Ethernet at the Physical Layer

Started by VelMurugan, Nov 04, 2008, 11:50 PM

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Ethernet at the Physical Layer

Ethernet is the most popular Local Area Network architecture that was jointly developed by Digital Equipment Corporation, Intel Corporation and Xerox Corporation. It consists of certain specifications and standards as well as hardware devices and components. Ethernet provides services corresponding to physical layer and data link layer of the OSI reference model. Each Ethernet physical layer protocol has a three part name that summarizes its characteristics. The components specified in the naming convention correspond to LAN speed, signalling method, and physical media type.

Ethernet Topology

Ethernet topology is based on following three categories:

    * Ethernet 10 Mbps: A single LAN specification that operate at 10 Mbps over coaxial Ethernet 10base2 or 10base5 cable.
    * 100-Mbps Ethernet: A single LAN specification, also known as Fast Ethernet that operates at 100 Mbps over UTP Ethernet cable.
    * 1000-Mbps Ethernet: A single LAN specification, also known as Gigabit Ethernet that operates at 1 Gbps (1000 Mbps) over fiber optics and twisted pair Ethernet cables.

100BaseT Overview

100BaseT uses the existing IEEE 802.3 CSMA/CD specification. As a result, 100BaseT retains the IEEE 802.3 frame format, size and error-detection mechanism. In addition, it supports all applications and networking software currently running on 802.3 networks. 100BaseT maintains dual speeds of 10 and 100 Mbps using 100BaseT Fast Link Pulses (FLPs). 100BaseT hubs must detect dual speeds much like Token Ring 4/16 hubs, but adapter cards can support 10Mbps, 100 Mbps, or both.

100BaseT Signalling

100BaseT supports two signalling types:

    * 100BaseX
    * 4T+

Both signalling types are interoperable at the station and hub levels. MII ( Media Independent Interface) which same like AUI interface provides interoperability at base level. The hub provides interoperability at the hub level.

The 100BaseX signalling scheme has a convergence sublayer that adapts the full duplex continuous signalling mechanism of the FDDI Physical Medium Dependent (PMD) layer to the half duplex, start-stop signalling of the Ethernet MAC sublayer. 100BaseTX's use of the existing FDDI specification has allowed quick delivery of products to market. 100BaseX is the signalling scheme used in the 100BaseTX and the 100BaseFX media types.

The 4T+ signalling scheme uses one pair of wires for collision detection and the other three pairs to transmit data. It also permits 100BaseT to work with existing Category 3 cabling while all four pairs are installed on desktop. 4T+ is the signalling scheme used in the 100BaseT4 media type, and it supports half duplex operation only.

100BaseT Hardware

Components used for a 100BaseT physical connection include the following: Physical medium: This device carries signals between computers and can be one of three 100BaseT media types:

    * 100BaseTX
    * 100BaseFX
    * 100BaseT4

Medium Dependent Interface (MDI): The MDI is a mechanical and electrical interface between the transmission medium and the physical layer device.

Physical Layer Device (PHY): the PHY provides either 10 or 100 Mbps operation and can be a set of integrated circuits (or a daughter board) on a Ethernet port, or an external device supplied with an MII cable that plugs into an MII port on a 100BaseT device (similar to a 10 Mbps Ethernet transceiver). Media Independent Interface (MII): It is used with a 100 Mbps external transceiver to attach a 100 Mbps Ethernet device to several of these three media types. The MII has a 40 pin plug and cable that stretches up to 0.5 meters.

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