Lesson 2

Selecting Data-Link Layer Protocols

1-21

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 Segment

Type

Designation

Cable Type

Speed

Length

Standard

10Base-T

Category 3 UTP

10 Mbps

100 meters

Ethernet

Fast

100Base-TX

Category 5 UTP

100 Mbps

100 meters

Ethernet

Fast

100Base-FX

62.5/125 multimode

100 Mbps

412 meters at half

Ethernet

fiber optic

duplex and 2,000

meters at full duplex

Gigabit

1000Base-LX

9/125 single-mode

1000 Mbps

3 kilometers

Ethernet

fiber optic

Gigabit

1000Base-LX

50/125 or 62.5/125

1000 Mbps

550 meters

Ethernet

multimode fiber optic

Gigabit

1000Base-SX

50/125 multimode

1000 Mbps

500 meters

Ethernet

fiber optic (400 MHz)

Gigabit

1000Base-SX

62.5/125 multimode

1000 Mbps

220 meters

Ethernet

fiber optic (160 MHz)

Gigabit

1000Base-T

Category 5 (or 5E)

1000 Mbps

100 meters

Ethernet

UTP

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.