Do STL-DSTL Link System Relaibly?

Radio broadcasters have traditionally used 950MHz analog or digital point-to-point radio systems to transport their audio programming from the studio to the transmitter site. Leased T1 or E1 digital land-based circuits have also been popular where a line-of-sight radio path cannot be established. Each system has strengths and shortcomings;  the radio STL is one-way only and has little room for ancillary data. The landline T1/E1 solution implies a monthly lease expense, as well as high capital costs for the endpoint equipment, and still not much extra data bandwidth once the program audio is transported. With multi-station shared transmitter sites, HD Radio, off-site automation backup, security cameras, remote control, and Internet access all becoming necessities, a reliable, high-bandwidth data transport scheme becomes  critical. Enter the modern 2-way IP-radio system. These carrier-grade links can transport several stereo audio channels with bit-for-bit clarity, plus provide for all the other data services just mentioned and have room for growth. This paper describes not only the STL needs and challenges facing broadcast engineers, but provides clear, workable solutions both in the abstract and specific solution cases.

Traditional STL Systems

Studio-Transmitter Links (STLs) have for decades  been analog RF transmitters and receivers, typically in the 950 MHz band (in the USA). Some systems were monaural, others consisted  of 2 monaural transmitters and receivers, each pair being offset in frequency from the center of an STL channel to provide a stereo path. Many systems were and still are “composite” STL systems where the stereo  multiplex signal is generated at the studio and passed faithfully to the FM transmitter over a Composite STL radio system. In all of these  systems, some relatively slow-speed data could be sent from studio to transmitter site using subcarriers. The mid-1990’s  saw the introduction of digital STL systems. In these, analog or AES audio input was bit-rate-reduced using MPEG 1 Layer 2, MP3 algorithms and transmitted as a serial bit stream to the receiver for decoding. Later, linear audio digital STL systems became available. Still, these are one-way (simplex) studio-transmitter links with no return audio or data path. Moreover, there still isn’t much data rate available, even in these digital STLs.

More data bandwidth

Broadcasters are seeking more bandwidth - specifically, Internet Protocol (IP) bandwidth - between studio and transmitter. Commercial Internet service is often not available at remote transmitter sites, so engineers are looking to provide their own solutions where needed. Today, a variety of appliances and services are remotely usable with an IP connection. Security cameras, off-site file servers, VoIP telephone, and, of course, a high quality IP-based Studio-Transmitter Link highlight the need for reliable IP connectivity that is 100 percent under the broadcaster’s control.

IP works
IP connectivity between two points can take several forms.  If commercial Internet service is available at both endpoints, then simply paying a monthly fee for it can be a good way to connect.  In most cases, however, broadcasters desire more reliability than we often find is actually provided by typical ISPs.

Broadcast engineers would like to see “Five 9s of reliability” or better.  Five 9s equals 99.999% uptimes. This translates to downtime of 5 minutes and 15 seconds per year. Six 9s (99.9999%) uptime is even better, implying only 32 seconds of downtime per year.

Experience shows that many ISPs typically deliver only three or four 9’s of reliability. That level equates to between one and nine hours per year of downtime. Sadly, it’s not uncommon to experience two 9’s (99%) of uptime, equating to about 3½ days of downtime each year. Experience varies widely with commercial ISPs, with some broadcasters suffering through daily or weekly outages (worse than two 9s), while others are getting that five 9s reliability.For use as an audio STL poor reliability with frequent outages is wholly unacceptable  to broadcasters. Either a five or six 9s IP link is needed, or a main and backup IP connection for both ends is required.

Commercial Internet service linking two sites is likely worse when there are two different ISPs involved. With just a single ISP, there’s a good chance that site-to-site data is routed the shortest possible way, and will likely stay within the same city or region as the two endpoints.  If it’s necessary to use two different ISPs, there’s a very good chance that all the point-to-point data will be routed out of the area to a “gateway” location. This is a data center where several ISPs and backbone providers interconnect with each other. If one endpoint is on Verizon, for example, and the other is on CenturyLink, then all the data traveling between the two may be routed halfway across the country in order to interconnect. Staying within the same ISP from end to end will very likely result in the highest reliability for commercial Internet service.


IP radios

If a line-of-sight path is available between studio and transmitter sites, or even via an intermediary “hop” point, the option opens to consider installing IP radios. IP radios can provide a very reliable (five or six 9s of uptime) IP connection.  Moreover, modern IP radios convey IP packets with bandwidths approaching 1 gigabit per second, though more typical bandwidths may be 50 to 100 megabits per second.  Whatever bandwidth the IP radios will support over a given path, this option is likely to be very reliable and shouldn’t involve any recurring monthly costs.

IP radio antenna systems vary among models, too. While “flat panel” antennas are popular for general Internet access from Wireless Internet Service Providers (WISPs), they cannot provide the extra gain margin and off-axis interference  rejection of a parabolic antenna.

Some IP radio models feature a “split” electronics package, with the bulk of the circuitry in an indoor- mounted  unit. Then, the up/down converters, pre-amp,  and output amp in an outdoor unit, typically mounted  on the back of the antenna.  Many IP radio models - especially the crop of less-expensive one becoming popular - feature an all-in-one design, with the electronics package part-and-parcel with the antenna(s). Still others offer a mix-and-match topology where a small, outdoor electronics pack can be coupled to large, medium, or small antennas.

One additional differentiator in IP radio systems is whether they are half-duplex or full-duplex. Half-duplex systems cannot actually transmit  and receive simultaneously. Rather, they switch between transmit  and receive at a rate that is optimal for the path length, providing the most efficient throughput  possible in half-duplex scenario. 

Full-duplex systems don’t have to alternate transmit and receive; they can transmit  and receive simultaneously full-time. This allows not only better throughput  but affords less jitter in the IP packets delivered to each far-end network.  For ordinary IP transport, half-duplex works fine. However, for time-critical Audio over IP (AoIP) applications, full-duplex offers some benefits to reliable operation. An excellent explanation and visualization of simplex, half- and full-duplex systems is presented here.

LICENSED AND UNLICENSED
IP radios are available in a variety of sizes, frequency bands, power levels, and feature sets. They’re also available in licensed bands, requiring frequency coordination and regulatory licensing, as well as unlicensed bands. Unlicensed IP radio systems may be quick and easy to purchase and install, but may be subject to interference  from other users on the same or adjacent frequencies.

Whether a point to point wireless link is designed and deployed in either licensed microwave or unlicensed frequencies, the expense for equipment and the time it takes to deploy the equipment is the same.  The only practical cost difference is the licensing fee.
Licensed RF transmitters communicate  using a specific transmit  and receive frequency combination that is selected and assigned to the user (licensee). Licensed microwave wireless systems operate  within parts of the radio spectrum, such as: UHF/VHF, 900MHz, 2GHz, 3.65GHz (WiMax), 4.9GHz (public safety), 6GHz, 11GHz, 18GHz, 23GHz, and 80GHz (E-Band millimeter wave) as designated by the FCC.

Licensed microwave wireless systems are becoming more popular as a result of noise interference  in unlicensed wireless spectrum, most notably in built-up urban areas. Licensed microwave radios provide good security from the risk of interference  from other RF systems. In a licensed system  the channels that the radio system  transmits and receives on are allocated to the user and are registered  with the FCC after frequency coordination. Getting a license is reasonably inexpensive and can be obtained in the matter of weeks.

Prior to deploying and operating a licensed frequency an end user is responsible for performing a frequency coordination, filing a public notice, and submitting an application (FCC Form 601) with the FCC to ensure that no one else is already operating on the same frequency or a frequency that will inject interference on existing systems. This process provides full disclosure of the frequency assignment and typically avoids interference  from any existing licensee already assigned in the area. If licensed radios encounter  interference, it is typically resolved with the assistance of the FCC or other regulatory body.

With unlicensed systems there is no guarantee that a system  will operate  interference  free. However, many unlicensed systems can overcome interference  by having a good carrier to interference  ratio inherent with the hardware and by proper design and installation.  Indeed, the high-gain parabolic reflectors (antennas) used in point-to-point microwave systems typically provide excellent rejection of undesired interfering signals.

The major difference between licensed wireless and license-exempt systems, then, is that licensed radio users have a regulatory body that will assist  them in overcoming any interference  issues that may arise, while license-exempt users must resolve interference  issues without governmental assistance.

If you are interested in STL-DSTL Link System or any other broadcast equipment, please feel free to contact us:[email protected] 

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