In the half duplex mode, data are transmitted using the popular Carrier-Sense Multiple Access/Collision Detection (CSMA/CD) protocol on a shared medium. In the Ethernet standard, there are two modes of operation: half-duplex and full-duplex modes. This method is not popular because it requires excessive overhead that reduces performance.Įthernet : IEEE 802.3 Local Area Network (LAN) Protocols : Ethernet protocols refer to the family of local-area network (LAN)covered by the IEEE 802.3. If distances are long, time lags occur that may result in inappropriate carrier sensing, and hence collisions.ĬSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) : In CA collision avoidance), collisions are avoided because each node signals its intent to transmit before actually doing so. It is estimated that network traffic must be less than 40 percent of the bus capacity for the network to operate efficiently. This method handles collisions as they occur, but if the bus is constantly busy, collisions can occur so often that performance drops drastically. This is the technique used to access the 802.3 Ethernet network channel. A collision occurs, and both devices stop transmission, wait for a random amount of time, and then retransmit. There are two methods for avoiding these so-called collisions, listed here :ĬSMA/CD (Carrier Sense Multiple Access/Collision Detection) : CD (collision detection) defines what happens when two devices sense a clear channel, then attempt to transmit at the same time. On large networks, the transmission time between one end of the cable and another is enough that one station may access the cable even though another has already just accessed it. All devices have equal access to use the network when it is clear.Įven though devices attempt to sense whether the network is in use, there is a good chance that two stations will attempt to access it at the same time. MA (Multiple Access) indicates that many devices can connect to and share the same network.
If the channel is in use, devices wait before transmitting. Devices attached to the network cable listen (carrier sense) before transmitting. However, a node can listen to broadcasts on the medium, even its own, and determine whether the frames were transmitted.Ĭarrier Sensed Multiple Access (CSMA) : CSMA is a network access method used on shared network topologies such as Ethernet to control access to the network. If another node transmits at the same time, a collision occurs, and the frames that were transmitted are lost. In the ALOHA system, a node transmits whenever data is available to send. A shared communication system like ALOHA requires a method of handling collisions that occur when two or more systems attempt to transmit on the channel at the same time. The original system used terrestrial radio broadcasting, but the system has been implemented in satellite communication systems. It was developed in the 1970s at the University of Hawaii. Media access control, including initiation of frame transmission and recovery from transmission failure.įollowing Protocols are used by Medium Access Layer :ĪLOHA : ALOHA is a system for coordinating and arbitrating access to a shared communication channel. The MAC sub-layer has two primary responsibilities:ĭata encapsulation, including frame assembly before transmission, and frame parsing/error detection during and after reception.
In LAN nodes uses the same communication channel for transmission. Thanks to the LLC layer, it is possible to have at the same time, on the same network, multiple Layer 3 protocols.
The header of a packet LLC indicates the type of layer 3 protocol in it: most of the time, it is IP protocol, but it could be another protocol, such as IPX ( Internet Packet Exchange) for example. All WiFi data packets so carry a pack LLC, which contains itself packets from the upper network layers. The LLC layer is standardized by the IEEE as the 802.2 since the beginning 1980 Its purpose is to allow level 3 network protocols (for eg IP) to be based on a single layer (the LLC layer) regardless underlying protocol used, including WiFi, Ethernet or Token Ring, for example. In fact, the IEEE divided this layer into two layers “above” is the control layer the logical connection (Logical Link Control, LLC) and “down” the control layer The medium access (MAC). The MAC layer is the “low” part of the second OSI layer, the layer of the “data link”. The medium access layer was made necessary by systems that share a common communications medium. The Media Access Control (MAC) data communication Networks protocol sub-layer, also known as the Medium Access Control, is a sub-layer of the data link layer specified in the seven-layer OSI model.