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Technology Shift: Migration from TDM to packet-based solutions

April 30, 2012

The telecom space has witnessed a surge in data traffic during the past few years. With the introduction of smart devices and next-generation network (NGN) technologies, this trend is expected to continue in the future. However, the existing network architectures are not optimised to handle high volumes of packet-based voice and data services. Increasing bandwidth requirements and the changing nature of traffic have made it necessary for quality of service (QoS)-based prioritisation and new solutions to overcome the high cost of legacy backhaul. As a result, operators are migrating to a more cost-effective, converged and packet-based infrastructure. They are adopting backhaul technologies and solutions that can bring about significant cost savings without compromising on parameters such as service reliability and performance.



To meet backhaul requirements, telecom operators have been using solutions based on fibre, copper, satellite systems or microwave radio links. NGN operators prefer optic fibre-based technology as it meets the requirements of high bandwidth. Copper-based solutions are expensive and suffer from bandwidth limitations. Satellite systems also entail considerable costs and are usually deployed in areas where the laying of fibre poses a challenge. Recently, Wi-Fi technologies have also emerged as a promising backhaul solution. For packet switched backhaul, pseudowire solutions are used to transport time division multiplexing (TDM) services.

Traditional and emerging systems for backhaul are:

•    Copper cables: These are the traditional backhaul medium. TDM technology uses plesiochronous digital hierarchy for multiplexing several voice channels from base stations and transports them to the base station controller in various time slots. However, with data traffic on a continuous rise, Ethernet or xDSL technologies, in addition to TDM, are used on copper to deliver the required bandwidth.

•  Fibre-based solutions: These include point-to-point optic fibre links, synchronous digital hierarchy rings and variants of passive optical network (PON) technology such as gigabit PON and Ethernet PON. Optic fibre-based systems provide advantages like high capacity packet convergence and address the service level agreement needs of advanced wireless networks.

•     Microwave and satellite wireless: These are generally used in locations where the deployment of a wired backhaul system is difficult. Microwave transmission can be carried out in various frequency bands including the licensed (6 GHz to 38 GHz) and the unlicensed (2.4 GHz and 5.8 GHz) bands. For improved coverage, microwave-based backhaul systems can be deployed in point-to-point, point-to-multipoint (PMP) or multi-hop configurations. However, deploying the PMP topology in a microwave backhaul network will result in cost efficiency only if a minimum of five cells are served by each PMP system. While the deployment of microwave links results in higher capex as compared to E1 copper links, it is likely to reduce opex over time.

On the other hand, satellite systems offer solutions for locations where no other backhaul technology is feasible. These links are very expensive and the transmission is based on E1 techniques.

•     Pseudowire framework: The mechanism of transporting TDM traffic over a packet switched network is referred to as circuit emulation or TDM pseudowire. With next-generation systems such as long term evolution (LTE) expected to use packet-based solutions, the pseudowire framework will be used as a backhaul technology to transport services supported by TDM over packet switched networks – Ethernet, IP or multiprotocol label switching (MPLS). There is a consensus that pseudowire techniques reduce costs per megabit significantly.

•           Wi-Fi technology: This is a low-cost backhaul solution that can be used as a substitute for microwave links. It can be used in combination with pseudowire for traffic backhaul from nearby cell sites to the radio network controller present in the core network. Although Wi-Fi technology offers attractive cost benefits and deployment flexibility for backhaul networks, it involves design issues with regard to throughput, distance coverage, packet overhead, timing and synchronisation.

From TDM to IP

Cellular systems were primarily designed to carry only voice traffic and TDM technologies dominated the backhaul segment for many years. Later, with the emergence of data services and the need to offer these services on existing cellular systems, the focus shifted to providing data transmission through the prevailing voice infrastructure. TDM became the fundamental technology for data communication during the early 3G network deployments. However, as data traffic usage on 3G networks grew, TDM backhaul posed a serious challenge on two fronts – bandwidth capacity and cost. Smartphone users consumed multi-megabit data volumes, resulting in bandwidth scarcity. Increasing the number of TDM lines or their capacity was not a viable option given that the opex was too high as compared to the derived benefits.

With the rapid growth in data demand in the past few years, Ethernet has emerged as a popular backhaul technology. It offers significant cost and scale improvements over TDM backhaul systems. Upgrading to Ethernet enables operators to leverage statistical multiplexing, which is essential for backhaul aggregation, optimisation of traffic management in the network and reducing congestion.

However, there are challenges associated with the adoption of carrier Ethernet services. Operators are required to upgrade their networks, service and performance monitoring systems, and service assurance processes to accommodate Ethernet backhaul. The learning and flooding aspects of Ethernet networks also pose inherent scaling challenges. The spanning tree protocol, a network protocol that ensures a loop-free topology for any bridged Ethernet local area network (LAN), and its derivatives are used to address these issues on a low and medium scale. For large networks, several protocols like virtual private LAN service (VPLS), MPLS-transport profile and provider backbone bridging-traffic engineering have been developed to solve scaling issues. Further, the savings achieved are not substantial as carriers continue to operate separate TDM backhaul networks alongside Ethernet networks to carry voice and video traffic.

Another issue is that of timing network synchronisation, which is critical in cellular networks as voice and video traffic must be delivered in real time. Unlike TDM, Ethernet does not inherently support it. However, the industry has developed technology standards such as IEEE 1588 v2 and ITU Synchronous Ethernet (Sync-E), which enable timing synchronisation over Ethernet backhaul networks. Sync-E operates at Layer 1 of the communication stack and distributes timing signal in a manner similar to that of TDM networks. On the other hand, IEEE 1588 v2 is a protocol that runs in parallel  with data traffic. Its implementation provides synchronisation signals to frequency division duplex and time division duplex radio systems, and circuit emulation services-based transport systems.

Going forward, given the advancements in cellular networking systems, the proliferation of new devices, and the introduction of high speed services, an IP-based network will be best suited to serve the backhaul segment. The IP-centric backhaul network has many advantages over carrier Ethernet networks such as backhaul offload, which becomes simpler and scalable with an IP network, resulting in significant cost savings. Distributed mobility management architecture along with the IP backhaul network creates an optimised path for machine-to-machine and peer-to-peer communication. The mobility anchor point could be placed at the cell tower or at a local aggregation point, which will provide a much improved communication path for subscribers and machines connected to the wireless network.

Also, the transition from carrier Ethernet to IP backhaul is not difficult. The deployment of packet switched infrastructure has been largely accomplished. In addition, operators implementing carrier Ethernet with protocols like VPLS are already equipped with IP-compatible infrastructure. The most challenging aspect of this transition, however, is the development of operation, administration, maintenance and provisioning systems for an IP network, which will vary based on the operator’s existing Ethernet systems.

India scenario

Around 70-80 per cent of the cellular sites in India use TDM-based microwave backhaul systems. However, the increasing use of mobile smartphones and introduction of next-generation technologies such as 3G and LTE have generated a need for mobile infrastructure that can support high speed, high volume data services.

Industry analysts believe that the operators’ existing backhaul equipment will not be able to scale up once the uptake of 3G and 4G technologies starts rising. Existing fibre-based technologies can be scaled up to absorb the increase in data traffic but their deployment poses a challenge especially in dense urban or remote rural areas.

Over the past few years, operators have started upgrading their infrastructure with new Ethernet-ready microwave technologies. This is done by integrating the microwave radio technology (TDM and Ethernet), which results in optimised bandwidth, increased throughput and reduced capex.

Femtocell and E-band technologies are also likely to play a major role in microwave backhaul deployments in India. Further, analysts believe that the packet optical-based systems will dominate the Indian backhaul segment by 2015.

The road ahead

Given the surge in data traffic, developing a single technology that can meet the entire backhaul requirement is very difficult. Operators need to strike a balance between providing high-end data services like high speed internet access, mobile TV and video calling, and basic voice and data services such as SMS and GPRS, on the same backhaul network.

The ultimate approach for migrating to new-age backhaul systems is to start this transition by offloading best-effort internet traffic to a more cost-effective packet layer while retaining high-value QoS-sensitive services on the existing network. Later, as the business models for new data services mature, the legacy infrastructure can be withdrawn and replaced by an end-to-end pure packet system.


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