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ALOHA

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RANDOM-ACCESS METHODS The random-access methods we study in this chapter have evolved from a very interesting protocol known as ALOHA, which used a very simple procedure called Multiple Access (MA). The method was improved with the addition of a procedure that forces the station to sense the medium before transmitting. This was called carrier sense multiple access (CSMA). This method later evolved into two parallel methods: carrier sense multiple access with collision detection (CSMA/CD), which tells the station what to do when a collision is detected, and carrier sense multiple access with collision avoidance (CSMA/CA), which tries to avoid the collision. ALOHA ALOHA, the earliest random access method, was developed at the University of Hawaii in early 1970. It was designed for a radio (wireless) LAN, but it can be used on any shared medium. It is obvious that there are potential collisions in this arrangement. The medium is shared between the stations. When a station sen...
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CSMA,MAC

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Carrier Sense Multiple Access To minimize the chance of collision and, therefore, increase the performance, the CSMA method was developed. The chance of collision can be reduced if a station senses the medium before trying to use it. Carrier sense multiple access (CSMA) requires that each station first listen to the medium before sending. CSMA is based on the principle “sense before transmit” or “listen before talk.” CSMA can reduce the possibility of collision, but it cannot eliminate it. The reason for a space and time model of a CSMA network. Stations are connected to a shared channel (usually a dedicated medium). At time t1, station B senses the medium and finds it idle, so it sends a frame. At time t2 (t2 > t1), station C senses the medium and finds it idle because, at this time, the first bits from station B have not reached station C. Station C also sends a frame. The two signals collide and both frames are destroyed. Vulnerable Time The vulnerable t...
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Virtual LAN(VLAN)

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Virtual LAN(VLAN) The Formal definition of a virtual local area network (VLAN) as a local area network configured by software, not by physical wiring or connectivity. Above Figure shows the same switched LAN divided into VLANs. The whole idea of VLAN technology is to divide a LAN into logical, instead of physical, segments. A LAN can be divided into several logical LANs, called VLANs. Each VLAN is a work group in the organization. VLANs create broadcast domains. Here two switches are connected. This is a good configuration for a company with two separate buildings. Each building can have its own switched LAN connected by a backbone. People in the first building and people in the second building can be in the same work group even though they are connected to different physical LANs.
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