Computer network concepts and principles - arcnet.

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Mungai Andrew Gichuho

(JULY 2003)


Datapoint Corporation introduced ARCNET, classified as a local area network (LAN), as office automation in the late 1970s. ARCNET is coined from two words i.e. ARC & NET.  ARC in full is attached resource computer and NET is from the word network.  So ARCNET in full means attached resources computer network.

ARCNET’s use as an office automation network has diminished but continues to find success in the industrial automation industry because its performance characteristics are well suited for control.  It is very robust, fast and provides deterministic performance. In addition, it can span long distances thus making it a suitable fieldbus technology. The term fieldbus is used in the industrial automation industry to signify a network consisting of computers, controllers and devices mounted in the “field”.  ARCNET is an ideal fieldbus that delivers a message in predictable time fashion because of its token-passing protocol that it uses.

 ARCNET packet lengths vary from 0 to 507 bytes with little overhead and high data rate, typically 2.5 Mbps, yields quick responsiveness to short messages needed by feldbus.   It has a built-in CRC-16 (cyclic redundancy check) error checking and supports several physical cabling schemes including fiber optics.  The data link protocol is self-contained in the ARCNET controller chip.  Network functions such as error checking, flow control and network configuration are done automatically without software intervention.  In terms of the International Organization of Standards OSI (Open Systems Interconnect) Reference Model, ARCNET provides the Physical and Data Link layers of this model. In other words, ARCNET provides for the successful transmission and reception of a data packet between two network nodes. A node refers to an ARCNET controller chip and cable transceiver connected to the network. Nodes are assigned addresses called MAC (medium access control) IDs and one ARCNET network can have up to 255 uniquely assigned nodes.

Deterministic Performance

The key to ARCNET’s performance and its attractiveness as a control network is its token-passing protocol. In a token-passing network, a node can only send a message when it receives the “token.” When a node receives the token it becomes the momentary master of the network for very short time before passing it to next node.  The length of the message that can be sent is limited and, therefore, no one node can dominate the network before it relinquishes control of the token. Once the message is sent, the token is passed to another node allowing it to become the momentary master. By using token passing as the mechanism for mediating access of the network by any one node, the time performance of the network becomes predictable or deterministic.  Industrial networks require predictable performance to ensure that controlled events occur when they must and ARCNET provides this predictability.

Logical Ring

A token (ITT—Invitation to Transmit) is a unique signaling sequence that is passed in an orderly fashion among all the active nodes in the network. When a particular node receives the token, it has the sole right to initiate a transmission sequence by passing it to its logical neighbour. This neighbour, physically located anywhere on the network, has the next highest address to the node with the token.  This token-passing sequence continues in a logical ring fashion serving all nodes equally. Node addresses must be unique and can range from 0 to 255 and  are independently assigned with regardless of the physical location on the network.   Address 0  is reserved for broadcast messages.

Directed Messages

In a transmission sequence, the node with the token becomes the source node and any other node selected by the source node for communication becomes the destination node.  The selection of the destination node is done by away of the source node sending out a Free Buffer Enquiry (FBE) to find if the destination node is in a position to accept. The destination node responds by returning an Acknowledgement (ACK) meaning that a buffer is available or by returning a Negative Acknowledgement (NAK) meaning that no buffer is available. Upon an ACK, the source node sends out a data transmission (PAC) with either 0 to 507 bytes of data (PAC). If the destination node as evidenced by a successful CRC test properly received the data, the destination node sends another ACK. If the transmission was unsuccessful, the destination node does nothing, causing the source node to timeout. The source node will, therefore, infer that the transmission failed and will retry after it receives the token on the next token pass. The transmission sequence terminates and the token is passed to the next node. If the desired message exceeds 507 bytes, the message is sent as a series of packets—one packet every token pass. This is called a fragmented message. The packets are recombined at the destination end to form the entire message.

Broadcast Messages

ARCNET’s broadcast messages can be sent to all nodes with one transmission by specifying Node 0 as the destination address.  Nodes that have been enabled to receive broadcast messages will receive such messages. Node 0 does not exist on the network and is reserved for this broadcast function.  No ACKs or NAKs are sent during a broadcast message making broadcast messaging fast.

Automatic Reconfigurations

 ARCNET reconfigures the network automatically if a node is either added or deleted from the network.  Any new node joining the network has to jam the network with a reconfiguration burst that destroys the token-passing sequence.  Consequently the token is lost; all nodes cease transmitting and begin a timeout sequence based upon their own node address.  The node with the highest address will timeout first and begin a token pass sequence to the node with the next highest address.  If that node does not respond, it is assumed not to exist. The destination node address is incremented and the token resent. This sequence is repeated until a node responds. At that time, the token is released to the responding node and the address of the responding node is noted as the logical neighbor of the originating node. All nodes repeat the sequence until each node learns its logical neighbor. At that time the token passes from neighbor to neighbor without wasting time on absent addresses. If a node leaves the network the reconfiguration sequence is slightly different. When a node releases the token to its logical neighbor, it continues to monitor network activity to ensure that the logical neighbor responded with either a token pass or a start of a transmission sequence. If no activity was sensed, the node that passed the token infers that its logical neighbor has left the network and immediately begins a search for a new logical neighbor by incrementing the node address of its logical neighbor and initiating a token pass. Network activity is again monitored and the incrementing process and resending of the token continues until a new logical neighbor is found. Once found, the network returns to the normal logical ring routine of passing tokens to logical neighbors.  With ARCNET, reconfiguration of the network is automatic and quick without any software intervention.

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Unmatched Cabling Options

ARCNET is the most flexibly cabled network since it can support bus, star and distributed star topologies. In a bus topology, all nodes are connected to the same cable. The star topology requires either passive or active a hub to concentrate the cables from each of the nodes. The distributed star (all nodes connect to an active hub with all hubs cascaded together) offers the greatest flexibility and allows the network to extend to greater than four miles (6.7 km) without the use of extended timeouts. Media support includes coaxial, twisted-pair and glass fiber optics.


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