UNIX System Administration Handbook - Evi Nemeth [224]
15.2 ETHERNET: THE COMMON LAN
Having captured over 80% of the world-wide LAN market, Ethernet can be found just about everywhere in its many forms. It started as Bob Metcalfe’s Ph.D. thesis at MIT. Bob graduated and went to Xerox PARC; together with DEC and Intel, Xerox eventually developed Ethernet into a product. It was one of the first instances in which competing computer companies joined forces on a technical project.
Ethernet was originally specified at 3 Mb/s (megabits per second), but it moved to 10 Mb/s almost immediately. It was developed on the Xerox Alto, which didn’t have enough room on the circuit board for an external clock. The Ethernet interface had to use the Alto’s clock, which meant that the network speed had to be 2.94 Mb/s. This was rounded up to 3 Mb/s. Metcalfe and other early developers who had worked on the architecture of the ARPANET objected to a roundoff error that exceeded the ARPANET’s entire bandwidth, but marketing won out.
Ethernet weathered its early years in the 1980s, a time at which a variety of network operating systems, including UNIX, were also gaining basic networking skills and learning to play with each other. When Ethernet hit its mid-teenage years, it was ready to drive. In 1994, Ethernet caught attention as it was standardized at 100 Mb/s. Just after turning 19 years old in 1998, it was ready to fight a new war—at 1 Gb/s. Now an adult in its early 20s, Ethernet is headed for the new frontier of 10 Gb/s, having eclipsed all of its rivals.1
Table 15.1 on the next page highlights the evolution of the various Ethernet standards.
Table 15.1 The evolution of Ethernet
a. Unshielded twisted pair
b. Multimode and single-mode fiber
How Ethernet works
Ethernet can be described as a polite dinner party at which guests (computers) don’t interrupt each other but rather wait for a lull in the conversation (no traffic on the network cable) before speaking. If two guests start to talk at once (a collision) they both stop, excuse themselves, wait a bit, and then one of them starts talking again.
The technical term for this scheme is CSMA/CD:
• Carrier Sense: you can tell whether anyone is talking.
• Multiple Access: everyone can talk.
• Collision Detection: you know when you interrupt someone else.
The actual delay upon collision detection is somewhat random. This convention avoids the scenario in which two hosts simultaneously transmit to the network, detect the collision, wait the same amount of time, and then start transmitting again, thus flooding the network with collisions. This was not always true!
Exhibit A A polite Ethernet dinner party
Ethernet topology
The Ethernet topology is a branching bus with no loops; there is only one way for a packet to travel between any two hosts on the same network. Ethernet provides a mechanism to exchange three types of packets on a segment: unicast, multicast, and broadcast. Unicast packets are addressed to only one host. Multicast packets are addressed to a group of hosts. Broadcast packets are delivered to all hosts on a segment.
A “broadcast domain” is the set of hosts that receive packets destined for the hardware broadcast address, and there is exactly one broadcast domain for each logical Ethernet segment. Under the early Ethernet standards and media (such as 10Base5), physical segments and logical segments were exactly the same since all the packets traveled on one big cable, with host interfaces strapped onto the side of it.2
With the advent of modern switches, today’s logical segments usually consist of many (possibly dozens or hundreds) physical segments to which only two devices are connected: the switch port and the host. The switches