Data Communication & networking

This blog is for undergraduate/graduate students who require some basic information about their subjects or any other topic related to data communication.

Monday, 28 March 2011

The Ethernet

The most widely used high speed LANs today  are based on Ethernet and were developed by IEEE 802.3 standards committee. The original Ethernet was created in 1976 ar Xerox's Palo Alto Research Center (PARC).
Since it has gone through 4-generation.
  • Standard Etherner (10+ Mbps)
  • Fast Ethernet         (100Mbps)
  • Giga bit Ethernet    (1 Gbps)
  • Ten Giga bit Ethernet (10Gbps)
Ethernet Medium Access Techniques:
CSMA/CD and itls precursors can be termed as random access or contention techniques. The earliest  techniques were known as ALOHA and Slotted ALOHA.
ALOHA, was developed for packet radio networks. However, it is applicable to any shared transmission medium. ALOHA specifies that a staion may transmit a frame at any time.The station then listens for an amount of time equal to the maximum possible round-trip propagation delay on the network plus a small fixed time increment. If the station hears an acknowledgment during that time, fine; otherwise, it resends the frame. If the station fails to receive an acknowledgment after repeated transmissions, it gives up. 
A receiving station determines the correctness of an incoming frame by examining a frame check sequence field, as in HDLC. If the frame is valid and if the destination address in the frame header matches the receiver's address, the station immediately sends an acknowledgment. The frame may be invalid due to noise on the channel or because another station transmitted a frame at about the same time. In the latter case, the two frames may interfere with each other at the receiver so that neither gets through; this is known as a collision.
ALOHA is as simple as can be, and pays a penalty for it. Because the number of collisions rise rapidly with increased load, the maximum utilization of the channel is only about 18%.
Slotted ALOHA:
To improve efficiency, a modification of ALOHA, known as slotted ALOHA, was developed. In this scheme, time on the channel is organized into uniform slots whose size equals the frame transmission time. Some central clock or other technique is needed to synchronize all stations. Transmission is permitted to begin only at a slot boundary. Thus, frames that do overlap will do so totally. This increases the maximum utilization of the system to about 37%.
Both ALOHA and slotted ALOHA exhibit poor utilization. Both fail to take advantage of one of the key properties of both packet radio and LANs, which is that propagation delay between stations is usually very small compared to frame transmission time.

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