Chapter 9 - Ethernet

Ethernet is a new largest LAN technology uses today. The first foundation of the Ethernet has begun on 1970 with a program Alohanet. Alohanet was a digital radio network designed to transmit information over a shared radio frequency between the Hawaiian Islands. There first attempt was to use thick coaxial cables, directly connected in a backbone and can cover only 500 meters. But still, the Ethernet was designed to accommodate multiple computers that were interconnected on a shared bus topology. Because of LANs grew larger and  increasing demands , the thick coaxial cables are replaced by a UTP cables which are less expensive, easy to use and lightweight. And its physical topology was replaced by a hub. And recently today, the hubs are replaced a switch because switches can control the flow of data by isolating each port and sending a frame only to its proper destination (if the destination is known), rather than send every frame to every device, and later it called full duplex communication. While the hub shared a media, it means it can successfully transmit only one station at a time, this type of connection is called half duplex communication. The switch has enabled the development of 1Gbps Ethernet and beyond. Because of its increasing demand the Ethernet beyond the LAN has fully developed.

                In 1985, the Institute of Electrical and Electronics Engineers (IEEE) standards committee for Local and Metropolitan Networks published standards for LANs. The standard for Ethernet is 802.3 in the layer 1 and layer 2 of the Ethernet it is used. In the Data Link layer (which is known as Media Access Control sublayer) and the Physical layer this are the two layers of the OSI model which the Ethernet operates. At Ethernet Layer 1 involves signals, bit streams that travel on the media, physical components that put signals on media, and various topologies. Ethernet Layer 1 performs a key role in the communication that takes place between devices, but each of its functions has limitations. At Ethernet Layer 2 addresses limitations. The MAC sublayer is concerned with the physical components that will be used to communicate the information and prepares the data for transmission over the media. Media Access Control (MAC) is implemented by hardware, it is in the computer Network Interface Card (NIC).The Ethernet MAC sublayer has two primary responsibilities: Data Encapsulation and Media Access Control. In the data encapsulation this provides three primary functions: Frame delimiting; Addressing; and Error detection. In MAC sublayer controls the placement of frames on the media and the removal of frames from the media. This includes the initiation of frame transmission and recovery from transmission failure due to collisions. MAC addressing is a part of a Layer 2 PDU. An Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits.

                In Ethernet, different MAC addresses are used for Layer 2 unicast, multicast, and broadcast communications. In a unicast MAC address is the unique address used when a frame is sent from a single transmitting device to single destination device. In a broadcast the packet contains a destination IP address that has all ones (1s) in the host portion. The multicast represent a group of addresses (sometimes called a host group), they can only be used as the destination of a packet. The source will always have a unicast address.

                Using hubs in an extended star it can create large collision domains. Collision domain is a network segment where if one particular device sends a frame on a network segment, every other device on that same segment will process that frame. This also means that if two or more devices on that same segment transmit a frame at the same time there will be a collision. Importantly, only one device in the entire network can send data at any one time.

The Ethernet LAN switches use five basic operations:

Learning - the switch needs the source MAC address from the frame received on a port and stores it in the MAC address table for use in the forwarding of frames to host.

Aging - The entries in the MAC table acquired by the Learning process are time stamped. This timestamp is used as a means for removing old entries in the MAC table.

Flooding – The sending of frames through all outgoing ports.

Selective forwarding - is the process of examining a frame's destination MAC address and forwarding it out the appropriate port.

Filtering – a frame that is not forwarded.

The ARP protocol provides two basic functions: Resolving IPv4 addresses to MAC addresses and Maintaining a cache of mappings. Ethernet uses the Address Resolution Protocol to determine the MAC addresses of destinations and map them against known Network layer addresses.