The Real Story about Random Early Discard (RED)

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What is RED and its Effect on Queue Control?
Random Early Discard (RED) is a data queue control mechanism to improve data utilization during network congestion. Some radio vendors have made exaggerated claims about its capacity to improve “radio link utilization.”

To help control network congestion (i.e., overloading) the Internet Transmission Control Protocol (TCP) uses a mechanism known as the TCP sliding window, which is designed to maximize bandwidth usage while avoiding traffic congestion. Under control of the sliding window, TCP connections have their window size (i.e., share of bandwidth) increased as acknowledgements (ACKs) are received. With multiple connections in play this can reach a point where all bandwidth is consumed, resulting in network congestion and the dropping (i.e., tail dropping) of frames. At this point, the sliding window mechanism initiates a simultaneous reduction in window size for all TCP connections after which, with a return to stability, it ramps up their window size to create an oscillating traffic pattern. It results in inefficient use of the available traffic bandwidth. This oscillating behavior is termed “TCP global synchronization.”

Counteracting the Problem
In order to counteract this problem different queuing control mechanisms have been devised. A common one is Random Early Discard or Radom Early Detection (RED). In RED, when the queue exceeds a certain size the network component marks each arriving packet with a probability that depends on the queue size. When the buffer is full, the probability reaches 1 and all incoming packets are dropped. The chance that the network component notifies a particular sender to reduce its data transmission rate is proportional to the sender’s share of the bandwidth of the link—an improvement over tail dropping.

An issue that was realized early on about the RED algorithm is that it cannot differentiate between traffic types. A variation of the RED algorithm that addresses this problem is called Weighted Random Early Detection (WRED). In WRED, the probability of dropping packets is based on the size of the queue and the traffic flow type (IP precedence).

Improvement Using RED and WRED Algorithms are Modest
Although some microwave vendors claim to obtain up to 25 percent improvement in “radio link utilization” with the RED and WRED algorithms, independent studies show that RED improvement for real data applications is more modest. Also, bear in mind that RED is only beneficial where the bulk of the traffic is TCP/IP.

The first study from AT&T Labs and Stanford University using a simple analytical model showed that although RED may prevent the traffic rate from moving in lockstep, this algorithm was not enough to prevent the traffic rate from oscillating and wasting throughput for all traffic flows. The study suggests that if the buffer sizes are small, then randomized policies are unlikely to reduce aggregated periodic behavior.

A second study from the University of North Carolina concludes that below saturation point (90 percent utilization) there is little difference in performance between RED and tail dropping. For loads from 90 percent to 100 percent, RED can be tuned to outperform tail dropping but only with careful RED parameter settings and degradation in latency response. Analyzing the results from these and other studies it becomes clear that the claim of 25 percent link improvement is not realistic.

Eduardo Sanchez
Marketing Engineering Specialist
Aviat Networks

42GHz: A New Global Standard for Wireless Backhaul?

Parabolic antennas

With ever-increasing demand for spectrum in fixed services, FWCC has endorsed opening up the 42GHz band as a new global standard for microwave backhaul. (Photo credit: Miguel Ferrando via Wikipedia)

An ever-increasing demand for spectrum has recently turned focus on the 42GHz band. Initially opened in some European countries following the development of ECC REC(01)04, the recently published ECC Report 173 states 12 countries have opened this band including Germany, Norway, Poland, Switzerland and the United Kingdom. In the U.K., this band was part of a wider auction of fixed service bands in 2010, with three operators being granted blocks of spectrum in the 42GHz band as a result.

Building on this growth there is a move to make this band global and earlier this year saw the publication of ITU-R Rec F.2005, which in effect promoted the aforementioned CEPT recommendation to global status. Aviat Networks has been lobbying key regulators to open this band. We are eagerly awaiting a consultation from Canada and responses have already been submitted to recent consultations from France and Ireland containing considerations regarding opening this band. The process is also underway in Finland and Sweden to open up this band. Recently our attention has turned to the United States and whether the FCC will open this band for use by the fixed service.

Back in autumn 2011, Aviat Networks raised this topic within the FWCC (Fixed Wireless Communications Coalition) as the first stage of a petition of rulemaking to the FCC. At first there was only a lukewarm reception to our idea as there was concern that the FCC would refuse the request out-of-hand as some previously released spectrum below 40 GHz is underutilized and, therefore, why is more needed? We pointed out that much of this spectrum (e.g., 39 GHz) was block-allocated by auction and thus has not been readily available to all users and that the licensees have underutilized the spectrum. There is a growing need for spectrum that can be licensed on a flexible, site-by-site basis, and this is reflected by the fact that there are no underutilization issues in bands such as 18 and 23 GHz, which are licensed in this manner. It is no coincidence that auctioned bands tend to underperform in terms of efficiency and utilization. So, undeterred, we forged ahead and this resulted in the production of a FWCC petition to the FCC in May 2012. The FCC has recently placed this petition on public notice, per its procedures. This is a great success for Aviat Networks and our commitment to seeking more spectrum for the fixed service, but the story has not ended here as can be seen from a recent blog entry from the FWCC.

Aviat Networks will continue work with the FWCC to ensure that the FCC gives this proposal full consideration and, having learned important lessons from past spectrum allocations, we will lobby for a flexible approach to the licensing model.

Ian Marshall
Regulatory Manager
Aviat Networks

Is the Backhaul Really the Bottleneck for LTE?

Is Backhaul the LTE Bottleneck?The popularity of smartphones and tablets, motivated by the launch of the iPhone (2007) and the iPad (2010) have created a dramatic increase in mobile data consumption. The need to provide higher throughputs at the base station level to serve this demand has concerned operators, equipment vendors and industry watchers about a possible bottleneck in the backhaul network.

The basis for this concern is that microwave technology will not be able to provide enough capacity, and that only fiber is able to meet the capacity needs of 4G/LTE networks. This apprehension is being capitalized on by some optical network providers who argue that fiber connections are needed to provide gigabit levels at each base station. Although a gigabit connection in each base station is desirable, extremely high costs, slow deployment and inflexibility of fiber optic networks prevent this from being a viable option for operators who are CAPEX and OPEX constrained.

Aviat Networks’ studies, based upon our early involvement in some of the largest LTE network deployments, show that an average of 100 to 200 Mbps of backhaul capacity per LTE cell site is more than adequate and easily achievable with current microwave technologies. Read the white paper below or see our case study on a national U.S. LTE operator.

What is More Susceptible to Rain Outage, Vertical or Horizontal Radio Signals?

English: A map of the world divided into Inter...

The world is divided into separate International Telecommunication Union regions. In many regions of the world, ITU methods of calculating rain outage are most commonly used. In other regions, such as North America, the Crane model is used more often. (Image credit: Wikipedia)

Ever wonder which antenna polarization is more susceptible to rain outage? Vertical? Horizontal? Which should you use for very long hops?

What would you do besides add extra fade margin to mitigate rain outage? Design a shorter path or use a lower frequency band?

Aviat Networks’ microwave radio guru, Principal Engineer Dick Laine, tackles these tricky questions and others in the latest episode of our Radio Head Technology Series of videos.

Dick also talks about rain outage—as calculated by ITU using a simple scientific calculator, or computer programs (Starlink) that use the Crane model. He goes through an ITU-R availability calculation in one example, noting specifically about rain attenuation calculation above and below 30 degrees latitude. Dick then proceeds into a deep dive on calculating outage when you know the fade margin, followed by a discussion on the Crane rain attenuation model.

Aviat Networks invites our readers to register to be added to our Radio Heads distribution list to get notified of new Radio Head Technology Series releases and links to replays.

Mobile Network Modernization in Africa

Africa Mobile Penetration Rates

Total African Mobile Connections and Penetration Rate (million, percentage penetration). Source GSMA Africa Mobile Observatory 2011

Throughout Africa a wind of change is blowing as mobile network operators ponder, and in many cases implement, a wave of network modernization. The trigger for this is multi-faceted. Booming subscriber growth, introduction of new data services and arrival of new undersea fiber optic cable links are combining to strain existing network infrastructure to the breaking point.

Booming Mobile Subscriber Growth
According to the GSMA , as of September 2011 Africa has overtaken Latin America with 620m mobile connections, making it the second largest mobile market in the world after Asia-Pacific. The number of connections has more than doubled over the past four years, with growth expected to continue at the fastest rate of all global regions over the next four years.

First Voice, Now Increasingly Data
Most networks across Africa were built many years ago to serve the initial rollout of 2G/GSM mobile networks that were designed to provide basic voice services. Many operators have since introduced data services using EDGE, 3G WCDMA, and, more recently 3G HSPA, putting an incredible strain on these networks. These data services can be vital for the operator, as they are often supporting premium, prepaid subscribers or new fixed line data services being offered for small and medium-size businesses.

One example of network modernization in action is in East Africa, where a mobile network operator saw subscriber numbers increase 9 percent in 2011, with 3G customers increasing more than 85 percent. This operator was also offering fixed data services to private and corporate customers through the deployment of WiMAX base stations collocated with the existing mobile sites. All this new data traffic was growing exponentially and fast outstripping the legacy backhaul network capacity. The operator also had to ensure that existing voice traffic was protected.

Priorities Driving Network Upgrades
Today, several priorities are driving network operators to upgrade their networks including the need for:

  • Increased capacity
  • More efficient use of backhaul spectrum resources
  • Support for increasing volume of Ethernet/IP-based traffic
  • Network Simplification
  • Reduction in Capital and Operational Expenses

These five priorities are closely interrelated. For more details, download the complete article.

Stuart D. Little
Director Corporate Marketing
Aviat Networks

Record 180 km Hybrid Diversity IP Microwave Link

Survey view from Belize toward Honduras, at 1000 m AMSL

Survey view from Belize toward Honduras, at 1000 m AMSL

Link between Honduras and Belize Crosses Water and Land

Last year I wrote about the world’s longest all-IP microwave link, stretching 193 km over the Atlantic Ocean in Honduras. Aviat Networks and Telecomunicaciones y Sistemas S.A. (TELSSA) designed and implemented this link together. This year, Aviat Networks and TELSSA again worked together to build another link and achieve another record—an Eclipse microwave link between Honduras and Belize that crosses 75 km of the Atlantic Ocean and 105 km of rugged terrain for a total path length of 180 km. This is a new world record for a hybrid diversity microwave link!

After the success of implementing the 193km link over water, Aviat Networks and TELSSA were eager to meet the challenge to connect Honduras and the neighboring nation of Belize using a single microwave link. Aviat Networks network engineers and TELSSA engineers were able to use their extensive knowledge of local propagation conditions, thorough understanding of long path design principles and precise installation practices to successfully implement this 180km microwave link.

Long Path Design Considerations

As outlined in the article last year for the longest all-IP hop, a deep understanding of path design considerations and experience in microwave transmission path design are necessary to successfully complete a long path design. Key considerations involved:

  • The effect of antenna diameter on highly refractive paths
  • Precise alignment of the antennas to mitigate the effect of refractivity
  • Optimum RF and space diversity spacing to counter elevated divergent dielectric layers
  • Deterministic prediction of the variations of atmospheric conditions
  • Multi-path propagation delay

To read more about this world-record Hybrid Diversity IP microwave link, download the full article.

Ivan Zambrano
Senior Network Engineer
Aviat Networks

Wind Farm Interference on Microwave Links: Is it a Real Problem?

Wind Turbine

Microwave links can traverse orbits of large industrial wind turbines. New studies are needed to examine the effect of turbines on modern microwave link technology. Photo credit: mcdlttx (D Turner) via Flickr.

Since the 1980s, there has been concern about the potential interference that wind turbine farms can cause to wireless communication equipment. The focus has been on TV, civilian and military radar and point-to-point microwave systems. This led to studies to evaluate the degradation effects that wind turbine farms have on these systems. They concluded that physical propagation effects such as dispersion and diffraction of electromagnetic signals propagating through wind turbine farms produce low-level, long-delay, multipath distortion on telecommunications equipment [1] [2].

In recent years, these conclusions have been used to recommend an overzealous approach to the design of digital microwave paths that go through or over wind turbine farms. Overzealous recommendations with little supporting evidence have made it standard operating procedure to establish “exclusion zones” around wind turbine farms [3].

In the case of microwave links, the technology has made great improvements since the 1980s. In the 1980s, microwave links were analog and more vulnerable to interference and multipath distortion created by wind turbine blades. New digital microwave radios with Forward Error Correction (FEC), Adaptive Coding and Modulation (ACM) and high dispersive fade margin are better equipped to deal with interference and multipath distortion produced by wind turbine blades. These technological advances were not available in the 1980s and 1990s when the most rigorous studies about wind farm interference were completed.

Besides the improvements in digital microwave technology, wind turbines have also changed. In the 1980s they were smaller (compared to contemporary units) and mostly made of metal; today wind turbines are bigger, and the blades are mainly made of reinforced fiberglass, which is transparent to microwaves. Although obstruction due to the wind turbine pole and generator case will create path loss and possible diffraction of the signal, poles and casings are very thin (compared to the Fresnel zone radius) and would have to be in the direct line of sight of the link to produce significant penetration loss.

Although a conservative approach to microwave path planning is always recommended, and detailed planning and path surveying for each path are necessary, an overzealous design based on outdated studies can lead to unnecessary CapEx and OpEx. More rigorous studies based on detailed field measurements with high performance digital microwave wireless communication equipment must be undertaken to establish wind turbine clearance criteria based on current technology and field measurements.

A good starting point can be to check current links that are intentionally or accidentally traversing the orbits of a wind turbine blade. Aviat Networks would be like to hear from users that have this situation (leave a comment for this blog). Further analysis of a link in this condition can prove or disprove the hypothesis that modern microwave radios are relatively unaffected by modern fiberglass wind turbine blades and thin wind turbine pole structures. If this hypothesis is confirmed, current guidelines can be relaxed to avoid passive repeaters and bigger towers that represent additional costs.

Eduardo Sanchez
Marketing Engineering Specialist
Aviat Networks