LEASED-LINE REPLACEMENT
10 Things to Consider Before Choosing Your IP Gateway
WHITE PAPER
Outline
Leased-lines are still being used in many parts of the world to carry voice communications between radio repeater sites and from radio sites to dispatching centers. Telecom service providers charge for this service and these fees can be a significant proportion of the running costs of the radio network. Significant savings and productivity gains can be made by switching to an IP infrastructure and using Radio over IP (RoIP) gateways. However, choosing the right products can have a considerable impact on reliability, productivity and ultimately long-term cost to the organization. This document outlines ten important considerations when selecting gateways for leased line replacement.
Introduction
A leased-line is a dedicated voice path which is rented from Telco providers to
organizations to connect a base radio repeater to a dispatch console. Sometimes the line is used to connect two base radios together which are geographically separated to give better radio coverage. This type of line is different to the normal switched telephone line as it is available to the user 24/7 and can only be accessed by the organization leasing it. Cost per month will be based on distance between sites, availability of lines and type of line service.
Typical Leased-line infrastructure
Problem
Solution
An alternative to Leased-lines is to use IP networks. IP networks have the ability to carry voice as well as data. Most people are familiar with Skype or IP phone systems that use the internet or local area networks (LANs) to carry voice calls. This form of telecommunications is commonly known as VoIP, ‘Voice over the IP’. This general methodology can be used to replace leased-lines.
The adaptation of VoIP for radio communications has been referred to as “Radio over IP” or RoIP, although there is no actual standard. It can deliver several key benefits to
organizations using radio networks. These include: • Considerably lower running costs
• Improved reliability • Improved productivity • Increased interoperability
All of these are derived by utilizing the existing IP infrastructure - the IP backbones that make up both local and wide area networks.
Return on Investment
The cost of leased lines is variable, depending on supplier, availability and distance between end points; however it is not uncommon to find pricing of around $1250 per month, per line. Switching to a RoIP solution can provide a Return on Investment (ROI) of 3-4 months.* The conversion between radio audio and IP data is performed by a device typically known as a RoIP Gateway. This can be a stand-alone box or it can be part of a software program running on a PC. Both forms have their advantages and disadvantages depending on the application.
IP as an infrastructure opens up many possibilities to productivity and costs savings. Look for manufacturers that can provide a range of devices that collectively work together. Some applications require multiple leased-lines to be replaced at the same remote radio repeater site. Having a device that can multiplex, say 4 leased lines through a single Ethernet port can provide considerable cost savings both in capital and running costs.
The cost of providing an Ethernet connection point is generally independent of distance, unlike a leased-line service. This presents a host of new options for solving an organization’s connectivity needs. An organization having operations spread over a large geographical area can now economically connect multiple radio repeater sites together.
Examples include: A state-wide public safety group such as Highway patrol; an airline company requiring radio connectivity between airports; or an Oil & Gas company with several facilities both on-shore and off-shore. All of these can now connect all radio users together very economically. The interoperability and network design options are greatly increased by using IP as the medium.
New interoperability options for Oil & Gas organization
There are a few important differences between VoIP and RoIP that need to be considered when looking for a suitable method to replace a leased-line.
10 THINGS TO CONSIDER BEFORE CHOOSING YOUR IP GATEWAY
#1 - Control Line Function
#2 - CODEC options
The Gateway should have a full range of CODECs that can be selected for different radio network applications. High compression CODEC’s such as GSM can take advantage mobile phone service providers IP data networks such as 3G and 4G. This enables remote dispatch applications and backup radio links. Low compression CODEC’s such as G711 consume greater bandwidth but impact much less distortion on tones. Some radio signaling schemes will even pass with a degree of acceptability. It is important to remember that, in general, most common CODEC’s will always add some level of distortion to Tone signals and cannot reliably carry PTT / Busy functions.
#3 - Tone Reliability
In analog radio networks, signaling is not just limited to PTT and Busy. Often, complex tone signaling schemes are used to selectively call individuals or groups of radio users. Typical signaling schemes include SELCALL, DTMF, MDC1200, and CTCSS (also known as PL or QT tones). Each scheme has different levels of tolerance to CODEC distortion. This can impact on the ability to transport tones reliably. To overcome this problem, the RoIP gateway should have the ability to detect incoming tone signals prior to IP conversion. The tone information is then carried as separate IP data packets from the voice and regenerated and mixed with the audio at the remote end.
Background Knowledge: Most CODECS are optimized to recognize and convert speech but this
process doesn’t work well for pure tones. Often tones will get distorted or lost completely in the process. In some cases the tone distortion is so bad that when the tone is converted back to audio, it cannot be detected correctly by the radio or dispatching equipment. The radio simply fails to PTT or even worst, PTTs intermittently.
Different CODECs provide different levels of IP packet compression to reduce bandwidth
requirements. Higher levels of compression result in less bandwidth but unfortunately also result in higher levels of tone distortion.
Background Knowledge:
Most CODECS are optimized to recognize and convert speech but this process doesn’t work well for pure tones. Often tones will get distorted or lost completely in the process. In some cases, the tone distortion is so bad that when the tone is converted back to audio, it cannot be detected correctly by the radio or dispatching equipment. The radio simply fails to PTT, or even worse, PTTs intermittently. Different CODECs provide different levels of IP packet compression to reduce bandwidth
Block Diagram of Tone detection & transportation used in sophisticated RoIP gateways.
#4 - IP Packet Loss Compensation
Make sure the Gateway has the ability to minimize the impact of poor quality IP links on radio signaling, regardless of whether signals are carried in-band (mixed with the audio) or as special data packets. All IP networks are subject to various levels of data packet loss. Small amounts of packet loss may not be obvious for speech but can have a significant impact on signaling tones. For example, some radio signaling schemes such as EIA (Guard tone followed by a hold tone) require the constant presence of the tone for continued PTT. If the tone is lost, even briefly, the radio stops transmitting and will not simply restart with the re-detection of the tone. EIA PTT signaling is particularly vulnerable to packet loss, and in some applications cannot be used, at all, across IP links. Other signaling schemes such as DTMF may simply miss a tone digit and cause the wrong individual or group to be called. A good gateway should provide mechanisms to compensate for packet loss especially when using guard tones to control radio transmissions.
#5 - Serial Data Capability
Often, a radio repeater site may have devices that need to be controlled or monitored using RS-232. As this is separate to the RoIP function, it is not uncommon to see organizations using additional devices such as serial to Ethernet adapters. However, a RoIP gateway should be able to support this function using an RS-232 port. This process is sometimes referred to as “data tunneling” or “serial pass-through”.
Block diagram of Voice with Serial data tunneling
The result is that the RoIP gateway can provide a UDP connection that passes transparent data between two end points. In theory, the data can be anything since the gateways don’t inspect or modify packets. It’s the devices or applications that connect to the serial ports of the gateways that are responsible for transporting the data and correcting for possible errors. This kind of application is often used for passing telemetry between some central maintenance and support facility and remote repeater sites. A good example is the ability to monitor power supply levels and critical repeater functions using SCADA.
#6 - Remote IP Access and Diagnostics
It is essential that the gateway provide a rich set of features to enable both local and remote diagnostics.
Some gateways reduce costs by only providing device configuration through a local data port. This can greatly reduce productivity at installation & commissioning and impede on-going maintenance support. Devices that provide IP access to the gateway’s configuration should also have the ability to upload new firmware. This means the organization’s IT supervisor or Communications Technicians can comprehensively manage the whole radio network from a central point.
the ability to force generate sample voice files at the output of the gateway (into the radio) or to simulate received audio from the radio. The same applies to the various tone signaling schemes. In addition, the device must have the ability to collect important statistics about the gateway to facilitate the problem diagnosis and troubleshooting.
Using an Internet browser to access Gateway Diagnostic and Configuration tools
A further handy feature is a basic handset port which can be used to assist in
troubleshooting. Generating RF through a handheld in a radio equipment room or a repeater hut is often undesirable due to the sensitive nature of other equipment, or is not possible due to structural shielding. Therefore, by plugging in a handset, technicians can quickly and confidently test the radio interface.
#7 - Security and Encryption
Look for gateways that have the ability to support security and encryption. Public safety and other mission critical organizations need to protect their communications as it passes through IP networks. Sometimes the organization needs to use an Internet service provider to get connectivity between the remote radio sites and a dispatching center. Other
#8 - IP Connectivity Methods
Although RoIP gateways are designed for radio communications, they’re usually installed into IP networks of various complexities and with operational limitations. Therefore, it’s important that the gateway provides multiple methods of communicating over IP.
There are 3 common forms of IP communications: UDP, TCP and Multicasting. Of these, only UDP and Multicasting are really appropriate for VoIP and RoIP. UDP is typically used to carry voice packets from point-to-point. This is also known as “unicast” communications.
It is the easiest method to use but it does have limitations in terms of addressing and system size. Multicasting is the other common method which supports point-to-multipoint. It also uses UDP but it’s more efficient and overcomes many of the limitations of the unicast method. The problem is that Multicast is difficult to implement and troubleshoot in large networks or over the Internet. An alternative method is to use conferencing. This method may provide point-to-multipoint communications in networks where Multicast is not practical.
Background Knowledge
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9 - SIP Compatibility
SIP is an Internet standard that is used to setup calls across IP networks, similar to a call setup in a telephone network. Gateways that support SIP connectivity can provide greater interoperability to the organization. If the organization has an IP PBX for example, any user with a phone extension can call the gateway and gain access to the radio network. The limitation is that the calls can only be setup in one direction - phone users to the radio user. Block diagram of typical connectivity between radio and phone using SIP
Look for gateways that support connectivity from the radio network to IP PBX. Some gateway manufactures have taken advantage of existing simple conventional signaling schemes, such as DTMF, to provide a switched path from radio network back to the PBX. Because SIP is a standard protocol, connectivity is not just limited to phone extensions. A radio user can use SIP connectivity to contact another SIP gateway which could be attached to another base radio. With this type of architecture, users can selectively call different geographical regions or combine different radio groups together in the event of an emergency.
Block diagram showing radio users dynamically connect to remote radio sites or office phones
#10 - Future Digital Radio Connectivity
Organizations that are considering leased-line replacement are probably using analog radio. However, in the process, organizations may also consider the potential to upgrade to digital radio. In this case, the choice of gateways becomes even more critical. This may involve some form of migration over a period of time, often with both analog and digital systems running in parallel. Some manufacturers of RoIP gateways can provide compatibility between analog and digital protocols which is essential in making the smooth transition. Because digital radio systems provide higher levels of functionality than their analog counterparts, their interfacing requirements are much more sophisticated. In all cases, this involves interfacing to the radio using a digital protocol. Although there are standards, most of the radio vendors use proprietary protocols. The gateway needs to be flexible enough to support each protocol used by the different manufacturers. As new protocols are released or enhanced, the gateway must be able to keep up with the changing technology. This type of gateway provides the best interoperability between protocols and manufactures now and into the future.
Critical choice of gateway for replacement of Leased-line with a future to Digital radio
Summary
Replacing leased-lines with IP gateways can have immediate impact on the bottom line of the organization. The return on investment can be very quick. Astute IT and Radio
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