network operators to measure and report one-way and
round-trip service parameters for
• Frame Delay Measurement
• Frame Delay Variation Measurement
• Frame Loss Measurement.
Service demarcation and aggregation
units deployed in the backhaul
network must be engineered with a
hardware processing architecture to
provide the required attributes.
Carrier Ethernet OAM functions for connectivity fault management and
performance monitoring provide backhaul network operators with a complete
set of tools for assurance and reporting of SLAs to the mobile network operator.
While backhaul network operators typically use these mechanisms on a
constant basis, mobile operators may choose to only verify the service
quality periodically.
All Carrier Ethernet OAM functions must be implemented with special diligence
to guarantee superior user experience. Scalability, flexibility and, last but
not least, high measurement accuracy are elementary when designing
mobile backhaul networks for a large number of EVCs. Consequently, service
demarcation and aggregation units deployed in the backhaul network must
be engineered with a hardware processing architecture to provide the required
attributes. Scalability and high measurement accuracy in particular can only be
achieved by a hardware-centric design.
Radio Access Network Synchronization
Both Carrier Ethernet system vendors and the timing community worked on
methods to deliver synchronization information over packet networks. The
obvious goals were to keep it simple, cost-effective, predictable and reliable. Two
practical mechanisms for providing synchronization via packet-based networks
have emerged: Synchronous Ethernet (SyncE) and 1588v2. Both standards are
the result of efforts by international standards bodies, notably the ITU-T and the
IEEE.
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Two practical mechanisms for providing synchronization via packet-based
networks have emerged: Synchronous
Ethernet (SyncE) and 1588v2.
SyncE uses the Ethernet physical layer to synchronize neighboring nodes.
It is attractive to many network operators because it closely resembles the
familiar SONET/SDH model and its timing quality is completely independent
of the network load. However, SyncE only provides frequency synchronization
and requires that each node in the hierarchy supports it. If a single network
element in the chain does not support SyncE, all nodes lower in the hierarchy
do not receive accurate timing information.
1588v2, in contrast, specifies a master-slave exchange of packets that carry time
stamps for recovering frequency, phase and time-of-day information. Operators
can use 1588v2 to provide synchronization directly across any packet network.
However, operators must ensure that the synchronization flow is not distorted
by packet loss, delay or delay variation beyond the filtering capabilities of the
slave clock. The draft ITU-T Telecom Profile for 1588v2 requires that all nodes
in the network must support 1588v2 boundary clock functionality for the high
accuracy network phase synchronization required by LTE Advanced and other
TDD air interfaces. Both mechanisms provide additional information about the
delay conditions in the network and therefore support increased clock accuracy.
Table 3 summarizes the key differences.
Attribute
SyncE
IEEE 1588v2
Capability
Frequency
Frequency, Time, Phase
Layer
Physical
Ethernet, UDP
Distribution
Physical Layer
In-Band Packets
Sensitivity
Asynchronous
Switches
Delay, Jitter, Loss
Table 3: SyncE / 1588v2 comparison
SyncE and 1588v2 are complementary technologies that can co-exist in the
network and can be used on the same path. Both technologies have distinct
advantages and disadvantages over each other. SyncE is deterministic and
the performance is independent of the network load. 1588v2 can function
SyncE and 1588v2 are complementary
over asynchronous switches and additionally distributes phase and timetechnologies that can co-exist in the
of-day information. Slaves that support both can converge on accurate
network.
timing information quickly by using the SyncE frequency to discipline the
1588v2 local oscillator. SyncE in conjunction with 1588v2 also provides an
alternative holdover capability in case of failure at the packet layer. A combined
implementation promises to deliver the best overall performance.
Assured delivery with guaranteed
QoS metrics is a necessity not only for
data traffic streams but also for timing
services.
The ability to consistently monitor and accurately test and troubleshoot the
synchronization infrastructure when delivering timing information via SyncE
and 1588v2 is mandatory for assuring clock accuracy and therefore the
quality of the delivered timing service. Assured delivery with guaranteed
QoS metrics is a necessity not only for data traffic streams but also for timing
services. As 1588v2 packet flows potentially traverse different technologies
and operator networks, service assurance mechanisms as implemented in
Carrier Ethernet OAM are required.
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Network timing behavior is not a stationary process. It is subject to dynamic
conditions and changes over the short and longer term. Appropriate tools are
required for cost-effective and time-efficient end-to-end management of the
synchronization domain during all phases of the network lifecycle – installation,
turn-up testing, monitoring and troubleshooting.
Figure 7: Synchronization service assurance support tools
The Right Solution for LTE-Capable Mobile Backhaul
The explosive growth of video and data services on mobile devices has created
a challenge as mobile network operators look to provide them to an expanding
base of subscribers while simultaneously reducing the cost of transporting this
increased traffic load across the mobile backhaul network. While efficiency and
reduced cost per bit are important metrics, reliability of the mobile backhaul
network is essential for efficient network operations and providing a superior
user experience.
Our Etherjack™ and Syncjack™ suite
enable mobile backhaul network
operators to deliver reliable, highperformance data and synchronization
services.
ADVA Optical Networking has a comprehensive FSP 150 Carrier Ethernet access
and backhaul portfolio that offers a complete solution including scalable QoS
management, end-to-end service assurance and accurate delivery of timing
information for mobile backhaul networks of any size. Our Etherjack™ and
Syncjack™ suite, which are fully integrated into the FSP 150 platform, enable
mobile backhaul network operators to deliver reliable, high-performance
data and synchronization services supported by a rich and complete set of
tools for end-to-end service monitoring and assurance.
Our FSP 150 Carrier Ethernet solution provides operators with the capability
to evolve their mobile backhaul network without constraints and supports
seamless migration of radio access networks to LTE and later LTE Advanced.
It is architected to deliver 99.999% availability, supports end-to-end SLA
management per traffic flow and scales with your radio access network: a
complete and uniform solution for demarcation and aggregation applications in
mobile backhaul networks.
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About ADVA Optical Networking
ADVA Optical Networking is a global provider of intelligent telecommunications
infrastructure solutions. With software-automated Optical+Ethernet transmission
technology, the Company builds the foundation for high-speed, next-generation
networks. The Company’s FSP product family adds scalability and intelligence
to customers’ networks while removing complexity and cost. Thanks to reliable
performance for more than 15 years, the Company has become a trusted partner
for more than 250 carriers and 10,000 enterprises across the globe.
Product
FSP 150
ADVA Optical Networking
North America, Inc.
5755 Peachtree Industrial Blvd.
Norcross, Georgia 30092
USA
ADVA Optical Networking SE
Campus Martinsried
Fraunhoferstrasse 9 a
82152 Martinsried / Munich
Germany
For more information visit us at www.advaoptical.com
ADVA Optical Networking
Singapore Pte. Ltd.
25 International Business Park
#05-106 German Centre
Singapore 609916
Version 07 / 2012
ADVA Optical Networking’s family of intelligent Ethernet access products provides
devices for Carrier Ethernet service demarcation, extension and aggregation.
It supports delivery of intelligent Ethernet services both in-region and out-ofregion. Incorporating an MEF-certified UNI and the latest OAM and advanced
Etherjack™ demarcation capabilities, the FSP 150 products enable delivery of
SLA-based services with full end-to-end assurance. Its comprehensive Syncjack™
technology for timing distribution, monitoring and timing service assurance
opens new revenue opportunities from the delivery of synchronization services.
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