CCNA讲义之七

北京邮电大学思科网络技术学院.
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Context Index
7.1 Data Link Layer – Accessing the Media 7.2 Media Access Control Techniques 7.3 Media Access Control Addressing and Framing Data 7.4 Putting it All Together 7.5 Labs and Activities
北京邮电大学思科网络技术学院.
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7.2.3 Media Access Control for Non-Shared Media
针对非共享介质的介质访问控制协议需要少量甚至不需要控制。这些协议具有 更简单的介质访问控制规则和过程。如点到点。 Define Full Duplex and Half Duplex as it relates to Media Access Control for non-shared media


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.
什么是介质访问控制？Regulating the placement of data frames onto the media is known as media access control.
控制方法有多种，取决于：介质共享和拓扑
北京邮电大学思科网络技术学院.
cn
北京邮电大学思科网络技术学院.
cn
2
Objectives
说明数据链路层协议在数据传输中的作用。 阐述数据链路层如何准备数据，以便通过网络介质传输。 描述不同类型的介质访问控制方法。 认识几种常见的逻辑网络拓扑，还可以说明逻辑拓扑确定 网络介质访问控制方法的方式。 解释将数据包封装成帧以方便介质访问的意图。 描述第 2 层帧结构并认识通用字段。 解释帧头和帧尾主要字段（包括编址、服务质量、协议类 型以及帧校验序列）的功能。
北京邮电大学思科网络技术学院.
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7.1 Data Link Layer – Accessing the Media
数据链路层——访问介质
北京邮电大学思科网络技术学院.
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7.1.1 Data Link Layer – Supporting & Connecting to Upper Layer Services
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7.2.2 Media Access Control for Shared Media
There are two basic media access control methods for shared media: －Controlled - Each node has its own time to use the medium －Contention-based - All nodes compete for the use of the medium
北京邮电大学思科网络技术学院.
cn
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7.2.2 Media Access Control for Shared Media
There are two basic media access control methods for shared media: －Controlled - Each node has its own time to use the medium －Contention-based - All nodes compete for the use of the medium
数据链路层学思科网络技术学院.
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7.1.1 Data Link Layer – Supporting & Connecting to Upper Layer Services Numerous Data Link layer protocols are being used over various types of LANs and WANs.
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7.1.4 Data Link Layer – Connecting Upper Layer Services to the Media Data Link layer is often divided into two sub-layers: －Logical Link Control －Media Access Control
北京邮电大学思科网络技术学院.
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7.1.3 Data Link Layer – Creating a Frame
Data Link layer frame includes:
1.开始和停止字段 — 帧的开始和结束限制 2.命名或编址字段 3.类型字段 — 包含在帧中的 PDU 的类型 4.质量 — 控制字段
北京邮电大学思科网络技术学院.
cn
18
7.2.3 Media Access Control for Non-Shared Media
针对非共享介质的介质访问控制协议需要少量甚至不需要控制。这些协议具有 更简单的介质访问控制规则和过程。如点到点。 Define Full Duplex and Half Duplex as it relates to Media Access Control for non-shared media
北京邮电大学思科网络技术学院.
cn
13
7.2 Media Access Control Techniques
介质访问控制技术
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cn
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7.2.1 Placing Data on the Media
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
北京邮电大学思科网络技术学院.
cn
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7.2.3 Media Access Control for Non-Shared Media
针对非共享介质的介质访问控制协议需要少量甚至不需要控制。这些协议具有 更简单的介质访问控制规则和过程。如点到点。
Define Full Duplex and Half Duplex as it relates to Media Access Control for non-shared media
北京邮电大学思科网络技术学院.
cn
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7.1.4 Data Link Layer – Connecting Upper Layer Services to the Media
The Data Link layer exists as a connecting layer between the software processes of the layers above it and the Physical layer below it. In many cases, the Data Link layer is embodied as a physical entity, such as an Ethernet network interface card (NIC).
5.数据字段 — 帧负载（网络层数据包）
北京邮电大学思科网络技术学院.
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7.1.2 Data Link Layer – Controlling Transfer across Local Media
The technique used for getting the frame on and off media is called the media access control method. 图中演示路由器的数据链路层功能
北京邮电大学思科网络技术学院.
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7.2.2 Media Access Control for Shared Media
There are two basic media access control methods for shared media: －Controlled - Each node has its own time to use the medium －Contention-based - All nodes compete for the use of the medium
北京邮电大学思科网络技术学院.
cn
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7.1.5 Data Link Layer – Standards
Standards for the Data Link layer
国际标准化组织 (ISO) 电气电子工程师协会 (IEEE) 美国国家标准学会 (ANSI) 国际电信联盟 (ITU)
Data Link Layer Terms