📄 content.html
字号:
this.ns4 = (this.b=="ns" && this.v==4) this.ns5 = (this.b=="ns" && this.v==5) this.ie = (this.b=="ie" && this.v>=4) this.ie4 = (navigator.userAgent.indexOf('MSIE 4')>0) this.ie5 = (navigator.appVersion.indexOf('MSIE 5.0')>0) this.ie55 = (navigator.appVersion.indexOf('MSIE 5.5')>0) if (this.ie5) this.v = 5 this.min = (this.ns||this.ie) } // automatically create the "is" object is = new BrowserCheck()</script> </head> <body onLoad="window.focus();" alink="#000000" vlink="#000000" link="#000000" bgcolor="#E7E7E7" background="../../../../../images/ccna/common/bg.gif"> <table width="100%" cellspacing="0" cellpadding="2" border="0"><tr> <td bgcolor="#000000"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td class="rlohdr"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td valign="top" class="rlohdr">7.3</td><td width="100%" class="rlohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="rlohdr">Ethernet and IEEE 802.3</td> </tr> </table> </td> </tr> <tr> <td bgcolor="#000000"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td class="riohdr"><img height="1" width="2" border="0" src="../../../../../images/ccna/common/transdot.gif"></td><td valign="top" class="riohdr">7.3.6</td><td width="100%" class="riohdr"> <table width="90%" cellpadding="0" cellspacing="0" border="0"> <tr> <td class="riohdr">Ethernet 10BASE-T media and topologies</td> </tr> </table> </td> </tr> <tr> <td colspan="4"> <table width="90%" cellspacing="0" cellpadding="0" border="0"> <tr> <td class="smtext"> <p> </p><table cellpadding="0" cellspacing="0" width="100%" border="0" xmlns:java="http://xml.apache.org/xslt/java"> <tr> <td width="100%"><img height="10" width="1" border="0" src="../../../../../images/ccna/common/transdot.gif"></td> </tr> <tr> <td width="100%"> <table bgcolor="#000000" bordercolor="#000000" cellspacing="0" bordercolorlight="#000000" width="100%" border="0"> <tr> <td width="100%"> <table bgcolor="#FFFFFF" cellspacing="0" cellpadding="2" width="100%" border="0"> <tr> <td valign="middle"><img height="22" width="22" border="0" src="../../../../../images/ccna/common/inotes.gif"></td><td valign="middle"><span class="cstitle">Instructor Note</span></td> </tr> <tr> <td valign="top"> </td><td valign="top"><span class="cstext"> <p>A common Ethernet variety is 10BASE-T. So the media and topologies typically used for 10BASE-T are presented in some detail. The students should be told that other forms of Ethernet may or may not use the same media and topologies. But Cat 5 UTP media is used up to Gigabit per second speeds, as are extended star topologies. So this is a basic configuration for the students to learn.</p> </span></td> </tr> </table> </td> </tr> </table> </td> </tr> </table><p>In a LAN with a star topology, the networking media is run from a central hub out to each device attached to the network. The physical layout of the star topology resembles spokes radiating from the hub of a wheel.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121638/knet/v214adataimage1/1.gif" width="12" height="12"> As the figure shows, a central point of control is used in a star topology. When a star topology is used, communication between devices attached to the local area network is via point-to-point wiring to the central link or hub. All network traffic in a star topology passes through the hub.</p><p>The hub receives frames on a port. It then copies and transmits (repeats) the frame to all of the other ports. The hub can be either active or passive. An active hub connects the networking media as well as regenerates the signal. In Ethernet where hubs act as multiport repeaters, they are sometimes referred to as concentrators. By regenerating the signal, active hubs enable data to travel over greater distances. A passive hub is a device used to connect networking media and does not regenerate a signal.</p><p>One of the advantages of the star topology is that it is considered the easiest to design and install. This is due to the networking media being run directly out from a central hub to each workstation area. Another advantage is its ease of maintenance since the only area of concentration is located at the hub. In a star topology, the layout used for the networking media is easy to modify and troubleshoot. Workstations can be easily added to a network employing a star topology. If one run of networking media is broken or shorted, then only the device attached at that point is out of commission, the rest of the LAN will remain functional. In short, a star topology means greater reliability.</p><p>In some ways the advantages of a star topology can also be considered disadvantages. For example, while limiting one device per run of networking media can make diagnosis of problems easier, it also increases the amount of networking media required, which adds to the setup costs. And, while the hub can make maintenance easier, it represents a single point of failure (if the hub breaks, everyone's network connection is lost).</p><p>TIA/EIA-568-A specifies that the physical layout, or topology that is to be used for horizontal cabling, must be a star topology.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121638/knet/v214adataimage2/2.gif" width="12" height="12"> <img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121638/knet/v214adataimage3/3.gif" width="12" height="12"> This means the mechanical termination for each telecommunications outlet/connector is located at the patch panel in the wiring closet. Every outlet is independently and directly wired to the patch panel.</p><p>The TIA/EIA-568-A specification, for the maximum length of horizontal cabling for unshielded twisted pair cable, is 90 m.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121638/knet/v214adataimage4/4.gif" width="12" height="12"> The maximum length for patch cords at the telecommunications outlet/connector is 3 m, and the maximum length for patch cords/jumpers at the horizontal cross-connect is 6 m.</p><p>The maximum distance for a run of horizontal cabling, that extends from the hub to any workstation, is 100 m.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121638/knet/v214adataimage5/5.gif" width="12" height="12"> (actually 99 m. but it is commonly rounded up to 100 m.) This figure includes the 90 meters for the horizontal cabling, the 3 meters for the patch cords, and the 6 meters for the jumpers at the horizontal cross-connect. Horizontal cabling runs in a star topology radiate out from the hub, much like the spokes of a wheel. This means that a LAN that uses a star topology could cover the area of a circle with a radius of 100 m.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121638/knet/v214adataimage6/6.gif" width="12" height="12"></p><p>There will be times when the area to be covered by a network will exceed the TIA/EIA-568-A specified maximum length that a simple star topology can accommodate. For example, envision a building where the dimensions are 200 m x 200 m.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121638/knet/v214adataimage7/7.gif" width="12" height="12"> A simple star topology that adhered to the horizontal cabling standard specified by TIA/EIA-568-A could not provide complete coverage for that building.</p><p>As indicated in the Figure <img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121638/knet/v214adataimage8/8.gif" width="12" height="12">, workstations E, F, and C are located outside the area that can be covered by a star topology that adheres to TIA/EIA-568-A specifications. As shown, they are not part of the local area network. So users at these workstations wanting to send, share, and receive files, would have to use sneakernet. Because no one wants to return to the days of sneakernet, some cable installers are tempted to solve the problem of inadequate coverage in a star topology by extending the length of the networking media beyond the TIA/EIA-568-A specified maximum length.</p><p>When signals first leave a transmitting station, they are clean and easily recognizable. However, the longer the cable length, the weaker and more deteriorated the signals become as they pass along the networking media. If a signal travels beyond the specified maximum distance, there is no guarantee that it will be readable when it reaches the NIC.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121638/knet/v214adataimage9/9.gif" width="12" height="12"></p><p>If a star topology cannot provide enough coverage for an area to be networked, the network can be extended through the use of internetworking devices that compensate for the attenuation of the signal. This resulting topology is designated as an extended star topology. By using repeaters, the distance over which a network can operate is extended. Repeaters take in weakened signals, regenerate and retime them, and send them back out onto the network.<img border="0" src="../../../../../CHAPID=knet-v214aCH47507/RLOID=knet-v214aRLO47674/RIOID=knet-v214aRIO121638/knet/v214adataimage10/10.gif" width="20" height="12"></p><p> <a href="#" onClick="window.open('../../../../../static/cabling/index.html','CablingStandards','width=742,height=390,toolbar=0,scrollbars=0,left=26,top=38');void(null);">Upcoming Changes in Cabling Standards</a> </p></td> </tr> </table> </td> </tr></table> <p> </p> <table width="100%" cellspacing="0" cellpadding="2" border="0" valign="bottom"> <tr> <td></td> </tr> </table> </body></html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -