01 November 2004
Clouding the picture
Frame relay networks save money and speed connections.
By Lee Schmidt and Jeff Miller
With utilities pressured to reduce staffing levels and attain a higher level of automation, supervisory control and data acquisition (SCADA) network speed and reliability become increasingly critical. Typical SCADA communication systems (sometimes termed telemetry systems) must provide needed information in a timely manner. To increase the network's transmission speed and thereby increase throughput and bandwidth, a frame relay communication network might be the right choice.
Simply put, a frame relay system (FRS) permits a star-type Ethernet network connection over a large geographical area. Connections over a large geographical area are wide-area networks (WAN). Where a typical star-type Ethernet network contains a hub or switch at its center, a phone company provides the central connections for an FRS network instead.
A typical frame relay representation replaces the middle of the star network with a frame relay "cloud." You can consider the frame relay cloud as a WAN.
The individual connections to the WAN are high-speed circuits run to the telephone company switching center, where the telephone company networks establish permanent virtual connections (PVCs) or circuits between all user connections.
They call these connections virtual because they are actually dedicated data channels between the various connections on the carrier's public network. Frame relay is similar to leased-line technology in this manner, but frame relay has greater flexibility in that several different sites can communicate with each other simultaneously.
For transmission across the carrier's network, frame relay data transfers form into packets that encapsulate into "frames." To speed up communications across the network, data error detection and correction occurs at the transmission/detection end, rather than between each communicating portion of the carrier's network. Because network communication is digital as opposed to analog, data reliability and available bandwidth are greater.
Because transmissions require full access to the network, frame relay is ideally suited for data transmissions and less suited for voice or video transmissions. Voice and video require nearly constant communications to provide the quality expected.
All of the FRS provider network connections are restricted to a limited number of individuals who have authorized access to the network's provisioning system. Typically, each PVC must be set up to include the device's IP address and media access control address. This combination restricts access to the system to those who provide the needed information.
Another means of addressing security is using encryption in the device that provides access to the FRS. Frame relay access devices (FRADs) provide the proper interface to the phone company network on one side and the end user's Ethernet connection on the other. FRADs typically use 192-bit encryption, which is practically the highest level of encryption available.
Because the end user's data travels over a public network, a small security risk exists no matter how the carrier addresses security. Most users consider this risk minimal and do not employ any other security enhancements beyond those mentioned.
Numbers speak frankly
Frame relay networks are becoming financially competitive. A T1 connection can run anywhere from $500–$1,200 per month. Frame relay connectivity cost can range drastically based upon location, number of sites, and required bandwidth—anywhere from $500–$20,000 per month. However, you must take into consideration the cost of radios, towers, cabling, design, and installation for wireless or other communication systems. Frame relay speed, reliability, distance, and accuracy should undergo evaluation when comparing network price and feasibility.
Frame relay pricing can vary greatly depending on the application. For example, a bundled frame relay service customer that has 19 remote locations, each with a 56-kilobit-per-second frame relay port and committed information rate (CIR) at 16 kilobits per second, will pay about $6,000 per month. The numbers come with the understanding there will be a one-year contract and a preconfigured DSU/CSU (a communication card) at each site.
Another frame relay customer that has 99 remote locations, each with a 56-kilobit-per-second frame relay port and 16-kilobit-per-second CIR over 198 PVCs used to create a meshed network architecture, would pay about $19,000 per month. This price also comes from a one-year contract. This is five times the number of remote locations as the previous example, yet 38% cheaper per location.
Frame relay cost can also vary depending on the required bandwidth. If any additional hardware is necessary to complete the network, the user must purchase it. Normally, if not provided with the service, you will need a router and CSU/DSU card, together costing about $1,600.
The typical star network contains a hub or a switch at its center.
A frame relay cloud replaces the hub in the center of the network.
Frame relay advantages
Frame relay systems for SCADA telemetry have advantages over other types of phone circuits and radio telemetry options.
- Higher speeds: Typical dedicated phone circuits for telemetry purposes have a maximum speed of 56 kilobits per second, with attainable speeds being largely dependent upon the quality of the connection. FRS can attain speeds up to T1 (1.544 megabits per second) or T3 (45 megabits per second) for U.S. and Canadian service providers. European standard FRS data transmissions can vary between 64 kilobits per second and 2 megabits per second.
- Availability of state contract circuits: Although FRS requires monthly recurring fees paid to the provider, most utilities can typically qualify for state-assisted funding for frame relay connections. Most U.S. states buy a number of FRS contracts and dole them out to utilities on a first-come, first-served basis.
- Diagnostics and troubleshooting: Because FRS is a digital system controlled by the provider, the provider can perform nearly instantaneous trouble-shooting and diagnostics on each circuit. This can help in predicting potential problems.
- Provider responsibilities: FRS providers are responsible for network security and maintenance to the point of connection to your FRAD. This keeps utilities from having to do maintenance beyond the FRAD. This would not be the case for utilities that owned their entire communication infrastructure.
- Reliability: Expect reliability as good as or exceeding that of point-to-point leased digital lines.
Shortcomings are some
Frame relay systems for SCADA telemetry also have some disadvantages with respect to other types of phone circuits and radio telemetry options.
- Maintenance competition: Some disadvantages are the same as the advantages. While the provider is responsible for maintenance, that means utilities must compete at times with other FRS subscribers for maintenance. To get around this, some providers will increase your maintenance priority for additional fees, which is not something that most utilities can bear. Additionally, you should evaluate the frame relay provider for reliability and response times for maintenance requests.
- Recurring costs: All phone circuits have an associated monthly recurring cost, which is not required for self-owned wireless systems. A present cost analysis should occur when considering which technology to deploy.
- Availability of connections: FRS requires high-quality connections for each site. This typically relates to the proximity of sites to the nearest switching office. For residentially located sites, this is not usually a problem. For remotely located sites, it can be more difficult.
- Speed versus distance: As can be the case with other phone circuits and with some radio technologies, network speed will diminish as distance between sites increases. Distance between sites correlates to multiple transmissions between provider switching sites. Each transmission adds some small amount of data latency.
Why frame relay system?
So now that you know what a frame relay system is and that it has some advantages and disadvantages, you're probably still asking yourself why you should consider using a frame relay system as opposed to other telemetry options. Following are some reasons that may apply to your utility.
Reason 1: Control system architecture with Ethernet programmable logic controllers (PLCs) and standard controllers. Given the overwhelming preponderance in the controls industry of PLCs and controllers using Ethernet for communications, most utilities are implementing Ethernet communications for in-plant PLC and controller communications. Rather than using a different PLC or controller for remote sites or using a different model communication card, it's much easier to use similar components between in-plant and remote facilities. This also makes the programming software common for both in-plant and remote PLCs and controllers.
Reason 2: High speed. Some SCADA sites only need to be checked for status every couple of minutes. Many SCADA sites also require near-instantaneous control and feedback.
These critical sites may be sites with large gate/valve control, sites that have large volume pumping requirements, or sites that have critical security concerns, among other requirements for high-speed communications. Point-to-point communication systems, including phone and radio, can provide these high-speed requirements. Most utilities, however, have more than one or two of these types of sites. Providing individual point-to-point communication systems for each site becomes prohibitively expensive. A frame relay WAN becomes the better, cost-effective solution.
Reason 3: Connectivity. A frame relay system introduces different benefits for your SCADA system. One benefit is to link remotely located treatment facilities together so you can remotely monitor and/or control one from the other. This is especially helpful in a situation with a small operation staff. An FRS can also link the treatment plant(s) to a remotely located utility office. Another opportunity that presents itself is that you can access remotely located PLCs or controllers through the programming software from any FRS network connection. This allows maintenance staff to remotely upload programming changes or to monitor the operation of the PLC or controller without having to be at the site.
In addition to these reasons, many other possibilities exist for maximizing the capabilities of a WAN by using FRS, as opposed to the typical SCADA telemetry systems.
When it all comes down to it, if your utility has multiple, scattered SCADA sites and if you desire high-speed data with maximum possibilities, then a frame relay implementation might be the best choice for your SCADA telemetry needs. In your investigations, be sure to check into the availability of state-subsidized contracts to help minimize recurring monthly costs. ?
Behind the byline
Lee Schmidt is an electrical engineer and works in instrumentation and controls. He and Jeff Miller work in the water sector at Black & Veatch Corp.