Ethernet Networking- P3 docx

48 Fast and Gigabit Ethernet Media and Standards attenuation. Fiber optic media can therefore be used in situa- tions where wire media pose problems, such as on factory floors. # It is much harder for someone to tap than wire media. # It is much less susceptible to attenuation than wire media. # It has much higher bandwidth than most wire media. The same fi- ber optic media can carry Ethernet signals at any standard speed. On the other hand, fiber optic cabling is more difficult to work with than wire. It cannot be spliced and taped with electrical tape like wire, but in- stead requires special connectors that precisely line up the ends of two seg- ments of cable with one another. In addition, fiber optic equipment is more expensive than equipment for wire media. Nonetheless, in environments where many devices share network media (in particular, linking servers) or where severe electrical interference is a factor, fiber optic cabling is a viable choice. For example, in graphics and video design firms where large files move between workstations, fiber op- tic cabling can significantly speed up workflow by providing additional bandwidth. 5ing/e versus Multimode Fiber Optics There are two types of fiber optic cabling, single mode and multimode. Sin- gle mode, which can transmit a single wavelength of light long distances, is used primarily for WAN connections. Multimode can transmit multiple sig- nals at one tim, but is more limited in length and typically used in LANs. When light is introduced into an optical fiber, it can either go straight down the middle of the optical tube or it can travel at an angle, reflecting off the side of the tube as it travels. Each signal traveling down the tube at a time is known as a mode. The diameter of the core of a single-mode fiber is very small (for example, 9 microns). A single ray of light is transmitted down the core, and it travels without reflection straight to its destination. In theory, one single-mode fi- ber link can be as long as 10 kilometers. Fiber Optic Cabling 49 Multimode fiber has a larger core diameter and supports the transmission of multiple signals. Each ray of light has a different angle of reflection, making it possible for the receiving device to separate the individual sig- nals. (See Figure 3-6.) However, the reflection angles disperse over dis- tances (modal dispersion), spreading the signals and ultimately making it impossible to tell the signals apart. This limits the distance of multimode fiber. If the core is 62.5 microns in diameter, the maximum length is ap- proximately 275 meters; 50 micron fiber can go as far as 550 meters. Multiple modes I ~ transmitted [ concurrently~ Cladding i\ Cladding r- Modes ~, travel ~, different distances and therefore exist at different times Figure 3-6: Multiple signals traveling down multimode fiber Multimode fiber is generally easier to work with than single mode. Be- cause fiber optic cabling cannot be spliced, the ends of two pieces of single mode fiber must the aligned precisely when they are to be used as a single run of cable. Multimode fiber, because of its shorter runs, often doesn't need to be assembled out of multiple pieces of cabling; it can use a single unbroken piece of fiber. Fiber Optic Cable Bundles Just as UTP cable comes in several varieties, fiber optic cables come in seven basic types of bundle. As you can see in Table 3-4, they vary in where they are used and their strength. Each can use either single or multimode fiber. There are no standards for fiber optic cable assembly; therefore, these types of cable vary somewhat from one manufacturer to another. 50 Fast and Gigabit Ethernet Media and Standards Table 3-4: Types of Fiber Optic Cable Type Use Description Tight-buffered Inside Disribution Inside Breakout Inside Loose tube Outside Ribbon Outside Armored Outside Arial Outside Each fiber is surrounded by 900 microns of aramid yarn. Then the yarn is surrounded by plastic. There is no reinforcement of each individual fiber. Used primarily for patch cords and from the wall to the desktop. A cable with just a single fiber is known as simplex; a two-wire cable is duplex. Made of many tight-buffered fibers. No reinforcement of individual fibers. Must terminate in a breakout box or patch panel. Many tight-buffered cables bundled together reinforcing fibers (e.g., aramid yarn). Because the fibers are reinforced, does not need to terminate in a breakout box or patch panel; may use quick-install connectors. Although more expensive per foot than distribution cabling, may be cheaper and easier to install and maintain. A single fiber optic rod runs down the center to reinforce fibers wound around it. The outside coating can be filled with a gel to protect the fibers from water. Therefore, it can be buried. Made from layers of fiber optic tubes. Covered with metal for burying in areas where rodents are a problem. Designed for running on utility poles. Usually hung from a "messenger" cable or from another utility wire. Fast Ethernet Standards There are four Fast Ethernet media specifications, three of which use UTP wire and one of which uses fiber optic cable. Fast Ethernet Standards 51 Twisted- Pair Wire The three Fast Ethernet options that are designed for UTP wire are sum- marized in Table 3-5. Current installations, however, are almost exclusive- ly 100BASE-TX, using Category 5 or higher wire. Therefore, we will focus solely on 100BASE-TX. Table 3-5: Fast Ethernet Cabling Options Standard Cable type 100BASE-TX 100BASE-T4 100BASE-T2 Category 5 UTP (uses 2 pairs of wire) Category 3 UTP (uses 4 pairs of wire) Category 3 UTP (uses 2 pairs of wire) As you can see from the preceding table, the predominant UTP standard for Fast Ethernet uses only two of the four pairs of wires in the cable. The specific wire usage for 100BASE-TX can be found in Table 3-6. The mir- rored signals (-TD and-RD) are used to help identify and eliminate crosstalk (the cancellation technique). The receiving station can compare the positive and negative polarity signals. They should be the same except for the polarity. Any difference can be attributed to crosstalk and stripped out. Another way to look at the same issue to to call it differential signaling. Any interference that affects one wire will almost certainly affect the other as well. The receiver can then calculate the difference between the two sig- nals, which will always be constant, regardless of interference. Fiber Optics Fiber optic configurations for Fast Ethemet are covered by the 100BASE- FX standard. Like standard Ethernet wiring for fiber optic cables, Fast Ethernet fiber optics requires two cables, one for transmitting and one for receiving. 52 Fast and Gigabit Ethernet Media and Standards Table 3-6: Fast Ethernet UTP Wire Usage Wire color Use Comments White (paired with green) +TD Green -TD White (paired with orange) +RD Orange -RD White (paired with blue) unused Blue unused White (paired with brown) unused Brown unused Transmit data Copy of transmit data signal but with the opposite polarity Receive data Copy of receive data signal but with the oppostie polarity Gigabit Ethernet Standards Like Fast Ethernet, the Gigabit Ethernet standard has been written for two types of medium: fiber optics and copper wire. In this case, the fiber optic implementations came first and UTP implementations have become feasi- ble to the desktop since about 2004. (The technology was available before that, but wasn't particularly affordable.) Fiber Optics Three standards deal with Gigabit Ethernet networks built from fiber optic cable. I O00BASE-CX: Designed for the direct interconnection of clusters of equipment. This standard has been superceded by 1000BASE-T, the UTP implementation of Gigabit Ethernet. Gigabit Ethernet Standards 53 I O00BASE-SX: Designed for horizontal cabling using multi- mode fiber. In other words, you can use this stanadard for inter- connecting network segments on a single floor or for creating a group of servers (a server farm). I O00BASE-LX: Designed for interconnecting network seg- ments, including vertical runs through buildings, using single- mode fiber. Most small offices do not need to use media based on this standard. As you may have already concluded, the distance that you can run a fiber optic segment depends on the diameter of the fibers in the cable and the ca- ble's bandwidth. As you can see in Table 3-7, segment lengths in the pub- lished standards vary from 230 to 5000 meters. There is, however, a large gap between the last two entries in the table (5000 meter maximum) and the remaining entries because the last two are single-mode fiber specifications. Table 3-7: Sample Fiber Optic Cable Lengths Standard Diameter Bandwidth Cable length (in microns) (MHz*km) (in meters) 1000BASE-SX 62.5 160 2-230 62.5 200 2-275 50 400 2-500 50 500 2-550 1000BASE-LX 62.5 500 2-550 50 400 2-550 50 500 2-550 9 n/a 2-5000 5 5000 2-5000 Twis te d- Pair Wire 1000BASE-TX, which requires Cat 5 or better cabling, uses all eight wires in the UTP cable. In addition, each wire can handle a bidirectional signal 54 Fast and Gigabit Ethernet Media and Standards (both send and receive). The signal is then sent in four parts, mirrored on each pair of wires, as in Table 3-8. Table 3-8: Fast Ethernet UTP Wire Usage Wire color Use Comments White (paired with green) +BI_DA Bidirectional data A Green -BI_DA Mirror of +BI_DA White (paired with orange) +BI DB Bidirectional data B m Orange -BI_DB Mirror of +BI_DB White (paired with blue) -BI_DC Mirror of +BI_DC Blue +BI_DC Bidirectional data C White (paired with brown) +BI_DD Bidirectional data D Brown -BIDD Mirror of-BI_DD Note: To be completely accurate, Gigabit Ethernet over UTP cabling doesn't really run at I Gbps. It actually runs at the same speed as Fast Ethernet, but it uses all four pairs of wires at the same time and handles two signals per wire. That produces the I Gbps speed/ Creating Network Segments The basic building block of an Ethernet is a network segment, a group of devices whose message exchanges are controlled by a single interconnec- tion device. Hubs (once also known as repeaters) and switches are inter- connection devices that create individual network segments; switches and routers connect segments to make larger networks. Note: Routing is such a complex topic that it is covered in a chapter of its own (Chapter 6). Although you can still purchase hubs today, and they are in use in many ex- isting networks, the price of switches has dropped so significantly that there is rarely any reason to install a hub in a new or upgraded network. As you will see, switches provide better performance at about the price of a hub. 55 56 Creating Network Segments Hubs (Repeaters) As you will remember from Chapters 1 and 2, Ethernet was created to al- low multiple devices to share the same wire. The original topology (the layout of the devices and the wire) was a straight-through bus, as in Figure 4-1. All messages are broadcast to the network bus, where all devices can read them. Such topologies are based on coaxial cable. (See Appendix A for details on outdated Ethemet standards.) Such segments would often be equipped with repeaters, devices that read the broadcast signal and retrans- mitted it, thus extending the length of the bus. File server Connection M~~ Internet l l The bus J, networki T - T " ' Printer ,r ~ i Workstation Workstation Figure 4-1" A simple Ethemet bus topology When Ethemet standards for UTP appeared, the bus was collapsed into a single small box called a hub. (See Figure 4-2.) Each device is connected to the hub with a single UTP cable. However, because the bus is a single wire, it can carry only one signal at a time. That means that a device can either transmit or receive, but not both, at the same time. Communication is therefore half duplex (bidirectional but only one direction at a time). Transmission using a hub happens in the following sequence" 1. A device checks the bus. 2. If the bus is free, the device transmits using its transmit wire. If the bus is not free, the device waits, following the CDMA/CD protocol. Hubs (Repeaters) 57 Figure 4-2: The wiring inside a hub 3. The transmitting device handles a collision if one occurs. 4. When the hub receives a signal without a collision, it repeats the signal and broadcasts it out all its ports. 5. All attached devices recognize that there is a signal on their receive line. Each checks the MAC address of the frame to determine whether it is the receiptient of the frame. Most hubs today can handle multiple transmission speeds. For example, a Fast Ethernet hub can handle 100 Mbps connections as well as the older 10 Mbps connections. The ports are said to be autosensing because they can automatically detect the maximum speed of the NIC at the other end of the UTP cable. Unmanaged Hubs A hub of the type we have been discussing is known as an unmanaged or passive hub and has no intelligence of its own. In particular, it has no idea what devices are connected to its ports. All it can do is broadcast a signal out all ports. A hub is a simple device with no moving parts that is reliable, [...]... limits on standard Ethernet installations To prevent this on a Fast Ethernet network, you must take overall cable distances into account when planning the network layout You start with a table of the average round-trip delays, expressed in bit times, for the devices on your network (see Table 4-1) The maximum allowable bit times be- Hubs (Repeaters) 63 Table 4-1" Type Sample Fast Ethernet Round-trip... you use a switch, there is no longer any contention for a single bus For this reason, some people believe that a network that is not built on hubs isn't really Ethernet However, switches still use the Ethernet frame layout and adhere to all other Ethernet standards Whichever way you view it, go for the switch; forget the hub unless someone gives you one and you can't afford a switch Creating Network... simple and workable solution as long as no single network device is more than 100 meters from the hubs Figure 4-8: Fast Ethernet stackable hubs (Courtesy of 3Com Corporation) Propaga?ion Delay Cable length isn't the only problem that you can run into when you use hubs to build an Ethernet Another major issue is propagation delay, the time it takes for a signal to be broadcast and read by all devices... computers are equipped with it Otherwise, Fast Ethernet over UTP to the desktop is more than adequate Note: Of course there will be exceptions to the preceding there are always exceptions to just about everything! For example, if you are networking a graphics design firm and artists are exchanging large files over the network, then you may want Gigabit Ethernet throughout the network and feel that the... 100BASE-T4 device 127 Category 3 cable segment 1.14 114/100 meters Category 4 cable segment 1.14 114/100 meters Category 5 cable segment 1.112 111.2/100 meters Fiber optic cable 1.00 412/412 meters Fast Ethernet hub 92 tween any two devices is 512, Therefore, you add up the delays between the two devices on the network that are the farthest apart from one another If the result is less than 512, then the... Most switches operate at the Data Link layer in the joint TCP/IP and OSI protocol stack They therefore have access to the MAC addresses of both the sending and receiving devices, which are part of the Ethernet frame that the Data Link layer handles (A switch doesn't necessarily contain the entire TCP/IP-OSI protocol stack, but only the Physical and Data Link layers, which are all it needs to operate.)... outdated and that because switches cost only a tiny bit more, there is very little reason to use hubs (Why discuss hubs at all? Because you can't appreciate switches and how they are and aren't exactly Ethernet unless you understand hubs !) From the outside, a small switch doesn't look all that much different from a hub (for example, see the switches in Figure 4-10) They have RJ-45 ports and possibly... ensuring that there is only one path from one switch to another in a network A group of switches in the same "tree" select one switch as the "root" of the tree (This is usually the switch with the lowest Ethernet address.) Then, the root collects configuration information from all the switches in the tree and sends that information to every switch involved Each switch then computes a single path from its... certainly don't want employees and clients/customers to be walking over network cabling, nor do you want your switches accessible to anyone who might be in the office This is where wiring closets come in When Ethernets were new, a wiring closet was exactly t h a t ~ a place where all the network wiring came together The closet would be locked so that only a network engineer could gain access to make physical... require manipulating Creating Network Segments 76 the individual wires in a cable, but they often contain switches, hubs and other equipment that you need to keep away from unauthorized hands A hub-based Ethernet wiring closet would contain one or more patch panels, devices like those in Figure 4-17 into which RJ-45 plugs could be inserted The patch panels are rack-mountable, which means that they can . another utility wire. Fast Ethernet Standards There are four Fast Ethernet media specifications, three of which use UTP wire and one of which uses fiber optic cable. Fast Ethernet Standards 51. standard. Like standard Ethernet wiring for fiber optic cables, Fast Ethernet fiber optics requires two cables, one for transmitting and one for receiving. 52 Fast and Gigabit Ethernet Media and. Copy of receive data signal but with the oppostie polarity Gigabit Ethernet Standards Like Fast Ethernet, the Gigabit Ethernet standard has been written for two types of medium: fiber optics