THE D-LINK DSS-5+ 5-PORT ETHERNET SWITCH
Last updated: 12/2/99

INTRODUCTION.  A local area network (LAN) consists of one or more computers networked together, with cables and networking hardware and software components, in a small area such as home office, building, or even a group of buildings, such as a college campus.  The two most common unshielded twisted-pair (UTP) network standards are the10 Mhz 10BASE-T Ethernet and the 100Mhz 100BASE-TX Fast Ethernet.  The 100BASE-TX standard has become the predominant LAN standard.

Among the ways to connect the PC's in a small 100BASE-TX Ethernet together are a crossover cable, hub, and a switch.  A crossover cable is fine for a two PC network.  A hub or an Ethernet switch is needed for more than two PC's. A hub is the simpler of the two.  A switch is faster, more complex, and costs more than a hub.

THE DIFFERENCE BETWEEN A HUB AND A SWITCH.  When a computer sends an Ethernet packet to a hub, the hub forwards the packet to all of its ports and thus to all of the computers on the LAN.  That is, the hub and the computers connected to the hub form one network segment (or Ethernet collision domain).  A 100BASE-TX hub therefore has a total network bandwidth of 100 Mhz which is shared by all of the computers connected to the hub.  If one or more computers tries to send a packet to the LAN (or segment) at the same time a collision (garbage) results.  All of the involved computers must go through a routine which is part of the Ethernet Carrier Sense Multiple Access with Collision Detection (CSMA/CD) process to resolve which one them can send a packet next.  If one person on the network starts downloading a large file from another computer to his or her computer, it will congest the network and slow it down with collisions which result when other computer try to send packets.  In a 10 Mhz 10BaseT network the affect is to slow the network to nearly to a crawl.  The affect on a small 100 Mhz 100BASE-TX network is less noticeable unless several computers are transferring large amounts of data at the same time.

An Ethernet switch automatically divides the network into multiple segments, acts as a high-speed, selective bridge between the segments, and supports simultaneous connections of multiple pairs of computers which don't compete with other pairs of computers for network bandwidth.  It accomplishes this by maintaining a table of each destination address and its port.  When the switch receives a packet, it reads the destination address, establishes a temporary connection between the source and destination ports, sends the packet on its way, and then terminates the connection.

References.

Cisco Systems, Fast Ethernet Hub or Ethernet Switch?

Quick Reference Guides to 100 Mbps Fast Ethernet
By Charles Spurgeon