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Mobile broadband backhaul: Addressing the challenge

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  • 1. Mobile broadband backhaul: Addressing the challenge Cost considerations are the key decision-making criteria for planning backhaul networks. The success of mobile broadband calls for changes to radio access and backhaul network architectures. And technical adaptation is needed alongside the support of new business models. Rajesh Chundury The article gives an overarching perspective on backhaul for the radio network. Proceeding from individual operator requirements, it examines the main architec- tural and technology choices and suggests a path for evolving toward packet transport. Whatisbackhaul? The backhaul environment is the part of a mobile network that connects base stationstonetworkcontrollerswithina coveragearea(Figure1).Thetransport network, which interconnects the net- work controller and the core network (circuit,packetorboth),isreferredtoas the core transport network. Some defi- nitions in the industry extend the defi- nitionofabackhaulnetworktoinclude the core transportnetwork;thisarticle doesnot. Backhaul is also sometimes referred to as first mile and last mile (first mile from a fixed or wireline perspective, and last mile from a wireless or mobile perspective). To avoid confusion, this articleintroducestwonewterms:  Lowradioaccessnetwork(LRAN)–that is,thecellsiteaccesspartoftheRAN backhaul,whichtypicallyincludesmulti- plephysicalaccesstechnologies(micro- wave,copperandfiber)1 ;and  Highradioaccessnetwork(HRAN)– thatis,thepartofthenetworkthatcol- lects,aggregatesandconcentratestraf- ficfromLRANforconnectingintothe radiocontrollers.Thispartofthenet- workiscommonlybasedonopticalfiber networkinginfrastructure. Whynext-generationbackhaul? High-speed packet access (HSPA) sub- scriber growth and increased usage of mobilemultimediaservicesaredriving thesuccessofmobilebroadbandaround the world. This serious complement to fixed broadband has become a source of new revenue streams for mobile net- work operators. Indeed, the increase in data traffic in mobile networks has been dramatic – a study of Ericsson- enabled networks has shown that in 2007mobiledatatrafficsurpassedvoice trafficinmostoperators’HSPA-enabled networks. Many operators are seeing exponentialincreasesintraffic,thanks to attractive offerings and to consumer trends toward high-bandwidth appli- cations, such as video sharing, peer- to-peer and enterprise use of mobile internet and remote office connectivi- ty. These developments are driving the needfornext-generationbackhaul. RANrequirements All radio access networks (GSM/CDMA, WCDMA/CDMA2000 as well as LTE) put specific sets of requirements on backhaul. Other requirements stem from type and number of end-user ser- vices (voice, best-effort data, gaming, BOX A  Terms and abbreviations CESR Carrier Ethernet switch router DSCP Differentiated services code point DSL Digital subscriber line E-LAN Ethernet service type based on a multipoint-to-multipoint EVC E-LINE Ethernet service type based on a point-to-point EVC EoS Ethernet over Sonet/SDH E-TREE Ethernet service type based on a rooted multipoint EVC EVC Ethernet virtual connection HRAN High RAN HSPA High-speed packet access IPSec IP security (a suite of protocols for securing IP) L2TP Layer-2 tunnelling protocol LRAN Low RAN LTE 3GPP Long Term Evolution MAC Media access control MBMS Multimedia broadcast/multicast service MEF Metro Ethernet Forum MOS Mean opinion scores MPLS Multiprotocol label switching NNI Network-to-network interface NTP Network time protocol OCx Optical carrier level x (for example, OC3) OSI Open systems interconnection PBB Provider backbone bridge PBB-TE PBB traffic engineering PoS Packet over Sonet/SDH POTP Packet optical transport platform PTP Precision time protocol (IEEE 1588) PWE3 Pseudo wire emulation edge-to-edge RAN Radio access network SAToP Structure-agnostic TDM over packet SDH Synchronous digital hierarchy SFN Single frequency network SLA Service level agreement SoIP Synchronization over IP Sonet Synchronous optical network TCO Total cost of ownership TDD Time-division duplex TDM Time-division multiplexing UNI User network interface VLAN Virtual LAN VPLS Virtual private LAN service WCDMA Wideband code-division multiple access WDM Wavelength-division multiplexing Ericsson review • 3 2008 4 Planning backhaul networks
  • 2. mobileTV,andsoon).Forinstance,base stations require accurate synchroniza- tion in order to generate a radio signal, to ensure smooth handover of calls, to avoidinterference(inTDDmode),andto meet performance and availability lev- els (for instance, for mobile TV based on MBMSSFN).Asaresult,stringentrequire- ments (based on the services offered by the radio network) are put on backhaul networks. Table 1 summarizes some of thekeytechnicalrequirements. Voice,latency,jitter When operators move from fixed- latencycircuit-switchednetworkstovari- able-latency packet-data networks, they mustbecertaintomaintainvoicequality. Toattainhighmeanopinionscores(MOS) forvoicequality,regardlessofwhetheritis nativevoice(2G,3G)orVoIP(LTE),thevoice packets must arrive in sequence within a verytightdelaywindow(lowjitter). Video, frame loss rate, traffic separation With video driving the needs of next- generation mobile networks, backhaul networksmustalsoofferlowpacketloss rates and be able to handle at least four classes of service – in order to support the four broad types of traffic: network control, voice/video, best-effort data, andnetworkmanagement. Restorationandavailability To achieve high availability, higher- order radio nodes are typically built with carrier-class nodes which fea- ture redundant paths and which can restoreconnectivitywithin50to100ms (carrier-class nodes have multiple and redundantcontrolanddataplanes). Vision:IPintheRAN,Ethernetin thebackhaul As the radio nodes evolve toward O BSC 2G 3G LTE RNC CS core PS core SAE GW End-user IP endpoints outside the mobile network IP endpoints in the RAN LRAN HRAN Backhaul Radio access network (RAN) Core network (CN) Core transport Ethernet de-marc/handoff to backhaul Ethernet aggregationEthernet access Microwave Copper Fiber IP endpoints in the mobile core Figure 1  Radio, core and backhaul network boundaries. table 1  Backhaul requirements relative to end-user services. GSM (assuming the primary service is voice) WCDMA (assuming the primary services are voice and best-effort mobile broad- band connectivity) LTE (assuming the primary services are video (on-demand, P2P), gam- ing, VoIP and best-effort mobile broadband) One-way delay Max: 40ms Target: 10ms Max: 30ms Target: 10ms Max: 20ms Target: 10ms Delay variation Max: 10ms Target: 5ms Max: 10ms Target: 2ms Max: 2ms Target: 1ms Packet loss rate Max: 10-3 Target: 10-4 Max: 10-3 Target: 10-6 Max: 10-3 Target: 10-6 Synchronization (radio frequen- cy accuracy) 50–100ppb 50ppb 50ppb Classes of service (CoS) 2 2–4 2–4 Recommended availability 99.99 99.99 99.99 Typical backhaul per base station (low : med : high) 2:4: 8 T1/E1 6:20:50 Mbps 90:180:400+ Mbps 5 Ericsson review • 3 2008
  • 3. theworld,whereTDM-basedmicrowave is predominant, Ethernet upgrade kits areavailable. The offloaded packet traffic is aggre- gated in HRAN and can be overlaid with incremental investment to exist- ing optical transport equipment that can carry TDM and Ethernet natively andefficiently. In the final state, all new radio net- work elements will be based on IP/ Ethernet and any residual TDM servic- es can be carried over a native packet- basedbackhaulnetwork. Future-proofingforLTE Ethernet is a good fit for LTE backhaul. MEF has defined three categories of Ethernet virtual connections (EVC): point-to-point (E-Line), multipoint-to- multipoint (E-LAN), and rooted multi- point(E-Tree). For instance, given the LTE S1 inter- faceandthefactthat(a)thereisnomac- ro diversity (in the current LTE stan- dards) and (b) all handovers between twoLTEbasestations(eNodeB)arehard, itmakessensetobuildLTES1onrooted multipoint E-TREE. Likewise, it makes sense to build LTE X2 (a direct inter- face between neighboring eNodeBs) on multipoint-to-multipoint E-LAN.4 In summary, Carrier Ethernet and MEF- defined Ethernet constructs represent the best technological and architectur- al choices for building next-generation backhaulnetworks. Keytechnicalconsiderations L2versusL3 The “convergence” of packet data com- munication and technology with tradi- tional circuit-switched transport and transmission is forcing the “fusion” of OSI layer functions (Figures 3–4). Traditional layer-1 (L1) transport ven- dors, for example, have created packet optical transport platforms (POTP) by combining critical layer-2 (L2) Ethernet aggregation functionality with lega- cy Sonet/SDH and newer WDM nodes. Similarly, typical layer-3 (L3) data ven- dors have created Carrier Ethernet switch routers (CESR) by including crit- icalL2Ethernetaggregationfunctional- ityinIPorIP/MPLSrouters. In addition, steps have been taken to evolve pure L2 networks into scalable carrier-gradeEthernetby IP, the backhaul evolves toward Ethernet. The best and most cost- ­effectivewayoftransportingIPconnec- tivity is to use an Ethernet backhaul; all backhaul nodes do not need to be IP-aware,andbecausetheLRANismore orlesspoint-to-pointintermsoftopolo- gy, Ethernet is generally the most cost- effective form of transport for carry- ingIPtraffic.IPoverEthernetgivescost andcapacityscalabilityadvantagesand isfuture-proof. TheMetroEthernetForum(MEF)has identifiedfivekeyattributesthatdistin- guishCarrierEthernetfromtraditional LAN-based Ethernet, namely standard- ized services, scalability, service man- ageability, quality of service, and reli- ability. Carrier Ethernet can also pro- vide statistical multiplexing. It is thus qualifiedintermsofcharacteristicsand behaviortohavealeadingroleinmobile backhaul. Backhaulmigration In order to cater for the appropriate interconnectorinterfacewiththeback- haulnetwork,backhaulprovidersneed tounderstandtheevolutionofthetrans- port interfaces offered by radio hard- warevendors. Today,most2Gand3Gnetworkscon- nect over a TDM infrastructure. All TDM connections from the LRAN are multiplexed into higher-order Sonet/ SDH rings for final delivery to the net- work controllers. At present, most ref- erences to an IP connection on a radio nodeareoveran  Ethernet(FE/GE)portatthephysical interface;or  ML-PPPdatalinkconnectionoveraset ofTDM(T1,E1orJ1)physicalinterfaces. One incremental approach to evolving thebackhaulnetworkistoemployHSPA offload with packet overlay (Figure 2). The assumption here is that the major- ity of mobile broadband data is best- effort in nature and based on flat-rate pricing models. Notwithstanding, the availabilityofdual-stackradionetwork elements(TDMandIP/Ethernet)enables the radio networks to continue to oper- ate by means of tried-and-tested TDM connections for radio network control and delay-sensitive voice traffic, while offloading best-effort data (via the IP/ Ethernet port) to an Ethernet/L2 VPN connection. In North America, a variant of DSL canbeusedasanextensionofthenear- est fiber access point. In other parts of Node BNativeTDM IP/TDM TDM+IP TDM/IP Native IP Dual-stack RBS * SAE GW – includes both serving gateway (S-GW) and PDN gateway (PDN GW). IP RBSTDM RBS TDM Ethernet xDSL or microwave Circuit emulationIP/ML-PPP BTS eNode B Node B Node B Node B NodeB BTS BTS BSC SAEGW RNC BTS Figure 2  Transport interfaces and backhaul migration options. 6 Ericsson review • 3 2008 Planning backhaul networks
  • 4.  employingMAChiding(viaprovider backbonebridges/MAC-in-MAC); turningoffMAClearning;and employingsub-100mspathresiliency andrestorationmechanismsinaccor- dancewiththeIEEE802.1familyofstan- dards. ForEthernet in LRAN,the industryrec- ommendation is to build simple point- to-point L2 connections by employing providerbridging(VLANencapsulation) or similar L2 mechanisms, and then to carry TDM natively. Later, as the peak radiodataratesincrease,onecanintro- duce overbooking to achieve greater efficiencyviaEthernetswitching. HRANs are dependent on operator assets (mobile only, fixed only, or fixed and mobile) and can be either a simple opticalnetworkwithembeddedL2aggre- gation or a richer multiservice network withfullL3IPorIP/MPLScapabilities. Figure 5 describes the architecture that best serves a “transport-oriented” Ethernetbackhaulmodel.Inthiscase,the HRAN delivers connectivity in the man- neroftraditionalSonet/SDHnetworks. Figure 6 describes the architecture that best serves a “service-oriented” Ethernet backhaul model. In this case, theHRANdeliversconnectivitywith  pseudowire-based(PW)technology thatcanevolvefromstaticPWupto traffic-engineeredPW;and Ethernetservices,suchasE-Line,E-Tree, ormultipoint-to-multipointE-LAN. The decision to employ L3- or L2-based architectureisdependentonthenature and mix of applications. Moreover, this issuesolelyappliestotheHRAN–thatis, thebasicrulesstillapply:“Switchwhere you should (within the same domain), routewhereyoumust(acrossdomains), andtransporteverywhere.” Deterministicbehaviorwithconnection- orientedEthernet Next-generation packet backhaul net- works need to retain the inherent- ly deterministic nature of a circuit- switched or TDM-based backhaul. At present,therearetwowaysofbringing about determinism (applicable more in HRANthaninthesimpleLRAN):  EthernettunnelsbasedonPBBand PBB-TE(IEEEproposal);and  IP/MPLSEthernetPWE3andMPLS-TP (IETFandITU-Tproposal). Although the concept of Ethernet tunnels is appealing (given that the L2 issueissolvedinandbyL2itself),thereis meritintheargumentthatIP/MPLShas already solved the L2 issue (by employ- ingL3intelligence)andthereforeenjoys atime-to-marketadvantage. If time to market or the needs of a multiservice network predominate, thenanIP/MPLS-basedapproachmight be advantageous, albeit at greater cost (OPEX) because troubleshooting com- petencemustnowencompassL1/access, L2/Ethernet,L3/IPandMPLS. Note,anew,jointIETF/ITU-Tstandard called MPLS Transport Profile, which takes into account the traditional OAM toolsavailablefromSonet/SDH,isbeing developed to resolve the issue of native IP/MPLScomplexity. Notealsothatthecharacteristicplug- and-play simplicity associated with tra- ditional Ethernet LAN services is not applicable in the case of PBB(-TE) ser- vices which require explicit provision- ing,possiblybymeansofacontrolplane that handles the automatic creation of tunnels. The simple fact is we live in a TDM Sonet/SDH MSPP IP ML-PPP POTP (Transport tunnel) ATM Circuit emulation services/ PWE3 PBB (-TE)/ MPLS-TP EoS or PoS Ethernet WDM/OTN (ROADM) NMS-based provisioning Figure 3  POTP – Transport-oriented model. TDM IP routing ATM CESR (Transport service) ETH VPN IP/MPLS or MPLS-TP Ethernet WDM/OTN (ROADM) Sonet/SDH Interfaces IP ML-PPP EoS or PoS Policy-based provisioning Circuit emulation services/PWE3 Figure 4  CESR – Service-oriented model. BOX b  Backhaul In a hierarchical telecommunica- tions network, the backhaul por- tion of the net- work comprises the intermediate links between the core, or backbone, of the network and the small subnetworks at the edge. For example, while cell phones com- municating with a single cell tower constitute a local subnetwork, the connection between the cell tower and the rest of the world begins with a backhaul link to the core of the telephone com- pany’s network. 7 Ericsson review • 3 2008
  • 5. network interfaces (NNI) are relatively simple and well-defined (OCx), with no buffering in third-party networks. As a result, they have no impact on laten- cy. Packet-data networks, on the oth- er hand, necessitate SLAs to sort out the latency issues associated with the connectionless and store-and-forward nature of packet transport. MEF UNI, NNI & Service OAM, 802.1ag and ITU-T Y.1731bestaddressthemeansandmeth- odsthatcouldbeusedtoenableSLAsfor serviceassurance. Trafficseparationandprioritization Asmentionedabove,nomorethanfour classes of service are needed within a backhaulnetwork. Ordinarily, IP DSCP is one of the end- to-endpath-levelquality-of-service(QoS) mechanisms that span the RAN, core network and the internet. Likewise, Ethernetlink-levelQoSservestheinter- mediate connecting links. This makes good sense because there is no guaran- tee that Ethernet will be the sole data link of choice – indeed, the mobile IP client and server connect over various locallinktechnologiesthatspanmobile networksandtheinternet. Note that because most radio nodes map the radio classes of service to IP QoS (six-bit DSCP field in the IPv4 head- er) and to Ethernet QoS (three-bit user priorityfieldin802.1Q),abackhaulnet- work needs only respect and adhere to the802.1pvaluessetbytheradionodes. The individual radio bearers must be mapped correctly to IP and Ethernet QoS. Furthermore, policing, metering and shaping functions are needed to differentiate traffic and maintain ser- vicelevelsinanEthernetbackhaulthat spansacrossmultipledomains. Security Security is more inherent at the low- er levels of the network. Multilayered securityisbothakeyfeatureandachal- lenge of IP transport. A recommenda- tion is to designate separate site securi- tyzonesandtoassignprotectionaccord- ing to the assessed risk. To reduce the riskofintrusion,onecanrestrictaccess from one zone to another by firewall- ingtraversingtraffic.Also,whendecid- ing on the appropriate level of securi- ty, bear in mind that WCDMA payload traffic is encrypted. IPSec can be used at a non-secure site to interface a non- trusted network or shared network to protect non-encrypted payload from otherradiosystems. Circuitemulation To maintain a low and strict latency budget, one should only emulate cir- cuits when absolutely necessary. This O L2 (Ethernet) LRAN L3 (IP/MPLS) HRAN Backhaul (Ethernet interconnect) Radio access network (IP endpoints) Cell sites Radio controller sites Fiber Fiber BSC RNC Residental (IPTV)Enterprise (LAN) Enterprise (LAN) 2G 3G LTE SAE GW Microwave Copper Figure 6  “Service-oriented” L2.5 backhaul – L2 LRAN w/L3 HRAN multiprotocolworld.Evenso,thepri- maryissueisnotsomuchamatterofL2 versus L3 or switching versus routing; instead,itisaboutachievingdeterminis- ticbehaviorinordertoaddresskeyradio requirementsinthebackhaul. Serviceassurance In TDM environments, the network-to- O 2G 3G LTE L2 (Ethernet) LRAN L1 (POTP) HRAN Backhaul (Ethernet interconnect) Radio access network (IP endpoints) Microwave Cell sites Radio controller sites Copper Fiber BSC RNC SAE GW Figure 5  “Transport-oriented” L1.5 backhaul – L2 LRAN with L1 HRAN. 8 Ericsson review • 3 2008 Planning backhaul networks
  • 6. is because circuit emulation always incurs a certain amount of overhead or delay (Box C). Certainly, circuit emula- tion is applicable where bandwidth is not a premium commodity (in optical networks, for example) but should be avoided(tomaintainspectralefficiency) over microwave links. When used, cir- cuitemulationalsoaffectssynchroniza- tion, necessitating the regeneration of theservicetiming.Inaddition,thescal- abilityofTDMinterfacesinbasestations might become a problem. In summary, a circuit-emulation step in a migration path to completely native packet trans- port could carry additional cost due to repeatedupgradesandsitevisits. Synchronization There are several ways of distributing the reference timing signals in next- generation packet-data networks. Four key considerations to have in mind when employing packet-based timing are targetrequirements  packetdelayvariation;  abilityoftheclock-recoveryalgorithmto filterpacketdelayvariation;and  qualityoftheoscillatorinreceiving equipment(typicallyabasestation). The choice of packet format (NTP or PTP)isnotacriticalconsiderationespe- cially when the packet-based method is deployed end-to-end (that is, with- out support from the intermediate nodes). Technologies that require support fromtheintermediatenodes(forexam- ple,synchronousEthernet)aresuitable fordirectlyconnectedopticalnetworks (typicalinHRAN)andbenefitfromkeep- ingrelatedperformancelessdependent (or totally independent as in the case of synchronousEthernet)fromvariations inpacketdelay. However, these technologies might not always lend themselves as a com- plete solution, due to the heteroge- neous and varied nature of backhaul. Accordingly, the best solution for syn- chronization is to go with end-to-end packet-basedtiming-regenerationtech- nologiesthatareindependentfromthe underlyingtransportlayer.4 Synchronization over IP (SoIP), a refined NTP-based algorithm that has been optimized for distributing radio timing, is one example of an end-to- end packet-based timing-regeneration technique.EricssonusesSoIPinitsradio nodes. Recommendations While many technologies attempt to solve the technical and business chal- lengesdescribedinthisarticle,Ericsson recognizes that the key challenge of backhaul is in providing cost-effective capacityatthelowestpossibletotalcost ofownership(TCO). There is no single backhaul solution thatappliestoeverysituation.Butaslong as the key radio network requirements (latency,jitter,packetlossrate,synchro- nization and classes of service) are met, either of the “transport-­oriented” or “service-oriented” approaches can be used successfully. As the key stan- dardsforconnection-orientedEthernet mature, operators will continue to choose pragmatic solutions, such as HSPA offload, and take advantage of dual-stackRANtechnology. Ericssonistacklingthechallengesof backhaul evolution by working hand in hand with some of the world’s most innovative mobile broadband provid- ers.Giventhatavarietyofsolutionscan meettherequirementsathand,thekey is to remain focused on the vision of IP in the RAN and Ethernet in the back- haul(withWDMforveryhighcapacities andoptimumuseoffiber),andtochoose the simplest, most cost-effective solu- tionforenablingservicegrowth. BOX c  Circuit emulation overhead T1 (1.544Mbps) takes approxi- mately 1.9Mbps when emulated as a SAToP PWE3 using MPLS. This translates to about 25 percent overhead. One can reduce this figure by increas- ing the packeti- zation interval, but doing so increases the end-to-end delay of the T1 service. Rajesh Chundury joined Ericsson in 1996 with an M.Sc. in computer science from the Universi- ty of Texas at Arlington. He is a senior solutions manager within the VP Networks organization at MU North America (MUNA). He previously worked in the MUNA Global Services organization leading the national trans- port network design team for Cingular UMTS RAN deployments and similar assignments for T-Mobile USA and Sprint-Nextel. Rajesh also worked six years in R&D at the Richardson, Texas facility. Acknowledgements Matthew Smith, Mats Uhlin, Stian Kildal, Anders Eriksson U, Christian Gotare, David Giaina, James Glover, Jonas Edstam, Jonathan Olsson, Kåre Gustafsson, Marie Hogan, Panagiotis Saltsidis, Rishi Mehta, Staffan Karl- berg, Stefano Ruffini, Thomas Edwall and Tony Thun High Speed Drop Technologies for Mobile1. Backhaul. Ericsson White Paper Telecom Quality in ALL-IP Networks. Ericsson2. White Paper, March 2006 (www.ericsson.com/ technology/whitepapers) Network Synchronization in Packet Based3. Networks: Challenges and Solutions. Ericsson White Paper (www.ericsson.com/technology/ whitepapers) Green, H., Monette, S., Olsson, J., Saltsidis, P. and4. Takács, A.: Carrier Ethernet: The native approach. Ericsson Review, Vol. 84(2007)3, pp 84–89 Full Service Broadband Metro Architecture.5. Ericsson White Paper, June 2008 (www.ericsson. com/technology/whitepapers) Full Service Broadband with GPON. Ericsson6. White Paper, June 2008 (www.ericsson.com/ technology/whitepapers) HSPA, the undisputed choice for mobile7. broadband. Ericsson White Paper, May 2007 (www.ericsson.com/technology/whitepapers) Long Term Evolution (LTE): An Introduction.8. Ericsson White Paper, October 2007 (www.erics- son.com/technology/whitepapers) The New Ethernet Aggregation Network.9. Redback White Paper SM 480 Smart Ethernet Switch: A Carrier-class10. Platform Optimized for Delivery of Layer-2 Services. Redback White Paper References 9 Ericsson review • 3 2008