Data Link Layer Concepts
and IEEE Standards
In this chapter, we discuss the second layer of the OSI model, namely, the data link layer.
This layer handles the transfer of data between the ends of a physical link—it is responsi-
ble for transferring data from the network layer on the source machine to the network
layer on the destination machine. We begin the chapter with an overview of the data link
layer and provide information about IEEE, which developed data link layer protocols that
serve as the basis for various LAN technologies such as Ethernet and token ring. In the
remaining part of the chapter, we examine the data link layer from IEEE’s perspective. We
also include at the end of the chapter information about the concepts of data prioritization
and quality of service (QoS), which are integral to the delivery of time-sensitive data such
as real-time voice and video trafﬁc. An outline of the terms and concepts we deﬁne and
• Data Link Layer Overview and IEEE’s Perspective (Questions 1–6)
• Framing (Questions 7–11)
• Ethernet/802.3 Frames (Questions 12–15, 44–45)
• Flow Control and Flow Control Protocols (Questions 16–27)
• Error Control (Questions 28–43)
• MAC Sublayer (Questions 46–52)
• Random Access and Token Passing Protocols (Questions 53–58)
• Data Prioritization and Quality of Service (QoS) (Questions 59–69)
1. What is the data link layer?
In network architecture, the purpose of the data link layer is to regulate and format
transmission of data from software on a node to the network cabling facilities. The data
link layer is the “glue” between the wire and the software on a node. Without it, the partic-
ular network connection will not operate at all. The data link layer creates the network
environment for the wire and dictates data formats, timing, bit sequencing, and many other
activities for each particular type of network.
182 Networking Explained, Second Edition
As the second layer of the OSI reference model, the data link layer provides a service to
the network layer (layer 3) using the services of the physical layer (layer 1). Some of the
services the data link layer provides to the network layer include:
• provisioning links between network entities (generally these are adjacent nodes
within a subnetwork);
• framing, which involves partitioning data into frames with recognized frame
boundaries and exchanging these frames over the link;
• frame sequencing (if required), which involves maintaining the correct ordering
of frames as they are being exchanged;
• establishing and maintaining an acceptable level of flow control as frames are
being exchanged across a link;
• detecting (and possibly correcting) errors in the physical layer, which includes
error notification when errors are detected but not corrected;
• selecting quality of services (QoS) parameters associated with a specific trans-
mission, including ensuring that sufficient bandwidth is available and that
transmission delays (i.e., latency) are predictable and guaranteed.
In short, the data link layer enables data frames to be transmitted error-free between two
end nodes over the physical layer.
2. What is the IEEE and what does it have to do with local area networks?
In the early days of local area network development, there were no standards for LANs.
Chaos and instability were the order of the day. Proprietary vendor standards ruled, cus-
tomers became customers for life, and companies got fat. The dearth of industrywide stan-
dards effectively prevented customers from using “outside” products for fear of
incompatibility. In February 1980, the IEEE (pronounced “eye triple E”) assumed respon-
sibility for setting LAN standards, primarily for the physical and data link layers, using the
OSI reference model as a framework. IEEE, which stands for the Institute of Electrical and
Electronics Engineers, is a professional society founded in 1963. IEEE members include
engineers, scientists, and students. One of its many activities is to act as a coordinating
body for computing and communication standards. Many international standards from the
International Organization for Standardization (ISO) and the International Electrotechni-
cal Commission (IEC) are based on IEEE networking standards. For additional informa-
tion about these organizations, see http://www.ieee.org; http://www.iso.ch; and http://
www.iec.ch. IEEE conducted its LAN standards development under the auspices of the
IEEE Computer Society. A list of these standards is provided in Table 5.1, with corre-
sponding ISO/IEC standards given in parentheses.
3. Looking at Table 5.1, I notice that all of the IEEE standards start with 802. Why?
IEEE’s development of LAN standards was assigned the project number 802, for Feb-
ruary 1980 (get it? 2/80 or 802), and the committee’s collective body of work has become
known as Project 802. The standards that have resulted are identiﬁed as IEEE 802.x.