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CSMA/CD Introduction: A Teletypewriter Analogy
Perhaps the earliest demonstration of Ethernet LAN concepts occured with the implementation of teletypewriter “orderwire” circuits. These were relatively easy to construct, and were widely deployed for maintenance and troubleshooting functions between PSTN facilities up to the early 1980s.
Often, Current Loop interface technology (20/60mA) was employed, and circuits could be extended with low speed FSK modems. In such circuits, as explained later, the modem should support the use of the BREAK character (SPACING condition for a duration greater than one character length).
Now, on an Ethernet Local Area Network, an access method known as CSMA/CD is employed (as opposed to Token Ring or Token Bus). CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection. General CSMA/CD principles can be shown with the network illustrated above.
In the example above, if Station A wishes to talk to Station D, the operator will first LISTEN to make sure that nobody else is using the system. If the system is being used, the operator must wait until it is free. Station A then transmits the message information to Station D. But since this information is also made available to all other stations on the line, all station operators must continually check to see if there is traffic destined for them. If Station A detects distortion of the signal (errors), it will retransmit the message. Station operators often sent the transmission of a BREAK character (PREAMBLE) prior to message traffic; to alert other stations of inbound message traffic.
Later, teletypewriter technology advanced to a point where unique character strings could be sensed by the device, sounding an audible alarm. These unique strings essentially became ADDRESSES! Now Station A can send a message to Station D and then NOTIFY Station D by sending Station D’s unique character string. An audible alarm is generated at Station D notifying the operator, who can check the message. Also, codes are available to allow Station A to notify (alarm) all other stations on the line (BROADCAST).
Because of this notification method, operators at each teletypewriter facility can be used much more efficiently.
Additionally, speed of transmission could vary between operators and stations. Some stations and operators may output the message at a very fast rate, while other output at a lower rate. The only requirements are that the language PROTOCOL is the same between stations and that each station be able to accept the maximum transmission rate (usually determined by the type of line).
Now there are a few drawbacks to the system. First of all, each teletypewriter address must be unique to the system, or else the operator will have to service messages not destined for them. Also, distances were a concern. If stations were placed far apart, the relative time between stations increased to a point where GLARE conditions could occur. Glare is the result of two or more stations trying to access the line at the same time. All stations will see the GLARE (COLLISION) condition because the line current is interrupted (a BREAK condition). This usually results in excessive spacing conditions on the line, causing an audible “chatter” and/or garbled characters.
Even the two message originators will see the glare condition, abort (BACK OFF) the transmission, then reattempt to send the message (RETRANSMIT).
To conclude, Ethernet CSMA/CD systems operate in a similar fashion. Since the operator (from the example herein) function has been replaced with silicon, specific rules are defined for access to the line and how traffic is distinguished and sorted. These rules are discussed in other articles!