Ethernet was the first Local Area Network (LAN) technology and remains the most important one. It was developed during the early 1970s by Xerox PARC. The original Ethernet enabled computers located within a few hundred yards of one another to exchange messages. By adding repeaters and bridges between multiple LANs, that distance has been extended to a few thousand yards. Thus, it is suitable for connecting the computers in a building or campus.
Ethernet architecture is based on the concept of connecting multiple computers to a long cable, sometimes called the ether, thereby forming a bus structure. Each computer is fitted with an Ethernet adapter that includes a unique 48-bit address for that computer. Each computer is joined to the ether through a transceiver that forms a logical "T." The transceiver receives Ethernet messages on the cable, looks at the address, and either passes the message to its computer, if the address matches, or transmits it down the cable, if the address does not match.
A single (logical) Ethernet cable forms a local area network. Two LANs can be joined through so-called bridges. A bridge is a special computer connected to two LANs. When it receives a message, it determines which of the two networks the computer being addressed is on and forwards the message to the appropriate network.
Messages are variable length records that range form 64 to slightly more than fifteen hundred bytes (or "octets") of data. The format of an Ethernet message, called a frame, is shown, below.
Internets are formed by joining two or more local area networks, often Ethernets, through routers. When this is done, IP packets of data are enclosed as the data portion of an Ethernet (or other LAN protocol) frame.
Ethernet originally operated at 10Mbps. The current standard is 100Mbps,and we can anticipate Gbps within the next few years. Thus, as Ethernet continues to evolve, many believe it will obviate the need for ATN and other more complex network architectures.
- 1970s - developed by Xerox PARC
- Connect multiple computers relatively close to one another
- Bus connection
- adapter board connects to host interface
- attachment Unit Interface (AUI) cable
- thick coaxial
- thinnet coaxial, with internal transceiver
- 10BASE-T twisted pair, with hub
- Bus topology
- linear "wire" with connected computers
- repeaters extend wire
- bridges join Ethernets
- permanently assigned to physical devices
- 48 bits long
- communication takes place within variable length packets, called frames,
- 64 - 1518 octets (bytes) in length
- preamble: 8
- destination address: 6
- source address: 6
- frame type 2
- data: 64 - 1500
- CRC (error check): 4
Ethernet Frame. Adapted from Comer, Internetworking with TCP/IP, Figure 2.8.
- 10 Mbit broadcast bus technology with best-effort delivery semantics and distributed access control
- transceiver "listens" on wire
- when perceives wire idle, transmits
- when determines collision has occurred
- generates random number
- waits that interval
- if another collision occurs, doubles wait and retransmits
- applies recursively, with exponential waits
- when frame address matches host address, receives
- bridges block or forward frames between LANs, using adaptive learning strategies
- Interconnected LANs appear as single integrated (virtual) LAN