CCNA Exploration - Network Fundamentals

7 Data Link Layer

7.0 Chapter Introduction

7.0.1 Chapter Introduction Page 1:

To support our communication, the OSI model divides the functions of a data network into layers.

To recap:

• The Application layer provides the interface to the user.

• The Transport layer is responsible for dividing and managing communications between the processes running in the two end systems.

• The Network layer protocols organize our communication data so that it can travel across internetworks from the originating host to a destination host.

For Network layer packets to be transported from source host to destination host, they must traverse different physical networks. These physical networks can consist of different types of physical media such as copper wires, microwaves, optical fibers, and satellite links. Network layer packets do not have a way to directly access these different media.

It is the role of the OSI Data Link layer to prepare Network layer packets for transmission and to control access to the physical media.

This chapter introduces the general functions of the Data Link layer and the protocols associated with it.

Learning Objectives

Upon completion of this chapter, you will be able to:

• Explain the role of Data Link layer protocols in data transmission.

• Describe how the Data Link layer prepares data for transmission on network media.

• Describe the different types of media access control methods.

• Identify several common logical network topologies and describe how the logical topology determines the media access control method for that network.

• Explain the purpose of encapsulating packets into frames to facilitate media access.

• Describe the Layer 2 frame structure and identify generic fields.

• Explain the role of key frame header and trailer fields, including addressing, QoS, type of protocol, and Frame Check Sequence.

7.1 Data Link Layer - Accessing the Media

7.1.1 Data Link Layer - Supporting & Connecting to Upper Layer Services Page 1:

The Data Link layer provides a means for exchanging data over a common local media.

The Data Link layer performs two basic services:

• Allows the upper layers to access the media using techniques such as framing

• Controls how data is placed onto the media and is received from the media using techniques such as media access control and error detection

As with each of the OSI layers, there are terms specific to this layer:

Frame - The Data Link layer PDU

Node - The Layer 2 notation for network devices connected to a common medium

Media/medium (physical)* - The physical means for the transfer of information between two nodes

Network (physical)** - Two or more nodes connected to a common medium

The Data Link layer is responsible for the exchange of frames between nodes over the media of a physical network.

* It is important to understand the meaning of the words medium and media within the context of this chapter. Here, these words refer to the material that actually carries the signals representing the transmitted data. Media is the physical copper cable, optical fiber, or atmosphere through which the signals travel. In this chapter media does not refer to content programming such as audio, animation, television, and video as used when referring to digital content and multimedia.

** A physical network is different from a logical network. Logical networks are defined at the Network layer by the arrangement of the hierarchical addressing scheme. Physical networks represent the interconnection of devices on a common media. Sometimes, a physical network is also referred to as a network segment.

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Upper Layer Access to Media

As we have discussed, a network model allows each layer to function with minimal concern for the roles of the other layers. The Data Link layer relieves the upper layers from the responsibility of putting data on the network and receiving data from the network. This layer provides services to support the communication processes for each medium over which data is to be transmitted.

In any given exchange of Network layer packets, there may be numerous Data Link layer and media transitions. At each hop along the path, an intermediary device - usually a router - accepts frames from a medium, decapsulates the frame, and then forwards the packet in a new frame appropriate to the medium of that segment of the physical network.

Imagine a data conversation between two distant hosts, such as a PC in Paris with an Internet server in Japan. Although the two hosts may be communicating with their peer Network layer protocols (IP for example), it is likely that numerous Data Link layer protocols are being used to transport the IP packets over various types of LANs and WANs. This packet exchange between two hosts requires a diversity of protocols that must exist at the Data Link layer. Each transition at a router could require a different Data Link layer protocol for transport on a new medium.

Notice in the figure that each link between devices uses a different medium. Between the PC and the router may be an Ethernet link. The routers are connected through a satellite link, and the laptop is connected through a wireless link to the last router. In this example, as an IP packet travels from the PC to the laptop, it will be encapsulated into Ethernet frame, decapsulated, processed, and then encapsulated into a new data link frame to cross the satellite link. For the final link, the packet will use a wireless data link frame from the router to the laptop.

The Data Link layer effectively insulates the communication processes at the higher layers from the media transitions that may occur end-to-end. A packet is received from and directed to an upper layer protocol, in this case IPv4 or IPv6, that does not need to be aware of which media the communication will use.

Without the Data Link layer, a Network layer protocol, such as IP, would have to make provisions for connecting to every type of media that could exist along a delivery path. Moreover, IP would have to adapt every time a new network technology or medium was developed. This process would hamper protocol and network media innovation and development. This is a key reason for using a layered approach to networking.

The range of Data Link layer services has to include all of the currently used types of media and the methods for accessing them. Because of the number of communication services provided by the Data Link layer, it is difficult to generalize their role and provide examples of a generic set of services. For that reason, please note that any given protocol may or may not support all these Data Link layer services.

Internetworking Basics - http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Intro-to-Internet.html

MTU - http://www.tcpipguide.com/free/t_IPDatagramSizeMaximumTransmissionUnitMTUFragmentat.htm