Specifications

7.1.2 10Mbps - 100Mbps - 1Gbps - 10Gbps

Initially UTP Ethernet operated at 10 million bits per second (10Mbp/s). Fast Ethernet increased speed to

100 million bits per second over Category 5 wiring 100Mbp/s). Gigabit Ethernet is 10 times faster then Fast

Ethernet (1,000Mbp/s). During Gigabit Ethernet development the Cat5 specification was tightened

resulting in Cat5e. Work is in progress to increase Ethernet speed by another factor of 10 to 10 Gigabits per

second.

7.1.3 Ethernet Hubs and Switches

UTP Ethernet is a point-to-point topology electrically even though logically it is a party line. Each Ethernet

interface must be directly connected to another Ethernet Interface. Hubs regenerate Ethernet signals and

allow devices to talk to each other, remember the party line analogy. Cable must run directly between the

outlet and the hub it cannot be spliced or daisy chained. CDMA/CA scheme used by Ethernet places a

limit on the number of wire segments and how many hubs can be used. For 10Mbps Ethernet use the 5-4-3

rule, maximum of 5 wire segments and 4 hubs between devices, however only 3 of those hubs can have

devices attached. Because 100Mbps Ethernet is faster the rules are more stringent. A maximum of two

Class II hubs, and the distance between hubs is limited to less then 5 meters. Class I hubs cannot connect

directly to another hub. For all intents and purposes 100Mbps Ethernet networks are limited to a single hub.

Where hubs need to be cascaded the solution is to use an Ethernet switch. Switches do not simply repeat

incoming packets on all ports. A switch examines each incoming packet, reads the destination address and

passes it directly to the proper port. A switch allows multiple conversations to occur simultaneously as

opposed to being limited to only one with a hub. This allows total switch bandwidth to be greater then a

hub. A 100Mbp/s hub shares 100Mbp/s among all devices. A switch segments traffic betweens pairs of

ports. A non-blocking 16-port 100Mbp/s Ethernet switch has a maximum throughput capacity of

800Mbp/s. This assumes 8 pairs of connections evenly divided between the 16 ports; each one operating at

full 100Mbps. A switch has another advantage it eliminates collisions allowing full duplex communication.

This means individual computers can be transmitting at the same time they are receiving. This doubles

throughput of our hypothetical 16-port 100Mbp/s switch to 1.6Gbp/s as compared to 100Mbp/s for a hub.

In actual use the advantage will not be as great but switches offer tremendous performance advantage

The switch selects the proper port based on MAC address. Every Ethernet controller has a MAC address.

The switch reads packets as they arrive and associates a port with a specific MAC address. When the

switch does not know which port to use it broadcasts the incoming packet to all ports, much like a hub.

When the device responds the switch knows which port it is connected to.

Hub Hub

10Mbps 5-4-3 Rule

Hub Hub

100Mbps - Class II hub

5 Meters Max

Hub

100Mbps - Class I hub

Figure 23 Hub rules for Ethernet and Fast Ethernet