CIS 361 Data Communications and Networks

Chapter 10: Data Link Control Protocols



This chapter discusses protocols, rules for using communications circuits. The objectives important to this chapter are:

  • understanding what a protocol is
  • understanding the differences between contention and polling
  • understanding protocols that are oriented toward characters, bytes and bits
  • understanding what protocols are used on LANs
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:

  • Startup - rules about who can start communications and how to do so
  • Character identification and framing - rules to tell which bits are parts of which characters, and whether they are control characters or message characters
  • Message identification - rules for grouping characters into messages
  • Line control - rules about line turnaround and acknowledgment
  • Error control - rules for handling errors, including a break in communications
  • Termination - rules for ending communications

Three desirable attribute (characteristics) of data link protocols (from page 369):

  • Transparency - the ability to transmit control characters as message characters
  • Code independence - the ability to transmit a message in any code system (language)
  • Efficiency - more message, less overhead

On page 370, the text discusses methods of line access. These represent rules about who can transmit when.

  • Contention - each node on the system "listens" to the line and will try to use it when the line seems to be available. It is possible that the "contending" nodes will transmit at the same time and cause signal collisions. The text says it works best when either node can transmit in point to point circuits, when traffic is light in multipoint circuits, and when it does not take long to transmit the signal.
  • Polling - Several types are described:
  • Roll Call Polling - A "master" station asks each node, in turn, if they have anything to transmit
  • Fast Select Polling - The master polls several stations at time. If none respond, the next group is polled.
  • Hub Polling - The master polls a node. If the node does not want the line, the node polls the next node, and so on, reporting to the master when a node wants the line.
  • Token Passing - a token is a small character group that is passed from station to station until one of them wants to transmit. That station keeps the token until it is done transmitting its message.

    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:

    • Character-oriented protocols make use of control characters to mark header text and trailer sections of the packets.
    • Byte-count-oriented protocols count the bytes in the message block and send that count in the header.
    • Bit-oriented protocols send a stream of bits. The beginning and end of the message packet are marked, but the protocol depends on the header, text and trailer to be specific lengths.

    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:

    • Consistent or bursty transmissions - do we send often or seldom?
    • Amount of data transmissions - large or small amounts in each transmission?
    • Time sensitivity of data - how soon does it have to be transmitted?
    • Number of active devices - how many will the net support?
    Contention systems work by letting each device try to send a message on the net as needed, contending or competing with all the other devices for the bandwidth. Two examples of protocols that support such systems are CSMA/CD (Carrier Sense, Multiple Access, with Collision Detection) and CSMA/CA (Carrier Sense, Multiple Access, with Collision Avoidance). A collision occurs when two signals collide on the medium, causing signal loss. These are bursty protocols, best supporting intermittent transmissions. Only about 60 to 70 users can be supported on a segment. Time sensitivity is good, as users do not often have to wait for media access.

    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.