The Case for Wireless and the OSP

DAS, Wireless Backhaul, Crowded Poles and the Real World of the Wireless OSP

by: Ernie Gallo

OSP Magazine

Wireless networks have transformed how people function in their daily lives. They affect how people communicate, conduct business, shop, travel, learn, and are entertained. Increasing numbers of applications and services are being developed every day for use with wireless devices. These new applications and services require greater amounts of bandwidth and that means significant impact on outside plant equipment, resources and systems, especially as networks are upgraded through multiple generations of wireless technology.

How Is Wireless Related to the OSP?
While it might sit in the background, the Outside Plant (OSP) is the backbone of wireless communication. While your mobile phone may be totally wireless, the cell tower it is connected to is not. Every cell tower is connected to a mobile switching center via outside plant copper or fiber cables. These mobile switching centers are, in turn, connected to the carrier backhaul network, which is the connection between the cell site base station and the mobile switching center. Each “wireless” call is broadcast by 3 cell towers with sectors aimed at the center of the cell.

Outside the cell, signal strength falls off very rapidly. This allows efficient reuse of the cells. Seven (7) cells together, using 7 total frequency sets, form a cluster.

With cellular honeycomb frequency reuse, cellular clusters are repeated over and over in all directions like cellular honeycomb. In this way, cell service can cover the entire country with only 7 frequency sets (A-G).

Cell towers are connected to the network, via the OSP, to centralized mobile switching centers. These mobile switching centers are required to:

• Monitor what cell you are located in so the system knows where to find you if someone calls.
• Monitor your signal strength from each of the three base stations serving the cell, plus your signal strength in adjacent cells.
• Pass off your call from one cell to another as you move.
• Keep track of your billing information.

The Needs and Requirements of the Backhaul Network
As stated earlier, the backhaul network connects the cell site base station and the mobile switching center. While we hear more frequently about the wireless spectrum and the need for data speed, the entire wireless spectrum would not be very useful without a properly sized, protected backhaul network.

For a 2G (second generation) Global System for Mobile Communications (GSM) wireless network, a bandwidth of 2 Mb/s is needed. This would require that the tower be served with one, possibly two, T1 (1.544 Mb/s) circuits.

A 3G network requires 8 to 20 Mb/s, or at least 10 T1 circuits, and when we move to a 4G network up to 300 Mb/s, at least 50 Mb/s is required to serve the tower. A Digital Signal level 3 (DS3) circuit at 45 Mb/s or a 100 BaseT Ethernet at 100 Mb/s could also be used.

Many of the issues seen with wireless networks are due to choking in the backhaul. Many dropped calls are due to backhaul issues, but most cell users do not even realize that.

It is important to remember that each cell tower will have several wireless carriers with different generations (2G, 3G, 4G) of wireless services, with each requiring backhaul bandwidth. Increasingly, the only way to provide sufficient bandwidth is to provide Fiber-to-the-Tower (FTTT). This requires new equipment and cabinets to be placed in the OSP. Therefore, wireless is changing the topologies and architectures of OSP deployments.

Solutions to Backhaul Congestion
As greater numbers of wireless device users having more and more applications come on-line, the size of the cell and clusters is going to decrease because each cell can only support a certain number of users with limited bandwidth. The smaller the cell, the more towers that are needed. This poses a problem in that, while everyone wants cell service, no one wants a large cell tower in his or her backyard. Even though such towers may be disguised as inordinately big pine trees, communities are resistant to the installation of new cell towers.

There are solutions to relieve this backhaul congestion. One is the use of a femtocell, which is a mini-wireless network that can be placed in the home. A femtocell is a base station/antenna that serves one residence. It takes the customer’s cell traffic and moves it off the backhaul network to the Public Switched Telephone Network (PSTN). With a femtocell, the customer has the advantage of a 5-bar cell signal anywhere in his or her home; however, this requires broadband service in the home. Traffic from femtocells need to be backhauled to a cellular provider network. The use of femtocells could be enhanced revenue for Digital Subscriber Line (DSL) and fiber deployments.

Cellular equipment can be placed on buildings as a way to avoid having to build cell towers. An advantage to this is that it reduces the need for towers. The disadvantages are that the demands of building owners and requirements from the National Electrical Code (NEC) must be addressed.

Cell service providers are starting to mount the radio equipment on top of towers and monopoles, which can be thought of as extensions of roof-mounted equipment, but with much less room to work. There are several advantages to moving the equipment from the hut below the tower to mounting radio head equipment directly on the cell tower:

• It eliminates long, expensive coax placement for new installations.
• It also leads to a minimal Radiofrequency (RF) power loss of about 3 dB, which can result in a 40% savings of generated RF power. With a 20% efficiency for RF generation, this yields a significant power savings.
• The use of shunt-fed DC grounded antennas eliminates the need for RF protectors.
• It reduces hut or cabinet space at ground level and air conditioning capital and energy costs by eliminating the high heat generating transmitters in telecommunication huts and congestion in huts or cabinet space at ground level.
• Since the tower leasing cost is determined by the number of cables on the tower; this number would be lower with tower-mounted radio equipment.
• It would reduce tower wind load due to the elimination of coax.

Areas of concern for doing this include:

• Existing services on towers must coexist with radio-mounted tower equipment. Each “sector” requires its own radio in a limited space.
• To power the radio head requires heavy-gauge cabling for DC power, and fiber cable will probably be needed for data transmission. Hybrid cables that carry both DC power and fiber will soon come to market to address this opportunity.
• In the event of a failure, equipment that requires repair and replacement will be much more expensive and difficult for tower-mounted equipment. Installation costs are expensive as well.
• Very few people are trained for tower work, and they can take very limited equipment with them. Use of connectorized equipment will be key to success. Telcordia is working with the industry to develop a new set of criteria for wireless equipment. This criteria will be published in GR-3171-CORE, Generic Requirements for Network Elements Used in Wireless Networks Physical Layer Criteria, which is anticipated for release in mid-2012.

An Alternative to the Cell Tower
Since the number of users of wireless systems, users, and applications are increasing, the cells become congested. The choice of building more cell towers is becoming difficult due to expense and community objections. A new alternative to cell towers is the use of Distributed Antenna Systems (DAS). DAS was originally proposed to provide uniformly good signal coverage. It provides equally distributed downlink power to all antennas, and it obtains dramatic reduction in signal attenuation and multipath delay spread. A DAS uses a multiple antenna system interconnected via coaxial cables or optical fibers belonging to a single access point/base station dispersed across a coverage area.

The single antenna radiating at high power that is used in cell towers is replaced in a DAS by a group of low-power antennas to cover the same area. Other advantages of a DAS include the ability to provide service for multiple wireless carriers (e.g., T-Mobile, AT&T Wireless, Verizon, and Sprint/Nextel) without the need to have separate antenna sites for each carrier at each location, and the ability to place the antennas on existing vertical structures such as light or utility poles.

As with any new technology, there are pros and cons and unknowns to it. Some of these include:

• DAS antennas can be installed on buildings, poles, etc. To achieve maximum height, or due to space requirements, antennas may be placed on top of a pole. This raises joint use/ownership issues that will need to be addressed. The installation and maintenance needs will probably require trained and qualified supply personnel.
• DAS electronics can be located on the pole, on the street, or in a hand-hole. Electromagnetic compatibility issues may arise from the use of DAS and traditional telephony and broadband networks. These will need to be studied further.
• There are also mechanical concerns regarding installation of DAS antennas on the top of a pole (top 6 inches):

          • Premature splitting or weakening of the pole top.
          • The top 6” is usually classified as unusable space.
          • Pole top extensions.
          • Loading calculations are needed.

Accessibility of equipment and antennas may also be an issue. The antennas and associated power supply systems may not be readily accessible. For associated infrastructure, while the antenna itself may be in the supply space, many other items of equipment such as control boxes, meters, or connections can be located in or around the communication or neutral space.

One Last Thing: The Need for Automation
As should now be very apparent: There is a tremendous amount of information to be handled in servicing the wireless OSP. Engineers in the field can do their jobs efficiently only if the information they are presented with is accurate, comprehensive, and consistent. And with the amount of network change caused by all of the issues described above, it’s essential that service providers prioritize having systems that will automate as much of the detailed design, sizing, placement, and connectivity and so on as possible.

Service providers often face the most difficulty when they go to upgrade in-place network resources. That’s when deficiencies in their information, systems and processes can cause real problems, as records are inaccurate, supposed spaces are unavailable, or it is not known whether equipment is carrying live traffic. Automated discovery of equipment is now increasingly available, reconciling back to a central, reliable, inventory -- though of itself it does not provide a solution to determining whether the right piece of equipment is in the right place!

The key to managing the wireless OSP is full visibility of the network in all its detail, processes, and systems that keep that information accurate, and automation tools that standardize and ensure consistent application of appropriate design rules and policies.

Given all of these considerations for the future, we must be aware that OSP will continue to be critical to the support of wireless networks. Though behind the scenes, it’s clear that we must evolve the OSP quickly to support the future of the wireless world.

Ernie Gallo is Telcordia’s Project Manager for Product Development in the area of network integrity solutions addressing the physical layer of the networks providing voice, video, and data telecommunications service. Ernie has more than 30 years of experience, and has received awards from the FCC, ATIS and IEEE for his work. Contact Ernie via egallo@telcordia.com or visit www.telcordia.com.

What’s your take on this subject? Leave a comment and get the conversation going.

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It’s a Bird, It’s a Plane -- It’s a… Crowded Pole!
Ever looked up to find your eye drawn to a very crowded pole? Here’s where the OSP meets the wireless world in a very big and crowded way.

DAS systems and the continual evolution of wireless services lead to very crowded poles. Today, it is possible to see the following on a pole:

• Power, CATV, and telephony wiring and closures.
• Equipment that is pole-mounted due to right-of-way issues.
• Solar arrays that feed power back on the grid as part of renewable energy.
• Cellular equipment.

Check out the equipment that can be found on poles even as you read this article. Then, think about what you’ll likely see on those crowded poles in the very near future.

Cabinet on poles