This chapter discusses protocols, rules for using communications circuits. The objectives important to this chapter are:
Concepts:Protocols are the rules used in communications. If a data system could not understand the protocols used by another system, there could be no communication between them.
Data link protocols are rules about the circuits. Other
protocols exist, but this chapter is about the ones that are used on the
second layer of the ISO-OSI model. Some of the tasks that protocols have
to handle are listed on page 368:
Three desirable attribute (characteristics) of data link protocols (from
On page 370, the text discusses methods of line access. These
represent rules about who can transmit when.
When sending a message, the format of the message must follow a pattern. A common pattern is shown on page 372. You see a header, the text of a message and a trailer. This general format is followed by all the protocols described in this chapter.
Mr. Rowe describes the main features of several asynchronous protocols pages 373 through 376. The various protocols described have differing packet sizes. This is the formatted block of characters that are sent between headers and trailers. They also have differing error trapping methods, such as checksum (which adds the ASCII values of all the characters in a block and divides by 255) and several variations of Cyclic Redundancy Check (CRC) (differing in the number of terms in the polynomial).
Synchronous data link protocols are described next, and the author
makes a distinction between the three concepts of character-oriented,
byte-count-oriented and bit-oriented protocols. This is not really relevant
to actual usage. Let's limit our interest to page 376:
A more useful section of the chapter begins on page 385, describing
LAN protocols. A useful way of to think about these is to consider these
as methods of getting access to the network. When an analyst is planning
what kind of network to set up, the following factors are recommended
by Novell (a major network operating system provider) for consideration:
In a CSMA/CD system (example: Ethernet), the collision is detected and the devices each wait a random number of seconds before sending again. This usually results in one device going ahead of the other. In a CSMA/CA system, devices can be assigned time slices or can be required to ask permission to send, avoiding collisions. Apple LocalTalk is an example of this.
Token-Passing involves passing a token, a small data frame, from station to station. When a station has the token, it is that station's turn to access the medium. Examples of this method are token ring, FDDI and token bus. This type of media access is predictable and consistent, allowing large or small transmissions. It is not the best for time sensitive data since waits are built in, but it will support more devices that contention.
Contention is best when the load is light, token passing is better with heavier loads, and both schemes crash under too much load.
Polling is discussed quite a bit in this chapter. You should know it exists, and that it is not used by any major network vendor. It is too slow and controlled to give the users the speed they expect and are given by other network methods.
Protocols vary from one network to another. Protocol converters are devices or software that translate from one protocol to anther. Whether the conversion is done in hardware or software, it can be done on either or both ends of a circuit.