MaximumPC 2008 07

W

e tend to take things for granted when

they work exceptionally well. Take

Ethernet, for instance; it’s almost

magical: Plug a simple cable into a computer,

and it can exchange data with another rig—or

many others. Peek behind the curtain and you’ll

discover a brilliantly simple yet continually

evolving networking system.

But let’s clear up one thing fi rst: Ethernet

technology doesn’t actually contain ether (in-

cluding its form as a chemical analgesic, so don’t

bother sniffi ng the cables). Robert Metcalfe, one

of the technology’s inventors, copped the name

from an early scientifi c theory (discredited in

1887) that luminiferous ether was a passive sub-

stance that permitted the propagation of light.

Metcalfe thought the name suitable because

the cable used to build a network is a passive

medium that permits the propagation of data.

ETHERNET’S ORIGINS

The original Ethernet was created at Xerox’s

Palo Alto Research Center in the mid 1970s. It

allowed all the computers on the network to

be connected to a single cable, using a protocol

known as CSMA/CD (Carrier Sense Multiple

Access with Collision Detect). When one com-

puter wanted to transmit, it would fi rst check to

see if any other machines were using the line.

If the line was free, the transmitting computer

tagged the data it needed to send with a MAC

(Media Access Control) address and loaded

it onto the network. The MAC

address identifi ed the intended

recipient so that the machine

possessing that unique MAC

address would accept the data

and all the other machines on the

network would ignore it.

If the transmission line

was busy, the computer would wait until it

detected a lull, but if two machines tried to

transmit at the same time, each would react to

the collision by waiting a random number of

milliseconds before attempting to retransmit.

The process was simple, but it was also very

limited. Multiple collisions could quickly throt-

tle the performance of a large network, for ex-

ample, and it was relatively easy to eavesdrop

on the network’s traffi c—all it took was a fake

MAC address. The network wasn’t very robust,

either: Damage to any cable in the network

could cause the entire system to crumble.

SWITCHED ETHERNET

The introduction of the hub enabled larger

Ethernet networks. A hub rebroadcasts

network traffi c to extend the network’s reach,

and it eliminates the problem of one damaged

cable bringing down the entire network. But

the development of the switch was a far more

signifi cant improvement to Ethernet topology.

The switch inspects the source and destination

addresses of every message carried on the

network, and it uses this information to con-

struct a look-up table so that it knows which

machine is connected to each of its ports.

When the switch receives a packet of data

tagged with a specifi c MAC address, it can

transmit the data directly to the port that the

recipient machine is connected to. This leaves

all the other ports free, and it minimizes the

possibility of collisions.

BUT LET’S CLEAR UP ONE

THING FIRST: ETHERNET

TECHNOLOGY DOESN’T

ACTUALLY CONTAIN ETHER.

62 |MAMAMAXIMXIMXIMXIMUUUUMMPPPCC| JUL 08 | http://www.maximumpc.com

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EXAMINING TECHNOLOGY AND PUTTING IT TO USE

How the world’s most common LAN technology works —ZACK STERN

Ethernet

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Ethernet

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HOW IT WORKS

80 00 20 7A 3F 3E 80 00 20 20 3A AE 08 00 IP, ARP, ETC. 00 20 20 3A

MAC HEADER

(14 bytes)

TOTAL SIZE

(64 to 1,518 bytes)

DATA

(46–1,500 bytes)

VERIFICATION

(4 bytes)

Destination MAC address Source MAC address Ether type Payload CDC checksum

Ethernet packets look the same whether they’re traveling over Cat5e, Cat6, or radio waves. The MAC header identifi es where the data came from and

where it’s going. The payload is the data itself, and the CRC checksum is a means of verifying the packet’s integrity.

Anatomy of an Ethernet Frame