Wireless Backhaul for LTE - Requirements, Challenges and Options

8/6/2019 Wireless Backhaul for LTE - Requirements, Challenges and Options

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Wireless Backhaul for LTE - Requirements,Challenges and Options

G Kalimani Venkatesan, Kishor Kulkarni, Wipro Technologies, Bangalore 560100Email: {kalimani.venkatesan.kishor.kulkarni}@wipro.com

Fig.I. LTEBackhaul- Connectivity requirement

A summary of the key requirements for LTE Backhaul isgiven below -

support this inter-Base station communication requirement.

Additional intrinsic requirements on the Backhaul networksuch as (i) Scalability (in Bandwidth provisioning forincremental rollouts, in Cell Sites), (ii) Logistical (AccessNetwork Reach, Ease/Speed of new deployment), (iii)Migration / Upgrade paths (from existing 2G/3GUMTS/CDMA backhaul networks, as well as scaling for thefuture), (iv) Service continuity (support 2G/3G services andNetwork elements over the backhaul, TDM/ATM emulation,and Timing/Synchronization transport), all at LowerCapex/Opex and at service levels equal to or better thanexisting TDM based network infrastructures.

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1. Higher data rates: From 100/50 Mbps DL/UL up to 1Gbps DL between eNB and MME/SAE-GW.2. Flat Architecture: MMEs/ SAE-GW and eNB shall be

connected in a many-to-many relationship.3. Low latency: Providing lower user and control planelatency [e.g. less than 5ms in ideal conditions].4. Connectivity: Point-to-point (P2P), point-to-multipoint(P2MP, Multicast) and multipoint-to-multipoint(MP2MP, Broadcast) traffic between various networkentities.5. Guaranteed QoS, multi CoS: Handle different IP trafficclasses - Real-time, Non-Real time, andMission critical.6. High Availability, Resiliency, Fault and Performancemanagement

LTE ARCHITECTURE AND BACKHAULREQUIREMENTS

Abstract-NextGeneration Broadband Wireless Technologiessuch as 3GPP Long Term Evolution (LTE) and WiMax offersignificantly higher data rates and requi re suitably highercapacity backhaul networks. While some service providers haves tar ted rolling out WiMax, 3GPP LTE is expected to bestandardized during 2009, and many service providers areplanning to offer LTE services by 2010-2012. Apart fromsignificantly higher speeds, LTE Base stations (eNBs) requirelogical full mesh connectivity due to the flat all-IP architecture.This paper explores the Wireless backhaul networkinfrastructure options for addressing the LTE bandwidth andconnectivity challenges. The paper details the backhaulrequirements for 3GPP LTE as specified in the LTEspecifications. The paper analyses different architectures for theBackhaul Access and Aggregation networks. The paperproposes a logical topology model for the Aggregation network,examines its realization via Carrier Ethernet Transport andIP/MPLS, and identifies technology gaps in realizing the logicaltopology model. The paper concludes that while MPLS satisfiesthe requirements better today, given the time available for LTEevolution, native Carrier Ethernet Transport could emerge as astrong candidate for future deployments.

3GPP Long Term Evolution (LTE) [1] is a significantevolutionary step for UMTS in terms of capacity andarchitecture. LTE aims to offer a minimum of 100/50 MbpsDL/UL (Downlink/Uplink, 1 sector, 20MHz spectrum) and upto 1 Gbps DL (3 sectors, DL/UL of 300/150 Mbps per sector)per LTE Base Station (eNB). This is significant increase over3G with peak data rates around 30 Mbps. Consequently, theexisting 3G Wireless Backhaul Access network capacitiesneed to be significantly increased to cater to LTE backhaul.The Radio Access Network (RAN) connectivity has been

evolving towards a flat, all- IP architecture with reducednumber of Network elements for lower latency. As seen inFig.l. LTE defines direct connectivity ("SI") between theLTE eNB and the LTE Gateways (Mobility ManagementEntity-MME) without any Radio Network Controller. Hence,LTE eNB carries more intelligence for functionalities likeRadio Resource Management towards distributed decisionmaking, which requires inter-eNB logical connectivity("X2"). Again, existing Backhaul networks do not naturally

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II. LTEBACKHAULARCHITECTURALOPTIONSA. AccessNetwork Technology OptionsBandwidth, Cell-site Reach, available Upgrade paths are

key criteria for evaluating the Access options. BroadbandAccess technologies such as (i) VDSL2 (with bonding forcapacity, co-located DSLAMs at Cell Sites), (ii) PON (GPON,G EPON; Upgrades via with lOG GPON/10G GEPON, WDMPON), (iii) Ethernet over Fiber (WDM - 1 to 10 Gbps, withCarrier Ethernet Metro rings), (iv) Microwave Radios:PDHlSDH Radios or native Ethernet Framed Radios withSwitched Ethernet functionality and (v) NG SONET/SDH(investment utilization, till transformation to Carrier GradePSNs) can be considered for the LTE Access Network.B. Aggregation Network - Technology OptionsDepending on the user traffic and network control traffic

patterns, and associated CoS requirements, appropriateconnectivity models can be adopted for the Aggregationnetwork. As in the following figure, the LTE eNB NetworkController connectivity can be modeled as a set of P2P orP2MP tunnels, or a mix of both. The Inter-eNB connectivity,for Network control traffic such as handoffs, would fit aMP2MP model.

Fig.2. LTE Backhaul- Logical Connectivity with P2P, P2MP andMP2MP

- P2P model requires N (number of eNBs) trafficengineered tunnels. Multiple traffic classes can be supportedwithin one tunnel via CoS identifiers. Connection orientedP2P tunnels with admission control (CAC), and Pathprotection (by an Active/Standby P2P tunnel pair) is ideal formeeting stringent QoS requirements.- P2MP model, which allows for bidirectional eNB NC

connectivity can also be used to carryall traffic classes. OneP2MP tunnel can be provisioned across all eNBs, or onetunnel per set of eNBs, or one tunnel per service class. Theillustration depicts two P2MP tunnels, one per service class.Multicast applications, such as Multicast Video and Network

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Synchronization via Packet based Timing, suit this model.- MP2MP model avoids having N tunnels from each eNB

to every other eNB (N tunnels), and requires a single tunnelat a eNB, leaving the interconnection complexity to thenetwork.When mapped to MEF services [2][3] (via IP/MPLS or

native Carrier Ethernet Transport), P2P maps to EthernetVirtual Private Line (EVPL) service, P2MP maps to EthernetVirtual Private Tree (EVP-Tree) service, and MP2MP mapsto Ethernet Virtual Private LAN (EVP-LAN) service. Thefollowing sections evaluate native Carrier Ethernet Transportand MPLS for realizing the logical connectivitymodels.1) Carrier Ethernet TransportRealizing the connectivity models is through a mix ofProvider Bridging (PB) [4]/ Backbone Bridging (PBB) [5]/Backbone Bridging with Traffic Engineering (PBB-TE) [6]technologies, and the options are listed below in the order ofsuitability. Carrier Ethernet Transport evolution andchallenges have been evaluated in [12].

TABLE IREALIZING CONNECTIVITYMODELSWITH PB / PBB/ PBB-TE

Connectivity Options [* -Not standardized]P2P - PBB-TE with NMS or GMPLS, involving CAC, pinned(EVP-Line) paths and sub-50 msec protection via CCM.

- PBB with PLSB as control plane*- PB with STPIIVL, or VLAN tunneling or cross connect.P2MP - PBBw ith PLSB a s control plane *(EVP-Tree) -PB with STPIIVL, or VLAN tunneling or cross connect(P2MPwith more thanone SVLAN)MP2MP -PBB withPLSB as control p lane *(EVP-LAN) - PB/ PBB wi th IVL P2P PBB-TE ( like VPLS withEoMPLS Pseudowires)

For P2P transport, PBB-TE offers a Connection orientedmodel. PBB-TE addresses network/node scalability, avoidsxSTP limitations and learning based forwarding with aconfigured or a GMPLS control plane based approach,provides resilience with preconfigured protection path andsub-50 msec switchover. However, currently, PBB-TE islimited to P2P. P2MP or MP2MP support can be derived viaPLSB (Provider Link State Bridging). Future enhancementsto PBB-TE for native P2MP support can be via GMPLS [7].Also, PBB-TE today does not support Pseudowires natively[8], which is a key requirement for 3G services continuity.

Instead of PBB-TE, PB and PBB can be configured to offerP2P, P2MP and MP2MP connectivity. A P2P VLAN tunnelwith PB can be constructed by configuring each eNB in aunique S-VLAN, and the Network Controller attaching tomultiple S-VLANs. This would impose the known VLANscaling issues (4094) for eNB. With PBB, a P2P VLAN canbe constructed without scaling limitations, by mapping the 1SID for each eNB as a unique service instance. However, withPB and PBB, it would be more common to use aShared/Service based EVC model, with an S-VLAN sharedacross a set of eNBs for a specific Traffic class.



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2) MPLSBeing the technology of choice in Mobile core network,MPLS can logically be extended into the Aggregation. Also,MPLS is more than a mere transport, as the many servicesthat have spawned off it (Pseudowires, L3 VPN, L2 VPN)have become indispensable. Pseudowires are indispensable tomigrating the 2G TDM and 3G ATM elements to a Packetbackhaul, and MPLS is the only defined PSN today forcarrying Pseudowires. The table below lists the possibleMPLS options and availabilities in the order of suitability.

TABLE IICONNECTIVITY MODELS WITH MPLS

Connectivity Options [* -Not standardized]P2P - EoMPLS Pseudowires or native IP over MPLS(EVP-Line) - CACwithRSVP-TE or NMS provisioned

- Protectionwith FRRand PW redundancy [8]*- Multi-segment PW with PW-stitching for scalability[7] *- Dry-Martini for simplification at Access *

P2MP - Multicast MPLS - RSVP-TE, LDP with P2MP extensions,(EVP-Tree) which doesn 't require IPMulticast Routing.

- FRR for P2MPMPLS-TE *MP2MP - VPLS(EVP-LAN) - Scalingwith H-VPLS;VPLS with 802.lah/802. la d [9]*

MPLS provides P2P Traffic Engineered tunnels, setup byRSVP-TE. These P2P tunnels can be protected by fast-reroute(FRR), which can offer 50 msec restoration times. End-to-endPseudowire path protection can also be provisioned via PWRedundancy.MPLS support for P2MP has been standardized, with

P2MP extensions for RSVP-TE and LDP. With one MPLSP2MP tunnel (from NC to eNB) and multiple P2P tunnels(from eNB to NC), an EVP-TREE topology can be realized.This can be used for both Unicast andMulticast traffic.MP2MP support to communicate across eNBs can be via

VPLS, or H-VPLS with 802.lad (PB) or 802.lah (PBB).- MPLS P2P solution would require tunnels as the number

of eNBs (O[N]). To reduce the scalability burden, Multisegment Pseudowires with PW-stitching can be used. EacheNB/CSG gets connected via a PW tunnel till the MobileAggregation Gateway (Access segment), where many suchPW are "stitched" together to provide a single tunnel towardsthe NC (Aggregation segment). This would ease the numberof tunnels at the Aggregation.- The Access segment Pseudowires can also be statically

configured, reducing the complexity. At the Access nodes,running a full IP routing and MPLS signaling functionalitynot needed. It is possible to simplify this by having staticconfiguration for routes, MPLS labels and Pseudowire label,and the data plane encapsulation alone for Pseudowires.Again, using PW stitching, Aggregation gateway can connect

3such static Pseudowires to Dynamic P2P tunnels.

3) Hybrid: Carrier Ethernet MPLSTABLE III

CARRIER ETHERNET TRANSPORT AND IPIMPLS COMPARISONCarrier Ethernet Transport IPIMPLS

Strengths - PossibleLowerCapex/Opex. - Proven- Good for P2P with PBB-TE - Pseudowires- S t r o n ~ e r 0 AM toolkit - P2PIP2MP withTEWeakness - P2MP withxSTP, noTE/QoS - Perceivedcomplexity,- Not proven higher Capex and

Opex.

The table above compares Carrier Ethernet and IP/MPLSfor the LTE backhaul. Rather than adopting single technologyapproaches, solutions can take advantage of eachtechnology's strengths for specific application. An example isa Hybrid solution for providing MP2MP inter eNBconnectivity. Assuming the Aggregation is PB/PBB based,the P2MP/MP2MP approach with the multicast PLSB supportis only a future option. Instead, it is possible to emulate theVPLS model by having PB clouds (connecting sets of eNBs)connecting via PBB-TE tunnels to each other. Or, a first levelaccess/aggregation network could provide PBB-TE basedVPLS, interconnecting over an MPLS Aggregation Network.

III. CONCLUSIONThis paper has analyzed the LTE backhaul requirements at

Access and Aggregation. The LTE connectivity model goesbeyond the existing 'single tunnel aggregation' model andrequires traffic engineered P2P, P2MP and MP2MPconnectivity. Between native Carrier Ethernet and MPLS,MPLS is seen as a better option in the current assessment tosatisfy LTE backhaul requirements. However, innovations inCarrier Ethernet transport, and time factor for LTEdeployment could bring Carrier Ethernet into contention.

REFERENCES[I] E-UTRANArchitecture description, 3GPPTS 36.401, 3GPP

specifications [online].:http://www.3gpp.orgj[2] Ethernet Services Attributes Phase 2,MEF 10.1.1 specification, June

2008,Metro Ethernet Forum [online]. http://www.metroethemetforum.org[3] Mobile Backhaul Implementation Agreement, MEF D00065_003, June

2008,Metro Ethernet Forum [online]. http://www.metroethernetforum.org[4] ProviderBridges, IEEE 802.lad, draft 6, March2008 [online] Available:

http://ieee802.orgj[5] Provider BackboneBridges, IEEE 802.lah draft, Jan 2008,[online][6] ProviderBackboneBridging with Traffic Engineering, IEEE 802.1Qay,March 2008, [online][7] Don Fedyk, et aI., "GMPLS controlof Ethernet PBB-TE", draft-fedyk

gmpls-ethernet-pbb-te-02, IETF draft [online]..http://www.ietf.orgj[8] D. Allan, et aI., "Carrying PWE3 Pseudo Wires over Provider Backbone

Transport", draft-allan-pw-o-pbt-03, IETF [online]. http://www.ietf.orgj[9] Bocci, et aI., "An Architecture for Multi-Segment Pseudo Wire Emulation

Edge-to-Edge" draft-ietf-pwe3-ms-pw-arch, IETF [online][10] Muley, et aI, "Pseudowire Redundancy", draft-muley-pwe3-pw

redundancy, IETF draft [online]. Available: http://www.ietf.orgj[II] A. Sajassi, et aI., "VPLS Interoperabilitywith Provider Backbone

Bridges", draft-sajassi-12vpn-vpls-pbb-interop-Ol, IETF draft [online].[12] KishorKulkarni, G Kalimani Venkatesan, "Carrier Ethernet - State ofArt

and ChallengesAhead"; IEEE ANTS Conference, Dec 2007.


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Wireless Backhaul for LTE - Requirements, Challenges and Options