By Dick Caro, CMC Associates
Karlsruhe, Germany – The SP100.11a standards working group met here for four full business days to make progress toward the standard. By the end of the meeting, the committee had agreed upon an architecture, selected a single application layer and formed the editorial team to actually write the standard.
While cooperation among competing suppliers seems to be working very well, there continues to be a feeling that each of the two groups—CI (Collaboration Initiative, lead by Emerson) and WNSIA (Wireless Network for Secure Industrial Applications, lead by Honeywell)—will try to gain an advantage. After last month’s user objections, the committee agreed that the Honeywell attempt to get its narrowband frequency-hopping spread spectrum physical layer adopted as an alternative would be deferred to Release 2. Meanwhile, Release 1, based on the CI recommendation using an IEEE 802.15.4-2006 radio with channel-hopping, continues to take shape.
The architecture of the SP100 network has been the subject of many heated discussions. The architecture adopted for Release 1 is a full mesh network in which all nodes may contain a routing function, but it can be disabled or omitted for simplicity or power conservation reasons. The mesh allows for a network to become physically large without installation of a higher-speed backbone, but each “hop” comes with a time delay that may prevent the use of the data from far points in fast control loops. This is why Release 1 is targeted at non-critical functions that can tolerate a latency (time lag) of 100 milliseconds. This limitation is often expressed as a control loop cycle time of about one second.
The Honeywell alternative architecture proposed at the Phoenix meeting included a wireless backbone and field-mounted SP100 access points that were interconnected using the IEEE 802.11s (still in draft stage) wireless mesh protocol. This architecture will most likely emerge in Release 2 of SP100.11a as an option. Honeywell has shown that its proposed architecture is more time-efficient than the full mesh network proposed for Release 1, and is more suited for faster control loops and even safety systems. However, it was agreed that nothing in the architecture for SP100, even in Release 1, would prevent operation with latencies as low as 10 milliseconds.
After presenting a proposal for the application layer (AL) in Phoenix, Honeywell delivered an almost complete document for consideration to the Application/Gateway Task Group in Karlsruhe. This AL is highly compatible with Foundation fieldbus, HART and Profibus, since it is based on the use of electronic device descriptions (EDD) taken from IEC 61804-3, an international standard now being balloted as ISA/ANSI 104.
SP100 AL will not specify any EDDs, but will allow the use of EDDs by applications connected through an SP100 network. All of the required AL services are included in this document, including a tunneling or encapsulation protocol for transmitting messages other than SP100. The remaining committee work is to convert the Honeywell document to the correct form for a standard. This committee also heard an alternative AL presentation from Rockwell Automation for its CIP protocol. It was generally agreed that CIP would be considered for a later release that was more oriented toward factory automation; however, CIP will be reviewed to see if any of its features are suitable for inclusion in Release 1.
The committee is planning to include a gateway function for SP100.11a. The gateway may be contained in a separate device, or it may be distributed to other network devices. The gateway is designed to translate from SP100 data streams to those of the supported networks—Foundation fieldbus, Profibus/PROFInet or HART. SP100 will not specify the translation utility, but will provide the network interfaces required so that the respective organizations can create one.
Since HART is using a wireless network totally incompatible with SP100, we cannot plan that that these signals can be communicated via the SP100.11a network in any way. All that we can do for wireless HART is to specify the gateway functions that a device could use to be compatible with the SP100 interface to the host control system.
The work of the Network/Transport Task Group has now been defined. In SP100, the mesh routing (intranet) will not be done in the network layer (NL), but is assigned to the upper data link layer. The NL will only handle routing between different SP100 network segments. The services that the transport layer (TL) will provide to applications are cyclic and acyclic reads and writes, alerting and multicast messages. The requirements for these services were obtained from the draft AL specification.
The exact protocol for the NL has not yet been formalized. Since the NL must allow routing between connected SP100 networks, it must provide routing services in the protocol stack. One such stack proposal is called 6LoPAN, which is designed to use a compressed version of Internet Protocol, version 6, over a low-powered, low-data-rate network. 6LoPAN has recently been approved by the Internet Engineering Task Force (IETF), the Internet standards body, as a draft standard for low-powered networks. A decision on the NL protocol must be made soon.
Release 1 will have only one physical layer (PHY). This is similar to the original proposal by CI group members Dust Networks and Emerson. It will operate at 2.4 GHz using direct-sequence spread spectrum with added channel-hopping. This PHY will use IEEE 802.15.4-2006 radios.
The committee discussed the process of adding new PHYs in the future. The committee regards the radio itself as the most rapidly changing technology, and the SP100.11a specification must make the integration of new radio technology as seamless as possible.
Honeywell has already made a proposal for a narrowband frequency-hopping radio, and the IEEE 802.15.4a committee has published a new low-power specification using ultra-wideband technology operating in the 3.8 GHz to 11 GHz band. These radios should be available in the next year and may offer some attractive features for use in the industrial environment. The result of these decisions is that the data link layer will be split into a lower half called the MAC (media access control) that must match the radio features, and a stable upper data link layer that provides services, including message routing, to the network, independent of the radio.
Work also continues on both the network management and security management areas. It has been agreed that for Release 1 of SP100.11a, both management functions will be considered to be centralized rather than distributed. However, the design will not preclude distribution of either or both management functions.
Part of network management is the provisioning process, which is the telecommunications word for how a new field device becomes a member of the network so that it can send and receive messages. The committee agreed that provisioning will be via a radio link and will not require a wired connection. The field device must first be given a tag name either in the shop or after it is installed. Once the device tries to gain access to the network, it will be given a network address (invisible to the user), and any configuration data previously defined for that tag name will be downloaded. This process is very similar to that used by Foundation fieldbus.
Naming the device with a numeric tag to allow membership in a wireless version of a Profibus network is also a possibility. Provisioning from either a handheld terminal or a computer in the shop will be possible if both are equipped with an SP100 interface. Part of the provisioning process is the recording of the user name, date and location of the field device.
The details of security management are not yet completed, but it is expected that all transmissions will be encrypted to prevent intrusion, interception, spoofing and sabotage. A default level of security will be used during provisioning and startup. In general, the following are the agreements by the Security Management Task Group:
- Provide hop-by-hop security at the data link layer,
- Provide end-to-end security at the transport layer,
- Use the same cryptographic technologies for data link and transport layers,
- Select cryptographic technologies supported by 802.15.4-2006 chip sets,
- AES128 Encryption and decryption,
- CCM (a mode of operation for cryptographic block ciphers),
- Message integrity checking with non-repeating nonce,
- Data link and transport security options will include:
- No security to minimize packet overhead and transceiver on-time,
- Message integrity and authentication only mode,
- Message integrity, authentication and encryption.
Undercurrents of Distrust
Despite the high degree of cooperation exhibited by the major suppliers, there continues to be an undercurrent of distrust. At this point, it seems to be unwarranted. The CI group has its most favored radio network architecture, while WNSIA has its most favored application architecture, and a process to add its desired radio network at a later time. The need to support control in field devices is also being met with the understanding that some careful site planning to can minimize latency on a mesh network enough to support one-second control loops in the first release. Rockwell and Siemens, both more interested in factory automation networks, also have the promise of participation in a future network oriented toward the high-speed acquisition and control of discrete sensors and actuators.
The SP100 committee has agreed to work together with ISA to develop a demonstration of a wireless field network at ISA 2007 in Houston. While it will be too early to include all of SP100.11a, the demonstration will show the degree of cooperation between competing suppliers to work on and achieve a successful demonstration. Members of both the CI and WNSIA group will be participating in the demonstration that is currently planned to include working wireless field instruments from several suppliers.
Analysis and Prediction
Although there continues to be some undercurrent of distrust between the CI and WNSIA groups, the committee seems to be making very serious steps toward arriving at the goal of completing a first draft standard ready for voting early in 2008. The only goal not being met is that of compatibility with wireless HART. The committee seems to still be on track to meet the goal of becoming the wireless field network for use by Foundation fieldbus and Profibus. While users have expressed their desire for SP100 to be the network path to channel wireless HART signals to the control system, lack of coordination by the wireless HART working group stands in the way.
ISA has now provided funding for SP100 committee member and IEC committee chair Pat Kinney to assume the duties of lead editor as well as committee co-chair. The goal of releasing a draft standard for SP100.11a Release 1 for committee vote by the first quarter 2008 will probably be met. This standard, once approved, will be useful for suppliers to construct interoperable field instruments and networks suitable for non-critical, real-time process industry applications.
Dick Caro is principal of CMC Associates, is a recognized expert on fieldbus technology and a founding member and Chairman of the Fieldbus standards committees. You can reach him at RCaro@CMC.us.