Network Protocols in Digital Communication:
“Intelligent” (microprocessor-controlled) switching systems have become the hubs of intelligent networks. Terminal devices and line connection equipment have also been given microprocessor “brains,” and thus the introduction of intelligent devices into the data communications field has brought a sophistication to the interconnection possibilities. With terminals capable of establishing circuit connections and communicating with computers and other sites, the need for rules governing the interchange of data became essential. These rules, developed over a number of years, fall into several categories. Procedures were needed to define interchanges between computers and remote sites. These rules, or protocols, were called “handshaking.” As the systems grew, procedures became necessary to determine standard methods of communicating within data channels, and so protocols for integration of control signals with data in standard formats and sequences were developed. Also, the expansion of network complexity permitted numerous stations access to transmission circuits. To prevent interference between users, Network Protocols in Digital Communication were devised which established communications priorities and control sequences to be used to initiate and terminate switched interconnections.
Data communications Network Protocols in Digital Communication typically have three phases: establishment, message transfer and termination. The contents of these phases differ for different system arrangements and equipment types. In point-to-point systems which involve a master station and one or more slave stations, the flow of data is determined by the master station. The master station has direct control of each slave station. It establishes the connection, controls the transfer of data and terminates the connection.
Systems which interconnect several stations on a shared basis can use either Polling Protocol or Contention Protocol. In polling systems, one station is designated the master station, and queries, or polls, the other stations to determine which interconnections are to be established. This type of polling is referred to as roll calling. The master station remains at the center of the system. It polls each remote station in turn, retains control of the circuit and directs the other stations to send or receive data as required.
Contention systems do not designate a master station, Instead, the interconnected stations contend for the role of master station. Whichever station seizes control of the communication channel first directs the flow of data until it terminates the communication. The channel will remain vacant until the next station with data to transmit seizes the line and establishes communication. The Network Protocols in Digital Communication must provide for instances of simultaneous line seizure attempts by several stations as well as establishing priority schemes among the users.
Switched or dial-up systems must have protocols which direct the establishment of communication via dial-in requests. These systems are very popular and often involve the use of automatic circuits at both send and receive stations to effect the dial-up interconnection. This requires that Network Protocols in Digital Communication be standardized so that equipment from different systems can communicate without intervention.
Some networks interconnect the stations in the form of a loop, with each station connecting to the next station. Data to be transferred to a station around the loop must pass through each intermediate station. The loop arrangement has the benefit of reducing the number and length of data circuits required as compared to a central master station network. Protocols for the loop system must provide for data direction and system control. Polling can be used in loop systems. When used, it is referred to as forward polling, in that each station polls the next station in line.
It was indicated earlier that protocols must provide for integration of control characters within the data stream. Control characters are indicated by specific bit patterns, but it is possible that these patterns could accidentally occur in the data stream at places where control characters are not intended. This is particularly true when the data represents digitization of an analog function or some similar situation in which the data is not alphanumeric in nature. To prevent this problem, a data transmission protocol called Character Insertion (also referred to as character stuffing) is sometimes used. Under this protocol, the transmitting equipment checks the data stream as it is transmitting, to determine whether character patterns identical to control characters exist in the data. If these patterns are encountered, the control character pattern is inserted into the data stream after the data pattern. The result is to have the control character pattern occur twice. At the receive site, the data is evaluated two characters at a time. If a control character is detected, the receiver checks the following character to see whether it duplicates the control character. If it does, the control character pattern is recognized as false, and the second character is removed from the data stream. If the pattern occurs only once, it is a valid control character, and the appropriate action is taken. This method of control character recognition is called Transparency.