18-07-2013, 04:24 PM
GIGABIT ETHERNET
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INTRODUCTION
Ethernet is the world's most pervasive networking technology, since the 1970's. It is estimated that in 1996, 82% of all networking equipment shipped was Ethernet. In 1995, the Fast Ethernet standard was approved by the IEEE . Fast Ethernet provided 10 times higher bandwidth, and other new features such as full-duplex operation, and auto-negotiation. This established Ethernet as a scalable technology.
Now, with the emerging Gigabit Ethernet standard, it is expected to scale even further .The Fast Ethernet standard was pushed by an industry consortium called the Fast Ethernet Alliance.
A similar alliance, called the Gigabit Ethernet Alliance, was formed by, 11 companies in May 1996, soon after IEEE announced the formation of the 802.3z Gigabit Ethernet Standards project. At last count, there were over 95 companies in the alliance from the networking, computer and integrated circuit industries. A draft 802.3z standard was issued by, IEEE in July 1997.
EVOLUTION OF ETHERNET
Today, Ethernet is synonymous with the IEEE 802.3 standard for a "1-persistent CSMA/CD LAN". The 802.3 standard has an interesting history. The beginning is generally considered to be the University of Hawaii ALOHA network. This system is the ancestor of all shared media networks.
The original Ethernet, developed by Xerox was based on the ALOHA system. It was a 2.94 Mbps CSMA/CD system and was used to connect over 100 personal workstations on a 1 Km cable. It was so successful, that Xerox, DEC and Intel came up with a 10 Mbps standard. The IEEE 802.3 standard was based on the 10 Mbps Ethernet.
CSMA/CD refers to the protocol used by stations sharing the medium, to arbitrate use of the medium. A sender has to "listen" to the medium. If no one else is transmitting, then the sender may transmit. If two senders start transmitting at the same time, then a collision is said to have occurred. Transmitting stations, therefore, have to listen to the medium for collisions while transmitting, and retransmit a packet after some time, if a collision occurs.
The original 802.3 standard was published in 1985. Originally two types of coaxial cables were used called Thick Ethernet and Thin Ethernet. Later unshielded copper twisted pair (UTP), used for telephones, was added.
NEED OF UPGRADING TO GIGABIT ETHERNET
1. Applications in the modern enterprise make exacting demands from the network, and put a lot of pressure on the desktop, server, hub, and the switch for increased bandwidth. Megabytes of data will have to flow unhindered across intranets as communication within enterprises will move on from text-based e-mail messages to bandwidth-intensive real-time audio, video and voice.
2. An increasing number of enterprises are employing data warehousing for strategic planning. This implies increased volumes of data and low transmission latency. These warehouses may comprise of terabytes of data distributed over hundreds of platforms and accessed by thousands of users, and must be updated regularly to provide users near-real-time data for critical business reports and analyses.
3. Archiving an enterprise's mission critical information scattered across a network usually occur during off hours. Even then, it requires large amounts of bandwidth for a fixed period of time (4 to 8 hours). The backup involves gigabytes or terabytes of data distributed over hundreds of servers and storage systems throughout an enterprise.
Enterprise critical applications will proliferate and demand ever- greater shares of bandwidth at the desktop. As the number of users grows rapidly, enterprises will need to migrate critical portions of their networks (if not the whole network itself) to higher-bandwidth technologies.
Hence, crossing over from megabit to gigabit Ethernet will be inevitable, starting at the backbone.
MAC LAYER
The MAC Layer of Gigabit Ethernet uses the same CSMA/CD protocol as Ethernet. The maximum length of a cable segment used to connect stations is limited by the CSMA/CD protocol. If two stations simultaneously detect an idle medium and start transmitting, a collision occurs.
Ethernet has a minimum frame size of 64 bytes. The reason for having a minimum size frame is to prevent a station from completing the transmission of a frame before the first bit has reached the far end of the cable, where it may collide with another frame. Therefore, the minimum time to detect a collision is the time it takes for the signal to propagate from one end of the cable to the other. This minimum time is called the Slot Time.
Packet Bursting
Carrier Extension is a simple solution, but it wastes bandwidth. Up to 448 padding bytes may be sent for small packets. This results in low throughput. In fact, for a large number of small packets, the throughput is only marginally better than Fast Ethernet. Packet Bursting is an extension of Carrier Extension. Packet Bursting is "Carrier Extension plus a burst of packets".
When a station has a number of packets to transmit, the first packet is padded to the slot time if necessary using carrier extension. Subsequent packets are transmitted back to back, with the minimum Inter-packet gap (IPG) until a burst timer (of 1500 bytes) expires. Packet Bursting substantially increases the throughput. Fig. 2 shows how Packet Bursting works.
GMII (Gigabit Media Independent Interface)
The various layers of the Gigabit Ethernet protocol architecture are shown in Fig. 3. The GMII is the interface between the MAC layer and the Physical layer. It allows any physical layer to be used with the MAC layer. It is an extension of the MII (Media Independent Interface) used in Fast Ethernet. It uses the same management interface as MII. It supports 10, 100
and 1000 Mbps data rates. It provides separate 8-bit wide receive and transmit data paths, so it can support both full duplex as well as half-duplex operation.
CONCLUSION
Ethernet was initially developed in the 1970s, and is now the most widespread network technology in the world. As a result of the standardization of Gigabit Ethernet in June 1998, the scalability of Ethernet was again significantly improved. With a bandwidth of 1000 Mbps (1 Gbps), standard is compatible with Ethernet and Fast Ethernet. From today’s perspective, Gigabit Ethernet will be used as the backbone technology in corporate networks.
Gigabit Ethernet is the third generation Ethernet technology offering a speed of 1000 Mbps. It is fully compatible with existing Ethernets, and promises to offer seamless migration to higher speeds. Existing networks will be able to upgrade their performance without having to change existing wiring, protocols or applications. Gigabit Ethernet is expected to give existing high - speed technologies such as ATM and FDDI a run for their money.
The technology is further primarily suited to increasing the data transfer rate between clients and server farms, and to connecting Fast Ethernet switches. One other area of application is linking workstations and servers with very high bandwidth requirements, as in image editing or CAD. It is to be assumed that Gigabit Ethernet will be used predominant in the more powerful full-duplex mode.