Reading Networking Devices（2）
1．NIC（Network Interface Card）
Network Interface Card，or NIC is a hardware card installed in a computer so it can communi-cate on a network.The network adapter provides one or more ports for the network cable to connect to，and it transmits and receives data onto the network cable.
1.1 Wireless LAN card
Every networked computer must also have a network adapter driver，which controls the network adapter.Each network adapter driver is configured to run with a certain type of network adapter.
1.2 Network card
Network interface adapters perform a variety of functions that are crucial to getting data to and from the computer over the network.
These functions are as follows：
1.2.1 Data encapsulation
The network interface adapter and its driver are responsible for building the frame around thedata generated by the network layer protocol，in preparation for transmission.The network interface adapter also reads the contents of incoming frames and passes the data to the appropriate network layer protocol.
1.2.2 Signal encoding and decoding
The network interface adapter implements the physical layer encoding scheme that converts the binary data generated by the network layer—now encapsulated in the frame—into electrical voltages，light pulses，or whatever other signal type the network medium uses，and converts received signals to binary data for use by the network layer.
1.2.3 Transmission and reception
The primary function of the network interface adapter is to generate and transmit signals of the appropriate type over the network and to receive incoming signals.The nature of the signals depends on the network medium and the data-link layer protocol.On a typical LAN，every computer receives all the packets transmitted over the network，and the network interface adapter examines the destina-tion address in each packet to see if it is intended for that computer.If so，the network interface adapter passes the packet to the computer for processing by the next layer in the protocol stack；if not，the network interface adapter discards the packet.
1.2.4 Data buffering
Network interface adapters transmit and receive data one frame at a time，so they have builtin buffers that enable them to store data arriving either from the computer or from the network until a frame is complete and ready for processing.
1.2.5 Serial／parallel conversion
The communication between the computer and the network interface adapter runs in parallel，that is，either 16 or 32 bits at a time，depending on the bus the adapter uses.Network communica-tions，however，are serial（running one bit at a time），so the network interface adapter is responsi-ble for performing the conversion between the two types of transmissions.
1.2.6 Media access control
The network interface adapter also implements the MAC mechanism that the data-link layer pro-tocol uses to regulate access to the network medium.The nature of the MAC mechanism depends on the protocol used.
2．ISDN（Integrated Services Digital Network）adapters
Integrated Services Digital Network adapters can be used to send voice，data，audio，or video over standard telephone cabling.ISDN adapters must be connected directly to a digital telephone network.ISDN adapters are not actually modems since they neither modulate nor demodulate the digital ISDN signal.
Like standard modems，ISDN adapters are available both as internal devices that connect di-rectly to a computer’s expansion bus and as external devices that connect to one of a computer’s se-rial or parallel ports.ISDN can provide data throughput rates from 56 Kbps to 1.544 Mbps（using a T1 carrier service）.
ISDN hardware requires a NT（network termination）device，which converts network data sig-nals into the signaling protocols used by ISDN.Sometimes，the NT interface is included，or integrat-ed，with ISDN adapters and ISDN-compatible routers.In other cases，an NT device separate from the adapter or router must be implemented.ISDN works at the physical，data link，network，and transport layers of the OSI Model.
3．WAP（Wireless Access Point）
A wireless network adapter card with a transceiver，sometimes called an access point，broad-casts and receives signals to and from the surrounding computers and passes back and forth between the wireless computers and the cabled network.
Access points act as wireless hubs to link multiple wireless NICs into a single subnet.They also have at least one fixed Ethernet port to allow the wireless network to be bridged to a traditional wired Ethernet network.
4．Transceiver（media converter）
Transceiver，short for transmitter-receiver，is a device that both transmits and receives analog or digital signals.The term is used most frequently to describe the component in local-area networks（LANs）that actually applies signals onto the network wire and detects signals passing through the wire.For many LANs，the transceiver is built into the network interface card（NIC）.Some types of networks，however，require an external transceiver.
In Ethernet networks，a transceiver is also called a Medium Access Unit（MAU）.Media con-verters interconnect different cable types twisted pair，fiber，and thin or thick coax，within an exist-ing network.They are often used to connect newer 100-Mbps，Gigabit Ethernet，or ATM equipment to existing networks，which are generally 10BASE-T，100BASE-T，or a mixture of both.They can also be used in pairs to insert a fiber segment into copper networks to increase cabling distances and enhance immunity to electromagnetic interference（EMI）.
5．Firewalls
In computing，a firewall is a piece of hardware and／or software which functions in a networked environment to prevent some communications forbidden by the security policy，analogous to the func-tion of firewalls in building construction.
A firewall has the basic task of controlling traffic between different zones of trust.Typical zones of trust include the Internet（a zone with no trust）and an internal network（a zone with high trust）.The ultimate goal is to provide controlled connectivity between zones of differing trust levels through the enforcement of a security policy and connectivity model based on the least privilege principle.
There are three basic types of firewalls depending on：
● whether the communication is being done between a single node and the network，or between two or more networks
● whether the communication is intercepted at the network layer，or at the application layer
● whether the communication state is being tracked at the firewall or not With regard to the scope of filtered communication exist these firewalls：
● Personal firewalls，a software application which normally filters traffic entering or leaving a single computer through the Internet.
● Network firewalls，normally running on a dedicated network device or computer positioned on the boundary of two or more networks or DMZs（demilitarized zones）.Such a firewall filters all traffic entering or leaving the connected networks.
In reference to the layers where the traffic can be intercepted，there are two main categories of firewalls：
● network layer firewalls，eg.ip tables.
● application layer firewalls，eg.TCP Wrapper.
These network-layer and application-layer types of firewall may overlap，even though the per-sonal firewall does not serve a network；indeed，single systems have implemented both together.
Lastly，depending on whether the firewalls track packet states，two additional categories of fire-walls exist：
● stateful firewalls
● stateless firewalls
a variety of 多种的
telephone cabling 电话线
parallel port 并行接口
transport layer 传输层
access point 访问点，接入点
wired Ethernet network 有线以太网
analog signal 模拟信号
digital signal 数字信号
thick coax 粗同轴电缆
stateful firewall 状态防火墙，基于状态检测的防火墙
stateless firewall 无状态防火墙
NIC（Network Interface Card） 网络接口卡，网卡
ISDN（Integrated Services Digital Network） 综合服务数字网
Kbps（Kilobits per second） 千位／秒
Mbps（Megabytes per second） 兆／秒
NT（network termination） 网络终端
WAP（Wireless Access Point） 无线接入点，无线访问点
coax（coaxial cable） 同轴电缆
ATM（Asynchronous Transfer Mode） 异步传输模式
EMI（ElectroMagnetic Interference） 电磁干扰
DMZ（DeMilitarized Zone） 隔离区