A new way to simplify small cell backhaul

How PoE+ and a new class of Media Converters are simplifying small cell backhaul.

As UK mobile network operators (MNOs) seek to expand 4G coverage in large cities, they have increased their budgets for network infrastructure and are adopting small cells to add needed capacity to the mobile network and to give users more coverage, signal strength and bandwidth.

The operators have been public about their investments in small cells in London and other major cities. In addition, the operators see small cell technology as key for “not spots,” those rural areas that have poor coverage today and where small cell technologies provide a cost-effective option for improved wireless coverage.

In major cities or rural villages, mobile backhaul – the transporting of small cell data to a mobile switching center (MSC) or other network interconnection point – is a growing concern. One big issue for MNOs is how to cost-effectively connect these cells to the network.

Backhaul is just one of the factors that challenge carriers when siting and installing a small cell, others include government permits, objections from the public, lack of skilled installation and maintenance labour. However, MNOs often report that backhaul is one of their top concerns, and it’s important to have the maximum flexibility for connectivity so that they can move forward aggressively when they find a site that meets all the criteria.

Media converters are one tool that MNOs are using to meet these alternative connectivity needs.

Today’s small cells require backhaul bandwidth ranging from 40 Mbps to 400 Mbps and often come with built-in 10/100/1000 copper unshielded twisted pair (UTP) Ethernet network connections. This meets a cell’s bandwidth needs for the 100-meter distance limitation of Ethernet over UTP, however there are a growing number of circumstances where a MNO needs to use cabling other than Ethernet UTP for connecting the cell to its network.

Media converters are one tool that MNOs are using to meet these alternative connectivity needs. Use of media converters can include connecting a small cell, or cluster of small cells, that is more than 100 meters away, utilising network cabling to deliver power, or making use of legacy cabling to reduce installation costs and speed time to service.

Fibre Optics for Extended Distances and Backbones

If the small cell is more than 100 metres from the Internet connection, it will need to use fibre-optic cabling for network connectivity. Although it is not typically an issue for small cells, Fibre can also be used to handle high data rates. Fibre-optic media converters often utilise small form-factor pluggable (SFP) optics, which are separate modules that slide into the media converter unit allowing MNOs to mix and match the connector and fibre type based on their network needs.

Up to eight small cells can be connected over a single pair of fibre, eliminating the need for separate fibre home runs for each of the cells.

Another use case where fibre is used is for connecting a cluster of small cells back to an internet connection, allowing organisations to better utilise their existing fibre cable plant. Specific SFPs are available with coarse wave division multiplexer (CWDM) capabilities that can be used to create fibre-optic backbone networks. Up to eight small cells can be connected over a single pair of fibre, eliminating the need for separate fibre home runs for each of the cells. Each small cell in the cluster is connected into a chassis-based media converter with the CWDM module installed inside.

Power-over-Ethernet Plus

In many cases, media converters supporting Power-over-Ethernet Plus (PoE+) can also provide a power solution for small cells. PoE+ (IEEE 802.3at-2009) enables the delivery of 25.5 watts of power via Category 5/6 UTP data cable. The standard calls for 30 watts of power at the power source, with a transport distance of up to 100 meters.

With PoE+, MNOs can make single wire connections, avoiding bulky DC power supply equipment and can power the small cells with PoE from the local power source.

PoE+ is a great solution for powering “all-in-one” small cells that are designed to be mounted on a power pole or on the side of a building. In these applications, the cost effectiveness of PoE+ is a benefit, but aesthetics is also critical as many siting authorities seek solutions that will not be an eyesore. With PoE+, MNOs can make single wire connections, avoiding bulky DC power supply equipment and can power the small cells with PoE from the local power source.

Modern media converters are also able to support PoE+ delivery with fibre-optic, 2-wire and coax cabling as well as UTP. In the case of fibre-optic cabling, a special hybrid cable is used that bundles copper cable for the power with fibre-optic cable for data. This solution helps deliver full PoE+ power and multiple gigabit data speeds for up to 750 meters.

Reusing Existing Cable to Reduce Backhaul Costs

With thousands of small cells being deployed, cutting installation costs by leveraging installed cabling, where applicable, is a good idea. A rule of thumb we hear from customers is that installing new cable increases a budget by 40% for indoor projects, and in one case a customer saved 80% by reusing cabling. These costs can go even higher in outdoor applications. New media converters can carry Ethernet packets over installed coaxial cable, 2-wire and fibre-optic cable, as well as all categories of UTP.

Key Technology Decisions for Backhaul Media Converters

MNOs who are considering media converters for backhaul applications should keep in mind some media converter features that help these solutions remain flexible and deliver necessary performance.

End-to-End Signal Integrity: Fibre-optic media converters typically re-amplify the light waves as they pass through the device, which reduces signal skew and other errors that can cause data transmission problems. Some media converters fully regenerate the signal by re-amplification, reshaping, and retiming the signal to ensure a quality link for the full distance of the connection.

Timing Support: If 2G/3G voice data is supported on the backhaul link, then the media converter must support some type of timing mechanism to ensure that packets are received in the same order they are sent — a critical need for voice calling applications. One popular option is Synchronous Ethernet, which transmits a synchronisation signal that is tied to an external clock as the mechanism for enforcing the order of packets. Another timing alternative is precision time protocol (IEEE1588), which offers the same functionality without the need for an external clock.

Detecting Cable Breaks: Detecting fibre cable breaks is a significant component of the cost and manpower required to manage a fibre-optic network. Simplifying this for a small cell network that could number into the tens of thousands of units is essential for the MNO. Optical time-domain reflectometer (OTDR) technology injects a series of light pulses into the fibre-optic cable and reads any light coming back from the network. With this input, the OTDR instrument analyses the change in the impedance of the cable from which it can determine the location of the cable break. Handheld OTDR analysers are standard equipment for network technicians, but with OTDR technology built into the optical SFP, technicians can go straight to the area of the break to fix the problem faster.

Management Data Security: Just as it’s important to secure network data, it’s also important to secure the management data that comes from media converters. Managed media converters should be password protected with access logs, and management data should be encrypted using secure sockets layer (SSL) or secure shell (SSH & TLS) encryption.

Operations, Administration and Maintenance Protocol Support: Many mobile applications utilise operations, administration and maintenance (OAM) protocols to examine information from the data link layer of each Ethernet packet to provide insight into link discovery, link monitoring, remote fault detection and remote loopback. IEEE 802.3ah and Y.1731 are the primary OAM and performance monitoring standards and they define the data that is used by network management to verify connectivity, detect faults and monitor performance.

Conclusion

Media converters are a key part of an MNO’s backhaul solution as they can be used to power small cells, reduce cabling costs, connect small cells beyond 100 metres from an interconnect point or build a backbone link. As wireless and wireline backhaul technology evolves, media converters will continue to play a key role for operators.

About the author: Curt Carlson is Head of Product Management, Transition Networks.

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