Computer Networking:

a Top-Down Approach (8th ed.):

Notes of "Select" Lectures

Chapter 6 The Link Layer and LANs

6.1 Introduction to the Link Layer

Link-layer: services, implementation context.

    Some important terminology:

•    Node (节点): any device that runs a link-layer (i.e., layer 2) protocol. Nodes include hosts, routers, switches, and WiFi access points

•    Links (链路): the communication channels that connect adjacent nodes along the communication path as.

•    In order for a datagram to be transferred from source host to destination host, it must be moved over each of the individual links in the end-to-end path.

•    Link-layer frame (链路层帧): over a given link, a transmitting node encapsulates the datagram in a link-layer frame and transmits the frame into the link.

6.1.1 The Services Provided by the Link Layer

The basic service: to move a datagram from one node to an adjacent node over a single communication link

    The details: vary from one protocol to the next. Possible services:

•    Framing (成帧).

•    Almost all link-layer protocols encapsulate each datagram within a frame before transmission over the link. A frame consists of a data field and a number of header fields.

•    Link access.

•    A medium access control (MAC, 媒体访问控制) protocol: specifies the rules by which a frame is transmitted onto the link.

•    for point-to-point links: simple.

•    when multiple access problem: coordinates the frame transmissions of the many nodes.

•    Reliable delivery.

•    guarantees to move each network-layer datagram across the link without error.

•    can be achieved with acknowledgments and retransmissions.

•    often used for links that are prone to high error rates, such as a wireless link.

•    can be considered an unnecessary overhead for low bit-error links.

•    Error detection and correction.

•    Bit errors are introduced by signal attenuation and electromagnetic noise.

•    Error detection: done by

•    having the transmitting node include error-detection bits in the frame, and

•    having the receiving node perform an error check.

•    Error correction: a receiver

•    not only detects when

•    but also determines exactly where

•    (and then corrects).

6.1.2 Where Is the Link Layer Implemented?

•    For the most part, implemented on a chip called the network adapter (网络适配器), also sometimes known as a network interface controller (NIC, 网络接口控制器). Thus, much of a controller's functionality is implemented in hardware.

•    Interface communicating

•    On the sending side, the controller takes a datagram, encapsulates the datagram in a frame, and then transmits the frame into the communication link.

•    On the receiving side, a controller receives the entire frame, and extracts the datagram.

•    If error detection, then

• the sending controller: sets the error-detection bits in the frame header

• the receiving controller: performs error detection

•    The link layer is a combination of hardware and software.

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来源： https://www.cnblogs.com/zjnu/p/kurose_ross_6.html