a managed pseudo-wire backhaul service for mobile network · pdf file ·...

A Managed Pseudo-Wire Backhaul Service for Mobile Network Operators

WHITE PAPER

White Paper p an y® Managed Pseudo-Wire Backhaul

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Th e Pseu do -W ire Co m

Important Notice This document is delivered subject to the following conditions and restrictions: This document contains proprietary information belonging to Axerra Networks, Inc. Such information is supplied solely for the purpose of evaluating AXN Pseudo-Wire Gateways and Access Devices™. Axerra Networks owns the proprietary rights to all information contained herein. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, magnetic, photocopy, recording, or otherwise, now or in the future, without prior written consent from Axerra Networks. The text and graphics are for illustration and reference only. The specifications on which they are based are subject to change without notice. Due to a policy of continuous development, Axerra Networks reserves the right to alter specifications and descriptions outlined in this publication without prior notice, and no part of this publication, taken separately or as a whole, shall be deemed to be part of any contract. Copyright © 2009. Axerra Networks, Inc. Axerra®, Axerra Networks®, AXN®, HPCR®, AXN Pseudo-Wire Gateways and Access Devices™, AXNVision™, and The Pseudo-Wire Company® are trademarks or registered trademarks of Axerra Networks, Inc. All other product names are trademarks or registered trademarks of their respective owners.


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Executive Summary Mobile operators have already begun the process of deploying HSPA data services across their UMTS networks. The lure of high-value and compelling services such as mobile-TV and truly mobile broadband Internet access, is rapidly driving investment in this new technology. HSPA represents a significant improvement in data rates over the initial 3G specification and for the mobile operators it also represents a significant challenge in engineering their networks. With downstream data rates capable of delivering a theoretical 21 Mbit/s, the demand for bandwidth in the radio access network (RAN) is increasing by an order of magnitude from where it is today. Ever growing user uptake and expected bandwidth increases beyond this 21 Mbit/s in the future are all putting increased pressure on the RAN backhaul infrastructure. As the bandwidths in the RAN increase, revenue per bit falls. For the fixed line operators offering broadband technologies, such as DSL and cable modems, this situation is nothing new. T1/E1 TDM circuits currently used within RAN backhaul networks are ideal for carrying the high- value voice services but are clearly not optimized in terms of cost or capacity for transport of high bandwidth data services. Therefore the mobile operators face a significant challenge; how to grow the data service revenues without having an associated cost per bit that makes the economics of the solution unacceptable. One possible and compelling solution to this problem is the hybrid approach. With the hybrid approach, the data traffic associated with mobile devices is separated from mobile voice services directly at the cell site. Voice traffic can be carried reliably and cost effectively over existing T1/E1 infrastructure whilst HSPA traffic can be backhauled using lower cost broadband technologies such as xDSL, cable modem, and where economically viable, carrier Ethernet. The traffic characteristics of mobile data are closely aligned with the asymmetrical nature of existing broadband solutions and as such a new synergy is developing between mobile and fixed broadband providers. The opportunity exists for advanced fixed line operators to step forward with a full end-to-end managed service that offers HSPA offload capability to the mobile operator. This should be a complete technical solution that delivers to the Mobile Network Operator (MNO) the interfaces needed to directly interface to the radio subsystems at both cell site and core aggregation locations. It should offer guaranteed SLAs in-line with broadband data availability, monitoring, pro-active fault isolation and recovery. A mixture of transport technologies may be employed to deliver the service and perhaps even multiple operators, but these underlying complexities should be transparent to the MNO. Combine these elements with rapid and flexible bandwidth upgrades and the need for a NGN packet-switched infrastructure is evident. Many fixed line operators have invested heavily already in such infrastructure, exploiting these investments to offer a tailor-made managed service solution which would allow the MNO to concentrate on deployment and optimization of the air interface, service creation and subscriber uptake. Axerra Networks’ Pseudo-Wire solution is ready today to offer a network operator all the features needed to build a complete HSPA backhaul managed service. In summary, the Axerra platforms deliver a solution allowing the transport of TDM, HDLC, and ATM based services over packet-based technologies such as MPLS, IP and Ethernet networks. Whether the underlying transport is xDSL, bonded SHDSL/EFM, native Ethernet on fiber or even point-to-point microwave, the Axerra solution is designed to make efficient use of the offered bandwidth to ensure MNOs can effortlessly grow their mobile broadband business and the managed service provider can profit from the new and growing revenues associated with mobile data services and its millions of subscribers.


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Radio Access Network (RAN) technologies and challenges 3GPP, the standards body and multi-vendor forum which defines and ratifies the UMTS technical standards, defined ATM on T1/E1 to be the interface of choice in the UMTS RAN. This decision was understandable considering existing GSM installations used TDM technologies throughout and initial data rates from 3G where not as high as we see today from HSPA. However because UMTS adopted ATM as a packet-switched technology, this led to a gap in the last-mile access rates between T1/E1 2Mbit/s and 155Mbit/s SONET OC-3/SDH STM-1. 3GPP mandates the use of IMA over multiple T1/E1 circuits to effectively fill this bandwidth gap. This is essentially where the problem arises; although IMA can offer flexible growth in bandwidth, the underlying TDM infrastructure is relatively expensive on a cost-per-bit basis, having never been designed to efficiently carry bursty and bandwidth-hungry data traffic. By looking at today's requirements it might be argued that 3GPP were overzealous in their commitment to TDM and ATM as an underlying technology, but this is not so. The facts are that not only must a UMTS cell site be serviced by appropriate levels of bandwidth but it also requires highly accurate clock synchronization. This is essential to ensure the successful functioning of two key elements of a mobile service; rapid inter-cell hand-off, employed whenever a mobile handset moves between different base stations and power-save techniques used to ensure reasonable battery life in the mobile devices. Both of these requirements rely on different cell sites being clock synchronous to within 16ppb. Clearly TDM was a logical choice when considering these requirements.

A basic GSM/UMTS RAN architecture can be seen below: In this diagram, the cell site has both 2G and 3G radio equipment and the backhaul link is directly between a pair of cross connects. In this configuration the link between radio base stations and RNC/BSC is TDM carrying both ATM AAL2/AAL5 voice/data/signaling traffic in the case of the UMTS Iub interface and also compressed 2G voice traffic carried across the Abis interface between the GSM base station and the GSM BSC. This rigid TDM-based architecture is exactly what must be addressed to allow the backhaul network to meet the growing demands of new data services. The future of UMTS networks lie in two clearly defined development paths. Firstly, the air interface between the mobile device and the radio base station will increase the usable data rates from today's 384Kbit/s in 3GPP R.99, to 21 Mbit/s with HSPA. It is expected that by the end of 2009 HSPA Evolved (HSPA+) will begin to deliver rates of around 42Mbit/s and still further into the future 3GPP are working on the definition of UMTS LTE (Long Term Evolution), with


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data rates exceeding 100Mbit/s. Secondly the network layer protocol used in the base stations and Radio Network Controllers (RNC) will eventually transition to an all-IP model, replacing and enhancing the ATM-based infrastructure that we see today. This transition will take place hand-in-hand with air-interface bandwidth increases and service uptake. New revenue opportunities will be driven out from capabilities inherent in the IP Multimedia subsystems (IMS) being developed and deployed in the MNO core infrastructure. Eventually these IMS functions will reside in part within the radio base stations, requiring fully-featured NG packet network connectivity directly to the cell sites. The challenge for the managed service provider is to offer a flexible and cost-effective backhaul solution that addresses the tough economic and technical challenges faced by the MNO during growth and migration.

HSPA Offload Topologies The solutions proposed within this white paper represent configurations based around Axerra Networks’ AXN product family, suitable for creating managed services capable of addressing the needs of mobile operators wishing to begin the migration to packet RAN environments. The AXN product family is able to support all mobile traffic types including GSM, UMTS, and CDMA as well as synchronization and clock recovery over packet-switched based networks suitable to meet tolerances found within the UMTS RAN. It is expected that a managed service provider would implement multiple solutions as part of the complete service. Different locations will utilize a variety of technologies dependant on many factors including distances from serving exchanges, service availability, MNO requirements and bandwidth demands as well as the technical capabilities of the UMTS Base station (NodeB). Several offload topologies options are described below: Physical interface separation at the NodeB As it became evident that the use of IMA over T1/E1 was less than ideal the radio equipment vendors began a process of developing possible solutions. One such solution is to split apart the HSPA data traffic from the existing 3G R.99 traffic and to present these ATM circuits on physically separate T1/E1 interfaces at the NodeB. In this way, the 3G voice/R.99 traffic could continue to use T1/E1 TDM backhaul while high bandwidth HSPA services could be dealt with using an alternative backhaul transport. Axerra's AXN10 cell site access device takes advantage of this physical/logical separation by terminating the HSPA IMA group presented from the NodeB and encapsulating the ATM VP/VC cell stream into a Pseudo-Wire for delivery over a packet switched transport. The same HSPA ATM circuits are then extracted at the RNC site and presented to the RNC over STM-1 VC4 ATM. From a MNO perspective the managed service would be demarcated at the IMA links from the NodeB and the STM-1 ATM interface at the RNC. The technologies used within the backhaul network would be under the discretion of the service provider and various possibilities are presented. It is expected that from a cost and availability perspective the most likely candidate for last-mile packet transport will be ADSL2 in many cases using the Annex M extensions. Other possibilities are shown in the diagrams for completeness. Further discussions on the various technologies available can be found later in this document.


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HSPA Offload Service using physical separation It may be that the managed service also includes the delivery of T1/E1 circuits as shown at the bottom of the diagram. In either case these T1/E1 circuits must be presented to the NodeB with suitable synchronization tolerances as it will be these circuits which are used to propagate timing to the NodeB air interface. VP/VC switching separation at the NodeB A second configuration with slightly different functionality is required to deliver the solution depicted in the diagram below. In this scenario the NodeB does not have physical level separation of the HSPA traffic, but instead presents the services on different ATM virtual circuits on the same IMA group. In today’s network, the NodeB can separate the HSPA traffic from the R.99 traffic. To avoid the cost and added complexity of such a solution, the AXN10 cell site access device can be used to terminate the NodeB IMA group, separate the R.99 ATM VCs from HSPA VCs, locally switch the R.99 traffic into another T1/E1 ATM port for backhaul over TDM whilst Pseudo-Wire encapsulating the HSPA traffic in the same manner as described above with physical separation. From the perspective of the MNO, this managed service brings considerable added value, avoiding unnecessary investment in ATM switching hardware whilst achieving clean separation of traffic and offload.


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HSPA Offload Service using VP/VC switching The previous two examples have focused on the offload of HSPA-only traffic onto a packet-based backhaul. It is expected that some MNOs will go further still, particularly those who do not have extensive TDM networks of their own and who therefore rely heavily on bandwidth supplied by a leased line operator. In this case, in order to maximize the cost saving benefits of packet transport, both the R.99 and HSPA traffic will be transported over a packet-based managed service. Native Ethernet backhaul services In many locations particularly within dense urban areas, native carrier Ethernet services are becoming much more common. Many existing operators have already deployed such infrastructure either directly using Metro Ethernet platforms or as enhancements to their TDM investments. As can be seen throughout this document, the link between the last-mile technology and the AXN platforms is always Ethernet. Therefore the ideal solution is one where all cell sites are serviced directly with native Ethernet transport. In some cases, the Ethernet interface will be delivered over bonded copper, such as the standardized EFM/802.3ah. In other cases, the physical layer could be an adaptive modulation microwave solution such as WiMAX or even legacy microwave technologies. Regardless of the underlying link technology, the presentation from the NTU in all circumstances will be Ethernet. When high bandwidth carrier-grade Ethernet is present, the following solution is proposed. In the diagram below a symmetrical Ethernet service of typically 10Mbit/s and above is used to transport both 3G R.99 traffic and HSPA data services. Once again the value of the Axerra solution is in encapsulating and forwarding the ATM virtual circuits presented over IMA from the NodeB and adapting them to transport over the packet based Ethernet backhaul using Pseudo-Wires. In this case, where all services share the same backhaul bandwidth, another key element of the Axerra solution is highlighted. The different ATM virtual circuits are mapped to different service classes within the AXN10 cell site access device. R.99 voice traffic is assigned to a high priority Diffserv class,


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with HSPA traffic being assigned a lower priority. Packets leaving the cell site are marked with the appropriate Diffserv classes and corresponding 802.1p priority bits in order that they are suitably treated within the carrier Ethernet domain.

Native carrier Ethernet UMTS/HSPA backhaul

Complete 2G and 3G RAN backhaul As described in the introductory section on MNO RANs, in a typical deployment, the UMTS NodeBs are actually co-located with GSM base stations. When carrier Ethernet service is available at the cell sites, it then becomes possible and logical to include the GSM Abis interface in the same optimized and converged packet backhaul solution for additional cost savings. This once again takes advantage of Pseudo-Wire encapsulation within the AXN10 cell site access device. However unlike the ATM Pseudo-Wires in the UMTS solution, the Abis interface is strictly TDM in nature. Therefore to adapt this to packet transport, circuit emulation Pseudo-Wires are employed. Axerra's solution has full implementations of both unstructured T1/E1 (SAToP: Structure Agnostic TDM over Packet – IETF RFC 4553) and structured (CESoPSN: Circuit Emulation over Packet Switched Networks – IETF RFC 5086) as defined within the IETF PWE3 working group. Either solution is applicable in this scenario, with CESoPSN offering the slight advantage of improved bandwidth efficiency by suppressing the unused timeslots within the Abis interface.

Native carrier Ethernet UMTS/HSPA + GSM backhaul


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The following figure illustrates the protocol stack used for encapsulation and transport of TDM-based circuit over Ethernet:

Figure 1: Protocol Stack for Transport Abis Interface across Carrier Ethernet Network The following figure illustrates the protocol stack used for encapsulation and transport of ATM-based circuit over Ethernet:

Figure 2: Protocol Stack for Transport Iub Interface across Carrier Ethernet Network


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Clock Synchronization As a direct consequence of using an all packet-based transport, the managed service assumes the responsibility for timing synchronization. This in itself is a significant task, one for which Axerra Networks is a clear market leader with a mature and field-proven solution and the first manufacturer to make available clock synchronization over packet which conforms to the requirement as specified in ETSI EN 300 912 and TS 125 402 "Synchronization in UTRAN stage 2." HPCR® (High Precision Clock Recovery) is Axerra's timing recovery protocol first made available in 2005. HPCR® uses three main elements to provide accurate clock recovery:

Specialized hardware in the AXN10 cell site access device

A messaging protocol running between the RNC Pseudo-Wire Gateway such as the AXN800/1600 and the AXN10 Pseudo-Wire Access Device.

An advanced algorithm to analyze and recover clock synchronization in a robust

manner. Capable of dealing with packet loss, significant levels of network jitter, network failures, re-routing and real-time latency changes.

HPCR® performance is in compliance with ITU-T G.8231 and G.8242 as well as G.82613 deployment case 2 clock recovery requirements and test cases. The HPCR® enables frequency accuracy of ±15 parts-per-billion (ppb) Fractional Frequency Offset (FFoFF) from the Primary Reference Source (PRS) clock. Axerra's solution also offers a dedicated synchronization service using a separate 'sync' Pseudo-Wire connection. This has the advantage of allowing the clock recovery topology to be defined independently of the traffic flows. This leads to lower bandwidth utilization for clocking information, and a more flexible architectural structure with clock recovery independent of physical interfaces. An example of such a topology would be a single central AXN chassis that is configured as a network-wide clock distribution server. Work in the IEEE has recently resulted in the specification of a new version of the precision timing protocol. Now officially termed IEEE 1588-2008, this standard describes a messaging protocol that is used to derive accurate reference timestamps across packet networks. However, this standard does not provide any clock recovery algorithm, which is crucial to any solution for clock over packet. Axerra's HPCR® solution today offers a complete, scalable and mature adaptive clock recovery solution that will continue to evolve into the future with the option of IEEE 1588-2008 messaging, thereby allowing a managed service to interwork seamlessly with other standards-based timing solutions as they become available in the market.

1 The control of jitter and wander within digital networks that are based on the 2048 kbit/s hierarchy. 2 The control of jitter and wander within digital networks that are based on the 1544 kbit/s hierarchy. 3 Timing and synchronization aspects in Packet networks.


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Example of Distributed Timing Solution Using Sync-PW and HPCR® The diagram above shows a sample configuration that uses the flexibility of sync-pw to distribute timing sources across Pseudo-Wire Gateways, offering timing independence from RNC locations and helping to protect radio network subsystems.

Network Aggregation In a UMTS RAN, the traffic converges onto a number of logically/regionally-organized locations at which the MNO has an RNC (Radio Network Controller). This platform deals with allocation of resources across the RAN and also the critical functions of mobile handset handoff. All traffic which comes to or from a NodeB must pass through its associated RNC. In order to aggregate the Pseudo-Wires for onward delivery onto an RNC, Axerra has developed the AXN1600 and AXN800 Pseudo-Wire Gateways™. These are full carrier-class platforms, offering a high density, high availability which can be used to terminate thousands of Pseudo-Wire connections. They also act as a centralized clock distribution server in situations where adaptive clock recovery is deployed. Typically a MNO will look to deploy aggregation in two possible locations:

A hub site or aggregation PoP where operator-owned microwave access terminates IMA groups into Pseudo-Wire to be further backhauled on carrier Ethernet.

Centrally co-located with the RNC. Terminating Pseudo-Wires from NodeB/aggregation

PoPs In both cases the managed service providers' role is to condition the traffic either from the access network and/or terminate the Pseudo-Wire traffic for delivery onto an RNC.


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Access Network Aggregation Managed Service

Central co-located Pseudo-Wire aggregation In this final stage of the end-to-end managed service, the MNO requires that the ATM Pseudo-Wires from all the connected NodeBs are terminated and the associated ATM VCs re-constructed and delivered onto the RNC via OC-3/STM-1 ATM interfaces. The AXN1600/800 has the interface density and scalability to fulfill this function. Axerra has carefully developed the platforms to offer the levels of Pseudo-Wire aggregation expected by MNOs at this point in the network. The diagram above shows a typical example of the configuration proposed by many MNOs. The ATM switch is an existing component used to terminate IMA groups from NodeBs that are supported directly over TDM backhaul.


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Summary The proposed managed service solution for UMTS backhaul, based on the AXN product family is the most proven and flexible solution for the packet RAN available today. It offers a managed service provider peace of mind knowing that future developments in this rapidly changing technology can be accommodated within the existing platform. It has the industry’s widest choice of Pseudo-Wire encapsulations and the most scalable and flexible product line which deliver the industry’s most mature and robust high accuracy timing recovery solution using HPCR®.

About the AXN Product Family The AXN product family consists of AXN1600 and AXN800 Pseudo-Wire Gateways and AXN10 and AXN1 Pseudo-Wire Access Devices. The AXN1600 and AXN800 Pseudo-Wire Gateways are based on Axerra’s leading edge and field-proven Multiservice over Packet (MSoP) technology. The platforms are fully interoperable with the other members of the AXN family: AXN1 and AXN10, delivering the industry’s most scalable family of Pseudo-Wire gateways and access devices.

AXN1600 Pseudo-Wire Gateway - A high-capacity, modular, fully-redundant platform, intended for deployment in a PoP or Central Office. The AXN1600 supports up to 12 IOM line cards and 2 network interfaces cards.

AXN800 Pseudo-Wire Gateway –is a medium-capacity, modular, fully-redundant

platform, intended for deployment in a PoP or Central Office. The AXN800 supports up to 4 IOM line cards and 2 network interfaces cards.

Both platforms can be equipped with a broad range of serial and channelized TDM I/O interfaces including T1/E1 and OC-3/STM-1. The AXN1600 and AXN800 can start with minimum configurations of 16 T1/E1 interfaces or a single OC-3/STM-1 interface which enables a low-cost entry point. Growth is accommodated by simply adding more IOM line cards as required. It can scale up to 64 physical T1/E1s interfaces with the AXN800 or to 192 T1/E1s interfaces with the AXN1600.

AXN10 Pseudo-Wire Access Device - a compact, cost-effective and fixed configuration device. The AXN10 is designed for deployment in customer locations or cell sites. It supports the following interfaces:

o 4 or 8 T1/E1 interfaces o 2 x 100 BaseFx or 2 x 1000 BaseX (LAN or network uplink interfaces) o 3 x 10/100 BaseTx (LAN or network uplink interfaces)

AXN1 Pseudo-Wire Access Device - a compact, cost-effective and fixed configuration

device. The AXN1 is designed for deployment in customer locations or cell sites. It supports the following interfaces:

o 1 T1/E1 or 1 serial interface o 1 x 100 BaseFx (LAN or network uplink interfaces) o 2 x 10/100 BaseTx (LAN or network uplink interfaces)


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Key Benefits and Features Standards-based circuit and service emulation - IETF PWE3 compliance encapsulation

for T1/E1 SAToP, CESoPSN , Frame Relay, HDLC, and ATM

“Telco-grade” carrier-class availability through a complete set of redundancy capabilities

High-Precision Clock Recovery (HPCR®) - The industry’s most robust and accurate

clock recovery mechanism, compliant with ITU-T G.823, G.824 and G.8261

Advanced QoS mechanisms - including rate limiting on a per-port basis, multiple prioritized queuing, DiffServ, and 802.1P CoS on a per-service flow basis

Advanced jitter management mechanism - to handle jitter and reduce end-to-end delay

which typically exists in packet-switched networks.

Comprehensive management capabilities - through Axerra’s CLI and AXNVision™ NMS

The following managed backhaul services are supported:

HSPA offload

3G Iub voice/video and data (R99 plus HSPA)

2G Abis interface.

NodeB synchronization using HPCR® and, as the standard matures, IEEE 1588-2008. Axerra Networks furnishes the industry's most complete range of service aggregation and Interworking solutions. Axerra's multiservice over packet (MSoP) technology enables incumbent carriers, mobile/wireless operators, and cable/MSOs to extend both profitable legacy and emerging services over their Carrier Ethernet networks, such as: IP/MPLS, Ethernet, DOCSIS HFC networks, etc. For more information visit Axerra Networks web site at: www.axerra.com Note: This document is provided for informational purposes only and may be subject to change without notice

Americas EMEA APAC General Inquiries [email protected] [email protected] [email protected] [email protected]

Addresses, phone numbers and fax numbers are listed on Axerra’s website: www.axerra.com/our_offices.asp

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a managed pseudo-wire backhaul service for mobile network · pdf file ·...

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