- Email: [email protected]
CD, or token ring?
ISATransactions 32 (1993) 193-198 Elsevier
193
Process control communications: Token bus, C S M A / C D , or token ring? John D. Wheelis
Fisher-Rosemount Systems, Austin, TX 78754, USA
This paper discusses the features of the token bus network, with special consideration given to its suitability for use as the communications backbone for process control applications. Comparisons are given between the token bus network and its chief rivals for use in process control, the token ring and the popular Carrier Sense Multiple Accesswith Collision Detection (CSMA/CD) network, commonlyimplemented as Ethernet.
Introduction Current requirements for communications in distributed control systems (DCS) include reliable transmission of data, deterministic delivery of messages under all communications media load conditions, end to end message confirmation, and message and device integrity. This paper discusses three communications network protocols and compares them for suitability for use in DCS.
Overview of the three protocols Carrier Sense Multiple Access with Collision Detection (CSMA / CD) The I E E E 802.3 network standard specifies the C S M A / C D protocol [1]. The most common implementation of this protocol is Ethernet, commonly used for local area networks of computers, workstations, terminals, and terminal servers. The cable topologies allowed by the I E E E 802.3 protocol are the linear, multi-drop, tree,
and segmented layouts (Fig. 1). The segmented topology requires the use of bridges to connect the segments. When the station wants to transmit, it listens to the network for a carrier. If the network has traffic on it, the station waits until the network is idle; otherwise it transmits immediately. If two or more stations hear the idle network and decide to transmit simultaneously, the messages of the transmitting stations collide. While each station is transmitting, it must also listen to detect a collision of messages. On detecting a collision between two or more messages, a transmitting station stops transmitting and waits a random time to retry its transmission. This random time starts as either 0 or 1 slot time; if a collision occurs again, the time to wait grows to 0, 1, 2, or 3 slot times. If yet another collision occurs, the number of slots to wait increases again as a power of 2, and repeats the process until a maximum interval of 1023 slots is reached (after 10 collisions). The idea of the growing interval is to keep the delay time short under low network load, but allow it to reach a feasible delay time under increasingly heavy loads.
Advantages Correspondence to: John D. Wheelis, Senior Product Applications Engineering, Fisher-Rosemount Systems,8301 Cameron Rd., Austin, TX 78754, USA.
The advantages of the C S M A / C D protocol include a simple algorithm for operation of the network and almost no delays in a station's access
0019-0578/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
194
J.D. Wheelis / Process control communications
A
B A
~"-
(
C
B D
)
(
C
T D
E
a) Linear
b) Multi-drop
A
B
o
o
E
O.---
d) Segmented
c) Tree Fig. 1. CSMA/CD network topology.
time to the network at low network load. This is due to low media access overhead. No communication bandwidth is used to gain access to the network; if a station is ready to transmit and the network is silent, the station can send its message.
Disadvantages The disadvantages of the protocol include a limited cable length (2.5 km with repeaters). The round trip cable length determines the slot time of the network, which directly impacts the performance. Priority levels of messages are not supported, so in a short-term burst of data transmissions, the protocol cannot grant access to the highway in prioritized fashion. The media access time is nondeterministic; this can cause a station that needs to transmit to wait an undetermined amount of time before it is able to send a message. Since data for alarms, interlocks, and control values cannot be guaranteed to arrive at the proper destination in a timely fashion, the network is inappropriate for real-time work. The worst case delay in media access time is unbounded. The 802.3 protocol can
only support type 2 connectionless or type 1 connected messages of the I E E E 802.2 data link layer; it does not support the type 3 Send Data With Acknowledge message. The C S M A / C D protocol does not offer a low-level acknowledgement of transmitted messages. Therefore, if a sending station sends a message, and does not sense a collision during transmission, it assumes that the receiver has received the message with no errors. Each station has to be able to detect a signal from the weakest station, even while transmitting a message. This forces the collision detection circuitry to be analog technology, which is more complex than an all-digital implementation. The minimum valid frame size is 64 bytes, because of the need to detect a collision between frames. This minimum frame size represents substantial overhead for short data lengths. The network efficiency drops as the network speed increases, and the network is not well suited to fiber optics due to difficulty of installing taps on the network cable. At high network load, message collisions are a major problem because they affect data through-
J.D. Wheel& / Processcontrolcommunications put greatly. Even under short-term burst conditions, a station's message can run into a series of collisions, and the station can discard the message after a number of attempts. This means the network can lose a message without ever transmitting it. Token ring The token ring ( I E E E 802.5) [2] consists of a true physical ring, and not a multi-drop configuration (Fig 2). The ring is actually a collection of point-to-point links that happen to form a circle. With no traffic on the network, a three-byte frame, the token, circulates around the devices on the network. When a station needs to communicate, it seizes the token and sets a specific bit in the second byte of the token to a 1. This converts the first two bytes of the token into the start of frame sequence. The station then adds its data into the frame and continues to transmit the frame's bits. Here the concept of the physical length of a bit needs to be discussed, in order to understand the impact on the way the protocol operates. If the data rate of the ring is R Mbps, then a bit is transmitted every 1 / R microseconds. With
Fig. 2. Token ring network topology.
195
the typical propagation speed on the cable of 200 meters/microsecond, each bit then has a length of 2 0 0 / R meters on the ring. Therefore, on a 1000 meter ring with a 1 Mbps data rate, the ring can contain only five bits on it at a time. Since the data bits are transmitted point to point from station to station on the ring, each station has to copy a bit it receives into a one-bit buffer, then copy it back out onto the ring. This means that each station introduces a one-bit delay. The first bit of a frame being transmitted normally goes around the ring and returns to the sender before a full frame has been transmitted. This requires that the station drain the ring of the incoming bits of the frame while it continues to transmit the remaining bits of the frame. Advantages The token ring uses a point-to-point connection, so engineering is easy and fully digital. Rings can be built using almost any transmission medium, from twisted pair to fiber optics. In an all-fiber implementation, the physical topology of the ring matches well to the characteristics of the fiber optics, which of necessity are point-to-point. The all-fiber system is also much less sensitive to electromagnetic interference. Use of wire centers, which can switch a station in or out of the ring, can enable the token ring to automatically detect and eliminate cable failures. Other advantages include: creation of priorities for messages are possible; short data frames are allowed, as are arbitrarily long frames; and the token ring allows the longest network lengths of the three protocols. Disadvantages Because of the physical implementation of the ring, a break in the cable would bring the whole network down. The ring is also a poor fit to most processes and assembly lines found in manufacturing operations. The ring requires a centralized monitor, and although a dead monitor can be replaced on-line, a sick monitor can cause extreme problems. Some delay in gaining access to the ring is encountered at lo W network load because the station has to wait for the token.
196
J.D. Wheelis / Process control communications
A
B
)-c
(
A B
D
)
(
C
D
E
a) Linear
b) Multi-drop
A
B
E
O
O
0-.-..--
c) Tree
d) Segmented Fig. 3. Token bus network topology.
Token bus
The token bus protocol ( I E E E 802.4) [3] allows a linear, multi-drop, tree-shaped, or segmented topology (Fig. 3), and uses 75 ohm coaxial cable as the communications media. The standard permits three different modulation schemes: (1) phase continuous Frequency Shift Keying (FSK); (2) phase coherent FSK; (3) multilevel duobinary amplitude-modulated phase shift keying.
The stations on the token bus are arranged logically into a ring, and each station knows the address of its predecessor and its successor. During operation of the network, the station with the token transmits data frames until either it runs out of data frames to transmit, or the time it has held the token exceeds the token holding time. Then the station regenerates the three-byte token and transmits it to its logical successor on the network. The physical location of the successor is
Oeice t
Device 2
/I
t /
I J
I I
)
Device 1
/ I
1
/
Device 4 Fig. 4. Token passing.
Device 3
Device 6
J.D. Wheelis / Processcontrol communications
not important, as shown in Fig. 4; the token is sent to the logical neighbor. In this manner, the token propagates around the logical ring. Collisions of data frames do not occur, as only one station at a time can transmit. The rules of the protocol also guarantee that the time to access the network media from the last time the station had access to the media does not exceed a predetermined maximum time. The protocol has provisions to regenerate the token if the token holder stops transmitting and does not pass the token on to its successor. Stations can also be added dynamically to the ring, and can ask to be dropped from the logical ring. Priority levels for messages are also allowed. The protocol defines four priority classes (0, 2, 4, and 6) for the messages, with a 0 denoting the lowest priority and the 6 denoting the highest priority. When the token is passed to a station, the station can begin transmitting priority 6 data frames. When it is finished sending the priority 6 frames, it can begin sending the priority 4 frames, then priority 2, then priority 0 frames, until it has either sent all of the frames it has to send, or its times for the token holding time expires. Advantages
The token bus protocol can support message priorities, which ensures that a station's high-priority messages, such as process alarms, interlock data, and critical process variables, can be given access to the network first. This allows the stations to use the available bandwidth of the network for low-priority traffic without affecting access for high-priority messages. The protocol is also deterministic, providing a station access to the network within a predetermined maximum time since the station last had access. The 802.4 protocol has excellent throughput and efficiency at high loads, can handle short data frames, and can dynamically add devices to the network. The token bus supports type 3 connectionless messages, allowing the receiving station to immediately acknowledge receipt of a Send Data with Acknowledge message, providing true end-to-end confirmation of the delivery of a data frame. The confirmed delivery of messages is a requirement established by the Instrument Society of America
197
process communication standards, and is recognized by the Manufacturing Automation Protocol (MAP) in its communication standards.
Disadvantages
The token bus is a complex protocol, which can raise the cost of the communications equipment. Similar to the C S M A / C D protocol, the token bus uses a substantial amount of analog circuitry. It also is not as well suited to use of fiber optics as the token ring. A large number of nodes ( > 100) in one logical ring tends to use a large percentage of the bandwidth in passing the token between stations. For process control, a token bus should probably be organized into networks of 64 nodes or less, with bridges used to connect local areas.
Comparison Under low to medium network loading, C S M A / C D has a distinct advantage, because it uses no media overhead to determine which device has access to the network. Testing by NBS [4] shows that Ethernet offers superior performance to token bus at highway loads less than 40% of network capacity. For highway loads above 40%, the response time grows exponentially, and quickly becomes significantly worse than the token bus protocol. DCS vendors cannot guarantee that the system communications loading will remain under 40% under plant upset conditions, and since an overloaded C S M A / C D network can collapse totally, it is not appropriate for process control applications. In comparing the two token passing schemes, the token ring wins marks for the capability to more easily use fiber optics to extend network distances and its ability to use many network media in its implementations. The token bus, on the other hand, gains ground because its message priority scheme is more fair than the token ring's scheme, devices can be dynamically added to the bus, and end-to-end confirmation of messages is available.
198
J.D. Wheelis / Process control communications
Conclusion Of the three network protocols discussed in this paper, the token bus is more suited to process control communication applications. The C S M A / C D protocol is well suited for standard computer network applications, where the network loading rarely exceeds 10-15%, and if data frames are lost or delayed a considerable time, the upper layers of the O p e n System Interconnection (OSI) model can determine that frames were lost, and correct for them, For process control applications, the requirement is to not lose data at any time, and to ensure the data is trans-
mitted in a deterministic and timely manner. The token bus best satisfies these requirements.
References [1] [2] [3] [4]
IEEE 802.3, ISO/DIS 8802/3, 1985. IEEE 802.5, ISO/DIS 8802/5, 1985. IEEE 802.4, ISO/DIS 8802/4, 1985. K. Mills, M. Wheatley and S. Heatley, "Prediction of transport protocol performance through simulation", Comput. Commun. Rev. 16(3) (1986) 75-83. [5] Andrew S. Tanenbaum, Computer Networks, Prentice-Hall, Englewood, Cliffs, NJ, 2nd edn., 1989.
193
Process control communications: Token bus, C S M A / C D , or token ring? John D. Wheelis
Fisher-Rosemount Systems, Austin, TX 78754, USA
This paper discusses the features of the token bus network, with special consideration given to its suitability for use as the communications backbone for process control applications. Comparisons are given between the token bus network and its chief rivals for use in process control, the token ring and the popular Carrier Sense Multiple Accesswith Collision Detection (CSMA/CD) network, commonlyimplemented as Ethernet.
Introduction Current requirements for communications in distributed control systems (DCS) include reliable transmission of data, deterministic delivery of messages under all communications media load conditions, end to end message confirmation, and message and device integrity. This paper discusses three communications network protocols and compares them for suitability for use in DCS.
Overview of the three protocols Carrier Sense Multiple Access with Collision Detection (CSMA / CD) The I E E E 802.3 network standard specifies the C S M A / C D protocol [1]. The most common implementation of this protocol is Ethernet, commonly used for local area networks of computers, workstations, terminals, and terminal servers. The cable topologies allowed by the I E E E 802.3 protocol are the linear, multi-drop, tree,
and segmented layouts (Fig. 1). The segmented topology requires the use of bridges to connect the segments. When the station wants to transmit, it listens to the network for a carrier. If the network has traffic on it, the station waits until the network is idle; otherwise it transmits immediately. If two or more stations hear the idle network and decide to transmit simultaneously, the messages of the transmitting stations collide. While each station is transmitting, it must also listen to detect a collision of messages. On detecting a collision between two or more messages, a transmitting station stops transmitting and waits a random time to retry its transmission. This random time starts as either 0 or 1 slot time; if a collision occurs again, the time to wait grows to 0, 1, 2, or 3 slot times. If yet another collision occurs, the number of slots to wait increases again as a power of 2, and repeats the process until a maximum interval of 1023 slots is reached (after 10 collisions). The idea of the growing interval is to keep the delay time short under low network load, but allow it to reach a feasible delay time under increasingly heavy loads.
Advantages Correspondence to: John D. Wheelis, Senior Product Applications Engineering, Fisher-Rosemount Systems,8301 Cameron Rd., Austin, TX 78754, USA.
The advantages of the C S M A / C D protocol include a simple algorithm for operation of the network and almost no delays in a station's access
0019-0578/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
194
J.D. Wheelis / Process control communications
A
B A
~"-
(
C
B D
)
(
C
T D
E
a) Linear
b) Multi-drop
A
B
o
o
E
O.---
d) Segmented
c) Tree Fig. 1. CSMA/CD network topology.
time to the network at low network load. This is due to low media access overhead. No communication bandwidth is used to gain access to the network; if a station is ready to transmit and the network is silent, the station can send its message.
Disadvantages The disadvantages of the protocol include a limited cable length (2.5 km with repeaters). The round trip cable length determines the slot time of the network, which directly impacts the performance. Priority levels of messages are not supported, so in a short-term burst of data transmissions, the protocol cannot grant access to the highway in prioritized fashion. The media access time is nondeterministic; this can cause a station that needs to transmit to wait an undetermined amount of time before it is able to send a message. Since data for alarms, interlocks, and control values cannot be guaranteed to arrive at the proper destination in a timely fashion, the network is inappropriate for real-time work. The worst case delay in media access time is unbounded. The 802.3 protocol can
only support type 2 connectionless or type 1 connected messages of the I E E E 802.2 data link layer; it does not support the type 3 Send Data With Acknowledge message. The C S M A / C D protocol does not offer a low-level acknowledgement of transmitted messages. Therefore, if a sending station sends a message, and does not sense a collision during transmission, it assumes that the receiver has received the message with no errors. Each station has to be able to detect a signal from the weakest station, even while transmitting a message. This forces the collision detection circuitry to be analog technology, which is more complex than an all-digital implementation. The minimum valid frame size is 64 bytes, because of the need to detect a collision between frames. This minimum frame size represents substantial overhead for short data lengths. The network efficiency drops as the network speed increases, and the network is not well suited to fiber optics due to difficulty of installing taps on the network cable. At high network load, message collisions are a major problem because they affect data through-
J.D. Wheel& / Processcontrolcommunications put greatly. Even under short-term burst conditions, a station's message can run into a series of collisions, and the station can discard the message after a number of attempts. This means the network can lose a message without ever transmitting it. Token ring The token ring ( I E E E 802.5) [2] consists of a true physical ring, and not a multi-drop configuration (Fig 2). The ring is actually a collection of point-to-point links that happen to form a circle. With no traffic on the network, a three-byte frame, the token, circulates around the devices on the network. When a station needs to communicate, it seizes the token and sets a specific bit in the second byte of the token to a 1. This converts the first two bytes of the token into the start of frame sequence. The station then adds its data into the frame and continues to transmit the frame's bits. Here the concept of the physical length of a bit needs to be discussed, in order to understand the impact on the way the protocol operates. If the data rate of the ring is R Mbps, then a bit is transmitted every 1 / R microseconds. With
Fig. 2. Token ring network topology.
195
the typical propagation speed on the cable of 200 meters/microsecond, each bit then has a length of 2 0 0 / R meters on the ring. Therefore, on a 1000 meter ring with a 1 Mbps data rate, the ring can contain only five bits on it at a time. Since the data bits are transmitted point to point from station to station on the ring, each station has to copy a bit it receives into a one-bit buffer, then copy it back out onto the ring. This means that each station introduces a one-bit delay. The first bit of a frame being transmitted normally goes around the ring and returns to the sender before a full frame has been transmitted. This requires that the station drain the ring of the incoming bits of the frame while it continues to transmit the remaining bits of the frame. Advantages The token ring uses a point-to-point connection, so engineering is easy and fully digital. Rings can be built using almost any transmission medium, from twisted pair to fiber optics. In an all-fiber implementation, the physical topology of the ring matches well to the characteristics of the fiber optics, which of necessity are point-to-point. The all-fiber system is also much less sensitive to electromagnetic interference. Use of wire centers, which can switch a station in or out of the ring, can enable the token ring to automatically detect and eliminate cable failures. Other advantages include: creation of priorities for messages are possible; short data frames are allowed, as are arbitrarily long frames; and the token ring allows the longest network lengths of the three protocols. Disadvantages Because of the physical implementation of the ring, a break in the cable would bring the whole network down. The ring is also a poor fit to most processes and assembly lines found in manufacturing operations. The ring requires a centralized monitor, and although a dead monitor can be replaced on-line, a sick monitor can cause extreme problems. Some delay in gaining access to the ring is encountered at lo W network load because the station has to wait for the token.
196
J.D. Wheelis / Process control communications
A
B
)-c
(
A B
D
)
(
C
D
E
a) Linear
b) Multi-drop
A
B
E
O
O
0-.-..--
c) Tree
d) Segmented Fig. 3. Token bus network topology.
Token bus
The token bus protocol ( I E E E 802.4) [3] allows a linear, multi-drop, tree-shaped, or segmented topology (Fig. 3), and uses 75 ohm coaxial cable as the communications media. The standard permits three different modulation schemes: (1) phase continuous Frequency Shift Keying (FSK); (2) phase coherent FSK; (3) multilevel duobinary amplitude-modulated phase shift keying.
The stations on the token bus are arranged logically into a ring, and each station knows the address of its predecessor and its successor. During operation of the network, the station with the token transmits data frames until either it runs out of data frames to transmit, or the time it has held the token exceeds the token holding time. Then the station regenerates the three-byte token and transmits it to its logical successor on the network. The physical location of the successor is
Oeice t
Device 2
/I
t /
I J
I I
)
Device 1
/ I
1
/
Device 4 Fig. 4. Token passing.
Device 3
Device 6
J.D. Wheelis / Processcontrol communications
not important, as shown in Fig. 4; the token is sent to the logical neighbor. In this manner, the token propagates around the logical ring. Collisions of data frames do not occur, as only one station at a time can transmit. The rules of the protocol also guarantee that the time to access the network media from the last time the station had access to the media does not exceed a predetermined maximum time. The protocol has provisions to regenerate the token if the token holder stops transmitting and does not pass the token on to its successor. Stations can also be added dynamically to the ring, and can ask to be dropped from the logical ring. Priority levels for messages are also allowed. The protocol defines four priority classes (0, 2, 4, and 6) for the messages, with a 0 denoting the lowest priority and the 6 denoting the highest priority. When the token is passed to a station, the station can begin transmitting priority 6 data frames. When it is finished sending the priority 6 frames, it can begin sending the priority 4 frames, then priority 2, then priority 0 frames, until it has either sent all of the frames it has to send, or its times for the token holding time expires. Advantages
The token bus protocol can support message priorities, which ensures that a station's high-priority messages, such as process alarms, interlock data, and critical process variables, can be given access to the network first. This allows the stations to use the available bandwidth of the network for low-priority traffic without affecting access for high-priority messages. The protocol is also deterministic, providing a station access to the network within a predetermined maximum time since the station last had access. The 802.4 protocol has excellent throughput and efficiency at high loads, can handle short data frames, and can dynamically add devices to the network. The token bus supports type 3 connectionless messages, allowing the receiving station to immediately acknowledge receipt of a Send Data with Acknowledge message, providing true end-to-end confirmation of the delivery of a data frame. The confirmed delivery of messages is a requirement established by the Instrument Society of America
197
process communication standards, and is recognized by the Manufacturing Automation Protocol (MAP) in its communication standards.
Disadvantages
The token bus is a complex protocol, which can raise the cost of the communications equipment. Similar to the C S M A / C D protocol, the token bus uses a substantial amount of analog circuitry. It also is not as well suited to use of fiber optics as the token ring. A large number of nodes ( > 100) in one logical ring tends to use a large percentage of the bandwidth in passing the token between stations. For process control, a token bus should probably be organized into networks of 64 nodes or less, with bridges used to connect local areas.
Comparison Under low to medium network loading, C S M A / C D has a distinct advantage, because it uses no media overhead to determine which device has access to the network. Testing by NBS [4] shows that Ethernet offers superior performance to token bus at highway loads less than 40% of network capacity. For highway loads above 40%, the response time grows exponentially, and quickly becomes significantly worse than the token bus protocol. DCS vendors cannot guarantee that the system communications loading will remain under 40% under plant upset conditions, and since an overloaded C S M A / C D network can collapse totally, it is not appropriate for process control applications. In comparing the two token passing schemes, the token ring wins marks for the capability to more easily use fiber optics to extend network distances and its ability to use many network media in its implementations. The token bus, on the other hand, gains ground because its message priority scheme is more fair than the token ring's scheme, devices can be dynamically added to the bus, and end-to-end confirmation of messages is available.
198
J.D. Wheelis / Process control communications
Conclusion Of the three network protocols discussed in this paper, the token bus is more suited to process control communication applications. The C S M A / C D protocol is well suited for standard computer network applications, where the network loading rarely exceeds 10-15%, and if data frames are lost or delayed a considerable time, the upper layers of the O p e n System Interconnection (OSI) model can determine that frames were lost, and correct for them, For process control applications, the requirement is to not lose data at any time, and to ensure the data is trans-
mitted in a deterministic and timely manner. The token bus best satisfies these requirements.
References [1] [2] [3] [4]
IEEE 802.3, ISO/DIS 8802/3, 1985. IEEE 802.5, ISO/DIS 8802/5, 1985. IEEE 802.4, ISO/DIS 8802/4, 1985. K. Mills, M. Wheatley and S. Heatley, "Prediction of transport protocol performance through simulation", Comput. Commun. Rev. 16(3) (1986) 75-83. [5] Andrew S. Tanenbaum, Computer Networks, Prentice-Hall, Englewood, Cliffs, NJ, 2nd edn., 1989.