White Paper: Choosing the Right Microwave Radio for P25 Backhaul Mission-Critical Communications Backhaul: If you don t choose the right backhaul radio, your emergency communications radios won t work. This paper explains the benefits of a native TDM solution, and the benefits of the 4.9 GHz band, as well as other frequency bands. There are two key points: Any radio system MUST meet the requirements of P25 (and many don t). The 4.9 GHz band is right for MOST P25 deployments - but not all.
Introduction The FCC has moved aggressively to allocate spectrum and assist the industry in developing the necessary standards. In support of this, the FCC has defined several topics related to reliable backhaul radio. These topics include operability, interoperability, security, efficiency, reliability, resiliency, redundancy and scalability. A key output of this activity is the P25 digital trunk standard, developed to facilitate radio interoperation, as well as other goals. To ensure interoperability, the FCC has set strict standards for P25 systems. Among these standards are requirements for digital traffic transport at very low latency and very low jitter. Backhaul for P25 systems can be accomplished using a variety of different frequency bands and radio configurations, some more applicable than others depending upon the deployment scenario. Frequency options include licenseexempt 2.4 and 5 GHz, licensed 6 GHz and licensed 4.9 GHz. The FCC has, in fact, promoted the use of the 4.9 GHz band for Public Safety applications, and made certain key changes to license conditions and terms in support of P25 backhaul deployments. P25 and Backhaul Radio Trunks P25, a suite of system standards that define digital radio communications system architectures, addresses the needs of Public Safety and government organizations. P25 involves digital land mobile radio services for local, state/provincial and national Public Safety organizations and agencies. A key part of P25 systems is a set of elements known as digital trunk radios. As the name implies, a digital trunk radio carries a large collection of voice and date traffic among, or between, P25 radio systems. These trunks are used to backhaul all types of communications among field sites and to headquarters sites. Thus, it is obvious that the availability, reliability and capacity of backhaul trunks are critical to overall P25 network performance. P1 Exalt Communications, Inc. WP-P25-A-0809
Backhaul Radio Requirements Overview Key issues for P25 trunk radios are performance, reliability, resiliency, redundancy and scalability. Performance P25 mandates certain latency and jitter requirements, equivalent to other TDM-type systems. Reliability The IEEE defines reliability as, The ability of a system or component to perform its required functions under stated conditions for a specified period of time. In the case of P25, the FCC has set standards for the specific reliability of the radio link (in bit-error rate and other parameters). Resiliency The ability of a communications system to respond to change and/or to recover from mishap is crucial in establishing the resilience of the system. A resilient system must be fault-tolerant, and reasonably easy to reconfigure or expand. A P25 radio link should be remotely monitorable, manageable and able to recover from intermittent errors. Redundancy This characteristic refers to the functionality of a communications system achieved by means of replication of equipment. No single piece of communications equipment can ever be 100% reliable, but by deploying backups, spares or alternatives, system reliability can be assured. Scalability Scalability refers to the system s ability to adapt and grow with expanded requirements and users. It is a measure of the flexibility of the system to adjust transparently to changes in operational scope of the system. A standardized P25 trunk radio system that can be sited, moved and reconfigured quickly supports the goal of scalability. Scenarios and Constraints in P25 Backhaul Networks In addition to the requirements defined above, P25 networks like all networks exhibit a variety of application-specific, site-specific, topology-specific and geography-specific challenges that may dictate the use of certain frequency bands and/or equipment types. Portability Some P25 networks (or portions of those networks) will be of a temporary nature, for use in emergency situations, while others will be fixed in place, available for use at all times. Clearly, backhaul systems for temporary networks must be portable, or at least easily movable; this requirement often drives the use of all-outdoor systems. Ease of Licensing In the case of temporary networks, ease of licensing is just as important as portability. Time is of the essence, thus a days- or weeks-long process for obtaining a license will exclude use of certain frequency bands and related equipment. Capacity Sites at the edge of a network typically have the lowest capacity requirements while sites closer to the center, where traffic is aggregated, have higher capacity requirements. The band in which backhaul radios operate must be able to accommodate the required capacity, and the radio itself must be configured to handle that capacity as well as anticipated capacity growth. Interference The RF noise floor naturally varies from site to site and region to region with urban areas typically subject to higher noise floors, particularly in the license-exempt bands. The risk of interference may dictate the use of one band over another or, at the very least, an assessment of the interference rejection characteristics of any systems being considered. P2 Exalt Communications, Inc. WP-P25-A-0809
In all but the smallest P25 networks, it is likely that the backhaul portion of the network will consist of at least two different radio configurations and, often, at least two different frequencies of operation. Selecting Frequencies and Backhaul Radio Equipment Within the scope of alternatives that can meet P25 trunk radio requirements for a range of scenarios, there are several possible frequency and equipment-type choices. Each has advantages and disadvantages. Selection of the best frequency and radio configuration for the application will have a dramatic effect on real-world performance. Frequency Choices In practice, there are four frequency alternatives when considering P25 trunk radio system backhaul: 2.4 GHz (license-exempt), 5 GHz (license-exempt), 4.9 GHz (licensed) and 6 GHz (licensed). Outside of North America, 7 GHz and 8 GHz licensed systems are applicable as well. All of the bands have the potential to be equivalent in terms of their ability to meet P25 requirements described above, but the bands do vary in their ability to meet the constraints outlined. Looking at each band in turn, in relation to these requirements and constraints, shows the similarities and differences. 2.4 GHz and 5 GHz Many radio manufacturers make 2.4 and 5 GHz radio equipment based on 802.11, commonly known as Wi-Fi. Additionally, the wide deployment of commercial and consumer Wi-Fi equipment makes these bands crowded and, thus, subject to interference. To be fair, consumer-grade 802.11 products can play many useful roles in wireless LAN services for emergency personnel, but P25 backhaul is not one of them. The unlicensed 2.4 / 5 GHz bands are the easiest to license, as no license is required. However, the likelihood that some other radio (or microwave oven) will be operating on the same frequency is quite high. Thus, it may not be possible to depend on this band for critical communications. These bands offer the most potential for portability and movability. Since no license is required, mobile command posts can be deployed at will. The two bands differ greatly in capacity - the 2.4 GHz band is 83 MHz wide, whereas the 5 GHz bands (there are three sub-bands, actually) offer 480 MHz of spectrum. Peak capacity can be much higher at 5 GHz. 4.9 GHz The FCC has reserved spectrum in the 4.9 GHz band for exclusive use by Public Safety agencies. While it is technically licensed spectrum, it is also what might be termed License-Lite. A Public Safety agency must obtain an FCC license (see Exalt Technical Note, 4.9 GHz Licensing Process ) in order to operate equipment in the 4.9 GHz band, but the agency does not need to license each transmitter and installation. This is in contrast to all other licensed frequencies (e.g. FCC Part 101), which require a specific FCC license for each transmitter and location. Why is this important? Public Safety emergencies are unpredictable. Communications personnel need to be able to deploy all types of equipment and quickly place transmitters wherever needed, in minutes or hours at the most. License-Lite 4.9 GHz radios are easy to move, the chance of interference is low and, thus peace-of-mind is high. The FCC allocated 50 MHz of spectrum at 4.9 GHz; slightly less than the available spectrum in the 2.4 GHz band,. 6 GHz The 6 GHz band is largely interference-free, but licensing requirements are stricter and therefore most expensive to meet. Radios in this band might be appropriate for permanent links between various agency headquarters, but can t be deployed quickly in emergency situations. The fully-licensed 6 GHz band largely resolves the interference problem, but equipment on this band cannot be moved on short notice. Transmitter locations must be licensed in advance. This band offers a total of 850 MHz of spectrum, so total available capacity is quite high. P3 Exalt Communications, Inc. WP-P25-A-0809
2.4/5 GHz 4.9 GHz 6 GHz Easy of Licensing Very Easy (none) Easy (license lite) Complex Bandwidth Moderate Small Large Likelihood of Interference Moderate Low Very Low Illustration of Tradeoffs in Microwave Radio Frequencies Ethernet: Probabilistic Access One of the great innovations of Ethernet was to provide a method for several transmitters to share a physical medium without a central traffic cop controlling who talked when. Transmitters use a random-number based variable delay to control who talks at what time. Such a system is called probabilistic, because it depends on probabilities, not certainties, for transmission access. History has shown that it works well for computer data traffic. It does not, however, meet the requirements of P25, which needs a deterministic, fixed access system, such as was developed by the telephone industry. RF Modulation Scheme Choices Microwave radio specifications can be an alphabet soup of acronyms. OFDM, DSSS, FDD, TDM, 802.11, WiFi - the list is (nearly) endless. Only one of these is critical to a P25 system, however - the access method. The technique must provide a method for duplexing, that is, moving traffic in both directions. Several techniques are in use, including 802.11 (WiFi), FDD and TDD. Many radio systems, including WiFi, use a technique borrowed from Ethernet to control which transmitters can transmit at any time. These are probabilistic, not deterministic, systems.these systems do not (and cannot) meet the strict requirements of P25 systems. Frequency-division duplexing (FDD) is a method for establishing a full-duplex communications link that uses two different radio frequencies for transmitter and receiver operation. The transmit direction and receive direction frequencies are separated by a defined frequency offset. One key advantage of FDD is that it offers very low latency operation, and the latency does not vary. Furthermore, the full capacity is always available in both directions. However, FDD systems are relatively complex to install. Any given path requires the availability of a pair of frequencies; if either frequency in the pair is unavailable, it may not be possible to deploy the system in that band. Furthermore, radios require pre-configured channel pairs, making sparing more complex. Any traffic allocation other than a 50:50 split between transmit and receive yields inefficient use of one of the two paired frequencies, lowering spectral efficiency. Last but not least, co-location of multiple radios is difficult. Time-division duplexing (TDD) is a method for emulating full-duplex communication over a half-duplex communication link. The transmitter and receiver both use the same frequency but transmit and receive traffic is switched in time. It has two key advantages. First, it is more spectrum friendly, requiring only a single frequency and doubling spectrum utilization, especially in license-exempt or narrow-bandwidth frequency bands. Second, it allows for the variable allocation of throughput between the transmit and receive directions, making it well suited to applications with asymmetric traffic requirements, such as video surveillance and broadcast. In addition, because the radios at each end are the same, only a single type of spare is needed for backup. TDD has higher latency than FDD because of the need to switch between transmit and receive. In addition, multiple co-located radios must be synchronized to avoid interference. P4 Exalt Communications, Inc. WP-P25-A-0809
The Exalt Solution Exalt developed CarrierTDD to address the shortcomings of Wi-Fi and FDD systems. CarrierTDD-capable radio systems are the first and only radios to offer the latency performance of FDD radio systems with the convenience and spectral efficiency of TDD. Latency and throughput are guaranteed, independent of load. Guaranteed throughput is fundamental to P25 operation. Exalt CarrierTDD radio systems are designed to support a guaranteed, sustained throughput level over both time and distance. Exalt CarrierTDD radios offer another advantage that standard TDD cannot match: they carry Ethernet traffic, without any sacrifice in TDM performance or latency. While critical P25 systems must have low-latency connections, many current and future emergency service systems are IP-based. They perform well on Ethernet systems, so a radio that supports both is a plus. Exalt CarrierTDD vs. Wi-Fi/OFDM @ 4.9 GHz Table 1 summarizes the advantages of Exalt CarrierTDD over Wi-Fi systems for P25 applications in the public safety band. Base technology Access/Arbitration Wi-Fi/OFDM Best-effort OFDM, carries TDM inside Ethernet packets Ethernet-style probabilistic Exalt CarrierTDD Isochronous, carries TDM natively Fixed; deterministic RF link speed Varies Fixed Spectral efficiency Poor, can be below Excellent, over 99% 50% Transmitter sync Some products Available Tunability None 5 MHz steps across 50 MHz band Interference resistance Poor; scales back data rate Excellent due to high system gain and low C/I ratio. Also, radio can be tuned around interference. Simply put, Exalt CarrierTDD is the best choice for all emergency-service deployments. It provides the low, fixed latency that P25 requires, but does not give up efficient packet capability - Ethernet - for applications that require it. It s a better, more spectrally-efficient approach than either FDD or traditional TDD. Meeting Backhaul Requirements with Exalt Communications Performance Every Exalt radio system is able to transport TDM in its native mode, as well as Ethernet. Why is it important to transport TDM traffic natively? Because when a system doesn t, that traffic must undergo TDM to IP protocol conversion, and that increases latency and jitter. Applications such as TDM-based voice services and simulcast radio for Public Safety depend upon both low and constant latency in order to meet precise timing constraints. This TDM traffic is latency sensitive, so the introduction of delay above and beyond propagation delay threatens the integrity and availability of the P25 system. The risk associated with packet delay is especially acute in multi-hop connections. With Exalt CarrierTDD, the radio system delivers the low and consistent latency performance of circuit-switched connections regardless of IP traffic behavior. A second advantage of native TDM is that is more spectrum efficient than packet-based methods, and requires less overhead processing. P25 needs low, consistent latency, but many other applications work across Ethernet, and this segment will grow in the future. Thus, it s necessary to have a system that can carry Ethernet as well as TDM. Two separate radio systems could be used but this is neither cost-effective nor spectrum-efficient. With native Ethernet built into Exalt CarrierTDD radios, Ethernet packets are processed at wireline speed, but separately from TDM, so they can never interfere with TDM traffic. Table 1: CarrierTDD vs. Wi-Fi/OFDM P5 Exalt Communications, Inc. WP-P25-A-0809
Reliability Reliability is more that just a laboratory measurement. In the real world, multiple radios are in operation. Such a configuration is commonplace, for example, in situations where a hub location serves as the termination point for multiple microwave paths. If the transmitters are properly synchronized, problems are eliminated. ExaltSync synchronization technology enables the high-density co-location of multiple Exalt CarrierTDD radio systems, providing the ability to mount radios and antennas in close proximity without the self-interference that would normally undermine such a configuration. By ensuring that all co-located radios share the same timing signal, ExaltSync reduces antenna separation requirements and dramatically simplifies frequency planning, enabling over 8:1 channel reuse. Redundancy & Resiliency Traditional long-haul telephony systems are engineered with a high degree of redundancy, typically in the form of a 1+1 sparing system. While desirable, this may not always be necessary in an emergency-driven temporary deployment. Rather than engineer the cost of 1+1 into every radio, it may make more sense to use other off-theshelf redundancy solutions where required, but avoid the cost when they are not required. Exalt offers the first and only carrier-class TDD microwave radios on the market to support an optional integrated 1+1 MHS for the 2.4 GHz and 5 GHz unlicensed bands. This option, which is available on the EX-2.4i-16 and EX-5i-16, provides Public Safety agencies the ability to ensure guaranteed equipment uptime with flexibility and ease of deployment of TDD. Alternatively, carriers can use other off-the-shelf solutions. Scalability, Portability and Redeployment Emergencies are unpredictable. Communications planners must allow not only for the anytime, anywhere nature of an emergency, they must plan for growth in personnel, equipment and technologies. Today s communications system will not be adequate for tomorrow s needs. That means it is important to choose microwave radio systems that can be scaled up, moved, redeployed and generally adapted to contingencies. Exalt systems meet all these requirements. They scale exceptionally well; for TDD systems, ExaltSync makes it easy to add radio links as needed. Exalt radios can be remotely configured to increase (or decrease) TDM capacity versus Ethernet capacity. Finally, Exalt native Ethernet and native TDM support means that networked computers and VoIP equipment can be added without a forklift upgrade of the network infrastructure. Exalt Backhaul Systems Applicable to P25 Exalt offers a range of microwave radio systems that meet the requirements of P25, regardless of the frequency band - or bands - required. Table 2 on the following page summarizes the key features, and provides a brief description and summary of advantages. P6 Exalt Communications, Inc. WP-P25-A-0809
Table 2: Comparison of Exalt Backhaul Radios Portability License TDM Capacity Aggregate Ethernet Capacity High density co-location 1+1 Deployment Environment Summary EX-2.4i 1RU None 4xT1/E1 200 Mbps Yes Indoor Good capacity, but band is crowded. All-indoor 2.4 GHz TDD radio for P25. Base configuration of 100 Mbps Ethernet plus 4xT1/E1. Software upgradeable to 200 Mbps Ethernet. Longest range, highest capacity radio available at 2.4 GHz. EX-2.4i-16 1.5RU None 16xT1/E1 200 Mbps Yes Yes Indoor All-indoor 2.4 GHz TDD radio for P25. Base configuration of 100 Mbps aggregate Ethernet plus 4xT1/E1 is software upgradeable to 200 Mbps Ethernet and 16xT1/E1. Only 2.4 GHz, 16xT1/E1 radio on the market. Available in 1+1 protected configuration. EX-4.9i 1RU Lite 4xT1/E1 55 Mbps Yes Indoor Best option for All-indoor licensed 4.9 GHz TDD radio for P25. Base configuration of 27 Mbps Ethernet plus 2xT1/E1. Software upgradeable to 55 Mbps Ethernet and 2xT1/E1 or 4xT1/E1. EX-4.9r 14 Panel Lite 4xT1/E1 55 Mbps Yes Outdoor All-outdoor licensed 4.9 GHz TDD radio for P25. Base configuration of 27 Mbps Ethernet. Software upgradeable to 55 Mbps Ethernet and 2xT1/E1 or 4xT1/E1. Available with integrated panel antenna or connectorized. P25 unless higher capacity is needed. EX-5i 1RU None 4xT1/E1 200 Mbps Yes Indoor Best for portable/ movable where high All-indoor, tri-band 5 GHz TDD radio for P25. Base configuration of 100 Mbps Ethernet plus 4xT1/E1. Software capacity needed. upgradeable to 200 Mbps Ethernet. EX-5i-16 1.5RU None 16xT1/E1 200 Mbps Yes Yes Indoor All-indoor, tri-band 5 GHz TDD radio for P25. Base configuration of 100 Mbps Ethernet plus 4xT1/E1. Software upgradeable to 200 Mbps Ethernet and 16xT1/E1. Available in 1+1 protected configuration. EX-5r 14 Panel None 4xT1/E1 200 Mbps Yes Outdoor All-outdoor, tri-band 5 GHz TDD radio for P25. Base configuration of 100 Mbps Ethernet. Software upgradeable to 200 Mbps Ethernet and 2xT1/E1 or 4xT1/E1. Integrated panel antenna or connectorized. EX-6i-DS3- GigE 1.5RU Fixed 1xDS3 374 Mbps No Yes Indoor Best for fixed or pre-planned All-indoor 6 GHz FDD radio for P25. Base configuration of 23 Mbps Ethernet plus 16xT1/E1 and 1xDS3. Software upgradeable to 187 Mbps Ethernet. Available in 1+0 non-protected, 1+1 protected and 1.5+0 semi-protected configurations. locations. Conclusion & Recommendations The bottom line in any emergency response situation is that the tool that actually works is the best tool for the job. P25 requires low-latency, low-jitter native TDM connections, and Exalt CarrierTDD systems are the only ones on the market that can meet TDM to P25 requirements, yet also meet the needs of future emergencyresponse equipment without a forklift upgrade. But which band? In most cases, 4.9 GHz is the best choice. It is free from interference, easy to license, and equipment can be re-deployed at will in an emergency. Are there exceptions? A licensed 6 GHz link offers more capacity, and can be used between fixed locations if there is a need for the capacity. Only when you need movability AND your capacity needs exceed 4 T1/E1 per link should you consider the 5 GHz band. What about the 2.4 GHz band? Interference from consumer products can be an issue, but there are some situations where it fits. Exalt 2.4 GHz systems offer greater range, and so are especially well-suited to rural environments, where distances may be long and interference is likely to be low In short, most P25 links should be on 4.9 GHz, and any public safety agency s P25 deployment should include Exalt microwave radios. Nothing else works as well. P7 Exalt Communications, Inc. WP-P25-A-0809
Exalt Communications, Inc. 580 Division Street Campbell, CA 95008 Produced in the United States of America. 2009 Exalt Communications, Inc. All rights reserved. More details on Exalt point-to-point microwave radio systems may be found at: www.exaltcom.com Exalt, the Exalt logo, CarrierTDD and ExaltSync are trademarks of Exalt Communications, Inc. Other company and product names may be trademarks of others.information contained in this document may be subject to change without notice. P8 Exalt Communications, Inc. WP-P25-A-0809