A system based on collision detect must control the time required for the worst round trip through the LAN. This round trip is limited to 50 microseconds, and at a signaling speed of 10 Mbps, this is enough time to transmit 500 bits. At 8 bits per byte, this is slightly less than 64 bytes. To make sure that the collision is recognized, Ethernet requires that a station must continue transmitting until the 50 microsecond period has ended. If the station has less than 64 bytes of data to send, then it must pad the data by adding zeros at the end [3]. .
GIGABIT ETHERNET (IEEE 802.3z).
The standard for gigabit Ethernet increases the speed of the original Ethernet to 1 billion bits per second (1000 Mb/s or 1 Gb/s) for interconnecting switches, routers, and servers in the backbone of local area networks. This standard consists of a series of updates to the 802.3 base standard with a group of new clauses which pertain only to 1 Gb/s operation. These standards address the physical and data link layers, which are divided into sublayers [4]. The various layers of the Gigabit Ethernet protocol architecture are shown in Fig. 3. .
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PHYSICAL LAYER.
The Physical Layer of Gigabit Ethernet uses a mixture of proven technologies from the original Ethernet in addition to Fibre Channel Specification. It is expected to support four physical media types. These will be defined in 802.3z (1000Base X) and 802.3ab (1000Base T) [1]. .
1000Base X.
The 1000Base X standard is based on the Fibre Channel Physical Layer. It has a four layer architecture. The lowest two layers FC 0 (Interface and media) and FC 1 (Encode/Decode) are used in Gigabit Ethernet. Since Fibre Channel is a proven technology, re using it will greatly reduce the Gigabit Ethernet standard development time. Three types of media are included in the 1000Base X standard: 1000 Base-SX (850 nm laser on multi-mode fiber), 1000 Base-LX(1300 nm laser on single-mode and multi-mode fiber), 1000 Base-CX(short haul copper - shielded twisted pair cable).