Fig. 11.22. IEEE 802.3 MAC format.

The preamble provides signal synchronization and consists of seven bytes of alternating 1 and 0 bits. The preamble and start Frame Delimiter (SFD) Synchronize the receiver by using a 64 bit sequence of alternating 1's and 0's ending with 11 (10101010 ... 11). The destination address (DA) is the hardware address (unique in the entire world). The hardware address of the NIC is also called a MAC address or physical address. IEEE assigns 22 bits of physical address to the manufacturers of NIC. The 46 bit address is burned into the ROM of each NIC and is called the universal administered number. Source address (SA) shows the address of the source from which the frame is originated.

The "length" field indicates frame length. This two byte field defines the number of bytes in the data field. The data field contains the actual information. The IEEE specifies that the minimum size of a data field must be 46 bytes and maximum sizes 1500 bytes. If information in the data field is less than 46 bytes, extra information is added in the PAD field to increase the size to 46 bytes. Frame check sequence (FCS) is used to detect errors and corrupted information during transmission. IEEE uses CRC-32 for error detection.

Ethernet Media. The Ethernet network uses four different media. They are 10 Base 5, 10 Base 2, 10 Base T and 10 Base-F.

10 Base 5 (Thick Net). It uses 10 Mbps Ethernet media with base band signalling with maximum segment lengths of 500 meters. The maximum length of the Attachment Unit Interface cable (AVI) is 50 meters. A maximum of five segments can be connected with a total of 2.5 km and not more than four repeaters in a path. The minimum distance between transceivers is 2.5 meters, allowing a maximum of 1000 stations at all. Both ends of the segments must be terminated by a 50 Q resistor.

10 Base 5 is occasionally used for network back bones. Connector DIX is used for 10 Base 5 that come with an NIC.

10 Base 2 (ThinNet). It uses 10 Mbps Ethernet media with maximum segment lengths of 185 m. The maximum length of a network cable is 925 meters with four repeaters. The transceiver is built into the NIC. No more than 30 connections are allowed per segment. 10 Base 2 uses thin coaxial cable.

BNC connectors are used for 10 Base 2 connections. Hence, the connectors and cables in this case are NICs thin coaxial cable, and a T shaped device called BNC-T connector.

Fig. 11.23 shows the NIC and components of ThinNet.

10 Base T. It uses 10 Mbps 22 to 26 AWG twisted pair cable (unshielded) instead of coaxial cable. A maximum segment length of 100 meter is supported. The tranceiver is built into NIC. The devices are connected to 10 Base T hub in a star topology. 10 Base T allows a maximum of four repeaters connected together and the maximum diameter is 500 m.

RJ 45 connector is used for 10 Base T connections. Devices with standard AVI connectors may be attached to the hub by using a 10 Base T receiver. Many companies currently use 10 Base T media.

10 Base F. It uses 10 Mbps Ethernet media over fiber optic cable. Low cost, Interference free electrical characteristics and high speed are the advantage of this media.

High Speed Ethernets. Due to recent advances in microprocessor technology, end devices (computers) communicates large amount of data, high end graphics including 3-D images at a high speed. The transfer of this type of data between modern computers through Ethernet network is very slow. Many companies upgraded their networks to Fast Ethernet which offers a data rate of 100 Mbps. The goal of the fast Ethernet is to increase the bandwidth of Ethernet network while using the same CSMA/CD protocol. Rapid researches are under process to cope up with the advances in microprocessor technology. Following are the currently available fast Ethernet.

Terminator (For first or last workstation)

RJ-45 Register Jack (a) NIC

(b) Components of thinnet

Thin ethernet cable

(For intermediate workstation)

Terminator (For first or last workstation)

RJ-45 Register Jack (a) NIC

(b) Components of thinnet

1. IEEE 802.3m is an extension of IEEE 802.3 and was approved as the standard for Fast Ethernet in 1995. Fast Ethernet offers a data rate of 100 Mbps. The media for Fast Ethernet are 100 Base T4, 100 Base TX and 100 Base FX.

2. Gigabit Ethernet is a new technology that is compatible with Ethernet and Fast Ethernet. It transfers data at one gigabit per second or 100 times faster than standard Ethernet. By adding Quality of Service (QoS), the Gigabit Ethernet is able to handle all types of data transmission, even voice and video information.

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