Upgrading and Repairing Networks (5th Edition)
| In the early 1990s, a faster version of Ethernet was developed, commonly referred to as Fast Ethernet. This standard (IEEE 802.3u) allows for Ethernet communications over both copper wire and optical fiber cables at a speed of 100Mbps. The different standards are named in the traditional way, as a concatenation of the speed, signaling method, and medium type, as described earlier in this chapter. Fast Ethernet encompasses the 100BASE-class of Ethernet, whereas 1000BASE-denotes the Gigabit Ethernet standards. Gigabit Ethernet network cards and switches are widely available today. The specification for 10Gigabit Ethernet was finished in July 2002, and many vendors, such as Cisco, Intel, Dynatem, and others are now marketing 10Gigabit hardware. Fast Ethernet
Fast Ethernet was designed to be compatible with existing 10BASE-T networks. It uses the same frame format, and it still uses the CSMA/CD medium access method defined in the 802.3 standard. What makes it even nicer as an upgrade path for an existing network is that it can interoperate on the same wiring as 10BASE-T. That is, with an intelligent hub (or switch) that can detect the speed being used by a particular workstation's network adapter card (autosensing), you can use both types on the same network and they can talk to each other. The hub/switch takes care of buffering data to enable transfers between ports operating at different speeds. If your network still contains computers connected using 10BASE-T, it is time to upgrade to Fast Ethernet. You can do so gradually, as your schedule permits, because autosensing ports and network adapter cards allow for both 10BASE-T and 100BASE-T nodes on the same LAN. 0. 100BASE-T
One of the nice things about migrating to 100BASE-T from older technology is that you can use existing wiring if the building has Category 3 cabling already in place. The 100BASE-T standard is defined for use with either twisted-pair wiring (100BASE-TX and 100BASE-T4) or optical fiber (100BASE-FX). 100BASE-T4 is the only standard that allows for the use of Category 3 wiring, so an upgrade path exists for those who cannot afford the expense of rewiring a building at this time. Running new cabling is one of the more expensive items when upgrading a network. Yet if you are still using anything less than Category 5 cabling, you should consider this expense something you should incur now, rather than later. A speed of 10Mbps is just too slow for most applications today in a large enterprise network. Applications and data files continue to grow, and 100Mbps is now considered to be the minimum bandwidth for most wired LANs. There is an important difference between 100BASE-T4 and 100BASE-TX: They do not use the same cable pairs to transmit and receive data. 100BASE-T4 uses all four cable pairs and a different signaling technique. For sites that were forward-thinking and installed Category 5 cables when creating a 10BASE-T network, upgrading to a 100Mbps network will prove that the investment was worthwhile. This twisted-pair version of the 100BASE-T specification can be used on this cabling or on the shielded twisted-pair (STP) cables that are usually found on Token-Ring networks. The 100BASE-TX standard is based on the ANSI TP-PMD (Twisted-Pair Physical Medium Dependent) specification. The maximum segment length is 100 meters, but again you must remember to include the distance from where the horizontal wiring terminates at the work area faceplate to the workstation. The total distance through the LAN can be as many as 200 meters, incorporating up to two hubs. There are two classes of hubs: Class I and Class II. Keep in mind that hubs are considered to be legacy devices today, and you will find this equipment only in older networks. You will probably not be able to purchase new devices of this sort. However, this information may prove useful if your network has not yet been upgraded to newer technology. Here's a rundown on the classes of hubs:
0. 100BASE-T4
For those networks that have a heavily installed base of Category 3 or Category 4 cabling, this version of 100BASE-T provides an upgrade path. This standard uses half-duplex signaling on four pairs of wires, as opposed to the two pairs used by 10BASE-T and 100BASE-TX. Three of the wire pairs are used for actual data transmission, and the fourth pair is used for collision detection. The three pairs used in transmission each operate at only 33.3Mbps, for a total of 100Mbps (called the 4T+ signaling scheme). Additionally, a three-level encoding scheme is used on the wire instead of the two-level scheme used for most other media. Because 100BASE-T4 requires special hardware, such as network adapter cards and hubs, and because it operates only in half-duplex mode, it shouldn't be considered for a new installation, but only as a possible upgrade path when other options cannot be justified. 0. 100BASE-FX
Fiber-optic cable provides the greatest distance for Fast Ethernet. 100BASE-FX, using a two-strand cable (one strand for transmission and one for receiving data and detecting collisions), can achieve a distance of up to 2 kilometers. Fiber is a good choice for use as a backbone in the network. Unlike copper wire cables, which use electrical impulses for communications, fiber uses pulses of light. This also makes fiber cable a better choice in an environment with a lot of electrical interference. Because fiber-optic cable emits no electrical signals itself (which can be intercepted to eavesdrop on the network), it is also ideal in a situation in which security is a great concern. Finally, optical fiber provides a built-in capability that will certainly be pushed to greater transmission speeds as new standards develop. Gigabit Ethernet
In 1998, the 802.3z standard for Gigabit Ethernet was finished and includes the following:
Note The UTP version of Gigabit Ethernet is known as the IEEE 802.3ab standard. Because of its short range (25 meters) it is intended mainly for use in connecting equipment in wiring closets.
Gigabit Ethernet is designed to mesh seamlessly with 10/100Mbps networks but you will need special hardware that is rated for gigabit standards. Gigabit Ethernet uses the same CSMA/CD medium access protocol and the same frame format and size. It is ideally suited for use as a network backbone to connect routers and hubs or other types of repeaters, due to both its compatibility with existing technology and the speeds of transmission that can be accomplished. For example, another feature that will make Gigabit Ethernet a choice for the network backbone is the capability to run in full-duplex mode on nonshared connections. In this mode two connectionsone for send, one for receiveare used to transmit data so that collision detection will not be needed. This will enable faster data transmissions between switches used to connect LANs. The IEEE 802.3z standard for Gigabit Ethernet added another field to the basic 802.3 frame: the Extension field. This field is appended to the frame after the Frame Check Sequence field and is used to pad the frame so that its minimum size is 512 bytes instead of the 64 bytes used by slower standards. This increased size is needed only when operating Gigabit Ethernet in half-duplex mode when collision detection is still involved. This field is not needed in full-duplex mode. Another method for making faster transmissions with Gigabit Ethernet is to reduce the overhead involved with using CSMA/CD for every single frame that is sent on the network. A mode of operation called burst mode was added in the 802.3z standard that provides for sending multiple frames, one after the other, after gaining access to the network media. This is accomplished by inserting special "extension bits" in the interframe gaps between normal frames. These extension bits keep the wire active so that other stations do not sense it as being idle and attempt to transmit. Tip Another proposal that is being considered by many companies is one called jumbo frames. This proposal, the work of Alteon Networks, Inc., raises the overall length of an Ethernet frame (on a full-duplex mode link) to 9,018 bytes. It is not practical to go much further past this 9,018-byte limit, because the CRC error detection/correction mechanism used by Ethernet cannot be as precise when frames get much larger than this. However, this is a dramatic increase over the standard 1,500-byte Ethernet frame. Gigabit Ethernet is currently being widely deployed for use in local area network backbones to connect high-capacity servers or switches. This role was earlier played by Fast Ethernet (and, of course, before that by 10Mbps Ethernet). As one technology advances to the desktop, another replaces it in the backbone. As we get into the area of high-speed transport protocols, Gigabit Ethernet might now start competing in areas that were previously the domain of ATM and Frame Relay, which, in the past, were typically used to carry IP data. Although SONET is widely deployed as a metropolitan area network (MAN) solution, the faster Ethernet gets, the harder it is to justify carrying it by other transport protocols when used in a switched environment. As IP approaches and passes the 10Gigabit speed limit, it will no doubt become an important player beyond network backbone usage. Because Ethernet is Ethernet (as long as you use the right interconnecting switches!), it's easier to manage a single transport protocol than to try to manage mapping one onto another. Gigabit Ethernet is definitely in your future, whether or not it is viable for you now, and 10Gigabit Ethernet products are already on the market, if you can justify the cost. 0. 10Gigabit Ethernet (IEEE 802.3ae)
With other WAN protocols already in use on long-distance backbones for large networks and the Internet, you might not think that Ethernet, basically a LAN protocol, would need to be developed beyond what is required in a typical LAN. With switching, increasing speeds, and full-duplex connections, Ethernet has faired far better than other LAN technologies in the past 30 years. Just compare it to Token-Ring. However, there is no reason why Ethernet should not be pushed further, and there are advantages to doing so. 10Gigabit Ethernet keeps the standard 802.3 frame format and the same minimum/maximum frame sizes as previous versions of Ethernet. However, half-duplex operation is no longer supported, and 10Gigabit Ethernet does not have any provisions for using a shared network mediayou'll use switches, not hubs. By removing the half-duplex feature and removing the need for CSMA/CD, the distances that Ethernet can now cover are limited only by the physical network media and the signaling method used. Another important reason for dropping the half-duplex option is the fact that although Gigabit Ethernet supports half- and full-duplex modes, customers have almost unanimously chosen full-duplex products. At the Physical layer, the 802.3ae specification provides for two Physical layer (PHY) typesthe LAN PHY and the WAN PHY. The PHY layer is further subdivided into the Physical Media Dependent (PMD) part and the Physical Coding Sublayer (PCS). The PCS is concerned with how data is coded onto the physical network. The PMD represents the physical components, such as the laser or the light wavelength used. The LAN PHY and the WAN PHY will both support the same PMDs. The PMDs for 10Gigabit Ethernet range from using an 850 nm laser on multimode optical fiber (50.0 microns) for short distances (up to 65 meters) to using a 1550 nm laser on single-mode fiber (9.0 microns) for up to 40 kilometers. The LAN PHY will be designed to operate with existing Gigabit Ethernet LAN encoding, but at a faster rate. The WAN PHY is a separate physical interface that allows for longer distances, with an optional interface under consideration that would allow 10Gigabit Ethernet to use SONET/SDH as a transport. SONET OC-192 provides a payload that is close to the 10Gigabit speed offered by Gigabit Ethernet. All that is necessary to connect the two is to provide some simple buffering mechanisms in connecting network equipment. Because SONET/SDH is a widely deployed technology, this means that it will not require WAN providers to make a huge investment in new cables to carry 10Gigabit Ethernet traffic. Instead, it becomes a value-added service they can offer to their customers. End-to-end Ethernet connections for a wide area network without the time-consuming need to convert from one frame format will make managing WANs simpler because there will be fewer factors that can go wrong. However, the current outlook is to use 10Gigabit Ethernet as the WAN protocol. It is estimated that it will be cheaper to implement 10Gigabit Ethernet services than to provide a similar T3 solution in a MAN or WAN environment. Of course, there are critics who point out that Ethernet cannot provide the same guaranteed Quality of Service (QoS) that ATM does. And, when compared to SONET and other high-speed transmission protocols, Ethernet comes up lacking in the management tools area. However, the simplicity of Ethernet and the fact that it costs so much less than other WAN solutions make it very attractive for many markets. |
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