Network Media
The media provides the channel over which the message travels from source to destination. Different types of network media have different features and benefits. There are three basic forms of network media on which data is represented:
Copper
Fiber
Wireless
Cooper
Copper media: coaxial cable (铜轴线）
Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single inner wire made of two conducting elements. One of these elements, located in the center of the cable, is a copper conductor. Surrounding the copper conductor is a layer of flexible insulation. Over this insulating material is a woven copper braid or metallic foil that acts as the second wire in the circuit and as a shield for the inner conductor. This second layer, or shield reduces the amount of outside electro-magnetic interference. Covering this shield is the cable jacket.
同轴电缆由一个中空的外圆柱形导体组成，围绕着由两个导电元件组成的单根内线。其中一个元件位于电缆的中心，是铜导体。围绕铜导体的是一层柔性绝缘材料。在这层绝缘材料上是一层编织的铜编织层或金属箔，作为电路中的第二根导线和内导体的屏蔽层。这第二层或屏蔽层可以减少外界的电磁干扰。覆盖这个屏蔽层的是电缆护套。屏蔽罩可以保护里面不受外面影响。
Coaxial cable: thicknet and thinnet ，粗缆和细缆都支持10兆以太网标准。
Copper media: Unshielded twisted-pair cable
铜轴线最大问题贵，成本高。用比较低成本达到类似性能。粗细缆被双绞线代替。
双绞线一系列的技术升级。所有线两两一对绞一起，好处是可以降低干扰，抵抗外界干扰。一般网线有四对双绞线。
Unshielded twisted-pair cable (UTP) is a four-pair wire medium. Each of the 8 individual copper wires in the UTP cable is covered by insulating material. In addition, each pair of wires is twisted around each other. This type of cable relies solely on the cancellation effect produced by the twisted wire pairs, to limit signal degradation caused by EMI and RFI. UTP cable must follow precise specifications as to how many twists or braids are permitted per foot of cable.
非屏蔽双绞线（UTP）是一种四对线介质。UTP电缆中的8根独立铜线中的每一根都由绝缘材料覆盖。此外，每对电线都是互相缠绕的。这种类型的电缆完全依靠双绞线对产生的消除效应，来限制由EMI和RFI引起的信号衰减。UTP电缆必须遵循精确的规格，即每英尺电缆允许有多少根绞线或编织线。
Crosstalk involves the transmission of signals from one wire to a nearby wire. When voltages change on a wire, electromagnetic energy is generated. Adjacent wires in the cable act like antennas, receiving the transmitted energy, which interferes with data on those wires. Crosstalk can also be caused by signals on separate, nearby cables. When crosstalk is caused by a signal on another cable, it is called alien crosstalk.
串扰是指信号从一根导线传输到附近的导线上。当电线上的电压发生变化时，就会产生电磁能。电缆中相邻的电线就像天线一样，接收传输的能量，从而干扰这些电线上的数据。 串扰也可能是由单独的、附近的电缆上的信号引起的。当串扰是由另一条电缆上的信号引起时，称为外来串扰。
Cancellation, when two wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. Thus, the two magnetic fields will cancel each other out. Twisting one pair of wires in a cable helps to reduce crosstalk of data or noise signals from an adjacent wire pair. In twisted-pair cable, a pair of wires is used to transmit one signal. The wire pair is twisted so that each wire experiences similar crosstalk. Because a noise signal on one wire will appear identically on the other wire, this noise be easily detected and filtered at the receiver.
抵消，当电路中的两根导线靠近放置时，它们的磁场是完全相反的。因此，两个磁场会相互抵消。 扭转电缆中的一对导线有助于减少来自相邻线对的数据或噪声信号的串扰。在双绞线中，一对导线用于传输一个信号。线对被扭曲，使每根线都经历类似的串扰。由于一条线上的噪声信号在另一条线上会出现相同的信号，因此这种噪声很容易在接收器上被检测到并被过滤。
通过叠加完全反向信号，降低辐射。两个反向信号，通过翻转，原始信号增强，噪音抵消。
双绞线设计实现对高带宽的支持。
UTP connector: RJ-45 plug
EIA/TIA specifies an registered jack (RJ-45) connector for UTP cable. The RJ-45 transparent end connector shows eight colored wires. The RJ-45 connector is the male component, crimped on the end of the cable. When looking at the male connector from the front, the pin locations are numbered 8 on the left down to 1 on the right.
UTP连接器。RJ-45插头
EIA/TIA为UTP电缆指定了一个注册插孔（RJ-45）连接器。RJ-45透明端连接器显示八根彩色导线。RJ-45连接器是公头组件，压接在电缆末端。从正面看公头连接器时，针脚位置从左边的8号到右边的1号。
The jack is the female component in a network device, wall outlet, or patch panel.
RJ-45 plugs fit into RJ-45 jacks or receptacles. The RJ-45 jack has eight conductors, which snap together with the RJ-45 plug. On the other side of the RJ-45 jack is a punch down block where wires are separated and forced into slots with a fork-like tool called a punch down tool.
插座是网络设备、墙上插座或跳线板中的母体组件。
RJ-45插头适合于RJ-45插孔或插座。RJ-45插孔有八个导体，与RJ-45插头扣在一起。在RJ-45插孔的另一侧是一个冲下块，在这里，电线被分离出来，并用一个类似叉子的工具强制插入槽中，称为冲下工具。
Fiber
Fiber-optic cable is usually made up of five parts. The parts are the core, the cladding, a buffer, a strength material, and an outer jacket.
光缆通常由五部分组成。这五部分分别是光芯、包层、缓冲器、强度材料和外护套。
Single-mode and Multimode fiber
Single-mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber. The single-mode core is eight to ten microns in diameter. Nine-micron cores are the most common. A 9/125 marking on the jacket of the single- mode fiber indicates that the core fiber has a diameter of 9 microns and the surrounding cladding is 125 microns in diameter. Single-mode fiber can carry LAN data up to 3000 meters. Multimode fiber has a large core that there are many paths that light can take through the fiber. A standard multimode fiber-optic cable uses an optical fiber with either a 62.5 or a 50-micron core and a 125-micron diameter cladding. This is commonly designated as 62.5/125 or 50/125 micron optical fiber. Multimode is only capable of carrying up to 2000 meters.
单模光纤的纤芯要小得多，只允许光线沿着光纤内的一种模式传播。 单模光纤的纤芯直径为八到十微米。九微米的纤芯是最常见的。在单模光纤的护套上有9/125的标记，表示纤芯的直径为9微米，周围的包层直径为125微米。单模光纤可以传输3000米以内的局域网数据。
多模光纤的纤芯很大，光可以有很多路径，因此，多模光纤可以传输的数据也很多。采取通过光纤。标准的多模光缆使用的是一根具有62.5或50微米纤芯和125微米直径包层的光纤。这通常被指定为62.5/125或50/125微米光纤。多模光缆最多只能传输2000米。(有色散，长距离不太行，光电转换头成本低一点）
Fiber-optic connectors: SC, ST and LC Connectors are attached to the fiber ends so that the fibers can be connected to the ports on the transmitter and receiver. The types of most commonly used connectors are with is the Straight Tip (ST), used for single-mode fiber. Subscriber Connector (SC), used for multimode fiber. Lucent Connector (LC), a small connector used with single-mode fiber and also supports multi-mode fiber.
光纤连接器: SC、ST和LC
连接器连接到光纤末端，使光纤可以连接到发射机和接收机的端口。最常用的连接器的类型是用的是
直头（ST），用于单模光纤。
用户连接器（SC），用于多模光纤。
朗讯连接器（LC），用于单模光纤的小型连接器，也支持多模光纤。
Types of Wireless Networks
The IEEE and telecommunications industry standards for wireless data communications cover both the physical and data link layers. Four common data communications standards that apply to wireless media are:
IEEE 802.15 - Wireless PAN standard, commonly known as "Bluetooth", uses a device pairing process to communicate over distances from 1 to 100 meters.
IEEE 802.11 - Wireless LAN standard, commonly referred to as Wi-Fi, uses a contention or non-deterministic system with a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) media access process.
IEEE 802.16 - Wireless MAN standard, commonly known as Worldwide Interoperability for Microwave Access (WiMAX), uses a point-to- multipoint topology to provide wireless broadband access.
2G (e.g. GSM) / 3G (e.g. WCDMA) / 4G (e.g. LTE) - Wireless WAN standard, provides data transfer over mobile cellular telephony networks.
IEEE和电信行业的无线数据通信标准涵盖物理层和数据链路层。常见的适用于无线介质的数据通信标准有四种。
IEEE 802.15--无线PAN标准，俗称 "蓝牙"，使用设备配对过程，在1至100米的距离内进行通信。
IEEE 802.11--无线局域网标准，俗称 "Wi-Fi"，采用争夺或非决定性系统，采用载波感知多址/避免冲突（CSMA/CA）媒体接入过程。
IEEE 802.16--无线MAN标准，俗称全球微波接入互通（WiMAX），采用点对多点的拓扑结构提供无线宽带接入。
2G(如GSM)/3G(如WCDMA)/4G(如LTE)--无线广域网标准，提供移动蜂窝电话网络的数据传输。
