Migrating to IP Based Backhaul - Operators Strategies WP

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Migrating to IP-based Mobile Backhaul: Operator’s Perspective

Written by Ariel Shuper, Senior Director, Head of Product Management & Strategy Celtro Ltd

February 2009

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Migrating to IP-based Mobile Backhaul: Operator’s Perspective Why migrate to IP/MPLS? The evolution of mobile radio technologies has dramatically increased the bandwidth capacity of end user equipment (UE). This poses new challenges for the mobile backhaul network because of the significant increase in traffic volumes, and consequently the required broadband services. Planning and dimensioning the RAN has become very difficult given the new data-oriented mobile services, characterized by high burstiness and complex statistical behavior. In a parallel development, establishing core networks on IP/packet-based technologies, and the widespread use of MPLS for efficient network partitioning and segmentation have resulted in the gradual replacement of legacy PDH/SDH networks. IP/MPLS-based networks have become dominant not only in the core portion of the network, and a drift toward “all IP” has begun in the metro and access networks as well, resulting in the gradual replacement of legacy PDH/SDH devices. The combination of increased data-centric (rather than voice-oriented) mobile capacity and of network migration from PDH/SDH to IP/MPLS has placed significant new demands on the mobile backhaul architecture.

The challenges of IP/MPLS mobile backhaul The increase in 3G HSPA services (IP-based traffic) and the adoption of IP/Ethernet interfaces in the radio elements (Node B/RNC) are reinforcing trends already followed by most telecom operators who have been deploying residential/business packet-switched networks (PSN). In this new situation, there is great motivation among operators to maximize revenues by converging mobile flows over a unified multi-service PSN infrastructure. Migration of this type can reduce both CAPEX and OPEX, gradually retire expensive legacy equipment, and achieve better utilization of the available bandwidth. To accomplish this migration, however, several technological challenges must be solved first: •

Several types of base stations must be backhauled simultaneously using TDM transport in the BSS and ATM transport in the UTRAN

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Remote OAM, fault isolation, performance monitoring, and measurement capabilities

Migrating to IP-based Mobile Backhaul: Operator’s Perspective

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Carrier grade reliability and guarantee of advanced QoS

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Strict synchronization capabilities

The benefits of IP-based mobile backhaul What is the incentive of mobile operators to migrate to IP-based backhaul? The short answer is: to keep up with the sharp increase in traffic volume. With each successive generation of radio technology (the omnipresent Gs), over-the-air rates have increased dramatically, and with them the range of available end-user applications. HSPA (3G), and soon HSPA+ and LTE (4G) are opening the door to mobile broadband services that generate unprecedented volumes of data, exceeding the capabilities of the traditional networks that were designed to carry voice data. According to Heavy Reading's “3G network evolution to LTE,” by mid-2007, larger operators offering 3G HSDPA service in the five big European markets were transporting 1,000 GB per day as a baseline, with substantially higher peaks. In September 2007, Ericsson released figures on the average throughput of its Radio Network Controllers (RNCs, which typically support 200-300 Node Bs). Ericsson's figures, shown in the chart below, are considered representative of the worldwide average.

Source: Ericsson Strategy & Technology Summit (London, September 2007)

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Stay one step ahead The chart shows that total traffic volume increased by ~50% to 37.5 Gbit/RNC/hour, packet traffic increased by ~100% percent to 20 Gbit/RNC/hour, and HSPA traffic increased by ~240% to 12 Gbit/RNC/hour. To achieve the low-cost, low-latency, flexible network that can carry the mobile broadband services operators are eager to offer, they need to adopt an IP-based mobile backhaul network architecture. The benefits of such a migration are many. •

Simplified networks using a small number of elements to save both OPEX and CAPEX

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The ability to move high volumes of data without a commensurate increase in the cost of service delivery

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A flexible network that can support both mobile and generic IP access networks

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Mobile broadband services that are competitive with wired broadband services in price/performance

Typical migration scenarios Given the broad mix of technologies and architectures used by mobile carriers, there are many possible migration scenarios. We illustrate here three typical cases of migration to IP: •

MPLS over new and legacy transport

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GSM (Abis/Ater) over IP

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Carrier Ethernet

Celtro solutions, which support all technologies and architectures, allow operators to choose the migration path that best suits them, based on their legacy equipment and technologies.

Migration to MPLS over new and legacy transport Migrating Radio Access Network (RAN) to packet-switched technologies is one way in which mobile operators can increase network flexibility and reduce operating costs. Operators generally prefer to do it gradually, to avoid having to “forklift” or “truck roll” their existing backhaul infrastructure. At the same time they must protect the quality of their voice services, which still generate higher revenues than the data services despite the reverse ratio in traffic volumes. MPLS is at present the preferred technology used to meet these goals. MPLS is a packet-switched technology that enables efficient backhaul of data-based services using existing transport resources

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Stay one step ahead and providing secure and protected methods to ensure that voice services are maintained at the same level of user experience. Migrating to an MPLS-based backhaul network requires defining dedicated virtual tunnels to connect the radio network elements (Node B/BTS with RNC/BSC-MSC). The tunnels, called Label Switched Paths (LSP), contain additional virtual tunnels (called Pseudo Wires) which are differentiated by QoS and used to backhaul the mobile traffic (voice, data, and signaling) generated at the NobeB/BTS and RNC/BSC. Pseudo wires (PWE) are used to carry mobile services through the designated LSPs. The function of the PWE is to emulate legacy traffic such as ATM (RFC 4717) and/or TDM (RFC 4553/5086), which the radio network elements produce in a packet-based format. After the various types of traffic are emulated, they are tunneled into the appropriate paths. An additional benefit of the tunneling method is the creation of a protection scheme to assure service continuity. The typical protection scheme is based on the RSVP-TE protocol (RFC 3209), which creates a standby LSP in parallel to the working LSP to ensure service continuity (within a 200ms switching time) in case of failure of one or more network elements along the path between the NodeB/BTS and the RNC/BSC. A significant advantage of MPLS in mobile backhaul networks is their ability to reuse existing network elements that are not packet-switch oriented. Typical mobile backhaul networks contain a significant number of legacy PDH/SDH network elements installed in the past for ATM/TDM transport. MPLS can be used over native Ethernet elements alongside PDH/SDH legacy elements. MPLS requires an intermediate protocol such as PPP or Multi-Link PPP (RFC 1990) when using PDH links and Packet over SDH (RFC 1619) when using SDH links. At the cost of a relatively small overhead, these protocols allow to transport MPLS-based traffic over legacy network elements, which in return creates significant saving through the reutilization of the transport infrastructure. The figure below illustrates a solution based on Celtro technology for migrating 2G and 3G traffic to an MPLS-based network. TDM/ATM traffic is emulated at the cell-site by Celtro’s DMT 1000, which carries it over Ethernet or PDH/SDH links using various MPLS PW/LSPs with differentiated QoS, according to the Differentiated Services mapping model. At the edge of the backhaul network, Celtro’s DMT 4000 receives the MPLS stream and terminates the MPLS session, de-encapsulating the various traffic types into their original format (2G-TDM, 3G-ATM) and forwarding them to their destination (2G-MSC, 3G-RNC):

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MPLS migration in a mixed backhaul network using Celtro technology This scenario has been successfully implemented with Celtro equipment by large operators in Europe and Africa.

GSM (Abis/Ater) over IP Some operators have been migrating their backhaul to IP not only to accommodate high 3G data rates but also to lower backhaul costs. This is the motivation of mobile operators who migrate their GSM backhaul to packet-switched solutions. With the rapid introduction of Ethernet lease lines and packet-switched infrastructure, the transport cost of packet-switched traffic have become more attractive, especially when the transport provider is a sister or parent company of the mobile operator. Several technical issues must be addressed to use packet-switched backhaul for GSM services. GSM traffic is based on TDM technology, with a deterministic bandwidth consumption that is not proportional to the actual bandwidth being used. Adapting the bandwidth consumption to the actual required bandwidth reduces dramatically the required capacity, and with it the backhaul costs. To be transported as packet-switched traffic, GSM traffic requires pseudo wire (PWE) emulation. IETF defines two different PWE options suitable for GSM services: SAToP (RFC 4553) and CESoPSN (RFC 5086). These options emulate TDM traffic (without the recommended adaptation of the actual bandwidth to the consumed bandwidth) as packet-based traffic in various formats. SAToP is

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Stay one step ahead designed for packet-based conversion of full E1s, CESoPSN performs packet-based conversion of Nx64kbps (specific time-slots). In addition to traffic emulation as packet-based traffic (with or without bandwidth adaptation), it is necessary to define an MPLS-based LSP as a point-to-point logical connection between each BTS and the aggregating BSC, and between each BSC and the responsible MSC. These LSPs ensure that all packets are transported along a direct path, without the disorder, packet loss, or differentiated delay that can occur in packet-based networks. As explained above, the logical path architecture allows adding the protection mechanism that ensures the service availability and network reliability necessary for voice services. Another significant challenge in migrating to a packet-based architecture is meeting the mobile requirement for accurate network synchronization of the radio elements (BTS/BSC). Typically, the radio elements obtain the required frequency synchronization from the E1/STM-1 (PDH/SDH) interfaces (G.823). When migrating to packet-switched technologies, which are inherently asynchronic, there is a need to provide adequate synchronization for all elements in the network. Several methods have been suggested for expanding packet-based technologies (Ethernet, IP/MPLS) to distribute frequency and clock information. IEEE developed a protocol for precision clock synchronization. The IEEE-1588v2 enables sub-microsecond synchronization of clocks by using a master clock that sends multicast synchronization message frames containing timestamps. All IEEE-1588v2 receivers correct their local time based on the received timestamp and an estimation of the one-way delay from transmitter to receiver. Another timing method, called Network Time Protocol (NTP), was defined by IETF to be used to distribute clock time over IP networks. Time servers send timestamps in packets, usually in response to requests from a client that may be in contact with several different time servers. New ITU-T recommendations attempt to define additional methods to transport timing reference over Ethernet networks. The ITU-T G.8261 standard defines a new timing method called Synchronous Ethernet, designed for commonly used dedicated-media full-duplex Ethernet interfaces, in both copper and optical physical layers that transmit continuously. At the physical layer, Ethernet (similarly to SDH) adopts the serial code stream in transmission. Data flows are transmitted from the Ethernet interface with a high-accuracy clock, and the receiver can recover and extract the clock and maintain the high-accuracy performance at the same time. All the above mechanisms, including traffic emulation, bandwidth adaptation, MPLS LSP settings, and network synchronization are essential for enabling GSM (Abis/Ater) backhaul over packet-

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Stay one step ahead switched technologies. Celtro has developed innovative solutions for coping with all the challenges listed above. The figure below illustrates a GSM over IP solution using Celtro's all-in-one-box switching solution, with the new DMT 1000 (DPS family) at the base stations and DMT 4000 at BSCs.

GSM (Abis/Ater) backhaul over IP/MPLS transport using Celtro technology This solution has been successfully implemented with Celtro equipment at a large African and FSU Operators.

Carrier Ethernet in mobile backhaul networks Mobile operators often want to share the resources of a parent/sister company that has wire-line network assets. This can produce significant savings because it eliminates the need to build and maintain two separate networks in parallel. Moreover, recently installed radio elements (NodeB/RNC) have native Ethernet switching capabilities that allow them to merge with the wire-line networks that have already migrated to Ethernet-based networks long ago. In the converged network, each service (wire-line and wireless) uses a dedicated Virtual Private Network (VPN), a method that allows traffic separation to avoid congestion and breaches in network security that can occur between different networks/services. There is ongoing debate about the

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Stay one step ahead preferred method of network virtualization, and whether it should be based on layer 3 (L3VPN) or layer 2 (L2VPN) mechanisms. When merging mobile and wire-line networks, Layer 2 VPN appears to be more suitable for the mobile backhaul network architecture, although there are exceptional cases in which L3VPN is preferable. . Among the different L2VPN options, Virtual Private LAN Services (VPLS), or more precisely the Hierarchical-VPLS (H-VPLS) architecture seems to meet the needs of the current mobile backhaul network. VPLS allows mobile operators to create a virtual LAN environment between their radio elements (Node B/BTS and RNC/BSC). H-VPLS partitions the network into several domains that are interconnected by means of an MPLS core. Considerations for edge devices at the cell-site are thus simplified, requiring either simplified MPLS/IP capabilities (route to the local n-PE) or Ethernet switching mechanisms such as VLAN Tag Stacking (Q-in-Q, IEEE 802.1ad provider bridges). Using Ethernet as the edge technology simplifies the operation of the edge domain and reduces the cost of the edge devices dramatically. When using Ethernet mechanism such as VLAN Stacking, each Node B receives a dedicated VLAN tag that specifies traffic origination or destination. The various services/traffic types generated at the cell-site receive their respective VLAN tags, which are shared between similar traffic types/services throughout the backhaul network. Having a dedicated VLAN for every service throughout the network makes it possible to create service differentiation within the network, while using the 2nd VLAN tag to identify the source/destination of each service for more efficient transport and reduced delay. MPLS is still required for H-VPLS because it creates the virtual tunnels (LSPs) throughout the network and enables service differentiation. Advancements in Ethernet technology (e.g., PBT, PBBT-TE) further improve this model (for example, eliminating the need for MAC learning in the network, expanding VLAN tag options, etc.). The figure below illustrates how using the Celtro DMT family of products allows a VPLS network to support mobile backhaul services, sharing the Metro Ethernet infrastructure with wireless networks. In the illustration, the different VLAN networks represent different services or traffic types that are carried throughout the Metro Ethernet network between the Node B/BTS and RNC/BSC with underline MPLS tunnels. This solution has been successfully implemented by a large operator in Russia using Celtro equipment.

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Carrier Ethernet backhaul in converged networks using Celtro technology

Summary The higher demand for service delivery and bandwidth produced by 3G applications places new challenges before mobile operators To be able to offer the profitable broadband data services that customers demand and that the RAN is already making possible, mobile operators must address several issues on the backhaul side. For adequate service delivery they must be able to ensure broadband with sufficient capacity to handle peak traffic, including future growth. They must also acquire the flexibility to handle both legacy and current technologies, ATM alongside IP/MPLS. And they must be able to add new services without the added cost of new infrastructure. Migration to a packet-switched network (PSN) is the key to meeting these challenges. But given the many infrastructures and technologies that operators have adopted over the years, migrating to PSN while protecting their investment in the legacy equipment is a challenging task.

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