- August 11, 2011
- antenna, antenna decoupling, Aviat Networks, backhaul, beamwideth, bigger isn't better, Construction Installation and Maintenance, dB, decibels, digital microwave, Director of Corporate Marketing, Equipment, frequency diversity, k-factor, microwave communication, Microwave transmission, Radio, receive signal level, RSL, Stuart Little, Telecommunication, wind load, wireless, Wireless network, wireless transmission engineers
Terrestrial microwave radio system with two antennas employing space diversity. (Image via Wikipedia: Photo credit David Jordan)
Antenna gain is directly related to the size (diameter) of the antenna, and wireless transmission engineers looking for more system gain to improve link performance on long or tough paths in frequency bands below 10 GHz may resort to using very large antennas with diameters of 12 feet (3.7 m) or more. However, bigger is not always better. In fact, large antennas should only be used under the most unusual of circumstances.
Use of large, oversized antennas was commonplace during the 1960s and 1970s, for analog FM-FDM heterodyne microwave communication high-capacity links operating in the L6 GHz band. This was for good reason. Communications paths consisting of multiple radio links required very high receive signal levels, and fade margins of up to 50 dB, on each link to meet end-to-end noise objectives. The large antennas helped cut baseband thermal noise by more than 3 dB, which is half that of smaller antennas. Many of these paths were relatively short and many of these analog wireless links employed frequency diversity, so higher fade margins were needed to reduce outage—especially in N+1 hops. This reliance on large antennas is often still prevalent in the minds of many wireless transmission engineers.
Today’s Digital Microwave Systems
In contrast to old analog systems, digital microwave operates essentially error-free (i.e., with a bit error rate of 1 in 1,013 transmitted bits), even with much smaller fade margins. Adequate path clearance, optimal selection of diversity arrangements using smaller antennas and the precise alignment of antennas are far more effective to ensure that error performance objectives for microwave communications are met.
Big Antennas = High TCO
So because big antennas are not really needed to ensure high path availability, they do directly impact the total cost of deploying and operating a microwave link, namely:
- Wind Loading—There is more wind loading because of the larger surface area. A 12-ft antenna has 45 percent more loading (e.g., 1,400 lbs wind load in a 70mph wind) compared to a 10-ft antenna (e.g., 980 lbs wind load). This means the microwave tower needs to be stronger to be less susceptible to the sway that results in antenna misalignment. Stronger towers mean more costly new towers, or expensive upgrades to existing towers
- Beamwidth—Beamwidth of a 12-ft dish is 25 percent narrower compared to a 10-ft antenna, which further increases the tower’s rigidity requirements and thus cost
- Non-Diversity vs. Diversity—Large 12-ft antennas are sometimes justified by assuming that the single large dish is more cost-effective and/or has performance characteristics as good as two smaller diversity dishes. A single 12-ft dish with its 1,400-lb single-point wind load—and narrower beamwidth—puts far more stress on a structure than dual 8-ft diversity dishes with a distributed wind load of 1,260 lbs (2x630lbs) and much wider beamwidths. Smaller diversity dish arrangements also increase the wireless link’s performance by reducing multipath outage by more than 80 percent compared to a single 12-ft dish deployed in a non-diversity hop
- Antenna Decoupling and Alignment—The smaller beamwidth of larger antennas also increases the difficultly to align accurately, and the risk of antenna decoupling due to angle-of-arrival variations during nocturnal atmospheric (k-factor) changes. Antenna decoupling, directly proportional to path length, is increased on those longer paths in difficult geoclimatic areas that attract the use of 12-ft dishes. It can be a death spiral—the longer, more difficult paths that attract the use of larger, narrower beamwidth antennas are those that are even more sensitive to the resulting geoclimatic conditions!
- Aesthetics—Bigger isn’t better when deploying dishes on towers, buildings and—especially—mountaintop sites, due to aesthetic concerns, building/tower owners’ concerns and local planning limitations. These can often be mitigated by using smaller antennas
- Deployment Costs—The overall deployment cost differential between a single 10-ft and 12-ft antenna can exceed $10,000 when transport, installation and ancillary hardware are taken into consideration, and this does not include the potential cost of added tower strengthening and increased monthly tower lease charges
So before you consider using large 12-ft+ antennas, think again and consider the bigger picture. You may well end up spending a lot more money for a path that may perform more poorly than it would have if smaller antennas had been used.
For more tips, we’ve also included some wireless transmission engineering guidelines for antennas and other wireless equipment.
Director of Corporate Marketing, Aviat Networks
Even though microwave communications have some built-in security-like features such as scrambling, narrow beamwidth, proprietary airframe, coding and other factors, it is not very hard for them to be broken by those with the proper expertise. Some vendors even openly offer digital microwave interception systems for “legitimate” monitoring. This and the growing sophistication and willingness of those attempting to break into wireless networks makes a high level of security for microwave more important than ever.
Historically, security and encryption measures were primarily employed by government or defense agencies or by the financial industry to protect sensitive information. But in today’s connected world the issue of network security can apply to any type of communications network, whether it is fixed, mobile or private.
Is Microwave Ready?
In general, microwave packet radio security is a concern. However, there are different aspects of microwave radio protection that must be considered. The information payload of microwave communications is the most obvious part. For operators that participate in the public switched telephone network (PSTN), the main issue is the security of the communications traffic they are carrying. That would involve both voice and data traffic.
Both popular and scholarly publications have been rife with stories of how easy it has become to tap into mobile calls. For example, the GSM code has been ineffective arguably since a hack was announced in August 2009. With GSM encryption broken, degraded or bypassed, mobile phone calls and text messages can be monitored and diverted by snooping parties. This can happen even before they get to the basestation. The BBC recently demonstrated GSM hacking in an online video.
Once calls and messages are in the mobile backhaul network, in many cases, no encryption is applied at all—not even the broken GSM code. In the past, hackers would have had to buy or by some other means obtain radio equipment identical to that they wanted to take over illegally. This was not an obstacle for those intent on industrial or governmental espionage, but it put it beyond the means of the run-of-the-mill hacker who has become familiar since the mid-1990s. Even if the hacking was not beyond the average hacker’s technical capabilities, it was beyond his economic capabilities. Now commercially available microwave monitoring equipment can be employed to pick out communications channels, to listen and record all conversation and ambient noises for up to 72 hours. One research firm also demonstrated how cell towers can be spoofed to intercept communications.
Another aspect of microwave security encompasses how secure is the management of the network. Even if the payload of a microwave backhaul network is secure, the management may not be, allowing hackers or others with malevolent motives to drop or kill traffic. Unsecure management channels can allow them to create mismatched frequency settings between radio pairs, reconfigure circuitry or reroute payload traffic to another radio if a cross-connect is present. For example, there was an instance where unauthorized users took control of a motorized antenna and repeatedly sent instructions for the motor to adjust the position of the antenna, eventually draining the batteries for the entire site, rendering it “dead.” However, with the shift to the all IP/Ethernet network of the future, hackers are finding ways to wreak havoc on backhaul networks from their desktop PCs, smartphones and other powerful mobile computing devices.
Access control of the microwave network is also a cause for concern. It is critical that only authorized personnel are allowed to log onto the administration of a microwave backhaul network. Like many computer-based systems, microwave radios are set up with some basic logon access procedures. Oftentimes, the logon screen will not look very dissimilar from the typical Windows or Macintosh workstation. There will be a dialog box for a username and a password. However, unlike the typical desktop computer, a microwave radio’s graphical user interface is not logged onto that much. Therefore, as per human nature, their usernames and passwords become all too predictable. “Root” and “admin” and “123456” and “password” were very popular as usernames and passwords, respectively, according to one security study. A “mechanized” or “dictionary” attack can randomly generate username-and-password combinations and succeed in unlawfully logging onto a radio on this premise: that the logon will be subject to people being creatures of habit. Thus, there must be a way for microwave network administration to enforce a hard-to-guess username/password security policy.
Another aspect to access control is the issue of the level of control. It is also essential to control what each legitimate user is allowed to perform once logged in—to prevent voluntary and involuntary damaging actions. Not only must users be limited to their area of responsibility and knowledge and avoid involuntary commands that could damage the network but also reserve critical activity for designated key personnel (e.g., cryptography officers).
Would my Radio Network be Secure?
Given the security issues around microwave payload, management and access control, many questions have been raised. Would my microwave radio network be safe from intrusion? What would be the impact of breached calls or text messages? There could always be potential for a Greece type of incident. More importantly, the proactive questions to ask about microwave network security include:
- Who does need a high level of security?
- What comprises the high level of security necessary to protect my microwave backhaul?
- What precautions will a high level of security invoke to protect my network?
- How is this high level of security implemented?
- What are the options for high-level security?
- How do I get a high level of security for my network?
- Is this high-level security solution standards-based?
- What type of threats does my high-level security solution need to protect against?
We’ll examine these questions more in future posts. Or see our white paper.