Redefining Backhaul Networks

Mar 15, 2001 12:00 PM, Reini Florin

The rapid growth in the number and diversity of cell sites and core switching platforms is having a corresponding influence on the size and complexity of transmission backhaul and WANs.

The backhaul network is defined here as that portion of the network that interconnects the carriers’ RF platforms with their core switching and IN elements. This part of the network infrastructure carries wireless calls from cell-site RF service platforms back to the MSC and then on to the appropriate service-termination points such as the PSTN and the Internet.

Carriers today view their backhaul and transmission networks as strategic assets. Cost containment is important because this portion of a network can run as high as 20% of an annual expense budget. Many carriers also are looking for ways to use these network assets to deliver fixed services such as broadband Internet access and business leased lines.

Carriers require a backhaul transmission and WAN network that supports several objectives.

First is to maximize billed minutes. Cell sites in the United States produce on average $500,000 to $1 million of revenue each per year. Carriers require a reliable backhaul network that minimizes the amount of downtime and the resulting lost revenue.

Second is to reduce operational expenses. The backhaul network has a direct impact on three key cost drivers: backhaul circuit charges, PSTN access fees, and operations and support expenses.

Finally, carriers are interested in new revenue-producing services. They require solutions that allow rapid installation of new cell sites and switching centers. Traditional voice-based services are becoming a commodity. As new data-service requirements emerge, carriers want to ensure that they become more than just bandwidth pipe providers. Many are evaluating a host of non-traditional business models to maintain competitive advantage including the option of offering fixed services for both wireless and wireline off of their backhaul networks.

Two Mobile Business Environments

Carriers are feeling intense cost pressures, adding additional revenue-producing and service-enhancing RF and data-oriented platforms while they deploy new cell sites to improve service quality and geographic coverage.

As they look for ways to differentiate themselves in the market, the need increases to interconnect their networks to an expanding list of roaming, interconnection, and value-added data service and content providers. And as a result of recent mergers, many of today’s cellular and PCS providers have a need to consolidate and re-deploy much of their MSC assets. This results in a backhaul networking environment that is more complicated, harder to manage and more difficult to grow under current design methods.

Cellular Backhaul Environment

As a result of a less-than-optimum original analog network build-out years ago (when quick network availability was the major business objective), most cellular-network providers have found themselves with an alphabet soup of network termination devices and service platforms. In some cases, their cell sites are populated with multivendor, transmission backhaul devices. Multiple T1/E1 and data-port connections (as many as 12 at some sites) are required to support all of the various RF service platforms. In addition, many of the cell sites that are in a “hut” environment have additional POTS service lines for technician access to the public network. Cell-site downtime as a result of circuit failures is regular, with some sites experiencing multiple failures each month.

As cellular carriers continue to add digital radios, additional carrier frequencies and data service platforms to their analog-based cell sites, a prime opportunity exists for backhaul network optimization. More than 75% of the existing cellular cell-site locations in the United States may undergo some type of RF network upgrade over the next two years.

Carriers can take advantage of their situation by also upgrading their backhaul networks. Eliminating their non-intelligent, single-purpose legacy backhaul devices with a next-generation integrated access device (IAD) can combine all functionality required into a single, remotely managed device. An IAD can efficiently aggregate the diverse RF services traffic over a shared set of route diverse T1/E1 public or private backhaul circuits.

PCS Network Environment

The bulk of new mobile wireless cell-site and network infrastructure installations over the next five years primarily will be based on PCS technologies.

PCS backhaul networks can differ significantly from their cellular counterparts for several reasons:

• Networks tend to be smaller in scale and incorporate more cell sites per geographic footprint due to the RF characteristics of the higher frequencies.

• Cell sites tend to deploy fewer discrete RF-service platforms because many of the platforms have been integrated into the latest versions of base-station radios.

• No legacy services exist that require a managed phase out of infrastructure.

• Smaller coverage footprints result in fewer backhaul circuits per site, yet more circuits per coverage area, creating a more complex backhaul network compared with cellular.

PCS providers have the advantage of designing an efficient backhaul network from the ground up, unlike their cellular counterparts that are somewhat tied to supporting their legacy services and customers. PCS providers should have a clearer picture of how their networks will migrate from existing 2G technologies to 2.5G and to 3G, and plan for growth accordingly in the design and equipment selection for their backhaul networks. The trade-off to this is the fact that PCS backhaul networks are more complicated and involve the interconnection of more cell sites per equivalent geographic area compared with cellular.

Traditional Approach

The equipment used and the approach to deploying a traditional backhaul transmission network is simply unsatisfactory for today’s competitive wireless environment and for meeting the needs listed before. Most carriers have and continue to deploy multiple single-purpose backhaul products at their cell sites.

These products, most of which are not carrier class and were designed for commercial use, are connected via public or private narrowband circuits in a star configuration back to a MSC digital-cross-connect switch (DACS) or multiplexer. The DACS/multiplexer provides connectivity to both the mobile switch and to the termination services.

This traditional use of products and approach to network design is hard to manage and costly. Further it does not allow for route diversity, does not take advantage of PSTN least-cost routing or access-tariff arbitration opportunities, and allows no smooth migration to broadband or IP-service deployment.

Cell-Site Access

At tower sites, a carrier can deploy a variety of RF service platforms, such as existing 1G analog radios, 2G digital radios, fraud-control radios and location service radios. In addition, carriers must plan for and soon begin deploying 2.5G- and/or 3G-based radio platforms. Additional WAN support also is required for signaling channels and other OSS traffic.

In many cases, cell-site locations also might deploy a LAN and a POTS-line connection for technician and support-services use. All of these applications require some form of transmission backhaul to a core switching platform.

As carriers deploy more cell sites to improve coverage, they increasingly require intelligent backhaul devices that can provide information about the health of the circuits and equipment that they interface with as well as a complete comprehensive view of the network and its status.

Several different design topologies for access backhaul can be implemented depending on a specific carrier’s environment and user needs.

A single-mode star configuration is used for cell sites that deploy only digital radios and where it is economical to dedicate T1/E1 circuits to each cell site. This is typically for cell sites that are located geographically in the same or immediately adjacent to wireline carriers’ central office as the mobile switch.

In this environment, the distance charges relating to a T1/E1 circuit might be economical enough to justify a dedicated circuit to each cell site. A private point-to-point microwave connection also can be appropriate under these circumstances.

A dual-mode star configuration is the most common cellular-network topology. It is used for cell sites that deploy both digital and analog radios. These cell sites tend to have multiple additional RF service platforms, such as CDPD and RF fingerprinting installed. The amount of traffic traversing this type of cell site requires multiple (two to four) T1/E1 circuits for transmission backhaul.

Backhaul bandwidth is at a premium in this environment, so voice-compression transcoding for the analog traffic commonly is required. Circuit- and traffic-route diversity becomes important in this topology should T1/E1 circuits fail or degrade.

A loop or cascade configuration can be used for digital BTS radio-only cell sites that are sufficiently distanced (generally greater than 20 miles) from the MSC. In this environment, T1/E1 circuit charges become costly, and an opportunity exists for traffic consolidation between multiple cell sites in a central-office service area prior to backhaul to a broadband hub site or the MSC.

Under these circumstances, only the T1/E1 circuit that provides transmission to the MSC incurs distance-related charges. An optional circuit can be added to a set of cascaded cells to complete a redundant loop should the carrier want to invoke route diversity between the cell sites.

A PSTN bypass application can be integrated into any of these topologies as long as excess bandwidth exists in the backhaul-circuit infrastructure. In this scenario, wireline termination traffic from the mobile switch shares the same circuit infrastructure as the ingress wireless traffic coming from the cell sites.

The wireline traffic is carried as long as possible on the wireless carrier’s backhaul circuit infrastructure before being handed off to a LEC end-office switch. Terminating these calls at an end-office switch (from a cell-site location) instead of a carrier tandem office (from an MSC) can save as much as 50% on PSTN access fees.

Radio-Access Network Hubbing

A new backhaul-network element, the radio-access network (RAN) hub, is becoming more prevalent in many wireless networks. This element acts as a cell-site traffic-aggregation point for multiple cell sites in a given geographical area. Equipment at this hub site combines narrowband fractional or full T1/E1 circuits into higher-level broadband pipes.

Traffic from hundreds of individual RF platforms can be aggregated at a single hub site before being sent on into the network core. The emergence of this network element has been driven by several factors:

• Wireless carriers want to extend their backbone networks closer to the edge and rely less on incumbent wireline carriers for their backhaul aggregation needs. By installing its own hubbing equipment, a carrier gains better control and better real-time information about the status of its network as it grows and becomes increasingly complex.

• The increasing availability of new, low-cost DS3 and SONET-based broadband services being offered from a variety of new carriers allows wireless carriers to create hub sites in co-location environments economically.

• Carriers want to view their backhaul networks as strategic assets that can be used to either resell bandwidth to others, consolidate internal network traffic with subscriber traffic or to deploy new fixed revenue-producing services.

Industry mergers and acquisitions are creating opportunities for the consolidation and elimination of geographically redundant MSCs. Reducing the number and cost of switch-center facilities by replacing them with RAN hub nodes allows carriers to redirect, consolidate and backhaul cell-site traffic on to a fewer number of MSCs.

Core Network Connectivity

Things are changing rapidly at MSCs. As the number of both data and voice services offered to subscribers proliferates, the need to interconnect core infrastructure platforms is increasing rapidly. All of these platforms need to interface across a wide area with an ever-increasing number of external networks for Internet access, content delivery, SS7 signaling and other value-added services.

This has created the need for a next-generation broadband-services node that can handle a variety of different interfaces and networking protocols flexibly, as well as provide a front end to the carrier’s core infrastructure elements. With careful planning and the right products, a carrier can take control of its backhaul network and develop it into a strategic asset.

Florin is Paragon Networks (www.paragon-networks.com) senior vice president.