Ethernet LANs Essay
Ethernet LANs: A collection of devices, including user devices, LAN switches, routers, old hubs, and cables, all of which use IEEE Ethernet standards at the physical and data link layers, so that the devices can send Ethernet frames to each other. 802.3: The name of the original IEEE Ethernet standard, as well as the overall base name of all IEEE Ethernet LAN working committees. Fast Ethernet: The informal name for one particular Ethernet standard, originally defined formally as 802.3u, which was the first Ethernet standard to surpass the original 10-Mbps speed to run at 100 Mbps. Gigabit Ethernet: The informal name for one particular Ethernet standard, defined formally in 802.3z (for fiber) and 802.3ab (for UTP), with a speed of 1 Gbps. Autonegotiation: A process defined by the IEEE so that nodes on the same Ethernet link can exchange messages for the purpose of choosing the best speed and duplex option that both nodes support. Ethernet frame: The bytes of data that flow in an Ethernet LAN, which begins with the Ethernet header, followed by data (which actually holds headers from other layers as well as end-user data) and ends with the Ethernet trailer. Ethernet LANs deliver Ethernet frames from one Ethernet device to another.
MAC address: A data link layer address, 48 bits in length, usually written as 12 hexadecimal digits and used to represent different devices connected to LANs. MAC address table: On a LAN switch, a table of MAC addresses and local switch ports that the switch uses when making its decision of where to forward Ethernet frames that arrive at the switch. Wired LAN: A local-area network (LAN) that uses cables/wires; the word wired refers to the wires inside UTP cables. Wireless LAN: A group of wireless clients, plus one or more wireless access points, with the access points all using a common SSID (wireless LAN name). Star topology: A network topology in which links extend outward from a central node, somewhat like rays of light going out from a star/sun. Ethernet frame: The bytes of data that flow in an Ethernet LAN, which begins with the Ethernet header, followed by data (which actually holds headers from other layers as well as end-user data) and ends with the Ethernet trailer.
Ethernet LANs deliver Ethernet frames from one Ethernet device to another. 10BASE-T: The common name for one of several standards that are part of the IEEE Ethernet 802.3i standard. This standard uses two twisted pairs in a UTP cable, with a bit rate of 10 Mbps. 100BASE-T: A term that refers to all Fast Ethernet standards, including 100BASE-Tx, which refers to the one Fast Ethernet standard that uses two pairs in a UTP cable. 1000BASE-T: A particular Ethernet standard shortcut name, also known by the formal standard 802.3ab, which defines 1000-Mbps (1-Gbps) operation, star topology, using four-pair UTP cabling. 10GBASE-T: A particular Ethernet standard shortcut name, also known by the formal standard 802.3an, that defines 10-Gbps operation, star topology, using four-pair UTP cabling. Metro Ethernet: A type of multiaccess WAN service that uses Ethernet as the physical access link and usually uses an Ethernet switch as the customer site device, with the customer sending Ethernet frames from one customer site to the other.
Token Ring: An old LAN technology, popularized by IBM and standardized by IEEE as standard 802.5, that competed with Ethernet LANs in the 1980s and 1990s. LAN Edge: A reference to the part of the campus LAN with the end-user devices and the switches to which they connect, through an Ethernet switch or a wireless LAN access point, that contains the largest number of physical links. Wireless-only LAN edge: A campus LAN design term referring to campus LANs with only wireless connections between end-user devices and APs, and no wired Ethernet LAN connections at the edge. Wired/wireless LAN edge: A campus LAN design term referring to campus LANs, with the edge of the LAN having both wireless connections plus wired Ethernet LAN connections.
Shorthand name (IEEE): The term for a type of name for IEEE standards. These names begin with a speed, list “BASE-” in the middle, and end with a suffix, for example, 10BASE-T. Edge switch: In a campus Ethernet LAN design, this term refers to the Ethernet LAN switch to which the end-user devices connect. Duplex: A networking link that allows bits to be sent in both directions. Half duplex: A networking link that allows bits to be sent in both directions, but only one direction at a time. Full duplex: A networking link that allows bits to be sent in both directions and at the same time. Straight-through cable: A UTP cabling pinout in which the wire at pin x on one end of the cable connects to pin x on the other end of the cable. Crossover cable: A UTP cabling pinout in which the wires in a wire pair connect to different pins on opposite ends so that one node’s send logic connects to the other node’s receive logic.
In Ethernet, pins 1,2 connect to 3,6, and pins 4,5 connect to 7,8. Ethernet header: A data structure that an Ethernet node adds in front of data supplied by the next higher layer to create an Ethernet frame. The header holds these important fields: Preamble, SFD, Destination Address, Source Address, and Type. Ethernet trailer: A data structure that an Ethernet node adds after the data supplied by the next higher layer to create an Ethernet frame; the trailer holds one field, the FCS field. Destination MAC address: A field in the Ethernet header that lists the MAC address of the device to which the Ethernet frame should be delivered.
Source MAC address: A field in the Ethernet header that lists the MAC address of the device that originally sent the Ethernet frame. Media Access Control: The formal IEEE 802.3 Ethernet term for the data link layer, data-link header, and other data-link features, including addresses. Error detection: In networking, the process by which a node determines whether a received message was changed by the process of sending the data. Ethernet broadcast: A special Ethernet address, FFFF.FFFF.FFFF, used to send frames to all devices in the same Ethernet LAN. Address:
Flooding: Part of an Ethernet LAN switch’s forwarding logic in which the switch forwards a frame out all ports, except the port in which the frame arrived. Forwarding: Part of an Ethernet LAN switch’s forwarding logic that refers to the choice a switch makes to take a received frame and send it out a single outgoing port, because the frame has a destination MAC address known to the switch (as listed in the switch’s MAC address table). Learning: Part of an Ethernet switch’s logic related to the forwarding process by which the switch learns MAC addresses and their associated port numbers. Unknown unicast frame: An Ethernet frame with destination MAC address FFFF.FFFF.FFFF.
Broadcast frame: From the perspective of a single Ethernet LAN switch, a frame whose destination MAC address is not known to the switch, in that the switch’s MAC address table does not list the frame’s destination MAC address. Known unicast frame: From the perspective of a single Ethernet LAN switch, a frame whose destination MAC address is known to the switch, in that the switch’s MAC address table lists the frame’s destination MAC address. Universal MAC address: A MAC address assigned to an Ethernet device (NIC, switch port, and so on) by the manufacturer, following rules defined by the IEEE, so that the device’s universal MAC address is unique among all other universal MAC addresses in the universe.