Mobile Backhaul Networks - The Next Generation

This article originally appeared in the Sept. 2012 issue of INTERNET TELEPHONY

Mobile network operators across the globe are experiencing enormous growth – although voice communications are growing in a linear fashion, the demand for data services is increasing exponentially, with consumers increasingly opting for bandwidth-hungry mobile services such as Internet access, photo sharing and music downloads. 

At the same time, mobile operators are under economic pressure, and their backhaul networks are experiencing bottlenecks due to scalability, flexibility and cost concerns. Operators are therefore looking for ways to future-proof their backhaul architecture to support 3G traffic, with the global wave of 4G (LTE/WiMAX (News - Alert)) adoption. They also want to be able to provide a clear migration path toward all-IP 4G networks. 

More and more consumers are pushing an increasing number of applications via the mobile network rather than over wires, creating greater pressure on the backhaul network to meet the growing demand and maintain end-to-end user experience. While many consumers are still migrating from 2G to 3G, operators are already exploring and adopting 4G (LTE (News - Alert)).

Long-term evolution offers high data rates at a reduced price per bit, better spectrum efficiency and latency. LTE also offers expected throughputs in the range of 100mbps and latency should be in the range of 20ms. This can offer a rich user experience, comparable to that of fixed connections. LTE will therefore enable new business models around emerging services such as real-time online gaming, HD video streaming, video blogging, and Peer2Peer file exchange.

Mobile backhaul is a crucial part of the mobile network, linking the radio access network and the mobile core network. In designing the end-to-end mobile infrastructure, no area of the mobile network feels the strain more than backhaul networks (in scalability, performance, cost and ease of migration from one generation to the next).

A new approach to mobile backhaul infrastructure

Cell sites are becoming increasingly complex as operators adopt 3G technologies such as HSPA and EVDO and already look toward 4G technologies.  Even as they migrate to these next-generation services and architectures, they realize that 4G technologies (IP/Ethernet) and emerging 3G (ATM) services will need to coexist with legacy 2G (TDM) for some time. However, as traffic for high-bandwidth data services continues to grow, operators must find ways to reduce the mobile backhaul costs. U.S. mobile operators have typically used leased T1/E1 lines in their mobile backhaul networks. In Western Europe and the greater part of Asia-Pacific, microwave-based (TDM encapsulations) backhaul is widely deployed. TDM is known for its reliability, but it is expensive and does not scale easily.

Clearly mobile operators need a new approach to scale their mobile backhaul networks cost-effectively, and many are shifting to Ethernet-based connectivity at cell sites. This approach must, in addition to being highly scalable and reliable, bridge the gap between legacy and next-generation networks and services – and provide the flexibility to support both.

Defining next-generation mobile backhaul networks

The Broadband Forum (News - Alert), a global standards organization focused on end-to-end IP network optimization, is tackling these backhaul challenges via its MPLS in Mobile Backhaul Initiative (MMBI). This proposes a framework for the use of IP/MPLS technology to transport radio access network backhaul traffic over access, aggregation and core networks. It describes possible deployment scenarios and provides recommendations on how to deploy MPLS in these scenarios to design flexible, scalable and economical backhaul network.

The forum has an MMBI Framework and Requirements technical specification, and members are working together to define standards-based, interoperable architecture frameworks for 2G and 3G networks and for LTE networks. 

Scope of MPLS in Mobile Backhaul Initiative

MMBI architecture allows operators to leverage their existing last mile access technology, such as TDM, point-to-point microwave links, DSL, and satellite. It is flexible and offers various options that enable the deployment of MPLS as close to cell sites as will fit within their architecture. Satellite is expensive, but essential in regions such as Africa and remote parts of India, where fiber or copper is unavailable or uneconomical.

IP/MPLS is increasingly seen as the best strategic solution for backhaul. It offers the combination of cost, scalability and flexibility that mobile operators need to leverage existing investments while building out capacity for growing data traffic, and it supports features such as:

  • Coexistence of TDM (2G), ATM (3G) and IP/Ethernet (4G) transport;
  • ATM-like quality of service and traffic engineering techniques;
  • Rapid service restoration after failure detection
  • Future-proof investment for migrating from 3G to all-IP based 4G/LTE.

Many equipment vendors offer MPLS features within their products, but the lack of a commonly agreed framework, architectures and deployment scenarios often results in additional avoidable costs. To address this, the Broadband Forum now offers a certification program for vendors, enabling service providers to choose standards-based, deployment-ready products and expediting the deployment of backhaul solutions.

Evolving from 2G/3G to 4G in the backhaul network

The Broadband Forum defines two architecture frameworks, one for 2G/3G and the other for 4G/LTE, corresponding to the 3rd Generation Partnership Project work. In 2G/3G RAN, base transceiver stations or simply base stations handle the radio interface with the mobile station and the base station controller manages one or more base stations to provide control functions such as radio-channel setup, handovers etc. A hub-and-spoke topology enables communication from base station to controller and controller to base station. This topology is also known as centralized topology. In this architecture, T1/E1 connections (TDM for 2G and ATM for 3G) between BTS and the BSC are carried over IP/MPLS based packet backhaul using pseudo-wire technologies.                                                                                                                                                                  


In LTE RANs, the base station itself consists of controller functionality and communicates with another base station directly via any-to-any topology. LTE base stations communicate with access gateways via a star topology as shown in figure 3.

Coexistence, interoperability, roaming, and handover between LTE and existing 2G/3G networks and services are inherent design goals, so that full mobility support can be given from day one. In LTE networks, IP is the only protocol used to support connectivity between the different mobile nodes as defined by 3GPP. To achieve, any-to-any topology for LTE backhaul, the Broadband Forum has recommended leveraging Layer 2 virtual private networks, and Layer 3 VPN such as virtual private LAN service and border gateway protocol/MPLS-based VPNs.

Hybrid IP/MPLS and TDM backhaul architecture for 2G/3G

In a hybrid model, carriers can build out capacity to accommodate the data traffic growth without having to re-engineer the voice network. Operators can leverage cost-effective alternatives such as metro Ethernet networks or existing assets to support data traffic (e.g. using the DSL infrastructure to offload data traffic from the cell site). They are thus able to develop greater familiarity with IP/MPLS technology and integrate voice traffic into the packet-based infrastructure at a later date.

Timing synchronization

Synchronization is critical to maintain good voice quality, reduce interference and manage call handovers between base stations. There are several approaches to achieve this timing synchronization, including synchronous Ethernet, adaptive clock recovery and IEEE (News - Alert) 1588 v2.  The Broadband Forum is currently assessing various requirements to support clock distribution to base stations, including frequency, phase, and time synchronization and provides recommendations in terms of QoS, resiliency, and efficient distribution based on topology (point-to-point or point-to-multipoint).

Matching SONET/SDH-type reliability with IP/MPLS

Synchronous optical networking and synchronous digital hierarchy are known for  high reliability and fast recovery from failures, and carriers expect new packet-based backhaul to match this level of resiliency. IP/MPLS has already proved itself in core networks to be as reliable as SONET/SDH. The combination of fast re-route and carefully engineered primary and secondary label-switched paths allows IP/MPLS-based network to recover in tens of milliseconds – on a par with SONET-based networks.

Managing and troubleshooting IP/MPLS-based networks

In moving to a new transport technology, carriers need to feel confident in its operation, administration and management tools. With years of successful service provider deployments around the world, IP/MPLS includes a robust set of standards-based OAM tools as well.

Robin Mersh is CEO, and Nikhil Shah is chairman of international development, at the Broadband Forum (www.broadband-forum.org).

Edited by Braden Becker