What is Ethernet 802.3

Ethernet is a family of computer networking technologies for local area networks (LANs) commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies. IEEE 802.3 is a working group and a collection of IEEE standards produced by the working group defining the physical layer and data link layer’s media access control (MAC) of wired Ethernet.

IEEE 802.3 Frame Format
IEEE 802.3 is a format frame which is the result of a merger of the specification IEEE 802.2 and IEEE 802.3, and consists of a header and a trailer IEEE of 802.3 and an IEEE 802.2 header.

Structure of data
An IEEE 802.3 frame consists of several fields as follows:

IEEE 802.3 header:

  •         Preamble
  •         Start Delimiter
  •         Destination Address
  •         Source Address
  •         Length

Header IEEE 802.2 Logical Link Control :

  •         Destination Service Access Point (DSAP)
  •         Source Service Access Point (SSAP)
  •         Control

    Payload
    IEEE 802.3 Trailer:

  •         Frame Check Sequence (FCS)


Ethernet frames
A data packet on the wire is called a frame. A frame begins with preamble and start frame delimiter, followed by an Ethernet header featuring source and destination MAC addresses. The middle section of the frame consists of payload data including any headers for other protocols (e.g., Internet Protocol) carried in the frame. The frame ends with a 32-bit cyclic redundancy check, which is used to detect corruption of data in transit.

Varieties of Ethernet
The Ethernet physical layer evolved over a considerable time span and encompasses quite a few physical media interfaces and several magnitudes of speed. The most common forms used are 10BASE-T, 100BASE-TX, and 1000BASE-T. All three utilize twisted pair cables and 8P8C modular connectors. They run at 10 Mbit/s, 100 Mbit/s, and 1 Gbit/s, respectively. Fiber optic variants of Ethernet offer high performance, electrical isolation and distance (tens of kilometers with some versions). In general, network protocol stack software will work similarly on all varieties.

Ethernet protocols refer to the family of local-area network (LAN) covered by the IEEE 802.3. In the Ethernet standard, there are two modes of operation: half-duplex and full-duplex modes. In the half duplex mode, data are transmitted using the popular Carrier-Sense Multiple Access/Collision Detection (CSMA/CD) protocol on a shared medium. The main disadvantages of the half-duplex are the efficiency and distance limitation, in which the link distance is limited by the minimum MAC frame size. This restriction reduces the efficiency drastically for high-rate transmission. Therefore, the carrier extension technique is used to ensure the minimum frame size of 512 bytes in Gigabit Ethernet to achieve a reasonable link distance.

Four data rates are currently defined for operation over optical fiber and twisted-pair cables:

  • 10 Mbps – 10Base-T Ethernet (IEEE 802.3)
  • 100 Mbps – Fast Ethernet (IEEE 802.3u)
  • 1000 Mbps – Gigabit Ethernet (IEEE 802.3z)
  • 10-Gigabit – 10 Gbps Ethernet (IEEE 802.3ae).

As with all IEEE 802 protocols, the ISO data link layer is divided into two IEEE 802 sublayers, the Media Access Control (MAC) sublayer and the MAC-client sublayer. The IEEE 802.3 physical layer corresponds to the ISO physical layer.

The MAC sub-layer has two primary responsibilities:

  • Data encapsulation, including frame assembly before transmission, and frame parsing/error detection during and after reception
  • Media access control, including initiation of frame transmission and recovery from transmission failure

The MAC-client sub-layer may be one of the following:

  • Logical Link Control (LLC), which provides the interface between the Ethernet MAC and the upper layers in the protocol stack of the end station. The LLC sublayer is defined by IEEE 802.2 standards.
  • Bridge entity, which provides LAN-to-LAN interfaces between LANs that use the same protocol (for example, Ethernet to Ethernet) and also between different protocols (for example, Ethernet to Token Ring). Bridge entities are defined by IEEE 802.1 standards.

Protocol Structure – Ethernet: IEEE 802.3 Local Area Network protocolsThe basic IEEE 802.3 Ethernet MAC Data Frame for 10/100Mbps Ethernet:

7 1 6 6 2 46-1500bytes 4
Pre SFD DA SA Length Type Data unit + pad FCS
  • Preamble (PRE)– 7 bytes. The PRE is an alternating pattern of ones and zeros that tells receiving stations that a frame is coming, and that provides a means to synchronize the frame-reception portions of receiving physical layers with the incoming bit stream.
  • Start-of-frame delimiter (SFD)– 1 byte. The SOF is an alternating pattern of ones and zeros, ending with two consecutive 1-bits indicating that the next bit is the left-most bit in the left-most byte of the destination address.
  • Destination address (DA)– 6 bytes. The DA field identifies which station(s) should receive the frame..
  • Source addresses (SA)– 6 bytes. The SA field identifies the sending station.
  • Length/Type– 2 bytes. This field indicates either the number of MAC-client data bytes that are contained in the data field of the frame, or the frame type ID if the frame is assembled using an optional format.
  • Data– Is a sequence of n bytes (46=< n =<1500) of any value. (The total frame minimum is 64bytes.)
  • Frame check sequence (FCS)– 4 bytes. This sequence contains a 32-bit cyclic redundancy check (CRC) value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames.

MAC Frame with Gigabit Ethernet Carrier Extension (IEEE 803.3z)

1000Base-X has a minimum frame size of 416bytes, and 1000Base-T has a minimum frame size of 520bytes. The Extension is a non-data variable extension field to frames that are shorter than the minimum length.

7

1

6

6

2

  Variable

4

Variable

Pre SFD DA SA Length Type Data unit + pad FCS Ext

source: global networking sites

 

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Posted on March 15, 2012, in System Networking and tagged , . Bookmark the permalink. Leave a comment.

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