Sangar Rajagopal

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.

CONNECTING DEVICES Hosts and networks do not normally operate in isolation. We use connecting devices to connect hosts together to make a network or to connect networks together to make an internet. Connecting devices can operate in different layers of the Internet model. We discuss three kinds of connecting devices: hubs, link-layer switches, and routers. Hubs today operate in the first layer of the Internet model. Link-layer switches operate in the first two layers. Routers operate in the first three layers. Hubs A hub is a device that operates only in the physical layer. Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data. A repeater receives a signal and, before it becomes too weak or corrupted, regenerates and retimes the original bit pattern. The repeater then sends the refreshed signal. In the past, when Ethernet LANs were using bus topology, a repeater was used to connect two segments

ISO-OSI Model OSI stands for Open Systems Interconnection . It has been developed by ISO – ‘ International Organization of Standardization ‘, in the year 1974. It is a Seven layer architecture with each layer having specific functionality to perform. All these seven layers work collaboratively to transmit the data from one person to another across the globe.   A layer should be created where different level of abstraction is needed. Each layer should perform a well defined function. The function of each layer should be chosen according to the internationally standardized protocols. The number of layers should be large enough that distinct functions should not be put in the same layer and small enough that the architecture does not become very complex. Physical Layer It is the bottom layer of OSI Model. It is responsible for the actual physical connection between the devices. Such physical connection may be wired or wireless. It is concerned

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

Carrier Sense Multiple Access with Collision Avoidance Carrier sense multiple access with collision avoidance (CSMA/CA) was invented for wireless networks. Collisions are avoided through the use of CSMA/CA’s three strategies: the interframe space, the contention window, and acknowledgments, Interframe Space (IFS)   First, collisions are avoided by deferring transmission even if the channel is found idle. When an idle channel is found, the station does not send immediately. It waits for a period of time called the interframe space or IFS. Even though the channel may appear idle when it is sensed, a distant station may have already started transmitting. The distant station’s signal has not yet reached this station. The IFS time allows the front of the transmitted signal by the distant station to reach this station. After waiting an IFS time, if the channel is still idle, the station can send, but it still needs to wait a time equal to the contention window. The IFS variable

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Carrier sense multiple access with collision detection (CSMA/CD) augments the algorithm to handle the collision. CSMA/CD method, a station monitors the medium after it sends a frame to see if the transmission was successful. If so, the station is finished. If, however, there is a collision, the frame is sent again. To better understand CSMA/CD, let us look at the first bits transmitted by the two stations involved in the collision. Although each station continues to send bits in the frame until it detects the collision, we show what happens as the first bits collide. In below figure stations A and C are involved in the collision. At time t1, station A has executed its persistence procedure and starts sending the bits of its frame. At time t2, station C has not yet sensed the first bit sent by A. Station C executes its persistence procedure and starts sending the bits in its frame, which propagate bo