Notes on Ethernet
Adapted From: http://en.wikipedia.org/wiki/Ethernet and related sites
Ethernet:
Ethernet is a large and diverse family of frame-based computer networking technologies for local area networks (LANs). The name comes from the physical concept of the ether. It defines a number of wiring and signaling standards for the physical layer, two means of network access at the Media Access Control (MAC)/data link layer, and a common addressing format.
LAN:
A Local Area Network (LAN) is a computer network covering a small local area, like a home, office, or small group of buildings such as a home, office, or college. Current LANs are most likely to be based on switched Ethernet or Wi-Fi technology running at 10, 100 or 1,000 Mbit/s (1,000 Mbit/s is also known as 1 Gbit/s).
The defining characteristics of LANs in contrast to WANs (wide area networks) are: a) much higher data rates, b) smaller geographic range - at most a few kilometers - and c) they do not involve leased telecommunication lines. "LAN" usually does not refer to data running over local analog telephone lines, as on a private branch exchange (PBX).
IEEE 802.3 :
IEEE 802.3 is a collection of IEEE standards defining the physical layer and data link layer of wired Ethernet. This is generally a LAN technology with some WAN applications. Physical connections are made between nodes and/or infrastructure devices (hubs, switches, routers) by various types of copper or fiber cable.
802.3 is a technology that can support the IEEE 802.1 network architecture.
The maximum packet size is 1518 bytes, although to allow the Q-tag for Virtual LAN and priority data in 802.3ac it is extended to 1522 bytes. If the upper layer protocol submits a PDU (Protocol data unit) less than 64 bytes, 802.3 will pad the data field to achieve the minimum 64 bytes.
Although it is not technically correct, the terms "packet" and "frame" are used interchangeably. The ISO/IEC 8802-3 ANSI/IEEE 802.3 Standards refer to MAC sub-layer frames consisting of the Destination Address, Source Address, Length/Type, data, and FCS fields. The Preamble and SFD are (usually) considered a header to the MAC Frame. This header plus the MAC Frame constitute a "Packet".
10-gigabit Ethernet:
10GbE is the most recent (as of 2005) and fastest of the Ethernet standards. It defines a version of Ethernet with a nominal data rate of 10 Gbit/s, ten times faster than gigabit Ethernet. 10GbE over fiber and InfiniBand is specified by the IEEE 802.3-2005 standard. 10GbE over twisted pair is forthcoming under the IEEE 802.3an amendment.
(http://en.wikipedia.org/wiki/10_gigabit_ethernet)
Ether:
In the late 19th century luminiferous aether ("light-bearing aether") was the term used to describe a medium for the propagation of light. Later theories including special relativity were formulated without the ether concept, and today the aether is considered to be an obsolete scientific theory.
The word "aether" stems via Latin from the Greek αιθηρ, from a root meaning "to kindle/burn/shine", which signified the substance thought in ancient times to fill the upper regions of space, beyond the clouds.
Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium. The methods used show some similarities to radio systems (though there are major differences, like the fact that it is much easier to detect collisions in a cable broadcast system than a radio broadcast). The common cable providing the communication channel was likened to the ether (a reference to the luminiferous ether) and it was from this reference that the name 'Ethernet' was derived.
From this early and comparatively simple concept Ethernet evolved into the complex networking technology that today powers the vast majority of local computer networks. The coaxial cable was later replaced with point-to-point links connected together by hubs and/or switches in order to reduce installation costs, increase reliability, and enable point-to-point management and troubleshooting. StarLAN was the first step in the evolution of Ethernet from a coaxial cable bus to a hub-managed, twisted pair network. The advent of twisted-pair wiring enabled Ethernet to become a commercial success.
On top of the physical layer Ethernet stations communicate to each other by sending each other data packets, small blocks of data that are individually sent and delivered. As with other IEEE 802 LANs, each Ethernet station is given a single 48-bit MAC address, which is used both to specify the destination and the source of each data packet. Network interface cards (NICs) or chips normally do not accept packets addressed to other Ethernet stations. Adapters generally come programmed with a globally unique address but this can be overridden either to avoid an address change when an adapter is replaced or to use locally administered addresses.
Despite the huge changes in Ethernet from a thick coaxial cable bus running at 10 Mbit/s to point-to-point links running at 1 Gbit/s and beyond, the different variants remain essentially the same from the programmer's point of view and are easily interconnected using readily available inexpensive hardware. This is because the frame format remains the same, even though network access procedures are radically different.
Due to the ubiquity of Ethernet, the ever-decreasing cost of the hardware needed to support it and the reduced panel space needed by twisted pair Ethernet, most manufacturers now build the functionality of an Ethernet card directly into PC motherboards obviating the need for installation of a separate network card.
OSI Model:
7 Application layer
6 Presentation layer
5 Session layer
4 Transport layer
3 Network layer
2 Data link layer
1 Physical layer
How framing works:
In telecommunications, a frame is a packet which has been encoded for transmission over a particular link.
This process involves, at a minimum, adding delimiters to distinguish the packet from dead air, address and control fields specific to the link, and checksums to detect errors. Sometimes the address, control, and checksum fields from the higher-level protocol are used directly.
Network Computing:
Computer networking is the scientific and engineering discipline concerned with communication between computer systems. Such networks involves at least two computers, which can be separated by a few centimeters (e.g. via Bluetooth) or thousands of kilometers (e.g. via the Internet). Computer networking is sometimes considered a sub-discipline of telecommunications.
MAC:
The Media Access Control (MAC) sublayer is the part of the OSI network model data link layer that determines who is allowed to access the physical media at any one time. It acts as an interface between the Logical Link Control sublayer and the network's physical layer.
The MAC sublayer is primarily concerned with the control of access to the physical transmission medium (i.e. which of the stations attached to the wire or frequency range has the right to transmit?) or low-level media-sharing protocols like CSMA/CD.
The Data-Link Layer:
The upper Data-Link sub-layer, the Logical Link Control (LLC) sub-layer takes care of: recognizing where frames begin and end in the bit-stream received from the physical layer (when receiving) delimiting the frames (when sending), i.e. inserting information (e.g. some extra bits) into or among the frames being sent so that the receiver(s) are able to recognize the beginning and end of the frames detection of transmission errors by means of e.g. inserting a checksum into every frame sent and recalculating and comparing them on the receiver side [1]
inserting the source and destination MAC addresses into every frame transmitted filtering out the frames intended for the station by verifying the destination address in the received frames
The data link layer is layer two of the seven-layer OSI model. It responds to service requests from the network layer and issues service requests to the physical layer.
This is the layer which transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment. The data link layer provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the Physical layer. Examples of data link protocols are Ethernet for local area networks and PPP, HDLC and ADCCP for point-to-point connections.
The data link is all about getting information from one place to a selection of other places. At this layer one does not need to be able to go everywhere, just able to go somewhere else. So in social contact, one needs to know at least one other person, but not necessarily know Fred Bob,
The data link provides data transfer across the physical link. That transfer might or might not be reliable; many data link protocols do not have acknowledgments of successful frame reception and acceptance, and some data link protocols might not even have any form of checksum to check for transmission errors. In those cases, higher-level protocols must provide flow control, error checking, and acknowledgments and retransmission.