The flood of mobility devices and plethora of applications supported is placing significant bandwidth demands on the cellular network. As a result, bandwidth requirements to the cell tower are increasing exponentially. To survive, mobile operators must strive to meet this demand and do so in the most cost-effective fashion. (See Figure 1. Backhaul Demands and Carrier Challenges.)

Today, the cellular backhaul network uses leased lines or PDH/SDH microwave radio to fulfill those bandwidth requirements. Unfortunately, legacy transport services do not scale cost-effectively. As such, smart operators must consider and evaluate a number of technology options as they look to roll out value-added mobile video and data services.

Interestingly, mobile operators, incumbent and alternative operators including CLECs and MSOs, as well as the tower providers, and utilities that serve their transport requirements, have turned to Ethernet as the solution to building more cost-effective backhaul networks. Mobile backhaul networks optimized for the requirements of profitable new mobile data and video services require far more capacity and scalability, granular QoS support, better transport utilization, and lower cost per bit than the backhaul networks of the past.

They must also continue to support the same resiliency and synchronization requirements that mobile telephony has always required, and support a variety of distance requirements to get the bandwidth where it is needed. Operators must consider not only the CapEx required to invest in such a solution, but also must take a close look at the total cost of ownership, including the cost of installing, provisioning, monitoring, managing, and maintaining their backhaul network.

Backing Into the Issues
To begin, it’s important to review the various access technologies that are used or can be used for mobile backhaul. These include leased lines (T1/E1 – 1.544 Mbps/2.048 Mbps), microwave radio, DSL, fiber, and Ethernet over bonded copper joined by an even newer technology that carries native Time-Division Multiplexing (TDM) simultaneously with native Ethernet.

The leased line T1/E1 (1.544 Mbps/2.048 Mbps) services that mobile operators utilize to a varying degree on a region by region basis for mobile backhaul do not scale cost-effectively. This is because they have to be provisioned separately and their cost increases linearly as mobile operators need more capacity. The crossover point at which these services become more expensive than Ethernet is reached with only a handful of T1s/E1s. In fact, leased lines become decreasingly cost-effective as backhaul capacity requirements continue to grow significantly – which they are today. Ethernet services can be transported over T1/E1 and SONET/SDH networks, however, the methodologies required can result in a solution that is not cost optimized. Below, is an examination of these technologies and their appropriateness for the backhaul applications needed today and well into the future:

Option A: Microwave
Microwave radio typically requires licensing of additional costly spectrum to ensure that the frequency used is not also being used for other applications in the surrounding area, which could potentially result in interference. These costs scale as backhaul bandwidth requirements scale and, in some markets, available spectrum in the bands most useful for mobile backhaul has become a real constraint.

Microwave requires specialized expertise and can have city-specific operational constraints as well. Line-of-site can be an issue and network availability can be affected by weather conditions. The point-to-point nature of the technology significantly increases its costs as bandwidth requirements increase. And though microwave radio vendors are introducing new “Ethernet” or “packet” microwave systems that support higher bandwidth requirements, this equipment is
costly to deploy at this time.

Indeed, microwave technology enables mobile providers the potential to bypass terrestrial or incumbent carriers, there are still spectral, line of site and weather issues, along with access to micro cell antennas and the high capital and installation costs of new infrastructure to support emerging mobility applications and their insatiable bandwidth requirements.

Option B: DSL Technologies
DSL technologies can provide enough bandwidth to satisfy many mobile backhaul requirements. However, there are very rigid distance limitations utilizing DSL. In addition, much of the installed base of ADSL technologies do not effectively support synchronous mobile applications (i.e., uploading pictures, gaming applications) that many operators expect to drive future growth and incremental revenue streams.

Some operators have considered VDSL2 for mobile backhaul, but its distance limitations constrains its applicability to only those places where the base station is located within 100 meters from the nearest fiber access point. Due to these limitations, traditional DSL is a tough option to consider.

Option C: Fiber
Fiber optics provides the panacea or optimal solution for mobile operators to support the mobile network’s bandwidth requirements in terms of scalability and virtually unlimited bandwidth. The reality, however, is that fiber is not available for the great majority of cell towers throughout the network today. In fact, less than 20% of cell towers today have access to fiber (Source: Heavy Reading). In today’s economy, it may be even more cost prohibitive to extend fiber to the cell site. The CapEx and OpEx costs as well as the time to market issues with deploying fiber to the cell tower make ubiquitous fiber access and delivery a difficult and impractical option for the near future.

Indeed, these three legacy access technologies are viable. That said, they also have the potential to significantly erode the business case to maximize profits in supporting new mobile applications. Therefore, alternatives must be explored.

Option D: Ethernet Over Bonded Copper
Ethernet over bonded copper (based on IEEE 802.3ah or 2BaseTL) backhaul solutions leverage copper plant and utilize bonding of copper pairs and modulation techniques to offer bandwidth and distance. They can be deployed wherever unbundled copper loops are available and provide far greater coverage than optical fiber. They also do not require the kind of line-of-site engineering required by microwave, the need to consider local weather patterns, or the expensive and potentially time consuming process of licensing spectrum. Ethernet over copper can support synchronous bandwidth requirements unlike much of the installed base of DSL, and can provide significantly more capacity over longer distances than do competing DSL technologies.

In addition, and most importantly, over 80% of cell tower locations only have access to copper facilities (Source: Heavy Reading). Ethernet over bonded copper can offer bandwidth capacity from 2 Mbps to 100s of Mbps in a variety of bonded copper configurations. (See Figure 2. Bonded Copper Pairs.)

For mobile operators and incumbent carriers who own the facilities and access to the cell towers, support for both TDM and Ethernet interfaces and the ability to converge backhaul for multiple generations of radio technologies is an anchor requirement.

Not all Ethernet over bonded copper solutions are created equal. The insatiable mobile user's bandwidth requirements for emerging mobility applications must be met, but not by sacrificing the sacred cow that is the intrinsic quality of the mobile voice call. The critical QoS and timing synchronization requirements of mobile voice and TDM has traditionally caused mobile operators hesitancy or pause on migration to Ethernet technologies.

Mobile backhaul applications and TDM have more stringent timing and quality requirements than traditional enterprise voice applications because they carry critical information for mobile hand-offs between towers as well as other control protocols that are not required with the average phone call. Additionally, the access network is often the most bandwidth-constrained part of the network, and typically suffers from significant congestion during peak usage times.

New technologies are being introduced to the market that will deliver native TDMtraffic simultaneously with native Ethernet traffic over the same bonded copper connection. Carrying TDM natively, rather than in a packet format, is extremely beneficial for mobile voice in the access portion of the network. (See Figure 3. Native Ethernet + Native TDM over Bonded Copper.)

These new technologies fundamentally partition a bonded copper connection into bits that carry TDM traffic and bits that carry Ethernet traffic. The TDM bits are reserved and cannot be interfered with by any Ethernet traffic. That means the delay and jitter of the TDM remain constant, regardless of other traffic or congestion in the network. TDM never has to wait in queue behind other, potentially larger, packet frames, essentially eliminating the packetization of TDM.

Moving Forward - Not Backward
The mobile operator needs to select a technology that supports a diverse set of mobile applications over a constantly evolving cellular network. The solutions they devise must deliver superior performance that is indistinguishable from the best Ethernet and TDM services, while simultaneously taking into account the cost of implementation and paying heed to the reliability of the access network. These issues can be solved by innovatively delivering both applications over a converged mobile infrastructure.

Going forward, mobile operators need a simple, cost effective, reliable, and efficient method to exponentially scale the transport of 2G, 3G, and 4G mobile voice, data, and control traffic to satisfy today's and tomorrow's bandwidth hungry mobile user; the cellular network must also support an evolving mixture of TDM and Ethernet interfaces, with stringent SLA requirements and the need to massively scale to support higher bandwidths to the cell tower in the most cost effective manner – the business case has to work.

Whatever direction or technology option mobile backhaul network operators decide to embark on, the solution must offer mobile operators strong legacy support, options for gradually migrating the cellular network, and efficient transport for the all-Ethernet RAN that operators have committed to, with the most cost effective approach that meets the needs of a generation of mobility centric applications and also supports the demands of its users.
 

About the Author
Vijay Raman serves as Vice President of PLM and Marketing for Hatteras Networks. Prior to Hatteras, Vijay worked at Aruba Wireless Networks where he was Head of Technical Marketing and Training Development. Vijay is a networking industry veteran with more than 15 years of experience working for premier equipment vendors (pre-IPO) that led the market in technology innovation such as Aruba Wireless Networks, FORE Systems, Avici Systems, and carriers such as Sprint Corporation, in a variety of leadership roles. For more information, visit www.hatterasnetworks.com.

What is your experience with this? Tell you fellow readers now!