Wired LANs: Ethernet

Definition of Terms

• Ethernet is the most widely used local area network protocol.
• The original Ethernet was created in 1976 at Xerox’s Palo Alto Research Center (PARC). Since then, it has gone through four generations.
• The IEEE 802.3 Standard defines I-persistent CSMA/CD as the access method for first-generation 10-Mbps Ethernet.
• The data link layer of Ethernet consists of the LLC sublayer and the MAC sublayer.
• The MAC sublayer is responsible for the operation of the CSMAlCD access method and framing.
• Each station on an Ethernet network has a unique 48-bit address imprinted on its network interface card (NIC).
• The minimum frame length for lO-Mbps Ethernet is 64 bytes; the maximum is 1518 bytes.
• The common implementations of lO-Mbps Ethernet are lOBase5 (thick Ethernet), 10Base2 (thin Ethernet), lOBase-T (twisted-pair Ethernet), and lOBase-F (fiber Ethernet).
• The 10Base5 implementation of Ethernet uses thick coaxial cable. lOBase2 uses thin coaxial cable. lOBase-T uses four twisted-pair cables that connect each station to a common hub. lOBase-F uses fiber-optic cable.
• A bridge can increase the bandwidth and separate the collision domains on an Ethernet LAN.
• A switch allows each station on an Ethernet LAN to have the entire capacity of the network to itself.
• Full-duplex mode doubles the capacity of each domain and removes the need for the CSMAlCD method.
• Fast Ethernet has a data rate of 100 Mbps. 
• Fast Ethernet was designed to compete with LAN protocols such as FDDI or Fiber Channel. IEEE created Fast Ethernet under the name 802.3u. Fast Ethernet is backward-compatible with Standard Ethernet, but it can transmit data 10 times faster at a rate of 100 Mbps.
• In Fast Ethernet, autonegotiation allows two devices to negotiate the mode or data rate of operation.
• The Fast Ethernet reconciliation sublayer is responsible for the passing of data in 4-bit format to the MII.
• The Fast Ethernet MII is an interface that can be used with both a 10- and a 100-Mbps interface.
• The Fast Ethernet PHY sublayer is responsible for encoding and decoding.
• The common Fast Ethernet implementations are 1OOBase-TX (two pairs of wistedpair cable), lOOBase-FX (two fiber-optic cables), and 100Base-T4 (four pairs of voice-grade, or higher, twisted-pair cable).
• Gigabit Ethernet has a data rate of 1000 Mbps.
• Gigabit Ethernet access methods include half-duplex using traditional CSMA/CD (not common) and full-duplex (most popular method).
• The Gigabit Ethernet reconciliation sublayer is responsible for sending 8-bit parallel data to the PHY sublayer via a GMII interface.
• The Gigabit Ethernet GMII defines how the reconciliation sublayer is to be connected to the PHY sublayer.
• The Gigabit Ethernet PHY sublayer is responsible for encoding and decoding.
• The common Gigabit Ethernet implementations are 1000Base-SX (two optical fibers and a short-wave laser source), 1000Base-LX (two optical fibers and a long-wave laser source), and 1000Base-T (four twisted pairs).
• The latest Ethernet standard is Ten-Gigabit Ethernet that operates at 10 Gbps. The three common implementations are lOGBase-S, 10GBase-L, and 10GBase-E. These implementations use fiber-optic cables in full-duplex mode.
• In the full-duplex mode of Gigabit Ethernet, there is no collision; the maximum length of the cable is determined by the signal attenuation in the cable.
• In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication among equipment from a variety of manufacturers. Project 802 is a way of specifying functions of the physical layer and the data link layer of major LAN protocols.
• Medium access methods can be categorized as random, controlled, or channelized.
• In the carrier sense multiple-access (CSMA) method, a station must listen to the medium prior to sending data onto the line.
• A persistence strategy defines the procedure to follow when a station senses an occupied medium.
• Carrier sense multiple access with collision detection (CSMA/CD) is CSMA with a postcollision procedure.
• Carrier sense multiple access with collision avoidance (CSMA/CA) is CSMA with procedures that avoid a collision.
• Reservation, polling, and token passing are controlled-access methods.
• In the reservation access method, a station reserves a slot for data by setting its flag in a reservation frame.
• In the polling access method, a primary station controls transmissions to and from secondary stations.
• In the token-passing access method, a station that has control of a frame called a token can send data.
• Channelization is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between stations on a network.
• FDMA, TDMA, and CDMA are channelization methods.
• In FDMA, the bandwith is divided into bands; each band is reserved fro the use of a specific station.
• In TDMA, the bandwidth is not divided into bands; instead the bandwidth is timeshared.
• In CDMA, the bandwidth is not divided into bands, yet data from all inputs are transmitted simultaneously.
• CDMA is based on coding theory and uses sequences of numbers called chips. The sequences are generated using Walsh tables.
• The preamble is a 56-bit field that provides an alert and timing pulse. It is added to the frame at the physical layer and is not formally part of the frame. SFD is a one byte field that serves as a flag.
• A multicast address identifies a group of stations; a broadcast address identifies all stations on the network. A unicast address identifies one of the addresses in a group.
• A layer-2 switch is an N-port bridge with additional sophistication that allows faster handling of packets.
• The least significant bit of the first byte defines the type of address. If the bit is 0, the address is unicast; otherwise, it is multicast.
• The broadcast destination address is a special case of the multicast address in which all bits are 1s.

Ethernet evolution through four generations

The Data Rates

• Standard Ethernet -10 Mbps
• Fast Ethernet - 100 Mbps
• Gigabit Ethernet - 1 Gbps
• Ten-Gigabit Ethernet - 10 Gbps

Categories of Standard Ethernet

Summary of Standard Ethernet implementations

The common Fast Ethernet implementations:

• 100Base-TX
• 100Base-FX
• 100Base-T4

Summary of Fast Ethernet implementations

The common Gigabit Ethernet implementations:

• 1000Base-SX
• 1000Base-LX
• 1000Base-CX
• 1000Base-T

Summary of Gigabit Ethernet implementations

The common Ten-Gigabit Ethernet implementations:

• 10GBase-S
• 10GBase-L
• 10GBase-E

Summary of Ten-Gigabit Ethernet implementations

Note: You can proceed to take the multiple choice exam regarding this topic. Wired LANs: Ethernet - Set 1 MCQs

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