Lecture CCNA Exploration 4.0 (Kỳ 1) - Chapter 7: Data Link Layer

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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,... Inviting you to refer.

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Nội dung Text: Lecture CCNA Exploration 4.0 (Kỳ 1) - Chapter 7: Data Link Layer

Module7- Data Link Layer 1
Overview 2
Overview 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. 3
Data Link Layer – Accessing the Media 4
Supporting & Connecting to Upper Layer Services The Data Link layer provides a means for exchanging data over a common local media. • The Data Link layer performs 2 basic services: 1. Allows the upper layers to access the media using techniques such as framing. 2. Controls how data is placed onto the media and is received from the media using techniques such as media access control and error detection. 5
Terms 7.1.1.1 6
Media • 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. 7
Logical Networks • Logical networks are defined at the Network layer by the arrangement of the hierarchical addressing scheme. 8
Physical Networks • Physical networks represent the interconnection of devices on a common media. Sometimes, a physical network is also referred to as a network segment. 9
Supporting & Connecting to Upper Layer Services 7.1.1.2 • 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. 10
Supporting & Connecting to Upper Layer Services • Without the Data Link layer ? 11
Controlling Transfer across Local Media 7.1.2 • Layer 2 protocols specify the encapsulation of a packet into a frame and the techniques for getting the encapsulated packet on and off each medium. • The technique used for getting the frame on and off media is called the media access control method. • The media access control methods described by the Data Link layer protocols define the processes by which network devices can access the network media and transmit frames in diverse network environments. 12
Creating a Frame 7.1.3.1 • Data - The packet from the Network layer • Header - Contains control information, such addressing, and is located at the beginning of the PDU • Trailer - Contains control information added to the end of the PDU Control information may tell: 1. Which nodes are in communication with each other 2. When communication between individual nodes begins and when it ends 3. Which errors occurred while the nodes communicated 4. Which nodes will communicate next 13
Creating a Frame 7.1.3.2 • If a node is receiving long streams of bits, how does it determine where a frame starts and stops or which bits represent the address? • Framing breaks the stream into decipherable groupings, with control information inserted in the header and trailer as values in different fields. 14
Creating a Frame Not all protocols include all of these fields Typical field types include: 1. Start and stop indicator fields - The beginning and end limits of the frame 2. Naming or addressing fields 3. Type field - The type of PDU contained in the frame 4. Quality - control fields 5. A data field -The frame payload (Network layer packet) 15
Connecting Upper Layer Services to the Media 7.1.4.1 • The Data Link layer exists as a connecting layer between the software processes of the layers above it and the Physical layer below it. 16
Connecting Upper Layer Services to the Media 7.1.4.2 • The upper sublayer defines the software processes that provide services to the Network layer protocols. • The lower sublayer defines the media access processes performed by the hardware. 17
Q/A • Why does the Data Link layer separate into sublayers ? • Allows for one type of frame defined by the upper layer to access different types of media defined by the lower layer. 18
Connecting Upper Layer Services to the Media • Logical Link Control – Logical Link Control (LLC) places information in the frame that identifies which Network layer protocol is being used for the frame. • Media Access Control – Media Access Control (MAC) provides Data Link layer addressing and delimiting of data according to the physical signaling requirements of the medium and the type of Data Link layer protocol in use. 19
Extra: LLC-MAC 20