Understanding Your LMR Backhaul

Land Mobile Radio (LMR) systems are critical in public safety organizations. Reliable and clear communications are necessary for groups such as firefighters, medical personnel, and rescue teams to operate effectively.

Advances in technology and gaps found in traditional LMR systems have spurred the process of modernizing these systems. For example, LMR system providers and microwave transport providers have led the move to discontinue traditional Time Division Multiplexing (TDM) telecommunication services and standardize the use of Internet Protocol (IP) for voice and data communications.

As a result of initiatives such as these, LMR systems have begun a transition towards packetization, which involves using IP instead of traditional TDM technology. This shift has led to the need for IP backhauls capable of supporting mission-critical networks. These backhauls prioritize packet quality over quantity and emphasize Quality of Service (QoS), latency, and jitter control in their design.

In this article, we’ll explore the purpose of the network backhaul in LMR communications, review TDM and IP networks, and discuss the advantages and disadvantages of the backhaul transition.

The Basics Of LMR And Backhaul

Land Mobile Radio (LMR) systems are a standard communication technology utilized by organizations that require instantaneous communication between personnel that are mobile and geographically dispersed. LMR Is typically used by public organizations such as emergency medical personnel, fire departments, and police, though private organizations also use LMR for activities such as construction and operations.

In the context of a communication network, a backhaul refers to the link between the core network and sub-networks existing on the network edge. The backhaul is responsible for the transportation of data between access points. As such, the backhaul is a critical component in QoS and network performance.

Historically, the network backhaul has been based on TDM services. TDM requires specialized equipment but has been a robust and reliable technology. However, there is a concentrated push to IP-based network backhauls to enable several key advantages.

To understand the benefits and concerns of transitioning the LMR backhaul, it’s important to understand the differences between the traditional TDM and modern IP technologies.

Traditional Time-Division Multiplexing

TDM has been the primary technology behind the backhaul for LMR systems for a few decades. TDM uses time to separate data streams from a transmitter into segments with very short durations. Each individual data stream can then be reassembled at the receiver based on the timing.

TDM in LMR communications is a proven technology, having been used extensively for public safety organizations. In fact, TDM was first developed in the late 1800s, with Bell Labs further developing the technology in the 1960s. Using TDM as a backhaul for LMR requires specialized equipment, and acts as a relatively independent network once operating. This comes with advantages, such as a lack of external interference and cybersecurity risks. However, this also means that the failure of a single piece of equipment could bring the entire network down.

Packetization Using Internet Protocol

At a high level, IP communications in LMR means transmitting radio communications over an IP connection. This is sometimes referred to as Radio over Internet Protocol (RoIP). RoIP is similar to Voice over Internet Protocol (VoIP) communications utilized widely in businesses and by individuals today, but with radio-specific communication functionality built in, such as half-duplex mode, push-to-talk, and digital-to-analog converting capabilities.

Though there are many topologies that can be used in IP communications, Multi-Protocol Label Switching (MPLS) is the topology of choice for LMR backhauls because it offers inherent fault tolerance and fast convergence.

Compared to TDM, IP backhauls are a relatively new concept, with implementations starting in the late 1990s. Due to advantages including lower capital costs, MPLS has been quickly gaining popularity in the communications industry.

Advantages Of Changing To The IP Backhaul

Considering the traditional reliance of LMR systems on TDM technology, why is there a push to move to IP backhauls? Utilizing IP backhaul has some significant potential advantages over the traditional TDM approach.

The advantages of IP over TDM may include:

  • Higher Capacity – available bandwidth is greater
  • Flexibility – Utilize unused bandwidth for non-LMR purposes
  • Lower Setup And Operational Costs – TDM requires expensive/specialized equipment to implement properly, whereas an IP backhaul would allow users to select from a wide variety of less expensive equipment or software applications.
  • Higher Reliability – TDM systems have single points of failure (i.e. specialized equipment) that could render the system virtually useless in the event of a disaster or equipment failure. Utilizing an IP backhaul allows for more redundancy. In the event that the main communication pathway goes down, it is relatively simple to redirect communication through another IP network path.
  • Low-Latency – Mission-critical information flows, especially in emergency situations, must be transmitted in real-time with no delay. Low latency is required from all network equipment and interacting with and all the way through the transport network.
  • Enhanced Diagnostics, Faster Remediation – By virtue of the greater ability to capture the quality of service parameters, pinpointing failures is available in real-time. This makes restoration possible much more quickly.
  • Future-Ready While Supporting Legacy – During the migration phase, backhaul networks will need to be flexible to support simultaneously TDM and Ethernet in the migration phase. Over time, they will need to migrate smoothly to all-Ethernet with support for remaining TDM services.

Concerns With The Backhaul Change

As with any technology change, there are challenges and concerns with moving to IP-based network backhauls. Though these concerns are not insurmountable, they are potentially significant and require serious consideration when planning a backhaul migration. These challenges may include:

  • QoS Issues – Anyone who has been on a VoIP call and had connectivity issues knows that internet communications are not always perfect. Dropped packets or network slowdown can lead to latency and connectivity issues. Though these issues may be a minor inconvenience for personal use, they can cause failures in a public safety LMR network.
  • Network Management Complexity – Most network monitoring tools are designed for complex IP networks, but not specifically designed with LMR in mind. LMR operators require simple, concise, and actionable information to maintain critical communications. Combined with operator skillset gaps, most “off-the-shelf” monitoring tools won’t work for LMR operations.
  • Network Troubleshooting/Operations – Shifting from specialized radio equipment to IP-based networks brings a significant change in communications operations technology. Traditional radio operators may not have the network engineering background necessary to troubleshoot communications issues. Organizations may need to invest significantly in training and hiring specialized personnel to support the backhaul transition.
  • Cybersecurity – Perhaps one of the greatest risks facing a shift to the IP backhaul is the issue of cybersecurity. Any data transmitted using an open protocol like IP faces the risk of cybersecurity threats, and radio communications would be no exception. Traditional radio networks had the advantage of utilizing “air gapping”, which ensures network security through physical isolation from unsecured networks. However, the use of IP-based networks removes the ability to rely on air gapping as a sufficient cybersecurity measure. As such, more robust cybersecurity protection will be a critical requirement for LMR systems using IP backhauls.

Stay tuned for the next blog post on this topic! We’ll dive further into the common issues encountered in an IP communications system, and how you can identify them.

About The Author

Dan Havens is the Chief Technology Officer of Municipal and Public Safety Networks. Dan oversees the identification and implementation of corporate technical direction for many of our products and services. Dan is also responsible for overall leadership of the San Diego, CA operations and the Municipal and Smart Grid services sectors. Dan’s technical background includes expertise in optical, satellite, VoIP, MPLS, SONET, metro, mesh, wireless, and LAN/WAN technologies with Cisco, Motorola, Tropos, Juniper, and many other leading networking/communications technology vendors. Dan holds a BS in Computer Science from the University of Washington.

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