Encuesta de medición del nivel de integración.

While 10-Mbps Ethernet equipment is still widely available, 100-Mbps Fast Ether- net is currently the industry standard. All Fast Ethernet network interface adapters are dual-speed devices, capable of automatically detecting the current speed of a network and adjusting themselves to operate at either 10 or 100 Mbps. Single- speed hubs are still available that run at either 10 or 100 Mbps, but there are also dual-speed hubs on the market with the same automatic speed adjustment capa­ bility as a dual-speed network adapter.

A Fast Ethernet network requires Category 5 UTP or better, or fiber-optic cable, but standard Ethernet requires only Category 3. This is not a major impediment, however, because virtually all UTP cable installed today is at least Category 5.

For a typical business LAN, 10 Mbps is usually sufficiently fast to read and write data files stored on network servers. However, a heavily trafficked network or one supporting high- bandwidth applications such as streaming audio or video can benefit from the 100-Mbps Fast Ethernet transmission speed. Despite the fact that many networks don’t need the addi­ tional bandwidth, Fast Ethernet is the recommended technology for new LAN installations today. Fast Ethernet equipment is almost the same price as standard Ethernet, and the addi­ tional speed provides room for growth in the type of applications your network uses.

Tip Remember that the transmission speed of the LAN usually has little or no effect on the users’ Internet access performance. Even the slowest Ethernet LAN, running at 10 Mbps, is six times faster than the 1.544-Mbps T-1 connection large organizations typically use to con­ nect to the Internet.

Gigabit Ethernet is still a relatively new technology, and as a result, it is still quite expensive when compared to Fast Ethernet. You use Gigabit Ethernet primarily for backbone networks and other applications that require its high speed. Although virtu- ally no networks run Gigabit Ethernet to the desktop, it is likely to occur in the future. Apart from the cost of the equipment, these different Ethernet speeds also vary in their cabling guidelines. Although all forms of Ethernet can still support the 100-meter maximum cable length, there are different limitations on the number of hubs permitted on a LAN, which is a factor you must consider when designing your network. As mentioned earlier in this lesson, a standard Ethernet LAN can have up to four hubs, while a typical Fast Ethernet LAN can have only two. Gigabit Ethernet supports only a single hub on a network.

Selecting an Ethernet Variant

Choosing a variant of the Ethernet protocol for your network is primarily a matter of selecting a cable type and a transmission speed. Table 1-1 lists the Ethernet variants that are currently in common use.

Table 1-1 Ethernet Variants

Ethernet Maximum

Type Designation Cable Type Speed Segment Length

Standard Ethernet Fast Ethernet Fast Ethernet Gigabit Ethernet Gigabit Ethernet Gigabit Ethernet Gigabit Ethernet Gigabit Ethernet

10Base-T Category 3 UTP

100Base-TX Category 5 UTP 100Base-FX 62.5/125 multimode

fiber optic 1000Base-LX 9/125 single-mode

fiber optic

1000Base-LX 50/125 or 62.5/125 multimode fiber optic 1000Base-SX 50/125 multimode fiber optic (400 MHz) 1000Base-SX 62.5/125 multimode fiber optic (160 MHz) 10 Mbps 100 meters 100 Mbps 100 meters 100 Mbps 412 meters at half duplex and 2,000 meters at full duplex 1000 Mbps 3 kilometers

1000 Mbps 550 meters 1000 Mbps 500 meters 1000 Mbps 220 meters 1000Base-T Category 5 (or 5E) UTP 1000 Mbps 100 meters

Using Token Ring

Token Ring is a data-link layer protocol that doesn’t support as many media types as Ethernet, and at 16 Mbps, it does not run as fast either. However, there is one advan­ tage to Token Ring that makes it a popular solution for some network engineers. The primary difference between Token Ring and Ethernet is that they use different media access control mechanisms. Media access control (MAC) is a sublayer of the data-link layer and is the method the computers on a LAN use to indicate they want to use the network to transmit data. If two computers transmit data at the same time, a conflict occurs called a collision, and both transmissions are lost. To prevent collisions, data- link layer protocols have a procedure that enables one computer at a time to take pos­ session of the network so that it can transmit its data without difficulty.

The Ethernet MAC mechanism is called Carrier Sense Multiple Access with Collision Detec­ tion (CSMA/CD). With this mechanism, a computer that has data to transmit listens to the network to see whether any other computer is transmitting. If the network is free, the com­ puter proceeds to transmit its data. During the transmission, the computer continues to

monitor the incoming signals from the network. If the transmitting computer detects a sig­ nal from another computer during its transmission (indicating that the two computers started sending data at nearly the same moment), a collision is said to have occurred. Both systems stop transmitting, pause for a randomly determined period of time, and then begin the entire transmission process over again. If one (or both) of the systems is involved in another collision, it waits twice as long before attempting to transmit again. This introduces a measure of waste, as the computers end up transmitting the same data more than once. With CSMA/CD, a certain number of collisions are normal and expected. As the traffic level on the network increases, so does the number of collisions. For this reason, Ethernet becomes a less effective protocol on a heavily trafficked network.

Token Ring, on the other hand, uses a MAC mechanism called token passing. With this method, a computer on the network generates a tiny packet called a token, which cir­ culates endlessly around the network. Only the computer in possession of the token is permitted to transmit its data. When a system wants to transmit data, it waits until the token passes by, grabs the token, and then proceeds to transmit its data. After complet­ ing the transmission, the computer then sends out the token again so that another sys­ tem can use it. The protocol specifications determine how long the system can keep the token, how long it can circulate the token, and how it can generate a new token if one isn’t currently in circulation. The advantage of token passing is that there are no collisions whatsoever on a properly functioning network. Consequently, there are also no retransmissions, so no bandwidth is wasted. This also means that a token-passing network functions just as well under heavy traffic conditions as it does in light traffic.

Off the Record The limited speed of Token Ring networks and the higher hardware prices have kept the protocol a distant second choice to Ethernet, but there are enough Token Ring advocates to keep the products on the market.