兔子銘的紅蘿蔔園區

Crossover Cable是什麼意思啊？為什麼要使用 Crossover Cable 呢？

Tx+ 表傳送正極資料腳位、Tx- 表傳送負極資料腳位、Rx+ 表接收正極資料腳位、Rx- 表接收負極資料腳位。至於另外四個腳位（Pin4、Pin5、Pin7及Pin8）目前在Ethernet上是屬於保留狀態，並未做任何使用。

因此要讓二台電腦可以彼此溝通及交換資料，則必須要將極性相同的傳送端及另一台電腦的接收端串連在一起（如下圖所示）。這也代表這條雙絞線兩邊接頭的線材排列順序是不一樣的，此時我們就稱此雙絞線纜線為 Crossover Cable。

Tx：1、2　Rx：3、6 的Device 為：

a)PC NICS

b)Routers

c)Wireless Access

d)Network Printer

Tx：3、6 Rx：1、2 的Device 為：

a)Switch

b)Hub

Transmission Media

This section outlines a variety of "guided" media for network communications. "Unguided" communication techniques are related to wireless networking. Figure 1 illustrates the primary types of cable used for data transmissions. These cable types are described here:

Figure 1

• Straight cable    This is the simplest type of cable. It consists of copper wires surrounded by an insulator. The wire comes in bundles or as flat "ribbon" cables and is used to connect various peripheral devices over short distances. Cables for internal disk drives are typically flat cables with multiple transmission wires running in parallel.

• Twisted-pair cable    This cable consists of copper-core wires surrounded by an insulator. Two wires are twisted together to form a pair, and the pair forms a balanced circuit (voltages in each pair have the same amplitude but are opposite in phase). The twisting protects against EMI (electromagnetic interference) and RFI (radio frequency interference). A typical cable has multiple twisted pairs, each color-coded to differentiate it from other pairs. UTP (unshielded twisted-pair) has been used in the telephone network and is commonly used for data networking in the United States. STP (shielded twisted-pair) cable has a foil shield around the wire pairs in a cable to provide superior immunity to RFI. Traditional twisted-pair LANs use two pairs, one for transmit and one for receive, but newer Gigabit Ethernet networks use four pairs to transmit and receive simultaneously. UTP and STP are constructed of 100-ohm, 24-AWG solid conductors.

• Coaxial cable    This cable consists of a solid copper core surrounded by an insulator, a combination shield and ground wire, and an outer protective jacket. In the early days of LANs, coaxial cable was used for its high bit rates. An Ethernet Thinnet (10Base-2) network has a data rate of 10Mbits/sec and implements a bus topology in which each station is attached to a single strand of cable. Today, hierarchical wiring schemes are considered more practical, and even though more twisted pair wire is required to cable such a network, cost has dropped, making such networks very practical.

• Fiber-optic cable    This cable consists of a center glass core through which light waves propagate. This core is surrounded by a glass cladding that basically reflects the inner light of the core back into the core. A thick plastic outer jacket surrounds this assembly, along with special fibers to add strength. Fiber-optic cable is available with a metal core for strength if the cable will be hung over distances.

Copper cable is a relatively inexpensive, well-understood technology. However, it has various electrical characteristics that impose restrictions on its use. For example, copper resists the flow of electrons, which limits the length of cables. It also radiates energy in the form of signals that can be monitored, and it is susceptible to external radiation that can distort transmissions. In contrast, fiber-optic cable transmits light (photons) through a core of pure silicon dioxide that is so clear, a three-mile-thick window of it would not distort the view. Thus, fiber cable has high transmission rates and is used for long distances. Photonic transmissions produce no emissions outside the cable and are not affected by external radiation. Thus, fiber cable is preferred where security is an issue.

A characteristic of cable that must not be overlooked is its fire rating. Cable installed in the plenum space, which is the airspace between the ceiling and the next floor or roof, must be installed in metal conduit, or must meet local fire codes. In the event of a fire, the cable must not produce noxious or hazardous gases that would be pumped to other parts of a structure through the plenum. Nonplenum cables have PVC (polyvinyl chloride) jackets while plenum-rated cables have jackets made with fluoropolymers such as Du Pont's Teflon.

The remainder of this section covers so-called guided media copper cabling, as opposed to unguided media. For a discussion of unguided media, see "Wireless Communications." Optical cabling is covered under "Fiber-Optic Cable." Also see "Optical Networks" and "WDM (Wavelength Division Multiplexing)."

Copper Cable Characteristics

Information is transmitted over copper cable by applying variable or discrete voltages at one end of the cable and reading those voltages at the other end. Data signals are discrete pulses of electricity (or light in the case of fiber cable). As mentioned, this discussion is oriented toward twisted-pair cable. Some of the characteristics discussed here only apply to wires that are twisted.

The following relationship exists between the frequency of the electrical signal and the rate at which data is transmitted:

• Bandwidth    The bandwidth of a communication system is the highest frequency range that it uses. This is defined by the engineering specification of the particular network. Some examples are listed in Table 1.

• Data rate    The actual data throughput of a cable, after applying encoding and compression schemes to more efficiently use the bandwidth of the cable.

Data rate is a more accurate measure of a transmission system's capabilities, but the term "bandwidth" is often used in a general way.

Table 1: Frequency range and data rate of communication systems

The relationship between bandwidth and data rate is illustrated in Figure 2. You can think of the cable as a pipe, where bandwidth is the size of the pipe and the data rate is the amount of information you can push through the pipe. An encoding and compression scheme lets you use the pipe more efficiently. See "Signals."

Figure 2

For example, 100Base-TX uses an encoding scheme called 4B-5B, which first maps data to a table of efficient codes and then transmits the codes over the cable using the NRZ signal encoding scheme. Thus, an 80-MHz signal supports a data rate of 100 Mbits/sec.

In contrast, Gigabit Ethernet (IEEE 802.3ab or 1000Base-T) transmits on four wire pairs in full-duplex mode, meaning that signals are transmitted in both directions on each wire pair. With encoding applied, each wire pair supports a data rate of 250 Mbits/sec in each direction. See "Gigabit Ethernet" for additional information.

Data transmissions over copper cable are subject to attenuation, delay distortion, noise, and environmental problems. Qualified cable installers should test cable both before the cable is installed (to test for cable quality) and after it is installed (to test for proper installation). A table of test parameters is provided later. Also see "Testing, Diagnostics, and Troubleshooting."

Gigabit Ethernet can use existing high-performance Category 5 cable (described later) if the cable passes appropriate testing. It requires much better cable performance than was defined in the original Category 5 specification, so existing cable must be retested to ensure it supports higher frequencies. The minimum recommendations for Gigabit Ethernet cabling are outlined in TIA TSB-95 as described under "TIA/EIA Structured Cabling Standards." Test equipment from Fluke (http:/www.fluke.com), Agilent (http://www.wirescope.com), and other vendors can help you determine the performance of a cable installation.

High-performance cable requires special handling procedures. The physical shape of the cable cannot be drastically altered, meaning that it should not be stretched, twisted, or bent beyond a radius that is 10 times the outside diameter of the cable. Figure 3 illustrates what can happen to wires that are excessively bent. The twisted pairs are pushed closer together, which causes signal interference between wire pairs and signal distortion.

Figure 3

Cable quality may be poor if a cable manufacturer has substituted some material because another material is in short supply, as happened several years ago during the worldwide shortage of FEP (Fluorinated Ethylene-Propylene Teflon). According to Anixter, there now exist more than 45 plenum and nonplenum cable designs that exhibit varying electrical performance characteristics but are still labeled Category 5 compliant. Moral: Have all existing cable installations tested and certified for use with the networking technology you plan to use. You may find that some existing cable runs will support high-speed networks, while others may need connector replacements, and still other require complete replacement.

The following sections discuss various cable characteristics and environmental conditions that affect performance and how those characteristics affect cable and network design.

Categories of Twisted-Pair Cable

Twisted-pair cable has been used for decades to transmit both analog and digital information. The existing telephone system is mostly wired with voice-grade twisted-pair wires (the wire is not twisted in some cases). Twisted-pair wire is now the preferred wire for network cabling. The twisting of pairs, the quality of the conductive material, the type of insulator, and the shielding largely determine the rate at which data can be transmitted over twisted-pair cable.

The following categories of cable are recognized throughout the industry, and Category 3, Category 4, and Category 5 cable are specified in the TIA/EIA 568-A specification.

• Category 1    Traditional unshielded twisted-pair telephone cable that is suited for voice. Most telephone cable installed before 1983 is Category 1 cable. It is not recommended for network use, although modems do a good job of transmitting over it.

• Category 2    Unshielded twisted-pair cable certified for data transmissions up to 4 Mbits/sec. This cable has four twisted pairs. It was commonly used for IBM mainframe and minicomputer terminal connections and was also recommended for low-speed ARCNET networks. This cable should not be used for high-speed networking.

• Category 3    This category is rated for signals up to 16 MHz and supports 10-Mbit/sec Ethernet, 4-Mbit/sec token ring, and 100VG-AnyLAN networks. The cable has four pairs and three twists per foot (although the number of twists is not specified). Costs are around 10 cents per foot. Plenum cable costs about 40 cents per foot. This cable is installed at many sites as telephone cabling.

• Category 4    This category is rated for signals up to 20 MHz and is certified to handle 16-Mbit/sec token ring networks. The cable has four pairs and costs under 20 cents per foot. Plenum cable costs under 50 cents per foot.

• Category 5    This category has four twisted pairs with eight twists per foot and is rated for signals up to 100 MHz at a maximum distance of 100 meters. Ethernet 100Base-TX, FDDI, and ATM at 155 Mbits/sec use this cabling. The cable has low capacitance and exhibits low crosstalk due to the high number of twists per foot. It costs under 30 cents per foot. Plenum cable costs under 60 cents per foot. This is the predominant cable installed in all new buildings since the early 1990s. Specifications for this cable are outlined in Table 2.

Even though Category 5 is widely used, there are many factors that can prevent a cabling system from delivering the intended data rate. Cable runs should not exceed 100 meters (300 feet). The TIA/EIA specification calls for 90-meter maximum runs from the wiring closet to the wall outlet. An extra 10 meters is allowed to connect computers to the wall outlet and to connect the cable runs to patch panels. Category 5 installations must use Category 5 connectors, patch panels, wall plates, and other components. In addition, proper twisting must be maintained all the way up to connectors.

Enhanced Cabling

Even though Category 5 was considered future-proof, new gigabit-per-second networking schemes have emerged that call for a better class of cable. As mentioned, you can have existing Category 5 cable tested to see if it supports Gigabit Ethernet, but if you are installing new cable for Gigabit Ethernet, choose Category 5E cable, or if you really want to future-proof your installation, consider Category 6 and Category 7 cable. The specifications for these cable types are outlined in Table C-2, earlier in this section.

• Category 5E (Enhanced)    This cable has all the characteristics of Category 5, but is manufactured with higher quality to minimize crosstalk. The cable has more twists than traditional Category 5. It is rated at frequencies up to 200 MHz, which is double the transmission capability of traditional Category 5. However, at these frequencies, crosstalk can be a problem, and the cable does not have shielding to reduce crosstalk. This cable is defined in TIA/EIA-568A-5 (Addendum 5).

• TIA Category 6 and ISO Class E    These cable types are designed to support frequencies over 200 MHz using specially designed components that reduce delay distortion and other problems. The TIA and ISO are cooperating on this category.

• TIA Category 7 and ISO Class F    These cable types are designed to support frequencies up to 600 MHz. Each pair is individually shielded and the entire cable is surrounded by a shielded jacket. Connectors are expected to be specially designed proprietary components. The TIA and ISO are cooperating on this category.

• Components of a Structured Cabling System
• In the 1980s, vendors and standards organizations saw a need to standardize cabling schemes, and they eventually created the TIA/EIA 568 structured cabling standard. The typical components of a structured wiring scheme are illustrated below. The patch panel provides a place to terminate the horizontal wiring that fans out to work areas. The twisted pairs in the cable are directly attached to the back of the patch panel. The front of the patch panel then provides a place to attach patch cables that connect to network hubs and switches. This arrangement makes moves and changes easily. When someone must be moved to another workgroup or subnetwork, the patch cable on the port leading to his or her computer is moved to another port on a network hub or switch.
• 
• As network bandwidth has increased, high-quality cable and components are essential, and they must be installed to exact specifications. A Category 5 cabling system must test within the allowed specifications across the entire cable plant, including all connectors, outlets, patch panels, and cross-connects. This is especially important as networks move to gigabit-per-second speeds. As mentioned, it might be possible to use existing Category 5 cable for gigabit networks, but testing may indicate that some components need replacement.

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