Wireless Backhaul For Lte - Requirements

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					              Wireless Backhaul for LTE - Requirements,
                       Challenges and Options
                        G Kalimani Venkatesan, Kishor Kulkarni, Wipro Technologies, Bangalore 560100
                                  Email: {kalimani.venkatesan.kishor.kulkarni}

                                                                                           support this inter-Base station communication requirement.
   Abstract-Next Generation Broadband Wireless Technologies
such as 3GPP Long Term Evolution (LTE) and WiMax offer
significantly higher data rates and require suitably higher                                                    ~
capacity backhaul networks. While some service providers have                                                     1\                                   ~

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                                                                                                                  1\                                  II

started rolling out WiMax, 3GPP LTE is expected to be
standardized during 2009, and many service providers are
planning to offer LTE services by 2010-2012. Apart from                                                          en
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significantly higher speeds, LTE Base stations (eNBs) require
                                                                                                                  I      \                     I       I

                                                                                                                  i      ~             ~               i
logical full mesh connectivity due to the flat all-IP architecture.                                               I


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                                                                                                                                                       :         E-UTRAN
   This paper explores the Wireless backhaul network                                                          eNS ' \ \ \        «» / /                    eNS
infrastructure options for addressing the LTE bandwidth and
connectivity challenges. The paper details the backhaul                                                                ~~O/~
requirements for 3GPP LTE as specified in the LTE                                                                                eNS
specifications. The paper analyses different architectures for the
                                                                                                            Fig.I. LTE Backhaul- Connectivity requirement
Backhaul Access and Aggregation networks. The paper
proposes a logical topology model for the Aggregation network,
examines its realization via Carrier Ethernet Transport and                                  A summary of the key requirements for LTE Backhaul is
IP/MPLS, and identifies technology gaps in realizing the logical                           given below -
topology model. The paper concludes that while MPLS satisfies
the requirements better today, given the time available for LTE                            1.    Higher data rates: From 100/50 Mbps DL/UL up to 1
evolution, native Carrier Ethernet Transport could emerge as a
                                                                                                 Gbps DL between eNB and MME/SAE-GW.
strong candidate for future deployments.
                                                                                           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 plane
                                                                                                 latency [e.g. less than 5ms in ideal conditions].

3   GPP Long Term Evolution (LTE) [1] is a significant
    evolutionary step for UMTS in terms of capacity and
                                                                                           4.    Connectivity: Point-to-point (P2P), point-to-multipoint
                                                                                                 (P2MP,      Multicast)    and     multipoint-to-multipoint
architecture. LTE aims to offer a minimum of 100/50 Mbps                                         (MP2MP, Broadcast) traffic between various network
DL/UL (Downlink/Uplink, 1 sector, 20MHz spectrum) and up                                         entities.
to 1 Gbps DL (3 sectors, DL/UL of 300/150 Mbps per sector)                                 5.    Guaranteed QoS, multi CoS: Handle different IP traffic
per LTE Base Station (eNB). This is significant increase over                                    classes - Real-time, Non-Real time, and Mission critical.
3G with peak data rates around 30 Mbps. Consequently, the                                  6.    High Availability, Resiliency, Fault and Performance
existing 3G Wireless Backhaul Access network capacities                                          management
need to be significantly increased to cater to LTE backhaul.
                                                                                              Additional intrinsic requirements on the Backhaul network
  The Radio Access Network (RAN) connectivity has been                                     such as (i) Scalability (in Bandwidth provisioning for
evolving towards a flat, all-IP architecture with reduced                                  incremental rollouts, in Cell Sites), (ii) Logistical (Access
number of Network elements for lower latency. As seen in                                   Network Reach, Ease/Speed of new deployment), (iii)
Fig.l. LTE defines direct connectivity ("SI") between the                                  Migration / Upgrade paths (from existing 2G/3G
LTE eNB and the LTE Gateways (Mobility Management                                          UMTS/CDMA backhaul networks, as well as scaling for the
Entity-MME) without any Radio Network Controller. Hence,                                   future), (iv) Service continuity (support 2G/3G services and
LTE eNB carries more intelligence for functionalities like                                 Network elements over the backhaul, TDM/ATM emulation,
Radio Resource Management towards distributed decision                                     and Timing/Synchronization transport), all at Lower
making, which requires inter-eNB logical connectivity                                      Capex/Opex and at service levels equal to or better than
("X2"). Again, existing Backhaul networks do not naturally                                 existing TDM based network infrastructures.

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    II. LTEBACKHAULARCHITECTURALOPTIONS                                                    Synchronization via Packet based Timing, suit this model.
                                                                                              - MP2MP model avoids having N tunnels from each eNB
 A. Access Network Technology Options                                                      to every other eNB (N2 tunnels), and requires a single tunnel
   Bandwidth, Cell-site Reach, available Upgrade paths are                                 at a eNB, leaving the interconnection complexity to the
key criteria for evaluating the Access options. Broadband                                  network.
Access technologies such as (i) VDSL2 (with bonding for                                       When mapped to MEF services [2][3] (via IP/MPLS or
capacity, co-located DSLAMs at Cell Sites), (ii) PON (GPON,                                native Carrier Ethernet Transport), P2P maps to Ethernet
G EPON; Upgrades via with lOG GPON/10G GEPON, WDM                                          Virtual Private Line (EVPL) service, P2MP maps to Ethernet
PON), (iii) Ethernet over Fiber (WDM - 1 to 10 Gbps, with                                  Virtual Private Tree (EVP-Tree) service, and MP2MP maps
Carrier Ethernet Metro rings), (iv) Microwave Radios:                                      to Ethernet Virtual Private LAN (EVP-LAN) service. The
PDHlSDH Radios or native Ethernet Framed Radios with                                       following sections evaluate native Carrier Ethernet Transport
Switched Ethernet functionality and (v) NG SONET/SDH                                       and MPLS for realizing the logical connectivity models.
(investment utilization, till transformation to Carrier Grade
PSNs) can be considered for the LTE Access Network.                                        1) Carrier Ethernet Transport
                                                                                              Realizing the connectivity models is through a mix of
 B. Aggregation Network - Technology Options
                                                                                           Provider Bridging (PB) [4]/ Backbone Bridging (PBB) [5]/
   Depending on the user traffic and network control traffic                               Backbone Bridging with Traffic Engineering (PBB-TE) [6]
patterns, and associated CoS requirements, appropriate                                     technologies, and the options are listed below in the order of
connectivity models can be adopted for the Aggregation                                     suitability. Carrier Ethernet Transport evolution and
network. As in the following figure, the LTE eNB ~ Network                                 challenges have been evaluated in [12].
Controller connectivity can be modeled as a set of P2P or
P2MP tunnels, or a mix of both. The Inter-eNB connectivity,                                                                       TABLE I
                                                                                                      REALIZING CONNECTIVITY MODELS WITH PB / PBB / PBB-TE
for Network control traffic such as handoffs, would fit a                                   Connectivity                     Options [* - Not standardized]
MP2MP model.                                                                                   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            - PBB with PLSB as control plane *
                                                                                            (EVP-Tree)        - PB with STPIIVL, or VLAN tunneling or cross connect
                                                                                                              (P2MP with more than one SVLAN)
                                                                                              MP2MP           - PBB with PLSB as control plane *
                                                                                            (EVP-LAN)         - PB / PBB with IVL + P2P PBB-TE (like VPLS with
                                                                                                              EoMPLS Pseudowires)

                                                                                              For P2P transport, PBB-TE offers a Connection oriented
                                                                                           model. PBB-TE addresses network/node scalability, avoids
                                                                                           xSTP limitations and learning based forwarding with a
                                                                                           configured or a GMPLS control plane based approach,
                                                                                           provides resilience with preconfigured protection path and
                                                                                           sub-50 msec switchover. However, currently, PBB-TE is
                                                                                           limited to P2P. P2MP or MP2MP support can be derived via
                                                                                           PLSB (Provider Link State Bridging). Future enhancements
  Fig.2. LTE Backhaul- Logical Connectivity with P2P, P2MP and MP2MP
                                                                                           to PBB-TE for native P2MP support can be via GMPLS [7].
                                                                                           Also, PBB-TE today does not support Pseudowires natively
   - P2P model requires N (number of eNBs) traffic                                         [8], which is a key requirement for 3G services continuity.
engineered tunnels. Multiple traffic classes can be supported
within one tunnel via CoS identifiers. Connection oriented                                   Instead of PBB-TE, PB and PBB can be configured to offer
P2P tunnels with admission control (CAC), and Path                                         P2P, P2MP and MP2MP connectivity. A P2P VLAN tunnel
protection (by an Active/Standby P2P tunnel pair) is ideal for                             with PB can be constructed by configuring each eNB in a
meeting stringent QoS requirements.                                                        unique S-VLAN, and the Network Controller attaching to
   - P2MP model, which allows for bidirectional eNB ~ NC                                   multiple S-VLANs. This would impose the known VLAN
connectivity can also be used to carryall traffic classes. One                             scaling issues (4094) for eNB. With PBB, a P2P VLAN can
P2MP tunnel can be provisioned across all eNBs, or one                                     be constructed without scaling limitations, by mapping the 1-
tunnel per set of eNBs, or one tunnel per service class. The                               SID for each eNB as a unique service instance. However, with
illustration depicts two P2MP tunnels, one per service class.                              PB and PBB, it would be more common to use a
Multicast applications, such as Multicast Video and Network                                Shared/Service based EVC model, with an S-VLAN shared
                                                                                           across a set of eNBs for a specific Traffic class.

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                                                                                           such static Pseudowires to Dynamic P2P tunnels.
    2) MPLS
  Being the technology of choice in Mobile core network,                                         3) Hybrid: Carrier Ethernet & MPLS
MPLS can logically be extended into the Aggregation. Also,
MPLS is more than a mere transport, as the many services                                                                   TABLE III
                                                                                                   CARRIER ETHERNET TRANSPORT AND IPIMPLS COMPARISON
that have spawned off it (Pseudowires, L3 VPN, L2 VPN)                                                      Carrier Ethernet Transport        IPIMPLS
have become indispensable. Pseudowires are indispensable to                                Strengths   - Possible Lower Capex/Opex.    - Proven
migrating the 2G TDM and 3G ATM elements to a Packet                                                   - Good for P2P with PBB-TE      - Pseudowires
                                                                                                       - Stron~er 0 AM toolkit         - P2PIP2MP with TE
backhaul, and MPLS is the only defined PSN today for                                       Weakness    - P2MP with xSTP, no TE/QoS     - Perceived complexity,
carrying Pseudowires. The table below lists the possible                                               - Not proven                    higher Capex and
MPLS options and availabilities in the order of suitability.

                                   TABLE II                                                   The table above compares Carrier Ethernet and IP/MPLS
               REALIZING CONNECTIVITY MODELS WITH MPLS                                     for the LTE backhaul. Rather than adopting single technology
Connectivity                    Options [* - Not standardized]
                                                                                           approaches, solutions can take advantage of each
   P2P          - EoMPLS Pseudowires or native IP over MPLS
(EVP-Line)      - CAC with RSVP-TE or NMS provisioned                                      technology's strengths for specific application. An example is
                - Protection with FRR and PW redundancy [8]*                               a Hybrid solution for providing MP2MP inter eNB
                - Multi-segment PW with PW-stitching for scalability[7] *
                - Dry-Martini for simplification at Access *
                                                                                           connectivity. Assuming the Aggregation is PB/PBB based,
   P2MP         - Multicast MPLS - RSVP-TE, LDP with P2MP extensions,                      the P2MP/MP2MP approach with the multicast PLSB support
 (EVP-Tree)     which doesn't require IP Multicast Routing.                                is only a future option. Instead, it is possible to emulate the
                - FRR for P2MP MPLS-TE *
  MP2MP         - VPLS                                                                     VPLS model by having PB clouds (connecting sets of eNBs)
(EVP-LAN)       - Scaling with H-VPLS;VPLS with 802. lah/802. lad [9]*                     connecting via PBB-TE tunnels to each other. Or, a first level
                                                                                           access/aggregation network could provide PBB-TE based
  MPLS provides P2P Traffic Engineered tunnels, setup by                                   VPLS, interconnecting over an MPLS Aggregation Network.
RSVP-TE. These P2P tunnels can be protected by fast-reroute
(FRR), which can offer 50 msec restoration times. End-to-end                                                             III. CONCLUSION
Pseudowire path protection can also be provisioned via PW                                     This paper has analyzed the LTE backhaul requirements at
Redundancy.                                                                                Access and Aggregation. The LTE connectivity model goes
                                                                                           beyond the existing 'single tunnel aggregation' model and
   MPLS support for P2MP has been standardized, with                                       requires traffic engineered P2P, P2MP and MP2MP
P2MP extensions for RSVP-TE and LDP. With one MPLS                                         connectivity. Between native Carrier Ethernet and MPLS,
P2MP tunnel (from NC to eNB) and multiple P2P tunnels                                      MPLS is seen as a better option in the current assessment to
(from eNB to NC), an EVP-TREE topology can be realized.                                    satisfy LTE backhaul requirements. However, innovations in
This can be used for both Unicast and Multicast traffic.                                   Carrier Ethernet transport, and time factor for LTE
                                                                                           deployment could bring Carrier Ethernet into contention.
  MP2MP support to communicate across eNBs can be via
VPLS, or H-VPLS with 802.lad (PB) or 802.lah (PBB).                                                                           REFERENCES
                                                                                           [I]  E-UTRAN Architecture description, 3GPP TS 36.401, 3GPP
   - MPLS P2P solution would require tunnels as the number                                      specifications [online].:http://www.3 gpp.orgj
of eNBs (O[N]). To reduce the scalability burden, Multi-                                   [2] Ethernet Services Attributes Phase 2, MEF 10.1.1 specification, June
                                                                                                2008, Metro Ethernet Forum [online].
segment Pseudowires with PW-stitching can be used. Each                                    [3] Mobile Backhaul Implementation Agreement, MEF D00065_ 003, June
eNB/CSG gets connected via a PW tunnel till the Mobile                                          2008, Metro Ethernet Forum [online].
                                                                                           [4] Provider Bridges, IEEE 802.lad, draft 6, March 2008 [online] Available:
Aggregation Gateway (Access segment), where many such                                           http://ieee802.orgj
PW are "stitched" together to provide a single tunnel towards                              [5] Provider Backbone Bridges, IEEE 802.lah draft, Jan 2008,[online]
the NC (Aggregation segment). This would ease the number                                   [6] Provider Backbone Bridging with Traffic Engineering, IEEE 802.1 Qay,
                                                                                                March 2008, [online]
of tunnels at the Aggregation.                                                             [7] Don Fedyk, et aI., "GMPLS control of 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
  - The Access segment Pseudowires can also be statically                                       Transport", draft-allan-pw-o-pbt-03, IETF [online]. http://www.ietf.orgj
configured, reducing the complexity. At the Access nodes,                                  [9] Bocci, et aI., "An Architecture for Multi-Segment Pseudo Wire Emulation
running a full IP routing and MPLS signaling functionality                                      Edge-to-Edge" draft-ietf-pwe3-ms-pw-arch, IETF [online]
                                                                                           [10] Muley, et aI, "Pseudowire Redundancy", draft-muley-pwe3-pw-
not needed. It is possible to simplify this by having static                                    redundancy, IETF draft [online]. Available: http://www.ietf.orgj
configuration for routes, MPLS labels and Pseudowire label,                                [II] A. Sajassi, et aI., "VPLS Interoperability with Provider Backbone
                                                                                                Bridges", draft-sajassi-12vpn-vpls-pbb-interop-Ol, IETF draft [online].
and the data plane encapsulation alone for Pseudowires.                                    [12] Kishor Kulkarni, G Kalimani Venkatesan, "Carrier Ethernet - State ofArt
Again, using PW stitching, Aggregation gateway can connect                                      and Challenges Ahead"; IEEE ANTS Conference, Dec 2007.

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