History: Initial Idea

￿Shared media ￿ CSMA/CD as access algorithm

￿COAX Cables

￿Half duplex communication

￿Low latency ￿ No networking nodes (except repeaters)

￿One collision domain and also one broadcast domain

10 Mbit/s shared by 5 hosts ￿ 2 Mbit/s each !!!

(C) Herbert Haas 2005/03/11

3

 

 

The initial idea of Ethernet was completely different than what is used today under the term "Ethernet". The original new concept of Ethernet was the use of a shared media and an Aloha based access algorithm, called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Coaxial cables were used as shared medium, allowing a simple coupling of station to bus-like topology.

Coax-cables were used in baseband mode, thus allowing only unicast transmissions. Therefore, CSMA/CD was used to let Ethernet operate under the events of frequent collisions.

Another important point: No intermediate network devices should be used in order to keep latency as small as possible. Soon repeaters were invented to be the only exception for a while.

An Ethernet segment is a coax cable, probably extended by repeaters. The segment constitutes one collision domain (only one station may send at the same time) and one broadcast domain (any station receives the current frame sent). Therefore, the total bandwidth is shared by the number of devices attached to the segment. For example 10 devices attached means that each device can send 1 Mbit/s of data on average.

Ethernet technologies at that time (1975-80s): 10Base2 and 10Base5

3