Chalk Talk

Mobile Backhaul Evolution

Over the last two months, we covered the following topics: the evolution of mobile wireless networks from circuit-switched architectures to packet-switched architectures and the definitions, standards, and technologies defining mobility today and in the future.

Continuing in the series of articles related to mobility (this one being number 3 of 4), this month’s article will focus on mobile backhaul evolution from circuit-switched networks to pseudowire networks to Ethernet/IP-based packet networks.

Mobile Backhaul Trends

As consumers’ insatiable demand for Internet access continues to grow, mobile traffic growth is increasing exponentially in two dimensions – the number of mobile devices is growing, and the amount of data generated by mobile devices is growing. A single high-end phone (such as an iPhone or Blackberry) generates more data traffic than 30 basic-feature cell phones. A laptop aircard generates more data traffic than 450 basic-feature cell phones.

According to Cisco’s Visual Networking Index, mobile broadband traffic will roughly double each year between 2008 and 2013, increasing 66 times during that span. With the advent of higher speed data networks – beginning with Third Generation EVDO and UMTS, and continuing with Fourth Generation WiMAX and LTE networks, mobile wireless backhaul networks also need to evolve to accommodate the data growth.

However, it is important to remember that today’s cellular technologies are not going away, so a backhaul network must evolve to support today’s current requirements – namely TDM transport and timing – as well as next generation mobile requirements – namely, high bandwidth and QoS-capable.

Mobile Backhaul Today

Today’s mobile backhaul networks are mainly ATM-based or TDM-based networks for the transport of voice and data services from the cell site to the mobile core infrastructure. Figure 1 illustrates the backhaul technologies of today’s common mobile standards.

Figure 1

These networks rely on the underlying backhaul technology to provide a number of services, including:

  • Transport – First and foremost, the backhaul network provides a mechanism to transport traffic from the air interface to the core network infrastructure.
  • Multiplexing – Backhaul networks multiplex multiple streams of information (that is, voice calls, data sessions) into TDM frames for transport.
  • Timing – Whether for frequency or time synchronization, TDM backhaul networks provide a clock signal that may be used by the radio for synchronization. Synchronization is important in mobile networks to maintain voice calls, support handoffs, and avoid inter-call interference.
  • OAM – Bit Error Rate Testing (BERT) and other TDM OAM mechanisms for link status/health detection are inherent to TDM.

Ethernet Backhaul and Pseudowires

From a sheer cost perspective, continuing to deploy TDM circuits and carry the increasing load of data traffic over these circuits is prohibitive. In addition, managing and maintaining a large number of TDM circuits per cell site is operationally complex. For this reason, many operators have begun migrating their backhaul networks from TDM to Ethernet/IP. These Ethernet/IP transport networks leverage a large number of transport mechanisms, including DSL and cable networks, optical transport networks (SONET or dark fiber), and microwave.

While the inherent values – 30% savings in CAPEX and OPEX – for Ethernet transport is well-known, it is also incumbent upon these technologies to provide the same set of capabilities that TDM provides. This is important in order to ensure that end devices, namely base stations at the cell site and the mobile core infrastructure, are unaware that the transport network in between has been altered.

Pseudowires provide an emulation service for the circuit-switched “native” services over a packet infrastructure. In order to provide this function, many IETF and ITU standards have emerged. These include:

  • Circuit Emulation Transport Services – Defined in many IETF standards, circuit emulation services provide for transport of TDM and ATM over Ethernet, IP, and MPLS networks. From an end-device perspective, the native TDM frames and ATM cells are received intact, with no modifications. The transport network is invisible to these nodes. Figure 2 illustrates a TDM pseudowire.

Figure 2

  • Timing – In order to ensure timing across the packet switched network, a number of mechanisms have been defined. The most prevalent of these technologies to date for mobile backhaul has been Synchronous Ethernet, or Sync-E, and IEEE 1588, or Precision Time Protocol (PTP).
    • Sync-E is a line-timing method for transporting timing information over the Ethernet physical layer. This model, similar to SONET/SDH, provides accurate frequency synchronization, but does not provide time/phase synchronization.
    • PTP protocol provides precise, real-time, network-wide synchronization accuracy in the sub-millisecond range.
  • OAM – Transport services over packet-switched networks require the same level of operational and measurement tools as their predecessors. IEEE and ITU have both standardized protocols for ensuring that Ethernet OAM can be provided. In addition, the Metro Ethernet Forum (MEF) has provided specifications for Wide Area Network (WAN) Ethernet applicability. These standards, IEEE 802.1ag/802.3ah and Y.1731, provide the following capabilities:
    • Service Layer OAM (IEEE 802.1ag Connectivity Fault Management and Y.1731): Service-Layer OAM operates on a per-VLAN or virtual connection, end-to-end basis. The mechanisms within these protocols allow for rapid fault detection and performance gathering.
    • Link Layer OAM (IEEE 802.3ah OAM) – Link-layer OAM operates on a per-segment basis, allowing for monitoring and troubleshooting of a single Ethernet link within the backhaul network. The main benefits of 802.3ah are “loopback” testing and unidirectional link detection/discovery.
    • Ethernet Local Management Interface (MEF-16 E-LMI) – E-LMI provides end-to-end Ethernet manageability. E-LMI allows for automatic provisioning and configuration of the cell site device in a mobile backhaul network, including VLAN to Ethernet Virtual Circuit (EVC) mapping, bandwidth profile, and QoS settings. In addition, E-LMI provides additional and complementary status information to 802.1ag/802.3ah and Y.1731.

With the standardization of the above capabilities over packet-switched networks, operators have begun migrating their infrastructure away from TDM transport networks. This migration may begin simply as an offload of data traffic, or it may take all voice and data traffic off TDM links altogether. In either scenario, operators are recognizing the need to migrate their networks to packet infrastructures today to achieve the OPEX advantages that packet-switched networks provide.

Ethernet Backhaul for Next Generation Mobile

As mobile networks evolve, the advantages of having a packet-switched network are further recognized. These next-generation mobile networks natively support Ethernet/IP in the base station and packet core network. End-to-end mobile networks will leverage Ethernet and IP as foundation building blocks. Pseudowires will continue to provide value for segmenting and isolating traffic across the network, including supporting both legacy voice/data services and next generation services over the same infrastructure and providing differentiated QoS on backhaul networks. Supporting services and end-to-end management over the packet-switched domain requires capabilities above and beyond those provided in the TDM domain. Figure 3 illustrates this concept.

Figure 3

These capabilities, however, align with the services that are deployed in today’s metro Ethernet networks. This provides just one example of convergence and how mobile networks and services are converging with today’s fixed broadband networks and services. As it happens, convergence will be the topic of our fourth, and final article in the mobile networking series.

Summary

Packet-switched backhaul networks are capable of more than just supporting packet-based services. Many standards organizations have developed protocols and specifications for transporting circuit-switched services, including today’s 2G/3G mobile traffic, over these packet-switched networks, while still meeting the requirements of the circuit-switched domain. As such, mobile operators have begun deploying packet-switched backhaul networks both as a cost-savings in today’s networks, but also as preparation for next-generation 4G mobile networks, where packet-switched infrastructure stretches end-to-end.

If you have any questions regarding the topics discussed in this article, you can email questions to Kevin from now through September 3, 2009.

About the Author:

Kevin Shatzkamer is a Customer Solutions Architect at Cisco Systems with responsibility for long-term strategy and architectural evolution of mobile wireless networks. He has worked at Cisco and the mobile wireless industry for nine years, focusing on various technologies ranging from GSM/UMTS to CDMA networks, packet gateway, network-based services and security, video distribution, Quality of Service, and end-to-end design theory. Kevin has 16 pending patents related to all areas of work. Kevin holds a Bachelors of Engineering from University of Florida and a Masters of Business Administration from Indiana University. He is also an author of IP Design for Mobile Networks, a Cisco Press book that detail’s IP’s role in current and future mobile networks.

Kevin Shatzkamer

IP Design for Mobile Networks

IP Design for Mobile Networks
Mark Grayson, Kevin Shatzkamer, Scott Wainner
ISBN: 158705826X
Pub Date: 6/15/2009
US SRP $60.00
Publisher: Cisco Press