Education Center, Archive for 2009

WAN: Why Not Synchronous? (part 5)

To understand the benefits of a synchronous protocol, it helps to look at the disadvantages of an asynchronous protocol. When a node using an asynchronous protocol such as 802.11 wants to transmit a frame, it normally will simply transmit the frame after it senses the channel is idle for a period of time (which is called Carrier Sense Multiple Access, or CSMA). If a collision is determined, due to the lack of an acknowledgment frame, the frame is re-transmitted after waiting an amount of time that increases exponentially for each retransmission. In order to minimize the impact of a collision and to maximize the chance of a successful reception of the data frame, 802.11 includes an optional collision avoidance (CA) function where a short Request-To-Send/Clear-To-Send (RTS/CTS) exchange is first performed, which causes devices overhearing those frames to not access the channel for a period of time. This collision avoidance function may be beneficial in some situations, but it comes with a large overhead, and it introduces problems of its own, and the impact of these problems is greatly increased in a long-range outdoor system.

WAN: Why Not Asynchronous? (part 4)

To understand the benefits of a synchronous protocol, it helps to look at the disadvantages of an asynchronous protocol. When a node using an asynchronous protocol such as 802.11 wants to transmit a frame, it normally will simply transmit the frame after it senses the channel is idle for a period of time (which is called Carrier Sense Multiple Access, or CSMA). If a collision is determined, due to the lack of an acknowledgment frame, the frame is re-transmitted after waiting an amount of time that increases exponentially for each retransmission. In order to minimize the impact of a collision and to maximize the chance of a successful reception of the data frame, 802.11 includes an optional collision avoidance (CA) function where a short Request-To-Send/Clear-To-Send (RTS/CTS) exchange is first performed, which causes devices overhearing those frames to not access the channel for a period of time. This collision avoidance function may be beneficial in some situations, but it comes with a large overhead, and it introduces problems of its own, and the impact of these problems is greatly increased in a long-range outdoor system.

WAN: Why Synchronous? (part 3)

To summarize so far, there are two primary reasons to use a synchronous, scheduled protocol within a mesh network: MAC layer coordination and to point directional antennas. Regarding the latter, to avoid the challenges of dynamically pointing antennas, some multi-antenna systems use a separate radio for each antenna (or subset of antennas). This has several problems, with the most obvious problem being cost. Even though there is now the availability of inexpensive 802.11 radios, these radios have many hidden costs due to:

• amplifiers
• increased processing power and processor interconnect
• increased node size
• increased power consumption

WAN: Why Synchronous? (part 2)

Beyond the reasons mentioned in Part 1, there is another equally important, if not more important, reason to use a synchronous protocol for broadband wireless mesh – to point antennas. One of the most effective tools an RF engineer uses to improve a wireless link and to minimize a link’s impact on others is to use directional antennas. The benefits of directional antennas include:

• increased link budget (both on transmit and receive), which allows higher modulation and longer range
• less susceptible to interference from others
• causes less interference to others
• increased power allowed in many regions

WAN: Why Synchronous? (part 1)

The most obvious reason that someone would choose a synchronous protocol for an outdoor wireless network is to have the ability to schedule transmissions. However, there are actually some crude ways to implement a scheduled system without being synchronous, such as by simple polling. In fact, 802.11 includes an optional Point Coordination Function (PCF) that uses polling (and 802.11e extends this functionality in its optional Hybrid Coordination Function). Additionally, 802.11 even includes some synchronous features in its base specification, specifically its Time Synchronization Function (TSF), which allows devices to periodically align their clocks, which can then be used by functions such as power-save where a sleeping device can periodically wake up at the right moment to see if there is data for it. However, there are many reasons that 802.11 is not considered a synchronous protocol. Some features traditionally associated with synchronous protocols, such as WiMAX or SecureMesh WAN, include:

WAN Mesh Capacity (Part 2): The Multi-Radio Myth

When we were designing the SecureMesh WAN multi-hop scheduling protocol, our task would have been much easier if we simply used one radio to talk to the parent node and another radio to talk to the child nodes. However, there are several reasons why we chose to tackle the much more difficult problem of single-radio multi-hop scheduling. Obvious reasons to use only a single radio include cost (radios might but cheap, but high power, industrial grade radios and the additional interconnect are not), power, size and the inability to find many clean channels…, but the main reason is that using multiple radios simply doesn’t work! Focusing on that last claim, simultaneous transmissions and receptions over long-distance links simply do not work in the real world. This is based on physics. If a high-power radio is transmitting at +30dBm while another co-located radio is trying to receive a signal at -90dBm, then the +30dBm transmission will completely swamp the -90dBm reception. That’s a 120dB difference in signal levels, and to put that in perspective, the transmitted signal level is 1 trillion times stronger than the received signal level. To combat this problem, multi-radio systems have traditionally tried using combinations of:

WAN Mesh Capacity (Part 1)

There has been an ongoing discussion in the mesh community about how much capacity is lost due to the relaying of data within a wireless mesh network. Proponents of multi-radio architectures have argued that they can deliver close to 1/n (where n is the number of hops) of the capacity of a radio simultaneously to each mesh device, while single radio architectures are closer to 1/2^n. For instance, a 4-hop path in a multi-radio system (assuming several clean channels are available) could deliver on the order of 1/4 the capacity of a radio simultaneously to all mesh devices, while a single-radio system may only be able to deliver 1/2^4, or 1/16, the capacity of a radio, due to multi-hop interference. This diagram shows how a traditional single radio mesh system has its bandwidth reduced due to a large interference domain allowing only a single device to transmit at a time (note: the circles show the communication range, while the interference range will usually have a radius many times larger). A multi-radio system could use several frequencies to allow multiple transmissions to take place at the same, reducing some of these interference conditions (however, not only does this require multiple clean channels, but there are some pitfalls that will be analyzed in a future post). So an obvious question is, “How does the SecureMesh WAN’s dynamic antenna switching affect system capacity?” The answer is that even though the SecureMesh WAN system uses a single backhaul radio, it can still provide 1/n the channel capacity simultaneously to each device due to the dynamic antenna switching. In addition to all of the previously discussed benefits of dynamic antenna switching, such as higher link budget, interference avoidance and point-to-point power levels, the largest benefit is probably from something called “spectral re-use”. Basically, spectral re-use is a benefit of using dynamically switched high-gain antennas where multiple transmissions can take place simultaneously, on the same frequency, in very close proximity. For example, the dynamic point-to-point link formed by the high-gain antennas allows a first-hop transmission to not interfere with a third-hop reception, even on the same channel. And while one first-hop device is relaying, spectral re-use allows many other devices to simultaneously communicate, such as allowing the gateway to transmit to another first-hop device. That is why we always recommend at least 2 first-hop devices. This allows the gateway, and most other devices within the mesh, to be continuously active, so the capacity of the overall system is equal to the capacity of the gateway radio. This allows at least 1/n to be delivered to each device simultaneously, equivalent to the multi-radio mesh system and much higher than traditional single radio systems. And by only consuming a single channel, additional channels can be employed in order to multiply overall system capacity (plus, it is often difficult to find the multiple clean channels that multi-radio architectures require). But, the use of multiple radios in context of traditional mesh networks and the SecureMesh WAN system will be explored in a future post.

Ethernet vs. IP at the edge of the Smart Grid

In every realm of networking, from backbone transport, to enterprise LAN, to access networks, to even data centers, there are debates about the use of layer 2 (Ethernet) versus layer 3 (IP) transport. The proponents of layer 2 argue that it’s inexpensive, efficient, and supports non-IP protocols while the proponents of layer 3 argue that it’s more secure and scalable than layer 2. There are obviously different answers for different networks, but having personally developed both IP and Ethernet systems for military wireless mesh, fixed wireless access and Wi-Fi clouds, I believe that in the case of last-mile wireless access the benefits of layer 2 (Ethernet) far outweigh the problems that need to be addressed. In order to compare the pros and cons of each transport technology, let’s look at the issues with each since the benefits of one technology are often the converse of the issues with the other.

Point to MultiPoint vs. Dynamic Antenna Switching

Point-to-MultiPoint (PtMP) systems typically require multiple frequencies in order to avoid self-interference (interference among base-stations within the same network, or among sectors of a single base-station). The degree that multiple frequencies are re-used within the network is called “frequency re-use”, and is quantified by a frequency re-use factor. The frequency re-use factor will vary based on the number of available frequencies, the deployed technology and the network architecture. The network architecture generally falls into two categories: omni-directional systems and sectorized systems.

Link Distance Flexibility

When deploying an outdoor wireless network, a choice is usually made between building a short-range mesh or a long-range PtMP/PtP system. A short-range mesh is normally used for downtowns, “hot zones” and campuses, and provides all the benefits normally attributed to meshing, such as fault tolerance due to re-routing and fast, easy installation with little need for link engineering due to the large amount of peers available. But the problems include:

• shorter links
• the need for more wired or wireless backhaul
• unpredictable service due to the large interference domain

HAN Gateway Architectures

Recent reports predict that 100 million smart meters will be deployed in the next five years and that half of these will have a built-in Home Area Network (HAN) gateway for in-home energy management programs and services. ON World’s survey of 77 utilities in the United States also found that 21% are planning to integrate a HAN gateway into every smart meter deployed.