Speed Considerations

Ethernet comes in several different varieties, which are summarized in Table 17.1. Each type of Ethernet has its advantages, but for most new networks, the extra speed afforded by 100BaseT makes it the variety of choice. I describe the different types of cabling later in this chapter, in "Cabling Choices."

Table 17.1 Varieties of Ethernet

Ethernet Type

Cable Type

Maximum Cable Length

10Base2 10Base5 10BaseT

100BaseT

1000BaseT

1000BaseSX

Thin coaxial

Thick coaxial

Category 3, 4, or 5 twistedpair

Category 5 twisted-pair

Category 5

twisted-pair

Fiber-optic

Maximum Speed

10Mbps 10Mbps 10Mbps

100Mbps

1,000Mbps

1,000Mbps

Note

The 2 and 5 in 10Base2 and 10Base5 refer to their respective maximum cable lengths in hundreds of meters. The T in 10BaseT and 100BaseT refers to twisted-pair cable. The 10 and 100 in the Ethernet names refers to the maximum speed in megabits per second.

Part IV

Broadly speaking, two types of Ethernet adapter are commonly available today:

• 10Mbps These boards support all the 10Mbps varieties of Ethernet. Today, they usually include connectors for 10Base2 and 10BaseT, although you can use an adapter to connect 10Base5 cabling if necessary.

• 10/100Mbps These boards support both 10Mbps and 100Mbps speeds. They normally support only twisted-pair cabling. Figure 17.1 shows a typical 10/100Mbps NIC.

Ethernet Cable Status LEDS Connector

Figure 17.1

A typical NIC includes one or more cable connectors and some activity LEDs.

Ethernet Cable Status LEDS Connector

Figure 17.1

A typical NIC includes one or more cable connectors and some activity LEDs.

Note

A few NICs are capable of using only 100Mbps protocols. These boards are quite rare, though. Most 100Mbps NICs also support 10Mbps speeds. The 1,000Mbps devices, which also go by the name gigabit Ethernet, are not common on workstations today. They're more commonly used to connect high-demand servers to a network by means of a switch or router.

Chapter 17

If you're building a new network or connecting to an existing twisted-pair network, I recommend you get a 10/100Mbps NIC. Even if your network can't now use the full 100Mbps speed, the 10/100 NICs don't cost substantially more than their 10Mbps-only counterparts, and the existence of 100Mbps-capable NICs can make it easier to upgrade your network speeds in the future.

Mixing 10- and 100Mbps hardware usually works well, although in some cases you might get only 10Mbps speeds, even between two 100Mbps-capable devices. Specifically, if you use a hub to connect your computers, all your devices default to the lowest speed supported by all your computers. If you use a switch instead of a hub, on the other hand, you'll get 100Mbps 17

transfers between those components that support the higher speed, and 10Mbps only when one computer supports only the lower speed. I describe the differences between hubs and switches ^ ^

in more detail later in this chapter, in "Hubs and Switches." g T

Ethernet works by encapsulating data in frames, which the NIC sends over the network cable. A g

A typical frame contains about 1.5KB of data. When two computers link directly to one m another, or when you use a switch to connect computers, the computers can transmit and receive data at the same time. This is known as full duplex operation. A 100Mbps Ethernet card operating in full duplex mode can theoretically send data at 100Mbps while it simultaneously transmits at the same speed. When you use a hub or coaxial cable, however, this isn't possible, so you can send at 100Mbps or receive at 100Mbps, or split the speed, as in 50Mbps sending and receiving. This is known as half duplex operation. In this configuration, too, there's an increased possibility of collisions. A collision occurs when two computers attempt to send data at the same time. As when two people talk at the same time, it becomes impossible to discern either signal. When an Ethernet card detects a collision, it pauses for a random but brief period of time and tries again. The other sender does the same. Because both pause for a random period of time, it's unlikely that their frames will collide again. Because of the need to retransmit, collisions can slow down Ethernet speed, particularly on networks that see a lot of traffic.

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