網路原理(一) Section 4.4

802.11 frame
After a wireless device sends an 802.11 frame, the receiver returns an acknowledgment so that the sender knows the frame arrived.

802.11 wireless ; Frame Relay ; HDLC ; Point-to-Point Protocol (PPP)
Data link layer protocol

Bus
All end systems are chained to each other and terminated in some form on each end.

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
is used in half-duplex Ethernet LANs.

Collision
碰撞

Contention-based access
Another form of CSMA that is used by IEEE 802.11 WLANs is Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). CMSA/CA uses a method similar to CSMA/CD to detect if the media is clear.

Control
Identifies special flow control services such as quality of service (QoS).

Controlled access
控制存取

Cyclic Redundancy Check (CRC) value
A transmitting node creates a logical summary of the contents of the frame, known as the cyclic redundancy check (CRC) value.

Data
Contains the frame payload (i.e., packet header, segment header, and the data).

Error Detection
These frame fields are used for error detection and are included after the data to form the trailer.

Ethernet
乙太網

Extended Star
In an extended star topology, additional Ethernet switches interconnect other star topologies.

Frame Check Sequence (FCS) Frame Relay
CRC value is placed in the Frame Check Sequence (FCS) field to represent the contents of the frame. 

Frame start and stop indicator flags
Used to identify the beginning and end limits of the frame.

Full-Duplex Communications
Both devices can transmit and receive on the media at the same time.The half-duplex mode is used in legacy bus topologies and with Ethernet hubs. WLANs also operate in half-duplex. Half-duplex allows only one device to send or receive at a time on the shared medium and is used with contention-based access methods.

Half-Duplex Communications
Both devices can transmit and receive on the media but cannot do so simultaneously. The data link layer assumes that the media is available for transmission for both nodes at any time. Ethernet switches operate in full-duplex mode by default, but can operate in half-duplex if connecting to a device such as an Ethernet hub. 

Header ; Trailer
Although there are many different data link layer protocols that describe data link layer frames, each frame type has three basic parts: 1.Header 2.Trailer 3.Data

Hub and Spoke
A WAN version of the star topology in which a central site interconnects branch sites using point-to-point links. 

Logical Point-to-Point Topology
The end nodes communicating in a point-to-point network can be physically connected via a number of intermediate devices. However, the use of physical devices in the network does not affect the logical topology.

Logical Topology 
Refers to the way a network transfers frames from one node to the next. This arrangement consists of virtual connections between the nodes of a network. These logical signal paths are defined by data link layer protocols. The logical topology of point-to-point links is relatively simple while shared media offers different access control methods. 

Mesh
This topology provides high availability, but requires that every end system be interconnected to every other system.

Point-to-Point
This is the simplest topology that consists of a permanent link between two endpoints. For this reason, this is a very popular WAN topology.


Physical Topology
Refers to the physical connections and identifies how end devices and infrastructure devices such as routers, switches, and wireless access points are interconnected. Physical topologies are usually point-to-point or star.

Media Access Control
媒體存取控制（英語：Media Access Control，縮寫：MAC）子層，是區域網路中資料鏈路層的下層部分，提供定址及媒體存取的控制方式，使得不同裝置或網路上的節點可以在多點的網路上通訊，而不會互相衝突，上述的特性在區域網路或者都會網路中格外重要。

Media Sharing
How the nodes share the media. The media sharing can be point-to-point, such as in WAN connections, or shared such as in LAN networks.

Quality of Service (QoS)
QoS is used to give forwarding priority to certain types of messages.

Ring
End systems are connected to their respective neighbor forming a ring.

Star
End devices are connected to a central intermediate device.

Topology
How the connection between the nodes appears to the data link layer.

type
類型

Virtual circuit
A virtual circuit is a logical connection created within a network between two network devices.